A BeppoSAX Observation of KS 1731–260 in Its Quiescent State: Constraints on the Magnetic Field of the Neutron Star

We report here the results of a 90 ks BeppoSAX observation of the low-mass X-ray binary and atoll source KS 1731-260 during a quiescent phase. From this observation we derive a source X-ray luminosity of ~1033 ergs s-1 (for a source distance of 7 kpc). If the neutron star is spinning at a period of a few milliseconds, as inferred from the nearly coherent oscillations observed during type I X-ray bursts, the quiescent X-ray luminosity constrains the neutron star magnetic field strength. We consider all the mechanisms that have been proposed to explain the quiescent X-ray emission of neutron star X-ray transients and compare the corresponding expectations with the measured upper limit on the X-ray luminosity. We find that, in any case, the neutron star magnetic field is most probably less than ~109 G. We have also observed KS 1731-260, still in its quiescent state, at 1.4 GHz with the Parkes radio telescope to search for radio pulses. We found that no radio signals with millisecond periods are present with an upper limit on the flux of 0.60 mJy using a 4 minute integration time (optimal for a close system with an orbital period smaller than a few hours) and of 0.21 mJy using a 35 minute integration time (optimal for a wide-orbit system).