A BeppoSAX Observation of KS 1731-260 in its Quiescent State: Constraints on the NS Magnetic Field
Abstract
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 (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 to less than 109 G. If the quiescent emission originates from the release of thermal energy accumulated through deep crust heating in the previous accretion phase, then the measured quiescent luminosity requires a neutron star mass of more than 1.7 M or a very long outbust recurrence time ( 103 yr). The former possibility would be in agreement with the expectation of the recycling scenario, namely that the neutron star has accreted a significant amount of mass, enough to spin it up to millisecond periods.
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