The discovery of a 2.78 hour periodic modulation of the X-ray flux from globular cluster source Bo 158 in M31

We report the discovery of periodic intensity dips in the X-ray source XMMU J004314.1+410724, in the globular cluster Bo 158 in M31. The X-ray flux was modulated by ~83% at a period of 2.78 hr (10,017 s) in an XMM-Newton observation taken 2002 January 6-7. The X-ray intensity dips show no energy dependence. We detected weaker dips with the same period in observations taken 2000 June 25 (XMM-Newton) and 1991 June 26 (ROSAT/PSPC). The amplitude of the modulation has been found to be anticorrelated with source X-ray flux: it becomes lower when the source intensity rises. The energy spectrum of Bo 158 was stable from observation to observation, with a characteristic cutoff at ~4-6 keV. The photoelectric absorption was consistent with the Galactic foreground value. No significant spectral changes were seen in the course of the dips. If the 2.78 hr cycle is the binary period of Bo 158, the system is highly compact, with a binary separation of ~10^11 cm. The association of the source with a globular cluster, together with spectral parameters consistent with Galactic neutron star sources, suggests that X-rays are emitted by an accreting neutron star. The properties of Bo 158 are somewhat reminiscent of the Galactic X-ray sources exhibiting diplike modulations. We discuss two possible mechanisms explaining the energy-independent modulation observed in Bo 158: (1) the obscuration of the central source by highly ionized material that scatters X-rays out of the line of sight and (2) the partial covering of an extended source by an opaque absorber that occults varying fractions of the source.