Massive stars in binary systems have long been regarded as potential sources of high-energy gamma rays. The emission is principally thought to arise in the region where the stellar winds collide and ...accelerate relativistic particles which subsequently emit gamma rays. On the basis of a three-dimensional distribution function of high-energy particles in the wind collision region-as obtained by a numerical hydrodynamics and particle transport model-we present the computation of the three-dimensional nonthermal photon emission for a given line of sight. Anisotropic inverse Compton emission is modeled using the target radiation field of both stars. Photons from relativistic bremsstrahlung and neutial pion decay are computed on the basis of local wind plasma densities. We also consider photon-photon opacity effects due to the dense radiation fields of the stars. Results are shown for different stellar separations of a given binary system comprising of a B star and a Wolf-Rayet star. The influence of orbital orientation with respect to the line of sight is also studied by using different orbital viewing angles. For the chosen electron-proton injection ratio of 10 super(-2), we present the ensuing photon emission in terms of two-dimensional projections maps, spectral energy distributions, and integrated photon flux values in various energy bands. Here, we find a transition from hadron-dominated to lepton-dominated high-energy emission with increasing stellar separations. In addition, we confirm findings from previous analytic modeling that the spectral energy distribution varies significantly with orbital orientation.
Colliding winds of massive star binary systems are considered as potential sites of nonthermal high-energy photon production. Motivated by the detection of synchrotron radio emission from the ...colliding wind location, we here investigate the properties of high-energy photon production in colliding winds of long-period WR+OB systems. Analytical formulae for the steady state proton- and electron-particle spectra are derived assuming diffusive particle acceleration out of a pool of thermal wind particles, taking into account adiabatic and all relevant radiative losses, and include advection/convection out of the wind collision zone. This includes analytical approximations for the electron energy losses in the Klein-Nishina transition regime. For the first time in the context of CWB systems, our calculations use the full Klein-Nishina cross section and account for the anisotropy of the inverse Compton scattering process. This leads to orbital flux variations by up to several orders of magnitude, which may, however, be blurred by the system's geometry. Both anisotropy and Klein-Nishina effects may yield characteristic spectral and variability signatures in the g-ray domain. Since propagation effects lead to a deficit of low-energy particles in the convection-dominated zone, one expects imprints in the radiation spectra. If protons are accelerated to at least several GeV, p super(0)-decay g-rays might be observable, depending on the injected electron-to-proton ratio. We show that photon-photon pair production is generally not negligible, potentially affecting the emitted spectrum above 650 GeV, depending on orbital phase and system inclination. The calculations are applied to the archetypal WR+OB systems WR 140 and WR 147 to predict their expected spectral and temporal characteristics and to assess their detectability with current and upcoming g-ray experiments.
Colliding-wind binaries (CWBs) constitute an emerging class of
γ
-ray sources powered by strong, dense winds in massive stellar systems. The most powerful of them are those binaries hosting a ...Wolf-Rayet (WR) star. Following the recent discovery of Apep – the closest known Galactic WR–WR binary – we discuss the non-detection of its putative high-energy emission by the
Fermi
Large Area Telescope (
Fermi
-LAT) in this Letter. The limits reported in the GeV regime can be used to set a lower limit on the magnetic field pressure density within the shocked wind-collision region (WCR), and to exclude Apep as a bright
γ
-ray emitting binary. Given that this WR–WR system is the most luminous CWB identified until now at radio wavelengths, this result proves unambiguously that non-thermal synchrotron emission is not a suitable identifier for the subset of
γ
-ray emitters in this class of particle accelerators. Rather, Apep could be an interesting case of study for magnetic field amplification in shocked stellar winds.
Context.
Colliding-wind binaries are massive stellar systems featuring strong, interacting winds. These binaries may be actual particle accelerators, making them variable
γ
-ray sources due to ...changes in the wind collision region along the orbit. However, only two of these massive stellar binary systems have been identified as high-energy sources. The first and archetypical system of this class is
η
Carinae, a bright
γ
-ray source with orbital variability peaking around its periastron passage.
Aims.
The origin of the high-energy emission in
η
Carinae is still unclear, with both lepto-hadronic and hadronic scenarios being under discussion. Moreover, the
γ
-ray emission seemed to differ between the two periastrons previously observed with the
Fermi
-Large Area Telescope. Continuing observations might provide highly valuable information for understanding the emission mechanisms in this system.
Methods.
We have used almost 12 yr of data from the
Fermi
-Large Area Telescope. We studied both low- and high-energy components, searching for differences and similarities between both orbits, and we made use of this large dataset to search for emission from nearby colliding-wind binaries.
Results.
We show how the energy component above 10 GeV of
η
Carinae peaks months before the 2014 periastron, while the 2020 periastron is the brightest one to date. Additionally, upper limits are provided for the high-energy emission in other particle-accelerating colliding-wind systems.
Conclusions.
Current
γ
-ray observations of
η
Carinae strongly suggest that the wind collision region of this system is perturbed from orbit to orbit, affecting particle transport within the shock.
Context.
γ
-ray binaries are systems composed of a massive star and a compact object whose interaction leads to particle acceleration up to relativistic energies. In the last fifteen years, a few ...binaries have been found to emit at high energies, but their number is still low. The TeV source HESS J1832−093 has been proposed as a binary candidate, although its nature is unclear. Neither a GeV counterpart nor a period was detected.
Aims.
The purpose of this work is to search for a GeV counterpart to understand the origin of the TeV signal detected by H.E.S.S. For an unambiguous identification of its binary nature, finding an orbital modulation is crucial.
Methods.
We analysed data spanning more than 10 years from the
Fermi
Large Area Telescope (
Fermi
-LAT), together with
Swift
archival observations taken between 2015 and 2018, using both the X-Ray Telescope and UV/Optical Telescope. We searched for periodicities in both X-ray and GeV bands.
Results.
We find a periodic modulation of ∼ 86 days in the X-ray source candidate counterpart XMMU J183245−0921539, together with indications of
γ
-ray modulation with a compatible period in the GeV candidate counterpart 4FGL J1832.9−0913. Neither an optical nor a UV counterpart is found at the X-ray source location. The overall spectral energy distribution strongly resembles the known
γ
-ray binary HESS J0632+057.
Conclusions.
Both the spectral energy distribution and the discovery of an orbital period allow the identification of the TeV source HESS J1832−093 as a new member of the
γ
-ray binary class.
ABSTRACT The dynamics of colliding-wind binary (CWB) systems and conditions for efficient particle acceleration therein have attracted multiple numerical studies in recent years. These numerical ...models seek an explanation of the thermal and nonthermal emission of these systems as seen by observations. In the nonthermal regime, radio and X-ray emission is observed for several of these CWBs, while gamma-ray emission has so far only been found in Carinae and possibly in WR 11. Energetic electrons are deemed responsible for a large fraction of the observed high-energy photons in these systems. Only in the gamma-ray regime might there be, depending on the properties of the stars, a significant contribution of emission from neutral pion decay. Thus, studying the emission from CWBs requires detailed models of the acceleration and propagation of energetic electrons. This in turn requires a detailed understanding of the magnetic field, which will affect not only the energy losses of the electrons but also, in the case of synchrotron emission, the directional dependence of the emissivity. In this study we investigate magnetohydrodynamic simulations of different CWB systems with magnetic fields that are strong enough to have a significant effect on the winds. Such strong fields require a detailed treatment of the near-star wind acceleration zone. We show the implementation of such simulations and discuss results that demonstrate the effect of the magnetic field on the structure of the wind collision region.
Context. Colliding wind binaries are massive systems featuring strong, interacting stellar winds which may act as particle accelerators. Therefore, such binaries are good candidates for detection at ...high energies. However, only the massive binary η Carinae has been firmly associated with a γ-ray signal. A second system, γ2 Velorum, is positionally coincident with a γ-ray source, but we lack unambiguous identification. Aims. Observing orbital modulation of the flux would establish an unambiguous identification of the binary γ2 Velorum as the γ-ray source detected by the Fermi Large Area Telescope (Fermi-LAT). Methods. We used more than ten years of observations with Fermi-LAT. Events are phase-folded with the orbital period of the binary to search for variability. We studied systematic errors that might arise from the strong emission of the nearby Vela pulsar with a more conservative pulse-gated analysis. Results. We find hints of orbital variability, indicating maximum flux from the binary during apastron passage. Conclusions. Our analysis strengthens the possibility that γ-rays are produced in γ2 Velorum, most likely as a result of particle acceleration in the wind collision region. The observed orbital variability is consistent with predictions from recent magnetohydrodynamic simulations, but contrasts with the orbital variability from η Carinae, where the peak of the light curve is found at periastron.
ABSTRACT It is widely accepted that cosmic rays (CRs) up to at least PeV energies are Galactic in origin. Accelerated particles are injected into the interstellar medium where they propagate to the ...farthest reaches of the Milky Way, including a surrounding halo. The composition of CRs coming to the solar system can be measured directly and has been used to infer the details of CR propagation that are extrapolated to the whole Galaxy. In contrast, indirect methods, such as observations of γ-ray emission from CR interactions with interstellar gas, have been employed to directly probe the CR densities in distant locations throughout the Galactic plane. In this article we use 73 months of data from the Fermi Large Area Telescope in the energy range between 300 MeV and 10 GeV to search for γ-ray emission produced by CR interactions in several high- and intermediate-velocity clouds (IVCs) located at up to ∼7 kpc above the Galactic plane. We achieve the first detection of IVCs in γ rays and set upper limits on the emission from the remaining targets, thereby tracing the distribution of CR nuclei in the halo for the first time. We find that the γ-ray emissivity per H atom decreases with increasing distance from the plane at 97.5% confidence level. This corroborates the notion that CRs at the relevant energies originate in the Galactic disk. The emissivity of the upper intermediate-velocity Arch hints at a 50% decline of CR densities within 2 kpc from the plane. We compare our results to predictions of CR propagation models.
Massive stars in binary systems (such as WR 140, WR 147, or eta Carinae) have long been regarded as potential sources of high-energy gamma -rays. The emission is thought to arise in the region where ...the stellar winds collide and produce relativistic particles that subsequently might be able to emit gamma -rays. Detailed numerical hydrodynamic simulations have already offered insight into the complex dynamics of the wind collision region (WCR), while independent analytical studies, albeit with simplified descriptions of the WCR, have shed light on the spectra of charged particles. In this paper, we describe a combination of these two approaches. We present a three-dimensional hydrodynamical model for colliding stellar winds and compute spectral energy distributions of relativistic particles for the resulting structure of the WCR. The hydrodynamic part of our model incorporates the line-driven acceleration of the winds, gravity, orbital motion, and the radiative cooling of the shocked plasma. In our treatment of charged particles, we consider diffusive shock acceleration in the WCR and the subsequent cooling via inverse Compton losses (including Klein-Nishina effects), bremsstrahlung, collisions, and other energy loss mechanisms.
Abstract
We combine for the first time all available information about the spectral shape and morphology of the radio halo of the Coma cluster with the recent γ-ray upper limits obtained by the ...Fermi-Large Area Telescope (LAT) and with the magnetic field strength derived from Faraday rotation measures. We explore the possibility that the radio emission is due to synchrotron emission of secondary electrons. First, we investigate the case of pure secondary models that are merely based on the mechanism of continuous injection of secondary electrons via proton-proton collisions in the intracluster medium. We use the observed spatial distribution of the halo's radio brightness to constrain the amount of cosmic ray protons and their spatial distribution in the cluster that are required by the model. Under the canonical assumption that the spectrum of cosmic rays is a power law in momentum and that the spectrum of secondaries is stationary, we find that the combination of the steep spectrum of cosmic ray protons necessary to explain the spectrum of the halo and the very broad spatial distribution (and large energy density) of cosmic rays result in a γ-ray emission in excess of present limits, unless the cluster magnetic field is relatively large. However, this large magnetic field required to not violate present γ-ray limits appears inconsistent with that derived from recent Faraday rotation measures. Secondly, we investigate more complex models in which the cosmic rays confined diffusively in the Coma cluster and their secondary electrons are all reaccelerated by magnetohydrodynamics turbulence. We show that under these conditions it is possible to explain the radio spectrum and morphology of the radio halo and to predict γ-ray fluxes in agreement with the Fermi-LAT upper limits without tension with present constraints on the cluster magnetic field. Reacceleration of secondary particles also requires a very broad cosmic ray spatial profile, much flatter than that of the intracluster medium, at least provided that both the turbulent and magnetic field energy densities scale with that of the intracluster medium. However, this requirement can be easily alleviated if we assume that a small amount of (additional) seed primary electrons is reaccelerated in the cluster's external regions, or if we adopt flatter scalings of the turbulent and magnetic field energy densities with distance from the cluster centre.
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