Fermi LAT Observations of Galactic X-ray binaries Adam Hill R. Dubois (SLAC), R. Corbet (UMBC/NASA, GSFC), G. Dubus (LAOG), T. Tanaka (SLAC), D. Torres (ICREA, Barcelona) on behalf of the Fermi-LAT collaboration

The Fermi instruments Fermi was successfully launched June 2008 LAT: 20 MeV – 300 GeV 1’ PSLA for bright sources 2 str FOV In survey mode scans the whole sky every 3 hours GBM: 8 keV- 30 MeV Views whole unocculted sky

Why do we expect to see binaries ? A number of X-ray binaries have been detected at > 1 TeV: PSR B1259-63 Radio pulsar in 3.5 yr orbit of Be star HESS detection at periastron LS 5039 HESS detects 3.9 day orbital period LS I +61°303 Be HMXB shows periodic radio flares MAGIC & VERITAS see VHE emission modulated on 26 day orbital period Sources seen by traditional X-ray missions, by INTEGRAL up to 100 keV. Seen by MAGIC, HESS & VERITAS at VHE. Why not at MeV-GeV energies Most radiation emitted above 10 MeV

What did EGRET see? 3EG J0241+6103 has been associated with LS I +61°303 but position is uncertain. Hartman et al. (1999) ASCA 2-10keV image with EGRET 68% and 95% confidence contours Evidence of variability found but no periodicity. Tavani et al. (1998)

What did EGRET see? 3EG1824-1514 is coincident with LS 5039 but has a 0.5° error box and no indication of variability Hartman et al. (1999) SED of LS 5039 including the spectra of EGRET & HESS. The 2 X-ray measurements are from RXTE & XMM. de Naurois. (2005)

Fermi view of LS I +61°303 LS I +61°303 has been fitted to R.A.=40.076, Dec.=61.233 with 95% error radius of 1.8’. This location is consistent with the known position of the optical counterpart. Flux variability is also clearly evident PRELIMINARY

First definitive detection at MeV-GeV energies We detect a periodicity in the LS I +61°303 light curve at 26.4±0.5 days Folded light curve indicates peaks of emission around periastron. PRELIMINARY PRELIMINARY

Orbit-by-orbit Investigating signs of orbit to orbit variability As more orbits are observed evidence will become clearer Data until January 2009 PRELIMINARY J.Casares et al (2005) N.B. The light curve is not background subtracted

VHE observations Veritas observations in 2006-2007 showed a strongly variable flux at 300 GeV – 5 TeV. Maximum flux is found during most orbital cycles at apastron. Acciari et al. (2008) MAGIC observations in 2005-2006 also showed variable emission (200 GeV – 4 TeV). See significant flux increase at phase 0.45-0.65 Albert et al. (2006)

LS I +61°303 spectrum Unbinned likelihood fitting of the Fermi flux to a power law yields F = A E-γ: Flux (E>100 MeV) = 0.83 ± 0.04 (stat) ± 0.21 (syst) 10-6 ph/cm2/s γ = 2.41 ± 0.03 (stat) ± 0.17 (syst) (Paper in preparation) (N.B. – The TeV spectral data is not contemporaneous and is not phase averaged) PRELIMINARY Points: Fitted energy bins Red: Fermi unbinned power law fit Grey: EGRET Blue: MAGIC (Only phase 0.4-0.7) Veritas data points overlaid (systematic errors not shown)

What else are we looking for? MAGIC reported a flare at VHE from Cyg-X1 in 2006. We are monitoring 68 sources on a daily basis. Challenging work due to the high contribution of diffuse emission in the galactic plane.

Hunting for new sources? The LAT has and will discover new unknown sources; some of which could potentially be gamma-ray binaries. 2 bright transients detected in the Galactic Plane (ATels 1771 &1788). Fermi J0910-5041 3EG J0903-3531 3EG J0903-3531 PRELIMINARY

Conclusions First detection in the GeV domain of flux modulation on the 26.5 day orbital period. Definitive detection of LS I +61°303 by Fermi. LS 5039 to be investigated. More binaries to look for both persistent and flaring. Lots more work to do...

Explaining the HE & VHE emission Pulsar wind model Massive star Pulsar wind Pulsar e+/e- Inverse Compton Synchrotron Assume orbital modulation of B (expected in pulsar wind model) Be star Pulsar wind Rshock High B Low B