Discovery of the optical counterpart to the ASCA transient AX 1845.0 - 0433

Very peculiar wind from BD+53°2790, the optical counterpart to 4U 2206+54

BD+53 2790, an O9.5Vp star, is the optical counterpart to the HMXRB 4U 2206+54. This system was classified initially as a BeX, but observational evidence soon stressed the need to revise this classification. The permanent asymmetry in the H-alpha line profiles (in contrast with the cyclic variations shown by Be stars), the variations in the profile of this line in time scales of hours (while time scales from weeks to months are expected in Be stars), and the lack of correlation between IR observables and H-alpha line parameters, strongly suggest that, while BD+53 2790 contains a circunstellar disc, it is not like the one present in Be stars. Furthermore, there is evidence of overabundance o…

Long-term optical/IR variability of the Be/X-ray binary LS V +44 17/RX J0440.9+4431

We present the first long-term study of the optical counterpart to the X-ray pulsar LS V +44 17/RX J0440.9+4431. The results of our photometric and spectroscopic analysis show that LS V +44 17/RX J0440.9+4431 contains a moderately reddened, E(B-V)=0.65+-0.05, B0.2V star located at about 3.3 kpc. The Ha line consistently shows a double-peak profile varying from symmetric shape to completely distorted on one side (V/R phases). A correlation between the equivalent width of the Ha line and the infrared magnitudes is seen: as the EW(Ha) decreases the IR magnitudes become fainter. This long-term optical/IR variability is attributed to structural changes in the Be star's circumstellar disc. The ob…

Gaia -ESO Survey: Analysis of pre-main sequence stellar spectra

This paper describes the analysis of UVES and GIRAFFE spectra acquired by the Gaia-ESO Public Spectroscopic Survey in the fields of young clusters whose population includes pre-main sequence (PMS) stars. Both methods that have been extensively used in the past and new ones developed in the contest of the Gaia-ESO survey enterprise are available and used. The internal precision of these quantities is estimated by inter-comparing the results obtained by such different methods, while the accuracy is estimated by comparison with independent external data, like effective temperature and surface gravity derived from angular diameter measurements, on a sample of benchmarks stars. Specific strategi…

The Be/X-ray Transient V0332+53: Evidence for a tilt between the orbit and the equatorial plane?

We present optical and infrared observations of BQ Cam, the optical counterpart to the Be/X-ray transient system V0332+53. BQ Cam is shown to be an O8-9Ve star, which places V0332+53 at a distance of ~7 kpc. H-alpha spectroscopy and infrared photometry are used to discuss the evolution of the circumstellar envelope. Due to the low inclination of the system, parameters are strongly constrained. We find strong evidence for a tilt of the orbital plane with respect to the circumstellar disc (pressumably on the equatorial plane). Even though the periastron distance is only ~ 10 R_*, during the present quiescent state the circumstellar disc does not extend to the distance of periastron passage. U…

INTEGRAL long-term monitoring of the Supergiant Fast X-ray Transient XTE J1739-302

In the past few years, a new class of High Mass X-Ray Binaries (HMXRB) has been claimed to exist, the Supergiant Fast X-ray Transients (SFXT). These are X-ray binary systems with a compact companion orbiting a supergiant star which show very short and bright outbursts in a series of activity periods overimposed on longer quiescent periods. Only very recently the first attempts to model the behaviour of these sources have been published, some of them within the framework of accretion from clumpy stellar winds.Our goal is to analyze the properties of XTE J1739-302/IGR J17391-3021 within the context of the clumpy structure of the supergiant wind. We have used INTEGRAL and RXTE/PCA observations…

Multiwavelength monitoring of BD+53°2790 , the optical counterpart to 4U 2206+54

We present the results of our long-term monitoring of BD+53 2790, the optical counterpart to the X-ray source 4U~2206+54. Unlike previous studies that classify the source as a Be/X-ray binary, we find that its optical and infrared properties differ from those of typical Be stars: the variability of the V/R ratio is not cyclical; there are variations in the shape and strength of the H$\alpha$ emission line on timescales less than 1 day; and no correlation between the EW and the IR magnitudes or colors is seen. Our observations suggest that BD+53 2790 is very likely a peculiar O9.5V star. In spite of exhaustive searches we cannot find any significant modulation in any emission line parameter …

Discovery of slow X-ray pulsations in the high-mass X-ray binary 4U 2206+54

Context. The source 4U 2206+54 is one of the most enigmatic high-mass X-ray binaries. In spite of intensive searches, X-ray pul- sations have not been detected in the time range 10−3–103 s. A cyclotron line at ∼30 keV has been suggested by various authors but never detected with significance. The stellar wind of the optical companion is abnormally slow. The orbital period, initially reported to be 9.6 days, disappeared and a new periodicity of 19.25 days emerged. Aims. The main objective of our RXTE monitoring of 4U 2206+54 is to study the X-ray orbital variability of the spectral and timing parameters. The new long and uninterrupted RXTE observations allow us to search for long (∼1 h) puls…

On the radio emitting high mass X-ray binary LS 5039

We present new optical - near-IR spectroscopic and photometric observations of the newly discovered galactic microquasar LS 5039, which indicate a classification for the mass donor in the system of O6.5V((f)). Optical spectroscopy and photometry shows no variability over a timescale of years, and we find no evidence of modulation by, or emission from the compact companion in these data. However significant photometric variability (~0.4 mag) is present in the H and K bands between 1995-2000. Such variability has been observed in other radio bright X-ray binaries where it has been attributed to synchrotron emission from the jet. However, given the non-thermal spectral index of the radio emiss…

The Large Observatory For x-ray Timing

The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supra-nuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m 2 effective area, 2-30 keV, 240 eV spectral resolution, 1 deg collimated field of view) and a WideFi…

Further evidence for the presence of a neutron star in 4U 2206+54. INTEGRAL and VLA observations

The majority of High Mass X-ray Binaries (HMXBs) behave as X-ray pulsars, revealing that they contain a magnetised neutron star. Among the four HMXBs not showing pulsations, and that do not show the characteristics of accreting black holes, there is the unusual HMXB 4U 2206+54. Here we present contemporaneous high-energy and radio observations of this system conducted with INTEGRAL and the VLA in order to unveil its nature. The high-energy spectra show clear indications of the presence of an absorption feature at ~32 keV. This is the third high-energy observatory which reveals marginal evidence of this feature, giving strong support to the existence of a cyclotron resonance scattering featu…

Wind accretion in the massive X-ray binary 4U 2206+54: abnormally slow wind and a moderately eccentric orbit

Massive X-ray binaries are usually classified depending on the properties of the donor star in classical, supergiant and Be X-ray binaries. The massive X-ray binary 4U 2206+54 does not fit in any of these groups, and deserves a detailed study to understand how the transfer of matter and the accretion on to the compact object take place. To this end we study an IUE spectrum of the donor and obtain a wind terminal velocity (v_inf) of ~350 km/s, which is abnormally slow for its spectral type. We also analyse here more than 9 years of available RXTE/ASM data. We study the long-term X-ray variability of the source and find it to be similar to that observed in the wind-fed supergiant system Vela …

The Be/X-ray transient 4U0115+63/V635 Cassiopeiae : II. Outburst mechanisms

We present multi-wavelength long-term monitoring observations of V635 Cas, the optical counterpart to the transient X-ray pulsar 4U0115+63. The evolution of emission lines and photometric magnitudes indicates that the Be star undergoes relatively fast ( ~3 - 5yr) quasi-cyclic activity, losing and reforming its circumstellar disc. We show that the general optical, infrared and X-ray behaviour can be explained by the dynamical evolution of the viscous circumstellar disc around the Be star. After each disc-loss episode, the disc starts reforming and grows until it reaches the radius at which the resonant interaction of the neutron star truncates it. At some point, the disc becomes unstable to …

The Be/X-ray binary LS 992/RX J0812.4-3114: Physical parameters and long-term variability

We present the first long-term optical and infrared study of the optical counterpart to the source RX J0812.4-3114, an X-ray pulsar with a Be type companion. During the period covered by the observations the profile of some Balmer lines changed from absorption to emission and back again to absorption. Contemporaneously, the infrared magnitudes varied by more than 0.8 mag. This long-term variability is interpreted as the formation and subsequent dissipation of the Be star's disc. The building up of the disc ended up in an active X-ray state characterised by regular outbursts occurring at 80 day intervals. The overall duration of the formation/dissipation of the disc is found to be < 4.3 year…

Discovery of the optical counterpart to the X-ray pulsar SAX J2103.5+4545

We report optical and infrared photometric and spectroscopic observations that identify the counterpart to the 358.6-s X-ray transient pulsar SAX J2103.5+4545 with a moderately reddened V=14.2 B0Ve star. This identification makes SAX J2103.5+4545 the Be/X-ray binary with the shortest orbital period known, Porb= 12.7 days. The amount of absorption to the system has been estimated to be Av=4.2+-0.3, which for such an early-type star implies a distance of about 6.5 kpc. The optical spectra reveal major and rapid changes in the strength and shape of the Halpha line. The Halpha line was initially observed as a double peak profile with the ratio of the intensities of the blue over the red peak gr…