2FGL J1653.6–0159: A NEW LOW IN EVAPORATING PULSAR BINARY PERIODS

We have identified an optical binary with orbital period P_b = 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 P_b = 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 ± 0.05 M _☉. 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