EDP Sciences
Free Access
Volume 413, Number 3, January III 2004
Page(s) 1065 - 1072
Section Stellar atmospheres
DOI https://doi.org/10.1051/0004-6361:20031593

A&A 413, 1065-1072 (2004)
DOI: 10.1051/0004-6361:20031593

BeppoSAX observation of PSR B1937+21

L. Nicastro1, G. Cusumano1, O. Löhmer2, M. Kramer3, L. Kuiper4, W. Hermsen4, T. Mineo1 and W. Becker5

1  IASF-CNR, via U. La Malfa 153, 90146 Palermo, Italy
2  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
3  University of Manchester, Jodrell Bank, Macclesfield SK11 9DL, UK
4  SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
5  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85740 Garching, Germany

(Received 30 June 2003 / Accepted 10 October 2003)

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 $\gtrsim$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 $F_{2-10} = 4.1\times 10^{-13}$ erg cm -2 s -1, implying a luminosity of $L_{\rm X} = 5.0\times 10^{31} \, \Theta$ ( d/3.6 kpc) 2 erg s -1 and a X-ray efficiency of $\eta = 4.5\times 10^{-5}\, \Theta$, 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 $N_{\rm H} \sim 2\times 10^{22}$ 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 $\sim 1\times 10^{22}$ 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.

Key words: stars: neutron -- stars: pulsars individual: PSR B1937+21 -- X-rays: stars

Offprint request: L. Nicastro, nicastro@pa.iasf.cnr.it

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© ESO 2004