The structure immediately around a supermassive black hole at the heart of an active galaxy can tell us about how material flows in and out of these monsters — but this region is hard to observe! A new study provides us with clues of what might be going on in these active and energetic cores of galaxies.
In- and Outflows
In active galactic nuclei (AGN), matter flows both in and out. As material flows toward the black hole via its surrounding accretion disk, much of this gas and dust can then be expelled from the vicinity via highly collimated jets.To better understand this symbiosis between accretion and outflows, we examine what’s known as the “corona” — the hot, X-ray-emitting gas that’s located in the closest regions around the black hole. But because the active centers of galaxies are generally obscured by surrounding gas and dust, it’s difficult for us to learn about the structure of these inner regions near the black hole.
Where are the X-rays of the corona produced: in the inner accretion flow, or at the base of the jet? How far away is this corona from the disk? And how does the corona’s behavior relate to that of the jet?
To address some of these questions, a group of scientists led by Ashley King (Einstein Fellow at Stanford University) has analyzed X-ray observations from NuSTAR and XMM-Newton of over 40 AGN. The team examined the reflections of the X-rays off of the accretion disk and used two measurements to learn about the structure around the black hole:
- the fraction of the corona’s X-rays that are reflected by the disk, and
- the time lag between the original and reflected X-rays, which reveals the distance from the corona to the reflecting part of the disk.
Modeling the Corona
The authors use these observations to build a self-consistent model of an AGN’s corona. In their picture, the corona is located at the base of the jet and moves mildly relativistically away from the disk, propagating into the large-scale jets.
As the velocity of the corona increases, more of its radiation is relativistically beamed away from the accretion disk, which decreases the fraction of X-rays that are reflected — explaining the inverse correlation between jet power and reflection fraction.
At the same time, the increased mass accretion further ionizes the inner disk region, pushing the dominant reflection region to further out in the disk — which explains the correlation between jet power and the distance from corona to reflection region.
King and collaborators show that this model is fully consistent with the X-ray observations of the 40 AGN they examined. Future X-ray observations of the strongest radio jet sources will help us to further pin down what’s happening at the heart of active galaxies.
Ashley L. King et al 2017 ApJ 835 226. doi:10.3847/1538-4357/835/2/226