Thanks to the data collected by the Srg / eRosita space mission, a team led by researchers from the Max Planck Institute for Extraterrestrial Physics in Germany, in which Gabriele Ponti dell’NAf also participated, has identified two galaxies hitherto considered quiescent which instead show eruptions quasi-periodic. The nuclei of these galaxies emit powerful X-ray glows every few hours, reaching peak brightness comparable to those produced by an entire galaxy. This pulsating behavior could be due to a star or celestial object of similar mass orbiting the supermassive black hole located in the central regions of the galaxy.
Quasars, or active galactic nuclei (Agn for short), are considered the beacons of the distant universe. The brightness of their central region, fueled by the supermassive black hole that increases enormous quantities of matter, can be thousands of times higher than that of a galaxy like our Milky Way.
“In the full-sky survey by eRosita, we found two previously quiescent galaxies that now show sharp and powerful almost periodic pulses in X-rays,” says Riccardo Arcodia , PhD student at the Max Planck Institute for Extraterrestrial Physics (Mpe), first author of a study published today in Nature . These objects are very rare: so far, in fact, only two sources with similar properties were known . “Since this new type of sources seems to be peculiar only in X-rays, we decided to use the data of the eRosita mission and we immediately found two more.”
The eRosita telescope currently scans the entire sky in X-rays and the data that is continuously collected is ideal for looking for transient events, just like these energy flares. Both new sources discovered by eRosita showed considerable variability in X-rays within a few hours, a feature that was confirmed by follow-up observations with the Xmm-Newton and Nicer space telescopes . Unlike the two similar objects known so far, the galaxies of these new sources discovered by eRosita were until now believed to host quiescent black holes. “These two galaxies appear completely normal, with a rather small mass and without any indication of any previous peculiar activity,” he explains.Andrea Merloni of the Mpe, principal investigator of eRosita. “Without these sudden and repeated X-ray eruptions we would have ignored them.”
Quasi-periodic emission such as that discovered by eRosita is typically associated with binary systems . The data indicate that if these eruptions were triggered by the presence of an object orbiting the black hole, its mass would have to be much smaller than it, such as that of a star or even a white dwarf. These celestial objects could interact with the black hole and surrounding matter during each orbit.
“We do not yet know what could trigger these X-ray eruptions,” Arcodia points out, “but we do know that the regions surrounding the black hole were quiet until recently, so it is not necessary to assume that an accretion disk like that was already present. around black holes in active galaxies to trigger these phenomena ». Further X-ray observation campaigns of these objects will help confirm or rule out this scenario.
“If the origin of these eruptions were a binary system, it would be a very intriguing solution because it would allow us in the future to combine both electromagnetic and gravitational wave observations, thus opening new frontiers of multi-message astrophysics”, concludes Gabriele Ponti , researcher INAF in Milan.
Featured image: Visible light image of the first galaxy from which X-ray eruptions with eROSITA were observed, with the trend of the X-ray emission recorded by the NICER mission in green. The galaxy was identified at a redshift of z ~ 0.05.
Approximately 18.5 hours pass between the peaks of X-ray eruptions. Credits: MPE;
Optical Image: DESI Legacy Imaging Surveys / D.
Lang (Perimeter Institute)
To know more:
- Read in Nature the article ” X-ray quasi-periodic eruptions from two previously quiescent galaxies “, by R. Arcodia, A. Merloni, K. Nandra, J. Buchner, M. Saved, D. Pasham, R. Remillard, J. Comparat, G. Lamer, G. Ponti, A. Malyali, J. Wolf, Z. Arzoumanian, D. Bogensberge, DAH Buckley, K. Gendreau, M. Gromadzki, E. Kara, M. Krumpe, C. Markwardt , ME Ramos-Ceja, A. Rau, M. Schramm and A. Schwope
Provided by INAF