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

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.

💡 Research Summary

This paper presents the first detection of pulsed gamma‑ray emission from two historically important millisecond pulsars (MSPs): PSR B1937+21, the original “fast” MSP, and PSR B1957+20, the first “black‑widow” system. Using 18 months of Fermi Large Area Telescope (LAT) data together with contemporaneous radio timing solutions derived from the Westerbork Synthesis Radio Telescope and the Nançay Radio Telescope, the authors were able to phase‑fold the gamma‑ray photons with sub‑microsecond precision.

The timing campaigns yielded high‑quality ephemerides: for B1937+21, 80 times‑of‑arrival (TOAs) with an RMS residual of 197 ns; for B1957+20, 38 TOAs at 1.4 GHz plus 426 low‑frequency TOAs, giving an RMS of 4.1 µs after correcting for significant dispersion‑measure (DM) variations. These solutions were incorporated into the Tempo2 package and made publicly available via the Fermi Science Support Center.

Gamma‑ray photons above 0.5 GeV were selected to reduce the overwhelming Galactic diffuse background that plagues low‑latitude sources. A standard H‑test gave detection significances of 4.6σ (B1937+21) and 5.4σ (B1957+20). Because the raw significances were modest, the authors also applied a weighted H‑test that accounts for each photon’s probability of originating from the target pulsar, further confirming the pulsations.

Spectral analysis employed a binned maximum‑likelihood method with the P6_V3 instrument response functions. Both pulsars are best described by a power‑law with an exponential cutoff: dN/dE = N₀ (E/1 GeV)⁻Γ exp