The near - UV pulse profile and spectrum of the pulsar PSR B 0656 + 14 ⋆

We have observed the middle-aged pulsar PSR B0656+14 with the prism and the NUVMAMA 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 ∼ αν = 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× 10erg s cm when corrected for E(B−V ) = 0.03 mag. At a distance of 288 pc this corresponds to a luminosity LNUV = 3.4× 10 28 erg s 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 opticalNUV 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.