WR140 is the archetype long-period colliding wind binary (CWB) system, and is
well known for dramatic variations in its synchrotron emission during its
7.9-yr, highly eccentric orbit. This emission is thought to arise from
relativistic electrons accelerated at the global shocks bounding the
wind-collision region (WCR). The presence of non-thermal electrons and ions
should also give rise to X-ray and gamma-ray emission from several separate
mechanisms, including inverse-Compton cooling, relativistic bremsstrahlung, and
pion decay. We describe new calculations of this emission and make some
preliminary predictions for the new generation of gamma-ray observatories. We
determine that WR140 will likely require several Megaseconds of observation
before detection with INTEGRAL, but should be a reasonably strong source for
GLAST.Comment: 4 pages, 1 figure, contribution to "Massive Stars and High-Energy
  Emission in OB Associations"; JENAM 2005, held in Liege (Belgium

Dougherty, S. M.

Pittard, J. M.

English

arXiv

WR140 is the archetype long-period colliding wind binary (CWB) system, and is well known for dramatic variations in its synchrotron emission during its 7.9-yr, highly eccentric orbit. This emission is thought to arise from relativistic electrons accelerated at the global shocks bounding the wind-collision region (WCR). The presence of non-thermal electrons and ions should also give rise to X-ray and gamma-ray emission from several separate mechanisms, including inverse-Compton cooling, relativistic bremsstrahlung, and pion decay. We describe new calculations of this emission and make some preliminary predictions for the new generation of gamma-ray observatories. We determine that WR140 will likely require several Megaseconds of observation before detection with INTEGRAL, but should be a reasonably strong source for GLAST

Pittard, JM

Dougherty, SM

White Rose Research Online

This is a repository copy of Non-thermal X-ray and gamma-ray emission from the colliding wind binary WR 140.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/130888/Version: Accepted VersionProceedings Paper:Pittard, JM orcid.org/0000-0003-2244-5070 and Dougherty, SM (2005) Non-thermal X-ray and gamma-ray emission from the colliding wind binary WR 140. In: Massive Stars and High Energy Emission in OB Associations: Proceedings of JENAM 2005 Distant Worlds. Joint European and National Astronomy Meeting "Distant Worlds", 04-07 Jul 2005, Liège, Belgium. , pp. 57-60. eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. arXiv:astro-ph/0509896v1  30 Sep 2005Non-thermal X-ray and Gamma-ray Emissionfrom the Colliding Wind Binary WR140J. M. Pittard 1 and S. M. Dougherty 21School of Physics and Astronomy, The University of Leeds, Leeds, UK2National Research Council, Herzberg Institute of Astrophysics, DRAO, Penticton, CanadaAbstract: WR140 is the archetype long-period colliding wind binary (CWB) system, and is wellknown for dramatic variations in its synchrotron emission during its 7.9-yr, highly eccentric orbit.This emission is thought to arise from relativistic electrons accelerated at the global shocks boundingthe wind-collision region (WCR). The presence of non-thermal electrons and ions should also give riseto X-ray and γ-ray emission from several separate mechanisms, including inverse-Compton cooling,relativistic bremsstrahlung, and pion decay. We describe new calculations of this emission and makesome preliminary predictions for the new generation of gamma-ray observatories. We determine thatWR140 will likely require several Megaseconds of observation before detection with INTEGRAL, butshould be a reasonably strong source for GLAST.1 IntroductionCWB systems are an important laboratory for investigating the underlying physics of particleacceleration since they provide access to higher mass, radiation and magnetic field energydensities than in supernova remnants, which have been widely used for such work. High-resolution observations (e.g., Williams et al. 1997; Dougherty et al. 2000, 2005) show thatthe particle acceleration site is where the massive stellar winds collide - the WCR. In additionto synchrotron radio emission, these non-thermal particles should give rise to X-ray and γ-rayemission from several separate mechanisms, including inverse-Compton (IC) cooling, relativisticbremsstrahlung, and pion decay, though definitive evidence for such emission does not yet exist.The advent of new and forthcoming observatories (INTEGRAL, GLAST and VERITAS) willdramatically improve the chance of detecting these systems, and in this paper we make newpredictions for the expected flux from WR140.Previous predictions of the IC emission have been based on the assumption that the ratioof the luminosity from IC scattering to the synchrotron luminosity is equal to the ratio of thephoton energy density, Uph, to the magnetic field energy density, UB:LicLsync=UphUB. (1)However, the use of this equation is rather unsatisfactory for a number of reasons. For example,Lsync is typically set to the observed synchrotron luminosity, whereas free-free absorption bythe extended wind envelopes can be significant (see Pittard et al. 2005). In such cases theintrinsic synchrotron luminosity, and thus the non-thermal X-ray and γ-ray luminosity, will beunderestimated. The predictive power of Eq. 1 is also greatly undermined by the fact that themagnetic field strength in the WCR of CWB systems is generally not known with any certainty.Since UB ∝ B2, small changes in the estimated value of B can lead to large changes in Lic (seeFig. 3 in Benaglia & Romero 2003).A more robust estimate of the IC emission from CWB systems can be acquired from modelfits to radio data, where the population and spatial distribution of non-thermal electrons isdetermined (see Dougherty et al. 2003 and Pittard et al. 2005). A key improvement inthese papers over earlier modelling of the radio emission from CWB systems is the use ofa hydrodynamical model to describe the stellar winds and WCR. The free-free emission andabsorption coefficients are determined from the local temperature and density values on thehydrodynamic grid. Particle acceleration at the shocks bounding the WCR creates a populationof non-thermal particles with a power-law energy distribution (i.e. n(γ) ∝ γ−p, where γ is theLorentz factor) - as these advect downstream IC cooling modifies the energy distribution of non-thermal electrons. Since the population of non-thermal particles is not determined from firstprinciples, their energy density is normalized to some fraction (ζrel,e and ζrel,i for the electronsand ions, respectively) of the thermal energy density (Uth) at the shocks. For the non-thermalelectrons, IC cooling reduces this fraction below ζrel,e in the downstream flow. Similarly, themagnetic field energy density is normalized by setting UB = ζBUth. When modelling specificsystems, ζB and ζrel,e are chosen to best match the observed radio emission. Predictions forthe IC and relativistic bremsstrahlung emission then follow since these arise from the samenon-thermal relativistic electrons which are responsible for the synchrotron radio emission.2 The High Energy Non-Thermal EmissionIn our current calculations we assume that IC scattering is isotropic and takes place in theThomson regime. Since the average photon energy of an early-type star, hν∗ ∼ 10 eV, Lorentzfactors of order 10− 104 are sufficient to produce IC X-ray and γ-ray radiation. The resultingemission has a spectral shape which is identical to the synchrotron emission at radio frequencies.Bremsstrahlung radiation at γ-ray energies is emitted from relativistic electrons since photonsof comparable energy to that of the emitting particle can be produced. Finally, it is possible toobtain γ-ray emission from hadronic collisions involving non-thermal ions, through the decay ofneutral pions, e.g., p+p → π0+X , π0 → γ+γ. The pion decay process yields information on thepopulation of non-thermal nucleons, in contrast to the IC and bremsstrahlung processes wherethe emission is dependent on the population of non-thermal electrons. We conservatively setthe energy density ratio of non-thermal ions to electrons, ζrel,i/ζrel,e, to 20. Further backgroundto the calculations can be found in Pittard & Dougherty (2006).3 Models of WR140We apply our model to WR140, the archetype of long-period CWB systems. It consists ofa WC7 star and an O4-5 star in a highly elliptical orbit (e ≈ 0.88), and in the context ofthe present paper is most noteable for its possible association with an EGRET source, lyingon the outskirts of the positional error box of 3EG J2022+4317 (Romero et al. 1999). Weadopt the orbital parameters and mass-loss rates determined by Dougherty et al. (2005), butallow the mass-loss rate of the O star (and thus the wind momentum ratio η) to vary, as this 1e-25 1e-20 1e-15 1e-10 1e-05 1e-10  1e-05  1  100000  1e+10  1e+15Flux (erg s-1 cm-2 keV-1)Energy (eV)INTEGRAL EGRETGLASTVERITAS 1e-09 1e-08 1e-07 1e-06 1e-06  1e-05  1e-04Figure 1: The radio, and non-thermal UV, X-ray and γ-ray emission calculated from our modelof WR140 at φ = 0.9, together with the observed radio and X-ray flux. The model radiodata that we show indicate the thermal free-free flux (displayed below the data points), theintrinsic synchrotron flux (displayed above the data points), and the total observed emission.This region is expanded in the inset. The IC (long dash), relativistic bremsstrahlung (shortdash), and pion decay (dotted) emission are shown as separate components, and their sum isalso displayed (solid). No absorption has been included in the calculation of the high-energyemission. We also show the observed X-ray spectrum obtained at φ = 0.84 with ASCA (dataset27022010, observed on 1999-10-22). It is reassuring that the IC flux predicted from our modelis less than the observed X-ray flux.parameter is not well determined. The system distance is assumed to be 1.85 kpc. Furtherdetails on the modelling can be found in Pittard & Dougherty (2006) and Dougherty et al.(these proceedings).Given the wealth of observational data which now exists for WR140, we have concentratedon obtaining a reasonable spectral fit to the radio data at a specific orbital phase, namelyφ = 0.9. Fig. 1 shows the resulting fit where it is clear that the intrinsic synchrotron emissionsuffers significant free-free absorption by the extended circumbinary envelope. A key point isthat the large ratio between the intrinsic and observed synchrotron emission implies that theIC luminosity predicted using Eq. 1 will be substantially underestimated.The high energy emission resulting from the radio fit is also shown in Fig. 1. The IC emissiondominates for photon energies less than 50 GeV, while the emission from pion decay reachesenergies up to 15 TeV. The relativistic bremsstrahlung emission is a minor contributor to thetotal non-thermal flux, and does not dominate at any energy. A gradual steepening of the ICspectrum occurs between 102 − 106 eV. The lack of a clearly defined spectral break reflects thefact that the break frequency is spatially dependent, and so is smoothed out once the flux isintegerated over the entire WCR (Pittard et al. 2005).The total photon flux in the 100 MeV - 20 GeV EGRET band from our model is 3 ×10−8 ph s−1 cm−2, which is an order of magnitude below the flux detected from 3EG J2022+4317during the December 1992 observation at φ = 0.97. The different phases prevent a directcomparison, but we expect that the IC emission will increase between φ = 0.9 − 0.97 as thestellar separation more than halves. Considering the limitations of the present model, this levelof agreement is satisfactory. We predict a photon flux in the 15 keV-10 MeV sensitivity range ofthe IBIS instrument onboard INTEGRAL of 1.4× 10−4 ph s−1 cm−2, which is likely to requirean exposure time greater than 3 Msec for a 5σ detection. The predicted 20 MeV-300 GeVphoton flux is 1.4 × 10−7 ph s−1 cm−2, and thus WR140 should be fairly easily detected byGLAST. Our flux predictions are approximately 7 times lower than those made by Benaglia &Romero (2003) - this is partly due to the greater distance which we assume for WR140.4 ConclusionsWe have presented predictions of the high energy non-thermal flux from WR140. We find thata long observation will be required for detection of WR140 with INTEGRAL, but it shouldbe a reasonably strong source in the GLAST energy band. The current neglect of two-photonpair production prevents us from making predictions for the flux in the sensitivity range ofVERITAS, a Cherenkov air shower telescope array. However, we expect that this absorptionmechanism will significantly attenuate the emission in the TeV range and suggest that otherCWB systems which are wider and less distant (e.g., WR146 and WR147) may be bettertargets at these energies. Improved high energy data will drive the development of theoreticalmodels which include anisotropic IC emission and two-photon pair production, and will allowa much more detailed comparison between observations and predictions.AcknowledgementsJMP gratefully acknowledges funding from the Royal Society.ReferencesBenaglia P., Romero G.E., 2003, A&A, 399, 1121Dougherty S.M., Beasley A.J., Claussen M.J., Zauderer B.A., Bolingbroke N.J., 2005,ApJ, 623, 447Dougherty S.M., Pittard J.M., Kasian L., Coker R.F., Williams P.M., Lloyd H.M., 2003,A&A, 409, 217Dougherty S.M., Williams P.M., Pollacco D.L., 2000, MNRAS, 316, 143Pittard J.M., Dougherty S.M., Coker R.F., Bolingbroke N.J., O’Connor E., 2005, A&A,submittedPittard J.M., Dougherty S.M., 2006, MNRAS, submittedRomero G.E., Benaglia P., Torres D.F., 1999, A&A, 348, 868Williams P.M., Dougherty S.M., Davis R.J., van der Hucht K.A., Bode M.F., Setia Gu-nawan D.Y.A., 1997, MNRAS, 289, 10

Non-thermal X-ray and Gamma-ray Emission from the Colliding Wind Binary WR140

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