Back in 2007, researchers observed a curious phenomenon at the center of galaxy RE J1034+396. The supermassive black hole there emitted a signal, or "heartbeat," about every hour from material orbiting around it. When the galaxy was observed once again in 2018, the researchers discovered that the heartbeat was still going strong.
The supermassive black hole weighs between 1 and 10 million times the Sun and the galaxy is located 600 million light-years from Earth. Reported in the Monthly Notices of the Royal Astronomical Society, these new observations are exciting because this system was the first one where these quasi-periodical oscillations, the heartbeats, were observed. The stability of these signals for more than a decade suggests that the source behind them is quite stable in the long-term.
"This heartbeat is amazing!" lead author Dr Chichuan Jin of the National Astronomical Observatories, Chinese Academy of Sciences, said in a statement. "It proves that such signals arising from a supermassive black hole can be very strong and persistent. It also provides the best opportunity for scientists to further investigate the nature and origin of this heartbeat signal."
Many supermassive black holes are surrounded by a disk of material. The black hole's incredible gravitational pull heats up this material, in the process turning into plasma. These regions can often shine in a powerful visible light such as X-rays, and any variations in light can be tracked down to the motion of material in the disk. It is rare that these changes repeat regularly, though.
This rare feature is providing important insights into the physics of the accretion disk, although there is plenty of uncertainty about what’s causing this periodic signal. In general, researchers describe it as an X-ray emitting blob but they don’t know what this "blob" actually is.
The European X-ray space telescope, XMM-Newton, observed the system in 2009 and then again in 2018. The team found that the period of the heartbeat has shortened a few minutes between these two observations. If the material was simply swirling into a black hole, the blob would have been gone in a few months. Instead, the disk is moving in an unexpected way.
Its long survival suggests that this is system is quite special. The team suggests similarities between what is seen here and systems that have two black holes, where a black hole is orbited by a neutron star, although the X-ray binary system GRS 1915+105, seems to be by far the better match, despite being radically different.
"The main idea for how this heartbeat is formed is that the inner parts of the accretion disk are expanding and contracting," co-author Professor Chris Done from Durham University said. "The only other system we know which seems to do the same thing is a 100,000 times smaller stellar-mass black hole in our Milky Way, fed by a binary companion star, with correspondingly smaller luminosities and timescales."
The team is planning more analysis of the data, to be published in a second paper, as well as more observations with upcoming observatories such as the European Space Agency's Athena X-ray observatory, to better understand such a system and its peculiar heartbeat.