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The bright high-mass X-ray binary GX 301-2 exhibits two periodic flare episodes along its orbit which are produced when the neutron star is close to the apastron and periastron passages. Time-resolved spectra were extracted and several models applied to describe all of them. The best description was obtained with a blackbody continuum modified by the Fe K-shell absorption edge, absorbed by a large column density on the order of $10^{23}$ cm$^{-2}$ and, if present, a Fe K$\alpha$ fluorescent emission line. Three of the nine apastron flares were as bright as the pre-periastron flare and two of them coincided with spin-up episodes of the neutron star. This fact points to the presence of a transient disc around the neutron star as it passes through the apastron that increases the accretion process. The size of emitting region on the neutron star surface showed some variability but quite consistent with a hot spot., Comment: 36 pages, 77 figures, accepted for publication in RMxAA. It will appear in Vol. 61, N{\deg}1, April 2025

The aim of this work is to study both light curve and orbital phase spectroscopy of this source taking advantage of the MAXI/GSC observation strategy. We have investigated the spectral and light curve properties of the X-ray emission from Cen X-3 along the binary orbit. These studies allow delimiting the stellar wind properties and its interactions with the compact object. A timing analysis of light curves in different energy bands was carried out. From the analysis of the light curve, we have estimated the orbital period of the binary system and also found possible QPOs around a superorbital period of $P_\mathrm{superorb} = 220\pm 5$ days. Both orbital phase-averaged and phase-resolved spectra were extracted and analysed in the 2.0-20.0 keV energy range. We have defined and compared the high and low states spectra with the averaged spectrum. Two models have described spectra satisfactorily, a partial absorbed Comptonization of cool photons on hot electrons plus a power law and a partial absorbed blackbody plus a power law, both modified by adding Gaussian lines. The radius of the blackbody emitting area has been determined and the high value of the X-ray luminosity in the averaged spectrum indicates that the accretion mode is not only due to the stellar wind., Comment: 41 pages (draft version), 21 figures. Accepted for publication in Revista Mexicana de Astronom\'ia y Astrof\'isica (RMxAA). This paper will appear in Vol. 58, No 2, October 2022

Hypothesis: A broad range of phenomena, such as emulsification and emulsion stability, foam formation or liquid evaporation, are closely related to the dynamics of adsorbing colloidal particles. Elucidation of the mechanisms implied is key to a correct design of many different types of materials. Experiments: Microspheres forced to rotate near a fluid interface exhibit a roto-translational hydrodynamic mechanism that is hindered by capillary torques as soon as the particles protrude the interface. Under these conditions, the time evolution in the ratio of moving spheres provides a direct description of the adsorption kinetics, while microscopy monitoring of particle acceleration\deceleration informs about the adsorption\desorption dynamics. In this work, the proposed strategy is applied at an air/water interface loaded with spherical magnetic particles negatively charged, forced to rotate by the action of a rotating magnetic field. Findings: The proposed method enables the adsorption/desorption dynamics to be followed during the earliest phase of the process, when desorption of a small fraction of particles is detected, as well as to estimate approximated values of the adsorption/desorption constants. The results obtained show that the addition of a monovalent salt or a cationic (anionic) surfactant promotes (inhibits) both adsorption and formation of permanent bonds between particles.

The aim of this work is to characterize the counterpart to the INTEGRAL High Mass X-ray Binary candidate IGR J19294+1816 so as to establish its true nature. We obtained H band spectra of the selected counterpart acquired with the NICS instrument mounted on the Telescopio Nazionale Galileo (TNG) 3.5-m telescope which represents the first infrared spectrum ever taken of this source. We complement the spectral analysis with infrared photometry from UKIDSS, 2MASS, WISE and NEOWISE databases. We classify the mass donor as a Be star. Subsequently, we compute its distance by properly taking into account the contamination produced by the circumstellar envelope. The findings indicate that IGR J19294+1816 is a transient source with a B1Ve donor at a distance of $d = 11 \pm 1$ kpc, and luminosities of the order of $10^{36-37}$ erg s$^{-1}$, displaying the typical behaviour of a Be X-ray binary., Comment: 8 pages, 6 figures, accepted to be published in MNRAS

Based on new Chandra X-ray telescope data, we present empirical evidence of plasma Compton cooling during a flare in the non pulsating massive X-ray binary 4U1700-37. This behaviour might be explained by quasispherical accretion onto a slowly rotating magnetised neutron star. In quiescence, the neutron star in 4U1700-37 is surrounded by a hot radiatively cooling shell. Its presence is supported by the detection of mHz quasi periodic oscillations likely produced by its convection cells. The high plasma temperature and the relatively low X-ray luminosity observed during the quiescence, point to a small emitting area about 1 km, compatible with a hot spot on a NS surface. The sudden transition from a radiative to a significantly more efficient Compton cooling regime triggers an episode of enhanced accretion resulting in a flare. During the flare, the plasma temperature drops quickly. The predicted luminosity for such transitions, Lx = 3 x 10^35 erg s-1, is very close to the luminosity of 4U1700-37 during quiescence. The transition may be caused by the accretion of a clump in the stellar wind of the donor star. Thus, a magnetised NS nature of the compact object is strongly favoured., Comment: Accepted for publication in MNRAS

Classical Supergiant X-ray Binaries (SGXBs) and Supergiant Fast X-ray Transients (SFXTs) are two types of High-mass X-ray Binaries (HMXBs) that present similar donors but, at the same time, show very different behavior in the X-rays. The reason for this dichotomy of wind-fed HMXBs is still a matter of debate. Among the several explanations that have been proposed, some of them invoke specific stellar wind properties of the donor stars. Only dedicated empiric analysis of the donors' stellar wind can provide the required information to accomplish an adequate test of these theories. However, such analyses are scarce. To close this gap, we perform a comparative analysis of the optical companion in two important systems: IGR J17544-2619 (SFXT) and Vela X-1 (SGXB). We analyse the spectra of each star in detail and derive their stellar and wind properties. We compare the wind parameters, giving us an excellent chance of recognizing key differences between donor winds in SFXTs and SGXBs. We find that the stellar parameters derived from the analysis generally agree well with the spectral types of the two donors: O9I (IGR J17544-2619) and B0.5Iae (Vela X-1). An important difference between the stellar winds of the two stars is their terminal velocities v_inf=1500km/s in IGR J17544-2619 and v_inf=700km/s in Vela~X-1, which has important consequences on the X-ray luminosity of these sources. Their specific combination of wind speed and pulsar spin favours an accretion regime with a persistently high luminosity in Vela X-1, while it favours an inhibiting accretion mechanism in IGR~J17544-2619. Our study demonstrates that the wind relative velocity is critical in the determination of the class of HMXBs hosting a supergiant donor, given that it may shift the accretion mechanism from direct accretion to propeller regimes when combined with other parameters., Comment: Accepted for publication in A&A. 25 pages, 32 figures

4U 1538-52, an absorbed high mass X-ray binary with an orbital period of 3.73 days, shows moderate orbital intensity modulations with a low level of counts during the eclipse. Several models have been proposed to explain the accretion at different orbital phases by a spherically symmetric stellar wind from the companion. The aim of this work is to study both the light curve and orbital phase spectroscopy of this source in the long term. Particularly, the folded light curve and the changes of the spectral parameters with orbital phase to analyse the stellar wind of QV Nor, the mass donor of this binary system. We used all the observations made from the Gas Slit Camera on board MAXI of 4U 1538-52 covering many orbits continuously. We obtained the good interval times for every orbital phase range which were the input to extract our data. We estimated the orbital period of the system and then folded the light curves and we fitted the X-ray spectra with the same model for every orbital phase spectrum. We also extracted the averaged spectrum of all the MAXI data available. The MAXI spectra in the 2-20 keV energy range were fitted with an absorbed Comptonization of cool photons on hot electrons. We found a strong orbital dependence of the absorption column density but neither the fluorescence iron emission line nor low energy excess were needed to fit the MAXI spectra. The variation of the spectral parameters over the binary orbit were used to examine the mode of accretion onto the neutron star in 4U 1538-52. We deduce a best value of $\dot{M}/v_\infty=0.65\times 10^{-9}$ $M_{\odot} \, yr^{-1}/(km \, s^{-1})$ for QV Nor., Comment: 12 pages, 5 figures, accepted to be published by A&A, corrected typos (changing bold font to normal one)

We present a comprehensive analysis of the whole sample of available XMM-Newton observations of High Mass X-ray Binaries (HMXBs) until August, 2013, focusing on the FeK{\alpha} emission line. This line is a key tool to better understand the physical properties of the material surrounding the X-ray source within a few stellar radii (the circumstellar medium). We have collected observations from 46 HMXBs, detecting FeK{\alpha} in 21 of them. We have used the standard classification of HMXBs to divide the sample in different groups. We find that: (1) FeK{\alpha} is centred at a mean value of 6.42 keV. Considering the instrumental and fits uncertainties, this value is compatible with ionization states lower than FeXVIII. (2) The flux of the continuum is well correlated with the flux of the line, as expected. Eclipse observations show that the Fe fluorescence emission comes from an extended region surrounding the X-ray source. (3) FeK{\alpha} is narrow (width lower than 0.15keV), reflecting that the reprocessing material does not move at high speeds. We attempt to explain the broadness of the line in terms of three possible broadening phenomena: line blending, Compton scattering and Doppler shifts (with velocities of the reprocessing material V=1000-2000 km/s). (4) The equivalent hydrogen column (NH) directly correlates with the EW of FeK{\alpha}, displaying clear similarities to numerical simulations. It highlights the strong link between the absorbing and the fluorescent matter. The obtained results clearly point to a very important contribution of the donors wind in the FeK{\alpha} emission and the absorption when the donor is a supergiant massive star., Comment: Accepted for publication in A&A. 13 pages, 16 figures + Appendices

High resolution X-ray spectroscopy is a powerful tool for studying the nature of the matter surrounding the neutron star in X-ray binaries and its interaction between the stellar wind and the compact object. In particular, absorption features in their spectra could reveal the presence of atmospheres of the neutron star or their magnetic field strength. Here we present an investigation of the absorption feature at 2.1 keV in the X-ray spectrum of the high mass X-ray binary 4U 1538-52 based on our previous analysis of the XMM-Newton data. We study various possible origins and discuss the different physical scenarios in order to explain this feature. A likely interpretation is that the feature is associated with atomic transitions in an O/Ne neutron star atmosphere or of hydrogen and helium like Fe or Si ions formed in the stellar wind of the donor., Comment: 8 pages, 6 figures (three of them multi-panel figures), accepted for publication in Astronomische Nachrichten/Astronomical Notes

Here we report the infrared identification of the X-ray source 2XMM J191043.4+091629.4, which was detected by XMM-Newton/EPIC in the vicinity of the Galactic supernova remnant W49B. The aim of this work is to establish the nature of the X-ray source 2XMM J191043.4+091629.4 studying both the infrared photometry and spectroscopy of the companion. We analyse UKIDSS images around the best position of the X-ray source, and obtain spectra of the best candidate using NICS in the 3.5 m TNG telescope. We present both photometric and spectroscopic TNG analysis of the infrared counterpart of the X-ray source identifying emission lines in the K-band. H-band spectra does not present any significant feature. We have showed that Brackett gamma H I, at 2.165 micrometre, and He I, at 2.184 micrometre and at 2.058 micrometre, are present significantly in the infrared spectrum. The CO bands are also not present in our spectrum. Based on these results and the X-ray characteristics of the source we conclude that the infrared counterpart is an early B-type supergiant star with an E(B-V)= 7.6 +/- 0.3 at a distance of 16.0 +/- 0.5 kpc. This would be, therefore, the first high-mass X-ray binary in the Outer Arm at galactic longitudes between 30 and 60 degrees., Comment: 5 pages, 4 figures (two of them multi-panel figures), accepted for publication in Astronomy & Astrophysics

The improvement of the capabilities of nowadays X-ray observatories, like Chandra or XMM-Newton, offers the possibility to detect both absorption and emission lines and to study the nature of the matter surrounding the neutron star in X-ray binaries and the phenomena that produce these lines. The aim of this work is to discuss the different physical scenarios in order to explain the meaning of the significant absorption feature present in the X-ray spectrum of 4U 1538-52. Using the last available calibrations, we discard the possibility that this feature is due to calibration, gain effects or be produced by the X-ray background or a dust region. Giving the energy resolution of the XMM-Newton telescope we could not establish if the line is formed in the atmosphere of the neutron star or by the dispersion of the stellar wind of the optical counterpart., Comment: 6 pages, 2 multipanel figures, accepted for publication on Proceedings of "An INTEGRAL view of the high-energy sky (the first 10 years)" the 9th INTEGRAL Workshop, October 15-19, 2012, Paris, France, in Proceedings of Science (INTEGRAL 2012), Eds. A. Goldwurm, F. Lebrun and C. Winkler, (http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=176), id PoS(INTEGRAL 2012)031

Context. The properties of the X-ray emission lines are a fundamental tool for studying the nature of the matter surrounding the neutron star and the phenomena that produce these lines. Aims. The aim of this work is to analyze the X-ray spectrum of 4U 1538-52 obtained by the XMM-Newton observatory and to look for the presence of diagnostic lines in the energy range 0.3-11.5 keV. Methods. We used a 54 ks PN & MOS/XMM-Newton observation of the high mass X-ray binary 4U 1538-52 covering the orbital phase between 0.75 to 1.00 (the eclipse-ingress). We have modelled the 0.3-11.5 keV continuum emission with three absorbed power laws and looked for the emission lines. Results. We found previously unreported recombination lines, in this system, at 2.4 keV, 1.9 keV and 1.3 keV, consistent with the presence of highly ionized states of S XV He?, Si XIII He? and Mg K? or Mg XI He?. On the other hand, both out of eclipse and in eclipse we detect a fluorescence iron emission line at 6.4 keV which is resolved into two components: a narrow ($\sigma \leq$ 10 eV) fluorescence Fe K? line plus one hot line from highly photoionized Fe XXV. Conclusions. The detection of new recombination lines during eclipse-ingress in 4U 1538-52 indicates that there is an extended ionized region surrounding the neutron star., Comment: 11 pages, 6 figures, accepted for publication in Astronomy and Astrophysics

We present pulse phase averaged spectra of the high mass X-ray binary pulsar 4U 1538-52/QV Nor. Observations of this persistent accreting pulsar were made with the Rossi X-ray Timing Explorer (RXTE). We study the variability of cyclotron resonant scattering feature (CRSF or simply cyclotron line) in the high energy spectra of this binary system. We show that the parameters of the CRSF are correlated. The first one is, as suggested by theory, between the width and the energy of the cyclotron line. The second one is between the relative width and the optical depth of the cyclotron line. We discuss these results with studies of other X-ray pulsars and their implications on the line variability., Comment: 11 pages, 5 figures, Proceedings of the 3rd Symposium of the Astrophysics Group of the Spanish Royal Physical Society (Real Sociedad Espa\~nola de F\'isica, RSEF) (Granada 2007), Spain. Edited by A. Ulla and M. Manteiga. Torculo Press, ISBN 84-936098-0-1

Cyclotron resonant scattering features are an essential tool for studying the magnetic field of neutron stars. The fundamental line provides a measure of the field strength, while the harmonic lines provide information about the structure and configuration of the magnetic field. Until now only a handful of sources are known to display more than one cyclotron line and only two of them have shown a series of harmonics. The aim of this work is to see the first harmonic cyclotron line, confirming the fundamental line at around 22 keV, thus increasing the number of sources with detected harmonic cyclotron lines. To investigate the presence of absorption or emission lines in the spectra, we have combined RXTE and INTEGRAL spectra. We modeled the 3-100 keV continuum emission with a power law with an exponential cut off and look for the second absorption feature. We show evidence of an unknown cyclotron line at around 47 keV (the first harmonic) in the phase-averaged X-ray spectra of 4U 1538-52. This line is detected by several telescopes at different epochs, even though the signal-to-noise ratio of each individual spectrum is low. We conclude that the line-like absorption is a real feature, and the most straightforward interpretation is that it is the first harmonic, thus making 4U 1538-52 the fifth X-ray pulsar with more than one cyclotron line., Comment: 6 pages, 5 figures, accepted for publication in Astronomy and Astrophysics

We show how to obtain all covariant field equations for massless particles of arbitrary integer, or half-integer, helicity in four dimensions from the quantization of the rigid particle, whose action is given by the integrated extrinsic curvature of its worldline, {\ie} $S=\alpha\int ds \kappa$. This geometrical particle system possesses one extra gauge invariance besides reparametrizations, and the full gauge algebra has been previously identified as classical $\W_3$. The key observation is that the covariantly reduced phase space of this model can be naturally identified with the spinor and twistor descriptions of the covariant phase spaces associated with massless particles of helicity $s=\alpha$. Then, standard quantization techniques require $\alpha$ to be quantized and show how the associated Hilbert spaces are solution spaces of the standard relativistic massless wave equations with $s=\alpha$. Therefore, providing us with a simple particle model for Weyl fermions ($\alpha=1/2$), Maxwell fields ($\alpha=1$), and higher spin fields. Moreover, one can go a little further and in the Maxwell case show that, after a suitable redefinition of constraints, the standard Dirac quantization procedure for first-class constraints leads to a wave-function which can be identified with the gauge potential $A_\mu$. Gauge symmetry appears in the formalism as a consequence of the invariance under $\W_3$-morphisms, that is, exclusively in terms of the extrinsic geometry of paths in Minkowski space. When all gauge freedom is fixed one naturally obtains the standard Lorenz gauge condition on $A_{\mu}$, and Maxwell equations in that gauge. This construction has a direct generalization to arbitrary integer values of $\alpha$, and we comment on the physically interesting case of linearized Einstein gravity ($\alpha =2$)., Comment: 16 pages, plain TeX (macros included)

We provide a description of W_3 transformations in terms of deformations of convex curves in two dimensional Euclidean space. This geometrical interpretation sheds some light on the nature of finite W_3-morphisms. We also comment on how this construction can be extended to the case of W_n and ``nicely curved'' curves in $\reals^{n-1}$., Comment: plain TeX (macros included). Slightly expanded version published in Phys. Lett. B

We prove that particle models whose action is given by the integrated $n$-th curvature function over the world line possess $n+1$ gauge invariances. A geometrical characterization of these symmetries is obtained via Frenet equations by rephrasing the $n$-th curvature model in $\reals^d$ in terms of a standard relativistic particle in $S^{d-n}$. We ``prove by example'' that the algebra of these infinitesimal gauge invariances is nothing but $\W_{n+2}$, thus providing a geometrical picture of the $\W$-symmetry for these models. As a spin-off of our approach we give a new global invariant for four-dimensional curves subject to a curvature constraint., Comment: plain TeX (macros included). Slightly modified version published in Nuc. Phys. B

Classical {\W}$_3$ transformations are discussed as restricted diffeomorphism transformations (\W-Diff) in two-dimensional space. We formulate them by using Riemannian geometry as a basic ingredient. The extended {\W}$_3$ generators are given as particular combinations of Christoffel symbols. The defining equations of \W-Diff are shown to depend on these generators explicitly. We also consider the issues of finite transformations, global $SL(3)$ transformations and \W-Schwarzians., Comment: 10 pages, UB-ECM-PF 94/20, TOHO-FP-9448, QMW-PH-94-25

We prove that $\W_3$ is the gauge symmetry of the scale-invariant rigid particle, whose action is given by the integrated extrinsic curvature of its world line. This is achieved by showing that its equations of motion can be written in terms of the Boussinesq operator. The $\W_3$ generators $T$ and $W$ then appear respectively as functions of the induced world line metric and the extrinsic curvature. We also show how the equations of motion for the standard relativistic particle arise from those of the rigid particle whenever it is consistent to impose the ``zero-curvature gauge'', and how to rewrite them in terms of the $\KdV$ operator. The relation between particle models and integrable systems is further pursued in the case of the spinning particle, whose equations of motion are closely related to the $\SKdV$ operator. We also partially extend our analysis in the supersymmetric domain to the scale invariant rigid particle by explicitly constructing a supercovariant version of its action. Comment: This is an expanded version of hep-th/9406072 (to be published in the Proceedings of the Workshop on the Geometry of Constrained Dynamical Systems, held at the Isaac Newton Institute for Mathematical Sciences, Cambridge, June 14-18, 1994.)., Comment: 14 pages, plain TeX (macros included). QMW-PH-94-23

We show that W_3 is the algebra of symmetries of the ``rigid-particle'', whose action is given by the integrated extrinsic curvature of its world line. This is easily achived by showing that its equation of motion can be written in terms of the Boussinesq operator. We also show how to obtain the equations of motion of the standard relativistic particle provided it is consistent to impose the ``zero-curvature gauge'', and comment about its connection with the KdV operator., Comment: 9 pages. Plain TeX (macros included). QMW-PH-94-16

We introduce a particle mechanics model with Sp($2M$) gauge invariance. Different partial gauge-fixings by means of sl(2) embeddings on the gauge algebra lead to reduced models which are invariant under diffeomorphisms and classical non-linear \W-transformations as the residual gauge symmetries thus providing a set of models of gauge and matter fields coupled in a \W-invariant way. The equations of motion for the matter variables give Lax operators in a matrix form. We examine several examples in detail and discuss the issue of integration of infinitesimal \W-transformations., Comment: 34 pages, LaTeX, no figures. Several changes (including Title) and new material added. Version to appear in Ann. Phys