A Stellar ‘Light Switch’ Orbiting a Black Hole

Title: The rebrightening of AT2018fyk as a repeating partial tidal disruption event

Authors: T. Wevers, E.R. Coughlin, D.R. Pasham, M. Guolo, Y. Sun, S. Wen, P.G. Jonker, A. Zabludoff, A. Malyali, R. Arcodia, Z. Liu, A. Merloni, A. Rau, I. Grotova, P. Short, Z. Cao

First Author’s Institution: European Southern Observatory, Santiago, Chile

Status: Submitted to ApJ Letters [open access]

Out in the center of a distant galaxy, a star is being torn apart as it circles the drain around an enormous black hole! Today’s paper reports on the re-emergence of X-ray and UV emission from a star orbiting a supermassive black hole (SMBH). After being discovered, this emission suddenly flicked off and stayed undetectable for ~600 days, before it quickly returned like a light switch being turned back on after a blackout– making this a very dynamic system to study.

In 2018, optical emission from the star was discovered by the All-Sky Automated Survey for Supernovae (ASASSN), a supernova search using 24 telescopes around the world which can see objects 50,000 times dimmer than we can see with our naked eyes! The event was called AT2018fyk, and further analysis found that the emission was coming from the nucleus of a galaxy named LCRS B224721.6−450748. These super catchy and memorable names are thanks to astronomers using astrometric coordinates and dates of discovery to name new objects, since there are too many in the sky to give each a unique name! But 600 days after the initial discovery, there was a sharp decrease in the brightness of the X-ray and UV emission, with the X-ray emission plummeting to less than 1/6,000th of its original brightness. For 600 days, this dimming persisted, suggesting that the star had been torn apart by the gravitational pull of the black hole, and all of the stellar material had fallen onto the surface of the black hole, leaving nothing behind. This is known as a tidal disruption event (TDE), since the tidal forces (yes, the same ones that cause the ocean tides on Earth!) rip the star apart.

However, today’s authors report that 600 days after the dimming began, the ‘light switch’ was flipped, and the X-ray and UV emission from AT2018fyk have returned to close to pre-dimming levels.  In most tidal disruptions, the star is totally torn apart and the emission slowly fades, never to return– so their hypothesis is that this event was only a partial TDE, where the core of the star remained intact while only the outer layers were stripped away.

Figure 1: Cartoons illustrating the evolution of the star/SMBH system over time. The binary system is torn apart in panels a) and b), the stellar material begins to fall onto the black hole in panel c), the star moves away from the black hole in panel e), and the tidal disruption begins once again in panel f). Figure 3 from today’s paper.

Figure 1 shows a schematic which illustrates the key phases of AT2018fyk’s history. The origins of this system are unique- given the previously estimated SMBH mass, a star should theoretically take at least a few thousand years to make one full orbit around the central black hole– way longer than the timescales of a few years that we’re seeing! But, if the star was originally part of a binary system, the black hole can disrupt the binary, pulling one star into an orbit around the black hole while the other star is shot at extremely high speeds away from the galaxy. Panels a) and b) of Figure 1 show this process, with the yellow dot representing the star’s ex-binary companion (now called a hypervelocity star) which is flung off into space. 

Panel c), at t=0, matches up with the initial discovery of the system, with material falling onto the surface of the supermassive black hole and getting heated up, which creates X-rays. This process is called accretion, or stellar fallback. Panel e), at  t=600 days after discovery, shows that at this point the core of the star has moved farther away from the SMBH, and the stellar material remains gravitationally attracted to the stellar core, so it has stopped falling onto the SMBH– this is the point at which the X-ray and UV emission got much dimmer. At t=1200 days (the focus of this paper), what remains of the star has moved back into the region where the outer material of the star will be pulled onto the SMBH, and the emission ‘turns on’ once again.

Figure 2- the light curve of the stellar/SMBH system over time, since its discovery. Both the UV (green diamond; from Swift) and X-ray (black, from Swift/XMM-Newton/Chandra/eROSITA) light curves are shown. The x-axis is measured in days, with t=0 equal to the discovery of the system. Top left panel of Figure 1 from today’s paper.

Figure 2 shows the light curve, or the luminosity of the emission over time, in the UV (green) and X-ray (black) wavelength ranges over the course of the observational history of AT2018fyk. Letters A-D represent the first 600 days of bright emission: at first, the UV emission is brighter (higher up on the y-axis) than the X-ray emission, but they switch around letter C. Why do we observe this behavior? At early times, the gas surrounding SMBH will be optically thick, but when the star moves away and the rate of fallback declines, the gas is able to expand and cool, becoming more optically thin (puffier) so it’s easier to see through to the hot inner region of the system, leading to brighter X-ray emission. At letter E, the dimming period begins as the star moves away from the SMBH, and the emission brightness drops sharply into its “quiescent” state. Finally, at letter F, the bright emission returns at similar luminosity levels to before, implying that the same star has orbited back around to a point where material is falling onto the SMBH.

The authors predict that there will be another sharp brightness decline in August 2023 and, if the star survives this second encounter, a third episode of re-brightening should begin around March 2025. This gives astronomers an exciting prediction to look forward to confirming or denying, as we continue to learn about exotic systems like this!

Astrobite edited by Sabina Sagynbayeva

Featured image credit: ESO/L. Calçada/M.Kornmesser

About Evan Lewis

Evan is a graduate student in astronomy at West Virginia University. His research focuses on transient radio sources, including pulsars, magnetars, and fast radio bursts. Outside of research, he enjoys playing percussion, hugging dogs, baking, and playing video games!

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