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

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

Further constraints on neutron star crustal properties in the low-mass X-ray binary 1RXS J180408.9−-342058

We report on two new quiescent {\it XMM-Newton} observations (in addition to the earlier {\it Swift}/XRT and {\it XMM-Newton} coverage) of the cooling neutron star crust in the low-mass X-ray binary 1RXS J180408.9$-$342058. Its crust was heated during the $\sim$4.5 month accretion outburst of the source. From our quiescent observations, fitting the spectra with a neutron star atmosphere model, we found that the crust had cooled from $\sim$ 100 eV to $\sim$73 eV from $\sim$8 days to $\sim$479 days after the end of its outburst. However, during the most recent observation, taken $\sim$860 days after the end of the outburst, we found that the crust appeared not to have cooled further. This suggested that the crust had returned to thermal equilibrium with the neutron star core. We model the quiescent thermal evolution with the theoretical crustal cooling code NSCool and find that the source requires a shallow heat source, in addition to the standard deep crustal heating processes, contributing $\sim$0.9 MeV per accreted nucleon during outburst to explain its observed temperature decay. Our high quality {\it XMM-Newton} data required an additional hard component to adequately fit the spectra. This slightly complicates our interpretation of the quiescent data of 1RXS J180408.9$-$342058. The origin of this component is not fully understood.Comment: Accepted for publication by MNRA

A window into the neutron star: Modelling the cooling of accretion heated neutron star crusts

In accreting neutron star X-ray transients, the neutron star crust can be substantially heated out of thermal equilibrium with the core during an accretion outburst. The observed subsequent cooling in quiescence (when accretion has halted) offers a unique opportunity to study the structure and thermal properties of the crust. Initially crust cooling modelling studies focussed on transient X-ray binaries with prolonged accretion outbursts (> 1 year) such that the crust would be significantly heated for the cooling to be detectable. Here we present the results of applying a theoretical model to the observed cooling curve after a short accretion outburst of only ~10 weeks. In our study we use the 2010 outburst of the transiently accreting 11 Hz X-ray pulsar in the globular cluster Terzan 5. Observationally it was found that the crust in this source was still hot more than 4 years after the end of its short accretion outburst. From our modelling we found that such a long-lived hot crust implies some unusual crustal properties such as a very low thermal conductivity (> 10 times lower than determined for the other crust cooling sources). In addition, we present our preliminary results of the modelling of the ongoing cooling of the neutron star in MXB 1659-298. This transient X-ray source went back into quiescence in March 2017 after an accretion phase of ~1.8 years. We compare our predictions for the cooling curve after this outburst with the cooling curve of the same source obtained after its previous outburst which ended in 2001.Comment: 4 pages, 1 figure, to appear in the proceedings of "IAUS 337: Pulsar Astrophysics - The Next 50 Years" eds: P. Weltevrede, B.B.P. Perera, L. Levin Preston & S. Sanida

Quiescent X-ray variability in the neutron star Be/X-ray transient GRO J1750-27

The Be/X-ray transient GRO J1750-27 exhibited a type-II (giant) outburst in 2015. After the source transited to quiescence, we triggered our multi-year Chandra monitoring programme to study its quiescent behaviour. The programme was designed to follow the cooling of a potentially heated neutron-star crust due to accretion of matter during the preceding outburst, similar to what we potentially have observed before in two other Be/X-ray transients, namely 4U 0115+63 and V 0332+53. However, unlike for these other two systems, we do not find any strong evidence that the neutron-star crust in GRO J1750-27 was indeed heated during the accretion phase. We detected the source at a rather low X-ray luminosity (~10^33 erg/s) during only three of our five observations. When the source was not detected it had very low-luminosity upper limits (<10^32 erg/s; depending on assumed spectral model). We interpret these detections and the variability observed as emission likely due to very low-level accretion onto the neutron star. We also discuss why the neutron-star crust in GRO J1750-27 might not have been heated while the ones in 4U 0115+63 and V 0332+53 possibly were.Comment: 13 pages, 6 figures, 5 tables. Accepted for A&

Current methods of diagnosis and treatment of scaphoid fractures

Fractures of the scaphoid bone mainly occur in young adults and constitute 2-7% of all fractures. The specific blood supply in combination with the demanding functional requirements can easily lead to disturbed fracture healing. Displaced scaphoid fractures are seen on radiographs. The diagnostic strategy of suspected scaphoid fractures, however, is surrounded by controversy. Bone scintigraphy, magnetic resonance imaging and computed tomography have their shortcomings. Early treatment leads to a better outcome. Scaphoid fractures can be treated conservatively and operatively. Proximal scaphoid fractures and displaced scaphoid fractures have a worse outcome and might be better off with an open or closed reduction and internal fixation. The incidence of scaphoid non-unions has been reported to be between 5 and 15%. Non-unions are mostly treated operatively by restoring the anatomy to avoid degenerative wrist arthritis