Mon. Not. R. Astron. Soc., 499, 4479-4489 (2020/December-2)
Cooling of accretion disc coronae by Type I X-ray bursts.
SPEICHER J., BALLANTYNE D.R. and MALZAC J.
Abstract (from CDS):
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 ≲1 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 ≲1 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 ≳60 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.