Orbital Modulation of Gamma Rays from PSR~J2339$-$0533

Kavli Affiliate: Roger W. Romani

| First 5 Authors: Hongjun An, Roger W. Romani, Matthew Kerr, ,

| Summary:

We report on orbital modulation of the 100-600 MeV gamma-ray emission of the
$P_{rm B}=4.6$ hr millisecond pulsar binary PSR J2339$-$0533 using 11 yr of
Fermi Large Area Telescope data. The modulation has high significance (chance
probability $papprox 10^{-7}$), is approximately sinusoidal, peaks near pulsar
superior conjunction, and is detected only in the low-energy 100-600 MeV band.
The modulation is confined to the on-pulse interval, suggesting that the
variation is in the 2.9-ms pulsed signal itself. This contrasts with the few
other known systems exhibiting GeV orbital modulations, as these are unpulsed
and generally associated with beamed emission from an intrabinary shock. The
origin of the modulated pulsed signal is not yet clear, although we describe
several scenarios, including Compton upscattering of photons from the heated
companion. This would require high coherence in the striped pulsar wind.

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