You MAD, flow?: The hot accretion flow of the M87 supermassive black hole - Astrobites

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Title: The Accretion flow in M87 is really MAD

Authors:  Feng Yuan, Haiyang Wang, Hai Yang 

First Author’s Institution: Shanghai Astronomical Observatory

Status: Accepted for publication in ApJ, available on arxiv.

Many galaxies that we observe have bright regions at their center, with some of this brightness due to the light coming from stars, and some of it coming from the accretion of matter by the supermassive black holes at their center. The active central regions of these galaxies have been named active galactic nuclei (AGN for short —some are so active that they can outburst relativistic jets!), and understanding their physical processes involves understanding the accretion flow of the black holes at their center. 

There are two types of accretion flow for black holes: hot and cold. The hot accretion flow around black holes is what astronomers currently believe powers low-luminosity AGNs. The hot accretion flow has a high temperature and fast radial velocity (gas flows inward more quickly), and can be subdivided into two categories, named SANE (standard and normal evolution) and MAD (magnetically arrested disk). The main reason for this differentiation is the disk’s magnetic field strength in the pole-to-pole (poloidal) direction: in SANE disks (i.e. “standard disks”), the angular momentum is transported outward by magneto-rotational instability (MRI), but in MAD disks the magnetic field is so strong that it can actually stop the flow of matter into the black hole, making the disk lose its axis symmetry in accretion flow.

 The authors of today’s paper try to determine whether the hot accretion flow of M87’s black hole (yes, the one with the famous picture you have likely seen before —thanks to the Event Horizon Telescope) is MAD or SANE. This target was chosen because of its known hot accretion flow, the presence of jets—which are likely linked to the accretion flow—and the large amounts of  observational data on this object. The authors use models and radio polarization observations of the jet in M87 from VLBA (Very Long Baseline Array) datasets ranging 8 years found in this 2019 paper. Although the radiated spectra are difficult to distinguish, SANE and MAD flows have different evolutionary traits that can be observed. One such trait is the rotation measure (RM). As it turns out, it is possible to estimate the strength of the magnetic field of a black hole using the rotation measure of their jets. The Faraday rotation measure is a quantity that can be calculated from parameters such as the electron density of the gas and the gas temperature, as well as the magnetic field strength.

 Since the rotation measure quantity can be determined by observations and is proportional to the field strength, the authors take the following approach: they first use a program to simulate the rotation measures we would expect if M87 was under a SANE or MAD accretion flow according to current models for supermassive black hole accretion flow, and then compare the obtained results to actual observational data for M87’s rotation measure. The results are presented in Figure 1.

Figure 1. The rotation measure absolute values in rad/m² as a function of the de-projected distance from the black hole (top x-axis in milliarcseconds; bottom y-axis in r_g, or the gravitational radius of the black hole, which is dependent on its mass —for M87, that is around 6.5 billion Solar masses!). The orange curve is the MAD rotation measure value range, while the green curves represent the SANE rotation measure values. The difference in curves is the mass accretion rate, represented in Solar masses/year. The black points with error bars correspond to the observational data. Figure 2 in the paper.

From Figure 1, it becomes quite clear that M87 has a MAD accretion flow (the orange curve), rather than a SANE one. The SANE rotation measures are at least 100x larger than the MAD and observed rotation measures. This paper shows us that we can characterize the accretion flow of supermassive black holes at the center of AGNs. There are two important questions to be answered from the concepts of today’s paper: first, that it would be interesting to determine whether most supermassive black holes in AGNs have MAD or SANE accretion flows, and second, these accretion flows could help us better understand the evolution of AGNs!

Edited by: Suchitra Narayanan

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