Gamma-ray Orbital Modulation in Spider Pulsars

My first PhD paper was finally published in ApJ!

Spider pulsars are known to exhibit orbital modulation of their flux across the entire electromagnetic spectrum, from radio to X-rays. Over the past decade, gamma-ray orbital modulation (GOM) has been discovered in a handful of spiders, but its origin remains largely unknown. Because GOM is very subtle, its detection requires high photon statistics and precise spin and orbital ephemerides.

Using timing solutions for 43 spider pulsars from the recently published Fermi’s Third Pulsar Catalog, we conducted a systematic search for GOM. We discover GOM from three spiders – PSR J1124-3653, PSR J1946-5403 and PSR J2215+5135 and confirm four previous detections. The X-ray orbital light curves are usually in anti-phase, 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 find that the modulated fractions of all seven systems are similar, with an average value of 22.0 ± 2.6%, independent of orbital inclination. 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 with 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.

The animation below illustrates how the significance of both pulsations and GOM increases over time for PSR J1124-3653 as more data are accumulated. The left panel shows spin phase versus time for detected photons, with color indicating the probability that each photon originates from the pulsar. The right panels show weighted counts as a function of spin and orbital phase.