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Abstract
Accretion onto magnetized neutron stars is expected to be centrifugally inhibited at low accretion rates. Several sources including 1A 0535+262, however, are known to pulsate in quiescence at luminosities below the theoretical limit predicted for the onset of the centrifugal barrier. Here we present the results of an analysis of a ~ 50 ks long XMM-Newton observation of 1A 0535+262 in quiescence. At the time of the observation, the neutron star was close to the apastron, and the source had remained quiet for two orbital cycles. In spite of this, we detected a pulsed X-ray flux of ~ 3×10−11 erg cm−2 s−1 . Several observed properties, including the power spectrum, remained similar to those observed in the outbursts. Particularly, we have found that the frequency of the break detected in the quiescent noise power spectrum follows the same correlation with flux observed when the source is in outburst. We argue that, along with other arguments previously reported in the literature, our results suggest that the accretion in quiescence also proceeds from an accretion disk around the neutron star.
Highlights
Like many accreting pulsars [5,6,7] lighcurves of 1A 0535+262 exhibit a break in power density spectrum
At the time of the observation, the neutron star was close to the apastron, and the source had remained quiet for two orbital cycles
Along with other arguments previously reported in the literature, our results suggest that the accretion in quiescence proceeds from an accretion disk around the neutron star
Summary
Like many accreting pulsars [5,6,7] lighcurves of 1A 0535+262 exhibit a break in power density spectrum. Using RXTE observations, [6] were able to show that the frequency of the break is correlated with flux, and proportional to the Keplerian frequency at the inner edge of the accretion disk, where it is disrupted by the magnetosphere 1A 0535+262 is best studied at high luminosities It is one of a few accreting pulsars observed to pulsate outside of outbursts at lower fluxes as well [8], which is commonly defined as quiescence.
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