2FGL J1653.6-0159: A new low in evaporating pulsar binary periods

Abstract

We have identified an optical binary with orbital period Pb = 4488 s as the probable counterpart of the Fermi source 2FGL J1653.6-0159. Although pulsations have not yet been detected, the source properties are consistent with an evaporating millisecond pulsar binary; this Pb = 75 minutes is the record low for a spin-powered system. The heated side of the companion shows coherent radial-velocity variations, with amplitude K = 666.9 ± 7.5 km s-1 for a large mass function of f (M) = 1.60 ± 0.05M⊙. This heating suggests a pulsar luminosity ∼3×1034 erg s-1. The colors and spectra show an additional blue component dominating at binary minimum. Its origin is, at present, unclear. This system is similar to PSR J1311-3430, with a low-mass H-depleted companion, a dense shrouding wind and, likely, a large pulsar mass.