Periodic X-ray sources in the Galactic bulge: application of the Gregory–Loredo algorithm

Abstract

ABSTRACT We present a systematic study of periodic X-ray sources in the Limiting Window (LW), a ∼70 arcmin2 field representative of the inner Galactic bulge and the target of ∼1 Ms Chandra observations. Using the Gregory–Loredo algorithm, which applies Bayes’s theorem to the phase-folded light curve and is well suited for irregularly sampled X-ray data, we detect 25 periodic signals in 23 discrete sources, among which 15 signals are new discoveries and two sources show dual periods. The vast majority of the 23 periodic sources are classified as magnetic cataclysmic variables (CVs), based on their period range, X-ray luminosities, spectral properties, and phase-folded light curves that are characteristic of spin modulation. Meanwhile, there is a paucity of non-magnetic CVs seen as periodic sources, which can be understood as due to a low detection efficiency for eclipsing sources. Under reasonable assumptions about the geometry of magnetic CVs and a large set of simulated X-ray light curves, we estimate the fraction of magnetic CVs in the inner Galactic bulge to be ≲23 per cent, which is similar to that in the solar neighbourhood. There is an apparent lack of long-period (≳3.3 h) CVs in the LW, when contrasted with the range of known CVs in the solar neighbourhood. We suggest that this might be an age effect, in the sense that CVs in the inner bulge are more evolved systems and have substantially shrunk their orbits.