EDP Sciences
Free access
Volume 420, Number 2, June III 2004
Page(s) 527 - 532
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20040113

A&A 420, 527-532 (2004)
DOI: 10.1051/0004-6361:20040113

The high energy X-ray tail of $\mathsf{\eta}$ Car revealed by BeppoSAX

R. F. Viotti1, L. A. Antonelli2, C. Rossi3 and S. Rebecchi4

1  Istituto di Astrofisica Spaziale e Fisica Cosmica, CNR, Area di Ricerca Tor Vergata, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
2  INAF-Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Roma, Italy
3  Dipartimento di Fisica, Università La Sapienza, Piazzale Aldo Moro 3, 00185 Roma, Italy
4  ASI Science Data Center (ASDC), c/o ESA-ESRIN, Via Galileo Galilei, 00044 Frascati, Roma, Italy

(Received 27 December 2003 / Accepted 10 February 2004 )

We report on the June 2000 long (100 ks) BeppoSAX exposure that unveiled a new very high energy component of the X-ray spectrum of $\eta$ Car above 10 keV extending to at least 50 keV. We find that the 2-150 keV spectrum is best reproduced by a thermal + non-thermal model. The thermal component dominates the 2-10 keV spectral range with kTh  = 5.5  $\pm$ 0.3 keV and log NH h  = 22.68  $\pm$ 0.01. The spectrum displays a prominent iron emission line centred at 6.70 keV. Its equivalent width of 0.94 keV, if produced by the thermal source, gives a slightly sub-solar iron abundance ([Fe/H]  = -0.15  $\pm$ 0.02). The high energy tail above 10 keV is best fitted by a power law with a photon index of 2.42  $\pm$ 0.04. The integrated 13-150 keV luminosity of ~12  $L_{\odot}$ is comparable to that of the 2-10 keV thermal component (19  $L_{\odot}$). The present result can be explained, in the $\eta$ Car binary star scenario, by Comptonisation of low frequency radiation by high energy electrons, probably generated in the colliding wind shock front, or in instabilities in the wind of the S Dor primary star. It is possible that the high energy tail had largely weakened near the minimum of the 5.53 yr cycle. Probably, the thermal component has a longer recovery time like that of the highest excitation optical emission lines. Both features can be associated with the large absorption measured by BeppoSAX at phase 0.05.

Key words: radiation mechanisms: non-thermal -- stars: individual: $\eta$ Car -- stars: winds, outflows - X-rays: stars

Offprint request: R. Viotti, uvspace@rm.iasf.cnr.it

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