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

TL;DR

This study models gamma-ray light curves of millisecond pulsars using 3D emission geometries, revealing that high-altitude emission and pair production are prevalent even in old, rapidly rotating neutron stars, and identifying two distinct MSP sub-classes.

Contribution

It introduces detailed 3D emission modeling of MSPs incorporating relativistic effects, showing the dominance of pair production and high-altitude emission in their gamma-ray light curves.

Findings

Most MSP light curves fit TPC and OG models, indicating active pair production.

High-altitude emission dominates gamma-ray pulse shapes across geometries.

Distinct MSP sub-classes are identified based on light curve shapes and phase lags.

Abstract

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 3D 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 two-pole 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.