Eclipses of the ablating binary pulsar PSR1744–24A

Abstract

PSR1744–24A, a binary pulsar in the globular cluster Terzan 5, is eclipsed for up to half of each 109-minute orbit^1,2. This is a much longer duration than its 0.09-solar-mass companion star could cause if it were confined within its Roche lobe, and has led to the suggestion that PSR1744–24A is ablating material from its companion, producing an extensive stellar wind that eclipses the radio signal. We have measured the pulsar light curve at a frequency of 1.67 GHz, and find that the eclipse is not total: maximum attenuation in some cases is only about 70 per cent. The pulsar signal is substantially delayed during the eclipse. We model these observations by means of free-free absorption and dispersion in an ionized wind, processes which also explain the frequency dependence of the eclipse duration³. A simplified model predicts a wind density at the Roche surface of ˜6x 10⁷ electrons cm⁻³ and an electron temperature in the wind of (3.6–15) x 10³ K, although some theoretical difficulties remain. Such a wind is unlikely to ablate the companion star entirely in less than a Hubble time, so that in this one case at least, ablation may not ultimately form an isolated millisecond pulsar.