HEAD Division Meeting 1999, April 1999
Session 15. X-Ray Binaries
Poster, Tuesday, April 13, 1999, 8:30am-6:02pm, Gold Room

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[15.03] Hunting for Cyclotron Lines with BeppoSAX: OAO1657--415

M. Orlandini, D. Dal Fiume, F. Frontera (TeSRE/CNR), S. Del Sordo, A. Santangelo, A. Segreto (IFCAI/CNR), A.N. Parmar (SSD/ESA)

The 38~s X--ray binary pulsar OAO1657--415 shows one of the hardest spectra among X--ray pulsars, without any evidence of steepening at high (>10 keV) energy. The high energy instruments aboard BeppoSAX, namely HPGSPC and PDS, are the best suited to detect high energy cutoff in hard sources. Indeed, a 30~ksec observation of OAO1657--415 detected for the first time a change of slope in its spectrum, at ~15 keV. The wide band spectrum is well fit by both a cutoff power law, or a power law modified by a high energy cutoff. The two models are equivalent. The inclusion of a cyclotron resonance feature (CRF) at 36 keV significantly improves the \chi2 for the cutoff model (Probability of Chance Improvement (PCI) 1.3\times 10-5), while the PCI for the power law plus high energy cutoff model is moderately high (4.5%). The statistical significance of our data is not sufficient to constrain the CRF width but by fixing its value to a {\em canonical} value of 10 keV, the relation between CRF energy and its FWHM (explained in terms of Doppler broadening of the electrons responsible of the CRF) is fulfilled.

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