Swift J053041.9-665426 is most likely a Be/X-ray binary
ATel #3753; R. Sturm, F. Haberl, W. Pietsch, (Max-Planck-Institut fuer extraterrestrische Physik)
on 9 Nov 2011; 17:15 UT
Credential Certification: Richard Sturm (rsturm@mpe.mpg.de)
Subjects: X-ray, Binary, Neutron Star, Transient
Swift J053041.9-665426 was discovered as new X-ray transient in the LMC on 2011 Nov. 6 (ATel #3747).
The proposed optical counterpart was found to be highly variable (ATel #3751).
A Swift on-axis follow-up observation (00032172001) with 986 s exposure on 2011 Nov. 8 from 16:43 to 17:00 (UT)
yields an improved X-ray position and spectrum.
The position is R.A. = 05:30:41.9 and Dec. = -66:54:29 with uncertainty of 3.6 arcsec.
This decreases the distance to the proposed optical counterpart from 4.5 to 1.9 arcsec,
affirming the correct identification.
The spectrum was modelled as described in ATel #3747.
From the new spectrum with higher statistics (535 compared to 163 net counts),
we obtain a LMC column density of less than 9.5 ×1020 cm-2
and a power-law photon index of 1.15±0.17.
The detected flux increased to (4.1±0.3) ×10-11 erg cm-2 s-1,
which corresponds to an unabsorbed luminosity of 1.3 ×1037 erg s-1.
The current power-law photon index is typical for a neutron star in a high mass X-ray binary.
The brightness of the optical counterpart and the high long term X-ray variability are atypical for a supergiant system.
This allows an identification of the system as classical Be/X-ray binary.
We thank the Swift team for scheduling the ToO observation.
