2004A&A...413.1065N

We present the results of a BeppoSAX observation of the fastest rotating pulsar known: PSR B1937+21. The ∼200ks observation (78.5ks MECS/34ks LECS on-source time) allowed us to investigate with high statistical significance both the spectral properties and the pulse profile shape. The pulse profile is clearly double peaked at energies >4keV. Peak widths are compatible with the instrumental time resolution and the second pulse lags the main pulse 0.52 in phase, like is the case in the radio. In the 1.3-4keV band we detect a ∼45% DC component; conversely the 4-10keV pulsed fraction is consistent with 100%. The on-pulse spectrum is fitted with an absorbed power-law of spectral index ∼1.2, harder than that of the total flux which is ∼1.9. The total unabsorbed (2-10keV) flux is F_2–10=4.1x10^–13erg/cm²/s, implying a luminosity of L_X=5.0x10³¹Θ(d/3.6kpc)²erg/s and a X-ray efficiency of η=4.5x10^–5Θ, where Θ is the solid angle spanned by the emission beam. These results are in agreement with those obtained by ASCA and a more recent Rossi-XTE observation. The hydrogen column density N_H∼2x10²²/cm² is ∼10 times higher than expected from the radio dispersion measure and average Galactic density of e^–. Though it is compatible (within 2 σ) with the Galactic (HI derived) value of ∼1x10²²/cm², inspection of dust extinction maps reveal that the pulsar falls in a highly absorbed region. In addition, 1.4GHz radio map shows that the nearby (likely unrelated) HII source 4C21.53W is part of a circular emission region ∼4' across.