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Astron. Astrophys. 339, 802-810 (1998)

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The comptonized X-ray source X 1724-308 in the globular cluster Terzan 2

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

1 Astrophysics Division, Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
2 Istituto di Fisica Cosmica ed Applicazioni dell'Informatica, C.N.R., Via Ugo la Malfa 153, I-90146 Palermo, Italy
3 Osservatorio Astronomico di Roma, Via dell'Osservatorio, I-00044 Monteporzio Catone, Italy
4 Istituto Tecnologie e Studio delle Radiazioni Extraterrestri, C.N.R., Via Gobetti 101, I-40129 Bologna, Italy

Received 16 June 1998 / Accepted 31 August 1998

Abstract

We report on the BeppoSAX observation of the X-ray source X 1724-308 in the globular cluster Terzan 2. The broadband spectrum can be described as the superposition of a power-law with photon spectral index [FORMULA] and a thermal component with typical temperature [FORMULA]1 keV. X 1724-308 is detected in the PDS up to 150 keV with a S/N ratio [FORMULA]3, notwithstanding a sharp exponential cutoff with [FORMULA] keV. The broadband spectrum can be best interpreted as due to the Comptonization of a Wien photon distribution with [FORMULA]1 keV by a spherically-symmetric hot plasma with electron temperature [FORMULA] keV and optical depth [FORMULA]. A comparison with the fainter and steeper state, which was observed by EXOSAT, suggests a correlation between the temperature of the Comptonizing electron distribution (or the plasma optical depth) and the X-ray intensity. The X-ray thermal component could originate from a boundary layer between the accretion disk and the compact object, or from the accretion disk itself.

Key words: X-rays: stars – globular clusters: individual: Terzan 2 stars: neutron stars: individual: X 1724-308 – binaries: close accretion, accretion disks

Send offprint requests to: M.Guainazzi, (mguainaz@astro.estec.esa.nl)

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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