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

By A. Abdo, et al., for the Belle collaboration. 2009

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This Letter presents the first results from the observations of LS I +61°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 Γ = 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.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/2cdb1bbb0b5bd7bb7de66b8f092d7f7d16e6b259

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Citations in the corpus

THE FIRST FERMI LARGE AREA TELESCOPE CATALOG OF GAMMA-RAY PULSARS (2009, methodology): “The majority of the data are collected at 1398 MHz, while for MSPs in particular, observations are duplicated at 2048 MHz to allow DM monitoring.”
Fermi Large Area Telescope Fourth Source Catalog (2019, methodology): “More information about the LAT is provided in Atwood et al. (2009), and the in-flight calibration of the LAT is described in Abdo et al. (2009d) and Ackermann et al. (2012a, 2012c).”
FERMI LARGE AREA TELESCOPE FIRST SOURCE CATALOG (2010, methodology): “7−7037 which is also associated with PKS 0601−70, a blazar that was significantly variable during the time span of the 1FGL catalog (Abdo et al. 2009f).”