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

We report the detection of pulsed gamma-rayemission 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 Nancay 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-rayemission 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-rayMSPs with phase-aligned peaks in different energy bands. The modeling of the radio and gamma-rayemission profiles suggests co-locatedemission regions in the outer magnetosphere.