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Astron. Astrophys. 356, 163-170 (2000)


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X-ray/optical observations of XTE J0421+560/CI Cam in quiescence

M. Orlandini 1, A.N. Parmar 2, F. Frontera 1,3, N. Masetti 1, D. Dal Fiume 1, A. Orr 4, A. Piccioni 5, G. Raimondo 6, A. Santangelo 7, G. Valentini 6 and T. Belloni 8

1 Istituto Tecnologie e Studio Radiazioni Extraterrestri, TeSRE/CNR, Via Gobetti 101, 40129 Bologna, Italy
2 Astrophysics Division, Space Science Department of ESA, ESTEC, 2200 AG Noordwijk, The Netherlands
3 Dipartimento di Fisica, Università di Ferrara, Via Paradiso 11, 44100 Ferrara, Italy
4 Institute for Astronomy and Astrophysics Tübingen, IAAT, Waldhaeuser Strasse 64, 72076 Tübingen, Germany
5 Dipartimento di Astronomia, Università di Bologna, Via Zamboni 33, 40126 Bologna, Italy
6 Osservatorio Astronomico di Teramo, Collurania, 64100 Teramo, Italy
7 Istituto Fisica Cosmica e Applicazioni all'Informatica, IFCAI/CNR, Via La Malfa 153, 90146 Palermo, Italy
8 Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate (Lc), Italy

Received 10 May 1999 / Accepted 27 January 2000

Abstract

We report on a BeppoSAX observation of the transient X-ray source XTE J0421+560 during quiescence performed [FORMULA]150 days after the 1998 April outburst. The source had an unabsorbed 0.5-10 keV flux of [FORMULA] erg cm-2 s-1 and was still remarkably soft with most of the emission below 2 keV. The X-ray spectrum can be fit with the same two-temperature model as the outburst observations. There is evidence for the presence of an Iron emission feature at [FORMULA]7 keV. We report also on a series of optical observations performed using the 72 cm Teramo-Normale Telescope (TNT) of the Teramo Observatory, and the 1.5 m Loiano Telescope of the Bologna Observatory. The optical spectrum includes very strong Balmer emission lines, He I, [Fe II], and [O I] features. From the observed [FORMULA] the quiescent optical emission cannot be due to re-processing of the X-rays, but has to be generated in the optical companion or the circumstellar material. Moreover, the quiescent X-ray luminosity cannot be due to the optical star if it is of spectral type OB. Although the nature of the compact object present in the XTE J0421+560/CI Cam system cannot be firmly established, we speculate that it is most probably a white dwarf.

Key words: stars: binaries: symbiotic – stars: individual: XTE J0421+560 – stars: novae, cataclysmic variables – X-rays: bursts – X-rays: stars

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

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000

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