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13th of September 2019
Quasi-periodic X-ray bursts from a nearby galaxy
by Miniutti et al.
In a paper published in Nature this week, Miniutti et al. report on quasi-periodic X-ray bursts from a low mass black hole galactic nucleus. XMM-Newton, the most sensitive X-ray observatory, discovered some mysterious X-ray flashes from the active black hole at the core of the galaxy GSN 069, some 250 million light years away. In December 2018, the source was seen to suddenly increase its brightness by up to a factor 100, then dimmed back to its normal levels within one hour and lit up again nine hours later. Further X-ray observations confirmed that the distant black hole was still keeping the tempo, emitting nearly periodic bursts of X-rays every nine hours. The researchers are calling the new phenomenon ‘quasi-periodic eruptions’, or QPEs.

Radio observations were carried out with the Australia Telescope Compact Array (ATCA), MeerKAT (in South Africa), and the Karl G. Jansky Very Large Array (in the USA) simultaneously with an observation with the Chandra X-ray satellite in 2019 February. While steady radio emssion was detected from GSN 069 at giga-hertz frequencies no radio variability was observed in exposures simultaneous with X-ray QPEs, providing hints as to the physical processes producing the phenomenon.

Although never before observed, Giovanni et al. suggest periodic flares like these might actually be quite common in the Universe. It is possible that the phenomenon hadn't been identified before because most black holes at the cores of distant galaxies, with masses millions to billions of times the mass of our Sun, are much larger than the one in GSN 069, which weighs about 400,000 times our Sun. The bigger and more massive the black hole, the slower the fluctuations in brightness it can display, so a typical supermassive black hole would erupt not every nine hours, but every few months or years, making a detection unlikely as observations rarely span such long periods of time.

The image above shows the variation in the X-ray signal over a 130 kilo-second Chandra observation, showing the quasi-periodic flares. More details are given in the paper in Nature.

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