A cooling neutron star crust after recurrent outbursts: Modelling the accretion outburst history of Aql X-1

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.Comment: 20 pages, 8 figures, 4 tables, accepted for publication in MNRA

A search for the 835 Hz835\,\text{Hz} superburst oscillation signal in the regular thermonuclear bursts of 4U 1636-536

Burst oscillations are brightness asymmetries that develop in the burning ocean during thermonuclear bursts on accreting neutron stars. They have been observed during H/He-triggered (Type I) bursts and Carbon-triggered superbursts. The mechanism responsible is not unknown, but the dominant burst oscillation frequency is typically within a few Hz of the spin frequency, where this is independently known. One of the best-studied burst oscillation sources, 4U 1636-536, has oscillations at $581\,\text{Hz}$ in both its regular Type I bursts and in one superburst. Recently however, Strohmayer \& Mahmoodifar reported the discovery of an additional signal at a higher frequency, $835\,\text{Hz}$, during the superburst. This higher frequency is consistent with the predictions for several types of global ocean mode, one of the possible burst oscillation mechanisms. If this is the case then the same physical mechanism may operate in the normal Type I bursts of this source. In this paper we report a stacked search for periodic signals in the regular Type I bursts: we found no significant signal at the higher frequency, with upper limits for the single trial root mean square (rms) fractional amplitude of 0.57(6)\%. Our analysis did however reveal that the dominant $581\,\text{Hz}$ burst oscillation signal is present at a weak level even in the sample of bursts where it cannot be detected in individual bursts. This indicates that any cutoff in the burst oscillation mechanism occurs below the detection threshold of existing X-ray telescopes.Comment: 6 pages, 2 figures. Accepted for publication by Ap

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

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 properties of the crust of a neutron star, 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 remain unchanged 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 observed difference in quiescent evolution after the 2016 outburst.Peer Reviewe

Reisicumhurdan şanlı Türk ordusuna taziyet:İsmet İnönü ile Mareşal Fevzi Çakmak arasında mektuplar teati edildi

Taha Toros Arşivi, Dosya Adı: Atatürk'ün Ölümü ve Cenaze Tören

Different accretion heating of the neutron star crust during multiple outbursts in MAXI J0556–332

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 MAXI, Swift, Chandra, and XMM-Newton data of MAXI J0556−332 obtained during and after three accretion outbursts of different durations and brightnesses. We report on new data obtained after outburst III. The source has been tracked up to ~1800 days after the end of outburst I. Outburst I heated the crust strongly, but no significant reheating was observed during outburst II. Cooling from ~333 eV to ~146 eV was observed during the first ~1200 days. Outburst III reheated the crust up to ~167 eV, after which the crust cooled again to ~131 eV in ~350 days. 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 ~17 MeV nucleon−1 and outburst III required ~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 ~0 to 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

Different Accretion Heating of the Neutron Star Crust during Multiple Outbursts in MAXI J0556-332

© 2017. The American Astronomical Society. All rights reserved.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 MAXI, Swift, Chandra, and XMM-Newton data of MAXI J0556-332 obtained during and after three accretion outbursts of different durations and brightnesses. We report on new data obtained after outburst III. The source has been tracked up to ~1800 days after the end of outburst I. Outburst I heated the crust strongly, but no significant reheating was observed during outburst II. Cooling from ~333 eV to ~146 eV was observed during the first ~1200 days. Outburst III reheated the crust up to ~167 eV, after which the crust cooled again to ~131 eV in ~350 days. 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 ~17 MeV nucleon-1 and outburst III required ~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 ~0 to 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.Peer Reviewe

CAGI, the Critical Assessment of Genome Interpretation, establishes progress and prospects for computational genetic variant interpretation methods

BackgroundThe Critical Assessment of Genome Interpretation (CAGI) aims to advance the state-of-the-art for computational prediction of genetic variant impact, particularly where relevant to disease. The five complete editions of the CAGI community experiment comprised 50 challenges, in which participants made blind predictions of phenotypes from genetic data, and these were evaluated by independent assessors.ResultsPerformance was particularly strong for clinical pathogenic variants, including some difficult-to-diagnose cases, and extends to interpretation of cancer-related variants. Missense variant interpretation methods were able to estimate biochemical effects with increasing accuracy. Assessment of methods for regulatory variants and complex trait disease risk was less definitive and indicates performance potentially suitable for auxiliary use in the clinic.ConclusionsResults show that while current methods are imperfect, they have major utility for research and clinical applications. Emerging methods and increasingly large, robust datasets for training and assessment promise further progress ahead

CAGI, the critical assessment of genome interpretation, establishes progress and prospects for computational genetic variant interpretation methods

Background: The Critical Assessment of Genome Interpretation (CAGI) aims to advance the state-of-the-art for computational prediction of genetic variant impact, particularly where relevant to disease. The five complete editions of the CAGI community experiment comprised 50 challenges, in which participants made blind predictions of phenotypes from genetic data, and these were evaluated by independent assessors. Results: Performance was particularly strong for clinical pathogenic variants, including some difficult-to-diagnose cases, and extends to interpretation of cancer-related variants. Missense variant interpretation methods were able to estimate biochemical effects with increasing accuracy. Assessment of methods for regulatory variants and complex trait disease risk was less definitive and indicates performance potentially suitable for auxiliary use in the clinic. Conclusions: Results show that while current methods are imperfect, they have major utility for research and clinical applications. Emerging methods and increasingly large, robust datasets for training and assessment promise further progress ahead