The effect of diffusive nuclear burning in neutron star envelopes on cooling in accreting systems

Abstract

Valuable information about the neutron star interior can be obtained by comparing observations of thermal radiation from a cooling neutron star crust with theoretical models. Nuclear burning of lighter elements that diffuse to deeper layers of the envelope can alter the relation between surface and interior temperatures and can change the chemical composition over time. We calculate new temperature relations and consider two effects of diffusive nuclear burning (DNB) for H-C envelopes. First, we consider the effect of a changing envelope composition and find that hydrogen is consumed on short timescales and our temperature evolution simulations correspond to those of a hydrogen-poor envelope within ∼100 days. The transition from a hydrogen-rich to a hydrogen-poor envelope is potentially observable in accreting NS systems as an additional initial decline in surface temperature at early times after the outburst. Second, we find that DNB can produce a non-negligible heat flux, such that the total luminosity can be dominated by DNB in the envelope rather than heat from the deep interior. However, without continual accretion, heating by DNB in H-C envelopes is only relevant for <1-80 days after the end of an accretion outburst, as the amount of light elements is rapidly depleted. Comparison to crust cooling data shows that DNB does not remove the need for an additional shallow heating source. We conclude that solving the time-dependent equations of the burning region in the envelope selfconsistently in thermal evolution models instead of using static temperature relations would be valuable in future cooling studies.

X-ray bounds on cooling, composition, and magnetic field of the Cassiopeia A neutron star and young central compact objects

“…For isolated or non-accreting binary systems, a hydrogen atmosphere is likely due to even a small amount of accretion from the interstellar medium (Blaes et al 1992). Helium or carbon atmospheres are possible for the youngest neutron stars that are still hot after formation since hydrogen and helium can be depleted by residual nuclear burning (Chang & Bildsten 2003Chang et al 2010;Wijngaarden et al 2019Wijngaarden et al , 2020. Such appears to be the case for the CCO in Cassiopeia A, i.e, its observed properties (kpc) using the observations examined here).…”

“…This evolution in atmosphere composition is due initially to the hot neutron star at birth. The high temperatures prompt formation of carbon and other heavy elements from nuclear fusion of any residual surface hydrogen and helium (Chang & Bildsten 2003Chang et al 2010;Wijngaarden et al 2019Wijngaarden et al , 2020. These nuclear reactions cease to be effective after several hundred years as the neutron star surface cools.…”

Model-independent constraints on superfluidity from the cooling neutron star in Cassiopeia A

“…Here we adopt the expressions for the iron envelopes given by Potekhin, Chabrier & Yakovlev (1997) and C−Fe envelopes from Beznogov, Potekhin & Yakovlev (2016). Notice that the composition of the envelope, and hence the 𝑇 𝑠 (𝑇 𝑏 ) relation, can change in time due to the diffusive nuclear burning of light elements (Chang & Bildsten 2003;Wijngaarden et al 2019Wijngaarden et al , 2020. We also note that according to the previous analysis (Shternin & Yakovlev 2015), a large amount of carbon in the envelope cannot be reconciled with the CasA NS observations.…”

Deep crustal heating for realistic compositions of thermonuclear ashes

“…Comparison of the NS crust cooling curves to the theoretical models of heat relaxation (Rutledge et al 2002;Shternin et al 2007;Brown & Cumming 2009;Cackett et al 2010;Page & Reddy 2013;Degenaar, Wijnands & Miller 2013;Meisel et al 2018;Parikh et al 2019;Wijngaarden et al 2020;Potekhin & Chabrier 2021), as well as modelling of the steady-state thermal configurations (Brown et al 1998;Yakovlev, Levenfish & Haensel 2003;Yakovlev et al 2004;Heinke et al 2007Heinke et al , 2009Wijnands, Degenaar & Page 2013;Beznogov & Yakovlev 2015;Han & Steiner 2017;Fortin et al 2018;Brown et al 2018;Potekhin et al 2019;Fortin et al 2021) allows one to constrain the properties of NS matter. To model the thermal evolution one needs to know the profile of energy release, the crust equation of state (EOS) and composition, which can be found by studying the chain of nuclear reactions in the crust of an accreting NS.…”