Probing millisecond pulsar emission geometry using light curves from the Fermi/Large Area Telescope

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Date
2009Author
Venter, Christo
Harding, A.K.
Guillemot, L.
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An interesting new high-energy pulsar sub-population is emerging following early discoveries of gamma-ray
millisecond pulsars (MSPs) by the Fermi Large Area Telescope (LAT). We present results from three-dimensional
emission modeling, including the special relativistic effects of aberration and time-of-flight delays and also
rotational sweepback of B-field lines, in the geometric context of polar cap (PC), outer gap (OG), and twopole
caustic (TPC) pulsar models. In contrast to the general belief that these very old, rapidly rotating neutron
stars (NSs) should have largely pair-starved magnetospheres due to the absence of significant pair production,
we find that most of the light curves are best fit by TPC and OG models, which indicates the presence of
narrow accelerating gaps limited by robust pair production—even in these pulsars with very low spin-down
luminosities. The gamma-ray pulse shapes and relative phase lags with respect to the radio pulses point to
high-altitude emission being dominant for all geometries. We also find exclusive differentiation of the current
gamma-ray MSP population into two MSP sub-classes: light curve shapes and lags across wavebands impose
either pair-starved PC (PSPC) or TPC/OG-type geometries. In the first case, the radio pulse has a small lag with
respect to the single gamma-ray pulse, while the (first) gamma-ray peak usually trails the radio by a large phase
offset in the latter case. Finally, we find that the flux correction factor as a function of magnetic inclination and
observer angles is typically of order unity for all models. Our calculation of light curves and flux correction factor
for the case of MSPs is therefore complementary to the “ATLAS paper” of Watters et al. for younger pulsars