Swift/XRT detection of a possible 1.2 day period the faint persistent X-ray source AXJ1538.3-5541

ATel #1209; J. A. Kennea (PSU), S. Campana (INAF-OAB), R. Wijnands (UvA), D. N. Burrows (PSU), E. Cackett (U. Michigan), N. Degenaar (UvA), D. Grupe (PSU), V. La Parola (INAF-IASF), A. Moretti (INAF-OAB), C. Pagani (PSU), R. L.C. Starling (U. Leicester) and N. Gehrels (NASA/GSFC)
on 8 Sep 2007; 17:40 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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Over the period of August 30th to September 7th, 2007, during the reactivation phase of Swift, XRT has been performing frequent observations of AXJ1538.3-5541, a faint ASCA detected point source first reported by Sugizaki et al. (2001, ApJS, 134, 77) and thought to be a low-mass X-ray binary. Aside from the recent monitoring, four observations have also been performed of this target in 2007, between January 5th and March 16th. XRT detects a point source inside the ASCA error circle located at the following coordinates:

RA(J2000) = 15h 38m 13.8s
Dec(J2000) = -55d 42m 11.9s

with an estimated error of 3.6 arcseconds (90% confidence level), approximately 1 arcminute away from the ASCA measured position. Note that this position is based on the observations of AXJ1538.3-5541 taken before the August 11th Swift attitude control system problems (GCN #6760). We note the presence of two 2MASS point sources close to this error circle, one, 2MASS 15381365-5542096, lies inside the error circle 2.3 arcseconds from the XRT position, and another, 2MASS 15381404-5542085, lies just outside the XRT 90% error circle 3.8 arcseconds from the XRT position. Both 2MASS objects have similar IR brightnesses (J~14.4).

The spectrum of AXJ1538.3-5541 is well described by an absorbed power-law model with NH = (8.6 +/- 2.0) x 10²² cm, and a photon index of 2.4 +/- 0.5 (errors 90% confidence) with an average absorbed flux of 10-11 erg/s/cm² (0.2 - 10 keV) or approximately 10³⁵ erg/s assuming a distance of 5 kpc and correcting for absorption. The spectral parameters and X-ray flux are consistent with those reported by Sugizaki et al. (2001).

The lightcurve of AXJ1538.3-5541 is highly variable and timing analysis shows hints of a periodicity of 1.23 +/- 0.05 days, with an amplitude of approximately 40-50% (although we note that the light-curve is not sinusoidal). We speculate that this period seems too long to be the spin period of either a white dwarf or a neutron star so it is likely to be the binary period of the system. Given its length this period might represent the orbital period of a neutron star LMXB system.

Observations of this source are on-going during Swift's reactivation phase.