Getting too close to a supermassive black hole can be fatal for stars. They experience a tidal disruption event (TDE) and are ripped apart before spiraling into the black hole. How efficiently this material organizes around the black hole has been a matter of contention in the last few years. Now a new study, to be published in The Astrophysical Journal, helps resolve the issue.
If material from a TDE forms an accretion disk around a black hole, powerful X-ray emissions are expected to accompany it. However, researchers had not observed these emissions from TDEs in the last few years, leading them to assume there was no disk formation after all. Now researchers have found indirect evidence of the disk that suggests something is preventing the X-rays from reaching us.
“In classical theory, the TDE flare is powered by an accretion disk, producing X-rays from the inner region where hot gas spirals into the black hole,” lead author Dr Tiara Hung, from UC Santa Cruz, said in a statement. “But for most TDEs, we don’t see X-rays – they mostly shine in the ultraviolet and optical wavelengths – so it was suggested that, instead of a disk, we’re seeing emissions from the collision of stellar debris streams.”
The theoretical framework for this was developed in the last few years. Simulations suggest that depending on how the disk is inclined with respect to our line of view, this will allow for certain types of emissions to come through. In certain orientations, X-rays will be detectable. In others, observations will see a double-peaked emission. This discovery provides support for the double-peaked theory.
The crucial new observations come from the TDE known as AT 2018hyz. The event was first detected in November 2018, but on January 1, 2019, the team obtained the light spectrum for the event using the Shane Telescope at the Lick Observatory. This allowed the researchers to establish the element present in the event, but it also did something more in this case.
“My jaw dropped, and I immediately knew this was going to be interesting,” co-author professor Ryan Foley added. “What stood out was the hydrogen line – the emission from hydrogen gas – which had a double-peaked profile that was unlike any other TDE we’d seen.”
“I think we got lucky with this one,” co-author professor Enrico Ramirez-Ruiz, who developed the theoretical framework, added. “Our simulations show that what we observe is very sensitive to the inclination. There is a preferred orientation to see these double-peak features, and a different orientation to see X-ray emissions.”
The double-peaked profile is a hallmark of rotating hydrogen gas. The peculiar emissions are due to the doppler effect. Just like how the pitch of an ambulance’s siren changes when it’s coming towards you and moving past you, wavelengths of light are shifted one way or another if the emitting body is coming towards you or moving away from you. In a rotating disk, some gas will move away and others towards Earth. The emissions of hydrogen are then turned into a double-peaked feature.
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