Evidence for Pulsar X-ray Emission from Outer Gap in Psr

The fastest millisecond pulsar PSR B1937+21 presents an interpulse separated from the main pulse by nealy 180 at radio frequencies. Recently, the ASCA observations (Takahashi et al. 2001) detected pulsed X-ray emission from this pulsar. Only a single narrow X-ray pulse is observed, which is coincident with the radio interpulse in phase. We investigate the validity of the polar cap (PC) model and outer gap model (OG) for the X-ray emission from PSR B1937+21 by assuming a dipolar magnetic field structure for the pulsar. It shows that the PC model can be almost ruled out while the OG model can explain the main observational facts. The uncertainty of the maximum rate of position angle sweep and the implication of large values of this rate are discussed. Subject headings: pulsar: PSR B1937+21: X-rays: emission mechanism