Confirmation of the supergiant fast X-ray transient nature of AX J1841.0-0536 from Swift outburst observations

Swift observed an outburst from the supergiant fast X-ray transient (SFXT) AX J1841.0-0536 on 2010 June 5, and followed it with X-ray Telescope (XRT) for 11 d. The X-ray light curve shows an initial flare followed by a decay and subsequent increase, as often seen in other SFXTs, and a dynamical range of ~1600. Our observations allow us to analyse the simultaneous broad-band (0.3-100 keV) spectrum of this source, for the first time down to 0.3 keV, which can be fitted well with models usually adopted to describe the emission from accreting neutron stars in high-mass X-ray binaries, and is characterized by a high absorption (N_H ~ 2 × 10²² cm^-2), a flat power law (Γ ~ 0.2) and a high-energy cut-off. All of these properties resemble those of the prototype of the class, IGR J17544-2619, which underwent an outburst on 2010 March 4, whose observations we also discuss. We show how well AX J1841.0-0536 fits in the SFXT class, based on its observed properties during the 2010 outburst, its large dynamical range in X-ray luminosity, the similarity of the light curve (length and shape) to those of the other SFXTs observed by Swift and the X-ray broad-band spectral properties.