A Rare Middle-Weight Black Hole in a Nearby Galaxy
Author: Dheeraj R. Pasham1,2
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 . 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 , 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 . 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 .
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)  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 , 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.
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.
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