Title: Light bending and X-ray echoes from behind a supermassive black hole
Authors: D. R. Wilkins, L. C. Gallo, E. Costantini, W. N. Brandt et al.
First Author’s Institution: Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, United States
Status: Published in Nature [closed access]
You might be familiar with the term echolocation, a special method that some animals use to “see” where they are going using sound instead of their eyes. For example, bats use echolocation to map out their flight path, sending out a series of audible clicks and then waiting to hear back the echos reverberating off of nearby objects. Similar to the little brown bat Myotis lucifugus, scanning the walls of a cave using the echoes of its audible clicks, the author of today’s paper uses echoes from the superheated, highly magnetic corona of a black hole to map out its local environment, and confirm a key prediction of general relativity along the way.
What causes X-ray flares?
To first understand these X-ray echos, it helps to build up a picture of what actually causes X-ray flaring in black holes. While black holes themselves do not emit any light, some of them can form coronae. These coronae are dense regions around the black hole where surrounding gas and materials accumulate as they accrete towards the black hole. This gas superheats to extremely high temperatures and ionizes, forming a plasma. This magnetized plasma is spun around and around by the angular momentum of the spinning black hole, and can whip up high above the black hole. In this chaotic environment, magnetic field lines break apart and reconnect violently, creating high energy electrons. These high energy electrons “Comptonise” the material in the accretion disk through inverse Compton scattering, effectively boosting the X-ray emission energy. The name corona itself is borrowed from solar science, due to similarities between the dynamics in this region and the physics of solar flares in the sun’s photosphere.
Some of these newly formed X-rays will be shot back towards the accretion disk, bouncing and reflecting off the material in the disk. The reflection of these X-rays is what causes the ‘echoes’! Because the accretion disk around a black hole can move at relativistic speeds, X-rays can be relativistically boosted or dimmed, depending on the motion of travel. The most common emission line for these luminous black holes is from the fluorescence of the K shell of iron at 6.4 keV, and it is in these emission line spectra that the characteristic relativistic boosting or dimming appears. As the high energy X-ray electrons smash into the iron atoms, they can dislodge one of the two K-shell electrons. This leads to an electron from an outer shell to drop down and replace the missing electron, resulting in either the production of an X-ray emission line photon, or the removal of another electron, called an Auger electron. Because iron is one of the most common cosmic elements with a low Auger yield, this emission line is very prominent when examining spectra from luminous black holes.
A flare for the dramatic
Today’s paper specifically investigates a supermassive black hole in I Zwicky 1, a spiral type galaxy 800 million light years away. X-ray data was taken in 2020 using two space-based X-ray observatories, XMM-Newton and NuSTAR. XMM-Newton observed from 0.3-10 keV for two periods of 76 kiloseconds (ks) and 69 ks, while NuSTAR observed in the hard X-ray band, from 3-50 keV for 5.3 continuously. From the light curves, two flares, each lasting approximately 10 ks, were observed. The X-ray photon count rate peaked at 2.5 times the mean level observed before the flaring event.
It has been hypothesized for decades that one can use the time delay between the flare and its reflection to create a “transfer function” that connects the geometry of the black hole (meaning values like black hole mass, spin, and corona height) to the light curve observed. By modeling the spectra emission for the various paths of travel for the X-rays, one can start to build an idea of the local environment.
There are two distinct features of the X-ray reflection embedded in the iron K line emission. Firstly, due to the extreme gravity of the area surrounding the black hole, there is relativistic broadening. Secondly, due to the Compton scattering mentioned earlier, there is a characteristic hump in the spectra that deviates from the power-law continuum expected from the corona. The authors find both features in their X-ray spectra, confirming that they have observed an X-ray reflection event.
These emission line photons experience variable shifting, depending on the position of their origin. Since we know what the rest frame spectra is for iron K, we can start to map out where various photons came from based on the shift in spectra that is measured. There is a gravitational redshift, which gets stronger near the black hole, and there is a varying line of sight velocity across the disk. The authors find that after the decline of each flare, there are a series of short peaks in the flux. These begin in the blueshifted iron K photons, then it is seen in the redshifted ones. For both flares, three offset peaks are detected, with 99.99% confidence. What would cause such an offset? The very same reflections and reverberations we’ve been talking about this whole time!
One shift, two shift, redshift, blueshift
Now for the most novel part of this paper. In the process of modeling these reverberations, the authors find that they can only explain the shift in iron K line if they include a portion of photons that reverberate on the far side of the accretion disk, which is normally hidden by the black hole. Due to gravitational lensing, these photons are bent into the line of sight of our telescopes and magnified. This is the first time reverberated photons like this have been observed, and it confirms once again that general relativity works.
To confirm their findings, the authors also investigate how the X-ray spectra changes on short time scales. First they subtract away the power-law continuum representing the initial coronal emission. Then they sum the two flares and also suppress Poissonian noise using a 2D Gaussian filter. Between 5-8 ks, the X-ray flare reverberation can be seen. They also plot the difference between a model spectra and the actual data from the iron K line spectrum at various points in time, spaced by 150 second bins. The response of the spectra shows the movement of the reverberation through the disk. The spectra starts redshifted, due to material in the inner disk, which is close to the corona. Then the shift transfers to the rest frame energy of 6.4 keV, due to the response of the outer disk. Finally, more and more redshifting occurs due to the reflection from the back side of the disk, behind the black hole. The line also brightens due to lensing. The authors conclude that other models cannot explain this specific variability.
Using their model, the authors are able to make predictions about properties of the corona and black hole. The corona is still not well understood, especially its geometry, so being able to more accurately constrain its features is key to unraveling its secrets. They find a height of ~3.7 gravitational radii, where the gravitational radius is the radius of the event horizon in the equatorial plane of a spinning black hole. They also find a black hole mass of 3.1 x 10^7 solar masses.
In their model, they also find that the disk only responds to the first part of the flare, otherwise the narrow peaks they observe would be smeared out. The authors interpret this as the corona accelerating away from the disk during the flare, thus reducing the emission reaching the disk. An artist’s rendition of such an event can be seen in Figure 1.
By building on the science of luminous black holes and coronas, the authors of this paper have managed to reveal much about the behavior of X-ray flaring. Using iron K emission line spectra to estimate black hole mass is a pre-existing technique, but for the first time, reverberated X-ray photons from behind a black hole are observed. In terms of future work, new and improved telescopes will give us better insight into these flaring events. The lead author, Dan Wilkins, is working on the Wide Field Imager for a new X-ray observatory being developed by the European Space Agency. Athena, the Advanced Telescope for High-ENergy Astrophysics, will offer a larger mirror and better temporal resolution than telescopes like XMM-Newton and NuSTAR. Athena is planned for launch in 2037.
Astrobite edited by Lucas Brown
Featured image credit: Bat Conservation Ireland, ESA Science XMM-Newton (2009),
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