NICER / ISS Science Nugget
for March 27, 2025

From disruption to eruption

Quasi-periodic eruptions (QPEs) are a new and growing class of X-ray transient originating from the cores of certain nearby, low-mass galaxies. Most theoretical efforts have converged upon explaining these recurring flares via interactions - likely collisions - between a stellar-mass object orbiting a supermassive black hole (SMBH, a million or more times the mass of our Sun) and the disk of matter flowing into it. Since the serendipitous discovery of QPEs in 2019, ten sources have been found, first through searches within archived data, then through all-sky X-ray surveys, and recently, through targeted follow-up of tidal disruption events (TDEs), the energetic outbursts that arise when an SMBH tears apart a star that wanders too nearby, forming a compact accretion flow in its wake. In the first five years following the discovery of QPEs, several lines of inconclusive-but-compelling evidence linked them to the aftermath of TDEs; last year, NICER's discovery of QPEs in the host galaxy of the transient AT2019qiz cemented this association, as it was the first robust detection of QPEs from a previously-known TDE.

With the X-ray archives and all-sky surveys now mostly searched, follow-up of TDEs may soon become the most fruitful avenue for further QPE discovery. In a peer-reviewed study recently accepted for publication in The Astrophysical Journal Letters, J. Chakraborty (MIT) and collaborators report the discovery of QPEs in the host galaxy of transient AT2022upj, the second instance of QPEs confirmed following a TDE. The newly identified eruptions are erratic, exhibiting large scatter in their measured arrival times, which previous NICER studies have suggested can be due to extreme-gravity precession and frame-dragging effects around the SMBH, affecting both the accretion disk and stellar orbiter. AT2022upj also shows unusually strong emission lines, at visible-light wavelengths, originating from iron and silicon atoms; such "extreme coronal-line emitters" (ECLEs) comprise a small handful, 5-20%, of TDEs, and represent gas- and dust-rich environments. Perhaps most interesting is that AT2019qiz also exhibited these extreme coronal lines. While a population of two is not conclusive, the low probability of a chance coincidence warrants further investigation of a possible QPE-ECLE association, a potential signpost for QPE discovery.

To date, about a hundred optically-discovered TDEs are known, predominantly found (like AT2022upj and AT2019qiz) by Mt. Palomar Observatory's Zwicky Transient Facility. Detecting QPEs following a TDE requires diligent long-term X-ray monitoring, potentially for several years, whereas existing X-ray observations of TDEs are heterogeneous and often sparse. But with such a large sample, one can probe "wide instead of deep," comparing observations across the entire TDE population against the rate of QPE detections to estimate the fraction of QPE-TDE associations. With some cautious Bayesian inference, Chakraborty et al. find that approximately 9% of optical TDEs result in QPEs, a fraction still subject to substantial uncertainty but that will improve with additional data. The QPE-TDE fraction is a promising avenue to constrain the existence and population properties of stellar orbiters around nearby SMBHs - "extreme mass-ratio inspiral" (EMRI) systems that will be a sought-after source of gravitational waves for the future ESA-NASA Laser Interferometer Space Antenna (LISA) mission. Studying QPEs thus lays important groundwork to enable multi-messenger astrophysics in the upcoming era of space-based gravitational wave astronomy.