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Symbiotic X-ray binaries are systems hosting a neutron star accreting form the wind of a late-type companion. These are rare objects and so far only a handful of them are known. One of the most puzzling aspects of the symbiotic X-ray binaries is the possibility that they contain strongly magnetized neutron stars. These are expected to be evolutionary much younger compared to their evolved companions and could thus be formed through the (yet poorly known) accretion induced collapse of a white dwarf. In this paper, we perform a broad-band X-ray and soft gamma-ray spectroscopy of two known symbiotic binaries, Sct X-1 and 4U 1700+24, looking for the presence of cyclotron scattering features that could confirm the presence of strongly magnetized NSs. We exploited available Chandra, Swift, and NuSTAR data. We find no evidence of cyclotron resonant scattering features (CRSFs) in the case of Sct X-1 but in the case of 4U 1700+24 we suggest the presence of a possible CRSF at similar to 16 keV and its first harmonic at similar to 31 keV, although we could not exclude alternative spectral models for the broad-band fit. If confirmed by future observations, 4U 1700+24 could be the second symbiotic X-ray binary with a highly magnetized accretor. We also report about our long-term monitoring of the last discovered symbiotic X-ray binary IGR J17329-2731 performed with Swift/XRT. The monitoring revealed that, as predicted, in 2017 this object became a persistent and variable source, showing X-ray flares lasting for a few days and intriguing obscuration events that are interpreted in the context of clumpy wind accretion.
We have analysed an archival XMM-Newton EPIC observation that serendipitously covered the sky position of a variable X-ray source AX J1714.1-3912, previously suggested to be a Supergiant Fast X-ray Transient (SFXT). During the XMM-Newton observation the source is variable on a timescale of hundred seconds and shows two luminosity states, with a flaring activity followed by unflared emission, with a variability amplitude of a factor of about 50. We have discovered an intense iron emission line with a centroid energy of 6.4 keV in the power law-like spectrum, modified by a large absorption (N-H similar to 10(24) cm(-2)), never observed before from this source. This X-ray spectrum is unusual for an SFXT, but resembles the so-called 'highly obscured sources', high mass X-ray binaries (HMXBs) hosting an evolved B[e] supergiant companion (sgB[e]). This might suggest that AX J1714.1-3912 is a new member of this rare type of HMXBs, which includes IGR J16318-4848 and CI Camelopardalis. Increasing this small population of sources would be remarkable, as they represent an interesting short transition evolutionary stage in the evolution of massive binaries. Nevertheless, AX J1714.1-3912 appears to share X-ray properties of both kinds of HMXBs (SFXT versus sgB[e] HMXB). Therefore, further investigations of the companion star are needed to disentangle the two hypothesis.
3A 1954+319 has been classified for a long time as a symbiotic X-ray binary, hosting a slowly rotating neutron star and an aged M red giant. Recently, this classification has been revised thanks to the discovery that the donor star is an M supergiant. This makes 3A 1954+319 a rare type of high-mass X-ray binary consisting of a neutron star and a red supergiant donor. In this paper, we analyse two archival and still unpublished XMM-Newton and NuSTAR observations of the source. We perform a detailed hardness ratio-resolved spectral analysis to search for spectral variability that could help investigating the structures of the inhomogeneous M supergiant wind from which the neutron star is accreting. We discuss our results in the context of wind-fed supergiant X-ray binaries and show that the newest findings on 3A 1954+319 reinforce the hypothesis that the neutron star in this system is endowed with a magnetar-like magnetic field strength (greater than or similar to 10(14) G).
Context. In supergiant X-ray binaries (SgXB), a compact object captures a fraction of the wind of an O/B supergiant on a close orbit. Proxies exist to evaluate the efficiency of mass and angular momentum accretion, but they depend so dramatically on the wind speed that given the current uncertainties, they only set loose constraints. Furthermore, these proxies often bypass the impact of orbital and shock effects on the flow structure. Aims. We study the wind dynamics and angular momentum gained as the flow is accreted. We identify the conditions for the formation of a disk-like structure around the accretor and the observational consequences for SgXB. Methods. We used recent results on the wind launching mechanism to compute 3D streamlines, accounting for the gravitational and X-ray ionizing influence of the compact companion on the wind. Once the flow enters the Roche lobe of the accretor, we solved the hydrodynamics equations with cooling. Results. A shocked region forms around the accretor as the flow is beamed. For wind speeds on the order of the orbital speed, the shock is highly asymmetric compared to the axisymmetric bow shock obtained for a purely planar homogeneous flow. With net radiative cooling, the flow always circularizes for sufficiently low wind speeds. Conclusions. Although the donor star does not fill its Roche lobe, the wind can be significantly beamed and bent by the orbital effects. The net angular momentum of the accreted flow is then sufficient to form a persistent disk-like structure. This mechanism could explain the proposed limited outer extension of the accretion disk in Cygnus X-1 and suggests the presence of a disk at the outer rim of the neutron star magnetosphere in Vela X-1 and has dramatic consequences on the spinning up of the accretor.
Context. X-ray radiation from accreting compact objects is an important part of stellar feedback. The metal-poor galaxy ESO 338-4 has experienced vigorous starburst during the last <40 Myr and contains some of the most massive super star clusters in the nearby Universe. Given its starburst age and its star-formation rate, ESO 338-4 is one of the most efficient nearby manufactures of neutron stars and black holes, hence providing an excellent laboratory for feedback studies. Aims. We aim to use X-ray observations with the largest modern X-ray telescopes XMM-Newton and Chandra to unveil the most luminous accreting neutron stars and black holes in ESO 338-4. Methods. We compared X-ray images and spectra with integral field spectroscopic observations in the optical to constrain the nature of strong X-ray emitters. Results. X-ray observations uncover three ultraluminous X-ray sources (ULXs) in ESO 338-4. The brightest among them, ESO 338 X-1, has X-ray luminosity in excess of 10(40) erg s(-1). We speculate that ESO 338-4 X-1 is powered by accretion on an intermediate-mass (greater than or similar to 300 M-circle dot)black hole. We show that X-ray radiation from ULXs and hot superbubbles strongly contributes to He II ionization and general stellar feedback in this template starburst galaxy.
We report the discovery of a circular shell centred on the Be X-ray binary (BeXB) SXP 1323 in the Small Magellanic Cloud. The shell was detected in an H α image obtained with the Very Large Telescope. Follow-up spectroscopy with the Southern African Large Telescope showed that the shell expands with a velocity of ≈100kms−1 and that its emission is due to shock excitation. We suggest that this shell is a remnant of the supernova explosion that led to the formation of SXP 1323’s neutron star ≈40000 yr ago. SXP 1323 represents the second known case of a BeXB associated with a supernova remnant (the first one is SXP 1062). Interestingly, both of these BeXBs harbour long-period pulsars and are located in a low-metallicity galaxy.
We report on observations of the pulsar/Be star binary system PSR J2032+4127/MT91 213 in the energy range between 100 GeV and 20 TeV with the Very Energetic Radiation Imaging Telescope Array and Major Atmospheric Gamma Imaging Cherenkov telescope arrays. The binary orbit has a period of approximately 50 years, with the most recent periastron occurring on 2017 November 13. Our observations span from 18 months prior to periastron to one month after. A new point-like gamma-ray source is detected, coincident with the location of PSR J2032+4127/MT91 213. The gamma-ray light curve and spectrum are well characterized over the periastron passage. The flux is variable over at least an order of magnitude, peaking at periastron, thus providing a firm association of the TeV source with the pulsar/Be star system. Observations prior to periastron show a cutoff in the spectrum at an energy around 0.5 TeV. This result adds a new member to the small population of known TeV binaries, and it identifies only the second source of this class in which the nature and properties of the compact object are firmly established. We compare the gamma-ray results with the light curve measured with the X-ray Telescope on board the Neil Gehrels Swift Observatory and with the predictions of recent theoretical models of the system. We conclude that significant revision of the models is required to explain the details of the emission that we have observed, and we discuss the relationship between the binary system and the overlapping steady extended source, TeV J2032+4130.
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. The large jet kinetic power and non-thermal processes occurring in the microquasar SS 433 make this source a good candidate for a very high-energy (VHE) gamma-ray emitter. Gamma-ray fluxes above the sensitivity limits of current Cherenkov telescopes have been predicted for both the central X-ray binary system and the interaction regions of SS 433 jets with the surrounding W50 nebula. Non-thermal emission at lower energies has been previously reported, indicating that efficient particle acceleration is taking place in the system. Aims. We explore the capability of SS 433 to emit VHE gamma rays during periods in which the expected flux attenuation due to periodic eclipses (P-orb similar to 13.1 days) and precession of the circumstellar disk (P-pre similar to 162 days) periodically covering the central binary system is expected to be at its minimum. The eastern and western SS 433/W50 interaction regions are also examined using the whole data set available. We aim to constrain some theoretical models previously developed for this system with our observations. Methods. We made use of dedicated observations from the Major Atmospheric Gamma Imaging Cherenkov telescopes (MAGIC) and High Energy Spectroscopic System (H.E.S.S.) of SS 433 taken from 2006 to 2011. These observation were combined for the first time and accounted for a total effective observation time of 16.5 h, which were scheduled considering the expected phases of minimum absorption of the putative VHE emission. Gamma-ray attenuation does not affect the jet/medium interaction regions. In this case, the analysis of a larger data set amounting to similar to 40-80 h, depending on the region, was employed. Results. No evidence of VHE gamma-ray emission either from the central binary system or from the eastern/western interaction regions was found. Upper limits were computed for the combined data set. Differential fluxes from the central system are found to be less than or similar to 10(-12)-10(-13) TeV-1 cm(-2) s(-1) in an energy interval ranging from similar to few x 100 GeV to similar to few TeV. Integral flux limits down to similar to 10(-12)-10(-13) ph cm(-2) s(-1) and similar to 10(-13)-10(-14) ph cm(-2) s(-1) are obtained at 300 and 800 GeV, respectively. Our results are used to place constraints on the particle acceleration fraction at the inner jet regions and on the physics of the jet/medium interactions. Conclusions. Our findings suggest that the fraction of the jet kinetic power that is transferred to relativistic protons must be relatively small in SS 433, q(p) <= 2.5 x 10(-5), to explain the lack of TeV and neutrino emission from the central system. At the SS 433/W50 interface, the presence of magnetic fields greater than or similar to 10 mu G is derived assuming a synchrotron origin for the observed X-ray emission. This also implies the presence of high-energy electrons with E-e up to 50 TeV, preventing an efficient production of gamma-ray fluxes in these interaction regions.
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.