Particle-accelerating Colliding-wind Binaries Research Articles

Search for particle acceleration in two massive Wolf–Rayet stars using uGMRT observations

ABSTRACT Large wind kinetic power of Wolf-Rayet (WR) stars make them ideal targets in low radio frequencies to search for non-thermal emission due to relativistic particle acceleration. In this paper, we present observations of two WR stars, WR 114 and WR 142, in Band 4 (550–950 MHz) and Band 5 (1050–1450 MHz) using the upgraded Giant Meterwave Radio Telescope. Neither star is detected in the observed frequency bands, nor extended emission associated with them. The upper limit to the free–free radio emission from the stellar wind enables us to constrain the mass-loss rate of WR 114 to $\lesssim \rm 10^{-5}\, M_{ \odot }\, yr^{-1}$; this is a factor three smaller than previously estimated using spectroscopic modelling. If we further assume that the WR stars are binaries, the non-detection of synchrotron emission from the putative wind collision region implies that the stars are either in very wide binary systems away from periastron, or that the stars are in close binary systems with an orbital separation <70 AU for WR 114 and <20 AU for WR 142. The non-detection of low-frequency radio emission from these two systems thus provides evidence that narrows their nature, though it does not rule them out as bona fide particle-accelerating colliding-wind binaries.

Are magnetic fields universal in O-type multiple systems?

ABSTRACTAlthough significant progress has been achieved in recent surveys of the magnetism in massive stars, the origin of the detected magnetic fields remains to be the least understood topic in their studies. We present an analysis of 61 high-resolution spectropolarimetric observations of 36 systems with O-type primaries, among them 10 known particle-accelerating colliding-wind binaries exhibiting synchrotron radio emission. Our sample consists of multiple systems with components at different evolutionary stages with wide and tight orbits and different types of interactions. For the treatment of the complex composite spectra of the multiple systems, we used a special procedure involving different line masks populated for each element separately. Out of the 36 systems, 22 exhibit in their least-squares deconvolution Stokes V profiles definitely detected Zeeman features, among them 7 systems with colliding winds. For 14 systems, the detected Zeeman features are most likely associated with O-type components, whereas for 3 systems we suggest an association with an early B-type component. For the remaining five systems, the source of the field is unclear. Marginal evidence for the detection of a Zeeman feature is reported for 11 systems and non-detection for 3 systems. The large number of systems with definitely detected Zeeman features presents a mystery, but probably indicates that multiplicity plays a definite role in the generation of magnetic fields in massive stars. The newly found magnetic systems are supreme candidates for spectropolarimetric monitoring over their orbital and rotation periods to obtain trustworthy statistics on the magnetic field geometry and the distribution of field strength.

Supercritical colliding wind binaries

Context. Particle-accelerating colliding-wind binaries (PACWBs) are systems that are formed by two massive and hot stars and produce nonthermal radiation. The key elements of these systems are fast winds and the shocks that they create when they collide. Binaries with nonaccreting young pulsars have also been detected as nonthermal emitters, again as a consequence of the wind–wind interaction. Black holes might produce nonthermal radiation by this mechanism if they accrete at super-Eddington rates. In such cases, the disk is expected to launch a radiation-driven wind, and if this wind has an equatorial component, it can collide with the companion star yielding a PACWB. These systems are supercritical colliding wind binaries. Aims. We aim to characterize the particle acceleration and nonthermal radiation produced by the collision of winds in binary systems composed of a superaccreting black hole and an early-type star. Methods. We estimated the terminal velocity of the disk-driven wind by calculating the spatial distribution of the radiation fields and their effect on disk particles. We then found the location of the wind collision region and calculated the timescales of energy gain and losses of relativistic particles undergoing diffusive particle acceleration. With this information, we were able to compute the associated spectral energy distribution of the radiation. We calculated a number of specific models with different parameters to explore this scenario. Results. We find that the interaction of winds can produce nonthermal emission from radio up to tens of GeV, with luminosities in the range of ∼1033–1035 erg s−1, which for the most part are contributed by electron synchrotron and inverse Compton radiation. Conclusions. We conclude that supercritical colliding wind binaries, such as some ultraluminous X-ray sources and some Galactic X-ray binaries, are capable of accelerating cosmic rays and producing nonthermal electromagnetic emission from radio to γ-rays, in addition to the thermal components.

Megahertz emission of massive early-type stars in the Cygnus region

AbstractMassive, early-type stars have been detected as radio sources for many decades. Their thermal winds radiate free–free continuum and in binary systems hosting a colliding-wind region, non-thermal emission has also been detected. To date, the most abundant data have been collected from frequencies higher than 1 GHz. We present here the results obtained from observations at 325 and 610 MHz, carried out with the Giant Metrewave Radio Telescope, of all known Wolf-Rayet and O-type stars encompassed in area of$\sim$15 sq degrees centred on the Cygnus region. We report on the detection of 11 massive stars, including both Wolf-Rayet and O-type systems. The measured flux densities at decimeter wavelengths allowed us to study the radio spectrum of the binary systems and to propose a consistent interpretation in terms of physical processes affecting the wide-band radio emission from these objects. WR 140 was detected at 610 MHz, but not at 325 MHz, very likely because of the strong impact of free–free absorption (FFA). We also report—for the first time—on the detection of a colliding-wind binary system down to 150 MHz, pertaining to the system of WR 146, making use of complementary information extracted from the Tata Institute of Fundamental Research GMRT Sky Survey. Its spectral energy distribution clearly shows the turnover at a frequency of about 600 MHz, that we interpret to be due to FFA. Finally, we report on the identification of two additional particle-accelerating colliding-wind binaries, namely Cyg OB2 12 and ALS 15108 AB.

Hints about the multiplicity of WR 133 based on multiepoch radio observations

Several tens of massive binary systems display indirect, or even strong evidence for non-thermal radio emission, hence their particle accelerator status. These objects are referred to as particle-accelerating colliding-wind binaries (PACWBs). WR 133 is one of the shortest period Wolf-Rayet + O systems in this category, and is therefore critical to characterize the boundaries of the parameter space adequate for particle acceleration in massive binaries. Our methodology consists in analyzing JVLA observations of WR 133 at different epochs to search for compelling evidence for a phase-locked variation attributable to synchrotron emission produced in the colliding-wind region. New data obtained during two orbits reveal a steady and thermal emission spectrum, in apparent contradiction with the previous detection of non-thermal emission. The thermal nature of the radio spectrum along the 112.4-d orbit is supported by the strong free–free absorption by the dense stellar winds, and shows that the simple binary scenario cannot explain the non-thermal emission reported previously. Alternatively, a triple system scenario with a wide, outer orbit would fit with the observational facts reported previously and in this paper, albeit no hint for the existence of a third component exists to date. The epoch-dependent nature of the identification of synchrotron radio emission in WR 133 emphasizes the issue of observational biases in the identification of PACWBs, that undoubtedly affect the present census of PACWB among colliding-wind binaries.

Prospects for radio observations of Particle-Accelerating Colliding-Wind Binaries with the Giant Metrewave Radio Telescope

Synchrotron radiation identified in the radio domain for several tens of binary systems made of massive stars provides compelling evidence that a particle acceleration process is at work in these objects, hence their Particle-Accelerating Colliding-Wind Binaries (PACWB) status. Measurements of the synchrotron radio emission allow to investigate the non-thermal physics and to derive some of their properties. Non-thermal spectra are known to increase at lower frequencies, while thermal ones increase toward higher frequencies. In this context, it is worth investigating the expectations from longer wavelengths such as those measured by the Giant Metrewave Radio Telescope in India, in complementarity with the most abundant measurements at centimetric wavelengths obtained with other radio observatories such as the VLA or ATCA.

Determination of physical parameters of particle-accelerating colliding-wind binaries based on generalized partition considerations

The sub-set of massive binaries known to accelerate particles, the so-called category of particle-accelerating colliding-wind binaries (PACWBs), constitutes a valuable laboratory for investigating the non-thermal physics in stellar environments. In particular, their synchrotron emitter status allows us to derive some basic properties of their population of relativistic electrons. In this paper, considerations about energy partitions (not restricted to equipartition or minimum energy) are developed to derive relevant physical parameters of PACWBs in an appropriate assumption context. This approach was applied to three PACWBs with known stellar wind and orbital parameters, along with rather well-known radio properties. For long-period systems, a local magnetic field of the order of 0.1–10 mG was determined for a wide range of assumptions on partition parameters, while values of a few G are obtained for the shorter period system we investigated. Normalization parameters of the relativistic electron populations were also determined. Synchrotron self-absorption appears to be an unlikely turn-over process for long-period systems, while it may compete with free-free absorption for a shorter period object. Our results are discussed in the context of the energy budget of non-thermal processes in PACWBs, and prospects for high energy emission are also addressed. The sensitivity of this approach, applied for the first time to PACWBs, was also investigated through a critical discussion of the dependence of determined physical quantities on adopted and assumed parameters. Even though this method is certainly not adequate to reproduce the physics of PACWBs in detail, it offers the advantage of allowing for the determination of valuable average quantities provided a few fundamental parameters and measurements are known, without the need of any detailed hydrodynamic and radiative modelling. In the absence of any relevant measurement of non-thermal radiation in the high-energy domain, the method presented here constitutes the most straightforward and accessible approach to date to discuss physical parameters that are relevant for the non-thermal physics of PACWBs.

The multi-messenger approach to particle acceleration by massive stars: a science case for optical, radio and X-ray observatories

Massive stars are extreme stellar objects whose properties allow for the study of some interesting physical processes, including particle acceleration up to relativistic velocities. In particular, the collisions of massive star winds in binary systems lead notably to acceleration of electrons involved in synchrotron emission, hence their identification as non-thermal radio emitters. This has been demonstrated for about 40 objects so far. The relativistic electrons are also expected to produce non-thermal high-energy radiation through inverse Compton scattering. This class of objects permits thus to investigate non-thermal physics through observations in the radio and high energy spectral domains. However, the binary nature of these sources introduces some stringent requirements to adequately interpret their behavior and model non-thermal processes. In particular, these objects are well-established variable stellar sources on the orbital time-scale. The stellar and orbital parameters need to be determined, and this is notably achieved through studies in the optical domain. The combination of observations in the visible domain (including e.g. 3.6-m DOT) with radio measurements using notably GMRT and X-ray observations constitutes thus a promising strategy to investigate particle-accelerating colliding-wind binaries in the forthcoming decade.

Long-term XMM–Newton investigation of two particle-accelerating colliding-wind binaries in NGC 6604: HD 168112 and HD 167971★

The long-term (over more than one decade) X-ray emission from two massive stellar systems known to be particle accelerators is investigated using XMM-Newton. Their X-ray properties are interpreted taking into account recent information about their multiplicity and orbital parameters. The two targets, HD168112 and HD167971 appear to be overluminous in X-rays, lending additional support to the idea that a significant contribution of the X-ray emission comes from colliding-wind regions. The variability of the X-ray flux from HD168112 is interpreted in terms of varying separation expected to follow the 1/D rule for adiabatic shocked winds. For HD167971, marginal decrease of the X-ray flux in September 2002 could tentatively be explained by a partial wind eclipse in the close pair. No long-term variability could be demonstrated despite the significant difference of separation between 2002 and 2014. This suggests the colliding-wind region in the wide orbit does not contribute a lot to the total X-ray emission, with a main contribution coming from the radiative shocked winds in the eclipsing pair. The later result provides evidence that shocks in a colliding-wind region may be efficient particle accelerators even in the absence of bright X-ray emission, suggesting particle acceleration may operate in a wide range of conditions. Finally, in hierarchical triple O-type systems, thermal X-rays do not necessarily constitute an efficient tracer to detect the wind-wind interaction in the long period orbit.

Search for magnetic fields in particle-accelerating colliding-wind binaries

Some colliding-wind massive binaries, called particle-accelerating colliding-wind binaries (PACWB), exhibit synchrotron radio emission, which is assumed to be generated by a stellar magnetic field. However, no measurement of magnetic fields in these stars has ever been performed. We aim at quantifying the possible stellar magnetic fields present in PACWB to provide constraints for models. We gathered 21 high-resolution spectropolarimetric observations of 9 PACWB available in the ESPaDOnS, Narval and HarpsPol archives. We analysed these observations with the Least Squares Deconvolution method. We separated the binary spectral components when possible. No magnetic signature is detected in any of the 9 PACWB stars and all longitudinal field measurements are compatible with 0 G. We derived the upper field strength of a possible field that could have remained hidden in the noise of the data. While the data are not very constraining for some stars, for several stars we could derive an upper limit of the polar field strength of the order of 200 G. We can therefore exclude the presence of strong or moderate stellar magnetic fields in PACWB, typical of the ones present in magnetic massive stars. Weak magnetic fields could however be present in these objects. These observational results provide the first quantitative constraints for future models of PACWB.

Catalogue of particle-accelerating colliding-wind binaries

Massive systems made of two or more stars are known to be the site for interesting physical processes -- including at least in some cases -- particle acceleration. Over the past decade, this topic motivated a particular effort to unveil the properties of these systems and characterize the circumstances responsible for the acceleration of particles and the potential role of pre-supernova massive stars in the production of high energy particles in our Galaxy. Although previous studies on this topic were mostly devoted to processes in general, or to a few individual objects in particular, a unified target-oriented census of particle-accelerating colliding-wind binaries (hereafter PACWBs) does not exist yet. This paper aims at making a general and unified census of these systems, emphasizing their main properties. A general discussion includes energetic considerations along with wind properties in relation with non-thermal emission processes that are likely at work in colliding-wind binaries. Finally, some guidelines for future observational and theoretical studies are drawn.