Observations of Short-Period Stars Orbiting the Supermassive Black Hole in our Galactic Center Are Probing the Gravitational Theory

From left to right: Aurelien Hees, Andrea M. Ghez, Tuan Do

Authors: Aurelien Hees, Andrea M. Ghez, Tuan Do

Affiliation:
Galactic Center Group, University of California Los Angeles, USA

Our current understanding of fundamental physics is based on four interactions: electromagnetism, the weak interaction, the strong interaction and gravitation. The first three ones are unified in a common framework, the Standard Model of particle physics based on a relativistic quantum field theory. On the other hand, Einstein’s theory of General Relativity, the gravitational theory, has not been successfully included in this quantum framework so far. The development of a quantum theory of gravity is important to understand phenomena that take place in very strong gravitational fields like for example in our very early universe or around black holes.

In addition, Dark Matter and Dark Energy are contributing to 26% and 69% to the mass-energy content of our Universe [1]. These two components of our Universe, essential to explain some cosmological and astrophysical observations, have not been directly observed so far and are also challenging General Relativity (see e.g. [2]).

For these reasons, in the last decades, theoreticians have developed many modified gravitational theories (see [3] and references therein). On the other hand, there has been a tremendous effort to confront General Relativity with different observations and to search for a deviation from General Relativity using a large number of experiments (see the review [4]). Historically, tests of gravitation have been first performed in the Solar System and in laboratories on Earth where extremely good accuracy in the measurements can be achieved. In these low gravitational fields, the agreement between General Relativity’s predictions and observations is extremely good. It is therefore highly interesting to perform such tests in other environments, such as in strong gravitational fields.

The motion of short-period stars orbiting around the supermassive black hole in the center of our Galaxy, the Milky Way, has been tracked since 1995 at the W. M. Keck Observatory in Hawaii by the UCLA Galactic Center Group. The stars observed close to the black hole have orbital periods of the orders of 10-50 years, allowing us to measure quasi-Keplerian motion (similar to
the motion of the planets around our Sun). These observations have led to many great discoveries, the most important one being the evidence for a supermassive black hole at the center of our Galaxy [5]. Two types of observations are made at the Keck observatory: (i) astrometric observations which give the 2-dimensional position of the stars in the plane of the sky and (ii) spectroscopic measurements which give the radial velocity of the stars. Nowadays, the typical accuracy of these measurements is at the level of 0.2 milliarcsecond for astrometry and of 30 km/s for radial velocity for a bright star. Recently, observations of two of these stars, S0-2 (period: 16 years) and S0-38 (period: 19 years), have been used to measure our distance to the Galactic Center and the mass of the central supermassive black hole with an accuracy of 5% [6] (Fig. 1 shows the orbit of these two stars).

Fig 1 : Orbital motion of two stars, S0-2 and S0-38, orbiting around the supermassive black hole at the center of our Galaxy. Observations of these two stars have been used to test the gravitational theory and to constrain the presence of a hypothetical 5th force.  Credit: S. Sakai and A. Ghez, W. M. Keck Observatory/UCLA Galactic Center Group.

In a recent work [7], we used these observations to perform a test of the gravitational theory using orbital dynamics in a strong gravitational field generated by a supermassive black hole. The main novelty in using these measurements to test General Relativity comes from the fact that we are probing the gravitational theory in a gravitational field much stronger than for example in the Solar System, around a central mass which is much more massive (the black hole mass is 4 x 10⁶ the mass of the Sun) and around an extremely interesting body: a black hole.

 More precisely, short-period stars are constraining the presence of a hypothetical fifth force in our Galactic Center. A fifth interaction is predicted by many modifications of General Relativity developed in order to unify it with the Standard Model of particle physics and by models of Dark Matter and Dark Energy. This fifth force is parametrized by a length of interaction λ and a strength of interaction  α, both of these parameters being constrained observationally by Solar System observations and by laboratory measurements (see e.g. [8]). We used observations of S0-2 and S0-38 to search for a fifth force in a strong gravitational field generated by a black hole [7]. No deviation from General Relativity has been observed and new constraint on this scenario has been derived (see Fig. 2).

 Two main factors are expected to improve such an analysis in the future. First, in 2018, the star S0-2 will experience its closest approach to the supermassive black hole. At that time, the gravitational effects experienced by the star are the strongest and the possibility to measure relativistic effects and to test General Relativity is enhanced. Observations during S0-2’s closest approach in 2018 will be the most sensitive to a hypothetical deviation from General Relativity. The UCLA Galactic Center Group is currently actively preparing this wonderful event. On the other hand, the development of the next generation of extremely large telescope, such as the Thirty Meter Telescope, will allow us to detect and to track stars that are even closer to the black hole. On the long-term, this will improve significantly tests of General Relativity.

Fig 2: (click on the image to view with higher resolution) Constraints on a hypothetical fifth interaction. In green are constraints coming from Earth observations or from the LAGEOS satellites, in blue the constraints coming from Lunar Laser Ranging observations, in orange constraints from planetary ephemerides and in red the new constraint obtained using observations of stars around our Galactic Center. Our current analysis probed the 5th interaction in a strong gravity field as emphasized in the right panel (red region).

References:
[1] Planck Collaboration, “Planck 2015 results. XIII. Cosmological parameters”. Astronomy & Astrophysics, 594, A13 (2016). Abstract.
[2] Ivan Debono, George F. Smoot, “General Relativity and Cosmology: Unsolved Questions and Future Directions”. Universe, 2, 23 (2016). Abstract.
[3] Timothy Clifton, Pedro G. Ferreira, Antonio Padilla, Constantinos Skordis, “Modified gravity and cosmology”. Physics Reports, 513, 1 (2012). Abstract.
[4] Clifford M. Will, “The Confrontation between General Relativity and Experiment”. Living Review in Relativity, 17, 4 (2014). Abstract.
[5] A. M. Ghez, B. L. Klein, M. Morris, E. E. Becklin, “High Proper-Motion Stars in the Vicinity of  
Sagittarius A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy”.  Astrophysical Journal, 509, 678 (1998). Abstract.
[6] A. Boehle, A. M. Ghez, R. Schödel, L. Meyer, S. Yelda, S. Albers, G. D. Martinez, E. E. Becklin, T. Do, J. R. Lu, K. Matthews, M. R. Morris, B. Sitarski, G. Witzel, “An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis”. Astrophysical Journal, 830, 17 (2016). Abstract.
[7] A. Hees, T. Do, A. M. Ghez, G. D. Martinez, S. Naoz, E. E. Becklin, A. Boehle, S. Chappell, D. Chu, A. Dehghanfar, K. Kosmo, J. R. Lu, K. Matthews, M. R. Morris, S. Sakai, R. Schödel, G. Witzel, “Testing General Relativity with stellar orbits around the supermassive black hole in our Galactic center”. Physical. Review Letters, 118, 211101 (2017). Abstract.
[8] E.G. Adelberger, J.H. Gundlach, B.R. Heckel, S. Hoedl, S. Schlamminger, “Torsion balance experiments: A low-energy frontier of particle physics”. Progress in Particle and Nuclear Physics 62, 102 (2009). Abstract.

Glycine, an Amino Acid and Other Prebiotic Molecules in Comet 67P/Churyumov-Gerasimenko

Kathrin Altwegg 

Author: Kathrin Altwegg and the ROSINA Team 

Affiliation: Physikalisches Institut, University of Bern, Switzerland.

By now it is an established fact that comets contain the most primitive material of all solar system bodies. In situ results from the Giotto flyby at comet Halley in 1986 and remote sensing in various wavelength ranges established the presence of many organic molecules in the coma of comets. The importance of comets for the origin of life on Earth has been the topic of many discussions in the past [1]. Among the key ingredients for life as we know it are amino acids and phosphorous. Many primitive meteorites contain amino acids. However most of them are formed by aqueous alterations [2,3]. Traces of amino acids were detected in samples from the Stardust mission to comet Wild 2 [4]. However, there always remained some doubts that these amino acids were actually cometary as the extraction and analysis of the material from the aerogel and aluminium frame always involves either hot water or even acids, which could then readily form amino acid on Earth.

In order to investigate the composition of the organics and to assess the importance of comets for the origin of terrestrial life the European Space Agency ESA launched in 2004 the spacecraft Rosetta towards comet 67P/ Churyumov-Gerasimenko . This comet belongs to the Jupiter family with an aphelion at 5.5 AU and a perihelion at 1.25 AU. Its orbital period is 6.5 y. In order to match the comet’s orbit Rosetta had to flyby three times the Earth and once Mars to arrive at the comet more than 10 years after launch in August 2014. Subsequently, Rosetta flew with the comet around the Sun from 3.6 AU to perihelion and out again to 3.8 AU, sometimes as close as 10 km from the comet centre. This allowed a thorough investigation of the comet from up close during the different phases of its orbit.

On board Rosetta was the ROSINA (Rosetta Orbiter Sensor for Ion and Neutral Analysis) suite consisting of two mass spectrometers (DFMS and RTOF) and the cometary pressure sensor COPS [5]. These sensors were built to analyse the composition of the cometary atmosphere along its path around the Sun, encountering very low densities for large heliocentric distances to much more violent outgassing during perihelion.

ROSINA measured almost continuously during more than two years. Most of the time, water was the dominant component of the cometary coma. However, due to its peculiar shape and the tilted rotation axis of the comet, the coma was very heterogeneous and varying along the orbit. Apart from water, ROSINA identified many more simple molecules like CO, CO₂, HCN, CH₄ and NH₃. But it detected also many complex organics like aliphatic carbon chains, alcohols with up to five C atoms and amines [6]. Most surprisingly it detected abundant O₂ [7]. O₂ is very reactive and was believed to have been non-existent in the protosolar nebula. Very few detections of O₂ outside the Earth have been made so far. The very good correlation with water led to the finding, that O₂ was most probably formed in the presolar stage due to radiolysis of water ice and that the water ice survived the solar system formation unchanged.

In March 2015 Rosetta performed a close flyby over the comet surface of just 15 km from the comet centre. During this flyby, dust production was high. In the mass spectra of ROSINA DFMS from this flyby an analysis of the mass spectrometry data of ROSINA DFMS revealed two mass peaks at mass 75 Da, one of which was identified as coming from the amino acid glycine. The exact mass of glycine is 75.0315 Da. There are several isomers on the exact same mass. In mass spectrometry, where ionization of the neutrals is done by electron impact, isomers can be distinguished by their fragmentation pattern as molecules are not only ionized to yield the parent ion, but also dissociate into ionized fragments according to the structure of a molecule. All of the isomers of glycine could be ruled out by looking at the specific fragmentation pattern from the electron impact ionisation in the ion source of DFMS.

Figure 1: (click on the image to view with higher resolution) a mass spectrum of mass 75 Da, taken by ROSINA/DFMS on March 28, 2015, integrated over 160 s at a distance of ~20 km from the comet.

Figure 1 shows a sample mass spectrum at 75 Da. The number of ionized particles registered on the detector is given as a function of the position on the detector which corresponds to m/z. The mass resolution of DFMS m/Δm is ~9000 at FWHM for mass 28 and decreases with increasing mass. Also on mass 75 Da we find C₃H₇O₂ (75.0441 Da) which might be a fragment of propylene glycol (C₃H₈O₂) or any of its isomers or/and of even heavier species like butanediol (C₄H₁₀O₂). Only a thorough analysis of all fragments can identify the parent of C₃H₇O₂. Details on the data analysis for ROSINA DFMS can be found in Ref.[6].

To detect glycine in the coma of 67P was quite surprising as glycine has a sublimation temperature of 140°C (8), a lot higher than the comet surface (9). Analysis of the flyby revealed that the density of glycine did not follow the expected 1/r² behaviour, which led to the conclusion that glycine sublimated from dust grains in the coma, which can become much hotter due to their small sizes of a few μm and their low albedo of a few % [10].

The way to form glycine on dust grains has been investigated by [11-13]. It can be formed from the precursor molecule methylamine which was also found in the mass spectra of DFMS together with CO₂. It is up to now the only amino acid where a path for formation is known without involving liquid water. It is therefore not surprising that a search for other amino acids like e.g. alanine was unsuccessful, as the comet most probably never had liquid water.

As glycine is probably mostly on dust grains and its sublimation temperature is high, it is not possible to determine the glycine abundance in the nucleus. The abundance in the coma varies relative to water between 0 and 0.0025. Glycine is not always detected in the spectra of DFMS. We preferentially see a signal from glycine during the perihelion passage between spring 2015 and September 2015 and only if the spacecraft was close enough to the comet.

The precursor molecules methylamine and ethylamine are seen in the mass spectra only when glycine also is detected. The three molecules seem to be closely related which is not surprising. Chemical models show that glycine could form on dust grains via three radical-addition mechanisms at temperatures from 40-120 K [11] which is compatible with temperatures in hot cores. Glycine can also be formed involving photochemistry and CO₂ [13]. In both cases methylamine is part of the process.

Another important species for living organisms is phosphorous found in adenosine triphosphate (ATP), in the backbone of DNA and RNA, and in cell membranes. The phosphorous atom with a mass of 30.9732 Da was detected by DFMS already in October 2014. However, the search for the parent (PH3, PO, PN or HCP) was unsuccessful although these species have been detected in the interstellar medium [14-17]. This is mostly due to overlaps in the mass spectra with other species, very often with abundant sulphur bearing species.

Many of the molecules found by ROSINA DFMS in the coma of comet 67P are compatible with the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and in particular to the early Earth [18] increasing drastically the concentration of life-related chemicals by impact on a closed water body. The fact that glycine was most probably formed on dust grains in the presolar stage also makes these molecules somehow universal, which means that what happened in the solar system could probably happen elsewhere in the Universe.

This article is based on our work published in 'Science Advances', 2016 [19].

References:
[1] Alyssa K. Cobb, Ralph E. Pudritz, and Ben K. D. Pearce, "Nature's Starships. II. Simulating the Synthesis of Amino Acids in Meteorite Parent Bodies", Astrophysical Journal, 809, 6 (2015). Abstract.
[2] Alyssa K. Cobb, Ralph E. Pudritz, "Nature's Starships. I. Observed Abundances and Relative Frequencies of Amino Acids in Meteorites", Astrophysical Journal, 783, 140 (2014). Abstract.
[3] Aaron S. Burton, Jennifer C. Stern, Jamie E. Elsila, Daniel P. Glavin, Jason P. Dworkin, "Understanding prebiotic chemistry through the analysis of extraterrestrial amino acids and nucleobases in meteorites", Chemical Society Reviews, 41, 5459-5472 (2012). Abstract.
[4] Jamie E. Elsila, Daniel P. Glavin, Jason P. Dworkin, "Cometary glycine detected in samples returned by Stardust", Meteoritics & Planetary Science, 44, 9, 1323–1330 (2009). Abstract.
[5] H. Balsiger, K. Altwegg, P. Bochsler, et al., SSR, 128, 745 (2007).
[6] Léna Le Roy, Kathrin Altwegg, Hans Balsiger, Jean-Jacques Berthelier, Andre Bieler, Christelle Briois, Ursina Calmonte, Michael R. Combi, Johan De Keyser, Frederik Dhooghe, Björn Fiethe, Stephen A. Fuselier, Sébastien Gasc, Tamas I. Gombosi, Myrtha Hässig, Annette Jäckel, Martin Rubin, Chia-Yu Tzou, "Inventory of the volatiles on comet 67P/Churyumov-Gerasimenko from Rosetta/ROSINA", Astronomy & Astrophysics, 583, A1 (2015). Abstract.
[7] A. Bieler, K. Altwegg, H. Balsiger, A. Bar-Nun, J.-J. Berthelier, P. Bochsler, C. Briois, U. Calmonte, M. Combi, J. De Keyser, E. F. van Dishoeck, B. Fiethe, S. A. Fuselier, S. Gasc, T. I. Gombosi, K. C. Hansen, M. Hässig, A. Jäckel, E. Kopp, A. Korth, L. Le Roy, U. Mall, R. Maggiolo, B. Marty, O. Mousis, T. Owen, H. Rème, M. Rubin, T. Sémon, C.-Y. Tzou, J. H. Waite, C. Walsh, P. Wurz, "Abundant molecular oxygen in the coma of comet 67P/Churyumov–Gerasimenko", Nature, 526, 678-681. (2015). Abstract.
[8] D. Gross, G. Grodsky, "On the Sublimation of Amino Acids and Peptides", Journal of the American Chemical Society, 77, 1678 (1955). Abstract.
[9] F. Peter Schloerb, Stephen Keihm, Paul von Allmen, Mathieu Choukroun, Emmanuel Lellouch, Cedric Leyrat, Gerard Beaudin, Nicolas Biver, Dominique Bockelée-Morvan, Jacques Crovisier, Pierre Encrenaz, Robert Gaskell, Samuel Gulkis, Paul Hartogh, Mark Hofstadter, Wing-Huen Ip, Michael Janssen, Christopher Jarchow, Laurent Jorda, Horst Uwe Keller, Seungwon Lee, Ladislav Rezac, Holger Sierks, "MIRO observations of subsurface temperatures of the nucleus of 67P/Churyumov-Gerasimenko", Astronomy & Astrophysics, 583, A29 (2015). Abstract.
[10] D.J. Lien, "Dust in comets. I - Thermal properties of homogeneous and heterogeneous grains", Astrophysical Journal, 355, 680-692 (1990). Abstract.
[11] Robin T. Garrod, "A Three phase chemical model of hot cores: The formation of Glycine", Astrophysical Journal, 765, 60 (2013). Abstract.
[12] Uwe J. Meierhenrich, Guillermo M. Muñoz Caro, Willem A. Schutte, Wolfram H.-P. Thiemann, Bernard Barbier, André Brack, "Precursors of Biological Cofactors from Ultraviolet Irradiation of Circumstellar/Interstellar Ice Analogues", Chemistry – A European Journal, 11(17), 4895-4900 (2005). Abstract.
[13] Jean-Baptiste Bossa, Fabien Borget, Fabrice Duvernay, Patrice Theulé, Thierry Chiavassa, Journal of Physical Organic Chemistry, 23, 333–339 (2010). Abstract.
[14] L.M. Ziurys, "Detection of interstellar PN - The first phosphorus-bearing species observed in molecular clouds", ApJ, 321, L81, (1987). Abstract.
[15] M.Guélin, J. Cernicharo, G.Paubert, B.E. Turner, "Free CP in IRC +10216", Astronomy & Astrophysics, 230, L9 (1990). Abstract.
[16] Marcelino Agúndez, José Cernicharo, Michel Guélin, "Discovery of Phosphaethyne (HCP) in Space: Phosphorus Chemistry in Circumstellar Envelopes", Astrophysical Journal, 662, L91 (2007). Abstract.
[17] E. D. Tenenbaum, N. J. Woolf, L. M. Ziurys, "Identification of Phosphorus Monoxide (X²Π_r) in VY Canis Majoris: Detection of the First P-O Bond in Space", Astrophysical Journal, 666, L29 (2007). Abstract.
[18] J. Oró, "Comets and the Formation of Biochemical Compounds on the Primitive Earth", Nature, 190, 389-390 (1961). Abstract.
[19] Kathrin Altwegg, Hans Balsiger, Akiva Bar-Nun, Jean-Jacques Berthelier, Andre Bieler, Peter Bochsler, Christelle Briois, Ursina Calmonte, Michael R. Combi, Hervé Cottin, Johan De Keyser, Frederik Dhooghe, Bjorn Fiethe, Stephen A. Fuselier, Sébastien Gasc, Tamas I. Gombosi, Kenneth C. Hansen, Myrtha Haessig, Annette Jäckel, Ernest Kopp, Axel Korth, Lena Le Roy, Urs Mall, Bernard Marty, Olivier Mousis, Tobias Owen, Henri Rème, Martin Rubin, Thierry Sémon, Chia-Yu Tzou, James Hunter Waite, Peter Wurz, "Prebiotic chemicals—amino acid and phosphorus—in the coma of comet 67P/Churyumov-Gerasimenko", Science Advances, 2(5), e1600285 (2016). Abstract.

Indications of an Influence of Solar Neutrinos on Beta Decays

From left to right: Peter A. Sturrock, Ephraim Fischbach, Jeffrey D. Scargle

Authors: Peter A. Sturrock¹, Ephraim Fischbach², Jeffrey D. Scargle<sup>3

1</sup>Kavli Institute for Particle Astrophysics and Cosmology and the Center for Space Science and Astrophysics, Stanford University, California, USA,
²Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana, USA,
³NASA/Ames Research Center, Moffett Field, California, USA.

Eckhard D. Falkenberg, who found evidence of an annual oscillation in the beta-decay rate of tritium, was either the first or one of the first to propose that some beta-decay rates may be variable [1]. He suggested that the beta-decay process may be influenced by neutrinos, and attributed the annual variation to the varying Earth-Sun distance that leads to a corresponding variation in the flux of solar neutrinos as detected on Earth. Supporting evidence for the variability of beta-decay rates could be found in the results of an experiment carried out at the Brookhaven National Laboratory. Alburger et al. had measured the decay rates of ³⁶Cl and of ³²Si from 1982.13 to 1986.13 (later extended to 1989.93), and reported finding “small periodic annual deviations of the data points from an exponential decay … of uncertain origin” [2].

In 2006, Fischbach and Jenkins of Purdue University set up an experiment to track the decay rate of ⁵⁴Mn as part of a project to determine whether or not beta decays are strictly random. They found, as had Falkenberg, that the decay rate appeared to be variable. This led them to examine the publication of Alburger et al. [2] and an article by Siegert et al. [3] concerning measurements of the decay rates of ²²⁶Ra acquired at the Physikalisch Technische Bundesanstalt in Germany. Jenkins and Fischbach proposed, as had Falkenberg, that the beta decay process may be influenced by solar neutrinos, and that the annual variation may be due to the varying Earth-Sun distance [4]. They also found evidence of a notable variation in the decay rate at the time of a solar flare on December 13, 2006, which led them to suggest that the beta decay process may somehow influence or be influenced by solar activity [5]. Although these two suggestions (that an annual oscillation in beta decays is due to the varying Earth-Sun distance and that beta decays are somehow correlated with solar activity) have not been substantiated by subsequent investigations, their articles were effective in drawing attention to the possibility of decay-rate variability.

Contrary to what one might hope for, but in line with reality, their suggestions quickly led to several nay-saying articles [6,7,8], to which Fischbach and his colleagues responded [9,10,11]. More recently, Kossert and Nahle have claimed to have evidence that beta-decay rates are constant [12], but their claims have been refuted [13]. The latest such article is one by Pomme et al. [14], who have published data concerning 67 measurements of a variety of decay processes, examining the measurements only for evidence of an Earth-Sun-Distance effect (which is known to be an inadequate test of variability) and claim to establish that there is no such effect. Their data have not yet been subjected to an independent analysis.

Although the first evidence for variability was the discovery of annual oscillations in decay rates, this approach to the problem has the obvious defect that an annual variation may be caused by any one of several experimental or environmental influences, which led us to seek evidence for an influence of solar rotation. We have learned from helioseismology that the synodic rotation rate (as seen from Earth) of the radiative zone is in the range 12.5 to 12.8 year^-1, whereas the synodic rotation rate of the photosphere extends to 13.8 year^-1 [15]. Variation of the solar neutrino flux may be attributable to the RSFP (Resonant Spin Flavor Precession) process [16], which can more easily occur in the deep solar interior (where there can be much stronger magnetic field) than in the convection zone [17]. This scenario would not lead one to expect an association between decay-rate variations and flare-like solar activity, which takes place in the outermost layer of the convection zone and in the solar atmosphere.

Power spectrum analysis of BNL data has in fact yielded evidence of oscillations in the frequency range 11 – 13 year^-1, supporting our conjecture that there may be a rotational influence on the solar neutrino flux [18,19]. However, these results raised the question of whether one could find corresponding evidence for solar rotation in measurements of the solar neutrino flux made by neutrino observatories such as Super-Kamiokande. An analysis of Super-Kamiokande data that takes account only of the mid-time of each bin, ignores the error estimates, and adopts an unrealistically wide search band, yields inconclusive results [20], but an analysis that takes account of the start and stop time of each measurement bin and of the upper and lower error estimates, and adopts an appropriate search band, yields strong evidence of an oscillation of frequency 9.43 year^-1 [21].

A crucial issue is whether or not solar influences are steady or time variable. Our investigations of GALLEX solar neutrino data indicated that the influence of rotation tends to be episodic [22]. This suggested that one should examine neutrino and beta-decay data by means of spectrograms rather than periodograms. These considerations led us to carry out a comparative analysis of spectrograms formed from the BNL data and from Super-Kamiokande data. The results have recently been published in Solar Physics [23].

Figure 1. Spectrogram formed from ³⁶Cl data for the frequency band 8 – 16 year^-1.

Figures 1 and 2, taken from that article, show spectrograms formed from BNL ³⁶Cl and ³²Si data, respectively. In order to focus on possible evidence of solar rotation, we show spectrograms only for the frequency range 8 to 16 year^-1. Figure 1 shows evidence of strong but transient oscillations at frequencies of approximately 11 year^-1 and 12.6 year^-1. Figure 2 also shows evidence of an oscillation at approximately 12.6 year^-1, but only slight evidence of an oscillation at approximately 11 year^-1. Evidence of these two oscillations has previously been found in power-spectrum analyses [18,19].

Figure 2. Spectrogram formed from ³²Si data for the frequency band 8 – 16 year^-1.

Figure 3 shows a spectrogram formed from Super-Kamiokande data, again for the frequency range 8 – 16 year^-1. This spectrogram shows evidence of a strong and steady oscillation at approximately 9.5 year^-1, as expected from our earlier power-spectrum analysis [21]. However, it also shows evidence of a transient oscillation with a frequency of approximately 12.6 year^-1, supporting the proposition that beta-decay variability may be attributed to an influence of solar neutrinos.

Figure 3. Spectrogram formed from Super-Kamiokande data for the frequency band of 8 – 16 year^-1.

The schedules of the relevant experiments were such that measurements leading to Figures 1 and 2 and measurements leading to Figure 3 were not acquired at the same time. It would clearly be desirable to compare beta-decay measurements and solar neutrino measurements that are acquired in the same time frame. The most extensive set of beta-decay measurements is the sequence currently (beginning in early 1992) being acquired by Steinitz and his colleagues at the Geological Survey of Israel (GSI) [24, 25]. The Borexino solar neutrino experiment began operation in 2007 and is still operational [26], so it may be possible at some time to compare beta-decay data with contemporaneous solar-neutrino data. It is however important to note that one may not find a perfect match between the two sets of data, even if beta decays are in fact influenced by neutrinos, since beta decays and neutrino detectors may respond to neutrinos of different energies and (since we have no theoretical understanding of beta-decay variability) conceivably of different flavors.

References:
[1] Eckhard Dieter Falkenberg, "Radioactive Decay Caused by Neutrinos?", Apeiron, 8, 32 (2001). Full Article.
[2] D.E. Alburger, G. Harbottle, E.F. Norton, "Half Life of ³²Si", Earth and Planetary Science Letters, 78, 168 (1986). Abstract.
[3] Helmut Siegert, Heinrich Schrader, Ulrich Schötzig, "Half-life measurements of Europium radionuclides and the long-term stability of detectors", Applied Radiation and Isotopes, 49, 1397 (1998). Abstract.
[4] Jere H. Jenkins, Ephraim Fischbach, John B. Buncher, John T. Gruenwald, Dennis E. Krause, Joshua J. Mattes, "Evidence of correlations between nuclear decay rates and Earth–Sun distance", Astroparticle Physics, 32, 42 (2009). Abstract.
[5] Jere H. Jenkins, Ephraim Fischbach, "Perturbation of nuclear decay rates during the solar flare of 2006 December 13", Astroparticle Physics, 31, 407 (2009). Abstract.
[6] Peter S. Cooper, "Searching for modifications to the exponential radioactive decay law with the Cassini spacecraft", Astroparticle Physics, 31, 267 (2009). Abstract.
[7] Eric B. Norman, Edgardo Browne, Howard A. Shugart, Tenzing H. Joshi, Richard B. Firestone, "Evidence against correlations between nuclear decay rates and Earth–Sun distance", Astroparticle Physics, 31, 135 (2009). Abstract.
[8] T.M. Semkowa, D.K. Hainesa, S.E. Beacha, B.J. Kilpatricka, A.J. Khana, K. O'Brienb, "Oscillations in radioactive exponential decay", Physics Letters B, 675, 415 (2009). Abstract.
[9] D.E. Krause, B.A. Rogers, E. Fischbach, J.B. Buncher, A. Ging, J.H. Jenkins, J.M. Longuski, N. Strange, P.A. Sturrock, "Searches for solar-influenced radioactive decay anomalies using spacecraft RTGs", Astroparticle Physics, 36, 51 (2012). Abstract.
[10] D. O’Keefe, B.L. Morreale, R.H. LeeJohn, B. Buncher, J.H. Jenkins, Ephraim Fischbach, T. Gruenwald, D. Javorsek II, P.A. Sturrock, "Spectral content of ²²Na/⁴⁴Ti decay data: implications for a solar influence", Astrophysics and Space Science, 344, 297 (2013). Abstract.
[11] Jere H. Jenkins, Daniel W. Mundy, Ephraim Fischbach, "Analysis of environmental influences in nuclear half-life measurements exhibiting time-dependent decay rates", Nuclear Instruments and Methods in Physics Research Section A. 620, 332 (2010). Abstract.
[12] Karsten Kossert, Ole J. Nähle, "Disproof of solar influence on the decay rates of ⁹⁰Sr/⁹⁰Y", Astroparticle Physics, 69, 18 (2015). Abstract.
[13] P.A. Sturrock, G. Steinitz, E. Fischbach, A. Parkhomov, J.D. Scargle, "Analysis of beta-decay data acquired at the Physikalisch-Technische Bundesanstalt: Evidence of a solar influence", Astroparticle Physics, 84, 8 (2016). Abstract.
[14] S. Pomméa, H. Stroh, J. Paepen, R. Van Ammel, M. Marouli, T. Altzitzoglou, M. Hult, K. Kossert, O. Nähle, H. Schrader, F. Juget, C. Bailat, Y. Nedjadi, F. Bochud, T. Buchillier, C. Michotte, S. Courte, M.W. van Rooy, M.J. van Staden, J. Lubbe, B.R.S. Simpson, A. Fazio, P. De Felice, T.W. Jackson, W.M. Van Wyngaardt, M.I. Reinhard, J. Golya, S. Bourke, T. Roy, R. Galea, J.D. Keightley, K.M. Ferreira, S.M. Collins, A. Ceccatelli, M. Unterweger, R. Fitzgerald, D.E. Bergeron, L. Pibida, L. Verheyen, M. Bruggeman, B. Vodenik, M. Korun, V. Chisté, M.-N. Amiot, "Evidence against solar influence on nuclear decay constants", Physics Letters B, 761, 281 (2016). Abstract.
[15] J. Schou, R. Howe, S. Basu, J. Christensen-Dalsgaard, T. Corbard, F. Hill, R. Komm, R. M. Larsen, M. C. Rabello-Soares, M. J. Thompson, "A Comparison of Solar p-Mode Parameters from the Michelson Doppler Imager and the Global Oscillation Network Group: Splitting Coefficients and Rotation Inversions", Astrophysical Journal, 567, 1234 (2002). Abstract.
[16] E. Kh. Akhmedov, "Resonant amplification of neutrino spin rotation in matter and the solar-neutrino problem", Physics Letters B, 213, 64 (1988). Abstract.
[17] João Pulido, C R Das, Marco Picariello, "Remaining inconsistencies with solar neutrinos: Can spin flavour precession provide a clue?", Journal of Physics: Conference series, 203, 012086 (2009). Abstract.
[18] P.A. Sturrock, J.B. Buncher, E. Fischbach, J.T. Gruenwald, D. Javorsek II, J.H. Jenkins, R.H. Lee, J.J. Mattes, J.R. Newport, "Power spectrum analysis of BNL decay rate data", Astroparticle Physics, 34, 121 (2010). Abstract.
[19] D. Javorsek II, P.A. Sturrock, R.N. Lasenby, A.N. Lasenby, J.B. Buncher, E. Fischbach, J.T. Gruenwald, A.W. Hoft, T.J. Horan, J.H. Jenkins, J.L. Kerford, f, R.H. Lee, A. Longman, J.J. Mattes, B.L. Morreale, D.B. Morris, R.N. Mudry, J.R. Newport, D. O’Keefe, M.A. Petrelli, M.A. Silver, C.A. Stewart, B. Terry, "Power spectrum analyses of nuclear decay rates", Astroparticle Physics, 34, 173 (2010). Abstract.
[20] J. Yoo et al. (Super-Kamiokande Collaboration), "Search for periodic modulations of the solar neutrino flux in Super-Kamiokande-I", Physical Review D, 68, 092002 (2003). Abstract.
[21] P.A. Sturrock, J.D. Scargle, "Power-Spectrum Analysis of Super-Kamiokande Solar Neutrino Data, Taking into Account Asymmetry in the Error Estimates", Solar Physics, 237, 1 (2006). Abstract.
[22] P.A. Sturrock, "Time–Frequency Analysis of GALLEX and GNO Solar Neutrino Data", Solar Physics, 252, 1 (2008). Abstract.
[23] P.A. Sturrock, E. Fischbach, J.D. Scargle, "Comparative Analyses of Brookhaven National Laboratory Nuclear Decay Measurements and Super-Kamiokande Solar Neutrino Measurements: Neutrinos and Neutrino-Induced Beta-Decays as Probes of the Deep Solar Interior", Solar Physics, 291, 3467 (2016). Abstract.
[24] G. Steinitz, O. Piatibratova, P. Kotlarsky, "Sub-daily periodic radon signals in a confined radon system", Journal of Environmental Radioactivity, 134, 128 (2014). Abstract.
[25] G. Steinitz, P. Kotlarsky, O. Piatibratova, "Observations of the relationship between directionality and decay rate of radon in a confined experiment", European Physical Journal, 224, 731 (2015). Abstract.
[26] S. Davini, G. Bellini, J. Benziger, D. Bick, G. Bonfini, D. Bravo, B. Caccianiga, F. Calaprice, A. Caminata, P. Cavalcante, A. Chepurnov, D. D'Angelo, A. Derbin, A. Etenko, K. Fomenko, D. Franco, C. Galbiati, C. Ghiano, A. Goretti, M. Gromov, Aldo Ianni, Andrea Ianni, V. Kobychev, D. Korablev, G. Korga, D. Kryn, M. Laubenstein, T. Lewke, E. Litvinovich, F. Lombardi, P. Lombardi, L. Ludhova, G. Lukyanchenko, I. Machulin, S. Manecki, W. Maneschg, S. Marcocci E. Meroni, M. Misiaszek, P. Mosteiro, V. Muratova, L. Oberauer, M. Obolensky, F. Ortica, K. Otis, M. Pallavicini, L. Papp, A. Pocar, G. Ranucci, A. Razeto, A. Re, A. Romani, N. Rossi, C. Salvo, S. Schönert, H. Simgen, M. Skorokhvatov, O. Smirnov, A. Sotnikov, S. Sukhotin, Y. Suvorov, R. Tartaglia, G. Testera, D. Vignaud, R. B. Vogelaar, J. Winter, M. Wojcik, M. Wurm, O. Zaimidoroga, S. Zavatarelli, G. Zuzel, "New results of the Borexino experiment: pp solar neutrino detection", Il Nuovo Cimento C, 38, 120 (2015). Abstract.

Recent Supernova Debris on the Moon

Thomas Faestermann (left) and Gunther Korschinek.

Authors: Thomas Faestermann, Gunther Korschinek

Affiliation: Technische Universität München, 85748 Garching, Germany.

Stars with a mass of more than about 8 times the solar mass usually end in a supernova explosion (SN). Before and during this explosion new elements, stable and radioactive, are formed by nuclear reactions and a large fraction of their mass is ejected with high velocities into the surrounding space. Most of the new elements are in the mass range until Fe, because there the nuclear binding energies are the largest. If such an explosion happens close to the sun it can be expected that part of the debris might enter the solar system and therefore should leave a signature on the planets and their moons. The interstellar space is not empty but contains dust and atomic particles, of course in minuscule densities.

A SN is cleaning up the surrounding space such that empty bubbles (around 0.06 atoms per cm³) are formed surrounded by denser space (around 10 atoms per cm3). The sun is embedded in a so called local bubble [1], indicating that one or even more SNe should have happened near the solar system in the past. Considering these ideas we have started already in the past to search for SN traces on our Earth. The best suited isotope for such a signature is ⁶⁰Fe. It has a half-life of 2.6 Myr [2] and it is not produced naturally on Earth, however it is also formed in small amounts by cosmic rays in interplanetary dust particles.

To detect and measure such extremely tiny amounts of ⁶⁰Fe an ultrasensitive method is needed. Accelerator mass spectrometry (AMS) is the only choice in this case. We have developed this method for many years using the Munich tandem accelerator, and achieve, besides a facility in Australia, the highest sensitivity worldwide [3]. The principle is the following: negative ions are formed in an ion source, acceleration with a voltage of a few kV and then pass a combination of electric and magnetic fields as in a conventional mass spectrometer. Subsequently they are accelerated in the tandem accelerator to a high velocity on the order of 7% of the speed of light. In the tandem the negative ions traverse a thin carbon foil where they lose a certain number of electrons to become multiply charged positive ions.

This process is so effective that absolutely no interfering molecules can survive. Thus a typical limitation in conventional mass spectrometry, molecular background, vanishes. In addition, because of the high energy of the ions, nuclear physics techniques are applied to reduce drastically possible interferences of stable isobars. In our case it is ⁶⁰Ni in our iron samples which is suppressed that much that an isotopic ratio ⁶⁰Fe/Fe of a few times 10^-17 can still be measured.

Our first studies in the past were focused on deep sea ferromanganese crusts. These depositions are very slowly growing, around 2 to 3 mm/Myr, on the bottom of the oceans, and accumulate elements present in the ocean water. As they collect also ¹⁰Be, a radioactive isotope with a half-life of 1.387 Myr, formed by cosmic rays on Nitrogen and Oxygen in our atmosphere, samples taken from different depths in the crust can be dated via the decreasing concentration of ¹⁰Be in deeper layers. The results of the most conclusive studies [4,5] are shown in fig.1.

Figure 1: (click on figure to view with higher resolution) The ⁶⁰Fe/Fe concentrations as measured in different depths of the ferromanganese crust 237KD (red points). The peak of an enhanced ⁶⁰Fe/Fe concentration at an age of around 2-3 Myr is due to the flux of SN-formed ⁶⁰Fe which has entered the solar system at that time. The blue triangles are from a separate measurement series where we have carefully leached out iron from crust samples and then analyzed. The vertical bars indicate 68% uncertainties, the horizontal ones the age range covered by the sample.

From the measured concentrations we had deduced the ⁶⁰Fe flux at that time and also the distance of one or more SNe. The critical point was however, the transport of the ⁶⁰Fe from the upper atmosphere through all atmospheric processes towards the biosphere in the ocean until the final deposition in the ferromanganese crust. To circumvent this difficulty we considered [6] to search for ⁶⁰Fe in lunar samples collected by US Astronauts between 1969 and 1972 and brought to earth. Together with colleagues from the Rutgers University, New Jersey (USA), we applied successfully for selected sample material from the astronomical laboratory of the Johnson space center (NASA).

An enhanced ⁶⁰Fe concentration in lunar material would be a clear proof of our previous measurements and the conclusions drawn. It must have been deposited everywhere in our solar system, on all the planets and their moons. In addition, the total amount of ⁶⁰Fe would provide solid data for the fluence and also the distance of the SNe because the ⁶⁰Fe has been collected directly on the surface of the Moon.

The drawback is however that the moon does not deliver the chronological information like the crust samples. The lunar surface (regolith) is constantly stirred and mixed by the impact of micrometeorites (a process called gardening) and also sporadic impacts by full-sized meteorites, thus losing any precise time information. A further drawback is that ⁶⁰Fe is also formed by the much higher cosmic ray flux via nuclear reactions on Ni which is present in lunar regolith, albeit only in tiny concentrations. To quantify this contribution we compared the lunar data with data from iron meteorites, which have been exposed for many millions of years to cosmic rays, and which we investigated as well. We know that cosmogenic ⁶⁰Fe is formed by nuclear reactions only on the heaviest stable nickel isotope ⁶⁴Ni. We know also that another long-lived radioisotope ⁵³Mn (T_1/2 = 3.7 Myr) is formed by cosmic rays on stable iron. In the case of additional SN produced ⁶⁰Fe the concentration ratios of ⁶⁰Fe/Ni to ⁵³Mn/Fe should be higher in the lunar samples than in the meteoritic data.

Fig. 2 shows the comparison of 11 lunar samples (red points) with meteoritic samples (green points). Instead of concentrations we plot by convention their activity (disintegrations per minute) relative to the amount of the target element Ni and Fe. The meteoritic data follow, as expected, a proportionality (the range between the green lines), indicating that ⁶⁰Fe like ⁵³Mn is produced by cosmic rays; the scatter of the activities is mainly due to differences in the meteoroid geometry. Most of the lunar samples have ⁶⁰Fe activities well above the expected relationship of the meteorite samples because of the SN contribution. Only three of the lunar samples have activities comparable to cosmic ray origin; they are from greater depth or have a complicated history; e.g. sample 3 is eroded material from the surface of a rock thus has no SN contribution.

Figure 2: (click on figure to view with higher resolution) The measured activities of ⁶⁰Fe versus ⁵³Mn in meteoritic and lunar samples. Units are disintegrations per minute per kg Fe and Ni, for ⁵³Mn and ⁶⁰Fe, respectively. Samples 1 through 11 (red) are lunar samples; the other values (green) are for iron meteorites. The area between the green straight lines indicates the 68% error band for cosmic ray produced ⁵³Mn and ⁶⁰Fe activities in meteorites.

Any ⁶⁰Fe signal is expected to be distributed downward due to gardening of the lunar surface [7]. In Fig. 3 we show the SN produced ⁶⁰Fe concentration (cosmic ray contribution subtracted) as a function of the depth (areal density) of the samples. The deposition of the ⁶⁰Fe on the lunar surface must have happened on a time scale of Myr, since already considerable gardening has happened and, on the other hand, cannot have happened more than some 3 half-lives, i.e. 8 Myr, ago to be still detectable. Thus it is very likely that it coincides with the ⁶⁰Fe surplus in the ferromanganese crust, which was collected between 1.7 and 2.6 Myr ago. In a time period of around 2.2 Myr, gardening is expected down to a few g/cm². It is reasonable, therefore, to integrate the measured ⁶⁰Fe concentration over this range, in order to estimate the local fluence of ⁶⁰Fe. Nevertheless we found also elevated concentrations of ⁶⁰Fe down to a depth of 20 g/cm² (Fig. 3), indicating possible excavations by meteorites and/or down-slope movements.

Figure 3: (click on figure to view with higher resolution) Depth dependence of the SN produced ⁶⁰Fe concentration and estimation of the local fluence of  ⁶⁰Fe on the Moon’s surface. The dashed curves represent two different integration scenarios. They symbolize a lower and an upper limit. The error bars indicate a 68% confidence level.

Thus, an inclusion of these deep samples yields an upper limit of ⁶⁰Fe for the integration to obtain a local interstellar fluence of ⁶⁰Fe. As the lower limit (smallest depth) we adopted that of sample 4. From the data we can estimate a range for ⁶⁰Fe/kg soil. Including corrections for the decay, and assuming a uniform spread over the lunar surface we end up with a fluence between 0.8 x 10⁸ atoms/cm² and 4 x 10⁸ atoms/cm² which was deposited during the past about 4 Myr. If we assume that this fluence came from a single SN and that the (typical) theoretical ⁶⁰Fe mass of 2x10^-5 solar masses has been ejected and formed dust to penetrate the solar system, then the SN would have happened 300 to 600 light years away.

In conclusion, our results show for the first time that the SN-formed ⁶⁰Fe has been also collected by the Moon, thus confirming the SN origin of previous measurements of ⁶⁰Fe on Earth. It delivers also more solid data for the fluence of ⁶⁰Fe which allow better theoretical estimation of other long-lived radioisotopes released by the SNe around 2 Myr ago. Theoretical considerations interpret our findings as SN activity in an association of young stars. They even seem to find good candidates like the Sco-Cen association [8] where the exploding stars could have been 2 Myr ago at a distance of around 300 light years or the Tuc-Hor association at about 150 light years [9].

In addition, further evidence for the SN activity has been added recently. An enhancement of ⁶⁰Fe has been found in ocean sediments at an Australian laboratory [10] and by our group [11]. This gives us a much better timing information than the crust and shows that the SN activity lasted for about 1 Myr and started about 2.7 Myr ago. Even in cosmic rays 15 nuclei of ⁶⁰Fe have been detected with the spectrometer CRIS aboard NASA’s satellite ACE (Advanced Composition Explorer) [12]. The authors conclude that at least two SNs must have occurred within 3000 light years from the sun during the last few Myr. Analysis of the spectra of high-energy cosmic rays leads to similar conclusions [13].

References:
[1] T. W. Berghöfer, D. Breitschwerdt, "The origin of the young stellar population in the solar neighborhood - A link to the formation of the Local Bubble?”, Astronomy & Astrophysics, 390, 299 (2002). Abstract.
[2] K. Knie, T. Faestermann, G. Korschinek, G. Rugel, W. Rühm, C. Wallner, "High-sensitivity AMS for heavy nuclides at the Munich Tandem accelerator”, Nuclear Instruments and Methods in Physics Research B, 172, 717 (2000). Abstract.
[3] G. Rugel, T. Faestermann, K. Knie, G. Korschinek, M. Poutivtsev, D. Schumann, N. Kivel, I. Günther-Leopold, R. Weinreich, M. Wohlmuther, “New Measurement of the ⁶⁰Fe Half-Life”, Physical Review Letters, 103, 072502 (2009). Abstract.
[4] K. Knie, G. Korschinek, T. Faestermann, E. A. Dorfi, G. Rugel, A. Wallner, "⁶⁰Fe Anomaly in a Deep-Sea Manganese Crust and Implications for a Nearby Supernova Source”, Physical Review Letters, 93, 171103 (2004). Abstract.
[5] C. Fitoussi, G. M. Raisbeck, K. Knie, G. Korschinek, T. Faestermann, S. Goriely, D. Lunney, M. Poutivtsev, G. Rugel, C. Waelbroeck, A. Wallner, “Search for Supernova-Produced ⁶⁰Fe in a Marine Sediment”, Physical Review Letters, 101, 121101 (2008). Abstract.
[6] L. Fimiani, D. L. Cook, T. Faestermann, J. M. Gómez-Guzmán, K. Hain, G. Herzog, K. Knie, G. Korschinek, P. Ludwig, J. Park, R. C. Reedy, G. Rugel, “Interstellar ⁶⁰Fe on the Surface of the Moon", Physical Review Letters, 116, 151104 (2016). Abstract.
[7] D.E.Gault, F. Hoerz, D.E. Brownlee, J.B. Hartung, "Mixing of the lunar regolith”, Proc. 5th Lunar Science Conference, Vol. 3, 2365 (1974). Abstract.
[8] D. Breitschwerdt, J. Feige, M. M. Schulreich, M. A. de. Avillez, C. Dettbarn, B. Fuchs,  “The locations of recent supernovae near the Sun from modelling ⁶⁰Fe transport”, Nature, 532, 73 (2016). Abstract.
[9] Brian J. Fry, Brian D. Fields, John R. Ellis, “Radioactive Iron Rain: Transporting ⁶⁰Fe in Supernova Dust to the Ocean Floor”,  Astrophysical Journal, 827, 48 (2016). Abstract.       
[10] A. Wallner, J. Feige, N. Kinoshita, M. Paul, L. K. Fifield, R. Golser, M. Honda, U. Linnemann, H. Matsuzaki, S. Merchel, G. Rugel, S. G. Tims, P. Steier, T. Yamagata, S. R. Winkler “Recent near-Earth supernovae probed by global deposition of interstellar radioactive ⁶⁰Fe”. Nature, 532, 69 (2016). Abstract.
[11] Peter Ludwig, Shawn Bishop, Ramon Egli, Valentyna Chernenko, Boyana Deneva, Thomas Faestermann, Nicolai Famulok, Leticia Fimiani, José Manuel Gómez-Guzmán, Karin Hain, Gunther Korschinek, Marianne Hanzlik, Silke Merchel, Georg Rugel, “Time-resolved 2-million-year-old supernova activity discovered in Earth’s microfossil record”, PNAS, 113, 9123 (2016). Abstract.
[12] W. R. Binns, M. H. Israel, E. R. Christian, A. C. Cummings, G. A. de Nolfo, K. A. Lave, R. A. Leske, R. A. Mewaldt, E. C. Stone, T. T. von Rosenvinge, M. E. Wiedenbeck, "Observation of the ⁶⁰Fe nucleosynthesis-clock isotope in galactic cosmic rays", Science, 352, 677 (2016). Abstract.
[13] M. Kachelrieß, A. Neronov, D. V. Semikoz “Signatures of a Two Million Year Old Supernova in the Spectra of Cosmic Ray Protons, Antiprotons, and Positrons”, Physical Review Letters, 115, 181103 (2016). Abstract.

Heavy Dark Matter Ignition of Supernovae

Joseph Bramante

Author: Joseph Bramante

Affiliation: Department of Physics, University of Notre Dame, Indiana, USA.

Dark matter makes up most of the mass we observe in our universe. Dark matter's gravitational pull has been identified in the structure of galaxies, the expansion of the universe, and the bending of light through galactic clusters. However, the mass of each dark matter particle, and dark matter's non-gravitational interactions, are still a mystery.

Type Ia supernovae result when a white dwarf's atomic nuclei rapidly fuse, prompting an explosion that spews forth heavier, decaying atomic nuclei. The light released by these decaying nuclei over a hundred days is collected by astronomers, who have noticed a pattern: the maximum brightness of Type Ia supernova can be used to predict how quickly the supernova's light will fade. This is the sense in which Type Ia supernovae are "standard candles" that have been used to measure the accelerating expansion of our universe.

For decades scientists thought that type Ia supernovae were standard candles, because they originated from "Chandraskhar mass" white dwarfs. The Chandrasekhar mass is the maximum mass a white dwarf can have, before it will collapse under its own weight. Contradicting expectations, recent precision studies of type Ia supernovae have shown that many white dwarfs which produce type Ia supernovae are significantly lighter than Chandrasekhar mass [1]. This is a puzzle, that could be solved by heavy dark matter.

A recent paper [2] shows that if dark matter particles are one million times heavier than protons, then enough dark matter could collect into white dwarfs to initiate a period of dark matter collapse. The collapsing dark matter particles would ricochet against white dwarf nuclei fast enough to fuse them, thereby sparking type Ia supernovae.

Figure 1: A schematic indicating that heavier, denser white dwarves would be ignited by dark matter more quickly than lighter, less dense white dwarves.

One prediction of this scenario is that more massive white dwarfs would explode sooner than less massive white dwarfs. This is because heavier white dwarfs are denser, and so the dark matter would be gravitationally bound into a smaller region inside them. Dark matter collected into a smaller region would collapse sooner (see Figure 1). Spurred by this possibility, the author looked for, and found some evidence for heavier white dwarfs exploding sooner in existing type Ia supernovae data (see Figure 2).

There are some plausible, competing explanations for sub-Chandrasekhar mass type Ia supernovae. For example, merging white dwarf stars may precipitate nuclear fusion [3]. The upcoming dark matter search experiments XENON [4] and Lux-Zeppelin [5] could detect or rule out some models of supernova-igniting dark matter. Finally, there is another dramatic consequence of supernova-igniting dark matter: it would collapse neutron stars into black holes at the center of our galaxy, where dark matter is more abundant [6].

Figure 2: (click on the image to view with higher resolution) There is an apparent correlation between the age of stars surrounding supernovae and the supernovae's initial masses. The thick red crosses indicate collected type Ia supernovae data, taken from reference [7]. The solid and dashed lines indicate the predictions of some heavy dark matter models.

References:
[1] R. Scalzo et al. (The Nearby Supernova Factory), "Type Ia supernova bolometric light curves and ejected mass estimates from the Nearby Supernova Factory", Monthly Notices of the Royal Astronomical Society, 440, 1498 (2014). Abstract.
[2] Joseph Bramante, "Dark matter ignition of type Ia supernovae", Physical Review Letters, 115, 141301 (2015). Abstract.
[3] Dan Maoz, Filippo Mannucci, Gijs Nelemans, "Observational clues to the progenitors of Type-Ia supernovae", Annual Reviews of Astronomy and Astrophysics, 52, 107 (2014). Abstract.
[4] E. Aprile et al. (XENON100 collaboration), "Limits on spin-dependent WIMP-nucleon cross sections from 225 live days of XENON100 data",  Physical Review Letters, 111, 021301 (2013). Abstract.
[5] D.S. Akerib et al. (LZ collaboration), "LUX-ZEPLIN (LZ) Conceptual Design Report", arXiv:1509.02910v2 [physics.ins-det].
[6] Joseph Bramante, Tim Linden, "Detecting dark matter with imploding pulsars in the galactic center",  Physical Review Letters, 113, 191301 (2014). Abstract.
[7] Y.-C. Pan, M. Sullivan, K. Maguire, I.M. Hook, P.E. Nugent, D.A. Howell, I. Arcavi, J. Botyanszki, S.B. Cenko, J. DeRose, H.K. Fakhouri, A. Gal-Yam, E. Hsiao, S.R. Kulkarni, R.R. Laher, C. Lidman, J. Nordin, E.S. Walker, D. Xu, "The Host Galaxies of Type Ia Supernovae Discovered by the Palomar Transient Factory", Monthly Notices of the Royal Astronomical Society, 438, 1391 (2014). Abstract.

A New Way To Weigh A Star

Nils Andersson (left) and Wynn Ho 

Authors: Nils Andersson, Wynn Ho

Affiliation: Mathematical Sciences and STAG Research Centre, University of Southampton, UK. 

You probably have a pretty good idea of your own weight. And if you need an exact answer it is relatively easy to find out. Get the bathroom scales out, step up and read off the result. But have you ever asked yourself what was actually involved in that measurement? Have you considered that what actually happened was that gravity’s pull was countered by the electromagnetic interaction of the atoms that make up the surface of the scale, and what you actually measured was how hard the atoms had to work to push back on your feet? Possibly not, and the chances are that you have never really considered how one would weigh a distant star, either.

As it turns out, this question has a fairly straightforward answer, which also involves gravity, electromagnetism… and a bit of luck. If you want to figure out how heavy a particular star is, and you are lucky enough that this star has a close companion, then all you need to do is track the star’s motion. The orbital motion of a double-star system is dictated by gravity and you can figure out how much the stars weigh using the same arguments that we use to figure out the mass of the moon (going around the Earth) and the Earth (circling the Sun). If you want to be a bit more precise you should use Einstein’s curved spacetime theory for gravity rather than Newton’s inverse square-law, but this may be a luxury in this exercise.

Using this technique, astronomers have weighed many stars with great precision and they have also managed to work out the masses of a number of pulsars [1]. This is particularly exciting as these systems stretch our understanding of several aspects of fundamental physics.

A pulsar is a highly magnetised rotating neutron star formed when a massive star runs out of nuclear fuel. At this point it can no longer hold itself up against gravity and it starts falling in on itself. This leads to a spectacular explosion – a supernova – the remains of which may be either a neutron star or a black hole. Neutron stars are nature’s own counterparts to the Large Hadron Collider [2]. In essence, they provide a link between astronomy and laboratory work in both high-energy and low-temperature physics. By weighing these stars we gain insight into physics under extreme conditions [3].

Pulsars are named for their rotating beam of electromagnetic radiation, which is observed by telescopes as it sweeps past the Earth, just like the familiar beam of a lighthouse [4]. They are renowned for their incredibly stable rate of rotation [5], but young pulsars occasionally experience so-called glitches where they are found to speed up for a very brief period of time [6]. The prevailing idea is that these glitches provide evidence of exotic states of matter in the star’s interior [3]. The glitches arise when a rapidly spinning superfluid within the star transfers rotational energy to the star's crust, a solid outer layer like a bowl containing a mysterious soup; the component that is tracked by observations. Imagine the bowl spinning at one speed and the soup spinning faster. Friction between the inside of the bowl and its contents, the soup, can cause the bowl to speed up. Whenever this happens, the more soup there is, the faster the bowl will be made to rotate.

Interestingly, it seems that the superfluid soup provides us with a new way of weighing these stars. This new technique is very different from the usual approach as it is not based on gravity, but nuclear physics, and it can also be used for stars in isolation. The star does not have to have a companion.

In a recent paper in Science Advances [7], we have developed this exciting new idea, which relies on a detailed understanding of neutron star superfluidity and the dynamics of the quantum vortices - a kind of ultra-slim tornadoes - by means of which these systems mimic large-scale rotation. Our results are promising and have important implications for the generation of revolutionary radio telescopes, like the Square Kilometre Array (SKA [8]) and the Low Frequency Array (LOFAR [9]), that are being developed by large international collaborations. The discovery and monitoring of many more pulsars is one of the key scientific goals of these projects. We now have a set of scales that may allow us to figure out how much these stars weigh, as well.

References: 
[1] See, for example, the list maintained at stellarcollapse.org. 
[2] See Large Hadron Collider. 
[3] J.M. Lattimer, M. Prakash, "The Physics of Neutron Stars", Science, 304, 536-542 (2004). Abstract. 
[4] The discovery of pulsars was recognized with the Nobel prize in physics in 1974. 
[5] Gravity tests using the stability of the timing of pulsars was recognized with the Nobel prize in physics in 1993. 
[6] A catalog of glitches is maintained by Jodrell Bank Centre for Astrophysics. 
[7] W.C.G. Ho, C.M. Espinoza, D. Antonopoulou, N. Andersson, "Pinning down the superfluid and measuring masses using pulsar glitches," Science Advances, 1, e1500578 (2015). Full Article. 
[8] See Square Kilometre Array.  
[9] See LOFAR. 

Finding Optical Transitions for Testing a Fundamental Constant’s Constancy

From left to right: José, Alexander and Hendrik  discuss the spectral analysis -- standing next to the Heidelberg electron beam ion trap, where the results were obtained.

Authors: Alexander Windberger, Hendrik Bekker, José R. Crespo López-Urrutia

Affiliation: Max-Planck-Institut für Kernphysik, Heidelberg, Germany.

We studied the uncharted optical spectra of the highly charged ions W¹⁴⁺, Re¹⁵⁺, Os¹⁶⁺, Ir¹⁷⁺, and Pt¹⁸⁺, and demonstrated generally applicable methods to identify the measured spectral lines. That allowed us to infer the transition energies for proposed ultra-stable frequency standards using Hf¹²⁺ and W¹⁴⁺ ions. In Ir¹⁷⁺, optical transitions with the highest sensitivity to a potential variation of the fine structure constant α ever predicted for a stable atomic system were determined. Highly advanced atomic structure calculations were benchmarked in the extreme regime of a triple level crossing.

Fundamental constants are taken as given by Nature. Our understanding of the origin of these constants is, however, rather poor: Their values are not set within the Standard Model but have to be determined empirically. Alternative theories, such as string or coupled dark energy theories, assume that fundamental constants emerge from dynamical fields and can vary at different times or places in the universe (see [1] for a review). Therefore, probing the stability of these constants allows us to search for physics beyond the Standard Model.

Our work focusses on testing a possible variation of the fine structure constant α, which characterizes the strength of interaction between charged particles and photons. Very small variations of α would lead to a detectable shift of wavelength, or color, of light which is emitted or absorbed by atoms. Following this approach the group of Webb et al. obtained the absorption spectra of interstellar clouds billions of light years away from us and in an extensive analysis found wavelength shifts for spectra observed at different angles [2]. This was interpreted as a spatial dipole-like variation of α.

We aim to test this extraordinary claim under well-defined laboratory conditions, preventing systematic uncertainties that the astrophysical observations might suffer from. Since the Earth, the Solar System, and our galaxy all move, a spatial variation translates into an effective temporal variation which was estimated at 10^-19/year [3]. Such a minuscule drift could be measured by monitoring the frequency ratio of two highly accurate optical atomic clocks. The clock transitions should be very sensitive to an α variation, but to nothing else. Highly charged ions fulfill these requirements. With an increasing ionic charge, the wavelengths of electronic transitions decrease and leave the range accessible to lasers. Systems with level crossings are an exception. When two or more configurations are almost equal in energy, optical transitions are possible. The level crossing of the 5s and 4f subshells predicted for Ir¹⁷⁺ should enable the highest sensitivity to the sought-after α variation in a stable atomic system [4].

No detailed knowledge of the electronic structure of these ion species existed. Most heavy highly charged ions, as Ir¹⁷⁺, are experimentally unexplored, and calculations are not sufficiently accurate for these complex systems. For our studies, we used the Heidelberg electron beam ion trap, which produces and traps the ions of interest. The continuously excited ions decay by emitting radiation, of which we analyzed the optical spectrum. An exemplary measurement can be seen in Fig. 1, showing the optical spectra of W¹⁴⁺, Re¹⁵⁺, Os¹⁶⁺, Ir¹⁷⁺, and Pt¹⁸⁺ (atomic numbers Z=74-78). These ions encompass the whole predicted 5s-4f level crossing region.

Figure 1. (Click on the figure to view with higher resolution) Typical spectral map of Ir ions measured using the Heidelberg electron beam ion trap. For this measurement we acquired spectra at 10 V intervals of the electron beam acceleration potential. New groups of fluorescence lines start to appear when the electron beam energy reaches the ionization potential of an ionic charge state. The new charge state is produced more efficiently as the electron beam energy further increases, and the fluorescence lines become stronger until the ionization threshold of the next higher charge state is reached. At this point the ion population is transferred to the next charge state, which starts to fluoresce, while the former one disappears. This dependence of the fluorescence intensity on the acceleration potential is depicted in the right graph. It is notable that this section of the optical spectrum assigned to Ir¹⁷⁺ already shows a dense manifold of spectral lines. In order to derive the level structure from the spectrum, sophisticated identification schemes had to be applied.

Subsequently, we assigned the measured spectral lines to their corresponding electronic transitions to establish the level scheme [5]. Given the large theoretical uncertainties, a direct comparison of calculated and measured spectra is futile. Instead, we used three alternative methods to identify the spectral lines.

First, we exploited the fact that these ions are isoelectronic, since they have the same number of electrons, and thus similar atomic structures. Over a limited range of atomic numbers, the transition energies depend on the respective nuclear charge with a simple polynomial scaling. By comparison between the measured scaling functions and theoretical predictions we were able to reliably identify all underlying transitions as can be seen in Fig. 2.

Figure 2. (Click on the figure to view with higher resolution) Identification of isoelectronic transitions using their characteristic energy scaling. (a) Measured spectra of W¹⁴⁺, Re¹⁵⁺, Os¹⁶⁺, Ir¹⁷⁺, and Pt¹⁸⁺ (black lines). An algorithm found nine isoelectronic transition energies that obey simple quadratic scaling laws (colored lines) as expected from theoretical considerations. (b) By comparing the experimentally determined constant offset A and the linear term B (full symbols) to the calculated ones (open symbols) an unambiguous identification of the underlying transitions could be achieved.

This method could be independently confirmed by measuring the identified transitions in Ir¹⁷⁺ with increased resolution and accuracy. The spectral lines revealed a characteristic line shape caused by the 8 T magnetic field present at the position of the trapped ions. The observed line shapes were individually modelled according to the Zeeman effect, leading to an independent verification in perfect agreement with the scaling method.

The identified transitions allowed us to test advanced atomic structure calculations for the first time in systems with such a complex level crossing of 4f ¹² 5s², 4f¹³ 5s, and 4f ¹⁴ configurations. We found that only relativistic multi-reference Fock-space coupled cluster calculations consistently showed a fair agreement with most of the observed lines.

A direct application is the determination of the transition energies of two proposed optical clock transitions with a potential relative frequency uncertainty of less than 10^-19 in W¹⁴⁺ and Hf¹²⁺ [6], another isoelectronic ion. Although we did not measure the spectrum of Hf¹²⁺, we were able to extrapolate the transition energy by applying the established energy scaling. Our experimental uncertainty is at least one order of magnitude smaller than that of predictions.

These clock transitions are exceptionally stable. However, they are not sensitive to a variation of the fine structure constant. For that, Ir¹⁷⁺ is ideal. By searching our data, we found closed transition cycles (Rydberg-Ritz principle) combining the identified with unidentified transitions. This enabled us to find two possible, but mutually excluding, candidates for the proposed α-sensitive transitions. We are currently performing more accurate measurements to remove this ambiguity.

Our method, line assignments by isoelectronic scaling of transitions (LINE ASSIST) is a straight-forward and general tool for exploring unknown spectra of highly charged ions. With the recent successful application of sympathetic cooling to highly charged ions [7], much higher accuracy can be achieved in future work: the ion temperature was reduced by nearly six orders of magnitude and the Doppler width accordingly. The experimental values for the transition energies obtained in the present work are needed for follow-up laser spectroscopy studies, applications as optical clock transitions, and testing the constancy of fundamental constants.

References:
[1] Jean-Philippe Uzan, "The fundamental constants and their variation: observational and theoretical status". Review of Modern Physics, 75, 403–455 (2003). Abstract.
[2] J. K. Webb, J. A. King, M. T. Murphy, V. V. Flambaum, R. F. Carswell, M. B. Bainbridge, "Indications of a Spatial Variation of the Fine Structure Constant". Physical Review Letters, 107, 191101 (2011). Abstract.
[3] J.C. Berengut, V.V. Flambaum, "Manifestations of a spatial variation of fundamental constants in atomic and nuclear clocks, Oklo, meteorites, and cosmological phenomena". Europhysics Letters, 97, 20006 (2012). Abstract.
[4] J.C. Berengut, V.A. Dzuba, V.V. Flambaum, A. Ong, "Electron-Hole Transitions in Multiply Charged Ions for Precision Laser Spectroscopy and Searching for Variations in α". Physical Review Letters, 106, 210802 (2011). Abstract.
[5] A. Windberger, J.R. Crespo López-Urrutia, H. Bekker, N.S. Oreshkina, J.C. Berengut, V. Bock, A. Borschevsky, V.A. Dzuba, E. Eliav, Z. Harman, U. Kaldor, S. Kaul, U.I. Safronova, V.V. Flambaum, C.H. Keitel, P.O. Schmidt, J. Ullrich, O.O. Versolato, "Identification of the Predicted 5s−4f  Level Crossing Optical Lines with Applications to Metrology and Searches for the Variation of Fundamental Constants". Physical Review Letters, 114, 150801 (2015). Abstract.
[6] V. A. Dzuba, A. Derevianko, V.V. Flambaum, "High-precision atomic clocks with highly charged ions: Nuclear-spin-zero f ¹²-shell ions". Physical Review A, 86, 054501 (2012). Abstract.
[7] L. Schmöger, O.O. Versolato, M. Schwarz, M. Kohnen, A. Windberger, B. Piest, S. Feuchtenbeiner, J. Pedregosa-Gutierrez, T. Leopold, P. Micke, A.K. Hansen, T.M. Baumann, M. Drewsen, J. Ullrich, P.O. Schmidt, J.R. Crespo López-Urrutia, "Coulomb crystallization of highly charged ions". Science, 347, 1233 (2015). Abstract.