The near-UV pulse profile and spectrum of the pulsar PSR B0656+14

Abstract

We have observed the middle-aged pulsar PSR B0656+14 with the prism and the NUV MAMA detector of the Space Telescope Imaging Spectrograph (STIS) to measure the pulsar spectrum and periodic pulsations in the near-ultraviolet (NUV). The pulsations are clearly detected, double-peaked and very similar to the optical pulse profile. The NUV pulsed fraction is 70±12%. The spectral slope of the dereddened phase-integrated spectrum in the ~1800-3200 Å range is ~α_ν=0.35±0.5 which together with the high pulse fraction indicates a non-thermal origin for the NUV emission. The total flux in the range ~1700-3400 Å is estimated to be 3.4±0.3×10^-15 erg s^-1 cm^-2 when corrected for E(B-V)=0.03 mag. At a distance of 288 pc this corresponds to a luminosity L_NUV=3.4×10²⁸ erg s^-1 assuming isotropy of the emission. We compare the NUV pulse profile with observations from radio to gamma-rays. The first NUV sub-pulse is in phase with the gamma-ray pulse marginally detected with EGRET, while the second NUV sub-pulse is similar both in shape and in phase with the non-thermal pulse in hard X-rays. This indicates a single origin of the non-thermal emission in the optical-NUV and in the X-rays. This is also supported by the observed NUV spectral slope, which is compatible with a blackbody plus power-law fit extended from the X-ray range, but dominated by the power-law component in most of the NUV range.