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Serving as the progenitors of electromagnetic and gravitational wave transients, massive stars have received renewed interest in recent years. However, many aspects of their birth and evolution remain opaque, particularly in the context of binary interactions. The centre of our galaxy hosts a rich cohort of very massive stars, which appear to play a prominent role in the ecology of the region. In this paper we investigate the binary properties of the Arches cluster, which is thought to host a large number of very massive stars. A combination of multi-epoch near-IR spectroscopy and photometry was utilised to identify binaries. 13 from 36 cluster members meet our criteria to be classed as RV variable. Combining the spectroscopic data with archival radio and X-ray observations - to detect colliding wind systems - provides a lower limit to the binary fraction of ~43%; increasing to >50% for the O-type hypergiants and WNLha. Dynamical and evolutionary masses reveal the primaries to be uniformly massive (>50M$_{\odot}$). Where available, orbital analysis reveals a number of short period, highly eccentric binaries, which appear to be pre-interaction systems. Such systems are X-ray luminous, with 80% above an empirical bound of $(L_{\rm x}/L_{\rm bol})\sim10^{-7}$ and their orbital configurations suggest formation and evolution via a single star channel; however, we cannot exclude a binary formation channel for a subset. Qualitative comparison to surveys of lower mass OB-type stars confirms that the trend to an extreme binary fraction (>60%) extends to the most massive stars currently forming in the local Universe., Accepted in MNRAS; In Memoriam: This work is dedicated to the memory of our dearest friend and colleague Simon Clark

We aim to identify and characterise binary systems containing red supergiant (RSG) stars in the Small Magellanic Cloud (SMC) using a newly available ultraviolet (UV) point source catalogue obtained using the Ultraviolet Imaging Telescope (UVIT) on board AstroSat. We select a sample of 560 SMC RSGs based on photometric and spectroscopic observations at optical wavelengths and cross-match this with the far-UV point source catalogue using the UVIT F172M filter, finding 88 matches down to m$_{F172M}$=20.3 ABmag, which we interpret as hot companions to the RSGs. Stellar parameters (luminosities, effective temperatures and masses) for both components in all 88 binary systems are determined and we find mass distributions in the ranges 6.1 to 22.3 Solar masses for RSGs and 3.7 to 15.6 Solar masses for their companions. The most massive RSG binary system in the SMC has a combined mass of 32 $\pm$4 M$_\odot$, with a mass ratio (q) of 0.92. By simulating observing biases, we find an intrinsic multipliciy fraction of 18.8 $\pm$ 1.5% for mass ratios in the range 0.3 < q < 1.0 and orbital periods approximately in the range 3 < log P [days] < 8. By comparing our results with those of a similar mass on the main-sequence, we determine the fraction of single stars to be ~20% and argue that the orbital period distribution declines rapidly beyond log P ~ 3.5. We study the mass-ratio distribution of RSG binary systems and find that a uniform distribution best describes the data below 14 M$_\odot$. Above 15 M$_\odot$, we find a lack of high mass-ratio systems., Accepted for publication in MNRAS; 17 pages

The presence of massive stars (MSs) in the region close to the Galactic Center (GC) poses several questions about their origin. The harsh environment of the GC favors specific formation scenarios, each of which should imprint characteristic kinematic features on the MSs. We present a 2D kinematic analysis of MSs in a GC region surrounding Sgr A* based on high-precision proper motions obtained with the Hubble Space Telescope. Thanks to a careful data reduction, well-measured bright stars in our proper-motion catalogs have errors better than 0.5 mas yr$^{-1}$. We discuss the absolute motion of the MSs in the field and their motion relative to Sgr A*, the Arches and the Quintuplet. For the majority of the MSs, we rule out any distance further than 3-4 kpc from Sgr A* using only kinematic arguments. If their membership to the GC is confirmed, most of the isolated MSs are likely not associated with either the Arches or Quintuplet clusters or Sgr A*. Only a few MSs have proper motions suggesting they are likely members of the Arches cluster, in agreement with previous spectroscopic results. Line-of-sight radial velocities and distances are required to shed further light on the origin of most of these massive objects. We also present an analysis of other fast-moving objects in the GC region, finding no clear excess of high-velocity escaping stars. We make our astro-photometric catalogs publicly available., Comment: 28 pages, 33 figures, 9 tables. Accepted for publication in MNRAS. [v3: Fixed bibliography]

We extend our previous study of equilibrium solutions of non-rotating force-free magnetospheres of neutron stars. We show that multiple solutions exist for the same sets of parameters, implying that the solutions are degenerate. We are able to obtain configurations with disconnected field lines, however, in nearly all cases these correspond to degenerate higher energy solutions. We carry out a wide parametric search in order to understand the properties of the solutions. We confirm our previous results that the lower energy solutions have up to $\sim 25\%$ more energy than the vacuum case, helicity of the order of $\sim 5$ (in some defined units), maximum twist of $\sim 1.5$ rad, and a dipole strength that is up to $\sim 40\%$ larger than the vacuum dipole. Including the degenerate higher energy solutions allows for larger theoretical limits of up to $\sim 80\%$ more energy with respect to the vacuum case, helicity of the order of $\sim 8$, and a dipole strength that can now be up to four times that of the vacuum dipole, while the twist can be significantly larger and even diverge for configurations with disconnected domains. The higher energy solutions are probably unstable, therefore, it is unlikely that such magnetospheres exist under normal conditions in magnetars and high magnetic field pulsars., Comment: 12 pages, 9 figures; added missing reference; accepted for publication in MNRAS

We study the torsional magneto-elastic oscillations of relativistic superfluid magnetars and explore the effects of a phase transition in the crust-core interface (nuclear pasta) which results in a weaker elastic response. Exploring various models with different extension of nuclear pasta phases, we find that the differences in the oscillation spectrum present in purely elastic modes (weak magnetic field), are smeared out with increasing strength of the magnetic field. For magnetar conditions, the main characteristic and features of models without nuclear pasta are preserved. We find in general two classes of magneto-elastic oscillations which exhibit a different oscillation pattern. For B_p < 4 x 10^{14} G, the spectrum is characterised by the turning points and edges of the continuum which are confined into the star's core, and have no constant phase. Increasing the magnetic field, we find, in addition, several magneto-elastic oscillations which reach the surface and have an angular structure similar to crustal modes. These global magneto-elastic oscillations show a constant phase and become dominant when B_p > 5 x 10^{14} G. We do not find any evidence of fundamental pure crustal modes in the low frequency range (below 200 Hz) for B_p >= 10^{14} G., 13 pages, 8 figures, 3 tables, version accepted for publication in MNRAS

The study of long-term evolution of neutron star (NS) magnetic fields is key to understanding the rich diversity of NS observations, and to unifying their nature despite the different emission mechanisms and observed properties. Such studies in principle permit a deeper understanding of the most important parameters driving their apparent variety, e.g. radio pulsars, magnetars, x-ray dim isolated neutron stars, gamma-ray pulsars. We describe, for the first time, the results from self-consistent magneto-thermal simulations considering not only the effects of the Hall-driven field dissipation in the crust, but adding a complete set of proposed driving forces in a superconducting core. We emphasize how each of these core-field processes drive magnetic evolution and affect observables, and show that when all forces are considered together in vectorial form, the net expulsion of core magnetic flux is negligible, and will have no observable effect in the crust (consequently in the observed surface emission) on megayear time-scales. Our new simulations suggest that strong magnetic fields in NS cores (and the signatures on the NS surface) will persist long after the crustal magnetic field has evolved and decayed, due to the weak combined effects of dissipation and expulsion in the stellar core., 15 pages, 11 figures, accepted by MNRAS

We revisit the various approximations employed to study the long-term evolution of the magnetic field in neutron star cores and discuss their limitations and possible improvements. A recent controversy on the correct form of the induction equation and the relevant evolution timescale in superconducting neutron star cores is addressed and clarified. We show that this ambiguity in the estimation of timescales arises as a consequence of nominally large terms that appear in the induction equation, but which are, in fact, mostly irrotational. This subtlety leads to a discrepancy by many orders of magnitude when velocity fields are absent or ignored. Even when internal velocity fields are accounted for, only the solenoidal part of the electric field contributes to the induction equation, which can be substantially smaller than the irrotational part. We also argue that stationary velocity fields must be incorporated in the slow evolution of the magnetic field as the next level of approximation., 6 pages, version accepted by MNRAS

We present a long-term phase-coherent timing analysis and pulse-phase resolved spectroscopy for the two outbursts observed from the transient anomalous X-ray pulsar CXOU J164710.2-45521. For the first outburst we used 11 Chandra and XMM-Newton observations between September 2006 to August 2009, the longest baseline yet for this source. We obtain a coherent timing solution with $P=10.61065583(4)$ s, $\dot{P} = 9.72(1) \times 10^{-13}\;$s s$^{-1}$ and $\ddot{P} = -1.05(5)\times10^{-20}\; $s s$^{-2}$. Under the standard assumptions this implies a surface dipolar magnetic field of $\sim 10^{14}$ G, confirming this source as a standard-$B$ magnetar. We also study the evolution of the pulse profile (shape, intensity and pulsed fraction) as a function of time and energy. Using the phase-coherent timing solution we perform a phase-resolved spectroscopy analysis, following the spectral evolution of pulse-phase features, which hints at the physical processes taking place on the star. The results are discussed from the perspective of magneto-thermal evolution models and the untwisting magnetosphere model. Finally, we present similar analysis for the second, less intense, 2011 outburst. For the timing analysis we used Swift data together with 2 XMM-Newton and Chandra pointings. The results inferred for both outbursts are compared and briefly discussed in a more general framework., Comment: 13 pages, 14 figures

The thermal X-ray spectra of several isolated neutron stars display deviations from a pure blackbody. The accurate physical interpretation of these spectral features bears profound implications for our understanding of the atmospheric composition, magnetic field strength and topology, and equation of state of dense matter. With specific details varying from source to source, common explanations for the features have ranged from atomic transitions in the magnetized atmospheres or condensed surface, to cyclotron lines generated in a hot ionized layer near the surface. Here we quantitatively evaluate the X-ray spectral distortions induced by inhomogeneous temperature distributions of the neutron star surface. To this aim, we explore several surface temperature distributions, we simulate their corresponding general relativistic X-ray spectra (assuming an isotropic, blackbody emission), and fit the latter with a single blackbody model. We find that, in some cases, the presence of a spurious 'spectral line' is required at a high significance level in order to obtain statistically acceptable fits, with central energy and equivalent width similar to the values typically observed. We also perform a fit to a specific object, RX J0806.4-4123, finding several surface temperature distributions able to model the observed spectrum. The explored effect is unlikely to work in all sources with detected lines, but in some cases it can indeed be responsible for the appearance of such lines. Our results enforce the idea that surface temperature anisotropy can be an important factor that should be considered and explored also in combination with more sophisticated emission models like atmospheres., 11 pages, 7 figures; accepted for publication in MNRAS

We study ambipolar diffusion in strongly magnetised neutron stars, with special focus on the effects of neutrino reaction rates and the impact of a superfluid/superconducting transition in the neutron star core. For axisymmetric magnetic field configurations, we determine the deviation from $\beta-$equilibrium induced by the magnetic force and calculate the velocity of the slow, quasi-stationary, ambipolar drift. We study the temperature dependence of the velocity pattern and clearly identify the transition to a predominantly solenoidal flow. For stars without superconducting/superfluid constituents and with a mixed poloidal-toroidal magnetic field of typical magnetar strength, we find that ambipolar diffusion proceeds fast enough to have a significant impact on the magnetic field evolution only at low core temperatures, $T \lesssim 1-2\times10^8$ K. The ambipolar diffusion timescale becomes appreciably shorter when fast neutrino reactions are present, because the possibility to balance part of the magnetic force with pressure gradients is reduced. We also find short ambipolar diffusion timescales in the case of superconducting cores for $T \lesssim 10^9$ K, due to the reduced interaction between protons and neutrons. In the most favourable scenario, with fast neutrino reactions and superconducting cores, ambipolar diffusion results in advection velocities of several km/kyr. This velocity can substantially reorganize magnetic fields in magnetar cores, in a way that can only be confirmed by dynamical simulations., Comment: 14 pages, 11 figures, version accepted for publication in MNRAS