Fermi-LAT upper limits on gamma-ray emission from colliding wind binaries

Context: Colliding wind binaries (CWBs) are thought to give rise to a plethora of physical processes including acceleration and interaction of relativistic particles. Observation of synchrotron radiation in the radio band confirms there is a relativistic electron population in CWBs. Accordingly, CWBs have been suspected sources of high-energy gamma-ray emission since the COS-B era. Theoretical models exist that characterize the underlying physical processes leading to particle acceleration and quantitatively predict the non-thermal energy emission observable at Earth. Aims: We strive to find evidence of gamma-ray emission from a sample of seven CWB systems: WR 11, WR 70, WR 125, WR 137, WR 140, WR 146, and WR 147. Theoretical modelling identified these systems as the most favourable candidates for emitting gamma-rays. We make a comparison with existing gamma-ray flux predictions and investigate possible constraints. Methods: We used 24 months of data from the Large Area Telescope (LAT) on-board the Fermi Gamma Ray Space Telescope to perform a dedicated likelihood analysis of CWBs in the LAT energy range. Results: We find no evidence of gamma-ray emission from any of the studied CWB systems and determine corresponding flux upper limits. For some CWBs the interplay of orbital and stellar parameters renders the Fermi-LAT data not sensitive enough to constrain the parameter space of the emission models. In the cases of WR140 and WR147, the Fermi-LAT upper limits appear to rule out some model predictions entirely and constrain theoretical models over a significant parameter space. A comparison of our findings to the CWB eta Car is made.Comment: 9 pages, 3 figure

High-energy particle transport in 3D hydrodynamic models of colliding-wind binaries

Massive stars in binary systems (as WR140, WR147 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 which subsequently might be able to emit $\gamma$-rays. Detailed numerical hydrodynamic simulations have already offered insight in 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 3D-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.Comment: 28 pages, 9 figures / accepted for publication in The Astrophysical Journa

X-ray and gamma-ray orbital variability from the gamma-ray binary HESS J1832-093

Context. Gamma-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 discovered to emit at high energies, but their number is still scarce. 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 found for it. Aims. The purpose of this work is to search for a gamma-ray source at GeV energies to understand the origin of the TeV signal. For an unambiguous identification of its binary nature, finding an orbital modulation is crucial. Methods. We have 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 (XRT) and UltraViolet and Optical Telescope (UVOT). We searched for periodicities in both X-ray and gamma-ray bands using a Lomb-Scargle periodogram. Results. We find a periodic modulation of \sim 86 days in the X-ray source XMMU J183245-0921539, together with indications of gamma-ray modulation with a compatible period in 4FGL J1832.9-0913. Neither an optical nor an UV counterpart is found at the X-ray source location. The overall spectral energy distribution strongly resembles the known gamma-ray binary HESS J0632+057. Conclusions. Both the spectrum and the discovery of an orbital period allow the identification of the TeV source HESS J1832-093 as a new member of the gamma-ray binary class.Comment: 9 pages, 9 figures. Accepted by A&

The new surprising behaviour of the two "prototype" blazars PKS 2155-304 and 3C 279

Recent VHE observations have unveiled a surprising behaviour in two well-known blazars at opposite sides of the blazar sequence. PKS 2155-304 have shown for the first time in an HBL a large Compton dominance, high gamma-ray luminosities and a cubic relation between X-ray and VHE fluxes. 3C 279 is the first FSRQ detected at VHE. The high luminosity required to overcome the significant absorption caused by the BLR emission cannot be easily reconciled with the historical and quasi-simultaneous SED properties. Both cases shed a new light on the structure and ambient fields of blazars. Contrary to previous claims, it is also shown that 3C 279 --as any FSRQ-- cannot in general provide robust constraints on the EBL.Comment: Proceedings of "4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy 2008" (Gamma 2008), July 7-11, 2008. Slightly refined text with updated reference

Parameter constraints for high-energy models of colliding winds of massive stars: the case WR 147

We explore the ability of high energy observations to constrain orbital parameters of long period massive binary systems by means of an inverse Compton model acting in colliding wind environments. This is particular relevant for (very) long period binaries where orbital parameters are often poorly known from conventional methods, as is the case e.g. for the Wolf-Rayet (WR) star binary system WR 147 where INTEGRAL and MAGIC upper limits on the high-energy emission have recently been presented. We conduct a parameter study of the set of free quantities describing the yet vaguely constrained geometry and respective effects on the non-thermal high-energy radiation from WR 147. The results are confronted with the recently obtained high-energy observations and with sensitivities of contemporaneous high-energy instruments like Fermi-LAT. For binaries with sufficient long periods, like WR 147, gamma-ray attenuation is unlikely to cause any distinctive features in the high-energy spectrum. This leaves the anisotropic inverse Compton scattering as the only process that reacts sensitively on the line-of-sight angle with respect to the orbital plane, and therefore allows the deduction of system parameters even from observations not covering a substantial part of the orbit. Provided that particle acceleration acts sufficiently effectively to allow the production of GeV photons through inverse Compton scattering, our analysis indicates a preference for WR 147 to possess a large inclination angle. Otherwise, for low inclination angles, electron acceleration is constrained to be less efficient as anticipated here.Comment: 33 pages, 9 figures; accepted by Ap