Using H-alpha as a Tracer of the Emission Region of LS I +61 303
22011 Fermi Symposium, Roma., May. 9-12 Using H α 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 modulatedemission across the electromagnetic spectrum. Here we present H α spectra of LS I +61 303 thatexhibit a dramatic emission burst shortly before apastron, observed as a redshifted shoulder in theline profile. A correlated burst in radio, X-ray, and GeV emission is observed at the same orbitalphase. We interpret the source of the emission as a compact pulsar wind nebula that forms whena tidal mass stream from the Be circumstellar disk interacts with the relativistic pulsar wind. TheH α 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 B0Ve and an unknown compact companion in a highlyeccentric, 26.5 day orbit [3]. While the system has arelatively low X-ray luminosity for a HMXB, LS I +61303 is the 15th brightest γ -ray source included in the Fermi
LAT 1-year Point Source Catalogue ([2]). TheBe disk interacts with the compact companion, pro-ducing emission that has been observed to vary withorbital 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 . − .
8, and they defined the arbitraryreference 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 performedan intense multiwavelength observing campaign on LSI +61 303 supported by a
Fermi
Cycle 1 program.We obtained optical H α spectra of LS I +61 303 atthe KPNO Coud´e Feed telescope over 35 consecutivenights to study the evolution of the emission during acomplete orbit [3], [8]. The H α line profile exhibits adramatic emission burst near φ (TG) ∼ .
6, observedas a redshifted shoulder in the line profile (see Fig.1) as the compact source moves almost directly awayfrom the observer.Smaller temporal changes in the red spectra sug-gest additional H α emission variability, so we sub-tracted the mean emission line profile to investigatethe residuals carefully (see Fig. 2). During about halfof the orbit, 0 . ≤ φ (TG) ≤ .
6, the difference spectrareveal a partial S-shaped pattern similar to a spiraldensity wave that is commonly observed in Be stardisks [9]. [12] also observed a strong blue peak near φ (TG) = 0 .
23, which supports the development of aspiral density wave near periastron. After this phase,the peculiar red shoulder develops.We measured the equivalent width of H α , W H α , for each spectrum by directly integrating over the lineprofile. (We use the convention that W H α is negativefor an emission line.) The errors in W H α are typicallyabout 10% due to noise and placement of the contin-uum. Figure 3 shows that during our Coud´e Feed run, W H α decreased slightly just before periastron. Since W H α is correlated to the radius of a Be star’s circum-stellar disk [5], we interpret the decline in emissionas a slight decrease in disk radius as gas is strippedaway by the compact companion. W H α then rises dra-matically with the onset of the red shoulder emissioncomponent near φ (TG) ∼ . W H α with thosemeasured by [6]. Their data were accumulated over sixdifferent observing runs over 1998–2000, and the longterm differences in emission strength are substantial.Also during 2008 October and November, G. Poo-ley obtained nearly simultaneous radio flux coveragewith the Arcminute Microkelvin Imager (AMI) array.The 15 GHz AMI light curve (Fig. 4) reveals emissionthat peaks at the same time as the H α “red shoulder”outburst. Contemporaneous RXTE light curves from[11]and
Fermi light curves ([4]) also reveal orbitallymodulated emission that peaks just before the H α redshoulder, although their wide phase bins may mask atrue correlation. The H α emission clearly traces thehigh energy emission region in this system. III. CONCLUSIONS
The unusual broadness of the H α red shoulder emis-sion is consistent with a Balmer-dominated shock(BDS; [7]). BDS are traditionally observed aroundsupernova remnants but are also sometimes producedwithin pulsar wind nebulae and other evolved stellarsystems. They form when high velocity (200–9000 kms − ) 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 thepre-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 a r X i v : . [ a s t r o - ph . H E ] O c t H J D - , , -600 -400 -200 0 200 400 600VELOCITY (km s -1 )1.0 0.5 0.0 O R B I T A L P H ASE ( T G ) FIG. 1: The upper plot shows the H α line profile over 35 continuous nights of observation at the KPNO Coud´e Feedtelescope. The lower plot shows a gray-scale image of the same line. Note that the lower plot of gray-scale spectra arenot folded by orbital phase but are placed in the same chronological order. From [8]. velocity, the pre-shock hydrogen atoms also exchangeelectrons with post-shock protons, manifesting them-selves as broad neutral hydrogen lines (widths ∼ − ). The H α line structure in LS I +61 303 iscomplicated by the superposition of emission from thecircumstellar disk; however, the broad red shoulder isconsistent with such a BDS. The temporary nature ofthe red shoulder, as well as the correlated GeV–radioemission, suggests that the BDS only forms when ahigh density tidal mass stream interacts with a pulsarwind in LS I +61 303. Acknowledgments
We thank Di Harmer and the staff at KPNO fortheir hard work to schedule and support the Coud´e Feed observations. Guy Pooley, Christina Aragona,Tabetha Boyajian, Amber Marsh, and Rachael Roet-tenbacher helped collect the data presented here andshould be cheered for their heroic efforts. This workis supported by NASA DPR numbers NNX08AV70G,NNG08E1671, NNX09AT67G, and an institutionalgrant from Lehigh University. eConf C110509
011 Fermi Symposium, Roma., May. 9-12 H J D - , , -1000 -500 0 500 1000VELOCITY (km s -1 )1.0 0.5 0.0 O R B I T A L P H ASE ( T G ) FIG. 2: The H α difference spectra of LS I +61 303, shown in the same format as Figure 1. Note the distinctive peak inemission that appears to follow a partial S-curve in the spectra. A fast moving “red shoulder” of emission produces astrong redshifted feature between φ (TG) = 0 . − .
7. From [8].[1] Abdo A. A., et al. 2009, ApJL, 701, 123[2] Abdo A. A., et al. 2010, ApJS, 188, 405[3] Aragona, C., McSwain, M. V., Grundstrom, E. D.,Marsh, A. N., Roettenbacher, R. M., Hessler, K. M.,Boyajian, T. S., & Ray, P. S. 2009, ApJ, 698, 514[4] Dubois, R. &
Fermi
LAT Collaboration, 2011, AASMeeting 217, eConf C110509
FIG. 3: The emission strength of H α over multiple orbits is indicated by the equivalent width of the line ( W H α ).Measurements from the six runs of Grundstrom et al. (2007; identified by the date HJD-2,450,000) and the mean W H α from each night of our 2009 WIRO run are shown folded by orbital phase. Note the consistent peak near φ (TG) ∼ . α spectroscopy campaign. Data courtesy of Guy Pooley.spectroscopy campaign. Data courtesy of Guy Pooley.