Abstract
We report on the Swift monitoring of the candidate supergiant fast X-ray transient (SFXT) IGR J16418-4532, for which both orbital and spin periods are known (~3.7d and ~1250s, respectively). Our observations, for a total of ~43ks, span over three orbital periods and represent the most intense and complete sampling of the light curve of this source with a sensitive X-ray instrument. With this unique set of observations we can address the nature of this transient. By applying the clumpy wind model for blue supergiants to the observed X-ray light curve, and assuming a circular orbit, the X-ray emission from this source can be explained in terms of the accretion from a spherically symmetric clumpy wind, composed of clumps with different masses, ranging from ~5E16 g to 1E21g. Our data suggest, based on the X-ray behaviour, that this is an intermediate SFXT.
Highlights
The X-ray transient IGR J16418−4532 was discovered by INTEGRAL during observations of the black hole X-ray transient 4U 1630−47 on 2003 February 1–5 (Tomsick et al 2004), with a 20– 40 keV flux of 3 × 10−11 erg cm−2 s−1
We considered the Burst Alert Telescope (BAT) data in the time range MJD 53414–55754
With a standard folding analysis of the system barycentre (SSB) light curves, we measured an orbital period of 3.740 ± 0.002 d (1σ error), which is fully consistent with the value obtained by Levine et al (2011), Porb = 3.73886 ± 0.00028 d
Summary
The X-ray transient IGR J16418−4532 was discovered by INTEGRAL during observations of the black hole X-ray transient 4U 1630−47 on 2003 February 1–5 (Tomsick et al 2004), with a 20– 40 keV flux of 3 × 10−11 erg cm−2 s−1. Sguera et al (2006) reported on further INTEGRAL observations, taken on 2004 February 26, during which fast (∼1 h) X-ray outbursts were observed that peaked at ∼80 mCrab (20–30 keV) This behaviour led Sguera et al (2006) to propose that IGR J16418−4532 is a candidate supergiant fast X-ray transient (SFXT). SFXTs (e.g. Sguera et al 2005) are highmass X-ray binaries (HMXBs) associated with OB supergiant stars via optical spectroscopy that show an X-ray dynamic range of three to five orders of magnitude They are characterized by bright (peak luminosities of 1036–1037 erg s−1), short (a few hours, as observed by INTEGRAL; Sguera et al 2005; Negueruela et al 2006) X-ray outbursts significantly shorter than those of typical Be/X-ray binaries, and a quiescent luminosity of ∼1032 erg s−1 (e.g. in’t Zand 2005; Bozzo et al 2010). This Swift monitoring represents the most intense and complete sampling along the orbital period of the light curve of this source with a sensitive X-ray instrument
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