FERMI LAT OBSERVATIONS OF LS I+61 degrees 303 : FIRST DETECTION OF AN ORBITAL MODULATION IN GeV GAMMA RAYS

• This Letter presents the first results from the observations of LS I + 61 degrees 303 using Large Area Telescope data from the Fermi Gamma-Ray Space Telescope between 2008 August and 2009 March. Our results indicate variability that is consistent with the binary period, with the emission being modulated at 26.6 +/- 0.5 days. This constitutes the first detection of orbital periodicity in high-energy gamma rays (20 MeV-100 GeV, HE). The light curve is characterized by a broad peak after periastron, as well as a smaller peak just before apastron. The spectrum is best represented by a power law with an exponential cutoff, yielding an overall flux above 100 MeV of 0.82 +/- 0.03(stat) +/- 0.07(syst) 10(-6) ph cm(-2) s(-1), with a cutoff at 6.3 +/- 1.1(stat) +/- 0.4(syst) GeV and photon index Gamma = 2.21 +/- 0.04(stat) +/- 0.06(syst). There is no significant spectral change with orbital phase. The phase of maximum emission, close to periastron, hints at inverse Compton scattering as the main radiation mechanism. However, previous very high-energy gamma ray (>100 GeV, VHE) observations by MAGIC and VERITAS show peak emission close to apastron. This and the energy cutoff seen with Fermi suggest that the link between HE and VHE gamma rays is nontrivial.

Ämnesord

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Nyckelord

binaries: close

gamma rays: observations

stars: variables: other

X-rays: binaries

X-rays: individual (LS I+61 degrees 303)

source 2cg 135+01

i+61 303

microquasar ls-i+61-303

binaries ls-5039

area telescope

pulsar wind

spaced data

egret data

h-alpha

cos-b

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