We present the results of a BeppoSAX observation of the fastest rotating pulsar known: PSR B1937+21. The ∼200 ks observation (78.5 ks MECS/34 ks 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 ≳4 keV. 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 1.3-4 keV band we detect a ∼45% DC component; conversely the 4-10 keV 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 (2-10 keV) flux is F2-10 = 4.1× 10-13 erg cm-2 s-1, implying a luminosity of LX = 5.0× 1031 Θ (d/3.6 kpc)2 erg s-1 and a X-ray efficiency of η = 4.5× 10-5 Θ, 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 NH ∼ 2× 1022 cm-2 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 (H I derived) value of ∼ 1× 1022 cm-2, inspection of dust extinction maps reveal that the pulsar falls in a highly absorbed region. In addition, 1.4 GHz radio map shows that the nearby (likely unrelated) H II source 4C21.53W is part of a circular emission region ∼4′ across.
