From AAS NOVA: “A Connection Between Corona and Jet”

AASNOVA

American Astronomical Society

15 March 2017
Susanna Kohler

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Artist’s impression of an AGN according to the unified model. Credit: ESA/NASA, the AVO project and Paolo Padovani

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.

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Top: The fraction of X-rays that is reflected decreases as jet power increases. Bottom: the distance between the corona and the reflecting part of the disk increases as jet power increases. [Adapted from King et al. 2017]

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?

Reflected Observations

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.

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A visualization of the authors’ model for an AGN. The accretion disk is red, corona is green, and jet is blue. The corona shines on the disk, causing the inner regions (colored brighter) to fluoresce, “reflecting” the radiation. As the accretion rate increases from the top to the bottom panel, the jet power increases and the dominant reflective part of the disk moves outward due to the ionization of the inner region (which puffs up into a torus). [Adapted from King et al. 2017]

King and collaborators find two interesting relationships between the corona and the jet: there is an inverse correlation between jet power and reflection fraction, and there is a correlation between jet power and the distance of the corona from the reflecting part of the disk the disk. These observations indicate that there is a relationship between changes in the corona and jet production in AGN.

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.

Citation

Ashley L. King et al 2017 ApJ 835 226. doi:10.3847/1538-4357/835/2/226

See the full article here .

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