Mass-loss rate estimation for the massive binary 4U 1538-52/QV Nor

"The X-ray Universe 2008" Symposium held in Granada, Spain, 27-30 May, 2008; Published online at https://www.cosmos.esa.int/web/xmm-newton/2008-symposium, id.56.We present an analysis of archival RXTE data of the X- ray binary source 4U 1538−52/QV Nor. The RXTE ob- servatory made one complete binary cycle observation on January 1997 and 2001, respectively. The X-ray con- tinuum data are well described by an absorbed Nega- tive Positive power laws Exponentials (NPEX) compo- nent modified by an iron emission line at 6.4 keV and a cyclotron absorption line at 20 keV. Using a simple spher- ically symmetric wind model to describe the X-ray ab- sorption variations as a function of orbital phase, we in- ferred a wind mass-loss rate from the companion star of (1.3 − 2.5) × 10−6 M⊙/yr. Our results are consistent with those obtained by the Ginga X-ray observatory. We have also analyzed X-ray flux variations over the binary orbit and we have found a sharp increase at orbital phase 0.34 in both set of data. However, the increase at orbital phase 0.66 is significantly different in the two observa- tions

The fundamental cyclotron line in 4U 2538-52

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.This research is partially supported by the Spanish INTEGRAL: Observaciones multifrecuencia de sistemas binarios de rayos X project number ESP2001-4541-PE and International Gamma Ray Astrophysics Lab. Operaciones. C3 project number ESP2002-04124-C03-03. This research has made use of data ob- tained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center

The first X-ray spectrum of the high-mass X-ray binary XTE J1855−026 during the compact object eclipse

We present the first XMM–Newton observation of the classical supergiant high-mass X-ray binary XTE J1855−026 taken entirely during the eclipse of the neutron star (NS), covering the orbital phases ϕ = 0.00–0.11. The analysis of the data allows us to (a) compare with the parameters obtained during the existing pre-eclipse observation and (b) explore the back-illuminated stellar wind of the B0I-type donor. The blackbody component, used to describe the soft excess during pre-eclipse, is not observed during eclipse. It must be then produced near the NS or along the donor–NS line. The 0.3–10 keV luminosity during eclipse (∼1034 erg s−1) is 70 times lower than pre-eclipse. The intensity of the Fe Kα line, in the average eclipse spectrum, is ∼7.4 times lower than the one measured during pre-eclipse. Since Kα photons cannot be resonantly scattered in the wind, the vast majority of Fe Kα emission must come from distances within 1R* from the NS. The eclipse spectrum is successfully modelled through the addition of two photoionized plasmas, one with low ionization (log ξ1,cold = 0.36) and high emission measure (EM1,cold ≈ 3 × 1059 cm−3) and another with high ionization (log ξ2,hot = 3.7) and low emission measure (EM2,hot ≈ 2 × 1056 cm−3). Assuming that the cold and hot gas phases are the clumps and the interclump medium of the stellar wind, respectively, and a clump volume filling factor of ≈0.04–0.05, typical for massive stars, a density contrast between clumps and the interclump medium of nc/ni ≈ 180 is deduced, in agreement with theoretical expectations and optical–ultraviolet observations of massive star winds

Astronomy outreach from the Busot observatory in lockdown times

Contribution to the XIV.0 Scientific Meeting (virtual) of the Spanish Astronomical Society, held 13-15 July 2020, online at https://www.sea-astronomia.es/reunion-cientifica-2020, id.258Astronomy outreach by live observation with remote telescope is one of the most powerful and attractive opportunities for members of the public to become involved in. During the Spanish state of emergency due to COVID-19, we developed outreach activities using the Busot Astronomical Observatory and social media spreading astronomy to the general public.OBRIMED1-16I and OBRIMEDI-20I

On the physical meaning of the 2.1 keV absorption feature in 4U 1538-52

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.This work was supported by the MICINN project number AYA2010-15431

Establishing the nature of X-ray binaries through infrared spectroscopy

European Space Agency's INTEGRAL (International Gamma-Ray Astrophysics Laboratory) satellite have discovered new X-ray sources due to its sensitivity above 20 keV. Most of these sources suffer from high absorption and the classical blue band spectral classification region is normally not accessible. This can be overcome, however, through infrared spectroscopy. The characterisation of these systems can influence the population synthesis models currently in use. In this work, we present the first H and K band spectra for three INTEGRAL sources using the NICS instrument mounted on the Telescopio Nazionale Galileo (TNG) 3.5-m telescope. This study was complemented with infrared photometry from UKIDSS, 2MASS, WISE and NEOWISE databases. Our spectra show all the significant features in emission and are, thus, consistent with a Be nature of the companions. Owing to their X-ray characteristics, these systems were classified as Be X-ray binaries. This allowed us to refine its distances to the sources using suitable calibrations that take into account the contamination by the circumstellar disk.This work was supported partially by the project ESP2017-85691-P AM acknowledges the support by the VIDI de la UA under visiting programme INV17-26

Cen X–3 as seen by MAXI during six years

The aim of this work is to study both light curve and orbital phase spectroscopy of Cen X-3 taking advantage of the MAXI/GSC observation strategy. These studies allow delimiting the stellar wind properties and its interactions with the compact object. 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 Psuperorb = 220 ± 5 days. Both orbital phase-averaged and phase-resolved spectra were extracted and analysed in the 2.0{20.0 keV energy range. 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 high value of the X-ray luminosity in the averaged spectrum indicates that the accretion mode is not only due to the stellar wind.Se pretende estudiar la curva de luz y la espectroscopía en fase orbital de Cen X-3 gracias a la estrategia de observación de MAXI/GSC. Estos estudios permiten delimitar las propiedades del viento estelar y sus interacciones con el objeto compacto. Por medio de este análisis hemos estimado el período orbital del sistema y hemos encontrado posibles QPOs alrededor de un superperíodo orbital de Psuperorb = 220 ± 5 días. Hemos extraído y analizado los espectros medio y en fase orbital en el intervalo 2.0{20.0 keV por medio de dos modelos: una comptonización parcialmente absorbida de fotones fróos en electrones calientes más una ley de potencias y un cuerpo negro parcialmente absorbido más una ley de potencias, ambos modificados por líneas Gaussianas. El alto valor de la luminosidad del espectro medio indica un modo de acreción no debido solamente al viento estelar.This work was supported by the Spanish Ministry of Education and Science project number ESP2017-85691-P

Discussing the physical meaning of the absorption feature at 2.1 keV in 4U 1538–52

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.This work was supported by the Spanish Ministry of Education and Science “De INTEGRAL a IXO: binarias de rayos X y estrellas activas”, project number AYA2010-15431, partially by GV2014/088 and MIF/14G04. JJRR acknowledges the support by the Spanish Ministerio de Educación y Ciencia under grant PR2009-0455 and by the Vicerectorat d’Investigació, Desenvolupament i Innovació de la Universitat d’Alacant under grant GRE12-35

Spectroscopy of high mass X-ray binaries with Swift/XRT

We present the X-ray spectroscopy study of three high mass X-ray binary systems, Vela X-1, Cen X-3 and V0332+53 using data obtained with Swift/XRT. The continuum emission of Vela X-1 is consistent with two absorbed power laws, each of them modified by different absorbing columns and with the same power law index. Moreover, the high energy part of the spectrum is modified by absorption components, like Gaussian profile, smedge} or edge} functions. We also detect emission lines and fit as Gaussians at 6.406^+0.004_-0.021 keV (Fe Kalpha) and 2.44^+0.04_-0.03 keV (S XV He_alpha). The continuum emission of Cen X-3 is consistent with two absorbed power laws, each of them modified by different absorbing columns and with the same power law index. We also detect emission lines and fit as Gaussians at 6.432^+0.020_-0.023 keV (Fe Kalpha), 6.84^+0.12_-0.10 keV (Fe XXVI), 2.90 +/- 0.18 keV Ar Kalpha and 1.12^+0.07_-0.06 keV (Ne X Ly_alpha). The presence of iron emission lines at 6.4 keV and 6.8 keV simultaneously indicates that there are at least two distinct emission sites. Fluorescence in a localized region of relatively low ionization may be responsible for the 6.4 keV emission. We could interpret the emission line at 6.8 keV as a blend of several narrow lines due to scattering of radiation from the neutron star in an extended highly ionized stellar wind. Finally, the continuum emission of V0332+53 is consistent with an absorbed power law and a Gaussian emission line to describe the soft excess at low energies. No emission lines are present in this system.Part of this work was supported by the Spanish Ministry of Education and Science Primera ciencia con el GTC: La astronomía española en vanguardia de la astronomía europea CSD200670 and Multiplicidad y evolución de estrellas masivas project number AYA200806166C0303. KLP and JPO acknowledge support from STFC. JMT & JJRR acknowledge support by the Spanish Ministerio de Educación y Ciencia (MEC) under grants PR2007-0176 & PR2009-0455. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester

Orbital phase-resolved spectroscopy of 4U 1538−52 with MAXI

Context. 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. Aims. The aim of this work is to study both the light curve and orbital phase spectroscopy of this source in the long term. In particular, we study the folded light curve and the changes in the spectral parameters with orbital phase to analyse the stellar wind of QV Nor, the mass donor of this binary system. Methods. 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 all orbital phase ranges, which were the input for extracting 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. Results. The MAXI spectra in the 2–20 keV energy range were fitted with an absorbed Comptonisation 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 in 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 Ṁ/v∞ = 0.65 × 10-9M⊙ yr-1/ (km s-1) for QV Nor.Part of this work was supported by the Spanish Ministry of Economy and Competitiveness project numbers ESP2013-48637-C2-2P, and ESP2014-53672-C3-3-P, by the Vicerectorat d’Investigació, Desenvolupament i Innovació de la Universitat d’Alacant project number GRE12-35, and by the Generalitat Valenciana project number GV2014/088. This research made use of MAXI data provided by RIKEN, JAXA, and the MAXI team. J.J.R.R. acknowledges the support by the Matsumae International Foundation Research Fellowship No14G04, and also thanks the entire MAXI team for the collaboration and hospitality in RIKEN. The work of A.G.G. has been supported by the Spanish MICINN under FPI Fellowship BES-2011-050874 associated to the project AYA2010-15431. T.M. acknowledges the grant by the Vicerectorat d’Investigació, Desenvolupament i Innovació de la Universitat d’Alacant under visiting researcher programme INV14-11