Professor Wang Tinggui and his research team from the Department of Astronomy at the University of Science and Technology of China (USTC) have recently conducted multi-band high-frequency monitoring of the tidal disruption event AT2023lli by a black hole. They have discovered the most significant "bump" feature ever observed in the optical-ultraviolet light curve before the peak, along with intermittent radiation characteristics in X-rays. These newly identified evolutionary features provide crucial clues for understanding the physical processes underlying such events. On March 26th, the findings were published online in the "Astrophysical Journal Letters".
A Schematic diagram showing the "intermittent" radiation of X-rays from AT 2023lli. Image provided by the University of Science and Technology of China.
When a star ventures too close to a supermassive black hole at the center of a galaxy, it gets torn apart by the immense tidal forces of the black hole and accreted, resulting in multi-wavelength electromagnetic flares lasting from months to years. This phenomenon is known as a Tidal Disruption Event (TDE). Following the discovery of a TDE, timely observations with ground-based and space telescopes are crucial to obtain photometric and spectroscopic evolution information, thereby inferring the primary physical processes and parameters of the black hole.
The observation of AT 2023lli this time involved domestic instruments such as the Mozi Survey Telescope, the 2.4-meter Telescope at the Lijiang Observatory of Yunnan Astronomical Observatory, as well as international facilities like the Global Telescope Network, NASA's Swift satellite, the European Space Agency's XMM-Newton satellite, and the Hale Telescope at Palomar Observatory.
Previous TDEs have exhibited relatively smooth light curves with a "fast rise, slow decline" trend. However, AT 2023lli is clearly an "anomaly." Optical monitoring results indicate a prominent "bump" during the early rise of its light curve, lasting for nearly a month and separated from the main peak by a time interval of up to two months, making it the most pronounced and longest-lasting "bump" observed in a TDE to date.
The research team analyzed the multi-wavelength light curves and suggested that this peculiar optical "bump" is likely caused by collisions of stellar debris torn apart due to the effects of general relativity. Additionally, the disruption of a binary star system by the black hole is also a possibility, with each disruption event resulting in the "bump" and the main peak, respectively.
In the context of the first image, the research team explained the phenomenon of X-ray emission delayed relative to ultraviolet-optical radiation and exhibiting "intermittent" radiation. They proposed that besides generating the "bump," the collision of debris streams also produces outflow ejecta, which obscures the inner regions of the accretion disk and absorbs soft X-ray photons, converting them into ultraviolet-optical radiation through reprocessing. Due to the uneven distribution of obscuring material, later stages become optically thin locally, allowing X-rays to "leak" out. Furthermore, the movement of material around the black hole leads to the "intermittent" characteristic.
For further details, refer to the related paper: https://doi.org/10.3847/2041-8213/ad319f.
