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Register a new userCrust cooling of the neutron star in Aql X-1: Different depth and magnitude of shallow heating during similar accretion outbursts
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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.
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With our neutron star crust cooling code {tt NSCool} we track the thermal evolution of the neutron star in Aql X-1 over the full accretion outburst history from 1996 until 2015. For the first time, we model many outbursts (23 outbursts were detected) collectively and in great detail. This allows us to investigate the influence of previous outbursts on the internal temperature evolution and to test different neutron star crust cooling scenarios. Aql X-1 is an ideal test source for this purpose, because it shows frequent, short outbursts and thermally dominated quiescence spectra. The source goes into outburst roughly once a year for a few months. Assuming that the quiescent {it Swift}/XRT observations of Aql X-1 can be explained within the crust cooling scenario (Waterhouse et al. 2016), we find three main conclusions. Firstly, the data are well reproduced by our model if the envelope composition and shallow heating parameters are allowed to change between outbursts. This is not the case if both shallow heating parameters (strength and depth) are tied throughout all accretion episodes, supporting earlier results that the properties of the shallow heating mechanism are not constant between outbursts. Second, from our models shallow heating could not be connected to one specific spectral state during outburst. Third, and most importantly, we find that the neutron star in Aql X-1 does not have enough time between outbursts to cool down to crust-core equilibrium and that heating during one outburst influences the cooling curves of the next.
The transient neutron star (NS) low-mass X-ray binary MAXI J0556$-$332 provides a rare opportunity to study NS crust heating and subsequent cooling for multiple outbursts of the same source. We examine {it MAXI}, {it Swift}, {it Chandra}, and {it XMM-Newton} data of MAXI J0556$-$332 obtained during and after three accretion outbursts of different durations and brightness. We report on new data obtained after outburst III. The source has been tracked up to $sim$1800 d after the end of outburst I. Outburst I heated the crust strongly, but no significant reheating was observed during outburst II. Cooling from $sim$333 eV to $sim$146 eV was observed during the first $sim$1200 d. Outburst III reheated the crust up to $sim$167 eV, after which the crust cooled again to $sim$131 eV in $sim$350 d. We model the thermal evolution of the crust and find that this source required a different strength and depth of shallow heating during each of the three outbursts. The shallow heating released during outburst I was $sim$17 MeV nucleon$^{-1}$ and outburst III required $sim$0.3 MeV nucleon$^{-1}$. These cooling observations could not be explained without shallow heating. The shallow heating for outburst II was not well constrained and could vary from $sim$0--2.2 MeV nucleon$^{-1}$, i.e., this outburst could in principle be explained without invoking shallow heating. We discuss the nature of the shallow heating and why it may occur at different strengths and depths during different outbursts.
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.
We have monitored the transient neutron star low-mass X-ray binary 1RXS J180408.9-342058 in quiescence after its ~4.5 month outburst in 2015. The source has been observed using Swift and XMM-Newton. Its X-ray spectra were dominated by a thermal component. The thermal evolution showed a gradual X-ray luminosity decay from ~18 x 10^32 to ~4 x 10^32 (D/5.8 kpc)^2 erg s^{-1} between ~8 and ~379 d in quiescence, and the inferred neutron star surface temperature (for an observer at infinity; using a neutron star atmosphere model) decreased from ~100 to ~71 eV. This can be interpreted as cooling of an accretion-heated neutron star crust. Modelling the observed temperature curve (using NSCOOL) indicated that the source required ~1.9 MeV per accreted nucleon of shallow heating in addition to the standard deep crustal heating to explain its thermal evolution. Alternatively, the decay could also be modelled without the presence of deep crustal heating, only having a shallow heat source (again ~1.9 MeV per accreted nucleon was required). However, the XMM-Newton data statistically required an additional power-law component. This component contributed ~30 per cent of the total unabsorbed flux in 0.5-10 keV energy range. The physical origin of this component is unknown. One possibility is that it arises from low-level accretion. The presence of this component in the spectrum complicates our cooling crust interpretation because it might indicate that the smooth luminosity and temperature decay curves we observed may not be due to crust cooling but due to some other process.
A number of studies have revealed variability from neutron star low-mass X-ray binaries during quiescence. Such variability is not well characterised, or understood, but may be a common property that has been missed due to lack of multiple observations. One such source where variability has been observed is Aql X-1. Here, we analyse 14 Chandra and XMM-Newton observations of Aql X-1 in quiescence, covering a period of approximately 2 years. There is clear variability between the epochs, with the most striking feature being a flare-like increase in the flux by a factor of 5. Spectral fitting is inconclusive as to whether the power-law and/or thermal component is variable. We suggest that the variability and flare-like behaviour during quiescence is due to accretion at low rates which might reach the neutron star surface.
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