The center of the Milky Way Galaxy hosts Sagittarius A*, a supermassive black hole with a mass of about 4 million solar masses that is very quiescent. Reflection of X-rays from Sagittarius A* by dense gas in the Galactic center offers a means to study its past flaring activity on timescales of hundreds and thousands of years. Now, astronomers using NASA’s Imaging X-ray Polarimetry Explorer (IXPE) have detected polarized X-ray emission in the direction of the molecular clouds in the Galactic center. The polarization angle is consistent with the supermassive black hole being the primary source of the emission, and the polarization degree implies that some 200 years ago, the X-ray luminosity of Sagittarius A* was briefly comparable to that of a Seyfert galaxy.
Sagittarius A*, which is more than 25,000 light-years from Earth, sits in the constellation of Sagittarius at the very heart of the Milky Way.
In previous X-ray studies, astronomers detected relatively recent X-ray emissions from giant gas clouds in its vicinity.
Given that these molecular clouds are cold and dark, the X-ray signatures of these clouds should have been faint. Instead, they shone brightly.
“One of the scenarios to explain why these giant molecular clouds are shining is that they are, in fact, echoing a long-gone flash of X-ray light, indicating that our supermassive black hole was not that quiescent some centuries ago,” said Astronomical Observatory of Strasbourg astronomer Frédéric Marin.
IXPE, which measures polarization of X-rays, observed the molecular clouds near Sagittarius A* for two periods of study in February and March 2022.
When the astronomers combined the resulting data with images from NASA’s Chandra X-ray Observatory and compared them to archival observations from ESA’s XMM-Newton mission, they could isolate the reflected X-ray signal and discover its point of origin.
“The polarization angle acts like a compass, pointing us toward the mysterious, long-gone…
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