Diffusive nuclear burning in cooling simulations and application to new temperature data of the Cassiopeia A neutron star

Abstract

A critical relation in the study of neutron star cooling is the one between surface temperature and interior temperature. This relation is determined by the composition of the neutron star envelope and can be affected by the process of diffusive nuclear burning (DNB), which occurs when elements diffuse to depths where the density and temperature are sufficiently high to ignite nuclear burning. We calculate models of H-He and He-C envelopes that include DNB and obtain analytic temperature relations that can be used in neutron star cooling simulations. We find that DNB can lead to a rapidly changing envelope composition and prevents the build-up of thermally stable hydrogen columns y H 10 7 g cm −2 , while DNB can make helium envelopes more transparent to heat flux for surface temperatures T s 2× 10 6 K. We perform neutron star cooling simulations in which we evolve temperature and envelope composition, with the latter due to DNB and accretion from the interstellar medium. We find that a time-dependent envelope composition can be relevant for understanding the longterm cooling behaviour of isolated neutron stars. We also report on the latest Chandra observations of the young neutron star in the Cassiopeia A supernova remnant; the resulting 13 temperature measurements over more than 18 years yield a ten-year cooling rate of ≈ 2%. Finally, we fit the observed cooling trend of the Cassiopeia A neutron star with a model that includes DNB in the envelope.

Force on a neutron quantized vortex pinned to proton fluxoids in the superfluid core of cold neutron stars

“…Nucleon superfluidity in the core of NSs has recently found additional support from the direct monitoring of the rapid cooling of the young NS in Cassiopeia A (Page et al 2011;Shternin et al 2011). However, the interpretation of these observations remains controversial (Posselt & Pavlov 2018;Wijngaarden et al 2019).…”

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

“…These temperature relations were calculated by computing a grid of envelope models for different combinations of surface temperatures and hydrogen columns and fitting an analytic relation to these results. More details on this approach can be found in Appendix A and Wijngaarden et al (2019). In Figure 1, we show the resulting relation between surface temperature and bottom boundary temperature for varying hydrogen column sizes.…”

“…Then, analytic fits of the temperature relations between the NS surface temperature (T s ) and the temperature at the bottom of the envelope, T b [≡ T (ρ b )], are used as a boundary condition. Analytic fits to numerical envelope calculations for different static envelope compositions and envelope sizes have been calculated for one- (Gudmundsson et al 1983) and two or more chemical component models (see, e.g., Potekhin et al 1997Potekhin et al , 2003Beznogov et al 2016;Wijngaarden et al 2019).…”

“…This process has been studied for H-C (Chang & Bildsten 2003, 2004 and He-C envelopes (Chang et al 2010). In addition, Wijngaarden et al (2019) calculated H-He and He-C temperature relations including the effect of DNB and investigated how DNB can alter the envelope composition over time for cooling isolated neutron stars.…”

X-ray spectral analysis of the neutron star in SNR 1E 0102.2−7219

“…The assumption of a hydrogen atmosphere should be valid for most NSs where fall-back or accretion has occurred, since the elements stratify quickly to leave the lightest (generally H) on top (Alcock & Illarionov 1980;Romani 1987;Brown et al 1998). Thermonuclear burning of light elements on the hot young NS surface may remove H and He, possibly leaving a C (or higher-Z) atmosphere, if fallback and accretion are kept to very low rates (Chang et al 2004(Chang et al , 2010Wijngaarden et al 2019). The heavy elements in such a mid-Z atmosphere can lead to detectable spectral features (Ho & Heinke 2009;Mori & Ho 2007).…”