Cooling of accretion disc coronae by Type I X-ray bursts

Abstract

ABSTRACT Although accretion disc coronae appear to be common in many accreting systems, their fundamental properties remain insufficiently understood. Recent work suggests that Type I X-ray bursts from accreting neutron stars provide an opportunity to probe the characteristics of coronae. Several studies have observed hard X-ray shortages from the accretion disc during an X-ray burst implying strong coronal cooling by burst photons. Here, we use the plasma emission code eqpair to study the impact of X-ray bursts on coronae, and how the coronal and burst properties affect the coronal electron temperatures and emitted spectra. Assuming a constant accretion rate during the burst, our simulations show that soft photons can cool coronal electrons by a factor of ≳ 10 and cause a reduction of emission in the 30–50 keV band to $\lesssim 1{{\ \rm per\ cent}}$ of the pre-burst emission. This hard X-ray drop is intensified when the coronal optical depth and aspect ratio is increased. In contrast, depending on the properties of the burst and corona, the emission in the 8–24 keV band can either increase, by a factor of ≳ 20, or decrease, down to $\lesssim 1{{\ \rm per\ cent}}$ of the pre-burst emission. An increasing accretion rate during the X-ray burst reduces the coronal cooling effects and the electron temperature drop can be mitigated by $\gtrsim 60{{\ \rm per\ cent}}$. These results indicate that changes of the hard X-ray flux during an X-ray burst probe the geometrical properties of the corona.