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Swift J010745.0-722740, a new SMC Be/X-ray binary with possibly a very long orbital period

ATel #5778; P. Maggi, R. Sturm, F. Haberl, G. Vasilopoulos (MPE, Germany), A. Udalski (Warsaw University Astronomical Observatory, Poland)
on 19 Jan 2014; 13:52 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Pierre Maggi (pmaggi@mpe.mpg.de)

Subjects: Optical, X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 5781, 5925, 14724

A series of observations performed with Swift/XRT (Target ID 33042) in the SMC detected a variable X-ray source within the field of view. The source appeared in observations performed on and after 2014 Jan 06, while it remained undetected in the period 2013 Dec 13 - 24. The XRT J2000 position is RA = 01:07:45.00, DEC = -72:27:40.9, (90% error radius of 4.0"). This position matches [MCS2008] 206, detected with Chandra on 2006 Feb 10.
All XRT detections and upper limits of Swift J010745.0-722740 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 NH=1.5×1021 cm-2 and a spectral index Γ=1, at the SMC distance of 60 kpc. We add the flux measured with Chandra, converting the value listed by McGowan et al. (2008, MNRAS 383, 330) to the 0.3-10 keV energy range, assuming the same power-law spectrum. Using archival XMM-Newton observations, we estimate a conservative upper limit of 10-14 erg cm-2 s-1. Strong X-ray variability is evident: up to now the source is ~90 times brighter than when detected with Chandra, and ~370 times brighter than the lowest upper limit, provided by XMM-Newton.
The strong X-ray variability argues against the AGN classification of McGowan et al. (2008), which was based on the source X-ray hardness and X-ray-to-optical flux ratio with the nearest neighbour, a V~19.8 mag object 1.32" away from the Chandra position. Instead, we propose that the emission-line star [MA93] 1641, only 1.37" away from the Chandra position, is the actual optical counterpart. In combination with the strong X-ray variability, we propose that Swift J010745.0-722740 is a new Be/X-ray binary (BeXRB) in the SMC.
The long-term OGLE light curve of the proposed companion exhibit strong (0.3-0.4 mag) outbursts, with a recurring time of 1180 d. The latest outburst started after 2014 Jan 9. The detection of a simultaneous X-ray outburst secures the association of the X-ray source with the Be star and thus our classification. If the outburst recurring time reflects the orbital period of the system, this would make Swift J010745.0-722740 the BeXRB with the longest orbital period known to date. We will follow the current outburst in optical and X-rays. Further observations are needed to reveal the spin period of the compact object. We encourage optical spectroscopy of the companion during the current outburst.

 
Sequence   Start date      Exp     Count rate         Flux              L  
             [MJD]         [s]   [10-3 count/s] [10-14 erg cm-2 s-1] [1035 erg s-1] 

Chandra 53776.82 9630 --- 3.6 0.16 XMM 54202.30 23000 --- < 1 < 0.04 Swift 26 56639.24 3903 < 2.27 < 16.4 < 0.71 Swift 27 56643.31 3516 < 1.88 < 13.6 < 0.59 Swift 28 56647.03 3594 < 1.84 < 13.3 < 0.57 Swift 29 56650.97 3756 < 2.58 < 18.7 < 0.81 Swift 30 56663.61 1848 6.26 ± 2.2 45.3 1.95 Swift 31 56667.34 1536 19.4 ± 4.1 140.4 6.05 Swift 32 56671.20 3452 23.6 ± 3.0 170.8 7.36 Swift 33 56675.13 3971 46.9 ± 3.7 339.4 14.62

We thank the Swift team for accepting and scheduling the observations.

OGLE-IV Light Curve for Swift J010745.0-722740 on the XROM web site (Udalski, 2008, Acta Astron., 58, 187)