arXiv:astro-ph/9906086v1 4 Jun 1999 THE BROAD BAND SPECTRAL PROPERTIES OF BINARY X–RAY PULSARS § D. Dal Fiume 1 , M. Orlandini 1 , S. Del Sordo 2 , F. Frontera 1,3 , T. Oosterbroek 4 , E. Palazzi 1 , A. N. Parmar 4 , S. Piraino 2 , A. Santangelo 2 and A. Segreto 2 1 TeSRE, Consiglio Nazionale delle Ricerche, via Gobetti 101, I–40129 Bologna, Italy 2 IFCAI, Consiglio Nazionale delle Ricerche, via La Malfa 153, I-90146 Palermo, Italy 3 Dipartimento di Fisica, Universit´ a di Ferrara, via Paradiso 1, I–44100 Ferrara, Italy 4 ESA–SSD, ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands ABSTRACT The X–ray telescopes on board BeppoSAX are an optimal set of instruments to observe bright galactic binary pulsars. These sources emit very hard and quite complex X–ray spectra that can be accurately measured with BeppoSAX between 0.1 and 200 keV. A prototype of this complexity, the source Her X–1, shows at least seven different components in its spectrum. A broad band measure is therefore of paramount importance to have a thorough insight into the physics of the emitting region. Moreover the detection of cyclotron features, when present, allows a direct and highly significant measure of the magnetic field intensity in the emission region. In this paper we briefly report the results obtained with BeppoSAX on this class of sources, with emphasis on the detection and on the measured properties of the cyclotron lines. 1 INTRODUCTION X–ray pulsars are studied since more than 25 years but still we are far from understanding the details of the mechanisms that produce the emission of their complex and variable spectra. The overall scenario was depicted with bright accuracy soon after their discovery (Davidson and Ostriker 1973). As the observational data became of better and better quality, it clearly emerged that the X-ray spectra emitted from these sources are quite complex. The spectra of almost all sources are characterized by a very hard power–law like emission in the 1–10 keV band (photon index α between 0 and -1) and by a high energy cutoff, approximately exponential, starting between 10 and 20 keV. While the task of understanding and convincingly modeling the X–ray emission is exceedingly difficult, the final reward to a success in this field is rather appealing. The emitting neutron stars are by themselves relativistic objects. Moreover the magnetic field, needed to channel the accreting matter onto the surface hot spots from which we observe the pulsed emitted radiation, is relativistic. Therefore these systems are an ideal laboratory to test relativistic effects by observational means. In this paper we report the observations of a small sample of bright X–ray pulsars, describing the observational results both on the broad band spectrum and on the cyclotron line features. § Accepted for publication in Advances in Space Research - Proceedings of 32nd COSPAR Scientific Assembly - Symposium E1.1: “Broad Band X-ray Spectroscopy of Cosmic Sources”