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Based on our new NuSTAR X-ray telescope data, we rule out any cyclotron line up to 60 keV in the spectra of the high-mass X-ray binary 4U2206+54. In particular, we do not find any evidence of the previously claimed line around 30 keV, independently of the source flux, along the spin pulse. The spin period has increased significantly, since the last observation, up to 5750 +/- 10 s, confirming the rapid spin-down rate (nu)over dot = -1.8 x 10(-14) Hz s(-1). This behaviour might be explained by the presence of a strongly magnetized neutron star (B-s > several times 10(13) G) accreting from the slow wind of its main-sequence O9.5 companion.
Aims. To gain a realistic picture of the donor star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model describing the wind stratification while properly reproducing the observed donor spectrum. To investigate how X-ray illumination affects the stellar wind, we calculated additional models for different X-ray luminosity regimes. Methods. We used the recently updated version of the Potsdam Wolf-Rayet code to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer. Results. The wind flow in Vela X-1 is driven by ions from various elements, with Fe III and S III leading in the outer wind. The model-predicted mass-loss rate is in line with earlier empirical studies. The mass-loss rate is almost unaffected by the presence of the accreting NS in the wind. The terminal wind velocity is confirmed at u(infinity) approximate to 600 km s(-1). On the other hand, the wind velocity in the inner region where the NS is located is only approximate to 100 km s(-1), which is not expected on the basis of a standard beta-velocity law. In models with an enhanced level of X-rays, the velocity field in the outer wind can be altered. If the X-ray flux is too high, the acceleration breaks down because the ionization increases. Conclusions. Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model reveals a low wind speed at the NS location, and it provides quantitative information on wind driving in this important HMXB.
Context. Classic massive binary evolutionary scenarios predict that a transitional common-envelope (CE) phase could be preceded as well as succeeded by the evolutionary stage when a binary consists of a compact object and a massive star, that is, a high-mass X-ray binary (HMXB). The observational manifestations of common envelope are poorly constrained. We speculate that its ejection might be observed in some cases as a transient event at mid-infrared (IR) wavelengths. Aims. We estimate the expected numbers of CE ejection events and HMXBs per star formation unit rate, and compare these theoretical estimates with observations. Methods. We compiled a list of 85 mid-IR transients of uncertain nature detected by the Spitzer Infrared Intensive Transients Survey and searched for their associations with X-ray, optical, and UV sources. Results. Confirming our theoretical estimates, we find that only one potential HMXB may be plausibly associated with an IR-transient and tentatively propose that X-ray source NGC4490-X40 could be a precursor to the SPIRITS 16az event. Among other interesting sources, we suggest that the supernova remnant candidate [BWL2012] 063 might be associated with SPIRITS 16ajc. We also find that two SPIRITS events are likely associated with novae, and seven have potential optical counterparts. Conclusions. The massive binary evolutionary scenarios that involve CE events do not contradict currently available observations of IR transients and HMXBs in star-forming galaxies.
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 quasi-spherical accretion on to a slowly rotating magnetized neutron star (NS). In quiescence, the NS 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 similar to 1 km, compatible with a hotspot on an 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, similar to 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 magnetized NS nature of the compact object is strongly favoured.
SXP 1062 is a Be X-ray binary (BeXB) located in the Small Magellanic Cloud. It hosts a long-period X-ray pulsar and is likely associated with the supernova remnant MCSNR J0127−7332. In this work we present a multiwavelength view on SXP 1062 in different luminosity regimes. We consider monitoring campaigns in optical (OGLE survey) and X-ray (Swift telescope). During these campaigns a tight coincidence of X-ray and optical outbursts is observed. We interpret this as typical Type I outbursts as often detected in BeXBs at periastron passage of the neutron star (NS). To study different X-ray luminosity regimes in depth, during the source quiescence we observed it with XMM–Newton while Chandra observations followed an X-ray outburst. Nearly simultaneously with Chandra observations in X-rays, in optical the RSS/SALT telescope obtained spectra of SXP 1062. On the basis of our multiwavelength campaign we propose a simple scenario where the disc of the Be star is observed face-on, while the orbit of the NS is inclined with respect to the disc. According to the model of quasi-spherical settling accretion our estimation of the magnetic field of the pulsar in SXP 1062 does not require an extremely strong magnetic field at the present time.
Context. Microquasars are potential gamma-ray emitters. Indications of transient episodes of gamma-ray emission were recently reported in at least two systems: Cyg X-1 and Cyg X-3. The identification of additional gamma-ray-emitting microquasars is required to better understand how gamma-ray emission can be produced in these systems. Aims. Theoretical models have predicted very high-energy (VHE) gamma-ray emission from microquasars during periods of transient outburst. Observations reported herein were undertaken with the objective of observing a broadband flaring event in the gamma-ray and X-ray bands. Methods. Contemporaneous observations of three microquasars, GRS 1915+105, Circinus X-1, and V4641 Sgr, were obtained using the High Energy Spectroscopic System (H.E.S.S.) telescope array and the Rossi X-ray Timing Explorer (RXTE) satellite. X-ray analyses for each microquasar were performed and VHE gamma-ray upper limits from contemporaneous H.E.S.S. observations were derived. Results. No significant gamma-ray signal has been detected in any of the three systems. The integral gamma-ray photon flux at the observational epochs is constrained to be I(>560 GeV) < 7.3 x 10(-13) cm(-2) S-1, I(>560 GeV) < 1.2 x 10-(12) cm s(-1), and I(>240 GeV) < 4.5 x 10(-12) cm(-2) s(-1) for GRS 1915+105, Circinus X-1, and V4641 Sgr, respectively. Conclusions. The gamma-ray upper limits obtained using H.E.S.S. are examined in the context of previous Cherenkov telescope observations of microquasars. The effect of intrinsic absorption is modelled for each target and found to have negligible impact on the flux of escaping gamma-rays. When combined with the X-ray behaviour observed using RXTE, the derived results indicate that if detectable VHE gamma-ray emission from microquasars is commonplace, then it is likely to be highly transient.