Swift observed an outburst from the supergiant fast X-ray transients (SFXT) AX J1841.0-0536 on 2010 June 5, and followed it with XRT for 11 days. 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, 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~2x10^22 cm-2), a flat power law (Gamma~0.2), and a high energy cutoff. 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.
Supergiant fast X-ray transients (SFXTs) are a new class of High Mass X-ray Binaries (HMXBs) discovered by INTEGRAL (e.g. Sguera et al. 2005) that are associated with OB supergiant stars via optical spectroscopy. In the X-rays they display outbursts significantly shorter than those of typical Be/X-ray binaries characterized by bright flares with peak luminosities of 10 36 -10 37 erg s -1 which last a few hours (as observed by INTEGRAL ; Sguera et al. 2005;Negueruela et al. 2006). As their quiescence is characterized by a luminosity of ∼ 10 32 erg s -1 (e.g. in't Zand 2005; Bozzo et al. 2010), their dynamic range is of 3-5 orders of magnitude. While in outburst, their hard X-ray spectra resemble those of HMXBs hosting accreting neutron stars, with hard power laws below 10 keV combined with high energy cut-offs at ∼ 15-30 keV, sometimes strongly absorbed at soft energies (Walter et al. 2006;Sidoli et al. 2006). So, even if pulse periods have only been measured for a few SFXTs, it is tempting to assume that all SFXTs might host a neutron star. The mechanism producing the outbursts is still being debated, and it is probably related to either the properties of the wind from the supergiant companion (in't Zand 2005;Walter & Zurita Heras 2007;Negueruela et al. 2008;Sidoli et al. 2007) or to the presence of a centrifugal or magnetic barrier (Grebenev & Sunyaev 2007;Bozzo et al. 2008). AX J1841.0-0536 was discovered during ASCA observations of the Scutum arm region performed on 1994 April 12, and 1999 October 3-4 as a flaring source which exhibited flux increases by a factor of 10 (up to ∼ 10 -10 erg cm -2 s -1 ) with rising times on the order of 1 hr (Bamba et al. 2001), a strong absorption NH = 3 × 10 22 cm -2 , and coherent pulsations with a period of 4.7394 ± 0.0008 s. A Chandra observation on 2004 May 12, which provided the coordinates refined to arcsecond accuracy [RA(J2000) = 18 h 41 m 0. s 54, Dec(J2000) = -5 • 35 ′ 46. ′′ 8, Halpern & Gotthelf 2004], found the source at a much fainter level (4 × 10 -12 erg cm -2 s -1 ), and with a spectrum that was fit with an absorbed power-law model [Γ = 1.35 ± 0.30, NH = (6.1 ± 1.0) × 10 22 The source was also detected in the 20-60 keV energy range in subsequent observations, at a flux roughly half that of the initial peak. Halpern & Gotthelf (2004) identified IGR J18410-0535 as AX J1841.0-0536. Halpern et al. (2004) established that the IR counterpart was 2MASS 18410043-0535465, a reddened star with a weak double-peaked Hα emission line, initially classified as a Be star, which Nespoli et al. (2008) later reclassified as B1 Ib type star; this corroborated the evidence that AX J1841.0-0536 is a member of the SFXT class, as proposed by Negueruela et al. (2006). Sguera et al. (2009) presented the first broad-band spectrum of this source, obtained with INTEGRAL (IBIS+JEM-X), that they fit with an absorbed power-law with Γ = 2.5 ± 0.6, NH = 23 +19 -14 × 10 22 cm -2 . In 2007 Swift (Gehrels et al. 2004) observed the outburst of the periodic SFXT IGR J11215-5952 (Romano et al. 2007), which allowed us to discover that the accretion phase during the bright outbursts lasts much longer than a few hours, as seen by lower-sensitivity instruments. This is contrary to what was initially thought at the time of the discovery of this new class of sources. Between 2007 October 26 and 2008 November 15, AX J1841.0-0536 was observed by Swift as part of a sample of 4 SFXTs which included IGR J16479-4514, XTE J1739-302, and IGR J17544-2619. The main aims were to characterize their longterm behavior, to determine the properties of their quiescent state, to monitor the onset of the outbursts and to measure the outburst recurrence period and duration (Sidoli et al. 2008;Romano et al. 2009cRomano et al. , 2010c)). Approximately two observations per week were collected with the X-ray Telescope (XRT, Burrows et al. 2005) and the UV/Optical Telescope (UVOT, Roming et al. 2005). During such an intense and sensitive monitoring, AX J1841.0-0536 was the only SFXT that did not go through a bright outburst, although several on-board Burst Alert Telescope (BAT, Barthelmy et al. 2005) detections have been recorded (Romano et al. 2009c).
In this paper we report on the observations of the first outburst of AX J1841.0-0536 observed by Swift on 2010 June 5 and we compare its properties with those of the prototype of the SFXT class, IGR J17544-2619, which went into a bright outburst on 2010 March 04. Romano et al. 2010a). This is the first outburst of AX J1841.0-0536 detected by the BAT for which Swift performed a slew, thus allowing broad-band data collection. The source was detected in a 1344 s BAT image trigger, during a pre-planned observation, and there is an indication that the source was already in outburst before this observation began and well after it ended. The XRT began observing the field rather late, at 17:51:50 UT (T + 1708 s), after the very long BAT image trigger. The automated target (AT, sequ
This content is AI-processed based on open access ArXiv data.
