Open your eager eyes, because I'm Mr.  Broad Line

Open your eager eyes, because I’m Mr. Broad Line

Title: A radio, optical, UV and X-ray view of the enigmatic changing appearance of Active Galactic Nucleus 1ES ~ 1927 + 654 from its state before to after flare

Author: Sibasish Laha, Eileen Meyer, Agniva Roychowdhury, et al.

First author’s institution: NASA Goddard Space Flight Center

Status: Open access on arXiv, sent to The Astrophysical Journal

If you are ambitious supermassive black hole Want to leave a mark on your host galaxy, magnetic fields would be a good place to start. They contribute a way for you to work with your surroundingswhether it is through launch relativistic jets, plasma colorviolent winds, or even, a rocket exhaust. But there is a catch: you have nothing to make up for, so you can not generate magnetic fields on your own. You are dependent on absorbing the magnetic flux from external plasma. And there comes a point where the flow becomes so strong that your incoming stream of supplements is disrupted, which interferes with your pleasant dinner time. So what does a voracious black hole have to do? Is there any way you can take a break from this magnetic stop of your accretion disk? Maybe today’s newspaper has some information that something like this is happening …

Coming out of my magnetic cage, and I’ve been fine

Figure 1: Schematic of an AGN in scales of Black shield radius of the central black hole. The wide line is believed to be located in the effluent gas launched by the accumulator disk. The corona is located right next to the black hole and possibly at the base of a relativistic jet. The angular size is shown for a black hole mass of 108 solar masses. Source: Figure 1 from Moustakas, L., Dowd, MO, Anguita, T., et al. arXiv e-prints, arXiv: 1904.12967

Active galactic nuclei (AGN) Optical spectra are generally classified as one of two types, Type 1 (with wide emission lines present) and type 2 (with broad emission lines absent). The broad emission lines come from fast-moving gas ionized by the ultraviolet light of the accretion disk, in a place called the broad-line region (see Figure 1). Because supermassive black holes are so big, have theirs accretion disks is believed to change production dramatically only on time scales that are much longer than we can observe (ie 100,000 years). For the most part, once an AGN is placed in one of these two types, they will stay in that type for the foreseeable future.

However, a very small minority of AGNs, called AGNs with altered appearance, are known to switch between different types. Perhaps the most dramatic example is 1ES1937 + 654, originally classified as a type 2 AGN. As of December 2017, it switched to a type 2 classification, brighter in optics and UV with four orders of magnitude and shows broad emission lines. Optical and ultraviolet light comes from the accretion disc in the black hole (see Figure 1), so this indicates a dramatic increase in the brightness of the disc. Even more strange, in June – August 2018, the source practically stopped emitting X-rays. AGN X-rays come from a population of hot (billion degrees!) electrons near the black hole called corona, often assumed to be located at the base of the relativistic beam of the black hole (see Figure 1). This is therefore an AGN where the corona of the black hole disappeared, a rare event. While this source has been subject to very earlier analysis, for the first time, the authors of today’s newspaper analyze not only data taken before and during AGN’s flare, but after flare, when it returned to its original brightness and type-2 spectrum.

Light curves for 1ES1937 + 654 starting at the time of change of appearance. Circles are measured values, inverted triangles mark upper limits. Panels 1 and 2 are X-ray curves. Panel 3 plots the relationship between high energy (hard) and low energy (soft) X-rays. Panel 4 is a curve for ultraviolet light. Panel 5 plots the relationship between UV and X-ray. Panel 6 is a radio light curve. The horizontal dashed line marks the value of each parameter before the Changing-look event. The vertical line marks when the X-rays returned. Picture 1 in the newspaper

It started with a Pole Flip How did it end like this?

The observations of 1ES1937 + 654 analyzed by today’s newspaper are shown as light curves in Figure 2. As can be seen in the figure, AGN flared violently into UV in December 2017 and then attenuated over time at a speed of approximately t-0.91. X-rays fall to a minimum in June-August 2018 independent of UV radiation before returning in October 2018. They reached their peak in November 2019 before falling to their pre-flare values. The fact that X-rays and UV change independently is very surprising, since the X-rays and UV brightness of AGN are normally closely correlated with each other. However, the radio flux reached a minimum at the same time as the X-rays. The radio is believed to come from the relativistic jets of an AGN, so it seems that the jets and coronae at their base have disappeared. What can cause the corona and rays to disappear while causing the accretion disc to lighten?

Previous articles have tried to answer this question with the help of one tidal disturbance (TDE). But that may not be entirely true due to the UV decay rate; for TDE it is generally t-5/3not t-0.91. The fact that the source returns to normal after the flare also indicates that the process that happened cannot be due to an increase in the amount of external material delivered to the accretion disk itself; AGN would remain in an elevated state for more than just a few years after that. The authors postulate that what is actually going on is a magnetic pole reversal, perhaps similar to what is happening in Earth or the Solar.

Figure 3 The artist’s impression of the magnetic flux inversion. The bright white and dark blue regions seen above and below the black hole represent corona and relativistic rays, while the brown to violet spiral around the black hole represents the accumulation disk. Picture 9 in the newspaper

The scheme of the process is shown in Figure 3. Initially, the accretion disk near the black hole is in a magnetically stopped state: the magnetic flux that enters the black hole is saturated at its maximum level, and plasma must struggle through the field to accumulate on the black hole. In this condition, the rays and the corona are strong but the accumulation rate is low (2011 in Figure 3). At some point, the magnetic polarity of the outer accretion disk reverses. The reverse field is inserted deeper into the disk as plasma flows toward the black hole at an accelerated speed. The accretion rate increases, which creates UV / optical flare (Dec 2017 in Figure 3). The inverted field is then inserted into the black hole by the inflowing plasma and removes the existing magnetic field on the hole. Without a magnetic field, the rays and coronae are turned off (August 2018 in Figure 3), which creates a decrease in X-rays that occurs separately from the UV radiation. Eventually, sufficiently opposite directional magnetic flux is absorbed by the black hole that it again has magnetic fields, but with opposite polarity. The corona and rays return, as do their accompanying X-rays (November 2019 in Figure 3). For a while, the X-rays are even brighter than they were from the beginning, as the rate of accumulation is still increased. Eventually, the rate of accretion drops to its previous low levels, and AGN returns to its initial state (2021 in Figure 3).

If today’s newspaper analysis of 1ES1937 + 654 is something to go by, there may be hope for supermassive black holes that are desperate for a magnetism-free meal. But it’s just a temporary respite, and you will not be able to hit anyone with your galaxy-destroying rays when you do. Yes Yes, this is life.

Astrobite edited by Kayla Kornoelje

Selected Image Credit: Figure 1 from Scepi et al. 2021, Monthly Announcements from the Royal Astronomical Society: Letter502, L25

About Lynnie Saade

I am a doctoral student at UCLA who works with X-ray observations of supermassive black holes and their active galactic nuclei. I have an unusual hobby of drawing comics and writing stories about personalized natural phenomena. I really want to see a story with a black hole used as a real character, just like how they (almost) are characters with a big impact on their galaxies in reality.

#Open #eager #eyes #Broad #Line

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