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Register a new userTime-Variable Emission from Transiently Accreting Neutron Stars In Quiescence due to Deep Crustal Heating
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Transiently accreting neutron stars in quiescence (Lx<10^34 erg/s) have been observed to vary in intensity by factors of few, over timescales of days to years. If the quiescent luminosity is powered by a hot NS core, the core cooling timescale is much longer than the recurrence time, and cannot explain the observed, more rapid variability. However, the non-equilibrium reactions which occur in the crust during outbursts deposit energy in iso-density shells, from which the thermal diffusion timescale to the photosphere is days to years. The predicted magnitude of variability is too low to explain the observed variability unless - as is widely believed - the neutrons beyond the neutron-drip density are superfluid. Even then, variability due to this mechanism in models with standard core neutrino cooling processes is less than 50 per cent - still too low to explain the reported variability. However, models with rapid core neutrino cooling can produce variability by a factor as great as 20, on timescales of days to years following an outburst. Thus, the factors of few intensity variability observed from transiently accreting neutron stars can be accounted for by this mechanism only if rapid core cooling processes are active.
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We study long-term thermal evolution of neutron stars in soft X-ray transients (SXTs), taking the deep crustal heating into account consistently with the changes of the composition of the crust. We collect observational estimates of average accretion rates and thermal luminosities of such neutron stars and compare the theory with observations. We perform simulations of thermal evolution of accreting neutron stars, considering the gradual replacement of the original nonaccreted crust by the reprocessed accreted matter, the neutrino and photon energy losses, and the deep crustal heating due to nuclear reactions in the accreted crust. We test and compare results for different modern theoretical models. We update a compilation of the observational estimates of the thermal luminosities in quiescence and average accretion rates in the SXTs and compare the observational estimates with the theoretical results. Long-term thermal evolution of transiently accreting neutron stars is nonmonotonic. The quasi-equilibrium temperature in quiescence reaches a minimum and then increases toward the final steady state. The quasi-equilibrium thermal luminosity of a neutron star in an SXT can be substantially lower at the minimum than in the final state. This enlarges the range of possibilities for theoretical interpretation of observations of such neutron stars. The updates of the theory and observations leave unchanged the previous conclusions that the direct Urca process operates in relatively cold neutron stars and that an accreted heat-blanketing envelope is likely present in relatively hot neutron stars in the SXTs in quiescence. The results of the comparison of theory with observations favor suppression of the triplet pairing type of nucleon superfluidity in the neutron-star matter.
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.
Detection of gravitational waves from accreting neutron stars (NSs) in our galaxy, due to ellipticity or internal oscillation, would be a breakthrough in our understanding of compact objects and explain the absence of NSs rotating near the break-up limit. Direct detection, however, poses a formidable challenge. Using the current data available on the properties of the accreting NSs in Low Mass X-Ray Binaries (LMXBs), we quantify the detectability for the known accreting NSs, considering various emission scenarios and taking into account the negative impact of parameter uncertainty on the data analysis process. Only a few of the persistently bright NSs accreting at rates near the Eddington limit are detectable by Advanced LIGO if they are emitting gravitational waves at a rate matching the torque from accretion. A larger fraction of the known population is detectable if the spin and orbital parameters are known in advance, especially with the narrow-band Advanced LIGO. We identify the most promising targets, and list specific actions that would lead to significant improvements in detection probability. These include astronomical observations (especially for unknown orbital periods), improvements in data analysis algorithms and capabilities, and further detector development.
We obtained four Chandra/ACIS-S observations beginning two weeks after the end of the November 2000 outburst of the neutron star (NS) transient Aql X-1. Over the five month span in quiescence, the X-ray spectra are consistent with thermal emission from a NS with a pure hydrogen photosphere and R_{infty}=15.9+{0.8}-{2.9} (d/5 kpc) km at the optically implied X-ray column density. We also detect a hard power-law tail during two of the four observations. The intensity of Aql X-1 first decreased by 50+/-4% over three months, then increased by 35+/-5% in one month, and then remained constant (<6% change) over the last month. These variations in the first two observations cannot be explained by a change in the power-law spectral component, nor in the X-ray column density. Presuming that R_{infty} is not variable and a pure hydrogen atmosphere, the long-term changes can only be explained by variations in the NS effective temperature, from kT_{eff, infty}=130+3-5 eV, down to 113+3-4 eV, finally increasing to 118+9-4 eV for the final two observations. During one of these observations, we observe two phenomena which were previously suggested as indicators of quiescent accretion onto the NS: short-timescale (<1e4 sec) variability (at 32+8-6% rms), and a possible absorption feature near 0.5 keV. The possible absorption feature can potentially be explained as due to a time-variable response in the ACIS detector. Even so, such a feature has not been detected previously from a NS, and if confirmed and identified, can be exploited for simultaneous measurements of the photospheric redshift and NS radius.
Neutrino emission in processes of breaking and formation of neutron and proton Cooper pairs is calculated within the Larkin-Migdal-Leggett approach for a superfluid Fermi liquid. We demonstrate explicitly that the Fermi-liquid renormalization respects the Ward identity and assures the weak vector current conservation. The systematic expansion of the emissivities for small temperatures and nucleon Fermi velocity, v_{F,i}, i=n,p, is performed. Both neutron and proton processes are mainly controlled by the axial-vector current contributions, which are not strongly changed in the superfluid matter. Thus, compared to earlier calculations the total emissivity of processes on neutrons paired in the 1S_0 state is suppressed by a factor ~(0.9-1.2) v_{F,n}^2. A similar suppression factor (~v_{F,p}^2) arises for processes on protons.
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