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Astron. Astrophys. 351, 225-232 (1999)

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BeppoSAX spectroscopy of the globular cluster X-ray source XB 1746-371 (NGC 6441)

A.N. Parmar 1, T. Oosterbroek 1, M. Guainazzi 1, A. Segreto 2, D. Dal Fiume 3 and L. Stella 4

1 Astrophysics Division, Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
2 Istituto di Fisica Cosmica ed Applicazioni all'Informatica, CNR, Via U. La Malfa 153, 90146 Palermo, Italy
3 Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Via Gobetti 101, 40129 Bologna, Italy
4 Osservatorio Astronomico di Roma, Via Frascati 33, Monteporzio Catone, 00040 Roma, Italy

Received 15 June 1999 / Accepted 18 August 1999

Abstract

During a BeppoSAX observation of the X-ray source XB 1746-371 located in the globular cluster NGC 6441 a type I X-ray burst, parts of 4 intensity dips, and energy dependent flaring were detected. The dips repeat every [FORMULA] hr and show no obvious energy dependence. If the dips are due to electron scattering this energy independence implies an abundance [FORMULA]130 times less than solar, confirming an earlier measurement. Since the overall cluster abundance is close to solar this low abundance is unexpected. Photoionization of the absorbing material, obscuration of an extended source, and variations in multiple components that combine to produce an apparent energy independence are all excluded. Thus, the nature of the dips remains uncertain. The best-fit model to the overall 0.3-30 keV spectrum is a disk-blackbody with a temperature of [FORMULA] keV, together with a cutoff power-law with a photon index of [FORMULA] and a cutoff energy of [FORMULA] keV. Absorption, consistent with the optical extinction to NGC 6441 of [FORMULA] atom cm-2 is required. All previous best-fit spectral models for this source are excluded at high confidence. The spectrum is dominated by the blackbody-like component, with the cutoff power-law only contributing an average of 12% of the 1-10 keV flux. During flaring intervals the contribution of this component decreases to [FORMULA]6% with variations in the intensity of the blackbody-like component being responsible for most of the flaring activity.

Key words: accretion, accretion disks – stars: individual: XB 1746-371 – stars: neutron – Galaxy: globular clusters: individual: NGC 6441 – X-rays: bursts – X-rays: general

Send offprint requests to: A.N. Parmar (aparmar@astro.estec. esa.nl)

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999
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