Gamma-Ray Orbital Modulation in Spider Pulsars: Three Discoveries and a Universal Modulated Fraction

Abstract Compact binary millisecond pulsars (also known as spiders) allow us to probe pulsar winds in their innermost regions, between the light cylinder (radius ∼10 7 cm) and the companion star (at ∼10 11 cm). Their flux is known to vary along the orbit, from radio to X-rays. During the past decade, gamma-ray orbital modulation (GOM) has been discovered in a handful of spiders, but its origin remains largely unknown. We present the results of a systematic search for GOM among 43 systems, selecting pulsed 0.1–1 GeV photons and using spin and orbital ephemeris from Fermi’s Third Pulsar Catalog. We discover GOM from three spiders—PSR J1124-3653, PSR J1946-5403, and PSR J2215+5135—and confirm four previous detections. In all seven cases so far, the GOM peaks near the pulsar’s superior conjunction. The X-ray orbital light curves are usually in antiphase, peaking when the pulsar is at inferior conjunction, but we find one case where both gamma rays and X-rays peak around superior conjunction: PSR J1946-5403. We measure the modulated fractions of the GOM and find consistent values for all seven spiders, with an average 22.0% ± 2.6%. Including eclipsing systems seen edge-on, we find no clear dependence of the modulated fraction on the orbital inclination (within ≃45°–90°). Our results challenge previous models proposed to explain GOM in spiders, based on inverse Compton and synchrotron emission close to the companion, since these predict a clear dependence on orbital inclination (stronger modulation at high inclinations). We nearly double the number of GOM detections in spiders, showing that it is more common than previously thought.