When a star gets too close to a supermassive black hole (SMBH), the star’s fate is sealed.
The SMBH’s gravity is overwhelming, and as the star is drawn toward the hole, it is stretched out and eventually consumed.
These are called tidal disruption events (TDE), and while they’re rare, their brilliant light catches astronomers’ attention.
A team of researchers have detected the second flare from a TDE, separated by about two years. They say that this is the first confirmed case of a star losing mass to an SMBH, then interacting again years later, losing more mass, and flaring again.
The TDE is named 2022dbl.
The detection is presented in research titled “The Double Tidal Disruption Event AT 2022dbl Implies that at Least Some “Standard” Optical Tidal Disruption Events Are Partial Disruptions.” It’s published in The Astrophysical Journal Letters, and the lead author is Dr. Lydia Makrygianni, a researcher at Lancaster University in the UK.
TDEs are rare events that give astrophysicists an opportunity to learn more about SMBHs, sometimes revealing the presence of otherwise undetectable black holes.
These behemoths occupy the center of large galaxies like ours, and their immense gravitational power dominates the surrounding region. The SMBH in our galaxy was discovered through the behaviour of stars in the region, whose orbits indicated something extremely massive in the center of the Milky Way.
Now scientists know that sometimes stars in a galaxy’s central region get too close to an SMBH and are destroyed in TDEs.
During a TDE, the star is stretched out in a process called “spaghettification” and a stream of stellar material is created that can loop around the black hole. Some of the material is captured by the SMBH’s accretion disk, where it flares brightly for several months.
TDEs can outshine their galaxies and emit light across the electromagnetic spectrum from radio waves to x-rays.
However, some of these TDEs don’t behave as expected.
There are two types of TDEs, defined by their emissions: X-ray TDEs and optical TDEs. X-ray TDEs were the first ones discovered, and they emit powerful X-rays and behave as expected. They exhibit a predictable power-law in their light curves, and their temperatures and luminosities are in line with direct accretion onto an SMBH.
Optical TDEs have been detected in automated surveys that look for transients. They emit little or no x-rays, but are strong in optical and UV light. These objects are puzzling, since TDEs are expected to be hot enough to emit X-rays.
The researchers think that AT 2022dbl can explain why the optical TDEs are puzzling. If SMBHs are taking small bites of stars instead of consuming large amounts of stellar material at once, that could explain why some TDEs are less brilliant.
The authors explain that AT 2022dbl’s characteristics mean it’s an optical-ultraviolet TDE. The only thing different about it is the fact that it flared twice, separated by about 700 days. “We show that the second flare exhibits a very similar light curve (albeit fainter and with a slower post-peak decline in the ultraviolet bands) and nearly identical spectra as the first flare,” the authors explain.
There are other possible explanations for the double flaring, but the authors ruled them out. Gravitational lensing of the same flare was ruled out because there were slight differences in the emitted light.
They also ruled out the idea that these could be two separate TDEs from two different stars. “We presented both analytical and numerical models that are consistent with the scenario of both flares being the disruption of the same star by the same SMBH, with at least the first flare being due to a partial disruption of the star,” the researchers explain.
“The question now is whether we’ll see a third flare after two more years, in early 2026,” says co-author Professor Arcavi in a press release. Arcavi is from the School of Physics and Astronomy at Tel Aviv University in Israel. “If we see a third flare,” continues Arcavi, “it means that the second one was also the partial disruption of the star. So maybe all such flares, which we have been trying to understand for a decade now as full stellar disruptions, are not what we thought.”
Theory shows that double TDEs are likely due to stars on highly eccentric orbits that are bound to SMBHs. In this scenario, the star loses some of its mass during each close pass to the SMBH, but the bulk of the star continues on its orbit. These are sometimes called partial tidal disruptions. “Stars grazing supermassive black holes on bound orbits may produce periodic flares over many passages, known as repeating partial tidal disruption events (TDEs),” the researchers explain in their paper.
If there is no third flare, that means the first and second flares were both partial disruptions. Since the light curves from the first and second flares are very similar, and the first one appeared to be a full disruption, that indicates that partial disruptions can appear to be full disruptions, as predicted by other researchers.
“Either way,” added Professor Arcavi, “we’ll have to re-write our interpretation of these flares and what they can teach us about the monsters lying in the centers of galaxies.”
There is, however, another potential explanation for the two types of TDEs. It could come down to a visual effect, and could depend on the viewing angle of the observer with respect to the orientation of the accretion disk around the SMBH. A 2018 paper explained that in optical TDEs, they could actually emit x-rays, but they’re absorbed by the surrounding debris disk and are emitted at lower energy levels.
Optical TDEs are primarily found in edge-on systems. In these systems, most of the X-ray emission are reprocessed by the outer disk . On the other hand, X-ray TDEs are primarily found in face-on systems, where we are viewing the TDE without intervening dust and debris.
However, if the authors are correct, it means that all optical TDEs are actually partial TDEs. We may not have spotted the other partial TDE events from these TDEs due to long timescales. Or they could be caused by stars that got close enough to an SMBH to have some of their matter stripped away, but due to dynamic interactions, were not trapped in the SMBH’s orbit.
“If this is the case, it requires a reassessment of the emission mechanisms, rates, and processes driving the host-galaxy preference of optical-ultraviolet TDEs,” the authors conclude.
Written by Evan Gough/Universe Today.
