New observations of black hole defeating a star show rapid disk formation

If a star goes too close to a supermassive black hole, tidal forces tear it apart, producing a bright torch of radiation as the star’s material falls into the black hole. Astronomers study the light of these “tide disturbance events” (TDEs) for clues to the feeding behavior of the supermassive black holes lurking in the centers of galaxies.

New TDE observations led by astronomers at UC Santa Cruz now provide clear evidence that the star’s debris forms a rotating disk, called an accretion disk, around the black hole. Theorists have debated whether an accreditation disc can form efficiently during an event of teasing, and the new findings, accepted for publication in the Astrophysical journal and getting online should help solve this question, said first author Tiara Hung, a postdoctoral researcher at UC Santa Cruz.

“In classical theory, the TDE flare is driven by an accretion disk, which produces X-rays from the inner region where hot gas spirals into the black hole,” Hung said. “But for most TDEs, we do not see X-rays – they shine mostly in the ultraviolet and optical wavelengths – so it was suggested that instead of a disk, we see emissions from the collision of stellar currents.”

Coauthors Enrico Ramirez-Ruiz, professor of astronomy and astrophysics at UCSC, and Jane Dai at the University of Hong Kong developed a theoretical model, published in 2018, that may explain why X-rays are not normally observed in TDEs despite the formation of a accretion disk. The new observations provide strong support for this model.

“This is the first solid confirmation that recovery disks form in these events, even if we do not see X-rays,” Ramirez-Ruiz said. “The region near the black hole is obscured by an optically thick wind, so we do not see the x-ray emissions, but we do see optical light from an extended elliptical disk.”

Narrative proof

The telltale evidence for an accreditation disk comes from spectroscopic observations. Co-author Ryan Foley, assistant professor of astronomy and astrophysics at UCSC, and his team began monitoring the TDE (called AT 2018hyz) after it was first discovered in November 2018 by the All Sky Automated Survey for SuperNovae (ASAS-SN ). Foley noticed an unusual spectrum when observing the TDE with the 3-meter Shane Telescope at UC’s Lick Observatory on the night of January 1, 2019.

“My jaw dropped, and I knew right away that this was going to be interesting,” he said. “What stood out was the hydrogen line – the emission of hydrogen gas – which had a double peak profile that was different from any other TDE we saw.”

Foley explained that the double peak in the spectrum arises from the Doppler effect, which emits the frequency of light emitted by a moving object. In an application disk that rotates around a black hole and is viewed at an angle, some of the material will move toward the observer so that the light it emits is moved to a higher frequency, and some of the material will move away from ‘ the observer, the light shifted to a lower frequency.

“It’s the same effect that the sound of a car on a race track causes from a high pitch when the car comes towards you to a lower pitch as it passes by and starts moving away from you,” Foley said. “When you sit in the bleacher, the cars on one bend all move towards you and the cars on the other bend move away from you. On an application disk, the gas moves in a similar way around the black hole , and that’s what gives the two peaks in the spectrum. “

The team continued to collect data over the coming months, observing the TDE with various telescopes as it evolved over time. Hung conducted a detailed analysis of the data, indicating that disk formation occurred relatively rapidly, within a few weeks of the star’s disturbance. The findings suggest that disk formation may occur more frequently among optically detected TDEs, despite the rarity of dual peak emission, which depends on factors such as the inclination of the disk relative to observers.

“I think we’re lucky with this,” Ramirez-Ruiz said. “Our simulations show that what we are observing is very sensitive to the tendency. There is a preferred orientation to see these dual-peak features, and another orientation to see x-ray emissions.”

He found that Hung’s analysis of observations with multiple wavelengths, including photometric and spectroscopic data, provides unique insights into these unusual events. “If we have spectra, we can learn a lot about the kinematics of gas and gain a much clearer understanding of the extraction process and what drives the emissions,” Ramirez-Ruiz said.

In addition to Hung, Foley, Ramirez-Ruiz, and other members of the UCSC team, the coauthors of the paper also include scientists at the Niels Bohr Institute in Copenhagen (where Ramirez-Ruiz holds a Niels Bohr professorship); University of Hong Kong; University of Melbourne, Australia; Carnegie Institute of Science; and Space Telescope Science Institute.

Observations were made at Lick Observatory, the WM Keck Observatory, the Southern Astrophysical Research (SOAR) telescope, and the Swope Telescope at Las Campanas Observatory in Chile. This work was supported in part by the National Science Foundation, the Gordon and Betty Moore Foundation, the David and Lucile Packard Foundation, and the Heising-Simons Foundation.