Neutron star crust cooling in KS 1731−260: the influence of accretion outburst variability on the crustal temperature evolution

Abstract

Using a theoretical model, we track the thermal evolution of a cooling neutron star crust after an accretion induced heating period with the goal of constraining the crustal parameters. We present for the first time a crust cooling model -NSCool -that takes into account detailed variability during the full outburst based on the observed light curve. We apply our model to KS 1731-260. The source was in outburst for ∼12 years during which it was observed to undergo variations on both long (years) and short (days-weeks) timescales. Our results show that KS 1731-260 does not reach a steady state profile during the outburst due to fluctuations in the derived accretion rate. Additionally, long time-scale outburst variability mildly affects the complete crust cooling phase, while variations in the final months of the outburst strongly influence the first ∼40 days of the calculated cooling curve. We discuss the consequences for estimates of the neutron star crust parameters, and argue that detailed modelling of the final phase of the outburst is key to constraining the origin of the shallow heat source.

The Thermal State of Ks 1731−260 After 14.5 Years in Quiescence

“…dStar gives the crust temperature profile at all times; thus we can study the temperature in the crust at the time of the superburst. As discussed in Ootes et al (2016), the accretion rate during the outburst varies and is significantly higher in the earlier part than at the end. The persistent flux before the superburst (taken from Kuulkers et al 2002) is about 1.66 times higher than the average mass accretion rate we assume.…”

“…However, a recent investigation by Ootes et al (2016) has shown that variations in the accretion rate throughout the outburst influence the cooling curve. Especially important are variations at the end of the outburst, which can strongly influence the early part of the cooling curve and hence have a significant impact on the implied amount of extra heating at shallow depths.…”

“…Their modeling of KS1731−260 (without including the latest observation) finds the need for a 1.4 MeV/nucleon shallow heat source when variations in accretion rate are included and only 0.6 MeV/nucleon when a constant accretion rate is assumed. (Although, note that Ootes et al 2016 do not optimize their fits to get the best-fit parameters.) This value for Q sh is significantly less than the value implied from our modeling, suggesting some dependence on the input parameters that we do not fit for, such as the depth of the shallow heating and crust composition included in the cooling code.…”

“…Once the system returns to quiescence, thermal X-ray emission below a few keV originates from the surface of the neutron star (Brown et al 1998) and the thermal relaxation of the crust can be observed directly. Direct observation of crustal cooling allows for the extraction of neutron star properties, such as thermal conductivity, crustal structure, and core temperature via cooling models (Shternin et al 2007;Brown & Cumming 2009;Page & Reddy 2013;Deibel et al 2015;Turlione et al 2015;Ootes et al 2016).…”

“…Since returning to quiescence, monitoring observations with Chandra and XMM-Newton have shown continued cooling up until 2009 (8 years into quiescence; Wijnands et al 2001Wijnands et al , 2002Cackett et al 2006Cackett et al , 2010. Physical models that track the thermal evolution of the neutron star reproduce the observed cooling in KS 1731−260 (e.g., Shternin et al 2007;Brown & Cumming 2009;Page & Reddy 2013;Turlione et al 2015;Ootes et al 2016). Fits to the crustal cooling of KS 1731−260 consistently suggest that the crust has a high thermal conductivity, indicating a low impurity parameter (i.e., an ordered lattice crustal structure; discussed later).…”