Were most Low Mass X ray Binaries born in Globular Clusters?

Abstract

We summarize the status of art of the secular evolution of low mass X-ray binaries (LMXBs) and take a close look at the orbital period distribution of LMXBs and of binary millisecond pulsars (MSP), in the hypothesis that this latter results from the LMXB evolution. The deficiency of systems below the period gap, which in cataclysmic binaries occurs between ~ 2 and 3 hr, points to a very different secular evolution of LMXBs with respect to their counterparts containing a white dwarf compact object. The presence of several ultrashort period LMXBs (some of which are also X-ray millisecond pulsars), the important fraction of binary MSPs at periods between 0.1 and 1 day, the periods (26 and 32hr) of two ``interacting'' MSPs in Globular Clusters are other pieces of the puzzle in the period distribution. We consider the possible explanations for these peculiarities, and point out that Grindlay's old proposal that all (most of) LMXBs in the field were originally born in globular clusters must be carefully reconsidered.