Testing the Shock-cooling Emission Model from Star-disk Collisions for Quasi-periodic Eruptions

TL;DR

The study critically tests the shock-cooling emission model from star-disk collisions as the origin of quasi-periodic eruptions by deriving $R_ullet$ constraints from observed $L_{ m p}$, $t_{ m e}$, and $T_{ m p}$ across a diverse QPE/QPE-like sample. It constructs a cohesive framework linking disk properties, collision hydrodynamics, and radiative diffusion to yield two independent constraints on the stellar radius and a temperature relation, then applies these to eight sources. The results show that, for most events, consistent parameter combinations are absent or require implausibly small stars that would be tidally disrupted, and the predicted temperatures are systematically lower than observed, highlighting significant tensions with the simplest star-disk collision picture. Only a subset of sources, notably eRo-QPE3, can be accommodated with a Sun-like star under tidal constraints, while retrograde-orbit scenarios offer marginal relief at low probability. Overall, the work challenges the universality of the shock-cooling star-disk collision model for QPEs and motivates consideration of alternative mechanisms or more complex physics.

Abstract

Quasi-periodic eruptions (QPEs), the repeated outbursts observed in soft X-ray bands, have attracted broad interest, but their physical origin is under debate. One of the popular models, the star-disk collision model, suggests that QPEs can be produced through periodic collisions of an orbiting star with the accretion disk of a central black hole (BH). However, previous tests of the star-disk collision model mainly focus on the timing analysis. Other observed properties, such as peak luminosities $L_{\rm{p}}$, durations $t_{\rm{e}}$, and radiation temperatures $T_{\rm{p}}$ of the eruptions, are not systematically investigated. For a sample of six QPE sources and two QPE-like sources, we test \textbf{the shock-cooling emission model from star-disk collisions} by using these observables to derive the constraints on the stellar radius $R_\star$. We find that, except for two sources (eRo-QPE3 and eRo-QPE4), the rest of the sample either has no allowed $R_\star$ to simultaneously reproduce the observed $L_{\rm{p}}$ and $t_{\rm{e}}$, or the required $R_\star$ is too large to avoid being disrupted by the central BH. For the two exceptions, a stellar radius of the order of $1\ R_{\rm{\odot}}$ is necessary to satisfy all the constraints. Another issue with the simplest version of this model is that it predicts $k T_{\rm{p}} \sim 10\ \rm{eV}$, one order of magnitude lower than the observed value.

Figures (4)

• Figure 1: The constraints on the stellar radius for GSN 069 (left panel) and RX J1301 (right panel). The red and blue lines stand for the constraints based on Eq. (\ref{['eq: combine']}) and Eq. (\ref{['eq:lp-lq']}), respectively. The solid and dashed lines are for different adopted values of $0.1$ and $0.01$ for the model parameter ($\alpha$ or $\eta$), respectively. The horizontal lines with downward arrows represent the full tidal disruption limit (green, Eq. \ref{['eq:r-star-max']}, $n = 1$ and $b=1$) and the partial tidal disruption limit (black, Eq. \ref{['eq:r-star-max']}, $n=2$ and $b=1$), respectively.
• Figure 2: The constraints on the stellar radius for other four QPE sources. The legends are the same as those in the left panel of Figure \ref{['fig:gsn']}.
• Figure 3: The constraints of stellar radius for longer time-scale repeating sources: ASASSN-14ko and J0230. The legends are the same as those in the left panel of Figure \ref{['fig:gsn']}.
• Figure 4: The comparison of the peak radiation temperatures $T_{\rm{p}}$ predicted by the shock-cooling emission model from star-disk collisions (solid lines) and those observed (crosses with error bars) for the sample. The predicted $T_{\rm{p}}$ is calculated from Eq. (\ref{['eq:Tp']}). The horizontal error bars of the observed $T_{\rm{p}}$ correspond to the estimated $M_{\rm{h}}$ range of each individual source, respectively.