Charting the Temperature of the Hot Neutron Star in a Soft X-Ray Transient

Abstract

We explore the thermal evolution of a neutron star undergoing episodes of intense accretion, separated by long periods of quiescence. By using an exact cooling code, we follow in detail the flow of heat in the star due to the time-dependent accretion-induced heating from pycnonuclear reactions in the stellar crust, to the surface photon emission, and to the neutrino cooling. These models allow us to study the neutron stars of the soft Xray transients. In agreement with recent work of Brown, Bildsten, & Rutledge, we conclude that the soft component of the quiescent luminosity of Aql X-1, of 4U 1608Ϫ522, and of the recently discovered SAX J1808.4 can be understood as thermal emission from a cooling neutron star with negligible neutrino emission. However, we show that, in the case of Cen X-4, despite its long recurrence time, strong neutrino emission from the neutron star inner core is necessary to understand the observed low ratio of quiescent to outburst luminosity. This result implies that the neutron star in Cen X-4 is heavier than the one in the other systems and the pairing critical temperature in its center must be low enough (well below 10 9 K) to avoid a strong suppression of the neutrino T c emission.

Constraints on the neutron star magnetic field of the two X-ray transients SAX J1808.4–3658 and Aql X–1

“…The presence of a weak, but not negligible, magnetic field in LMXBs has been invoked to explain some observational facts such as the QPO at ∼20−60 Hz (the so-called low-frequency QPO in atoll sources or horizontal branch oscillations, HBOs, in Z sources; , or the disappearance of the kHz QPOs at low and high inferred mass accretion rates (e.g. Campana 2000;Cui 2000). In particular, linking the kHz QPO observability to variations of the neutron star magnetospheric radius, in response to changes in the mass accretion rate, Campana (2000) estimates a magnetic field of B ∼ (0.3−1) × 10 8 Gauss for Aql X-1 and of B ∼ (1−8) × 10 8 Gauss for Cyg X-2.…”

An [ITAL]XMM-Newton[/ITAL] Study of the 401 H[CLC]z[/CLC] Accreting Pulsar SAX J1808.4−3658 in Quiescence

“…The soft component is usually ascribed to the cooling of the neutron star surface powered by the deep nuclear heating that the neutron star receives during each outburst (Brown et al 1998;Rutledge et al 2000;Colpi et al 2001; see also Campana et al 1998a 5 # 10 previous estimates (Bildsten & Chakrabarty 2001). This mean mass inflow rate translates into a soft quiescent luminosity of 10 32 ergs s Ϫ1 within the 0.5-10 keV energy band from deep crustal heating (Brown et al 1998;Colpi et al 2001). This is a factor of ∼10 higher than observed.…”

“…Given the fact that predictions from deep nuclear heating are quite robust, one is led to conclude that an additional source of cooling is present. A simple and well-known solution is when the direct Urca process is allowed in the neutron star core; then, in turn, the neutrino cooling does affect the neutron star thermal evolution (Colpi et al 2001). This can occur only for massive neutron stars with masses higher than 1.7-1.8 M , .…”

ABeppoSAXObservation of KS 1731−260 in Its Quiescent State: Constraints on the Magnetic Field of the Neutron Star

“…This scenario applies if the system has time to reach a steady state, in which the heat deposited during short and frequent outbursts in the NS is equal to the luminosity radiated in quiescence. Colpi et al (2001) have explored the thermal evolution of a NS undergoing episodes of accretion, lasting for a time t out , separated by periods of quiescence, lasting t rec , in the hypothesis that the thermal luminosity in quiescence is dominated by emission from the hot NS core, with which the crust and atmosphere of the NS are in thermal equilibrium. Adopting an outburst luminosity of 10 37 ergs s À1 and a quiescent luminosity of 10 33 ergs s À1 , this model predicts a ratio of the recurrence time t rec to the outburst time t out of $70 for a 1.4 M NS and less than 4 for a NS more massive than 1.7 M .…”