Fast X-Ray Transients from Dense Circumstellar Material Shock Breakout and Cooling Emission

Abstract In this Letter, we explore the soft X-ray shock breakout (SBO) and cooling emission of supernovae interacting with preexplosion dense circumstellar material (CSM) shells as a subclass of extragalactic fast X-ray transients. For geometrically thin CSM shells, we find that the SBO luminosities span a range of a few × 10 43 –10 45 erg s −1 , with peak times typically ≲10 4 s, both of which are primarily determined by the light-travel effect. When the shell’s outer radius is located at ∼100–500 R ⊙ , the SBO temperatures fall into the energy band covered by the current soft X-ray facilities. We derive analytic lightcurves for SBOs in dense CSM shells and explore how different physical parameters affect the observed lightcurves and the temperature evolution. Despite the multitemperature nature of the emission caused by the light-travel effect leading to nonthermal-like spectra, we find that the time-resolved spectra within the limited soft X-ray band can still be well described by a blackbody model. This provides a natural explanation for the observed blackbody-like time-resolved spectra of soft X-ray SBO events. Our model can reproduce the observed X-ray lightcurves and temperature evolutions of the three known X-ray SBO events, EP260321a, XRF 060218, and XRO 080109, revealing a diversity in the properties of the dense CSM shells ejected by stripped-envelope progenitors before the explosion. Future discoveries of more soft X-ray SBO events by facilities such as Einstein Probe will provide opportunities for probing the episodic mass-loss history of massive stars in the final stages before core collapse.