Spin down during quiescence of the fastest known accretion-powered pulsar

¹ Dipartimento di FisicaUniversitá degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy
e-mail: apapitto@oa-cagliari.inaf.it
² INAF - Osservatorio Astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy
³ Dipartimento di Scienze Fisiche ed Astronomiche, Universitá di Palermo, via Archirafi 36, 90123 Palermo, Italy

Received: 21 April 2010
Accepted: 20 January 2011

Abstract

We present a timing solution for the 598.89 Hz accreting millisecond pulsar, IGR J00291+5934, using Rossi X-ray Timing Explorer data taken during the two outbursts exhibited by the source on 2008 August and September. We estimate the neutron star spin frequency and we refine the system orbital solution. To achieve the highest possible accuracy in the measurement of the spin frequency variation experienced by the source in-between the 2008 August outburst and the last outburst exhibited in 2004, we re-analysed the latter considering the whole data set available. We find that the source spins down during quiescence at an average rate of  Hz s^-1. We discuss possible scenarios that can account for the long-term neutron star spin-down in terms of either magneto-dipole emission, emission of gravitational waves, and a propeller effect. If interpreted in terms of magneto-dipole emission, the measured spin down translates into an upper limit to the neutron star magnetic field, B ≲ 3 × 10⁸ G, while an upper limit to the average neutron star mass quadrupole moment of Q ≲ 2 × 10³⁶ g cm² is set if the spin down is interpreted in terms of the emission of gravitational waves.