Context. In the first paper of this series, we presented a numerical model for the non-thermal emission of gamma-ray binaries in a pulsar-wind-driven scenario. Aims. We apply this model to one of the ...best-observed gamma-ray binaries, the LS 5039 system. Methods. The model involves a joint simulation of the interaction between the pulsar wind and the stellar wind and the transport of electron pairs from the pulsar wind accelerated at the emerging shock structure. We compute the synchrotron and inverse Compton emission in a post-processing step while consistently accounting for relativistic beaming and γγ-absorption in the stellar radiation field. Results. The wind interaction leads to the formation of an extended, asymmetric wind collision region that develops strong shocks, turbulent mixing, and secondary shocks in the turbulent flow. Both the structure of the collision region and the resulting particle distributions show significant orbital variation. In addition to the acceleration of particles at the bow-like pulsar wind and the Coriolis shock, the model naturally accounts for the re-acceleration of particles at secondary shocks that contribute to the emission at very-high-energy (VHE) gamma-rays. The model successfully reproduces the main spectral features of LS 5039. While the predicted light curves in the high-energy and VHE gamma-ray band are in good agreement with observations, our model still does not reproduce the X-ray to low-energy gamma-ray modulation, which we attribute to the employed magnetic field model. Conclusions. We successfully model the main spectral features of the observed multi-band, non-thermal emission of LS 5039 and thus further substantiate a wind-driven interpretation of gamma ray binaries. Open issues relate to the synchrotron modulation, which might be addressed through a magnetohydrodynamic extension of our model.
Recent TeV-scale observations with the CANGAROO II telescope have led to the claim that the multi-band spectrum of RX J1713.7-3946 cannot be explained as the composite of a synchrotron and an inverse ...Compton component emitted by a population of relativistic electrons. It was argued that the spectrum of the high-energy emission is a good match to that predicted by pion decay, thus providing observational evidence that protons are accelerated in SNR to at least TeV energies. In this Letter we discuss the multi-band spectrum of RX J1713.7-3946 under the constraint that the GeV-scale emission observed from the closely associated EGRET source 3EG J1714-3857 is either associated with the SNR or an upper limit to the gamma-ray emission of the SNR. We find that the pion-decay model adopted by Enomoto et al. is in conflict with the existing GeV data. We have examined the possibility of a modified proton spectrum to explain the data, and find that we cannot do so within any existing theoretical framework of shock acceleration models.
Context. Gamma-ray binaries are systems that radiate the dominant part of their non-thermal emission in the gamma-ray band. In a wind-driven scenario, these binaries are thought to consist of a ...pulsar orbiting a massive star, accelerating particles in the shock arising in the wind collision. Aims. We develop a comprehensive numerical model for the non-thermal emission of shock-accelerated particles including the dynamical effects of fluid instabilities and orbital motion. We demonstrate the model on a generic binary system. Methods. The model was built on a dedicated three-dimensional particle transport simulation for the accelerated particles that were dynamically coupled to a simultaneous relativistic hydrodynamic simulation of the wind interaction. In a post-processing step, a leptonic emission model involving synchrotron and inverse-Compton emission was evaluated based on resulting particle distributions and fluid solutions, consistently accounting for relativistic boosting and γγ-absorption in the stellar radiation field. The model was implemented as an extension to the CRONOS code. Results. In the generic binary, the wind interaction leads to the formation of an extended, asymmetric wind-collision region distorted by the effects of orbital motion, mixing, and turbulence. This gives rise to strong shocks terminating the pulsar wind and secondary shocks in the turbulent fluid flow. With our approach it is possible for the first time to consistently account for the dynamical shock structure in particle transport processes, which yields a complex distribution of accelerated particles. The predicted emission extends over a broad energy range, with significant orbital modulation in all bands.
Aims.We present X-ray and 12CO($J=1{-}0$) observations of the very-high-energy (VHE) γ-ray source HESS J1813-178 with the aim of understanding the origin of the γ-ray emission. Methods. High-angular ...resolution X-ray studies of the VHE γ-ray emission region are performed using 18.6 ks of XMM-Newton data, taken on HESS J1813-178 in October 2005. Using this data set we are able to undertake spectral and morphological studies of the X-ray emission from this object with greater precision than previous studies. NANTEN 12CO($J=1{-}0$) data are used to search for correlations of the γ-ray emission with molecular clouds which could act as target material for γ-ray production in a hadronic scenario. Results.The NANTEN 12CO($J=1{-}0$) observations show a giant molecular cloud of mass $2.5 \times 10^5~M_{\odot}$ at a distance of 4 kpc in the vicinity of HESS J1813-178. Even though there is no direct positional coincidence, this giant cloud may have influenced the evolution of the γ-ray source and its surroundings. The X-ray data show a highly absorbed ($n_{{\rm H}} \sim 1. \times 10^{23}$ cm-2) non-thermal X-ray emitting object coincident with the previously known ASCA source AX J1813–178 exhibiting a compact core and an extended tail towards the north-east, located in the centre of the radio shell-type Supernova remnant (SNR) G12.82–0.02. This central object shows morphological and spectral resemblance to a Pulsar Wind Nebula (PWN) and we therefore consider that this object is very likely to be a composite SNR. We discuss the scenario in which the γ-rays originate in the shell of the SNR, and that in which they originate in the central object, in terms of a time-dependent one-zone leptonic model. We demonstrate, that in order to connect the core X-ray emission to the VHE γ-ray emission electrons have to be accelerated to energies of at least 1 PeV.
We present X-ray observations of the as of yet unidentified very high energy (VHE) Y-ray source HESS J1640-465 with the aim of establishing a counterpart of this source in the keV energy range, and ...identifying the mechanism responsible for the VHE emission. The 21.8 ks XMM-Newton observation of HESS J1640-465 in 2005 September represents a significant improvement in sensitivity and angular resolution over previous ASCA studies in this region. These new data show a hard-spectrum X-ray-emitting object at the centroid of the H.E.S.S. source, within the shell of the radio supernova remnant (SNR) G338.3-0.0. This Object is consistent with the position and flux previously measured by both ASCA and Swift XRT, but is now shown to be significantly extended. We argue that this object is very likely the counterpart to HESS J1640-465, and that both objects may represent the pulsar wind nebula of an as of yet undiscovered pulsar associated with G338.3-0.0.
We report the very first evidence of the possible detection of γ-ray resonant absorption along the line of sight towards γ-ray bright quasars (QSOs), like 3C 279, 3C 273, PKS 0528+0134, and BL ...Lacertae. These detections resulted from the analysis of COMPTEL and EGRET data that were collected either during monitoring campaigns of the Virgo and galactic anticenter regions by the Compton Gamma Ray Observatory (CGRO) or during ToO observations of QSOs flares. We discuss three resonant absorption mechanisms that affect the γ-ray spectrum of point-like sources when crossing the surrounding warm and cold absorbers, as well as the potential of this γ-ray photon absorption method to constrain the abundance of the absorber. We detected two absorbers along the line of sight towards γ-ray bright QSOs, one at the QSO rest frame redshift and another at approximately zero redshift. We tentatively identify the latter with an absorber in the Galactic halo, while the former is undoubtedly caused by photon absorption in the host galaxy of the QSO. We discuss the advantages and drawbacks of this method in studies of absorbers in different astrophysical environments and compare this new method to absorber studies at X-ray or other wavelengths. We applied this γ-ray absorption method to identify a few of the EGRET unidentified (EUID) sources as QSOs, and determine their redshifts.
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,
γ
2
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
γ
2
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
γ
2
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
Recent reports claiming an association of the massive star binary system
Velorum (WR 11) with a high-energy
γ
-ray source observed by
Fermi
-LAT contrast the so far exclusive role of
η
... Carinae as the hitherto only detected
γ
-ray emitter in the source class of particle-accelerating colliding-wind binary (CWB) systems. We offer support to this claim of association by providing dedicated model predictions for the nonthermal photon emission spectrum of
Velorum. We use 3D magnetohydrodynamic modeling (MHD) to investigate the structure and conditions of the wind-collision region (WCR) of
Velorum including the important effect of radiative braking in the stellar winds. A transport equation is then solved for the entire computational domain to study the propagation of relativistic electrons and protons. The resulting distributions of particles are subsequently used to compute nonthermal photon emission components. In agreement with observation in X-ray spectroscopy, our simulations yield a large shock-cone opening angle. We find the nonthermal
γ
-ray emission of
Velorum to be of hadronic origin owing to the strong radiation fields in the binary system, which inhibit the acceleration of electrons to energies sufficiently high for efficient inverse-Compton radiation. We also discuss the strong dependence of a hadronic
γ
-ray component on the energy-dependent diffusion used in the simulations. Of two mass-loss rates for the WR star found in literature, only the higher rate is able to accommodate the observed
γ
-ray spectrum with reasonable values for important simulation parameters such as the injection ratio of high-energy particles within the WCR.
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