An international team of astronomers has recently conducted an in-depth study of the long-term evolution of quasi-periodic eruptions (QPEs) originating from a source known as eRO-QPE2. This source, located in the galaxy 2MASX J02344872-4419325, has become a significant focus for astronomers studying the behavior of QPEs. The study, which spanned over three and a half years, provides crucial insights into the stability and properties of QPEs, marking a significant step forward in understanding these enigmatic cosmic phenomena. The findings were published on November 1, 2024, on the pre-print server arXiv.
QPEs are intense, short, and high-amplitude X-ray bursts that occur near the central supermassive black holes (SMBHs) of galaxies. These bursts, which are distinct from standard quasi-periodic oscillations (QPOs), are observed as rapid increases in X-ray flux that recur at quasi-regular intervals, but with a much higher amplitude. The term QPE was coined to distinguish these eruptions from the more gradual, sinusoidal variations associated with QPOs. The origin of QPEs is still under investigation, but their connection to the dynamics of SMBHs makes them crucial for understanding the behavior of matter in the extreme gravitational environments near black holes.
The eRO-QPE2 source was first identified in 2021 by the eROSITA (extended Roentgen Survey with an Imaging Telescope Array) instrument on the Spectrum-Roentgen-Gamma (SRG) spacecraft. This discovery added a new source to the growing list of QPE emitters. eRO-QPE2, located at a redshift of 0.0175, was found to be associated with a distant galaxy. Follow-up observations using ESA’s XMM-Newton satellite have provided valuable data on the properties of the QPEs emitted by eRO-QPE2, revealing that its luminosity oscillates dramatically between approximately 0.12 and 1.2 tredecillion erg/s in the 0.5–2 keV X-ray band. The duration of the rise-to-decay phases of these eruptions was found to average around 27 minutes, with the peak-to-peak separation between eruptions averaging about 2.4 hours. The duty cycle, which measures the fraction of time the system is erupting, was calculated to be 19%.
In the new study, led by Dheeraj Pasham of the MIT Kavli Institute for Astrophysics and Space Research, the team focused on the long-term behavior of eRO-QPE2 over a 1,277-day period from August 2020 to February 2024. This observation period, extending over three and a half years, allowed the researchers to monitor the source’s evolution and check for any significant changes in the eruption properties. One of the most remarkable findings was the stability of eRO-QPE2’s QPEs throughout this time. The eruption peak luminosity, eruption temperature, quiescent temperature, and quiescent luminosity all remained consistent over the entire observation period, showing little to no variation.
The researchers also studied the recurrence time between individual eruptions. The mean recurrence time of QPEs from eRO-QPE2 was calculated to be approximately 2.35 hours, which remained constant from 2022 to 2024. Interestingly, there was a slight decline in the recurrence time between August 2020 and June 2022, decreasing by about six minutes over that period. Despite this small change, the overall pattern of regular, periodic eruptions remained intact, indicating that eRO-QPE2 is highly stable compared to other known QPE sources, which typically show a decline in both eruption and quiescent flux over similar time frames.
This stability is notable because, in most known systems exhibiting QPEs, the frequency and intensity of eruptions tend to decline as the system evolves. In contrast, eRO-QPE2 has undergone many more cycles within the same time span, suggesting that its behavior is unusual. This unprecedented stability challenges current models of QPEs and may point to a unique physical mechanism at work within eRO-QPE2.
The origin of the QPEs from eRO-QPE2 remains an open question. In the paper, the authors propose a possible explanation: they suggest that the QPEs could be the result of a low-mass white dwarf—approximately 0.18 solar masses—being partially tidally disrupted by a supermassive black hole with a mass of about 230,000 solar masses. This hypothesis, though intriguing, requires further observational evidence to confirm. If correct, this would imply a highly dynamic and complex interaction between the white dwarf and the SMBH, possibly involving mass transfer, accretion, or tidal forces that produce the observed periodic eruptions.
The study of eRO-QPE2 and its QPEs is an exciting step in understanding the extreme environments around supermassive black holes. The stability of the QPEs from this source could provide valuable clues about the mechanisms governing such eruptions and may offer new insights into the nature of black holes and their interactions with surrounding matter. As astronomers continue to monitor eRO-QPE2 and other QPE sources, they hope to refine models of black hole accretion and tidal disruption events, ultimately leading to a deeper understanding of the most energetic phenomena in the universe. Further research, particularly through future X-ray observations and detailed modeling, will be crucial in testing the hypothesis regarding the white dwarf and SMBH interaction, shedding light on the true nature of these fascinating eruptions.