1. 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

2. 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

3. Why do we expect to see binaries ?
A number of X-ray binaries have been detected at > 1 TeV:
PSR B
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

4. What did EGRET see?
3EG J 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)

5. What did EGRET see?
3EG 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)

6. Fermi view of LS I +61°303
LS I +61°303 has been fitted to R.A.=40.076, Dec.= 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
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7. 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.
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8. Orbit-by-orbit Investigating signs of orbit to orbit variability
As more orbits are observed evidence will become clearer
Data until January 2009
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J.Casares et al (2005)
N.B. The light curve is not background subtracted

9. VHE observations
Veritas observations in 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 also showed variable emission (200 GeV – 4 TeV).
See significant flux increase at phase
Albert et al. (2006)

10. 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 ± (stat) ± 0.21 (syst) 10-6 ph/cm2/s
γ = 2.41 ± 0.03 (stat) ± (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 )
Veritas data points overlaid
(systematic errors not shown)

11. 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.

12. 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 J
3EG J
3EG J
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13. 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...

14. 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