Potential cooling of an accretion-heated neutron star crust in the low-mass X-ray binary 1RXS J180408.9−342058

Abstract

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 ∼18×10 32 to ∼4×10 32 (D/5.8 kpc) 2 erg s −1 between ∼8 to ∼379 days 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. Modeling 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.

The low-luminosity behaviour of the 4U 0115+63 Be/X-ray transient

“…When the accretion has stopped, the crust would then slowly cool down until equilibrium is reached again. This process would be similar to what has been observed for several low-magnetic field NS systems (for recent discussions see Degenaar et al 2015 andParikh et al 2017). Wijnands & Degenaar (2016) attempted to test this hypothesis in two Be/X-ray transients (4U 0115+63 and V0332+53) after the end of their type-II outbursts.…”

The quiescent state of the neutron-star X-ray transient GRS 1747−312 in the globular cluster Terzan 6

“…Also, the longer we study a neutron-star LMXB in quiescence, the deeper we are able to probe into its layers. Systems for which such studies are possible typically show quiescent spectra that are dominated by the soft spectral component (e.g., Cackett et al 2013b;Degenaar et al 2013Degenaar et al , 2014Homan et al 2014;Merritt et al 2016;Parikh et al 2017c), although sometimes a significant power-law component is present as well (e.g., Fridriksson et al 2010Fridriksson et al , 2011Degenaar et al 2015;Parikh et al 2017bParikh et al , 2018.…”

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

“…During accretion, compression of matter in the neutron star crust induces a series of nonequilibrium reactions, such as electron captures, neutron emissions, and pycnonuclear fusions (Sato 1979;Bisnovaty ȋ-Kogan & Chechetkin 1979;Haensel & Zdunik 1990). The energy generated by these reactions slowly diffuses into the neutron star core, a mechanism known Present sample of cooling observations after long accretion outbursts from the sources: MAXI J0556-332 outbursts 1 & 2 together (outburst 2 was small and had almost no effect on the cooling curve: see Figure 6 below) and 3 (Parikh et al 2017a and this work), XTE J1701-462 (Parikh et al 2020), EXO 0748-676 (Parikh et al 2020), two outbursts from MXB 1659-29 (Parikh et al 2019), IGR J17480-2446 (Ootes et al 2019), KS1731-260 (Merritt et al 2016), Swift J174805.3-244637 (Degenaar et al 2015), 1RXS J180408.9-342058 (Parikh et al 2017b(Parikh et al , 2018, and HETE J1900.0-2455 (Degenaar et al 2021). Dotted lines show simple fits of the form T (t) = T0 + ∆T exp(−t/τ ) to guide the eye and have no claim to be physically meaningful.…”

“…Figure 1.Present sample of cooling observations after long accretion outbursts from the sources: MAXI J0556-332 outbursts 1 & 2 together (outburst 2 was small and had almost no effect on the cooling curve: see Figure6below) and 3(Parikh et al 2017a and this work), XTE J1701-462(Parikh et al 2020), EXO 0748-676(Parikh et al 2020), two outbursts from MXB 1659-29(Parikh et al 2019), IGR J17480-2446(Ootes et al 2019), KS1731-260(Merritt et al 2016), Swift J174805.3-244637 (Degenaar et al 2015, 1RXS J180408.9-342058(Parikh et al 2017b(Parikh et al , 2018, and HETE J1900.0-2455(Degenaar et al 2021). Dotted lines show simple fits of the form T (t) = T0 + ∆T exp(−t/τ ) to guide the eye and have no claim to be physically meaningful.…”

“…Fitting a "double-thermal" model (Lin et al 2007) with n H fixed to 3.2×10 20 cm −2 (Parikh et al 2017b) gives black-body and disk-black-body temperatures of 1.45(1) keV and 0.42(1), respectively, with a reduced χ 2 of 1.00 (a typical hard state model, blackbody with power-law, resulted in a slightly worse reduced χ 2 of 1.07). The unabsorbed 0.5-10 keV flux was 5.8×10 −11 erg cm −2 s −1 , corresponding to a luminosity of 1.3×10 37 erg s −1 for a distance of 43.6 kpc (Parikh et al 2017b). Careful inspection of the data suggests that the burst is not the result of a short, sudden increase in the background.…”