The neutron stars of Soft X-ray Transients

Abstract

Abstract. Soft X-ray Transients (SXRTs) have long been suspected to contain old, weakly magnetic neutron stars that have been spun up by accretion torques. After reviewing their observational properties, we analyse the different regimes that likely characterise the neutron stars in these systems across the very large range of mass inflow rates, from the peak of the outbursts to the quiescent emission. While it is clear that close to the outburst maxima accretion onto the neutron star surface takes place, as the mass inflow rate decreases, accretion might stop at the magnetospheric boundary because of the centrifugal barrier provided by the neutron star. For low enough mass inflow rates (and sufficiently short rotation periods), the radio pulsar mechanism might turn on and sweep the inflowing matter away. The origin of the quiescent emission, observed in a number of SXRTs at a level of ∼ 10 32 − 10 33 erg s −1 , plays a crucial role in constraining the neutron star magnetic field and spin period. Accretion onto the neutron star surface is an unlikely mechanism for the quiescent emission of SXRTs, as it requires very low magnetic fields and/or long spin periods. Thermal radiation from a cooling neutron star surface in between the outbursts can be ruled out as the only cause of the quiescent emission.We find that accretion onto the neutron star magnetosphere and shock emission powered by an enshrouded radio pulsar provide far more plausible models. In the latter case the range of allowed neutron star spin periods and magnetic fields is consistent with the values recently inferred from the properties of kHz quasi-periodic oscillation in low mass X-ray binaries. If quiescent SXRTs contain enshrouded radio pulsars, they provide a missing link between X-ray binaries and millisecond pulsars.

The optical counterparts of accreting millisecond X-ray pulsars during quiescence

“…These properties clearly associate SXTs with LMXBs containing an old, weakly magnetized, neutron star (for a review see, e.g. Campana et al 1998). In the following we will refer to these systems as low-mass X-ray transients (LMXTs).…”

“…The same behaviour is seen in the quiescent optical light curve of IGR J00291+5934 (D'Avanzo et al 2007) 2 . In a theoretical study on SAX J1808.4−3658 Burderi et al (2003) proposed, on the base of previous works (Stella et al 1994;Campana et al 1998), that the irradiation is due to the release of rotational energy by the fast spinning neutron star, switched on, as a radio pulsar, during quiescence. Following this idea, Campana et al (2004) and D'Avanzo et al (2007) measured the required irradiating luminosity needed to match the optical flux of SAX J1808.4−3658 and IGR J00291+5934, respectively, and found that it is a factor of about 100 larger than the observed quiescent X-ray luminosity for both systems.…”

Improved mass and radius constraints for quiescent neutron stars in ω Cen and NGC 6397

“…The distances to globular clusters are well-known, with a 6% rms variation among different distance measurement techniques (Woodley et al 2012). NSs in qLMXBs produce thermal (blackbody-like) X-rays due to heating of the NS core (and crust) during periods of accretion (Brown, Bildsten & Rutledge 1998;Rutledge et al 2002b), c 0000 RAS and seem to produce both thermal X-rays and nonthermal X-rays (typically fit with a power-law) by accretion, though other physics may be involved in the poorly-understood power-law (Campana et al 1998;Cackett et al 2010;Deufel, Dullemond & Spruit 2001). NSs in qLMXBs are believed to have low magnetic fields, and in most cases pure hydrogen atmospheres, allowing for robust physical modeling (Zavlin, Pavlov & Shibanov 1996;Rajagopal & Romani 1996;Heinke et al 2006;Haakonsen et al 2012).…”

Neutrino emission from neutron stars

“…The main constituents of neutron star cores are thought to be strongly degenerate neutrons with the admixture of protons and electrons, although other particles may also be available. Observationally, neutron stars manifest themselves in different ways, for instance, as radio pulsars, X-ray pulsars, anomalous X-ray pulsars (e.g., Mereghetti et al 1998), X-ray bursters (e.g., Lewin et al 1995), X-ray transients (e.g., Campana et al 1998), sources of quasiperiodic X-ray oscillations (e.g., Psaltis et al 1998, Kluzniak 1998, and soft-gamma repeaters (e.g., Cline et al 1999 and references therein). They are the sources of electromagnetic radiation in all wavelength bands, from radio to hardest gamma-rays, and they are known to work as efficient accelerators of highly energetic particles.…”