A possible origin of the overlapping light curve of eRO-QPE1

TL;DR

This study investigates the unusual overlapping light-curve features of eRO-QPE1 and proposes gravitational lensing as an external modulation that could reproduce the observed morphology without altering the intrinsic QPE mechanism. By applying PM and SIS lens models and fitting the Obs1 light curve with a two-image lensing scenario, the authors find a time delay of about $1.9$ hours and a demagnification of the secondary image around $0.9$, with consistent lensing parameters across five energy bands. Spectral analysis supports a common origin for the two sub-eruptions, showing similar $N_H$ and disk-blackbody temperatures, which reinforces the lensing interpretation. An optical-depth calculation suggests such lensing events are rare, but microlensing scenarios could raise the likelihood under certain configurations; overall, the results point to gravitational lensing as a plausible mechanism for the observed complexity in eRO-QPE1 Obs1, with future work needed to test this across more QPEs and epochs.

Abstract

Quasi-Periodic Eruptions (QPEs) are recurrent X-ray eruptions discovered so far in the nuclei of low-mass galaxies. However, despite considerable observational progress, the origin of QPEs remains unclear. A variety of models have been proposed to explain their nature, but a definitive understanding has yet to be reached. Recently, chaotic mixtures of multiple overlapping eruptions with varying amplitudes have been observed in eRO-QPE1 obs1-features not reported in any other known QPE sources. This complex behavior presents a challenge to the existing QPE models. In this paper, we propose that the overlapping features may be the result of gravitational lensing. We analyze the light curve of eRO-QPE1 and compare its features to predictions from gravitational lensing scenarios. We discuss the implications for the trigger mechanism of QPEs in general. We show that the unique overlapping features observed in eRO-QPE1 may be naturally reproduced by gravitational lensing effects, without invoking a different physical origin from other known QPE sources.

Figures (8)

• Figure 1: The geometry of point mass lensing system. The source is marked as $\rm{S}$, and the observer is located at $\rm{O}$. $D_{\rm S}$ and $D_{\rm L}$ are the distance from the source to the observer and the lens object to the observer, respectively. $D_{\rm LS}$ is the separation between source and lens object. $\alpha, \beta, \theta$ and $\xi$ represents the deflection angle, angular position of the source without lens, image position and impact parameter, respectively.
• Figure 2: The upper panel shows the best fitting results of eRO-QPE1 obs1 data in the energy ranges of $0.2$ - $10$ keV (purple), $0.2$ - $0.4$ keV (blue), $0.4$ - $0.6$ keV (orange), $0.6$ - $0.8$ keV (green), and $0.8$ - $1.0$ keV (red). In all five energy bands, the solid lines are derived from Equation. \ref{['eq20']} combined with the posterior parameter distributions of the fitting results, the dashed lines from Equation. \ref{['eq18']} combined with the posterior parameter distributions of the fitting results, and the dot-dashed lines from Equation. \ref{['eq19']} combined with the posterior parameter distributions of the fitting results. The lower panel shows the residual of each data point.
• Figure 3: The phase-resolved spectra of obs1 in the energy bands of 0.2-0.4 keV, 0.4-0.6 keV, 0.6-0.8 keV and 0.8-1.0keV are presented. The left and right columns show the first and second sub-eruptions (or profile), respectively. These four energy bands are fitted with a power-law model. (Continued on Figure \ref{['spec-all-b']})
• Figure 4: (Continued from Figure \ref{['spec-all']}) The phase-resolved spectra of obs1 in the energy band of 0.2-10 keV are presented. The left and right columns show the first and second sub-eruptions (or profile), respectively. The 0.2-10 keV band is fitted with the $tbabs$ and $diskbb$ model.
• Figure 5: Temporal evolution of the spectral power-law index compared for the two sub-eruptions. The photon index of the first sub-eruptions are represented by squares, while those of the second sub-eruptions are denoted by circles. Different energy bands are color-coded as follows: 0.2–0.4 keV (blue), 0.4–0.6 keV (orange), 0.6–0.8 keV (green), and 0.8–1.0 keV (red).