Radioactive Iron - An Astrophysical Clock for Nucleosynthesis

Anton Wallner (photo credit: Stuart Hay, ANU)

Author: Anton Wallner^1,2

Affiliation:
¹Dept of Nuclear Physics, Australian National University, Canberra, Australia,
²VERA Laboratory, Faculty of Physics, University of Vienna, Austria.

Massive stars may end their life in a supernova explosion - one of the most violent events in our galaxy. Supernovae are thus massive exploding stars that return a large fraction of the star’s material back to the interstellar medium. Nucleosynthesis in massive stars shapes therefore the elemental abundance pattern and the galactic chemical evolution, e.g. our solar system is the product of many preceding star generations [1].

Extraterrestrial material in the form of interstellar dust can also enter the solar system and may be deposited on Earth [2]. Their nucleosynthetic history is locked in its isotopic signatures. Interstellar matter will contain stable isotopes but also freshly produced radionuclides. Thus, the existence of fresh radionuclides in the interstellar medium serves as radioactive clocks for their recent production.

Radioactive iron-60 (Fe-60) is a radionuclide with a half-life of about 2 million years. It is predominantly formed in massive stars at the end of their lives just before and during a supernova and then distributed by the explosion into the interstellar space. Fe-60 is thus an ideal candidate to monitor supernova explosions and recent element synthesis.

Since this radioactive iron is not naturally present on Earth, trace amounts of this isotope are a particularly sensitive astrophysical marker. Supernova-produced iron from the interstellar medium can be captured by the Earth on its way through the Milky Way. If one finds this radioactive iron-60 in the terrestrial environment (apart from artificial production), it must come from cosmic explosions; more precisely from the last few million years, otherwise it would have long since decayed.

With its half-life in the million year range, Fe-60 is suitable for dating astrophysical events, such as supernova explosions. The usability of this isotope, in particular as an astrophysical clock, was however limited, because the lifetime of this nuclide was not exactly known - an important prerequisite to serve as a chronometer. There were two measurements so far, one from 1984 [3] and another very precise one from 2009 [4], but both were almost a factor of 2 different.

Iron-60 – a monitor for element synthesis and nearby supernova explosions

This isotope has a variety of applications in astrophysics. The main reason is, it is observed in space through its radioactive decay and it is not naturally present on Earth.

Researchers can virtually monitor live nucleosynthesis in massive stars, e.g. active regions of element formation and also the distribution of ejected stellar material in the Milky Way. Iron-60 can be observed directly in our Milky Way via space-born satellites through its decay and the characteristic radiation emitted (similar to another radioactive isotope, Al-26) [5,6]. These observations clearly demonstrate its presence in the interstellar medium. Such radionuclides were produced 'recently", i.e. within a few half-lives. As their decay is observed, one needs the half-life to calculate the number of atoms present in the interstellar medium.

Knie et al., in a pioneering work at the Technical University of Munich, Germany, found Fe-60 at the ocean floor in a manganese crust indicating a possible near-Earth supernova activity about 2 to 3 million years ago [7,8]. Iron-60 was present at the birth of our solar system, more than four billion years ago. This is evidenced today in pre-solar material by overabundances of Fe-60’s decay products [9].

Establishing a connection between these observations of the radioactive decay of Fe-60 and the number of iron-60 atoms, however, requires a precise knowledge of its life-time, that is, its half-life.

How to measure a half-life of millions of years?

Firstly, one needs a sufficient number of atoms. We, a team of scientists from Australia, Switzerland and Austria [10] used artificially produced iron-60 extracted from nuclear waste of an accelerator facility in Switzerland. This iron fraction was separated by specialists in Switzerland and then analyzed for its Fe-60 content. The number of radioactive atoms must be measured in absolute terms, and this is a difficult task and was probably the reason for the discrepancy in earlier measurements.

Figure caption: Identification spectra with a clear separation of the main background Ni-60 from Fe-60: single atom counting of Fe-60 at the ANU - each point represents a single atom. Combining up to 5 different detector signals results in an unsurpassed sensitivity of Fe-60/Fe = 4 X 10^-17 (A. Wallner et al., [10]).

We used a very sensitive method to accurately determine the low number of Fe-60 atoms in their sample: accelerator mass spectrometry (AMS) [11,12], a technique that counts atoms directly and that is used for example, also for radiocarbon dating. The Fe-60 measurements were carried out at the Heavy Ion Accelerator Facility at the Australian National University in Canberra, one of the world's most sensitive facilities to detect tiny traces of rare elements in our environment. With this extremely sensitive facility no background could influence our results. Further, we counted Fe-60 relative to another radioactive iron isotope, namely Fe-55. Fe-55 is well known and easier to measure. By using the same measurement setup for Fe-60 and Fe-55, we are confident that potential unknown errors were minimized in our work.

The new value for the half-life of Fe-60 [10] shows a good agreement with the precise measurement by Rugel et al. from the year 2009 [4]. According to our result, they had done a very good job! Combining both measurements, this allows now the use of Fe-60 as a precise cosmic clock. It eliminates a long-standing discrepancy and thus establishes this radionuclide as a precise astrophysical chronometer.

As another additional outcome we encourage other groups to repeat such kind of measurements. With respect to the difficulty of performing measurements of long half-lives, independent and complementary techniques are essential for settling open and difficult-to-solve questions.

References:
[1] R. Diehl, D.H. Hartmann and N. Prantzos (eds.), "Astronomy with Radioactivities", Lecture Notes in Physics, vol. 812, Springer, Berlin (2011). Google Books Preview.
[2] A. Wallner, T. Faestermann, C. Feldstein, K. Knie, G. Korschinek, W. Kutschera, A. Ofan, M. Paul, F. Quinto, G. Rugel, P. Steier, "Abundance of live ²⁴⁴Pu in deep-sea reservoirs on Earth points to rarity of actinide nucleosynthesis", Nature Communications, 6:5956; DOI: 10.1038/ncomms6956 (2015). Full Article.
[3] Walter Kutschera, Peter J. Billquist, Dieter Frekers, Walter Henning, Kenneth J. Jensen, Ma Xiuzeng, Richard Pardo, Michael Paul, Karl E. Rehm, Robert K. Smither, Jan L. Yntema, Leonard F. Mausner, "Half-life of 60Fe", Nuclear Instruments and Methods in Physics Research, Section B, 5, 430 (1984). Abstract.
[4] G. Rugel, T. Faestermann, K. Knie, G. Korschinek, M. Poutivsev, D. Schumann, N. Kivel, I. Günther-Leopold, R. Weinreich, and M. Wohlmuther, "New Measurement of the ⁶⁰Fe Half-Life", Physical Review Letters, 103, 072502 (2009). Abstract.
[5] W. Wang, M. J. Harris, R. Diehl, H. Halloin, B. Cordier, A.W. Strong, K. Kretschmer, J. Knödlseder, P. Jean, G. G. Lichti, J. P. Roques, S. Schanne, A. von Kienlin, G. Weidenspointner, and C. Wunderer, "SPI observations of the diffuse ⁶⁰Fe emission in the galaxy", Astronomy & Astrophysics, 469, 1005 (2007). Abstract.
[6] Roland Diehl, "Nuclear astrophysics lessons from INTEGRAL", Reports on Progress in Physics. 76, 026301 (2013). Abstract.
[7] 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.
[8] 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.
[9] A. Shukolyukov, G.W. Lugmair, "⁶⁰Fe in eucrites", Earth and Planetary Science Letters, 119, 159 (1993). Abstract ; A. Shukolyukov, G.W. Lugmair, "Live iron-60 in the early solar system", Science, 259, 1138 (1993). Abstract.
[10] A. Wallner, M. Bichler, K. Buczak, R. Dressler, L. K. Fifield, D. Schumann, J. H. Sterba, S. G. Tims, G. Wallner, W. Kutschera, “Settling the half-life of ⁶⁰Fe – fundamental for a versatile astrophysical chronometer”, Physical Review Letters, 114, 041101 (2015). Abstract.
[11] Hans-Arno Synal, "Developments in accelerator mass spectrometry", International Journal of Mass Spectrometry, 349–350, 192 (2013). Abstract.
[12] Walter Kutschera, "Applications of accelerator mass spectrometry", International Journal of Mass Spectrometry, 349–350, 203 (2013). Abstract.

Sound Velocity Bound and Neutron Stars

Paulo Bedaque (left) and Andrew W. Steiner (right)

Authors: Paulo Bedaque¹, Andrew W. Steiner^2,3,4 

Affiliation: 
¹Department of Physics, University of Maryland, College Park, USA 
²Institute for Nuclear Theory, University of Washington, Seattle, USA
³Department of Physics and Astronomy, University of Tennessee, Knoxville, USA 
⁴Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA. 

Neutron stars are the final stage in the evolution of a star, the remnants of supernova explosion marking the end of the star’s life. They are incredibly compact objects: masses comparable to the Sun’s are compressed in a region of about 10 miles radius. At these densities, most matter is composed of neutrons. The repulsion between neutrons balances precariously against the strong gravitational fields generated by this high matter concentration: a little less repulsion or a little more mass leads to the collapse of the star into a black hole [1].

It has been possible to measure the mass of several neutron stars, and until recently, all accurate mass measurements were near 1.4 times the mass of our sun. However, within the past few years, two neutron stars have been discovered to have a mass around twice that of our sun [2]. What this discovery means is that the neutron matter composing the star is stiffer than previously expected.

The speed of sound in air is about 346 meters per second, and it tends to increase with either the density or the temperature of the medium in which it travels. Since neutron stars contain the most dense matter in the universe one might wonder how fast the speed of sound is inside neutron stars.

Everywhere else in the universe [3], the speed of sound seems to be limited to the speed of light divided by the square root of 3, that is, v < 0.577 c (see, for example, the figures here: Link to plots >> ). At high enough densities or temperatures, the speed of sound approaches this limiting value. This result comes from quantum chromodynamics (QCD) [4] - the physical theory which describes how neutrons and protons (made of quarks) interact. At high enough densities and temperatures, QCD exhibits "asymptotic freedom", meaning that the interaction becomes weaker [5]. Unfortunately, neutron star densities are not large enough so that quarks are weakly interacting.

In a paper published in Physical Review Letters (as an 'Editor's suggestion') on January 21st [6], we showed that the speed of sound in neutron stars must exceed this value at some point inside a neutron star. The reason is that models where the speed of sound is smaller than the limiting value at all densities (those like the black lines in the figure) are too soft to produce neutron stars with masses twice the mass of the sun. Thus, the only alternative is that the speed of sound must look something like either the blue dotted or red dashed lines.

This result is important because it tells us more about how neutrons and protons interact, not only in neutron stars, but also here on earth [7]. It gives us more insight into how QCD behaves at high densities. Finally, it also helps us understand some of the more extreme neutron star-related processes like core-collapse supernovae, magnetar flares, and neutron star mergers.

Notes & References:
[1] See a diagram of stellar evolution from the Chandra X-ray observatory, their neutron star page, or the wikpedia entry on neutron stars.
[2] P.B. Demorest, T. Pennucci, S.M. Ransom, M.S.E. Roberts, J.W.T. Hessels, "A two-solar-mass neutron star measured using Shapiro delay". Nature, 467, 1081–1083 (2010). Abstract; John Antoniadis, Paulo C. C. Freire, Norbert Wex, Thomas M. Tauris, Ryan S. Lynch, Marten H. van Kerkwijk, Michael Kramer, Cees Bassa, Vik S. Dhillon, Thomas Driebe, Jason W. T. Hessels, Victoria M. Kaspi, Vladislav I. Kondratiev, Norbert Langer, Thomas R. Marsh, Maura A. McLaughlin, Timothy T. Pennucci, Scott M. Ransom, Ingrid H. Stairs, Joeri van Leeuwen, Joris P. W. Verbiest, David G. Whelan, "A Massive Pulsar in a Compact Relativistic Binary". Science, 340, 6131 (2013). Abstract.
[3] The only possible exception is matter inside the event horizon of a black hole, which is not causally connected with the rest of the universe anyway.
[4] See the Wikipedia article on Quantum Chromodynamics.
[5] This finding led to 2004 Nobel prize in physics for David J. Gross, H. David Politzer and Frank Wilczek.
[6] Paulo Bedaque, Andrew W. Steiner, "Sound Velocity Bound and Neutron Stars". Physical Review Letters, 114, 031103 (2015). Abstract. Also available at: arXiv:1408.5116 [nucl-th].
[7] Neutrons and protons are the basic building blocks of all atomic nuclei.

A Rare Middle-Weight Black Hole in a Nearby Galaxy

Dheeraj R. Pasham

Author: Dheeraj R. Pasham^1,2

Affiliation:
¹Astronomy Department, University of Maryland, College Park, USA 
²Astrophysics Science Division and Joint Space-Science Institute, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

Black holes are among the most exotic and mysterious objects in the Universe, serving as one-way portals for matter, light, or anything else that gets too close. Our modern conception of black holes stems directly from Albert Einstein's theory of gravity, the general theory of relativity, proposed in 1915. However, it was not until the 1970s that new evidence emerged that these objects not only exist, but actually power the brightest objects in the Universe [1,2].

It is now established that there are at least two classes of black holes in the Universe: (1) the so-called "stellar-mass" black holes which weigh anywhere between 3-50 times the mass of our Sun and (2) "super-massive" black holes that are a million to a few billion times more massive than the Sun. Although we understand that the former are produced by spectacular deaths of the heaviest stars, the formation and the growth of super-massive black holes that are responsible for shaping the nature of many galaxies is still a mystery.

Understanding the formation of super-massive black holes holds a key to understanding the growth of galaxies that are the building blocks of our Universe. Current evidence indicates that super-massive black holes might have grown by accumulation of matter onto middle-weight black holes that are a few hundred to a thousand times more massive than the Sun and formed by the collapse the massive, first generation of stars formed when the Universe was only ~ 5% of its current age [3]. However, although candidates for such middle-weight black holes exist, no definitive mass measurements have yet been made. This is primarily because these objects are faint and thus the traditional methods used to weigh stellar-mass and supermassive black holes have not yet yielded any meaningful results [4,5].

In our recent result published in Nature [6], we used a new technique involving the 3:2 frequency ratio, X-ray resonance oscillations arising from close to the black hole in the galaxy M82, to measure its black hole mass to be 428±105 heavier than the Sun.

Figure 1 (click on the figure to view higher resolution version )

The basic idea behind the measurement is as follows. A subset of stellar-mass black holes exhibit the so-called high-frequency quasi-periodic oscillations (QPOs). Often in these systems the high-frequency QPOs occur in pairs of two, with their frequencies in a 3:2 ratio [7,8]. The power spectra of three such systems showing the twin pair QPOs are shown in Figure 1 [7]. The respective timescales of these oscillations (~0.01 seconds: 100-450 Hz) are comparable to the Keplerain orbital periods of test particles near the innermost stable circular orbit (ISCO) of these black holes. For example, for a non-rotating black hole weighing 10 solar masses, the Keplerian frequency of a test particle at ISCO is 220 Hz. In addition, for a given source, these frequencies appear to be stable to within a few percent for changes in the source luminosity. The fact that their frequencies are stable and appear to be originating from close to the ISCO (for a non-spinning black hole, the ISCO radius is 3 times the radius of the event horizon) suggests that they originate from very near to the black hole where strong gravity dominates and hence tied to the black hole's mass [9,10]. Under the assumption that these oscillations originate from a fixed characteristic radius within the accretion disk around the black hole, their frequencies should scale inversely with the black hole mass, and there is observational support that they do for stellar-mass black holes [7].

Some recent studies on X-ray variability of stellar-mass and supermassive black holes suggest that supermassive black holes behave as scaled-up stellar-mass black hole systems. More specifically, the qualitative nature of the variability of both the smaller stellar-mass and the heavier supermassive black holes appears to be the same (they appear to vary the same way) with the respective timescales of supermassive black holes being longer than than those of their stellar-mass counterparts. McHardy et al. (2006) [11] have demonstrated that these timescales scale inversely with the black hole mass after taking into account the rate at which matter fall onto the black hole. Under this black hole unification paradigm [11], if middle-weight black holes exist, some of them should exhibit these 3:2 pairs but at frequencies scaled down (longer timescales) according to their black hole masses.

Figure 2

Combing 6 years of archival X-ray data, we recently discovered such stable, twin-peak (3.3 and 5 Hz, 3:2 frequency ratio) X-ray oscillations from an object named M82 X-1 (see Figure 2) at frequencies roughly 50 times lower (or at timescales 50 times longer) than stellar-mass black holes. Scaling these frequencies to the oscillations of the black holes of known stellar mass implies that M82 X-1's black hole is 428±105 heavier than our Sun.

References:
[1] C.T. Bolton, "Identification of Cygnus X-1 with HDE 226868". Nature, 235, 271 (1972). Abstract.
[2] B. Louise Webster, Paul Murdin, "Cygnus X-1—a Spectroscopic Binary with a Heavy Companion?" Nature, 235, 37 (1972). Abstract.
[3] Piero Madau and Martin J. Rees, "Massive Black Holes as Population III Remnants". Astrophysical Journal Letters, 551, L27 (2001). Abstract.
[4] T.P. Roberts, J.C. Gladstone, A.D. Goulding, A.M. Swinbank, M.J. Ward, M.R. Goad, A.J. Levan, "(No) dynamical constraints on the mass of the black hole in two ULXs". Astronomische Nachrichten, 332, 398 (2011). Abstract.
[5] D. Cseh, F. Grisé, P. Kaaret, S. Corbel, S. Scaringi, P. Groot, H. Falcke, E. Körding, "Towards a dynamical mass of the ultraluminous X-ray source NGC 5408 X-1". Monthly Notices of the Royal Astronomical Society, 435, 2896 (2013). Abstract.
[6] Dheeraj R. Pasham, Tod E. Strohmayer, Richard F. Mushotzky, "A 400 solar mass black hole in galaxy M82". Nature, 513, 74 (2014). Abstract.
[7] Ronald A. Remillard and Jeffrey E. McClintock, "X-Ray Properties of Black-Hole Binaries". Annual Review of Astronomy & Astrophysics, 44, 49-92 (2006). Abstract.
[8] T. M. Belloni, A. Sanna, M. Méndez,"High-frequency quasi-periodic oscillations in black hole binaries". Monthly Notices of the Royal Astronomical Society, 426, 1701 (2012). Abstract.
[9] Marek A. Abramowicz, Włodek Kluźniak, Jeffrey E. McClintock, Ronald A. Remillard, "The Importance of Discovering a 3:2 Twin-Peak Quasi-periodic Oscillation in an Ultraluminous X-Ray Source, or How to Solve the Puzzle of Intermediate-Mass Black Holes". Astrophysical Journal Letters, 609, L63 (2004). Abstract.
[10] Robert V. Wagoner, "Diskoseismology and QPOs Confront Black Hole Spin". Astrophysical Journal Letters, 752, LL18 (2012). Abstract.
[11] I. M. McHardy, E. Koerding, C. Knigge, P. Uttley, R. P. Fender,"Active galactic nuclei as scaled-up Galactic black holes". Nature, 444, 730 (2006). Abstract. 

Differing Isomeric Responses used as a Barometer for Organic Materials in the Cosmos

From left to right: Wren Montgomery, Jonathan S. Watson, Mark A. Sephton

Authors: Wren Montgomery, Jonathan S. Watson, Mark A. Sephton

Affiliation: Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College, London, UK

We showed that a family of organic molecules having the same atomic composition but different structures have substantially different responses to the application of high pressure. This is the first potential cosmic barometer capable of recording high pressure events throughout the history of the universe.

Organic matter is common throughout the universe. It is found in interstellar and circumstellar clouds, asteroids, meteorites, comets, planets and satellites [1,2]. Taking a cue from terrestrial geochemistry, where isomer distributions of aromatic hydrocarbons can be used to determine formation environments (primarily temperature), we were intrigued by the possibility of a similar indicator for astronomical environments.

In order to reach high pressures in the laboratory, we used the diamond anvil cell (Figure 1). In this device, two modified brilliant-cut gem quality diamonds are mounted culet-to-culet and gently pressed against each other. The broad transparent window (UV to Far Infrared) of the diamond anvils means a variety of techniques can be used for in-situ measurements.

Figure 1. (a) Schematic of high-pressure synchrotron-source FTIR spectroscopy measurements parallel to the synchrotron beam. Not to scale. (b) Schematic of high-pressure synchrotron-source FTIR spectroscopy measurements in the plane of the sample. Not to scale. (c) Microphotograph of 1,7-dimethylnaphthalene loaded into the diamond anvil cell. The ruby is the sphere toward the center of the sample chamber ( © 2014. The American Astronomical Society).

In our recent work [3], we used synchrotron source FTIR spectroscopy in the diamond anvil cell (Figure 1) at the Swiss Light Source and SOLEIL Synchrotron (France) to collect FTIR spectra for 5 isomers of dimethylnaphthalene (DMN). DMNs were chosen because this type of organic structure has been suggested as the cause of some unidentified infrared bands [4] and is present in meteorites [5].

By plotting peak center against pressure, we determined the relative effects of pressure on the isomers (Figure 2). Overall, the greatest changes occurred in 1,8-DMN, possibly due to the greater void space in the crystal at ambient conditions. Meanwhile, 1,5-DMN shows very little change with the application of high pressure, contrary to the previously known instability at ambient conditions [6].

Figure 2. Compression and decompression peak fit data for 1,8-, 1,5-, 1,7-, 2,6-, and 2,7-dimethylnaphthalene. Each data point represents the center of a peak associated with that material at a single pressure ( © 2014. The American Astronomical Society).

Overall, the responses of dimethylnaphthalenes to pressure conform to those observed in response to heat. The response of 1,5-DMN to pressure is contrary to those changes brought about by high temperature and reveals that although the molecular consequences of heating and pressurizing are often the same, the two mechanisms are very distinct and can generate opposing effects.

This suggests that measurement of infrared spectra of organic samples from the cosmos, either on samples in the laboratory or using remote instruments on Rovers or Landers can be used to quantify the thermodynamic history of the universe.

References:
[1] Th. Henning, and F. Salama, “Carbon in the Universe”. Science, 282, 2204 (1998). Abstract.
[2] Scott A. Sandford, "Terrestrial Analysis of the Organic Component of Comet Dust".  Annual Review of Analytical Chemistry, 1, 549 (2008). Full Article.
[3] Wren Montgomery, Jonathan S. Watson, Mark A. Sephton, “An Organic Cosmo-barometer: Distinct Pressure and Temperature Effects for Methyl Substituted Polycyclic Aromatic Hydrocarbons”. The Astrophysical Journal, 784, 98 (2014). Abstract.
[4] Jun Shan, Masako Suton, L. C. Lee, “3.3 micron emission from ultraviolet excitation of some aromatic molecules”. The Astrophysical Journal, 383, 459 (1991). Abstract.
[5] M. A. Sephton, “Pyrolysis and mass spectrometry studies of meteoritic organic matter”. Mass Spectrometry Reviews, 31, 560-569 (2012). Abstract.
[6] Chick C. Wilson, “Locked-in methyl groups in 1,5-dimethylnaphthalene close to the melting point”. Chemical Communications, 1281-1282 (1997). Abstract.

A Cosmic Web Filament Revealed in Lyman α Emission around a Luminous High-redshift Quasar

[Left to Right] Sebastiano Cantalupo, Fabrizio Arrigoni-Battaia, J. Xavier Prochaska, Joseph F. Hennawi, Piero Madau.

Authors: Sebastiano Cantalupo^1,2, Fabrizio Arrigoni-Battaia², J. Xavier Prochaska^1,2, Joseph F. Hennawi², Piero Madau¹

Affiliation:
¹Department of Astronomy and Astrophysics & UCO/Lick Observatory, University of California, Santa Cruz, USA
²Max-Planck-Institut für Astronomie, Heidelberg, Germany

Galaxies are believed to be embedded in a “cosmic web”, the three-dimensional cellular foam arrangement of matter in the Universe predicted by the standard cold dark matter cosmological paradigm [1]. Most of the baryons do not reside in galaxies, but are spread along this web in highly ionized gaseous medium [2] that is too rarefied to form stars. While intergalactic gas may have been observed as absorption features in the spectra of background sources [3], direct constraints on the three-dimensional properties and morphology of the cosmic web are still missing. Limited by the rarity of bright background sources, absorption studies are only able to provide one-dimensional skewers of the cosmic web that are typically separated by several tens of Mpc. Direct detection of intergalactic gas in emission would instead provide a full three-dimensional image significantly improving our understanding of cosmological structure formation and the cycle of baryons in and out of galaxies.

Despite the predicted low surface brightness, there have been attempts to detect the cosmic web in Lyman α emission, e.g., by means of low-resolution spectroscopy [4] to search blindly for fluorescence generated by optically thick gas illuminated by the cosmic UV background [5]. Achieving a very deep flux limit of 8x10^-20erg s^-1 cm^-2 arcsec^-2, these observations failed to reveal the cosmic web. Positive fluctuations in the ionizing background may be used to increase the expected fluorescent signal [6]. In a pilot survey obtained in 2010 using a custom-built, narrow-band (NB) filter on the VLT-FORS we demonstrated indeed that bright quasars can, like a flashlight, “illuminate” the densest knots in the surrounding cosmic web and boost fluorescent Lyman α emission to detectable levels [7]. In this survey we found several compact ”dark galaxies” and extended nebulae (up to 65 physical kpc) around star forming galaxies, but none of them extending on intergalactic scales. Following the same experiment, we have initiated in 2012 a NB imaging campaign on Keck/LRISb centered on z~2 bright quasars and we have reported in a recent Nature letter [8] the first result of this new imaging survey.

Figure 1 : Processed and combined images of the field surrounding the quasar UM287. Each image is 2 arcmin on a side and the quasar is located at the center. In the narrow-band (NB3985) image (panel 'a'), which is tuned to the Lyman α line of the systemic redshift for UM287, one identifies very extended (≈ 55 arcsec across) emission – that we named "Slug Nebula". The deep V-band image (panel 'b') does not show any extended emission associated with UM287. This requires the Slug Nebula to be line-emission, and we identify it as Lyman α at the redshift of the quasar.

On November 12 and 13, 2012, we imaged the field of the quasar UM 287 with a custom NB filter tuned to Lyman α at z = 2.279 inserted into the Keck/LRISb camera on the 10m Keck-I telescope. We acquired 10 hours of integration in a series of dithered, 1200s exposures in clear conditions. In parallel (enabled by a dichroic), we obtained broad-band V images with the LRISr camera. Figure 1 presents the processed and combined images, centered on UM287. The V -band image is very deep and hundreds of compact sources are present in the field. We expect the majority of these are background galaxies, unrelated to the system. In the NB3985 image, however, one identifies a very extended source originating near the quasar with a projected size of about 1 arcmin (500kpc physical or 1.6 Mpc co-moving). We will refer to this extended emission as the ”Slug Nebula” in the reminder of this article. Within the nebula, very few sources are identified in the broad-band images nor is any extended emission observed. This requires the narrow-band light to be line-emission, and we identify it as Lyman α at the redshift of UM287.

Figure 2 : Lyman α image of the Slug Nebula. We subtracted from the NB image the continuum contribution estimated from the broad-band images. The location of the quasar UM287 is labeled with the letter “a”. The color map and the contours indicates, respectively, the Lyman α surface brightness and the signal-to-noise ratio (S/N) per arcsec² aperture. The extended emission spans a projected angular size of ≈ 55 arcsec (about 460 physical kpc), measured from the 2σ (~10⁻¹⁸ erg s^-1 cm^-2 arcsec^-2) contours. Object “b” is an optically faint (g~23AB) quasar at the same redshift of UM287. The Nebula appears broadly filamentary and asymmetric, extending mostly on the eastern side of quasar “a” up to a projected distance of about 35 arcsec (~285 physical kpc) measured from the 2σ isophotal.

Figure 2 presents the NB3985 image, continuum subtracted using standard techniques [8]. One identifies several compact sources including UM287 (labeled “a” in the figure) with excess Lyman α emission. The second brightest compact emitter (indicated by the letter “b”) is an optically faint (g~22 AB) quasar at the same redshift of UM287. The image is dominated, however, by the filamentary and asymmetric Slug Nebula. Although Lyman α nebulae extending up to about 250 kpc have been previously detected [9-13], the Slug Nebula represents so far a unique system as we show in Figure 3: with a size of about 55” or 460 physical kpc, it extends well beyond the virial radius of any plausible dark matter halo associated with UM287. Indeed, in order to be fully contained within the virial radius of a dark matter halo centered on UM287, the quasar host halo should have a total halo mass of 10^13.5 M_sun. This is ten times larger than the typical value associated with radio-quiet quasars (10^12.5 M_sun, see [8] for discussion) and it would make the host halo of UM287 one of the largest know at z > 2. However, this possibility is clearly excluded by the absence of an excess of Lyman α emitting galaxies around UM287 compared to other radio-quiet quasars. Our analysis of the galaxy distribution around UM287 suggests instead that this quasar is residing in a typical or under-dense environment for radio-quiet quasars and that its total halo mass therefore does not exceed 10^12.5 M_sun. Differently from any previous detection, the Slug Nebula is therefore the first possible image of intergalactic gas at z > 2 extending beyond any individual, associated dark matter halo. The rarity of these systems may be explained by the combination of anisotropic emission from the quasars (typically only about 40% of the solid angle around a bright, high-redshift quasar is unobstructed [14]), the anisotropic distribution of dense filaments and light travel effects that, for quasar ages younger than a few Myr, further limit the possible ”illuminated” volume.

Figure 3 : Luminosity-size relations for previously detected, bright Lyman α nebulae and the Slug Nebula around the quasar UM287. The plot includes nebulae surrounding AGN and Lyman α blobs (LAB). The dashed line indicates the virial diameter of a dark matter halo with total mass M ~ 10^12.5 M_sun, the typical host of radio-quiet quasars including UM287, as confirmed by the analysis of the galaxy overdensity in our field. The Slug Nebula, differently from any previous detection, extends on Intergalactic Medium scales that are well beyond any possible associated dark matter halo. Note that, even if we restrict the size measurement of the UM287 Nebula to the 4 × 10⁻¹⁸ erg s^-1 cm^-2 arcsec^-2 isophotal to be comparable with the majority of the previous surveys, the measured apparent size of the Slug Nebula will be reduced only by about 20%.

In order to constrain the physical properties of this, so far, unique system, we use a set of Lyman α radiative transfer calculations [15] combined with hydrodynamical simulation of cosmological structure formation around a quasar halo host similar to UM287. We consider two possible, extreme scenarios for the Lyman α emission mechanism of the intergalactic gas associated with the Slug Nebula: a) the gas is mostly ionized and the Lyman α emission is mainly produced by hydrogen recombinations. b) the gas is mostly neutral and the emission is mainly due to scattering of the Lyman α and continuum photons produced by the quasar Broad Line Region (BLR). In both cases, we performed a full three dimensional Lyman α radiative transfer calculation including gas temperature and velocity field effects on Lyman α scattering within the Nebula. The models are used to obtain the scaling relations between the observable Lyman α surface brightness from the intergalactic gas surrounding the quasar and the hydrogen column densities. Through these relations, we converted the observed SB into an estimated gas column density for the two extreme scenarios. Note that the estimated column densities for case ”a” are degenerate with the ionized gas clumping factor (C =2

>/², where n is the electron density) below the simulation resolution scale, ranging from ~10 proper kpc for diffuse intergalactic gas to ~100 pc for the densest regions within galaxies.

The results, using the observed BLR Lyman α luminosity and C = 1, are presented in Fig.4. The observed Lyman α emission from the intergalactic gas associated with the Slug Nebula requires very large column density of ”cold” (T < 5 x 10⁴ K) gas to be matched by current simulations. The implied total, cold gas mass ”illuminated” by the quasar is M_gas ~10^11.4±0.6 M_sun for the ”mostly neutral” case (”b”) and M_gas ~10^12±0.5M_sun for the ”mostly ionized” case (”a”) and C = 1. Note that the total estimated mass for the case ”a” scales as C^1/2. For comparison, a typical simulated filament in our cosmological simulation of structure formation with size and morphology similar to the Slug Nebula around a similar halo has a total gas mass of about 10^11.3 M_sun, but only about 15% of this gas is ”cold” (T < 5x10⁴ K), i.e. 10^10.5 M_sun and therefore able to emit substantial Lyman α emission. These estimates are consistent with other recent, grid-based hydrodynamical simulations of structure formation [16].

Figure 4 : Inferred hydrogen column densities associated with the Slug Nebula. We have converted the observed Lyman α Surface Brightness into gas column densities using a set of scaling relations obtained with detailed radiative transfer simulations. We have explored two extreme cases: a) the gas is mostly ionized by the quasar radiation (panel “a”), b) the gas is mostly neutral (panel “b”). Two circular regions with a diameter of 7 arcsec (~ 8 times the seeing radius) have been masked at the location of the quasars (black circles). The inferred hydrogen column density in panel “a” scales as C^−1/2, where C is the gas clumping factor below a spatial length of up to about 10 physical kpc at moderate overdensities (less than about 40 times the mean density of the Universe at z~2). The implied column densities and gas masses, in both cases, are at least a factor of ten larger than what is typically observed within cosmological simulations around massive haloes, suggesting, e.g., that a large number of small clumps within the diffuse Intergalactic medium may be missing within current numerical models.

How one can explain the large differences between the estimated, cold gas mass of the Slug Nebula and the available amount of cold gas predicted by numerical simulations on similar scales? The Slug Nebula seems to point in the direction of a second, fainter quasar companion of UM287. However, because of the large distance from UM287 -- at least 200 proper kpc and up to 4 proper Mpc considering the 1σ redshift error, and the morphology of the Nebula we can exclude that the UM287 Nebula is the result of tidal interaction due to a merging event between the two quasar hosts. Indeed, such a large separation would imply that any possible encounter between the two quasars is likely a high velocity interaction or an encounter with large impact parameter. We note that it is not impossible but extremely difficult to produce a long and massive tidal tail during a ”fast” encounter but the amount of gas stripped by the quasar host galaxies in the best scenario would likely be a very small fraction (< 10%) of its total ISM and certainly cannot account for the total amount of gas detected in the Nebula. Irrespective of the details of the possible interaction between the two quasar host galaxies, any resulting, long tidal tail would be very thin with sizes of the order of few kpc or less while the observed Nebula has a thickness of at least 100 physical kpc in its widest point.

Similarly, it would be very difficult to explain the properties of the Nebula assuming a galactic gas outflow origin produced by possible quasar feedback events. Indeed, although radio-quiet quasar outflows are highly unconstrained from current observations and poorly understood theoretically, the large size of the Nebula, extending well beyond the virial radius of the quasar host halo, would require a high velocity outflow that is incompatible with the “cold” temperature of the gas required by the Lyman α emission. A recent spectroscopic follow-up (Cantalupo et al., in preparation) provides additional evidences that the Nebula is kinematically quiet and therefore that it cannot be generated by “quasar feedback”. The size, morphology and kinematical properties of the gas are instead broadly consistent with our expectations from a filament of the “cosmic web”.

How one can then reconcile the intergalactic nature of the Slug Nebula with the large mass discrepancy with intergalactic gas simulations? One possibility is to assume that the simulations are not resolving a large population of small, cold gas clumps within the low-density Intergalactic medium that are illuminated and ionized by the intense radiation of the quasar. In this case, an extremely high clumping factor, namely C ~1000, on scales below few kpc would be required in order to explain the large luminosity of the Slug Nebula with the cold gas mass within the intergalactic filaments predicted by the simulations. On the other hand, if some physical process that is not fully captured by current grid-based simulations increases the fraction of cold gas around the quasar, e.g. a proper treatment of metal mixing, a smaller clumping factor may be required. In the extreme – and rather unrealistic - case that all the hot gas is turned into a cold phase, the required clumping factor would be C ~20. Even if the gas is not ionized by the quasar (case ”b” above), the simulations are able to reproduce the observed mass only if a substantial amount of hot gas is converted into a cold phase. Incidentally, this is exactly the same result found comparing the properties of Lyman α absorption systems around a large statistical sample of quasars with simulations [17].

The discovery of the Slug Nebula represents both a unique laboratory and a challenge for our knowledge of cosmological structure formation on Intergalactic scales around massive haloes. On one hand, it provides a fundamental confirmation that specifically designed, deep narrow-band surveys centered on bright quasars are able to provide - for the first time - an image of cosmic gas on intergalactic scales. The rarity of such detection however, may imply that several conditions regarding, e.g. the geometry of the quasar "illumination" are met arguing for the necessity of a very large sample of quasars. On the other hand, our observation indicates that current models of cosmological structure formation (at least numerical methods based on Adaptive Mesh Refinement algorithms) are far from providing an accurate picture of the gas properties - not only within galaxies - but also for diffuse Intergalactic gas within several hundreds of physical kpc from massive haloes at z ~2. In particular, the size and luminosity of the Slug Nebula suggest that a large population of cold, sub-kpc scale clumps may be present within the diffuse Intergalactic medium in proximity of quasars. Proper modeling of this gas phase will require a new generation of numerical models that are able - simultaneously - to spatially resolve these small intergalactic clumps within large simulation boxes, treat the multiphase nature of this gas and its interaction with galaxies and quasars.

References:
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The Formation of Two Supermassive Black Holes from A Single Collapsing Supermassive Star

From left to right: (top row) Christian Reisswig, Christian D. Ott, Ernazar Abdikamalov; (bottom row) Roland Haas, Philipp Mösta, Erik Schnetter

Authors: Christian Reisswig^1,*, Christian D. Ott^1,2,+, Ernazar Abdikamalov¹, Roland Haas¹, Philipp Moesta¹, Erik Schnetter^3,4,5

Affiliation:
¹TAPIR, California Institute of Technology, Pasadena, CA, USA
²Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo, Kashiwa, Japan
³Perimeter Institute for Theoretical Physics, Waterloo, ON, Canada
⁴Department of Physics, University of Guelph, Guelph, ON, Canada
⁵Center for Computation & Technology, Louisiana State University, Baton Rouge, LA, USA

^*NASA Einstein Fellow
⁺Alfred P. Sloan Research Fellow

The existence of supermassive black holes with masses a billion times the mass of our sun at high redshifts z>7 [1] is one of the mysteries in our understanding of the early history of the universe. At redshift z=7, the universe was less then one billion years old. This leads to a serious problem: how is it possible for black holes to acquire this tremendous amount of mass over a short timescale of just one billion years? A common theory of black hole growth assumes as a starting point the collapse of the very first stars, so called Population III stars. Population III stars may have had masses around 100 times the mass of our sun. The collapse of such a star can leave behind a black hole of similar mass that then grows via subsequent accretion of material from its surroundings. This process can yield quite massive black holes, but in order to reach supermassive scales within only one billion years, the accretion process must be rather extremely rapid to enable fast black hole growth. These high required accretion rates, however, seem to be difficult to be maintained due to, e.g. strong outgoing radiation that can blow away the surrounding gas that otherwise would be accreted onto the black hole [2]. The model, therefore, has difficulties of explaining the existence of very massive black holes in the early universe.

Another model which has recently regained attention is supermassive star collapse. Supermassive stars have originally been proposed by Hoyle & Fowler in the 1960s as a model for strong distant radio sources [3]. Such stars have masses up to a million times the mass of our sun and potentially formed in the monolithic collapse of primordial gas clouds that existed in the early universe [4, 5]. Unlike ordinary stars, which are mainly powered by nuclear burning, supermassive stars are mainly stabilized against gravity by their own photon radiation field that originates from the very high interior temperatures generated by gravitational contraction. During their short lives, they slowly cool due to the emitted photon radiation that keeps the stars in hydrostatic equilibrium. The colder stellar gas can be more easily compressed by the inward gravitational pull, and as a consequence, the stars slowly contract and become more compact. This process continues for a few million years until the stars reach sufficient compactness for gravitationally instability to set in. This general relativistic instability inevitably leads to gravitational collapse. One possible outcome of the collapse is a massive black hole containing most of the original mass of the star. Since the 'seed' mass of the nascent black hole is already pretty large, subsequent growth via accretion from the surroundings can easily push the black hole to supermassive scales within the available time without the need of extreme accretion rates and thus without any strong photon radiation that may blow away the surrounding accreting gas.

In our recent article published in Physical Review Letters [6], we study non-axisymmetric effects in the collapse of supermassive stars. The starting point of our models are supermassive stars which are at the onset of gravitational collapse. We use general relativistic hydrodynamic supercomputer simulations with fully dynamical non-linear space-time evolution to investigate the behavior and dynamics of collapsing supermassive stars. Such computer models have been considered in previous studies [7,8,9,10], however, mostly in axisymmetry.

 In an axisymmetric configuration, a supermassive star maintains a spherical shape during its collapse, which is possibly flattened due to rotation. In these previous studies, it has been shown that the possible outcome is either a single massive rotating black hole, or, alternatively, a powerful supernova explosion which completely disrupts the star. In our case, we select an initial stellar model which is rapidly rotating and leads to black hole formation. In fact, it is so rapidly rotating that the shape of our star is no longer spheroidal, but rather resembles the shape of a 'quasi'-torus where the maximum density is off-center and thus forming a central high-density ring (see upper left panel of Figure 1).

Figure 1: (To view higher resolution click on the image) The various stages encountered during the collapse of a supermassive star with an initial m=2 standing density wave perturbation. Each panel shows the density distribution in the equatorial plane.

Such a configuration is unstable to tiny density perturbations that may be present at the onset of collapse [10]. This instability is particularly strong for perturbations in the form of standing poloidal density waves with one (m=1) or two (m=2) maxima. Due to this instability, these perturbations grow exponentially during the collapse, and can lead to significant deformations away from axisymmetry. The nature of the instability typically leads to the formation of orbiting high-density clumps of matter inside the collapsing star (see upper right panel of Figure 1). Since the m=1 and m=2 perturbations grow fastest, either one or two high-density clumps will form, depending on the initial perturbation of the stellar density. These high density fragments continue to grow rapidly during the collapse, thus becoming denser and hotter. 

Once temperatures of more than one billion Kelvins are reached, a process sets, which is called electron-positron pair creation. The creation of particle pairs is possible because there is enough energy available in the surrounding gas to spontaneously create a particle and its anti-particle, in this case electrons and positrons. The pair creation process has the effect of taking out energy from the gas fragments, thus dramatically reducing their local pressure. The reduction in pressure support leads to a rapid increase in the central density within each fragment up to the point at which the fragments become so dense that event horizons appear around each of them (center left panel of Figure 1). In the case of an initial m=2 density perturbation, two black holes form that orbit each other. Since two black holes in close orbit emit very powerful gravitational radiation - ripples of space-time that travel at the speed of light - , the associated loss of energy causes the black hole orbits to shrink, leading to an inspiralling motion (red lines in the center left panel of Figure 1). The leading order mode of the corresponding emitted gravitational wave signal is shown in the lower panel Figure 2.

Figure 2: (To view higher resolution click on the image) The upper panel shows the time evolution of the density maximum until black hole formation. The center panel shows the mass and spin evolution of the black holes. The lower panel shows the emitted leading order gravitational wave signal.

It resembles the typical quasi-sinusoidal oscillatory signal expected from binary black hole mergers: as the orbit shrinks, the emitted radiation becomes higher in frequency. The inspiral continues until a common event horizon appears, marking the merger of the two black holes (lower left panel of Figure 1). The black hole merger remnant is initially deformed into a peanut shape, which quickly relaxes into a spherical shape by emitting exponentially decaying gravitational ring-down radiation. This is shown in the lower panel of Figure 2. The peak amplitude of the waveform corresponds to the black hole merger. From there, the signal quickly decays due to black hole ring-down. By the end of our simulation, the remnant black hole is rapidly rotating and is surrounded by a massive accretion disk (lower right panel of Figure 1).

The formation of two merging black holes requires a particular choice of initial stellar model parameters at the onset of collapse: (i) we require rapid rotation and (ii) a poloidal m=2 standing density wave perturbation must be present. This naturally leads to the question of the likelihood of our model. Recent cosmological simulations of collapsing primordial gas clouds - the potential birth sites for supermassive stars - indicate that rapid rotation is very likely [4]. Curiously, the same simulations also show that an m=2 deformation arises at the center of the clouds where the supermassive star will eventually form. Unfortunately, these simulations currently do not offer sufficient spatial resolution to investigate the formation of supermassive stars in the collapse of primordial gas clouds in detail. Further research will be necessary to self-consistently model the formation of supermassive stars that may inform us about the stellar conditions at the onset of collapse.


The new and exciting prediction that two black holes can form in the collapse of a single star gives rise to very efficient gravitational wave emission compared to models where only one black hole forms. The emitted gravitational radiation in our model configuration is so powerful that future space-borne gravitational wave observatories might see the signal from the edge of our universe. This has important implications for cosmology. If detected, the signal will inform us about the formation processes of supermassive stars and supermassive black holes in the early universe and will allow us to test the validity of the supermassive star collapse pathway to supermassive black hole formation.

Acknowledgements: This research is partially supported by NSF grant nos. PHY-1151197, AST-1212170, PHY-1212460, and OCI-0905046, by the Alfred P. Sloan Foundation, and by the Sherman Fairchild Foundation. CR acknowledges support by NASA through Einstein Postdoctoral Fellowship grant number PF2-130099 awarded by the Chandra X-ray center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. RH acknowledges support by the Natural Sciences and Engineering Council of Canada. The simulations were performed on the Caltech compute cluster Zwicky (NSF MRI award No. PHY-0960291), on supercomputers of the NSF XSEDE network under computer time allocation TG-PHY100033, on machines of the Louisiana Optical Network Initiative under grant loni_numrel08, and at the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the US Department of Energy under contract DE-AC02-05CH11231.

References:
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A New Nuclear ‘Magic’ Number in Exotic Calcium Isotopes

David Steppenbeck

Author: David Steppenbeck

Affiliation: Center for Nuclear Study, University of Tokyo, Japan

Physicists have come one step closer to understanding unstable atomic nuclei. A team of researchers from the University of Tokyo and RIKEN, among other institutions in Japan and Italy, has provided direct evidence for a new nuclear ‘magic’ number in the radioactive calcium isotope ⁵⁴Ca (a bound system of 20 protons and 34 neutrons). In an article published in the journal 'Nature' [1], they show that ⁵⁴Ca is the first known nucleus where N = 34 is a magic number.

The atomic nucleus, a quantum system composed of protons and neutrons, exhibits shell structures analogous to that of electrons orbiting in an atom. In stable, naturally occurring nuclei, large energy gaps exist between ‘shells’ that fill completely when the number of protons or neutrons is equal to 2, 8, 20, 50, 82 or 126 [2]. These are commonly referred to as the nuclear ‘magic’ numbers. Nuclei that contain magic numbers of both protons and neutrons are dubbed ‘doubly magic’ and these systems are more inert than others since their first excited states lie at relatively high energies.

However, recent studies have indicated that the traditional magic numbers (listed above) are not as robust as was once thought and may even change in nuclei that lie far from the stable isotopes on the Segrè chart. It is now known that while some magic numbers can disappear, other new ones can present themselves [3]. A few noteworthy examples of such phenomena are the vanishing of the N = 28 (neutron number 28) traditional magic number in ⁴²Si and the appearance of a new magic number at N = 16 in very exotic oxygen isotopes, one that is not observed in stable isotopes.

The explanation for such behaviour lies in the interplay between nucleons (protons and neutrons) in the nucleus and the ‘shuffling’ of nucleonic orbitals relative to one another, which is often referred to as ‘shell evolution’. In radioactive isotopes with extreme proton-to-neutron ratios, these orbitals may shuffle around so much to the extent that previously large energy gaps between orbitals can become rather small (causing the traditional magic numbers to disappear) while new enlarged energy gaps can sometimes appear (the onset of new magic numbers).

Nuclei around exotic calcium isotopes on the Segrè chart have also received much recent attention and experiments on ⁵²Ca, ⁵⁴Ti and ⁵⁶Cr have provided substantial evidence for a new magic number at N = 32. Another new magic number has been predicted to occur at N = 34 in the very exotic calcium isotope ⁵⁴Ca, but difficulties in producing this isotope in the laboratory have hindered experimental input—that is, until now. Owing to the world’s highest intensity radioactive beams being produced at the Radioactive Isotope Beam Factory [4] in Japan, the team of researchers was able to study the structure of the ⁵⁴Ca nucleus for the first time.

Figure 1: Detectors in the DALI2 γ-ray detector array used in the experimental study of ⁵⁴Ca. [Photo credit: Satoshi Takeuchi]

A primary beam of ⁷⁰Zn³⁰⁺ ions at an energy of 345 MeV/nucleon and an intensity of 6 X 10¹¹ ions per second was fragmented to produce a fast radioactive beam that contained ⁵⁵Sc and ⁵⁶Ti. These radioactive nuclei were directed onto a 1-cm-thick Be target to produce ⁵⁴Ca by removing one proton from ⁵⁵Sc or two protons from ⁵⁶Ti. The ⁵⁴Ca nuclei were produced either in their ground states or in excited states. In the case of the latter, the excited states decayed rapidly by emitting γ-ray photons to shed their excess energy. The energies of the γ rays were measured using an array of 186 sodium iodide detectors (Fig. 1) that surrounded the Be target. In turn, the Doppler-corrected γ-ray energies were used to deduce the energies of the nuclear excited states, which provide information on the nuclear structure.

The results of the study [1] indicate that the first excited state in ⁵⁴Ca lies at a relatively high energy, which not only highlights the doubly magic nature of this nucleus but confirms the presence of a new magic number at N = 34 in very exotic systems for the first time, ending over a decade of debate on the matter since its first prediction [5]. From a more general standpoint, understanding the nucleon-nucleon forces and evolution of nuclear shells in unstable nuclei plays a key role in the understanding of astrophysical processes such as nucleosynthesis in stars.

References:
[1] D. Steppenbeck, S. Takeuchi, N. Aoi, P. Doornenbal, M. Matsushita, H. Wang, H. Baba, N. Fukuda, S. Go, M. Honma, J. Lee, K. Matsui, S. Michimasa, T. Motobayashi, D. Nishimura, T. Otsuka, H. Sakurai, Y. Shiga, P.-A. Söderström, T. Sumikama, H. Suzuki, R. Taniuchi, Y. Utsuno, J. J. Valiente-Dobón, K. Yoneda. "Evidence for a new nuclear ‘magic number’ from the level structure of ⁵⁴Ca". Nature 502, 207–210 (2013). Abstract.
[2] Maria Goeppert Mayer. "On closed shells in nuclei. II". Physical Review, 75, 1969–1970 (1949). Abstract.
[3] David Warner. "Nuclear Physics: Not-so-magic numbers". Nature, 430, 517–519 (2004). Abstract.
[4] http://www.nishina.riken.jp/RIBF/
[5] Takaharu Otsuka, Rintaro Fujimoto, Yutaka Utsuno, B. Alex Brown, Michio Honma, Takahiro Mizusaki. "Magic numbers in exotic nuclei and spin-isospin properties of the NN interaction". Physical Review Letters, 87, 082502 (2001). Abstract.

The Observable Signature of Black Hole Formation

Anthony L. Piro


Author: Anthony L. Piro

Affiliation: Theoretical Astrophysics Including Relativity (TAPIR), California Institute of Technology, Pasadena, USA

Black holes are among the most exciting objects in the Universe. They are regions of spacetime predicted by Einstein's theory of general relativity in which gravity is so strong that it prevents anything, even light, from escaping. Black holes are known to exist and roughly come in two varieties. There are massive black holes at the centers of galaxies, which can have masses anywhere from a million to many billion times the mass of our Sun. And there are also black holes of around ten solar masses in galaxies like our own that have been detected via X-ray emission from accretion [1]. Although this latter class of black holes is generally believed to be formed from the collapse of massive stars, there is a lot of uncertainty that is the focus of current ongoing research. It is unknown what fraction of massive stars produce black holes (rather than neutron stars), what the channels for black holes formation are, and what corresponding observational signatures are expected. Through a combination of theory, state-of-the-art simulations, and new observations, astrophysicists are trying to address these very fundamental questions.

A computer-generated image of the light distortions created by a black hole [Image credit: 
Alain Riazuelo, IAP/UPMC/CNRAS]

The one instance where astronomers are fairly certain they are seeing black hole formation is in the case of gamma-ray bursts (GRBs). A GRB is believed to be the collapse of a massive, quickly rotating star that produces a black hole and relativistic jet. The problem is that these are too rare and are too confined to special environments to explain the majority of black holes. Astronomers regularly see stars exploding as supernovae, but it is not clear what fraction of any of these produce black holes. There is evidence, and it is generally expected, that in most cases these explosions in fact lead to neutron stars instead. This has led to the hypothesis that the signature of black hole formation is in fact the disappearance of a massive star, or "unnova," rather than an actual supernova-like event [2].

My theoretical work [3] hypothesizes that there may be an observational signature of black hole formation, even in circumstances where one might normally expect an unnova. Therefore I titled my work "Taking the 'Un' out of 'Unnovae'." The main idea is based on a somewhat forgotten theoretical study by D. Z. Nadezhin [4]. Before a black hole is formed within a collapsing star, a neutron star is formed first. This neutron star emits neutrinos [5,6], which stream out of the star (because neutrinos are very weakly interacting) carrying energy (and thus mass via E=mc²). This can last for a few tenths of a second before enough material falls onto the neutron star to collapse it to a black hole, and carrying away a mass equivalent to a few tenths of the mass of our Sun. From the point of view of the star's envelope, it sees the mass (and therefore gravitational pull) of the core abruptly decrease and the envelope expands in response. This adjustment of the star's envelope grows into a shock wave that heats and ejects the outer envelope of the star.

This process was also looked at in detail by Elizabeth Lovegrove and Stan Woosley at UC Santa Cruz [7]. They were focused on the heating and subsequent cooling of the envelope from this shock. They found that it would lead to something that looked like a very dim supernova that would last for about a year. In my work, I focused on the observational signature when this shock first hits surface of the star. When this happens, the shock's energy is suddenly released in what is called a "shock breakout flash." Although this merely lasts for a few days, it is 10 to 100 times brighter than the subsequent dim supernova. Therefore, this is the best opportunity for astronomers to catch a black hole being created right in the act.

The most exciting part of this result is that now is the perfect time for astronomers to discover these events. Observational efforts such as the Palomar Transient Factory (also known as PTF) and the Panoramic Survey Telescope and Rapid Response System (also known as Pan-STARRS) are surveying the sky every night and sometimes finding rare and dim explosive, transient events. These surveys are well-suited to find exactly the kind of event I predict for the shock breakout from black hole formation. Given the rate we expect massive stars to be dying, it is not out of the question that one or more of these will be found in the next year or so, allowing us to actually witness the birth of a black hole.

References:
[1] Ronald A. Remillard and Jeffrey E. McClintock, "X-Ray Properties of Black-Hole Binaries". Annual Review of Astronomy & Astrophysics, 44, 49-92 (2006). Abstract.
[2] Christopher S. Kochanek,John F. Beacom, Matthew D. Kistler, José L. Prieto, Krzysztof Z. Stanek, Todd A. Thompson, Hasan Yüksel, "A Survey About Nothing: Monitoring a Million Supergiants for Failed Supernovae". Astrophysical Journal, 684, 1336-1342 (2008). Fulltext.
[3] Anthony L. Piro, "Taking the 'Un' out of 'Unnovae'". Astrophysical Journal Letters, 768, L14 (2013). Abstract.
[4] D. K. Nadyozhin, "Some secondary indications of gravitational collapse". Astrophysics and Space Science, 69, 115-125 (1980). Abstract.
[5] Adam Burrows, "Supernova neutrinos". Astrophysical Journal, 334, 891-908 (1988). Full Text.
[6] J. F. Beacom, R. N. Boyd, and A. Mezzacappa, "Black hole formation in core-collapse supernovae and time-of-flight measurements of the neutrino masses". Physical Review D, 63, 073011 (2001). Abstract.
[7] Elizabeth Lovegrove and Stan E. Woosley, "Very Low Energy Supernovae from Neutrino Mass Loss". Astrophysical Journal, 769, 109 (2013). Abstract.