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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. Fermi LAT Observations of Galactic X-ray binaries. The Fermi instruments. Fermi was successfully launched June 2008 LAT:

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Fermi LAT Observations of Galactic X-ray binaries

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  1. 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 Fermi LAT Observations of Galactic X-ray binaries

  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 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

  4. What did EGRET see? • Evidence of variability found but no periodicity. • Tavani et al. (1998) 3EG J0241+6103 has been associated with LS I +61°303 but position is uncertain. Hartman et al. (1999)

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

  6. Fermi view of LS I +61°303 PRELIMINARY 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

  7. First definitive detection at MeV-GeV energies PRELIMINARY PRELIMINARY • 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.

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

  9. 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)

  10. LS I +61°303 spectrum 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) 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)

  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? Fermi J0910-5041 3EG J0903-3531 3EG J0903-3531 PRELIMINARY 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).

  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 Inverse Compton Synchrotron Assume orbital modulation of B (expected in pulsar wind model) Pulsar wind Massive star Rshock Be star Pulsar e+/e- High B LowB Pulsar wind

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