Using H-alpha as a Tracer of the Emission Region of LS I +61 303

2011 Fermi Symposium, Roma., May. 9-12 1 Using Hα as a Tracer of the Emission Region of LS I +61 303 M. Virginia McSwain Lehigh University, Bethlehem, PA 18015, USA The γ-ray binary LS I +61 303 is one of the brightest Fermi sources, with orbitally modulated emission across the electromagnetic spectrum. Here we present Hα spectra of LS I +61 303 that exhibit a dramatic emission burst shortly before apastron, observed as a redshifted shoulder in the line profile. A correlated burst in radio, X-ray, and GeV emission is observed at the same orbital phase. We interpret the source of the emission as a compact pulsar wind nebula that forms when a tidal mass stream from the Be circumstellar disk interacts with the relativistic pulsar wind. The Hα emission offers an important probe of the high energy emission morphology in this system. I. INTRODUCTION LS I +61 303 is a high mass X-ray binary (HMXB) that consists of an optical star with spectral type B0 Ve and an unknown compact companion in a highly eccentric, 26.5 day orbit [3]. While the system has a relatively low X-ray luminosity for a HMXB, LS I +61 303 is the 15th brightest γ-ray source included in the Fermi LAT 1-year Point Source Catalogue ([2]). The Be disk interacts with the compact companion, pro- ducing emission that has been observed to vary with orbital phase at every wavelength across the electro- magnetic spectrum, from radio to TeV (eg. [1], [10]). [10] found periodic radio outbursts that peak near φ(TG) = 0.6 −0.8, and they defined the arbitrary reference for zero phase at HJD 2,443,366.775 that re- mains the conventional definition for LS I +61 303. Periastron occurs at φ(TG) = 0.275 [3]. II. OBSERVATIONS During 2008 October and November, we performed an intense multiwavelength observing campaign on LS I +61 303 supported by a Fermi Cycle 1 program. We obtained optical Hα spectra of LS I +61 303 at the KPNO Coud´e Feed telescope over 35 consecutive nights to study the evolution of the emission during a complete orbit [3], [8]. The Hα line profile exhibits a dramatic emission burst near φ(TG) ∼0.6, observed as a redshifted shoulder in the line profile (see Fig. 1) as the compact source moves almost directly away from the observer. Smaller temporal changes in the red spectra sug- gest additional Hα emission variability, so we sub- tracted the mean emission line profile to investigate the residuals carefully (see Fig. 2). During about half of the orbit, 0.9 ≤φ(TG) ≤0.6, the difference spectra reveal a partial S-shaped pattern similar to a spiral density wave that is commonly observed in Be star disks [9]. [12] also observed a strong blue peak near φ(TG) = 0.23, which supports the development of a spiral density wave near periastron. After this phase, the peculiar red shoulder develops. We measured the equivalent width of Hα, WHα, for each spectrum by directly integrating over the line profile. (We use the convention that WHα is negative for an emission line.) The errors in WHα are typically about 10% due to noise and placement of the contin- uum. Figure 3 shows that during our Coud´e Feed run, WHα decreased slightly just before periastron. Since WHα is correlated to the radius of a Be star’s circum- stellar disk [5], we interpret the decline in emission as a slight decrease in disk radius as gas is stripped away by the compact companion. WHα then rises dra- matically with the onset of the red shoulder emission component near φ(TG) ∼0.6. Figure 3 also compares our recent WHα with those measured by [6]. Their data were accumulated over six different observing runs over 1998–2000, and the long term differences in emission strength are substantial. Also during 2008 October and November, G. Poo- ley obtained nearly simultaneous radio flux coverage with the Arcminute Microkelvin Imager (AMI) array. The 15 GHz AMI light curve (Fig. 4) reveals emission that peaks at the same time as the Hα “red shoulder” outburst. Contemporaneous RXTE light curves from [11]and Fermi light curves ([4]) also reveal orbitally modulated emission that peaks just before the Hα red shoulder, although their wide phase bins may mask a true correlation. The Hα emission clearly traces the high energy emission region in this system. III. CONCLUSIONS The unusual broadness of the Hα red shoulder emis- sion is consistent with a Balmer-dominated shock (BDS; [7]). BDS are traditionally observed around supernova remnants but are also sometimes produced within pulsar wind nebulae and other evolved stellar systems. They form when high velocity (200–9000 km s−1) shocks collide with the interstellar medium, man- ifesting themselves as optically emitting filaments. Energetic particles and/or photons may be gener- ated in the post-shock region of the collisionless, non- radiative shock. Direct collisional excitation of the pre-shock atoms produces a narrow emission line com- ponent that reflects thermal conditions within the pre- shock gas. If the energetic particles exceed the shock eConf C110509 arXiv:1111.00

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