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ATEL # 937; E. M. Cackett, J. M. Miller (University of Michigan)
on 7 Nov 2006; 23:45 UT
Password Certification: Edward Cackett (email@example.com)
Subjects: X-ray, Binaries, Black Holes, Neutron Stars
Using radial velocity curves from time resolved spectroscopy Geier et al. (2006, astro-ph/0610467) recently identified 4 subluminous B (sdB) stars in close binaries with possible neutron star or black holes. However, none of these sources have any X-ray counterparts in the ROSAT all sky survey, as would be expected if the sources were actively accreting low-mass X-ray binaries. We searched for the X-ray counterpart of one of the sources, HE 0532-4503 with a ~1 ksec Swift XRT observation. HE 0532-4503 has a period of 0.266 days and a compact object with a mass in the range 1.4 - 3.6 solar masses. However, no X-ray source was detected in the Swift observations. This provides an upper-limit to the source count-rate of 1.03E-3 counts/s, which corresponds to a flux of (1.0 - 7.0)E-14 erg/cm2/s, for a reasonable range in spectral shape, and N_H = 5e20 cm-2. The corresponding upper limit on the source luminosity is (2.0 - 9.0)(d/kpc)^2 E30 erg/s, which for any source within our galaxy is significantly less than the luminosity of actively accreting low-mass X-ray binaries. We therefore suggest that if HE 0532-4503 is a neutron star/black hole binary, it is not actively accreting.