Astron. Astrophys. 326, 662-668 (1997)

1. Introduction

The present pulsar birthrate and the number of supernovae required to account for the heavy-element abundance suggest up to 10⁹ neutron stars existing in the Milky Way. The young neutron stars are observable as radio pulsars, through their thermal emission from the cooling surface or in X-ray binaries. Recently a group of X-ray pulsars has been suggested as single neutron stars accreting from a disk, the remnant of common-envelope evolution of a high mass X-ray binary (Mereghetti & Stella 1995, van Paradijs et al. 1995).

After about yr when the radio emission has faded away and the temperature cooled below 10⁵ K, isolated neutron stars become invisible for us. However a fraction of the old neutron stars may be detected in the EUV to soft X-ray energy band when accreting from the interstellar medium (Ostriker et al. 1970). Estimates for the number of old, isolated neutron stars detectable in the ROSAT all-sky survey range from about 2000 to 10000 (Treves & Colpi 1991, Blaes & Madau 1993, Madau & Blaes 1994). However it will be very difficult to prove the nature of the majority of faint sources to confirm these numbers.

Despite of intensive searches however (e.g. Danner 1996, Belloni et al. 1997), evidence for only one candidate for an isolated old neutron star has been found today (RX J1856.5-3754, Walter et al. 1996, Neuhäuser et al. 1997). Walter et al. (1997) identified a blue object of about 26th magnitude as probable optical counterpart to the X-ray source. The ROSAT PSPC spectrum of RX J1856.5-3754 is soft (blackbody temperature kT = 57 eV) and little absorbed (N = 1.4  10 cm^-2 ). The distance is limited by a molecular cloud located behind RX J1856.5-3754 to 120 pc, yielding an observed luminosity (5 10³¹ (d/100pc)² erg s^-1 ) consistent with that predicted for a slow moving neutron star accreting from the ambient interstellar medium.

The discrepancy between the predicted number of neutron stars to be seen in the ROSAT all-sky survey and the observations is further strengthened by upper limits derived from nearly complete identifications of area-selected survey sources. Motch et al. (1997) found from the small number of unidentified sources in a 65 square degree field in Cygnus a factor of 10 lower limit than most model predictions.

We report here the discovery of RX J0720.4-3125, a likely isolated pulsating neutron star. The X-ray properties (Sect. 2.1) are very similar to those of RX J1856.5-3754. Optical observations, presented in Sect. 2.2, failed so far in finding a counterpart. We discuss the properties of RX J0720.4-3125 with respect to its evolutionary status.

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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