ITAL]RXTE[/ITAL] Observation of PSR B1951+32

Abstract

We report results of RXTE observations of PSR B1951+32 using the PCA instrument for 19k seconds during 1996 March 24th. We applied the contemporaneous radio ephemeris and various statistical tests to search for evidence of pulsation. These analyses yield intriguing yet inconclusive evidence for the presence of the pulsation in the time series: confidence level for the presence of pulsation is 94.5% in the 2.0-4.8 keV band and 97.6% in the 4.8-6.3 keV band based on the H-test. Under the premise of non-detection of pulsation, we derive estimated 2-sigma upper limits for the pulsed flux to be $3.3\times 10^{-6} cm^{-2} s^{-1} keV^{-1}$ in the 2.0-4.8 keV band, $2.7\times 10^{-6} cm^{-2} s^{-1} keV^{-1}$ in the 4.8-8.5 keV band, and $2.0\times 10^{-6} cm^{-2} s^{-1} keV^{-1}$ in the 8.5-13.0 keV band. These upper limits are consistent with the trend of spectral turn-over from high-energy gamma-ray emission as suggested by the OSSE upper limit. Such turn-over strongly suggests the outer magnetosphere as the emission site for pulsed gamma-rays. These RXTE upper limits for X-ray pulsation are, on the other hand, not consistent with the extrapolation of reported power-law spectra from the point source observed by ROSAT in the 0.1-2.4 keV band, assuming a constant pulse fraction: The pulsed soft X-ray emission detected by ROSAT must follow a much softer spectrum than that of the overall point source.