Pulsed Gamma Rays from the Original Millisecond and Black Widow Pulsars: a case for Caustic Radio Emission?

TL;DR

This paper reports the detection of pulsed gamma-ray emission from two millisecond pulsars using Fermi LAT data, revealing phase-aligned peaks across gamma-ray, radio, and X-ray bands, and suggests co-located emission regions in their outer magnetospheres.

Contribution

First detection of pulsed gamma-ray emission from MSPs B1937+21 and B1957+20, with evidence of phase alignment across multiple energy bands, supporting outer magnetosphere emission models.

Findings

Gamma-ray profiles show two peaks separated by half a rotation.

Phase alignment observed between gamma-ray, radio, and X-ray peaks.

Evidence of pulsed X-ray emission from PSR B1957+20 at ~4σ significance.

Abstract

We report the detection of pulsed gamma-ray emission from the fast millisecond pulsars (MSPs) B1937+21 (also known as J1939+2134) and B1957+20 (J1959+2048) using 18 months of survey data recorded by the \emph{Fermi} Large Area Telescope (LAT) and timing solutions based on radio observations conducted at the Westerbork and Nan\c{c}ay radio telescopes. In addition, we analyzed archival \emph{RXTE} and \emph{XMM-Newton} X-ray data for the two MSPs, confirming the X-ray emission properties of PSR B1937+21 and finding evidence ($\sim 4\sigma$) for pulsed emission from PSR B1957+20 for the first time. In both cases the gamma-ray emission profile is characterized by two peaks separated by half a rotation and are in close alignment with components observed in radio and X-rays. These two pulsars join PSRs J0034-0534 and J2214+3000 to form an emerging class of gamma-ray MSPs with phase-aligned peaks in different energy bands. The modeling of the radio and gamma-ray emission profiles suggests co-located emission regions in the outer magnetosphere.