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

We have identified an optical binary with orbital period P_b=4488s as the\nprobable counterpart of the Fermi source 2FGL J1653.6-0159. Although pulsations\nhave not yet been detected, the source properties are consistent with an\nevaporating millisecond pulsar binary; this P_b=75min is the record low for a\nspin-powered system. The heated side of the companion shows coherent radial\nvelocity variations, with amplitude K=666.9+/-7.5 km/s for a large mass\nfunction of f(M)=1.60+/-0.05 M_sun. This heating suggests a pulsar luminosity\n~3x10^34 erg/s. The colors and spectra show additional hard emission dominating\nat binary minimum. Its origin is, at present, unclear. This system is similar\nto PSR J1311-3430, with a low mass H-depleted companion, a dense shrouding wind\nand, likely, a large pulsar mass.\n