A Millisecond Pulsar in an Eclipsing Binary

Abstract

We report the discovery of a remarkable pulsar with period 1.6 ms, moving in a nearly circular 9.17 hour orbit around a low mass companion star. At an observing frequency of 430 MHz, the pulsar, PSR 1957+20, is eclipsed once each orbit for about 50 minutes. For a few minutes before an eclipse becomes complete, and for more than 20 minutes after the signal reappears, the pulses are delayed by as much as several hundred microseconds — presumably as a result of propagation through plasma surrounding the companion. The pulsar’s orbit about the system barycenter has a radius of 0.089 light seconds projected onto the line of sight. The observed orbital period and size, together with the fact that eclipses occur, imply a surprisingly low companion mass, only a few percent the mass of the sun. The eclipsing mass extends well beyond the Roche lobe of the companion, and appears to possess a comet-like tail, strongly uggesting that the pulsar is evaporating its companion.