The complex Halpha line profile of the bright companion to PSR J1740-5340 in NGC 6397

Abstract

We present a detailed study of the Halpha and He I spectral features of COM J1740-5340 (the companion to PSR J1740-5340 in the Galactic Globular Cluster NGC 6397), exploiting a series of high resolution spectra obtained at different orbital phases. The Halpha absorption line shows a complex two-component structure, revealing that optically thin hydrogen gas resides outside the Roche lobe of COM J1740-5340. The line morphology precludes the existence of any residual disk around the millisecond pulsar, and suggests the presence of a stream of material going from the companion toward the neutron star. This material never reaches the neutron star surface, being driven back by the pulsar radiation far beyond COM J1740-5340. By analyzing the He I absorption lines as a function of orbital phase, we infer the presence of an overheated longitudinal strip (about 150 times narrower than it is long) on the COM J1740-5340 surface facing the radio pulsar.