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Register a new userCrustal Emission and the Quiescent Spectrum of the Neutron Star in KS 1731-260
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Robert E. Rutledge
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(Abridged). The type-I X-ray bursting low mass X-ray binary KS 1731-260 was recently detected for the first time in quiescence by Wijnands et al., following an approximately 13 yr outburst which ended in Feb 2001. Unlike all other known transient neutron stars, the duration of this recent outburst is as long as the thermal diffusion time of the crust. The large amount of heat deposited by reactions in the crust will have heated the crust to temperatures much higher than the equilibrium core temperature. As a result, the thermal luminosity currently observed from the neutron star is dominated not by the core, but by the crust. Moreover, the level and the time evolution of quiescent luminosity is determined mostly by the amount of heat deposited in the crust during the most recent outburst. Using estimates of the outburst mass accretion rate, our calculations of the quiescent flux immediately following the end of the outburst agree with the observed quiescent flux to within a factor of a few. We present simulations of the evolution of the quiescent lightcurve for different scenarios of the crust microphysics, and demonstrate that monitoring observations (with currently flying instruments) spanning from 1--30 yr can measure the crust cooling timescale and the total amount of heat stored in the crust. These quantities have not been directly measured for any neutron star.
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Using a theoretical model, we track the thermal evolution of a cooling neutron star crust after an accretion induced heating period with the goal of constraining the crustal parameters. We present for the first time a crust cooling model $-text{ } NSCooltext{ } -$ that takes into account detailed variability during the full outburst based on the observed light curve. We apply our model to KS 1731-260. The source was in outburst for $sim$12 years during which it was observed to undergo variations on both long (years) and short (days-weeks) timescales. Our results show that KS 1731-260 does not reach a steady state profile during the outburst due to fluctuations in the derived accretion rate. Additionally, long time-scale outburst variability mildly affects the complete crust cooling phase, while variations in the final months of the outburst strongly influence the first $sim$40 days of the calculated cooling curve. We discuss the consequences for estimates of the neutron star crust parameters, and argue that detailed modelling of the final phase of the outburst is key to constraining the origin of the shallow heat source.
Crustal cooling of accretion-heated neutron stars provides insight into the stellar interior of neutron stars. The neutron star X-ray transient, KS~1731$-$260, was in outburst for 12.5 years before returning to quiescence in 2001. We have monitored the cooling of this source since then through {it Chandra} and {it XMM-Newton} observations. Here, we present a 150 ks {it Chandra} observation of KS~1731$-$260 taken in August 2015, about 14.5 years into quiescence, and 6 years after the previous observation. We find that the neutron star surface temperature is consistent with the previous observation, suggesting that crustal cooling has likely stopped and the crust has reached thermal equilibrium with the core. Using a theoretical crust thermal evolution code, we fit the observed cooling curves and constrain the core temperature (T$_c = 9.35pm0.25times10^7$ K), composition (Q$_{imp} = 4.4^{+2.2}_{-0.5}$) and level of extra shallow heating required (Q$_{sh} = 1.36pm0.18$ MeV/nucleon). We find that the presence of a low thermal conductivity layer, as expected from nuclear pasta, is not required to fit the cooling curve well, but cannot be excluded either.
Aims: A hard X-ray shortage, implying the cooling of the corona, was observed during bursts of IGR J17473-272, 4U 1636-536, Aql X-1, and GS 1826-238. Apart from these four sources, we investigate here an atoll sample, in which the number of bursts for each source is larger than 5, to explore the possible additional hard X-ray shortage during {it Rossi X-ray timing explorer (RXTE)} era. Methods: According to the source catalog that shows type-I bursts, we analyzed all the available pointing observations of these sources carried out by the {it RXTE} proportional counter array (PCA). We grouped and combined the bursts according to their outburst states and searched for the possible hard X-ray shortage while bursting. Results: We found that the island states of KS 1731-260 and 4U 1705-44 show a hard X-ray shortage at significant levels of 4.5 and 4.7 $sigma$ and a systematic time lag of $0.9 pm 2.1$ s and $2.5 pm 2.0$ s with respect to the soft X-rays, respectively. While in their banana branches and other sources, we did not find any consistent shortage.
We report on an approximately twelve hour long X-ray flare from the low-mass X-ray binary KS 1731-260. The flare has a rise time of less than 13 min and declines exponentially with a decay time of 2.7 hours. The flare emission is well described by black-body radiation with peak temperature of 2.4 keV. The total energy release from the event is 10^{42} erg (for an assumed distance of 7 kpc). The flare has all the characteristics of thermo-nuclear X-ray bursts (so-called type I X-ray bursts), except for its very long duration and therefore large energy release (factor of 1500-4000 longer and 250-425 more energy than normal type I X-ray bursts from this source). The flare is preceded by a short and weak X-ray burst, possibly of type I. Days to weeks before the flare, type I X-ray bursts were seen at a rate of ~3 per day. However, after the flare type I X-ray bursting ceased for at least a month, suggesting that the superburst affected the type I bursting behaviour. The persistent emission is not significantly different during the non-bursting period. We compare the characteristics of this event with similar long X-ray flares, so-called superbursts, seen in other sources (4U 1735-44, 4U 1820-30, 4U 1636-53, Ser X-1, GX 3+1). The event seen from KS 1731-260 is the longest reported so far. We discuss two possible mechanisms that might cause these superbursts, unstable carbon burning (as proposed recently) and electron capture by protons with subsequent capture of the resulting neutrons by heavy nuclei.
It is assumed that accreting neutron stars (NSs) in LMXBs are heated due to the compression of the existing crust by the accreted matter which gives rise to nuclear reactions in the crust. It has been shown that most of the energy is released deep in the crust by pycnonuclear reactions involving low-Z elements. We discuss if NSs in very-faint X-ray transients (VFXTs; those which have peak X-ray luminosities < 1E36 erg/s) can be used to test this model. Unfortunately we cannot conclusively answer this because of the large uncertainties in our estimates of the accretion rate history of those VFXTs, both the short-term (less than a few tens of thousands of years) and the one throughout their lifetime. The latter is important because it can be so low that the NSs might not have accreted enough matter to become massive enough that enhanced cooling processes become active. Therefore, they could be relatively warm compared to other systems for which such enhanced cooling processed have been inferred. However, the amount of matter can also not be too low because then the crust might not have been replaced significantly by accreted matter and thus a hybrid crust of partly accreted and partly original, albeit further compressed matter, might be present. This would inhibit the full range of pycnonuclear reactions to occur and thus very likely decreasing the amount of heat deposited in the crust. Furthermore, better understanding is needed how a hybrid crust affects other properties such as the thermal conductivity. We also show that some individual NS LMXBs might have hybrid crusts as well as the NSs in HMXBs. This has to be taken into account when studying the cooling properties of those systems when they are in quiescence. We show that the VFXTs are likely not the dominate transients that are associated with the brightest low-luminosity X-ray sources in globular clusters as was hypothesized.
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