Thermal state of transiently accreting neutron stars

Abstract

Abstract. We study thermal states of transiently accreting neutron stars (with mean accretion ratesṀ ∼ 10 −14 − 10 −9 M⊙ yr −1 ) determined by the deep crustal heating of accreted matter sinking into stellar interiors. We formalize a direct correspondence of this problem to the problem of cooling neutron stars. Using a simple toy model we analyze the most important factors which affect the thermal states of accreting stars: a strong superfluidity in the cores of low-mass stars and a fast neutrino emission (in nucleon, pion-condensed, kaon-condensed, or quark phases of dense matter) in the cores of high-mass stars. We briefly compare the results with the observations of soft X-ray transients in quiescence. If the upper limit on the quiescent thermal luminosity of the neutron star in SAX J1808. 4-3658 (Campana et al. 2002) is associated with the deep crustal heating, it favors the model of nucleon neutron-star cores with switched-on direct Urca process.

Thermal states of neutron stars with a consistent model of interior

“…We then obtain for a NS of given mass and composition of the envelope so-called Yakovlev et al 2003). We will compute such cooling and heating curves for various superfluid models using the gaps calculated with the same nuclear interaction model as the EOS.…”

Models of crustal heating in accreting neutron stars

“…The luminosity in quiescence depends on the structure of neutronstar core, and particularly on the rate of neutrino cooling. This opened a new possibility of exploring the internal structure and equation of state of neutron stars via confrontation of theoretical models with observations of qui-escent SXTs (see Colpi et al 2001;Rutledge et al 2002;Yakovlev et al 2003Yakovlev et al , 2004Levenfish & Haensel 2007 ).…”

Consistency between deep crustal heating of strange stars in superbursters and soft X-ray transients

“…They are in fact general enough to also describe neutron stars at the same level of detail, and indeed Eq. (7) corresponds entirely to case (ii.a) in Yakovlev et al (2003) for low-mass neutron stars with slow neutrino emission. Thus much of what follows would also apply for neutron stars -although in that case it has of course been done in much greater detail.…”

“…113 eV (Yakovlev et al 2003;Rutledge et al 2002a) and therefore T i,9 0.3 using a canonical mass-radius relationship, R = 10 km, M/R = 0.2, and…”

“…When accretion stops, the heat deposited at the surface quickly radiates away, and so in the absence of any residual accretion the surface temperature should reflect the temperature of the core including the effects of deep crustal heating. This has been thoroughly investigated for neutron stars in the literature (Brown et al 1998;Ushomirsky & Rutledge 2001;Colpi et al 2001;Brown et al 2002;Rutledge et al 2002a;Yakovlev et al 2003, and references therein) establishing deep crustal heating as an unavoidable heating mechanism in the core, which should at least provide a "rock bottom" luminosity for these sources. Residual accretion may still play a role however -indeed the power law components in the spectra of many of these sources are often taken as a sign of residual accretion or shock emission from a rotation powered pulsar -and furthermore the properties of the core remains uncertain, so the precise nature of the quiescent emission is not firmly established.…”