NICER / ISS Science Nugget
for May 9, 2024
The more the merrier
The discovery, in 2019, of quasi-periodic eruptions (QPEs) of X-rays from the core of an otherwise unremarkable galaxy came as a surprise - if not a complete shock. The supermassive black holes (SMBH; about 1 million times the mass of our Sun) that anchor most galaxies were expected to produce only slowly varying X-ray emission, on months-to-years timescales, but these eruptions recurred every few hours, with remarkable regularity. Armed with the knowledge that other examples of this unexplained and unexpected phenomenon must exist - but were likely missed because no one knew to look for them - teams of observers have combed data archives and enlisted telescopes to search for additional QPE-emitting galaxies. The number known today can be counted on the fingers of two hands, but is significantly exceeded by the number of published theoretical models attempting to explain the QPE phenomenon.
NICER is playing a key role in the discovery, confirmation, and ongoing characterization of QPE sources, thanks to its combination of sensitivity to soft X-rays and observing agility, which enables monitoring of targets on a wide range of timescales. In a peer-reviewed paper recently published in the European journal Astronomy & Astrophysics, R. Arcodia (MIT) and collaborators report the discovery of two new QPE sources, following up promising candidates uncovered in data from the eROSITA telescope on the German-Russian Spektrum-Roentgen-Gamma mission. The properties of the two new sources are consistent with those of the handful known at the time the paper was written, including: thermal X-ray emission spectra that reveal higher temperatures during the eruption rise and lower temperatures during the decay; eruption durations that scale with recurrence time (approximately 20%); and comparable host-galaxy SMBH masses. The emerging picture favors an especially exciting explanation for QPEs: a stellar-mass compact object (e.g., a neutron star or black hole) in orbit around the SMBH in a galaxy's core interacts with an accretion disk, punching through it once or twice each orbit to shock-heat gas to X-ray emitting temperatures. Such systems represent a highly anticipated class of sources of gravitational waves that can be detected by the future Laser Interferometer Space Array (LISA), a joint European-NASA mission currently in the early stages of development. The study of these so-called "extreme mass-ratio inspirals" (EMRIs) in gravitational waves (and potentially in X-rays) promises unique and exquisitely precise tests of our understanding of strong gravity, a prospect that drives the search for new QPE sources, to increase the sample size, confirm the association with EMRIs, and derive insights that connect electromagnetic and graviational-wave astronomy.
NICER's confirmation of two candidate quasi-periodic eruption sources (designated, in shorthand, eRO3 and eRO4 after the German eROSITA instrument that first detected their short-term X-ray variability). Red points are brightness measurements consistent with NICER's background rate; black points represent unambiguous detections (in photons per second) of X-rays from a cosmic source. In the left panel, vertical dashed lines mark evenly spaced intervals of 20 hours, the recurrence time of eruptions from eRO3. In the right panel, vertical dotted lines denote eruption times for eRO4, which are not evenly spaced. The zero points for the time axes (in Modified Julian Days) are 15:27 UT, April 28, 2022 for eRO3 and 15:17 UT, Jan 27, 2023 for eRO4. (Figure credit: Arcodia et al. 2024)
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