Quasi-periodic eruptions from eRO-QPE2 are remarkably stable, study finds

An international team of astronomers has inspected long-term evolution of quasi-periodic eruptions (QPEs) from a QPE source designated eRO-QPE2. The study found that QPEs from this source are remarkably stable over the investigated period of over three and a half years. The finding was reported November 1 on the pre-print server arXiv.

QPEs are defined as short, high amplitude quasi-periodic X-ray bursts over a stable flux level, originating near the central supermassive black holes (SMBHs) in galactic nuclei. They are observed as increases in the X-ray count rate over a quiescent level, and recur quasi-periodically.

The name QPE was chosen to differentiate them from the gentler, quasi-sinusoidal modulation of the standard quasi-periodic oscillations (QPOs).

Discovered in 2021 with the eROSITA instrument on the Spectrum-Roentgen-Gamma (SRG) spacecraft, eRO-QPE2 is a QPE source at a redshift of 0.0175, associated with the galaxy 2MASX J02344872-4419325.

Follow-up observations of eRO-QPE2 with ESA’s XMM-Newton satellite have revealed that it exhibits QPEs with intrinsic luminosities oscillating between approximately 0.12 and 1.2 tredecillion erg/s in the 0.5–2-keV band. The mean dispersion of the rise-to-decay duration of these eruptions was measured to be about 27 minutes, with a peak-to-peak separation of around 2.4 hours and a duty cycle of 19%.

Now, a new study conducted by a group of astronomers led by Dheeraj Pasham of MIT Kavli Institute for Astrophysics and Space Research in Cambridge, Massachusetts, provides more insights into the evolution and properties of quasi-periodic eruptions from eRO-QPE2.

“In this work, we studied the long-term evolution of eRO-QPE2 using XMM-Newton data taken between August 2020 to February 2024, i.e., a temporal baseline of 1,277 days or 3.5 years,” the researchers wrote.

Pasham’s team found that eRO-QPE2 is remarkably stable over the entire 3.5-year observation period in its eruption peak luminosity, eruption temperature, quiescent temperature, and quiescent luminosity. Moreover, the mean recurrence time of QPEs, calculated to be 2.35 hours, was found to be constant between 2022 and 2024, with a hint of a decay of approximately six minutes between August 2020 and June 2022.

By comparing eRO-QPE2 to other known systems experiencing quasi-periodic oscillations, the astronomers note that this stability is in contrast with the known sample of QPEs as they usually showcase declines in eruption and quiescent flux over comparable observation periods.

Furthermore, given the relatively short QPE recurrence time, eRO-QPE2 has undergone more cycles than these other systems, which underlines its unusual stability.

Trying to explain the origin of the eruptions of eRO-QPE2, the authors of the paper assume that it may be a low-mass (about 0.18 solar masses) white dwarf being partially tidally stripped by a black hole with a mass at a level of 230,000 solar masses. However, further studies are required in order to provide more evidence supporting this hypothesis.

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