BeppoSAX observation of PSR B1937+21

Abstract

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 Theta (d/3.6 kpc)2 erg s-1 and a X-ray efficiency of eta = 4.5? 10-5 Theta, where Theta 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 sigma) 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.