Publication Date:
2019-07-13
Description:
The Fermi Large Area Telescope (LAT) reported the first definitive gigaelectron volts detections of the binaries LS I +61 deg 303 and LS 5039 in the first year after its launch in 2008 June. These detections were unambiguous as a consequence of the reduced positional uncertainty and the detection of modulated gamma-ray emission on the corresponding orbital periods. An analysis of new data from the LAT, comprising 30 months of observations, identifies a change in the gamma-ray behavior of LS I +61 deg 303. An increase in flux is detected in 2009 March and a steady decline in the orbital flux modulation is observed. Significant emission up to 30 gigaelectron volts is detected by the LAT; prior data sets led to upper limits only. Contemporaneous terraelectron volt observations no longer detected the source, or found it-in one orbit-close to periastron, far from the phases at which the source previously appeared at terraelectron volt energies. The detailed numerical simulations and models that exist within the literature do not predict or explain many of these features now observed at gigaelectron volt and terraelectron volt energies. New ideas and models are needed to fully explain and understand this behavior. A detailed phase-resolved analysis of the spectral characterization of LS I +61 deg 303 in the gigaelectron volt regime ascribes a power law with an exponential cutoff spectrum along each analyzed portion of the system's orbit. The on-source exposure of LS 5039 is also substantially increased with respect to our prior publication. In this case, whereas the general gamma-ray properties remain consistent, the increased statistics of the current data set allows for a deeper investigation of its orbital and spectral evolution.
Keywords:
Instrumentation and Photography; Astronomy
Type:
GSFC-E-DAA-TN9555
,
The Astrophysical Journal; 749; 1; 54
Format:
application/pdf
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