Astronomy & Astrophysics

Free access article

A&A 385, 874-883 (2002)
DOI: 10.1051/0004-6361:20020048

BeppoSAX broad X-ray range observations of $\eta$ Carinae during high and low spectroscopic states

R. F. Viotti¹, L. A. Antonelli², M. F. Corcoran³, A. Damineli⁴, P. Grandi¹, J. M. Muller⁵, S. Rebecchi⁶, C. Rossi⁷ and M. Villada⁸

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica, CNR, Area di Ricerca Tor Vergata, Via Fosso del Cavaliere 100, 00133 Roma, Italy
² INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (Roma), Italy
³ Universities Space Research Association, 7501 Forbes Blvd, Ste 206, Seabrook, MD 20706, and Laboratory for High Energy Astrophysics, Goddard Space Flight Center, Greenbelt MD 20771, USA
⁴ Instituto Astronomico e Geofisico da Universidade de São Paulo, Av. Miguel Stefano 4200, 04301-904 São Paulo, Brazil
⁵ University Hospital Nijmegen, Department of Radiology, PO Box 9101, 6500 HB Nijmegen, The Netherlands
⁶ ASI Science Data Center (ASDC), c/o ESA-ESRIN, Via Galileo Galilei, 00044 Frascati (Roma), Italy
⁷ Dipartimento di Fisica, Università La Sapienza, Piazzale Aldo Moro 3, 00185 Roma, Italy
⁸ Dep. Astronomia Estelar, Observatorio Astronomico U.N.C., 5000 Cordoba, Argentina

(Received 22 May 2001 / Accepted 13 December 2001 )

Abstract
We present BeppoSAX spectra of the extremely luminous and massive object $\eta$ Car observed in a very broad X-ray range (0.1-200 keV) during high state (December 1996) and egress from the last low state (March 1998). Both spectra are composed of at least two components, a soft one with $kT_{\rm s} < 0.7$ keV, and a hard with $kT_{\rm h} =4.7$ keV. We found in March 1998 a large flux defect in the 1.5-4 keV range, while the flux remained constant below 1.5 keV and above 5 keV. We attribute this defect to a $\times$ 3.5 increase of the absorbing matter in front of the hard component, while its temperature and unabsorbed luminosity were nearly the same in the two epochs. In December 1996 the PDS X-ray flux in the 13-20 keV range is larger than the extrapolated hard spectrum, indicating the presence of an even harder additional component, which possibly declined during the March 1998 low state. Conversely, we find that at that time, the flux of the 6.7 keV iron line was 40% stronger. Coordinated optical and NIR spectroscopic observations indicate that in March 1998 $\eta$ Car was still in a state of low excitation of the emission line spectrum, with extended P Cygni absorptions. These results indicate that after the X-ray flux minimum, the hard component recovered its high state luminosity more rapidly than the high ionization spectral lines, but in the meantime it was partly occulted by a large amount of absorbing matter placed in front of the source. These results are discussed in the framework of the proposed binary model of $\eta$ Car.

Key words: stars: binaries: spectroscopic -- stars: emission-line, Be -- stars: individual: $\eta$ Car -- stars: mass loss -- X-rays: stars

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

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BeppoSAX broad X-ray range observations of Carinae during high and low spectroscopic states

BeppoSAX broad X-ray range observations of $\eta$ Carinae during high and low spectroscopic states