A new algorithm for detecting X-ray shots in Cyg X-1

TL;DR

This paper introduces a novel algorithm for detecting X-ray shots in Cyg X-1, enabling more sensitive identification of short, low-amplitude flares and revealing their dependence on accretion rate and spectral properties.

Contribution

The authors develop a new shot detection algorithm based on baseline detection and template fitting, improving sensitivity over previous methods.

Findings

Shot amplitude and recurrence rate scale with mean count rate.

Higher mass accretion rate leads to more and slightly larger shots.

No rapid spectral hardening process is needed to explain observed spectral changes.

Abstract

The short-term X-ray variability of Cyg X-1 can be interpreted as random occurrence of mini-flares known as the shots, whose physical nature is still unclear. We propose a new algorithm for shot identification in the X-ray light curve, based on baseline detection and template fitting. Compared with previous techniques, our algorithm allows us to detect shots with lower amplitudes and shorter time separations. With NICER observations, we find that, after correction for detection sensitivity, both the shot amplitude and recurrence rate are positively scaled with the mean count rate, while the recurrence rate has a much higher dependence on the count rate. These suggest that a higher mass accretion rate will drive more and slightly larger shots. We also find that the abrupt hardening near the shot peak found in previous studies is attributed to different shot profiles in different energy bands; there is no need to involve a rapid physical process to suddenly harden the emitting spectrum.