Repeated Short-term Spectral Softening in the Low/Hard State of the Galactic Black-Hole Candidate Swift J1753.5-0127

TL;DR

This study reports repeated short-term spectral softening events in the low/hard state of the galactic black-hole candidate Swift J1753.5-0127, characterized by increased soft X-ray emission and decreased hard X-ray emission, linked to short-term accretion rate increases.

Contribution

First detailed analysis of short-term spectral softening events in Swift J1753.5-0127, revealing changes in disk and Comptonized components during these episodes.

Findings

Softening events last for tens of days.

Inner disk temperature increases during softening.

Disk flux does not follow F ∝ Tin^4 relation.

Abstract

We report MAXI and Swift observations of short-term spectral softenings of the galactic black-hole X-ray binary Swift J1753.5-0127 in the low/hard state. These softening events are characterized by a simultaneous increase of soft X-rays (2-4 keV) and a decrease of hard X-rays (15-50 keV) lasting for a few tens of days. The X-ray energy spectra during the softening periods can be reproduced with a model consisting of a multi-color disk blackbody and its Comptonized component. The fraction of the Comptonized component decreased from 0.30 to 0.15 when the spectrum became softer; meanwhile the inner disk temperature (Tin) increased from 0.2 to 0.45 keV. These results imply that the softening events are triggered by a short-term increase of the mass accretion rate. During the observed spectral softening events, the disk flux (F) and Tin did not obey the relation: F is proportional to Tin^4, suggesting that the inner disk radius does not reach the innermost stable circular orbit.