Fermi LAT Observations of LS 5039

TL;DR

This paper reports the first detection of gamma-ray modulation at the binary period of LS 5039 using Fermi LAT data, revealing spectral cutoff and phase-dependent variability consistent with inverse Compton models.

Contribution

First observation of GeV gamma-ray modulation in LS 5039, with spectral analysis indicating a cutoff and phase-dependent variability, supporting magnetospheric emission models.

Findings

Detected gamma-ray modulation with a 3.903-day period.

Spectral cutoff at approximately 2.1 GeV observed.

Spectral variability correlates with orbital phase.

Abstract

The first results from observations of the high mass X-ray binary LS 5039 using the Fermi Gamma-ray Space Telescope data between 2008 August and 2009 June are presented. Our results indicate variability that is consistent with the binary period, with the emission being modulated with a period of 3.903 +/- 0.005 days; the first detection of this modulation at GeV energies. The light curve is characterized by a broad peak around superior conjunction in agreement with inverse Compton scattering models. The spectrum is represented by a power law with an exponential cutoff, yielding an overall flux (100 MeV - 300 GeV) of 4.9 +/- 0.5(stat) +/- 1.8(syst) x 10^-7 photon cm^-2 s^-1, with a cutoff at 2.1 +/- 0.3(stat) +/- 1.1(syst) GeV and photon index Gamma = 1.9 +/- 0.1(stat) +/- 0.3(syst). The spectrum is observed to vary with orbital phase, specifically between inferior and superior conjunction. We suggest that the presence of a cutoff in the spectrum may be indicative of magnetospheric emission similar to the emission seen in many pulsars by Fermi.