Are There Magnetars in High‐Mass X‐Ray Binaries? The Case of Supergiant Fast X‐Ray Transients

Abstract

In this paper we summarize some aspects of the wind accretion theory in high mass X-ray binaries hosting a magnetic neutron star and a supergiant companion. In particular, we concentrate on the different types of interaction between the inflowing wind matter and the neutron star magnetosphere that are relevant when accretion of matter onto the neutron star surface is largely inhibited; these include inhibition by the centrifugal and magnetic barriers. We show that very large luminosity swings (∼10 4 or more on time scales as short as hours) can result from transitions across different regimes. This scenario is then applied to the activity displayed by supergiant fast X-ray transients (SFXTs), a new class of high mass X-ray binaries in our galaxy recently discovered with INTEGRAL. According to this interpretation we argue that SFXTs which display very large luminosity swings and host a slowly spinning neutron star are expected to be characterized by magnetar-like fields. Supergiant fast X-ray transients might thus provide a unique opportunity to detect and study accreting magnetars in binary systems. 7th INTEGRAL Workshop

Transient outburst mechanisms in Supergiant Fast X-ray Transients

“…This property seems to be in contrast with the hypothesis suggested by Bozzo et al (2008) that SFXTs are magnetars. Moreover, Bozzo et al (2008) suggest that, to get the large dynamic ranges observed in SFXTs, these transients should host neutron stars with long spin periods (of the order of 1000 s or larger), which is not observed in SFXTs, to date: indeed, four SFXTs are X-ray pulsars with much shorter spin periods: IGR J11215-5952 (187 s, Swank et al 2007), AX J1841.0-0536 (4.7 s, Bamba et al 2001), IGR J18483-0311 (21 s, Sguera et al 2007b), IGR J16465-4507 (228 s, Lutovinov et al 2005). The large dynamic range between the quiescence and the maximum luminosity at the peak of the flares can, in any case, be explained already if the supergiant winds are clumpy with density contrasts as large as 10 5 (e.g.…”

“…the cutoff energies) are consistent with a pulsar magnetic field around a few 10 12 G, assuming the empirical correlation (although never theoretically confirmed) between the high energy cutoff in X-ray pulsars and the observed cyclotron energies (Coburn et al 2002). This property seems to be in contrast with the hypothesis suggested by Bozzo et al (2008) that SFXTs are magnetars. Moreover, Bozzo et al (2008) suggest that, to get the large dynamic ranges observed in SFXTs, these transients should host neutron stars with long spin periods (of the order of 1000 s or larger), which is not observed in SFXTs, to date: indeed, four SFXTs are X-ray pulsars with much shorter spin periods: IGR J11215-5952 (187 s, Swank et al 2007), AX J1841.0-0536 (4.7 s, Bamba et al 2001), IGR J18483-0311 (21 s, Sguera et al 2007b), IGR J16465-4507 (228 s, Lutovinov et al 2005).…”

“…Bozzo et al (2008) applied this idea to SFXTs hosting a highly magnetized neutron star [see also Grebenev & Sunyaev (2007) and references therein; Goetz et al (2007)]. Wind accretion in a HMXB containing a magnetized neutron star depends on three different radii: accretion radius [R acc ; Bondi & Hoyle (1944)], corotation radius (R cor ) and magnetospheric radius (R m ).…”

“…This only applies if the three radii driving the accretion display very similar values. Bozzo et al (2008) show that an abrupt X-ray luminosity jump from the quiescence (at about 10 32 erg s −1 ) to the peak of an outburst in a SFXT (10 36 -10 37 erg s −1 ) can be explained only if the the neutron star spins slowly (∼1000 s or larger) and if it displays a magnetar-like magnetic field (B∼10 14 G), in presence of a relatively mild change in the wind density (one or two orders of magnitude, instead of the at least 4 orders of magnitude requested by the clumpy wind model discussed above). Note that this model requires in any case a variability (although mild) in the wind parameters (such as clumps, or any other structure in the wind with higher density than the smooth wind).…”

Exploration of spin-down rate of neutron stars in high-mass X-ray binaries

“…Although many authors (e.g. Pringle & Rees 1972;Illarionov & Sunyaev 1975;Davies & Pringle 1981;Wang & Robertson 1985;Ikhsanov 2001;Bozzo, Falanga & Stella 2008;Shakura et al 2012) have investigated the propeller effect extensively, the efficiency of angular momentum loss during the propeller phase is still uncertain. Davies & Pringle (1981) suggested that the propeller phase can be divided into two subphases: supersonic propeller phase and subsonic propeller phase.…”