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Astron. Astrophys. 349, L9-L12 (1999)

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Letter to the Editor

The broad-band spectrum of OAO1657-415 with BeppoSAX: in search of cyclotron lines

M. Orlandini 1, D. Dal Fiume 1, S. Del Sordo 2, F. Frontera 1,3, A.N. Parmar 4, A. Santangelo 2 and A. Segreto 2

1 Istituto Tecnologie e Studio Radiazioni Extraterrestri, TeSRE/CNR, via Gobetti 101, I-40129 Bologna, Italy
2 Istituto Fisica Cosmica e Applicazioni all'Informatica, IFCAI/CNR, via La Malfa 153, I-90146 Palermo, Italy
3 Università di Ferrara, Dipartimento di Fisica, via Paradiso 11, I-44100 Ferrara, Italy
4 Astrophysics Division, Space Science Department of ESA, ESTEC, 2200 AG Noordwijk, The Netherlands

Received 8 June 1999 / Accepted 9 August 1999

Abstract

We report on a 30 ks observation of the high-mass X-ray binary pulsar OAO1657-415 performed by BeppoSAX on September 1998. The wide band spectrum is well fit by both a cutoff power law, or a power law modified by a high energy cutoff, plus a fluorescence Iron line at 6.5 keV. The two models are statistically equivalent. The inclusion of a cyclotron resonance feature at [FORMULA]36 keV - corresponding to a magnetic field strength [FORMULA] G, where z is the gravitational redshift - improves significantly the [FORMULA] for the cutoff model but only marginally for the power law plus high energy cutoff model. The statistical significance of our data is not adequate to discriminate between the two models, and even the Normalized Crab Ratio technique, successfully used to pinpoint cyclotron features in the spectra of other X-ray pulsars, is not conclusive in answering the question whether the feature is real or it is an artifact due to an improper modeling of the continuum used to fit the data.

Key words: magnetic fields – stars: binaries: close – stars: neutron – stars: pulsars: individual: OAO1657 – 415 – X-rays: stars

Send offprint requests to: orlandini@tesre.bo.cnr.it

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999
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