A sudden flare and cooling of gas around a black hole in a binary system has offered astronomers a rare opportunity to measure the system's magnetic field, finding it weaker than expected. Observations of black holes consuming material ripped from a nearby star reveal that these binary systems amass material into an accretion disk. Above this disk lies an accretion disk coronae (ADC), the behavior of which is governed by its magnetic field. Understanding the magnetic field within the ADC is important because it affects the flow of gas around a black hole, yet there have been very few measurements of such fields to date. On 15 June, 2015, astronomers detected a sudden flare from a binary system called V404 Cygni, which contains a black hole. They quickly mobilized radio, infrared, optical, and X-ray telescopes to collect data during the two week-long outburst. Yigit Dallilar et al. examined moments when there was a sudden drop in the system's electromagnetic radiation as the ADC cooled, allowing them to measure V404 Cygni's magnetic field, finding it was substantially weaker than expected. The precise measurements derived from this event will help constrain models of how black holes consume material, the authors say.
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