Multi-wavelength observations of IGR J17544-2619 from quiescence to outburst

TL;DR

This study presents a comprehensive multi-wavelength observational campaign of the supergiant fast X-ray transient IGR J17544-2619, capturing its spectral variability during quiescence and outburst, and analyzing its properties with high-sensitivity X-ray, optical, infrared, and UV data.

Contribution

It provides new simultaneous multi-wavelength data and spectral analysis of IGR J17544-2619, including the absence of cyclotron lines and spin period detection, enhancing understanding of its transient behavior.

Findings

Source remained mostly in low emission state during monitoring.

No cyclotron line or spin period confirmed in broad-band spectrum.

Optical and infrared magnitudes showed micro-variability consistent with supergiant stars.

Abstract

In this paper we report on a long multi-wavelength observational campaign of the supergiant fast X-ray transient prototype IGR J17544-2619. A 150 ks-long observation was carried out simultaneously with XMM-Newton and NuSTAR, catching the source in an initial faint X-ray state and then undergoing a bright X-ray outburst lasting about 7 ks. We studied the spectral variability during outburst and quiescence by using a thermal and bulk Comptonization model that is typically adopted to describe the X-ray spectral energy distribution of young pulsars in high mass X-ray binaries. Although the statistics of the collected X-ray data were relatively high we could neither confirm the presence of a cyclotron line in the broad-band spectrum of the source (0.5-40 keV), nor detect any of the previously reported tentative detection of the source spin period. The monitoring carried out with Swift/XRT during the same orbit of the system observed by XMM-Newton and NuSTAR revealed that the source remained in a low emission state for most of the time, in agreement with the known property of all supergiant fast X-ray transients being significantly sub-luminous compared to other supergiant X-ray binaries. Optical and infrared observations were carried out for a total of a few thousands of seconds during the quiescence state of the source detected by XMM-Newton and NuSTAR. The measured optical and infrared magnitudes were slightly lower than previous values reported in the literature, but compatible with the known micro-variability of supergiant stars. UV observations obtained with the UVOT telescope on-board Swift did not reveal significant changes in the magnitude of the source in this energy domain compared to previously reported values.