ALBERT

Description: XMM–Newton observations of the supergiant fast X-ray transient IGR J17544–2619 are reported and placed in the context of an analysis of archival INTEGRAL /IBIS data that provide a refined estimate of the orbital period at 4.9272 ± 0.0004 d. A complete outburst history across the INTEGRAL mission is reported. Although the new XMM–Newton observations (each lasting ~15 ks) targeted the peak flux in the phase-folded hard X-ray light curve of IGR J17544–2619, no bright outbursts were observed, the source spending the majority of the exposure at intermediate luminosities of the order of several 10 33 erg s –1 (0.5–10 keV) and displaying only low level flickering activity. For the final portion of the exposure, the luminosity of IGR J17544–2619 dropped to ~4 x 10 32 erg s –1 (0.5–10 keV), comparable with the lowest luminosities ever detected from this source, despite the observations being taken near to periastron. We consider the possible orbital geometry of IGR J17544–2619 and the implications for the nature of the mass transfer and accretion mechanisms for both IGR J17544–2619 and the supergiant fast X-ray transients (SFXTs) population. We conclude that accretion under the ‘quasi-spherical accretion’ model provides a good description of the behaviour of IGR J17544–2619 and suggests an additional mechanism for generating outbursts based upon the mass accumulation rate in the hot shell (atmosphere) that forms around the neutron star under the quasi-spherical formulation. Hence, we hope to aid in explaining the varied outburst behaviours observed across the SFXT population with a consistent underlying physical model.