Stable X-ray Quasi-Periodic Eruptions from eRO-QPE2 Observed over 3.5 Years

Core Concepts

eRO-QPE2, a source of X-ray quasi-periodic eruptions, exhibits remarkable stability in its eruption and quiescent properties over a 3.5-year observation period, challenging some existing models and raising new questions about the nature of these enigmatic events.

Abstract

• Bibliographic Information: Pasham, D., Kejriwal, S., Coughlin, E. R., Witzany, V., Chua, A. J. K., Zajaˇcek, M., Wevers, T., & Ajay, Y. (2024). Alive and Strongly Kicking: Stable X-ray Quasi-Periodic Eruptions from eRO-QPE2 over 3.5 Years. arXiv preprint arXiv:2411.00289v1.

• Research Objective: This study investigates the long-term temporal and spectral evolution of the quasi-periodic eruption (QPE) source eRO-QPE2 to gain insights into the physical mechanisms driving these recurring X-ray bursts.

• Methodology: The researchers analyzed a series of X-ray observations of eRO-QPE2 obtained by the XMM-Newton satellite over a period of 3.5 years. They used Bayesian blocks and skewed-Gaussian modeling to determine eruption peak times and analyzed the spectral evolution of both the eruptions and the quiescent emission between eruptions.

• Key Findings: Unlike other observed QPE sources that show declining eruption strength and quiescent luminosity over time, eRO-QPE2 exhibits remarkable stability in its eruption peak luminosity, temperature, quiescence luminosity, and temperature. The mean eruption recurrence time is also stable at 2.35 hours, with only marginal evidence for a slight decrease over the observation period.

• Main Conclusions: The stability of eRO-QPE2's QPEs over 3.5 years challenges models that predict significant evolution in eruption properties over such timescales. The authors suggest that this stability may disfavor models involving recent tidal disruption events (TDEs) or certain extreme mass ratio inspiral (EMRI) scenarios.

• Significance: This study provides crucial observational constraints for understanding the long-term behavior of QPEs, a recently discovered and enigmatic phenomenon. The unexpected stability of eRO-QPE2 highlights the diversity among QPE sources and underscores the need for further theoretical and observational work to unravel the underlying physical processes.

• Limitations and Future Research: The study relies on a limited number of observations of eRO-QPE2. Continued monitoring of this source is crucial to confirm the long-term stability of its QPEs and to search for potential variations or evolutionary trends. Further theoretical modeling, incorporating the observed stability, is needed to refine existing models and explore alternative explanations for QPEs.

Stats

Redshift of eRO-QPE2: z = 0.0175
Luminosity per eruption in the 0.2 – 2.5 keV band: ≃10^42.2 erg s^-1
Total integrated luminosity: ∼10^43.3 erg s^-1
Estimated accreted mass per eruption (assuming a radiative efficiency of 0.1 and an eruption duration of ∼2 ks): ∼2.2 × 10^-7 M⊙
Estimated lifetime of the system if the object feeding the accretion has a solar mass: ∼1200 years
Typical length scale of the thermally emitting area: Rerupt ∼ 28 R⊙ or 133 gravitational radii for M• = 10^5 M⊙
Estimated orbital period of the companion object: Porb ≈ 4.8 hours
Estimated orbital eccentricity: e ≈ 0.025
Estimated period of relativistic pericenter precession: Tprec ≈ 30 days (assuming M• = 10^6 M⊙)
Estimated change in orbital period over 3 years due to gravitational-wave emission: ∆T ≃ -0.085 hr (assuming M⋆ = 0.2 M⊙ and M• = 10^5 M⊙)

Quotes

"The stability of its peak eruption luminosity and that of the quiescent state are notably dissimilar from three previously tracked QPEs (GSN069, eRO-QPE1, eRO-QPE3), which show declines in eruption and quiescent flux over comparable temporal baselines."
"This stability is even more pronounced in eRO-QPE2 due to its 2.4 hour average recurrence time compared to GSN-069’s 9 hour, eRO-QPE1’s 16 hour, and eRO-QPE3’s 20 hour recurrence times, i.e., this system has undergone 4-8 times more cycles than these other systems over the 3.5 years of observations."

Key Insights Distilled From

Alive and Strongly Kicking: Stable X-ray Quasi-Periodic Eruptions from eRO-QPE2 over 3.5 Years

by Dhee... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2411.00289.pdf

Deeper Inquiries

How might the properties and evolution of the host galaxy of eRO-QPE2 provide further clues about the origin and nature of its QPEs?

Answer: Studying the properties of eRO-QPE2's host galaxy can provide valuable insights into the nature of its QPEs. Here's how:

Host Galaxy Type and Morphology: The type and morphology of the host galaxy can offer clues about the formation and availability of the central black hole's potential companions.

For instance, if the host galaxy is a post-starburst galaxy, as is common for QPE hosts, it suggests a recent burst of star formation. This could imply a higher likelihood of stellar-mass black holes or white dwarfs present in the vicinity of the central supermassive black hole, potentially leading to EMRI formation.
Additionally, examining the presence of tidal features or signs of recent mergers can indicate past interactions that might have driven material (and potential EMRI progenitors) towards the galactic center.

Gas Content and Dynamics: The presence and distribution of gas in the host galaxy can influence the evolution of an EMRI and the QPE mechanism.

A gas-rich environment might lead to more frequent interactions between the orbiting object and the accretion disk, potentially explaining the observed stability of eRO-QPE2's eruptions.
Studying the gas dynamics can also help constrain the mass of the central black hole, a crucial parameter for EMRI models.

Comparison with Other QPE Hosts: Comparing the properties of eRO-QPE2's host galaxy to those of other known QPE hosts can reveal commonalities or differences that could point towards specific formation scenarios or evolutionary pathways for QPE systems.
By carefully studying these aspects of eRO-QPE2's host galaxy and comparing them to theoretical models and other observed systems, astronomers can gain a deeper understanding of the environments where QPEs occur and refine models for their origin.

Could the observed stability of eRO-QPE2's eruptions be a temporary phenomenon, and might we expect to see significant evolution in its behavior over longer timescales?

Answer: It is certainly possible that the observed stability of eRO-QPE2's eruptions is a transient phase in its evolution. While the current data spanning 3.5 years show remarkable consistency, longer-term observations are crucial to determine if this stability persists.
Here are some factors that could lead to future evolution in eRO-QPE2's behavior:

Orbital Evolution: Even in the absence of significant orbital decay due to gravitational wave emission, interactions with the accretion disk can gradually modify the orbital parameters of the putative orbiting object. These changes could manifest as variations in the eruption recurrence time, amplitude, or profile over longer timescales.
Fuel Supply and Accretion Rate: The stability of the eruptions suggests a relatively steady fuel supply to the central black hole. However, this could change if the accretion disk structure evolves, or if the supply of material from the orbiting object varies. Such changes could lead to a decline or increase in the eruption strength or even a temporary cessation of the QPE activity, as observed in GSN 069.
Stochastic Processes: Accretion processes are inherently chaotic, and stochastic variations in the accretion flow can influence the observed properties of the eruptions. While these variations might be averaged out over short timescales, they could become apparent over longer observational baselines.
Therefore, continued monitoring of eRO-QPE2 with sensitive X-ray instruments is essential to track any long-term changes in its behavior. Detecting such changes would provide valuable constraints on the physical mechanisms driving the QPEs and the nature of the system itself.

If QPEs are indeed related to EMRIs, what are the implications of eRO-QPE2's stability for our understanding of the dynamics of such systems and their detectability through gravitational waves?

Answer: If the QPEs in eRO-QPE2 are generated by an EMRI, the observed stability has significant implications for our understanding of these systems and their detectability via gravitational waves:

Constraints on EMRI Evolution: The stability of eRO-QPE2's eruptions over 3.5 years implies that the orbital decay of the companion object due to gravitational wave emission is not rapid enough to cause significant changes in the observed QPE period. This suggests either a relatively low-mass companion object (e.g., a white dwarf) or a wider separation between the companion and the central black hole, both of which result in weaker gravitational wave emission and slower orbital decay.
Importance of Disk-Companion Interactions: The stability also highlights the potential role of interactions between the companion object and the accretion disk in shaping the EMRI evolution. These interactions can either counteract the orbital decay due to gravitational waves or introduce additional complexities in the orbital dynamics, potentially explaining the observed long-short variations in the eruption recurrence time.
Challenges for Gravitational Wave Detection: The relatively stable orbital period of the putative EMRI in eRO-QPE2, if confirmed, poses challenges for its detection through gravitational waves. Current and planned space-based gravitational wave detectors like LISA are most sensitive to EMRIs with shorter orbital periods (on the order of minutes to hours) that are close to merging. A stable, longer-period EMRI like that potentially present in eRO-QPE2 would require dedicated search strategies and longer observation times to disentangle its faint gravitational wave signal from instrumental noise and other astrophysical sources.
In conclusion, the stability of eRO-QPE2's eruptions, while challenging our understanding of EMRI evolution and detectability, underscores the importance of considering both gravitational wave emission and disk-companion interactions in modeling these systems. Continued monitoring of eRO-QPE2 and other QPE sources, combined with advancements in gravitational wave astronomy, will be crucial to unraveling the mysteries of these intriguing objects.