NICER / ISS Science Nugget
for May 6, 2021
X-ray quasi-periodic eruptions from two previously quiescent galaxies
In September 2019, Miniutti et al., Nature, stunned the astrophysics community by presenting puzzling observations of an Active Galactic Nucleus (AGN) that showed quasi-periodic high-amplitude brightness fluctuations, unlike any variability ever seen in another AGN. We do not know what causes these so-called Quasi-Periodic Eruptions (QPEs), but some suggested that they are related to instabilities in a pre-existing AGN accretion disc, that undergoes some periodic accretion episode.
This week in Arcodia et al., Nature, we have reported an important breakthrough in our understanding of these events with the discovery of two new galaxies exhibiting QPEs, doubling the current sample. These QPE candidates were first identified in a blind search with the eROSITA all sky survey. eROSITA was able to identify sources that showed extreme flux variability, but because of their low observing cadence, could not determine if the brightness fluctuations were QPEs or just regular stochastic variability, commonly observed in AGN. The eROSITA team requested observations with NICER to track the source variability. In 11 days of monitoring with NICER, we discovered 15 eruptions (see figure). These are the longest timescale QPEs ever discovered, suggesting they may originate from a higher mass black hole than the others.
Moreover, unlike the other two previously known QPEs, these QPEs did not originate in AGN. This rules out the interpretation of an accretion disc instability, and may suggest the presence of an orbiting compact object around the supermassive black hole. The timescale of the observed QPEs suggests that the orbiting compact object would have to be less massive than the supermassive black hole at the center. Such close separation binary events are expected to release gravitational waves that should be detected by the future space-based low-frequency gravitational wave observatory, LISA. It is possible that we have discovered the electromagnetic signature that will accompany these gravitational wave events.
In the binary scenario, as gravitational waves are emitted, the binary orbit should shrink, and we predict that we should be able to detect a shortening of the period between each eruption. NICER will continue to follow this source over time, to better constrain the period derivative, which is pivotal for understanding the origin of QPEs.
Figure: X-ray brightness - the rate of X-ray photons detected by NICER in the 0.3 to 1 keV energy band and on a logarithmic scale - is shown over 11 days of snapshot observations made during each ISS orbit. Increases in the brightness of this eROSITA galaxy, not previously known to be an X-ray source, are seen with a regular cadence of 17.6 hours.