The Discovery of an X-ray/UV Stellar Flare from the Late-K/Early-M Dwarf LMC 335

We report the discovery of an X-ray/UV stellar flare from the source LMC 335, captured by XMM-Newton in the field of the Large Magellanic Cloud. The flare event was recorded continuously in X-ray for its first 10 hours from the precursor to the late decay phases. The observed fluxes increased by more than two orders of magnitude at its peak in X-ray and at least one in the UV as compared to quiescence. The peak 0.1-7.0 keV X-ray flux is derived from the two-temperature APEC model to be ~(8.4 +/- 0.6) x 10^-12 erg cm-2 s-1. Combining astrometric information from multiple X-ray observations in the quiescent and flare states, we identify the NIR counterpart of LMC 335 as the 2MASS source J05414534-6921512. The NIR color relations and spectroscopic parallax characterize the source as a Galactic K7-M4 dwarf at a foreground distance of (100 - 264) pc, implying a total energy output of the entire event of ~(0.4 - 2.9) x 10^35 erg. This report comprises detailed analyses of this late-K / early-M dwarf flare event that has the longest time coverage yet reported in the literature. The flare decay can be modeled with two exponential components with timescales of ~28 min and ~4 hours, with a single component decay firmly ruled out. The X-ray spectra during flare can be described by two components, a dominant high temperature component of ~40-60MK and a low temperature component of ~10MK, with a flare loop length of about 1.1-1.3 stellar radius.Comment: 35 pages, 6 figures, 5 tables, accepted for publication in Ap

On the vacuum of the minimal nonsupersymmetric SO(10) unification

We study a class of nonsupersymmetric SO(10) grand unified scenarios where the first stage of the symmetry breaking is driven by the vacuum expectation values of the 45-dimensional adjoint representation. Three decade old results claim that such a Higgs setting may lead exclusively to the flipped SU(5) x U(1) intermediate stage. We show that this conclusion is actually an artifact of the tree level potential. The study of the accidental global symmetries emerging in various limits of the scalar potential offers a simple understanding of the tree level result and a rationale for the drastic impact of quantum corrections. We scrutinize in detail the simplest and paradigmatic case of the 45_{H} + 16_{H} Higgs sector triggering the breaking of SO(10) to the standard electroweak model. We show that the minimization of the one-loop effective potential allows for intermediate SU(4)_C x SU(2)_L x U(1)_R and SU(3)_c x SU(2)_L x SU(2)_R x U(1)_{B-L} symmetric stages as well. These are the options favoured by gauge unification. Our results, that apply whenever the SO(10) breaking is triggered by , open the path for hunting the simplest realistic scenario of nonsupersymmetric SO(10) grand unification.Comment: 22 pages, 1 figure. Refs added. To appear in Phys. Rev.

Real-Time RGB-D Camera Pose Estimation in Novel Scenes using a Relocalisation Cascade

Camera pose estimation is an important problem in computer vision. Common techniques either match the current image against keyframes with known poses, directly regress the pose, or establish correspondences between keypoints in the image and points in the scene to estimate the pose. In recent years, regression forests have become a popular alternative to establish such correspondences. They achieve accurate results, but have traditionally needed to be trained offline on the target scene, preventing relocalisation in new environments. Recently, we showed how to circumvent this limitation by adapting a pre-trained forest to a new scene on the fly. The adapted forests achieved relocalisation performance that was on par with that of offline forests, and our approach was able to estimate the camera pose in close to real time. In this paper, we present an extension of this work that achieves significantly better relocalisation performance whilst running fully in real time. To achieve this, we make several changes to the original approach: (i) instead of accepting the camera pose hypothesis without question, we make it possible to score the final few hypotheses using a geometric approach and select the most promising; (ii) we chain several instantiations of our relocaliser together in a cascade, allowing us to try faster but less accurate relocalisation first, only falling back to slower, more accurate relocalisation as necessary; and (iii) we tune the parameters of our cascade to achieve effective overall performance. These changes allow us to significantly improve upon the performance our original state-of-the-art method was able to achieve on the well-known 7-Scenes and Stanford 4 Scenes benchmarks. As additional contributions, we present a way of visualising the internal behaviour of our forests and show how to entirely circumvent the need to pre-train a forest on a generic scene.Comment: Tommaso Cavallari, Stuart Golodetz, Nicholas Lord and Julien Valentin assert joint first authorshi

Long-term X-ray Variability Study of IC342 from XMM-Newton Observations

We presented the results of an analysis of four XMM-Newton observations of the starburst galaxy IC342 taken over a four-year span from 2001 to 2005, with an emphasis on investigating the long-term flux and spectral variability of the X-ray point sources. We detected a total of 61 X-ray sources within 35' $\times$ 30' of the galaxy down to a luminosity of (1-2)$\times$1037 erg s-1 depending on the local background. We found that 39 of the 61 detected sources showed long-term variability, in which 26 of them were classified as X-ray transients. We also found 19 sources exhibiting variations in hardness ratios or undergoing spectral transitions among observations, and were identified as spectral variables. In particular, 8 of the identified X-ray transients showed spectral variability in addition to flux variability. The diverse patterns of variability observed is indicative of a population of X-ray binaries. We used X-ray colors, flux and spectral variability, and in some cases the optical or radio counterparts to classify the detected X-ray sources into several stellar populations. We identified a total of 11 foreground stars, 1 supersoft sources (SSS), 3 quasisoft sources (QSS), and 2 supernova remnants (SNR). The identified SSS/QSS are located near or on the spiral arms, associate with young stellar populations; the 2 SNR are very close to the starburst nucleus where current star formation activities are dominated. We also discovered a spectral change in the nuclear source of IC342 for the first time by a series of X-ray spectrum analysis.Comment: 45 pages, 6 figures accepted by Ap

A Luminous Be+White Dwarf Supersoft Source in the Wing of the SMC: MAXI J0158-744

We present a multi-wavelength analysis of the very fast X-ray transient MAXI J0158-744, which was detected by MAXI/GSC on 2011 November 11. The subsequent exponential decline of the X-ray flux was followed with Swift observations, all of which revealed spectra with low temperatures (~100eV) indicating that MAXI J0158-744 is a new Supersoft Source (SSS). The Swift X-ray spectra near maximum show features around 0.8 keV that we interpret as possible absorption from OVIII, and emission from O, Fe, and Ne lines. We obtained SAAO and ESO optical spectra of the counterpart early in the outburst and several weeks later. The early spectrum is dominated by strong Balmer and HeI emission, together with weaker HeII emission. The later spectrum reveals absorption features that indicate a B1/2IIIe spectral type, and all spectral features are at velocities consistent with the Small Magellanic Cloud. At this distance, it is a luminous SSS (>10^37 erg/s) but whose brief peak luminosity of >10^39 erg/s in the 2-4 keV band makes it the brightest SSS yet seen at "hard" X-rays. We propose that MAXI J0158-744 is a Be-WD binary, and the first example to possibly enter ULX territory. The brief hard X-ray flash could possibly be a result of the interaction of the ejected nova shell with the B star wind in which the white dwarf (WD) is embedded. This makes MAXI J0158-744 only the third Be/WD system in the Magellanic Clouds, but it is by far the most luminous. The properties of MAXI J0158-744 give weight to previous suggestions that SSS in nearby galaxies are associated with early-type stellar systems.Comment: 15 pages, 8 figures; ApJ accepte

Assortative Mixing Equilibria in Social Network Games

It is known that individuals in social networks tend to exhibit homophily (a.k.a. assortative mixing) in their social ties, which implies that they prefer bonding with others of their own kind. But what are the reasons for this phenomenon? Is it that such relations are more convenient and easier to maintain? Or are there also some more tangible benefits to be gained from this collective behaviour? The current work takes a game-theoretic perspective on this phenomenon, and studies the conditions under which different assortative mixing strategies lead to equilibrium in an evolving social network. We focus on a biased preferential attachment model where the strategy of each group (e.g., political or social minority) determines the level of bias of its members toward other group members and non-members. Our first result is that if the utility function that the group attempts to maximize is the degree centrality of the group, interpreted as the sum of degrees of the group members in the network, then the only strategy achieving Nash equilibrium is a perfect homophily, which implies that cooperation with other groups is harmful to this utility function. A second, and perhaps more surprising, result is that if a reward for inter-group cooperation is added to the utility function (e.g., externally enforced by an authority as a regulation), then there are only two possible equilibria, namely, perfect homophily or perfect heterophily, and it is possible to characterize their feasibility spaces. Interestingly, these results hold regardless of the minority-majority ratio in the population. We believe that these results, as well as the game-theoretic perspective presented herein, may contribute to a better understanding of the forces that shape the groups and communities of our society

X-ray Point Sources in The Central Region of M31 as seen by Chandra

We report on \chandra observations of the central region of M31. By combining eight \chandra ACIS-I observations taken between 1999 and 2001, we have identified 204 X-ray sources within the central $\sim 17'\times17'$ region of M31, with a detection limit of $\sim 2\times10^{35}$\lum. Of these 204 sources, 22 are identified with globular clusters, 2 with supernova remnants, 9 with planetary nebula, and 9 as supersoft sources. By comparing individual images, about 50% of the sources are variable on time scales of months. We also found 13 transients, with light curves showing a variety of shapes. We also extracted the energy spectra of the 20 brightest sources; they can be well fit by a single power-law with a mean photon index of 1.8. The spectral shapes of 12 sources are shown to be variable, suggesting that they went through state changes. The luminosity function of all the point sources is consistent with previous observations (a broken power-law with a luminosity break at $1.7\times10^{37}$\lum). However, when the X-ray sources in different regions are considered separately, different luminosity functions are obtained. This indicates that the star-formation history might be different in different regions.Comment: 42 pages, 9 figures, ApJ, accepted, Higher-resolution figures available on reques