XMM-Newtonobservations of 1A 0535+262 in quiescence

Abstract

Accretion onto magnetized neutron stars is expected to be centrifugally inhibited at low accretion rates. Several sources, however, are known to pulsate in quiescence at luminosities below the theoretical limit predicted for the onset of the centrifugal barrier. The source 1A 0535+262 is one of them. 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 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. This correlation has been associated with the truncation of the accretion disk at the magnetosphere boundary. We argue that our result, along with other arguments previously reported in the literature, suggests that the accretion in quiescence also proceeds from an accretion disk around the neutron star. The proposed scenario consistently explains the energy of the cyclotron line observed in 1A 0535+262, and the timing properties of the source including the spin frequency evolution within and between the outbursts, and the frequency of the break in power spectrum.

Awakening of the Fast-spinning Accreting Be/X-Ray Pulsar A0538–66∗

“…This is supported by the observation of accretion episodes at luminosities below the transition limit between the accretion and the centrifugal inhibition regimes observed in other X-ray binaries (e.g. Rutledge et al 2007;Doroshenko et al 2014). In the framework of the idea proposed by Zhang et al (1998) to explain the hard X-ray spectrum of Aql X−1, the soft spectral component of A0538−66 could be produced by the accretion of matter onto the NS surface.…”

The quiescent state of the accreting X-ray pulsar SAX J2103.5+4545

“…At least in two occasions, 1A 0535+262 (Negueruela et al 2000;Doroshenko et al 2014) and IGR J21343+4738 (Reig & Zezas 2014), X-ray pulsed emission was detected during an apparent disk-loss phase. SAX J2103.5+4545 is the third case of pulsations detected during the absence of the Be disk, but it is the first time that X-ray pulsations are detected at a luminosity level of ∼ 10 33 erg s −1 in the energy range 0.5-10 keV (see Table 2).…”

Meta-stable low-level accretion rate states or neutron star crust cooling in the Be/X-ray transients V0332+53 and 4U 0115+63

“…Proposed mechanisms include very low-level accretion onto the neutron star magnetic poles (strong evidence exist for this scenario for A0535+26; e.g. Rothschild et al 2013;Doroshenko et al 2014), although here it is also unclear how matter can leak through the magnetosphere at such low rates. It could also be due to emission from c 0000 RAS accretion onto the magnetosphere.…”