Consistent accretion-induced heating of the neutron-star crust in MXB 1659−29 during two different outbursts

Abstract

Monitoring the cooling of neutron-star crusts heated during accretion outbursts allows us to infer the physics of the dense matter present in the crust. We examine the crust cooling evolution of the low-mass X-ray binary MXB 1659−29 up to ∼505 days after the end of its 2015 outburst (hereafter outburst II) and compare it with what we observed after its previous 1999 outburst (hereafter outburst I) using data obtained from the Swift, XMM-Newton, and Chandra observatories. The observed effective surface temperature of the neutron star in MXB 1659−29 dropped from ∼92 eV to ∼56 eV from ∼12 days to ∼505 days after the end of outburst II. The most recently performed observation after outburst II suggests that the crust is close to returning to thermal equilibrium with the core. We model the crust heating and cooling for both its outbursts collectively to understand the effect of parameters that may change for every outburst (e.g., the average accretion rate, the length of outburst, the envelope composition of the neutron star at the end of the outburst) and those which can be assumed to remain the same during these two outbursts (e.g., the neutron star mass, its radius). Our modelling indicates that all parameters were consistent between the two outbursts with no need for any significant changes. In particular, the strength and the depth of the shallow heating mechanism at work (in the crust) were inferred to be the same during both outbursts, contrary to what has been found when modelling the cooling curves after multiple outburst of another source, MAXI J0556−332. This difference in source behaviour is not understood. We discuss our results in the context of our current understanding of cooling of accretion-heated neutron-star crusts, and in particular with respect to the unexplained shallow heating mechanism.

Crust cooling of the neutron star in Aql X-1: different depth and magnitude of shallow heating during similar accretion outbursts

“…An asterisk is used whenever the error range hit the hard boundary of that fit parameter in the model. In the standard fit for the 2016 data, the core temperature in the neutron star frame was fixed to the value obtained by Ootes et al (2018), T 0 = 8.9 × 10 7 K. In the alternative fit we allowed the core temperature to be lower, which yielded T 0 = 2.9 +1.0 −0.6 × 10 7 K. Degenaar et al 2014Degenaar et al , 2015Parikh et al 2017bParikh et al , 2019Ootes et al 2016Ootes et al , 2018. If crust cooling can be studied in Aql X-1, its frequent outbursts provide the opportunity to study the properties of this shallow heating after several different outbursts, thereby allowing to break degeneracies with neutron star specific parameters that may be involved in shallow heating and do not change between different outbursts (e.g.…”

“…There are two other neutron stars for which crust cooling has been studied after different outbursts; MAXI J0556-332 (Homan et al 2014;Parikh et al 2017a) and MXB 1659-298 (e.g. Wijnands et al 2004;Cackett et al 2013b;Parikh et al 2019). For MAXI J0556-332, which exhibited 3 outbursts of different duration and peak intensity, there is clear evidence that the depth and magnitude of shallow heating varies between the different outbursts.…”

“…The strength of this "shallow heating", if proportional to the mass-accretion rate, is inferred to be on the order of ∼1-2 MeV nucleon −1 (e.g. Degenaar et al 2011Degenaar et al , 2014Degenaar et al , 2015Page & Reddy 2013;Parikh et al 2017bParikh et al , 2019Ootes et al 2018), although one extreme case requires ∼15-17 MeV nucleon −1 (e.g. Homan et al 2014;Deibel et al 2015;Parikh et al 2017a).…”

“…It remains to be established to what extent shallow heating depends on neutron-star specific properties (e.g., spin, magnetic field strength, mass, radius, superfluid properties, age), and on the detailed properties of the accretion outburst (e.g., brightness, duration, accretion geometry). Studying multiple cooling curves of a single source may be a promising way to understand the origin of shallow heating; because the fundamental properties of a neutron star remain virtually unchanged, the effect of the outburst parameters on the crust heating and cooling can potentially be isolated (Waterhouse et al 2016;Parikh et al 2017aParikh et al , 2019.…”

Crucial role of neutron diffusion in the crust of accreting neutron stars

“…As a final remark, let us point that construction of diffusion equilibrium crust, in principle, can be important for the shallow heating problem -the phenomenological powerful heating source of unknown nature, which is introduced to the models of crustal cooling to explain observational data (Meisel et al 2018;Degenaar et al 2019;Parikh et al 2019). This source is typically assumed to be localized in outer crust, thus it is rather unlikely that neutron diffusion provides a direct mechanism to explain this source.…”