Object 68: NGC 6814
Podcast release date: 07 March 2022
Right ascension: 19:42:40.6
Corresponding Earth location: Ranchland in the state of Rondonia in Brazil
At first, NGC 6814 seems like a relatively ordinary-looking spiral galaxy. From Earth, we can see it face-on, which means that we have a nice view of the spiral structure. However, it's not particularly close; the galaxy is at a distance of 70.1 million light years (21.7 Mpc) , which is further than a lot of other more popular spiral galaxies (like the Andromeda Galaxy and M81 and M83 and M101 and so on and so forth), and it also seems a bit on the faint side, so amateur astronomers are going to have a hard time finding it in the sky and professional astronomers are going to go look at other galaxies if they want to study things like spiral arm structures.
However, professional astronomers do spend a lot of time looking at NGC 6814 because it contains a rather notable active galactic nucleus (AGN). As I've said multiple times in this podcast series, an AGN consists of a supermassive black hole millions or billions of times the mass of the Sun, a disk of thick gas and dust falling into the black hole, and jets of thin gas that emerge above the poles of the black hole. The jets originate from gas in the disk that was gravitationally compressed by the black hole, and the gas got so hot that, instead of falling into the black hole, it expanded and blew away from the black hole at close to the speed of light. While these AGN sound like rather exotic objects, especially since they involve really large black holes and people get really excited by black holes, a lot of galaxies, including ours, contain supermassive black holes or AGN. In fact, I seem to discuss an object with a supermassive black hole in about one of every four episodes of my podcast, so the black hole within the AGN in NGC 6814 has to be particularly special if that is why professional astronomers like to look at it so much.
And the reason why the AGN in NGC 6814 is so special is that it produces strong but highly variable X-ray emission [2, 3, 4, 5, 6]. This variable X-ray emission was first identified in the early 1980s when astronomers were just beginning to perform X-ray observations of astronomical objects . It was rather surprising when astronomers discovered variable X-ray emission from NGC 6814. In fact, it was so surprising that some people thought that astronomers were actually having problems with their X-ray detectors or other problems with their data , although it actually turned out that the X-ray emission from NGC 6814 really was varying on very short timescales.
The X-ray emission from NGC 6814 originates from both the really hot gas in the part of the gas disk that is really close to the surface of the black hole as well as the gas in the jets that is getting blown away from the inner regions of the AGN. The X-ray emission is variable because the gas from the disk does not fall inwards at a steady rate but instead at a variable rate and because gas gets ejected into the jets at a variable rate. In fact, the X-ray emission from the center of NGC 6814 can easily vary by a factor of 10 in brightness and can change on timescales of just a couple of minutes [2, 3, 4, 6].
Measurements of the timescales of the variations in the X-ray emission from NGC 6814 have allowed astronomers to estimate the mass of the black hole at the center of the AGN. One of the most interesting methods that has been used is something called X-ray reverberation mapping.
This mapping technique looks at what happens when the part of the jet of thin gas near the supermassive black hole briefly increases in brightness . When this happens, some of the X-rays are radiated directly at the Earth, while other X-rays are radiated onto the surface of the disk of thick gas falling into the black hole. Some of this X-ray emission will be absorbed by the disk and then re-radiated as new X-rays, while some of the X-ray emission from the jet will be scattered by the disk (a little like how light from the Sun is scattered to make the sky look blue, although the physics is different) . People like to use the term "reflected" in quotation marks to describe all of these processes together even though the X-rays aren't actually being reflected . I'll assume that, when you hear me say "reflected" during the rest of this episode, you can hear me enunciating these quotation marks.
Some of this "reflected" X-ray emission will be emitted towards the Earth from the disk of thick gas, but these X-rays will arrive slightly later than the X-rays coming from the jet itself. Also, the "reflected" light from the near side of the disk will arrive before the "reflected" light from the far side of the disk. Since all of these X-rays come from a region that looks very small from millions of light years away, astronomers are only able to see what looks like a sudden, brief but big increase in X-ray brightness followed by a few smaller, brief increases in X-ray brightness.
Now, because of the supermassive black hole, the disk has a hole in its center. The larger that hole is, the more time will pass between the X-ray emission from the jet and the "reflected" X-ray emission from the near side of the disk. There's also an increase in the amount of time that passes between when light is "reflected" from the near side and the far side of the hole. By looking at the time between the X-ray emission from the jet and the "reflected" X-ray emission from the disk, astronomers can estimate the size of the supermassive black hole in NGC 6814. The current mass measurement is around 2.6 million times the mass of the Sun [9, 10], although astronomers are still working on improving the accuracy of the measurement. Still, the use of X-ray reverberation mapping turned out very effective for estimating the mass of the central black hole in this galaxy.
Aside from NGC 6814, X-ray reverberation techniques have been used to measure black hole masses in a couple of other galaxies, although NGC 6814 has played a key role in the development of this technique. I expect that, in the future, the lessons learned from NGC 6814 will be applied to measuring the masses of the supermassive black holes at the centers of hundreds or thousands of other galaxies.
 Bentz, Misty C. et al., A Cepheid-based Distance to the Seyfert Galaxy NGC 6814, 2019, Astrophysical Journal, 885, 161
 Tennant, A. F. et al., Rapid X-ray variability in the Seyfert galaxy NGC 6814, 1981, Astrophysical Journal, 251, 15
 Mittaz, J. P. D. et al., The flux and spectral variability of NGC 6814 as observed with EXOSAT., 1989, Monthly Notices of the Royal Astronomical Society, 238, 1029
 Kunieda, Hideyo et al., Rapid variability of the iron fluorescence line from the Seyfert 1 galaxy NGC6814, 1990, Nature, 345, 786
 Done, C. et al., The X-Ray Variability of NGC 6814: Power Spectrum, 1992, Astrophysical Journal, 400, 138
 Mukai, K. et al., X-Ray Variability of the Magnetic Cataclysmic Variable V1432 Aquilae and the Seyfert Galaxy NGC 6814, 2003, Astrophysical Journal, 597, 479
 Beall, J. H. et al., On the Short Time Scale Variability of the Seyfert Galaxy NGC 6814, 1986, Astrophysical Journal, 308, 563
 Uttley, P. et al., X-ray reverberation around accreting black holes, 2014, \aapr, 22, 72
 Pancoast, Anna et al., Modelling reverberation mapping data - II. Dynamical modelling of the Lick AGN Monitoring Project 2008 data set, 2014, Monthly Notices of the Royal Astronomical Society, 445, 3073
 Pancoast, Anna et al., Erratum: Modelling reverberation mapping data - II. Dynamical modelling of the Lick AGN Monitoring Project 2008 data set, 2015, Monthly Notices of the Royal Astronomical Society, 448, 3070