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

## TL;DR

This study investigates the variability in shallow crust heating of the neutron star in Aql X-1 during similar accretion outbursts, revealing that the depth and magnitude of heating can differ significantly, affecting cooling behavior.

## Contribution

The paper provides observational evidence that shallow heating varies in depth and strength between outbursts, challenging assumptions of uniform crustal heating mechanisms in neutron stars.

## Key findings

- Shallow heating was stronger and deeper during the 2016 outburst compared to 2013.
- Neutron star parameters remain constant, indicating other factors influence shallow heating.
- Different outbursts with similar accretion profiles can produce diverse crust heating effects.

## 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 crust properties 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 do not change 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 difference in quiescent evolution after the 2016 outburst.