Confirmation of XMMU J010429.4-723136 as a high-mass X-ray binary

ATel #5674; P. Maggi, R. Sturm, F. Haberl, G. Vasilopoulos (MPE)
on 19 Dec 2013; 11:13 UT
Credential Certification: Richard Sturm (rsturm@mpe.mpg.de)

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A series of observations performed with Swift/XRT (Target ID 33042) in the SMC detected a relatively bright X-ray source at the edge of the field of view. The source is consistent with XMMU J010429.4-723136. It is classified as a high-mass X-ray binary (HMXB) candidate in the XMM-Newton point-source catalogue of the SMC (Sturm et al. 2013, A&A 558, A3), based on X-ray hardness ratios and association with the early-type star [M2002] SMC 59367 (= star 4018857 of the MCPS catalogue, Zaritsky et al. 2002, AJ, 123, 855). The first X-ray detection, with ROSAT PSPC, is reported in Haberl et al. (2000, A&AS 172, 41) where the source is given the identifier [HFP2000] 264. Analyses of the optical counterpart light curve reveal a period of 37.15 d (Rajoelimanana et al. 2011, ATel #3154) or 0.972 d (Schmidtke et al. 2013, MNRAS 431, 252).
Rajoelimanana et al. (2011, ATel #3154) associated a Chandra source (CXOU J010428.7-723134) to the same star, even though the optical counterpart was outside the X-ray error circle. The Chandra position precision was likely reduced because the source was at the edge of the detector. For the same reason, the period of 707 s reported in ATel #3154 is only measuring the dithering period of the Chandra spacecraft, which causes the source to move in and out of the field of view with that period. Therefore, this is not the spin period of the X-ray source.

All Swift/XRT detections of the source are listed in the Table below. Count rates (corrected for vignetting, dead times, and PSF losses) are converted to X-ray flux (0.3-10 keV) and luminosity assuming an absorbed power-law spectrum with N_H=1×10²¹ cm^-2 and a spectral index Γ=1, at the SMC distance of 60 kpc. X-ray variability over a few days is apparent. Furthermore, the source is ~50 times brighter than when detected with XMM-Newton (on MJD 55267.1), and ~400 times brighter than the 3σ upper limit derived on MJD 55149.1 in a 22 ks observation, when the source was not detected. The high X-ray variability is atypical for an AGN and provide the first evidence that XMMU J010429.4-723136 is indeed a HMXB.
Based on that X-ray variability and the secured association to the optical counterpart given by the XMM-Newton position, we confirm that XMMU J010429.4-723136 is a HMXB, with likely a Be star companion. Further observations are needed to reveal the spin period of the compact object, the orbital period of the system, and the spectral type of the companion.

 
Start date    Exp     Count rate          Flux               LX                                  
 [MJD]        [s]   [10-2 count s-1] [10-12 erg cm-2 s-1] [1035 erg s-1] 
56627.09      622     1.45 ± 0.63         1.02              4.4 
56627.57      849     1.03 ± 0.45         0.72              3.1 
56628.43      812     1.22 ± 0.50         0.86              3.7 
56630.51      512     1.43 ± 0.70         1.00              4.3 
56631.17      717     3.44 ± 0.86         2.41             10.4 
56631.78      679     3.70 ± 0.95         2.59             11.2 
56632.30      722     1.36 ± 0.56         0.95              4.1 
56632.57      819     1.09 ± 0.49         0.76              3.3 
56632.72      714     1.75 ± 0.62         1.23              5.3 
56633.44      829     2.32 ± 0.67         1.63              7.0 
56633.71      729     4.09 ± 0.92         2.87             12.4 
56639.24     3903     1.69 ± 0.27         1.19              5.1 
56643.31     3516     0.95 ± 0.21         0.66              2.8 

We thank the Swift team for accepting and scheduling the target of opportunity observations.