AG Draconis observed with XMM-Newton

¹ XMM Science Operations Centre, ESAC, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain e-mail: Rosario.Gonzalez@sciops.esa.int
² INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy
³ INAF - Osservatorio Astronomico di Padova, Sezione di Asiago, 36012 Asiago, Italy
⁴ Dipartimento di Fisica, Università La Sapienza, Piazzale Aldo Moro 3, 00185 Roma, Italy
⁵ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁶ INAF - Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁷ Astronomical Institute, Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovakia

Received: 31 August 2007
Accepted: 14 January 2008

Abstract

Context. AG Draconis is the brightest symbiotic star in X-rays and one of the prototypes of the supersoft X-ray source class.

Aims. Study of the X-ray spectrum of this peculiar binary system, covering both quiescence and activity periods, is necessary to investigate the physics of the high temperature spectral component, and to unveil the origin of the outbursts.

Methods. X-ray and UV observations with XMM-Newton during 2003–2005 and coordinated optical spectrophotometric monitoring, together with archive data, are employed to derive the behaviour of the high energy source of the AG Dra system during different orbital and activity phases.

Results. During quiescence the X-ray emission is very soft and is close in strength to the previous ROSAT observations, with an estimated luminosity of 2600  and a radius of 0.06 . We also found a 20% flux decrease in June 2005 at the time when a U-band minimum coincided with a V-band maximum. The X-ray flux in the XMM range largely decreases, and even vanishes near the optical light maxima (bursts). The UV fluxes measured with the XMM Optical Monitor is close to the fluxes observed with the IUE satellite. There is a marked anticorrelation between X-ray fluxes, and ultraviolet and optical fluxes, indicating that during outburst the WD is mostly emitting below 0.1 keV. Conversely, the large strengthening of the He II 4686 Å emission during the October 2003 and July 2005 light maxima indicates a marked increase of the far-UV brightness especially during the early stages of the outbursts. A high energy 0.5–0.6 keV X-ray excess seems to be present both in quiescence and outburst.

Conclusions. This is the first time that X-ray spectra of AG Draconis during an active phase are obtained. These data have allowed us to investigate the change of the energy distribution. The anti-correlation between X-ray and optical/ultraviolet emission appears to be a general feature of AG Dra independent of the type and strength of the outburst. We suggest that during outburst the WD radiation increases, but is strongly absorbed by the circumstellar ionised gas. The newly-found high energy X-ray component is probably an indication of the presence of a shocked region produced by interaction of the WD wind with the circumbinary envelope fed by the red giant's wind.