At the heart of our own Galaxy, the Milky Way, lies a supermassive black hole. Located in the constellation Sagittarius, hence its name Sgr A, this black hole is currently inactive. However, strange X-ray signatures have been observed around Sgr A, suggesting that this supermassive black hole may have been much more active in the past. But when? And why? The IXPE satellite, the result of a NASA-ASI collaboration, has been pointed towards the galactic centre to solve this mystery.
The IXPE satellite, in conjunction with the Chandra and XMM-Newton space telescopes, has observed giant molecular clouds near the supermassive black hole that are strangely bright in X-rays. IXPE discovered the origin of this radiation by measuring the polarisation of light, which is the average direction and intensity of the electric field of light waves.
The IXPE data, which show the echo of past activity, are visible in orange in the lower panel. They have been combined with data from Chandra, another NASA X-ray observatory, visible in blue, which show only direct light from the galactic centre. The upper panel is a much wider view of the centre of the Milky Way obtained thanks to Chandra.
Credits: NASA/CXC/SAO/IXPE
Thanks to this polarisation measurement, the team led by Frédéric Marin, an astrophysicist at the Strasbourg Astronomical Observatory, has been able to determine the origin of these X-rays, which are in fact an echo of a violent flash that is now extinguished. In fact, the angle of polarisation acted like a compass, pointing towards the mysterious source of illumination: the black hole Sgr A. But that’s not all: thanks to the degree of polarisation, it was possible to determine the distance between the molecular clouds and Sgr A, and therefore to date the echo seen in X-rays: nearly 200 years.
Based on this estimate, we were able to put a limit on the real intensity of the original flash. This corresponds to a flux comparable to that of a Seyfert galaxy, i.e. a galaxy whose core luminosity is comparable in intensity to the total luminosity of the stars that make it up. This means that Sgr A* was at least a million times brighter than it is today, in the relatively near past.
Dr Marin’s team will continue to observe the neighbourhood of Sgr A* to improve the estimate of the epoch of the flash, its original intensity and the three-dimensional distribution of the giant molecular clouds. In so doing, it will be possible to trace the origin of Sgr A*’s past activity and determine which physical processes are capable of temporarily reactivating a supermassive black hole.
Data sonification
The data was translated into sound using a circular scan, following the trajectory of the light emitted during the explosion of Sgr A* (not shown). The horizontal position corresponds to the stereo position of the sound. For IXPE data, the X-ray spectrum (of the echo region) is converted directly into an audio spectrum, 51 and 52 octaves below the true frequencies. Luminosity controls volume. For Chandra data, the brightness controls the pitch and volume of the music.
Article : X-ray polarization evidence for a 200 years-old flare of Sgr A∗, F. Marin et al., Nature, 2023.
Contacts: Frédéric Marin (frederic.marin@astro.unistra.fr), Thibault Barnouin (thibault.barnouin@astro.unistra.fr)
More information :
– NASA press release
– BBC