arXiv:1510.03885v2 [astro-ph.HE] 3 Mar 2016 o asv tr ferytpshv eysrn stellar strong very reach mass could the have rate Wolf-Rayet(WR) types of loss case early specific the of In winds. stars massive Hot INTRODUCTION 1 to rmsvrlo oldn idbnre CB)has (CWBs) ⋆ binaries wind colliding of radi- wind several synchrotron with from Non-thermal interact ation pres- pions. could the to that producing due hadrons material, arise accelerated could spectrum of the ence en 2009; of high al. the part Reimer to et (HE) & contribution Reimer ergy Another Reimer 2014). al. 2006; 2005; et Dougherty Reitberger Bednarek & 2000; Pittard Williams 2003; mecha- & 2006; Dougherty Compton Romero non-thermal very 1993; & strong inverse is Benaglia Usov a & and fields en- (Eichler of synchrotron magnetic nisms the production a via and where to radiation photon region leads the a 2013). This Raucq high. of in & density Becker place (De ergy takes power ul- acceleration wind to The percen kinetic several particles total to accelerate the up could harnessing pro- that energies, trarelativistic winds shocks supersonic sources strong Colliding prominent duce be radiation. could non-thermal winds of strong launch and types > ceed

o.Nt .Ato.Soc. Astron. R. Not. Mon. .S Pshirkov binaries. S. wind M. colliding the of view LAT Fermi The c 3 2 1 nttt o ula eerho h usa cdm fSci of Academy Russian the of Research Nuclear Pushchino, for 142290 Institute Observatory, Astronomy Radio State Moscow Pushchino Lomonosov Institute, Astronomical Sternberg -al [email protected] E-mail: 00k s km 1000 05RAS 2015 asv iaisweebt tr eogt early to belong stars both where binaries Massive 10 37 r s erg − 1 − idkntcpwro hs tr ol ex- could stars these of power kinetic wind , 1 . 10 1 − , 2 4 000 , 3 M ⋆ ⊙ 0–0 21)Pitd2 ac 01(NL (MN 2021 March 22 Printed (2015) 000–000 , yr (1 tteaotddistance adopted the At ntepprasml fsvnCB W 1 R7,W 3,W 140, WR 137, Fermi-L WR the 70, of WR years dee were 7 11, modelling, almost (WR theoretic using CWBs of analyzed seven means was by of candidates, which, sample 147) WR a 146, paper the In e)wsdtce t6.1 at detected was Vel) oe and power 10 tr:id,uflw tr:niiulW1,W7,WR12 WR70, stars:individual:WR11, WR147 WR146, - words: stars:winds,outflows energ high the Key on set systems. were 140 limits WR Upper and system. this of region a ore o oetm,hwvrn ytmohrthan other system no however time, some for sources ray oldn idbnre CB)hv encniee sapos a as considered been have (CWBs) binaries wind Colliding ABSTRACT − 1 . 31 8 ihtria velocities terminal with ± r s erg 0 . 6) − × 1 ∼ hslmnst mut to amounts This . 10 1 . − 6 9 × hcm ph 10 am-assas-sasbnre tr:Wl-ae - Wolf-Rayet stars: - stars:binaries - gamma-rays:stars of t − 4 nvriy nvriesypopk 3 192 ocw R Moscow, 119992, 13, prospekt Universitetsky University, - ne,171,Mso,Russia Moscow, 117312, ences, d − rcino h oe netdit h idwn interacti wind-wind the into injected power the of fraction σ Russia 2 340 = ofiec ee ihapoo u n0110GVrange GeV 0.1-100 in flux photon a with level confidence s − 1 eie rl exceptional truly 1997). besides al. et Williams synchrotro 1996; the al. et of (Dougherty region emission extended spatially obs reveal interferometric cases vations some in discovered, been already h SUC”caswr eetdadterecommended the and selected were class “SOURCE” the uut4t 05Jl .Aleet nteeeg range energy the in 2008 events from All data 1. LAT 15 July Fermi within GeV 2015 8 0.1-100 Pass to new 4 see of 2013), August years by al. usin reanalyzed 7 et detection were (Werner candidates almost these for in paper this candidates compiled In 1. promising Table was LAT most Fermi the the of list A METHOD AND DATA 2 th 4. interpretation, section introduced, their the are in and follows: presented used results as are method the conclusions organized the includes is and 3 paper data section this the 2 of section rest reconstruc- set event in The this latest 8. reanalyze the data and we Pass LAT data paper tion of Fermi this years the 7 In almost of 2013). se- using al. years were et 2 al. (Werner systems et using in Werner analyzed candidate 2004; and lim- promising al. lected upper most et (Tatischeff only Seven systems produced 2013). domain other HE for previ- the and its 2015) in al. searches et Reitberger ous 2011; Pabich & Bednarek n neeg flux energy an and cti orsod oaluminosity a to corresponds this pc oH aito rmteCB a enobserved been has CWBs the from radiation HE No ∼ A 6 T × E tl l v2.2) file style X 10 − ◦ (2 6 fteCBpstosblnigto belonging positions CWB the of rcino h oa idkinetic wind total the of fraction . 7 η ± a ytm(aaie l 2009; al. et (Tavani system Car 0 . 5) η a a endetected. been has Car × u rmteW 70 WR the from flux y Tdt.W 1( 11 WR data. AT ,W17 WR140, WR137, 5, 10 e otpromising most med il ihenergy high sible − 12 L (3 = r cm erg ussia . 7 ± − 0 2 . 7) WR s − er- on γ γ × 1 n g 2 e - . 2 M. S. Pshirkov

Table 1. List of the candidate sources Table 2. TS of the candidate sources (simple power-law model).

◦ ◦ Name α,hms δ,dms l, b, Distance, kpc Name TS

WR 11 08 09 32 -47 20 12 262.80 -07.69 0.34 WR 11 37.7

WR 70 15 29 45 -58 34 51 322.34 -1.81 1.9 WR 70 1.2

WR 125 19 28 16 +19 33 21 54.44 +1.06 3.1 WR 125 41.0

WR 137 20 14 32 +36 39 40 74.33 +1.10 2.4 WR 137 23.3

WR 140 20 20 28 +43 51 16 80.93 +4.18 1.7 WR 140 0.1

WR 146 20 35 47 +41 22 45 80.56 +0.44 1.2 WR 146 15.0

WR 147 20 36 44 +40 21 08 79.85 -0.31 0.65 WR 147 54.9 quality cut (zenith angle less than 90◦) was applied. After that, binned likelihood analysis was performed using Fermi science tools version v10r0p5 1. Implemented source models included the sources from the 3FGL catalogue (Fermi-LAT Collaboration 2015), the latest galactic interstellar emission model gll_iem_v06.fits, and the isotropic spectral template iso_P8R2_SOURCE_V6_v06.txt 2. Parameters of the sources inside the regions of interest (RoI) were allowed to change. Additional γ-ray sources from the 3FGL catalogue between 15◦ and 25◦ from the RoI centers were included with fixed parameters. The candidate sources were initially modelled with a simple power-law spectral model.

Figure 1. TS map of 2x2 degree square centered at the WR 147 3 RESULTS AND DISCUSSION position. TS excess is not well localized and the WR 147 position (indicated with the green circle) is away from the position of the The results are presented in the Table 2. It could be seen TS peak. that three candidates: WR 11, WR 125, and WR 147 have significance exceeding threshold TS = 25 and the signifi- cance for the WR 37 system is hovering around this thresh- 10 GeV. Overall spectral shape is very close to the one of η Car during its periastron passage (Reitberger et al. 2015). old (TS = 23.7). However, these sources reside close to 2 the galactic plane where the γ-ray background is especially The WR11 (γ Vel, WC8+O7.5) at a distance d = 340 strong. In case of its imperfect modelling the likelihood anal- pc is the closest to us Wolf-Rayet binary. The components have small separation (0.8 1.6 au), and the ysis could possibly produce unrealistic results. In order to − check the validity of detections, TS maps of the immediate is short, P = 78.53 days (North et al. 2007). The orbital ec- neighborhoods were calculated using the gttsmap tool. Un- centricity is non-negligible, e = 0.33. The relevant physical fortunately, in all cases except the WR 11, there were sev- parameters are presented in the Table 3. Total kinetic wind eral hotspots in < 0.5◦ radius with the significance larger than that of the “sources” (see for illustration WR 147, Fig. 1). That is not the case for WR 11, where there is a clear maximum of TS residing very close to the CWB (less than 0.05◦) (see Fig. 2). Thus it can be concluded that the source spatially coinciding with this system is genuine. Results of the fit with a simple power-law model for this source, from now on dubbed WR 11, gives a TS = 37.7 and a spectral index Γ = 2.16 0.2. However, the spectrum of ± the source is considerably curved ( Fig. 3), and the fits with more elaborated spectral models fare a bit better: TS = 41.5 for a log-parabola model and TS = 44.3 for a broken power- law model. Nevertheless, they all fail to fully reproduce the spectral shape: there is a hard tail at the energies larger than

Figure 2. TS map of 2x2 degree square centered at the WR 1 http://fermi.gsfc.nasa.gov/ssc/data/analysis/software/ 11 position. TS excess is localized around the WR 11 position 2 http://fermi.gsfc.nasa.gov/ssc/data/access/lat/ (indicated with the green circle). BackgroundModels.html

c 2015 RAS, MNRAS 000, 000–000 The Fermi LAT view of the CWBs. 3

Best-fit log-parabola Table 4. Upper limits on the flux (95% CL) in the 0.1-100 GeV energy range

-12 −9 −2 −1 10 Name F100, 10 ph cm s -1 s -2 WR 70 2.6

WR 140 1.1 dN/dE, erg cm 2 E

1036 erg s−1 and 1.6 10−4 fraction of the wind power 10-13 102 103 104 105 ∼ × Eγ, MeV that flows into the wind collision zone. The mechanism of the emission is uncertain, though the Figure 3. The spectrum of the WR11 system. It could be seen latest simulations imply that the hadronic processes domi- that the ’best-fit’ curve fits rather poorly due to the presence of an nate when the binary separation is small, like in the case of additional hard tail at the energies E > 10 GeV. This spectrum WR 11 ( 1013 cm) (Reitberger et al. 2014). On the other η ∼ strongly resembles the spectrum of Car during its periastron hand these simulations predict almost flat spectrum around passage (Reitberger et al. 2015). The upper limits indicated with E = 100 MeV for the hadronic mechanism and that could arrows correspond to 95% CL. be in mild tension with the observations (see Fig. 3). Future observations, including ones in the extended energy range (< 100 MeV and > 100 GeV), will allow to clarify this issue. Table 3. Adopted physical parameters of the WR11 system. If the emission is leptonic in origin then it is possible to estimate the magnetic field strength in the collision wind Parameter unit WC8 O7.5 zone: the ratio of the magnetic field energy density ǫB to Mass, M M⊙ 9.0 29.0 the seed photon field density ǫph is equal to the ratio of

−7 −1 the γ-ray luminosity to the non-thermal radio luminosity Mass-loss rate, M˙ 10 M⊙ yr 80 1.8 (1) (Tatischeff et al. 2004): Terminal wind velocity, v∞ km s−1 1450 2500 (1) −4 5 ǫB /ǫph = Lnon−thermal/Lγ K = 2.2 10 , Luminosity, L 10 L⊙ 1.7 2.8 ≡ × References: If not otherwise specified, all values are taken from (North et al. 2007); (1) (De Marco & Schmutz 1999).

2KLO 2KLO 36 −1 power Lw = 5.8 10 erg s is dominated by the contri- B = 2 = 2 = 1.7 G, (3) × s rOc ηr c bution from the Wolf-Rayet component. A fraction of the r total kinetic power that is dissipated in the wind collision 39 −1 where LO = 1.1 10 erg s is the O-type lu- zone – Lcwz – could be estimated from a purely geometri- × cal reasoning and it is equal to the wind momentum ratio η minosity, rO = √ηr is the distance from the star to the (De Becker & Raucq 2013): collision zone which is a fraction of the total separation r = 0.8 au. Magnetic field of this magnitude could be ex- ∞ M˙ OvO pected when the collision zone is located at such a small η = ∞ = 0.04, (1) M˙ WRv distance from the O-type star with a surface field of 100 WR ∼ G strength (Eichler & Usov 1993; Tatischeff et al. 2004). It 35 −1 is worth noting that even if the HE radiation is leptonic Lcwz = ηLw = 2.3 10 erg s . (2) × in origin it is produced by much more energetic population The X-ray emission from the colliding wind shock with an than the one producing the synchrotron emission: γ 104 − ∼ unabsorbed luminosity reaching 1033 erg s 1 (0.4-10 keV en- rather than several tens. ergy range) was also detected (Schild et al. 2004). WR11 is Finally, search for the periodicity corresponding to the a bright radio source at the cm wavelengths (26.5 mJy at binary period P = 78.53 days was performed. None was 28 −1 4.8 GHz, Lrad = 1.8 10 erg s ), but the bulk of the ob- found. Low statistics with the total number of the photons × 3 served emission has a thermal origin (Leitherer et al. 1997). from the source < 10 probably precluded any observations The non-thermal signal from the shock would be strongly of the periodical modulations. absorbed in the dense of the stellar winds, eventually The WR 11 system due to its proximity and relatively contributing up to a half of the total luminosity at 4.8 GHz high galactic latitude remains the only detected system, only 27 −1 (Lnon−thermal = 8.3 10 erg s )(Chapman et al. 1999). upper limits on their flux could be calculated for the rest of × The detected γ-ray source is rather weak: the flux in the CWBs in the list. It could be meaningless in the cases the HE range (0.1 -100 GeV) from the WR 11 is (1.8 of WR 125, WR 137, and WR 147 with their spurious high −9 −2 −1 ± 0.6) 10 ph cm s , and the energy flux (2.7 0.5) TSs, so the limits were set only for WR 70 and WR 140 (see −12× −2 −1 ± × 10 erg cm s . At the adopted distance d = 340 pc Tab. 4). WR 146 is a borderline case: with its TS 15 and 31 −1 ∼ this gives a luminosity L = (3.7 0.7) 10 erg s or complicated neighbourhood the calculated ULs could also −6 ± × 6 10 fraction of total kinetic wind power LW = 5.8 be difficult to interpret. ∼ × × c 2015 RAS, MNRAS 000, 000–000 4 M. S. Pshirkov

4 SUMMARY cial Publication, High-Energy Emission from the Collision in γ2 Velorum. p. 409 A search for HE emission from seven potential candidates Tavani M., et al., 2009, Astrophys. J., 698, L142 was conducted. HE emission from the nearest system WR Werner M., Reimer O., Reimer A., Egberts K., 2013, A&A, 11 (γ2 Vel) was detected at 6.1σ confidence level (TS = 555, A102 37.7) with a simple power-law model (spectral index Γ = Williams P. M., Dougherty S. M., Davis R. J., van der 2.16 0.2). The spectrum of the source is curved with a ± Hucht K. A., Bode M. F., Setia Gunawan D. Y. A., 1997, broad maximum around 1 GeV and shows a hardening ∼ MNRAS, 289, 10 at energies above 10 GeV. The photon flux in 0.1-100 GeV range is (1.8 0.6) 10−9 ph cm−2 s−1, the energy flux is ± −12 × −2 −1 (2.7 0.5) 10 erg cm s . At the adopted distance ± × d = 340 pc that corresponds to luminosity L = (3.7 0.7) 31 −1 −6 ± × 10 erg s , that is 6 10 fraction of total wind kinetic ∼−4× power and 1.6 10 fraction of power injected into the ∼ × wind interaction region of this system. Upper limits were set on the HE luminosity of WR 70 and WR 140 systems. The detection of WR 11 by the Fermi LAT endorses colliding wind binaries as a new separate class of high energy sources.

ACKNOWLEDGEMENTS

The author would like to thank the anonymous referee for the valuable comments that helped to improve the quality of the paper. This research was supported by RSF grant No. 14-12-00146. The author acknowledges the fellowship of the Dynasty Foundation. The numerical part of the work was done at the cluster of the Theoretical Division of INR RAS. This research has made use of NASA’s Astrophysics Data System.

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c 2015 RAS, MNRAS 000, 000–000