Oscillating X-rays from consumed stars offer new insights into the nature of black holes

The streams of electromagnetic energy released from a star destroyed by the tidal forces of a supermassive black hole nearly 290 million light years away encode valuable information about the physical properties of black holes, a new study finds. The results provide a new way to identify types of black holes that have been challenging to observe using other means. It's thought that a black hole lies at the center of nearly all massive galaxies. Most are inactive and do not produce any observable electromagnetic radiation, making them difficult to detect. Occasionally, however, the immense gravitational pull of these supermassive astronomical objects ensnares passing stars that stray too close - ripping them to pieces and casting the resulting debris into the maw. These stellar episodes, also known as tidal disruption events (TDE), are briefly observable through the electromagnetic energy, like X-rays, that is emitted and provide an opportunity to measure the defining characteristics of a black hole: mass and spin. While a black hole's mass can be inferred through the properties of its host galaxy, assessing its spin remains a challenge. According to the authors, energy emitted from stellar debris following a TDE could be used to determine a massive black hole's spin. Dheeraj Pasham and colleagues report on observations of ASASSN-14li, a TDE detected in 2014 by the All-Sky Automated Survey for Supernovae. Pasham et al. discovered that the X-rays emitted by TDE varied in a quasi-periodic oscillation every 131 seconds. Further analysis of the X-ray oscillation suggests that the stable, yet rapid rate of oscillation could only arise from stellar debris orbiting close to the black hole's event horizon and indicates that the black hole is rapidly spinning. Furthermore, the stable nature of the electromagnetic signal suggests that it is representative of the physical properties (i.e. mass and spin) of the black hole itself and not another transient astronomical event.

Credit: 
American Association for the Advancement of Science