Up until a beautiful May evening in 2019 at Hawaii’s Keck Observatory, Sagittarius A* (Sgr A*), the Milky Way’s central supermassive black hole was usually a passive flickering object that appeared like a massive, dormant volcano or a sleeping monster. But suddenly Sgr A*, located twenty-five thousand light years from Earth, brightened 75-fold. “The black hole is always variable,” observed astronomer Tuan Do, “but this was the brightest we’ve seen in the infrared so far. It was probably even brighter before we started observing that night!”
“The black hole was so bright I at first mistook it for the star S0-2, because I had never seen Sgr A* that bright,” Do said in an interview with ScienceAlert.
Eruptions as Bright as an Entire Galaxy
In a similar experience, astronomers at the Max Planck Institute for Extraterrestrial Physics using the SRG/eROSITA all-sky survey data, found two previously quiescent galaxies that now show quasi-periodic eruptions. The nuclei of these galaxies light up in X-rays every few hours, reaching peak luminosities comparable to that of an entire galaxy.
The origin of this pulsating behavior is unclear. A possible cause is a faint star or white dwarf orbiting too close to the central black hole, where tidal forces can begin to shred the stellar object apart. When a star comes too close to a very supermassive black hole with a mass exceeding 100 million solar masses, the black hole’s tidal forces completely disrupt the star in single passing. Such tidal disruption events produce powerful luminous transients that are sometimes mistaken for supernovae, and they never repeat as the star is completely destroyed.
The situation may be different for lower mass supermassive back holes, like Sgr A* at the center of our Milky Way, which is only 4 million solar masses. As Sgr A* and galactic nuclei exhibiting quasi-period eruptions are relatively close and small, this discovery could help scientists to better understand how black holes are activated in low-mass galaxies.
The image above shows the first galaxy found with quasi-periodic eruptions in the eROSITA all-sky data, the NICER X-ray light-curve is overlaid in green. The galaxy was identified as 2MASS 02314715-1020112 at a redshift of z~0.05. About 18.5 hours pass between the peaks of the X-ray outbursts. ( MPE; optical image: DESI Legacy Imaging Surveys/D. Lang -Perimeter Institute)
Quasars or “active galactic nuclei” (AGN) are often called the lighthouses of the distant universe. The luminosity of their central region, where a very massive black hole accretes large amounts of material, can be thousands of times higher than that of a galaxy like our Milky Way. However, unlike a lighthouse, AGN shine continuously.
“In the eROSITA all-sky survey, we have now found two previously quiescent galaxies with huge, almost periodic sharp pulses in their X-ray emission,” says Riccardo Arcodia, Ph.D. student at the Max Planck Institute for Extraterrestrial Physics (MPE), who is the first author of the study now published in Nature. These kinds of objects are fairly new: only two such sources were known before, found either serendipitously or in archival data in the past couple of years. “As this new type of erupting sources seems to be peculiar in X-rays, we decided to use eROSITA as a blind survey and immediately found two more,” he adds.
Sudden, Repeating X-ray Eruptions
The eROSITA telescope currently scans the entire sky in X-rays and the continuous data stream is well suited to find transient events such as these eruptions. Both new sources discovered by eROSITA showed high-amplitude X-ray variability within just a few hours, which was confirmed by follow-up observations with the XMM-Newton and NICER X-ray telescopes. Contrary to the two known similar objects, the new sources found by eROSITA were not previously active galactic nuclei.
“These were normal, average low-mass galaxies with inactive black holes,” explains Andrea Merloni at MPE, principal investigator of eROSITA. “Without these sudden, repeating X-ray eruptions we would have ignored them.” The scientists now have the chance to explore the vicinity of the smallest supermassive black holes. These have 100,000 to 10 million times the mass of our Sun.
Quasi-periodic emission, such as the one discovered by eROSITA, is typically associated with binary systems. If these eruptions are indeed triggered by the presence of an orbiting object, its mass has to be much smaller than the black hole’s—of the order of a star or even a white dwarf, which might be partially disrupted by the huge tidal forces close to the black hole at each passage.
Optical image of the second galaxy (above) found with quasi-periodic eruptions in the eROSITA all-sky data, the XMM-Newton X-ray light-curve is overlaided in magenta. The galaxy was identified as 2MASX J02344872-4419325 at a redshift of z~0.02. This source shows much narrower and more frequent eruptions, approximately every 2.4 hours. Credit: MPE; optical image: DESI Legacy Imaging Surveys/D. Lang (Perimeter Institute)
“We still do not know what causes these X-ray eruptions,” admits Arcodia. “But we know that the black hole’s neighborhood was quiet until recently, so a pre-existing accretion disk as the one present in active galaxies is not required to trigger these phenomena.”
“Billion Dollar Question”
“You are asking me the billion dollar question,” Arcodia wrote The Daily Galaxy in an email asking if he could offer any conjectures about what could cause these X-ray eruptions, if not by an orbiting object? “The orbiting object scenario (be it much smaller than the central supermassive black hole or a bit bigger than we suggested) is indeed still a conjecture at this point. The strength of it is that we can test it with data incoming over the next year. And truth is, if we’ll disfavor it I have very little left in the ideas tank at this stage, but I am confident that theorists in the community will come through, as theory and observations often march together providing input one to the other.”
Future X-ray observations will help to constrain or rule out the “orbiting object scenario” and to monitor possible changes in the orbital period. These kinds of objects could also be observable with gravitational waves signals, opening up new possibilities in multi-messenger astrophysics.