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

Abstract

The structure and composition of the crust of neutron stars plays an important role in their thermal and magnetic evolution, hence in setting their observational properties. One way to study the properties of the crust of a neutron star, is to measure how it cools after it has been heated during an accretion outburst in a low-mass X-ray binary (LMXB). Such studies have shown that there is a tantalizing source of heat, of currently unknown origin, that is located in the outer layers of the crust and has a strength that varies between different sources and different outbursts. With the aim of understanding the mechanism behind this "shallow heating", we present Chandra and Swift observations of the neutron star LMXB Aql X-1, obtained after its bright 2016 outburst. We find that the neutron star temperature was initially much lower, and started to decrease at much later time, than observed after the 2013 outburst of the source, despite the fact that the properties of the two outbursts were very similar. Comparing our data to thermal evolution simulations, we infer that the depth and magnitude of shallow heating must have been much larger during the 2016 outburst than during the 2013 one. This implies that basic neutron star parameters that remain unchanged between outbursts, do not play a strong role in shallow heating. Furthermore, it suggests that outbursts with a similar accretion morphology can give rise to very different shallow heating. We also discuss alternative explanations for the observed difference in quiescent evolution after the 2016 outburst.

A "Hyperburst" in the MAXI J0556-332 Neutron Star: Evidence for a New Type of Thermonuclear Explosion

“…This result is in agreement with the study of the MXB 1659-29 neutron star that could describe its evolution through two accretion outbursts, spanning a period of almost two decades (1999 till 2018), also with a single shallow heating parametrization (Parikh et al 2019). These results are, however, in contrast with the modeling of the evolution of the Aquila X-1 neutron star, which exhibits frequent but short accretion outbursts, that required a variation of the shallow heating parameters between different outbursts (Degenaar et al 2019).…”

Experimental constraints on shallow heating in accreting neutron-star crusts

“…The thermal relaxation of the crust is still not fully understood. In particular, the interpretation of the cooling data requires the existence of some unknown heat sources in the shallow layers of the crust [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], as summarized in Table I. In most cases, the extra heat, typically released in the outer crust at densities below about 10 10 g cm −3 , amounts to about 1-2 MeV per accreted nucleon with the notable exceptions of MAXI J0556−332 and Aql X-1.…”

Discovery of transition from marginally stable burning to unstable burning after a superburst in Aql X-1

“…However, there are few differences between the simulations and the observations in Aql X-1. Aql X-1 is a transient source, which has been observed X-ray outbursts recurring around one year, each of them lasting ∼1-6 months (Degenaar et al 2019). Considering that the superburst recurrence time t rec ∝ Ṁ −1 , we obtain t rec ≈ 1.28(0.0854 ṀEdd /0.3 ṀEdd ) −1 (79/365) −1 yr ≈ 20.8 yr, if we take a typical value of the averaged accretion rate of 0.0854 ṀEdd in total 79 days from the 2013 outburst in Aql X-1 (Ootes et al 2018).…”