Exploring the role of X-ray reprocessing and irradiation in the anomalous bright optical outbursts of A0538-66

In 1981, the Be/X-ray binary (Be/XRB) A0538-66 showed outbursts characterized by high peak luminosities in the X-ray and optical bands. The optical outbursts were qualitatively explained as X-ray reprocessing in a gas cloud surrounding the binary system. Since then, further important information about A0538-66 have been obtained, and sophisticated photoionization codes have been developed to calculate the radiation emerging from a gas nebula illuminated by a central X-ray source. In the light of the new information and tools available, we studied again the enhanced optical emission displayed by A0538-66 to understand the mechanisms responsible for these unique events among the class of Be/XRBs. We performed about 10^5 simulations of a gas envelope photoionized by an X-ray source. We assumed for the shape of the gas cloud either a sphere or a circumstellar disc observed edge-on. We studied the effects of varying the main properties of the envelope and the influence of different input X-ray spectra on the optical/UV emission emerging from the photoionized cloud. We compared the computed spectra with the IUE spectrum and photometric UBV measurements obtained during the outburst of 29 April 1981. We also explored the role played by the X-ray heating of the surface of the donor star irradiated by the X-ray emission of the neutron star (NS). We found that reprocessing in a spherical cloud with a shallow radial density distribution can reproduce the optical/UV emission. To our knowledge, this configuration has never been observed either in A0538-66 during other epochs or in other Be/XRBs. We found, contrary to the case of most other Be/XRBs, that the optical/UV radiation produced by the X-ray heating of the surface of the donor star irradiated by the NS is non-negligible, due to the particular orbital parameters of this system that bring the NS very close to its companion.Comment: Accepted for publication in Astronomy & Astrophysics. Abstract abridged to meet arXiv requirement

Pulse phase and precession phase resolved spectroscopy of Her X-1: studying a representative Main-On with RXTE

We performed a detailed pulse phase resolved spectroscopy of the accreting binary X-ray pulsar Her X-1 in the energy range 3.5-75 keV and have established pulse phase profiles for all spectral parameters. For the centroid of the cyclotron line, the photon index and the flux of the 6.4 keV iron line, we have studied the variation as a function of 35 d phase. We analyzed RXTE observations of the Main-On of November 2002. Four different time intervals of about 1 d duration were selected to provide a good coverage of a complete Main-On. The intervals are centered at 35 d phase 0.03, 0.10, 0.15, and 0.20, respectively. All spectral parameters show a strong modulation with pulse phase. While the centroid energy of the cyclotron line follows roughly the shape of the pulse profile, both the photon index and the iron line intensity exhibit distinct minima around the peak of the X-ray pulse. With respect to variations of the observed profiles with 35 d phase, we find that there is a clear evolution of the shape of the pulse profiles (flux versus pulse phase), a moderate increase of the maximum cyclotron line energy (found around pulse phase 0.7), but no significant evolution of the shape of the pulse phase profiles of the cyclotron line energy, the spectral power law index or the iron line intensity. The variation of spectral parameters as a function of the pulse phase provides important information about the system: 1. the disappearance of the Fe line flux near the highest continuum flux may be an indication of a hollow cone geometry of the accretion structure; ii. the apparent non-dependence of the cyclotron line energy profiles on 35 d phase provides a new possibility to test the model of free precession of the neutron star, proposed to be responsible for the systematic variations in the pulse profiles.Comment: 10 pages, 11 figures, Accepted by A&A on the 22/12/201

A non-pulsating neutron star in the supernova remnant HESS J1731-347 / G353.6-0.7 with a carbon atmosphere

Context: The CCO candidate in the center of the supernova remnant shell HESS J1731-347 / G353.6-0.7 shows no pulsations and exhibits a blackbody-like X-ray spectrum. If the absence of pulsations is interpreted as evidence for the emitting surface area being the entire neutron star surface, the assumption of the measured flux being due to a blackbody emission translates into a source distance that is inconsistent with current estimates of the remnant's distance. Aims: With the best available observational data, we extended the pulse period search down to a sub-millisecond time scale and used a carbon atmosphere model to describe the X-ray spectrum of the CCO and to estimate geometrical parameters of the neutron star. Methods: To search for pulsations we used data of an observation of the source with XMM-Newton performed in timing mode. For the spectral analysis, we used earlier XMM-Newton observations performed in imaging mode, which permits a more accurate treatment of the background. The carbon atmosphere models used to fit the CCO spectrum are computed assuming hydrostatic and radiative equilibria and take into account pressure ionization and the presence of spectral lines. Results: Our timing analysis did not reveal any pulsations with a pulsed fraction above ~8% down to 0.2 ms. This finding further supports the hypothesis that the emitting surface area is the entire neutron star surface. The carbon atmosphere model provides a good fit to the CCO spectrum and leads to a normalization consistent with the available distance estimates of the remnant. The derived constraints on the mass and radius of the source are consistent with reasonable values of the neutron star mass and radius. After the CCO in Cas A, the CCO in HESS J1731-347 / G353.6-0.7 is the second object of this class for which a carbon atmosphere model provides a consistent description of X-ray emission.Comment: 6 pages, 5 figures, accepted for publication in Astronomy&Astrophysic

Swift/BAT measurements of the cyclotron line energy decay in the accreting neutron star Her X-1: indication of an evolution of the magnetic field?

Context: The magnetic field is a crucial ingredient of neutron stars. It governs the physics of accretion and of the resulting high-energy emission in accreting pulsars. Studies of the cyclotron resonant scattering features (CRSFs) seen as absorption lines in the X-ray spectra of the pulsars permit direct measuremets of the field strength. Aims: From an analysis of a number of pointed observations with different instruments, the energy of CRSF, Ecyc, has recently been found to decay in Her X-1, which is one of the best-studied accreting pulsars. We present our analysis of a homogeneous and almost uninterrupted monitoring of the line energy with Swift/BAT. Methods: We analyzed the archival Swift/BAT observations of Her X-1 from 2005 to 2014. The data were used to measure the CRSF energy averaged over several months. Results: The analysis confirms the long-term decay of the line energy. The downward trend is highly significant and consistent with the trend measured with the pointed observations: dEcyc/dt ~-0.3 keV per year. Conclusions: The decay of Ecyc either indicates a local evolution of the magnetic field structure in the polar regions of the neutron star or a geometrical displacement of the line-forming region due to long-term changes in the structure of the X-ray emitting region. The shortness of the observed timescale of the decay, -Ecyc/(dEcyc/dt) ~ 100 yr, suggests that trend reversals and/or jumps of the line energy might be observed in the future.Comment: Accepted for publication in Astronomy&Astrophysic

Probing the stellar wind environment of Vela X-1 with MAXI

Vela X-1 is among the best studied and most luminous accreting X-ray pulsars. The supergiant optical companion produces a strong radiatively-driven stellar wind, which is accreted onto the neutron star producing highly variable X-ray emission. A complex phenomenology, due to both gravitational and radiative effects, needs to be taken into account in order to reproduce orbital spectral variations. We have investigated the spectral and light curve properties of the X-ray emission from Vela X-1 along the binary orbit. These studies allow to constrain the stellar wind properties and its perturbations induced by the compact object. We took advantage of the All Sky Monitor MAXI/GSC data to analyze Vela X-1 spectra and light curves. By studying the orbital profiles in the $4-10$ and $10-20$ keV energy bands, we extracted a sample of orbital light curves (${\sim}15$% of the total) showing a dip around the inferior conjunction, i.e., a double-peaked shape. We analyzed orbital phase-averaged and phase-resolved spectra of both the double-peaked and the standard sample. The dip in the double-peaked sample needs $N_H\sim2\times10^{24}\,$cm$^{-2}$ to be explained by absorption solely, which is not observed in our analysis. We show how Thomson scattering from an extended and ionized accretion wake can contribute to the observed dip. Fitted by a cutoff power-law model, the two analyzed samples show orbital modulation of the photon index, hardening by ${\sim}0.3$ around the inferior conjunction, compared to earlier and later phases, hinting a likely inadequacy of this model. On the contrary, including a partial covering component at certain orbital phase bins allows a constant photon index along the orbital phases, indicating a highly inhomogeneous environment. We discuss our results in the framework of possible scenarios.Comment: 10 pages, 9 figures, accepted for publication in A&

Footprints in the wind of Vela X-1 traced with MAXI

The stellar wind around the compact object in luminous wind-accreting high mass X-ray binaries is expected to be strongly ionized with the X-rays coming from the compact object. The stellar wind of hot stars is mostly driven by light absorption in lines of heavier elements, and X-ray photo-ionization significantly reduces the radiative force within the so-called Stroemgren region leading to wind stagnation around the compact object. In close binaries like Vela X-1 this effect might alter the wind structure throughout the system. Using the spectral data from Monitor of All-sky X-ray Image (MAXI), we study the observed dependence of the photoelectric absorption as function of orbital phase in Vela X-1, and find that it is inconsistent with expectations for a spherically-symmetric smooth wind. Taking into account previous investigations we develop a simple model for wind structure with a stream-like photoionization wake region of slower and denser wind trailing the neutron star responsible for the observed absorption curve.Comment: 5 pages, 3 figures, accepted in A&

Cyclotron lines in highly magnetized neutron stars

Cyclotron lines, also called cyclotron resonant scattering features (CRSF) are spectral features, generally appearing in absorption, in the X-ray spectra of objects containing highly magnetized neutron stars, allowing the direct measurement of the magnetic field strength in these objects. Cyclotron features are thought to be due to resonant scattering of photons by electrons in the strong magnetic fields. The main content of this contribution focusses on electron cyclotron lines as found in accreting X-ray binary pulsars (XRBP) with magnetic fields on the order of several 1012 Gauss. Also, possible proton cyclotron lines from single neutron stars with even stronger magnetic fields are briefly discussed.With regard to electron cyclotron lines, we present an updated list of XRBPs that show evidence of such absorption lines. The first such line was discovered in a 1976 balloon observation of the accreting binary pulsar Hercules X-1, it is considered to be the first direct measurement of the magnetic field of a neutron star. As of today (mid 2018), we list 36 XRBPs showing evidence of one ore more electron cyclotron absorption line(s). A few have been measured only once and must be confirmed (several more objects are listed as candidates). In addition to the Tables of objects, we summarize the evidence of variability of the cyclotron line as a function of various parameters (especially pulse phase, luminosity and time), and add a discussion of the different observed phenomena and associated attempts of theoretical modeling. We also discuss our understanding of the underlying physics of accretion onto highly magnetized neutron stars. For proton cyclotron lines, we present tables with seven neutron stars and discuss their nature and the physics in these objects.Comment: 32 pages, 15 figures, 8 Tables, accepted by A&A 201