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

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 approximately 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 detections 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 thousand 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.

The accretion environment of supergiant fast X-ray transients probed withXMM-Newton

“…In the case of the SFXT prototype IGR J17544-2619, large increases in the absorption column density were not detected during either the many outbursts observed with Swift /XRT (Romano et al 2008a(Romano et al , 2011a, XMM-Newton (Bozzo et al 2016a), and Suzaku (Rampy et al 2009), or during the fainter flares analyzed here. The spectral variability seems more related to changes in the power-law photon index and so to the emission mechanism intrinsic to the source (see the exhaustive discussion in Bozzo et al 2016b, and references therein), while the absorption column density does not change dramatically between quiescence and the very bright X-ray states (in 't Zand 2005).…”

“…In that occasion, an increase by a factor of ∼10 of the absorption column density in the direction of the source preceding the flare revealed that a massive and dense structure was rapidly approaching the NS, giving first rise to a large increase in flux (a factor of ∼1000) before obscuring the compact object and finally moving away from the observer line of sight. Bozzo et al (2016a) also fortuitously caught with XMM-Newton an episode of strongly enhanced X-ray activity from the SFXT IGR J17544-2619 that reached the typical luminosity of an X-ray outburst but displayed only a marginal increase in the local absorption column density (a factor of 2). Estimating the physical properties of the clumps in a reliable way from these observations have been proved complicated, due to our poor knowledge of the spin period and magnetic field of the NS in the SFXTs.…”

“…The HR-resolved spectral analysis probes directly when a source is undergoing a significant spectral variation, without the need to assume a priori that such changes can occur either during periods of lower/higher X-ray activity or in specific time intervals. This technique was already exploited in two of our previous publications and allowed us to prove that the bright flare observed by XMM-Newton from the SFXT IGR J18410-0535 was most likely related to the ingestion of a massive clump by the NS hosted in this system (Bozzo et al 2011), while the outburst caught from the SFXT prototype IGR J17544-2619 was shown to require the simultaneous inter-play of additional mechanisms to give rise to the measured dynamic range in the Xray luminosity (Bozzo et al 2016a). The conclusion in the case of IGR J17544-2619 was mainly derived from the fact that no particularly large increase of the accretion column density was recorded during this event, although it achieved an X-ray luminosity a factor of 10 larger than that of the X-ray flare caught from IGR J18410-0535 and the latter displayed evidence for an increase in the local N H by a factor of ∼10 preceding the Xray flare.…”

The THESEUS space mission concept: science case, design and expected performances

“…THESEUS will provide excellent coverage of the outbursts of classical neutron star and black hole X-ray binaries, for example Supergiant Fast X-ray Transient variability reaches up to a factor of 10 6 (Romano et al, 2015), with peak luminosities up to 10 38 erg s −1 ; the hourtimescale flares from these OB plus (presumed) NS systems have frequently triggered the Swift-BAT. THESEUS can constrain the temperature and optical depth of the accretion column (Farinelli et al, 2012;Bozzo et al, 2016), and the origin of the bright flares possibly due to wind accretion onto a magnetar (Bozzo et al, 2008). THESEUS will detect and provide localization for several black-hole transients, monitoring daily their X-ray spectral evolution throughout their full outburst.…”

Towards a Unified View of Inhomogeneous Stellar Winds in Isolated Supergiant Stars and Supergiant High Mass X-Ray Binaries

“…In this case the source is the SFXT prototype IGR J17544-2619 and the lightcurve is obtained from the longest continuous observational campaign performed in X-rays on one of these sources (ID. 0744600101; (Bozzo et al, 2016)). The more extreme variability compared to the classical SgXB can be immediately seen by comparing this lightcurve with that on the top panel.…”