by Krista Lynne Smith
In the centers of almost all galaxies lurk gigantic black holes, millions to billions of times the mass of the Sun. When there's not much mass in the vicinity (which is the case for most of them), they just mind their own business, but in about 10% of cases they are actively consuming huge clouds of matter and transforming much of it into vast amounts of energy. We call these gluttonous black holes “quasars”—supermassive black holes surrounded by bright, hot disks of gas called accretion disks (and previously, other ways to learn about black holes actively gobbling up matter were covered in this KIPAC Research Highlight by Dan Wilkins).
Although quasars are among the most luminous objects in the Universe, we still do not understand the detailed physics of how the matter being gobbled up behaves. This is now changing thanks to new insights and advances from an unexpected source: the highly sensitive timing satellites used to search for planets around other stars by continuously monitoring their brightness over time and searching for periodic dips caused by transiting planets.
The major problem facing the study of accretion disks is that they cannot be directly imaged: although they are large by human standards (about the size of our solar system) they are very distant. Supermassive black holes live in the centers of other galaxies, and even our best instruments are not capable of resolving them as anything larger than points of light. This means that direct observational probes of accretion disks are rare and precious. Fortunately, there is one ubiquitous property that we can exploit to facilitate our studies: optical variability.
It has been known since the discovery of quasars in the 1950's and 60's that they tend to exhibit strong variations in brightness over time, both over many years and also in the course of just a few hours (see image above for an example). Because these variations can be fast, we know that the region they come from must be quite small. Through spectral modelling, we know that the ultraviolet and optical continuum emissions in quasars comes from the accretion disks around their central supermassive black holes, and that this is likely the source of the rapid optical variability. So, studying the brightness of quasars over time is a direct observation of behavior within the disk.
Until recently, optical timing studies of quasars have been done with long campaigns on ground-based telescopes, where the rotation of the earth, annual cycle, weather, and scheduling constraints have made the even, regular sampling required for accretion physics studies very challenging. However, with the launch of the Kepler exoplanet-hunting satellite, everything has changed.
Kepler (and its subsequent version, K2) is a timing satellite that searches for exoplanets by staring, unblinking, at a large patch of the sky containing many stars for months to years at a time, waiting for the periodic dip in a star’s brightness that occurs when a planet moves in front of it. In order to see this tiny signal, it requires even sampling at very high photometric precision, making it the best optical timing instrument ever built. All of the things that make this satellite ideal for exoplanet hunting also make it ideal for accretion disk physics!
The location—in our own galaxy—where Kepler was originally pointed was chosen to maximize the number of planets it could find. As a consequence, the telescope's field of view did not overlap with many surveys of quasars and other galaxies. In order to find quasars in the Kepler field, my colleagues and I conducted an X-ray survey of that region using the Swift satellite, since quasars are well known to be bright X-ray emitters. We also did this for several of the K2 fields of view. In order to confirm whether or not the X-ray sources are actually quasars, my colleagues and I traveled to many observatories to collect spectra, from which we could also measure the mass of the black holes and their accretion rates. Once we had this information, we could ask Kepler/K2 to monitor our galaxies.
Since the Kepler data were never intended to be used for the type of analysis that we needed, a very large amount of effort was required to reduce and repair the data. Much of my work involved the creation of a software pipeline to facilitate the use of Kepler products for accretion disk physics.
Although this was quite challenging, the results have been worthwhile: the best-sampled, highest-precision optical light curves of quasars ever collected. Such fantastic data have enabled the discovery of characteristic variability timescales in some objects, usually around several days to weeks, that may tell us about important physical processes occurring near the black holes. Comparing these timing results with X-ray variability studies also tells us about the geometry of the gas near the black hole, and how the very hot, energetic X-ray emitting regions are related to the larger optical light-emitting disk. We may also have discovered a quasi-periodic oscillation in one galaxy, which means the variations seem to repeat on a period of about 44 days. Such oscillations haven't been explained—yet—but they've also been seen in the much smaller stellar-mass black holes that form when a massive star dies (they have also been the long-time subject of study by co-founding KIPAC faculty member, Bob Wagoner). Studying the similarities between light curves of stellar-mass black holes and supermassive black holes can tell us whether accretion occurs self-similarly across a very large range of mass scales (a few solar masses up to billions of solar masses). At this point in our research on accretion disks, there is still a lot we don't know. Determining whether they form in similar fashions in such wildly differing environments could be an important clue.
Although the K2 mission is nearing its end, the situation is about to get even better. In April, NASA’s new exoplanet hunting satellite, the Transiting Exoplanet Survey Satellite (TESS), was successfully launched and is now beginning to collect data.
Unlike the previous missions, TESS will monitor sources over almost the entire sky. The TESS data will begin to be released in December, so TESS results from us will have to wait until next year. Although using the TESS quasar data will very likely provide many challenges, we are prepared to leverage them to gather more insight into the accretion processes that power these luminous, energetic sources.
Light Variations in the Superluminous Radio Galaxy C3273 (subscription required; published in Nature, May 18, 1963)
The Kepler Light Curves of AGN (arXiv link; published in The Astrophysical Journal, April 25, 2018)
Evidence for an Optical Low-frequency Quasi-periodic Oscillation in the Light Curve of a Kepler Galaxy (arXiv link; published in The Astrophysical Journal, June 11, 2018)