The Spectrum of the 8.7 s X-ray Pulsar 4U 0142+61

Abstract

We investigate the properties of the 8.7 s X-ray pulsar 4U 0142+61 using new data obtained with the ASCA observatory and archival data from the Einstein and ROSAT observatories. New measurements of the pulse period from 1979 and 1994 confirm that 4U 0142+61 is spinning down on a timescale of 127,000 yr. The ASCA spectrum is featureless and requires two components consisting of a ~0.4 keV blackbody plus a power law with a photon index of ~3.7. The blackbody flux is ~40% the total and for a distance greater than 0.5 kpc covers more than 12% of the neutron star surface. This covering fraction is 2 orders of magnitude larger than expected for thin disk accretion onto a magnetized neutron star. These results suggest 4U 0142+61 is probably not a low-mass X-ray binary system, but rather is an isolated pulsar undergoing a combination of spherical and disk accretion. The observed properties seem consistent with the suggestion by van Paradijs, Taam, & van den Heuvel that this pulsar is powered by accretion from the remnant of a Thorne-Żytkow object (TŻO). The ROSAT PSPC image shows a dust-scattering halo that is a factor of 2 less than predicted by the measured equivalent hydrogen column density of 8 × 1021 cm-2, suggesting half of the absorbing material is located in the vicinity of the pulsar and possibly the remains of the TŻO envelope.