Black Holes Caught in the Act

Suvi Gezari, suvi@astro.umd.edu

Supermassive black holes (SMBHs) are a ubiquitous presence in galaxy nuclei. We see dynamical evidence for SMBHs in the centers of nearby galaxies, where telescopes like Hubble can resolve the stars and gas moving at high speeds under the influence of the central black hole’s gravitational forces. We see energetic evidence for SMBHs in distant quasars, which emit copious amounts of radiation (and sometimes jets) as gas accretes onto the central black hole. Remarkably, the masses of SMBHs measured from dynamics in nearby galaxies have a tight correlation with the masses of their host galaxies, indicating a link between their formation and growth. The fact that the majority of SMBHs in the universe lurk hidden, starved of fuel, and at distances that are too far to measure their dynamical influence, hinders probing the co-evolution of galaxies and SMBHs.

There is, however, the chance to catch a dormant black hole “light up” when it feeds on an unlucky star that wanders close enough for tidal gravitational forces to tear it apart. The burst of radiation from the accretion of the stellar debris is expected to be luminous and peak at ultraviolet (UV) and X-ray wavelengths, with a feeding rate that reaches a maximum a month after the disruption and declines with a power-law over several years. The time delay between the time of disruption and the peak of the flare scales with the mass of the black hole, as well as the mass and radius of the star disrupted. Thus, in principle, a flare from the tidal disruption of a star can be used as a means to weigh the mass of a central black hole. A tidal disruption event (TDE) is not only a probe of the demography of dormant SMBHs in distant galaxies, but observations of the events can be used as a “cosmic laboratory” to study accretion physics, the real-time formation of an accretion disk, and, in some cases, the launching of a jet. Unfortunately, such an event is rare in a galaxy, and depends on the density and orbits of stars in a galaxy’s center. Dynamical studies predict that a star will pass close enough to be disrupted by a central black hole only once every 1,000 to 100,000 years. Thus, in order to catch one black hole in the act, one must survey hundreds of thousands of galaxies in the UV and X-ray wavelengths.

The ROSAT satellite’s All-Sky Survey detected the first candidate TDEs at soft X-ray wavelengths (Komossa 2002). However, the best-studied candidates have emerged from searching for events at longer wavelengths, in the UV and optical. The Galaxy Evolution Explorer (GALEX) satellite is an ultraviolet telescope launched as a NASA Small Explorer Mission in 2003. The Mikulski Archive at Space Telescope (MAST) hosts nine years of GALEX observations taken during the baseline mission. In this archive are multiple epochs of far-UV and near-UV observations over hundreds of square degrees. By analyzing archival GALEX Deep Imaging Survey (DIS) observations, Gezari et al. (2006, 2008, 2009) discovered three luminous UV flares from the nuclei of otherwise normal galaxies. Flares from the tidal disruption of a single star by an otherwise dormant SMBH provide the best explanation for the combination of the archival UV light curves and the multiwavelength properties from target-of-opportunity observations.

Motivated by the serendipitous discovery of TDEs in GALEX DIS, the GALEX Science Team initiated a dedicated time-domain survey to monitor 40 square degrees of sky in the UV with a cadence of two days, in coordination with the Pan-STARRS1 Medium Deep Survey (PS1 MDS) in the optical. This resulted in the discovery of the spectacular UV and optical transient PS1-10jh (see Figure 1) reported in Nature by Gezari et al. (2012). The amplitude of variability of the flare—modulation by a factor of greater than 350 in the UV—is extreme compared to those observed from variable active galactic nuclei (AGNs), which typically are factors of a few. Furthermore, follow-up Chandra X-ray observations did not detect the flare, indicating that the event was a factor greater than 20 times fainter in the X-rays than expected for an AGN. The flare’s optical light curve, measured from simultaneous monitoring by PS1 MDS, follows the rise and decline expected for the feeding rate of gas from a tidally disrupted star onto a black hole of several million solar masses. Interestingly, follow-up spectroscopy of the transient with the MMT telescope indicated that the gas the black hole swallowed was predominantly helium, indicating that the star disrupted must have been the helium-rich core of a star previously stripped of its hydrogen envelope. The unprecedented detail of the observations, ranging from the optical to X-rays, enabled one to identify both the victim (the star disrupted) and the perpetrator (the central black hole).

The Large Synoptic Survey Telescope, representing the next generation of optical synoptic surveys, is expected to discover thousands of TDEs because of its increased sky area (20,000 square degrees) and depth. These large samples will be critical for constraining the rate of TDEs as a function of galaxy type, as well as looking for more exotic events (such as TDEs) around intermediate-mass black holes and very massive spinning black holes. The MAST archive will continue to be important for classification of new transients that appear in the sky, since their histories of variability in the UV can be used to identify previous AGN activity that would make a TDE interpretation less likely. Furthermore, the UV time domain provides new insight into a range of exciting phenomena, from flickering distant quasars to flaring M-dwarf stars in our own Galaxy.

References

Gezari, S. et al. 2006, ApJ, 653, L25
Gezari, S. et al. 2008, ApJ, 676, 944
Gezari, S. et al. 2009, ApJ, 698, 1367
Gezari, S. et al. 2012, Nature, 485, 217
Komossa, S. 2002, in Lighthouses of the Universe: The Most Luminous Celestial Objects and Their Use for Cosmology, ed. M. Gilfanov, R. Sunyeav, & E. Churazov (Berlin: Springer), 436