Detection of a new X-ray outburst from the SMC Be/X-ray binary Swift J010745.0-722740, confirming the 1180 d orbital period

ATel #10253; G. Vasilopoulos, F. Haberl (MPE, Germany), P. Maggi (IRFU, CEA/DRF - CNRS, France)
on 10 Apr 2017; 15:56 UT
Credential Certification: Georgios Vasilopoulos (gevas@mpe.mpg.de)

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Swift_J010745.0-722740 is a BeXRB candidate located in the SMC (ATel #5778) with no X-ray pulsations detected so far. Its companion exhibits strong (0.3-0.4 mag) outbursts, with a recurring time of 1180 d (ATel #5781, #5778).
The previous optical outburst started on 2014 Jan 6 and coincided with a serendipitous detection by Swift/XRT. We followed the outburst with XRT up to Lx~2.5×10³⁶ erg s^-1, detecting a complex multi-peaked behavior, while determining a rise time of the event of ~15-20 days
The next X-ray outburst was expected to occur in April 2017 and should peak on MJD 57857 (2017 April 14), thus we started to monitor the system with Swift/XRT to confirm and study the anticipated outburst. The system was not detected (upper limit: L_X≤2×10³⁵ erg s^-1) by Swift/XRT on 2017 March 25, while a 1ks Swift ToO on 2017 April 10 (following a period where the system was not visible for Swift) measures an L_X=2.3×10³⁶ erg s^-1 (0.067 c/s for XRT). The unabsorbed fluxes were estimated assuming an absorbed power-law spectrum with N_H=1.5×10²¹ cm^-2 and a spectral index Γ=1, at the SMC distance of 62 kpc. We note that the current X-ray luminosity is in agreement with the one detected during the 2014 outburst for the same phase around the peak luminosity (ATel #5778).
Further observations of this rare event occurring only once every 3.2 years are encouraged.