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The symbiotic binary and super-soft X-ray source AG Dra is going into outburst

ATel #7582; U. Munari (INAF Padova-Asiago), G. L. Righetti, U. Sollecchia, F. Castellani (ANS Collaboration)
on 1 Jun 2015; 10:49 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

Subjects: Optical, Ultra-Violet, X-ray, Binary, Cataclysmic Variable

Referred to by ATel #: 10390

The yellow symbiotic binary and luminous super-soft X-ray source AG Dra is on a steep rise in optical brightness, after seven years of flat quiescence following the 2006-08 multi-maxima outburst episode (Munari et al. 2009, PASP 121, 1070). We are tightly monitoring AG Dra both photometrically and spectroscopically with various ANS Collaboration telescopes and the Asiago 1.22m and 1.82m telescopes. On March 8.923 UT, when the current brightening begun, we measured B=11.301, V=9.885, Rc=9.005, and Ic=8.367, while our last measurement on May 29.883 UT provides B=10.989, V=9.683, Rc=8.822, and Ic=8.228. The start of what looks like as a new outburst episode is marked by a significant lowering of the usually very high ionization conditions: the HeII 4686 / Hbeta ratio has declined from 0.83 in Nov 2014 to 0.67 on May 29.86 UT, and the ratio between OVI Raman scattering at 6830, 7088 and HeI 6678, 7065 has lowered from 3.80 to 1.58. The profiles of emission lines are still broadly similar to those of quiescence, in particular no P-Cyg absorption has yet appeared.

AG Dra is a yellow symbiotic binary, with an orbital period of 550 days and a pulsation one of 355 days (Galis et al. 1999, A&A 348, 533). The high galactic latitude, large radial velocity (-135 km/s) and low metallicity of the K giant donor star ([Fe/H]=-1.3, Smith et al. 1996, A&A 315, 179), point to a partnership of AG Dra with the Galactic Halo. The luminous super-soft X-ray emission was discovered by ROSAT (Greiner et al. 1997, A&A 322, 576). AG Dra went through 3 major outburst periods during the last 50 years, in 1980-82, 1994-96 and 2006-08, each showing multiple maxima. These maxima have either been of the "cool" (probably related to an expansion and cooling of the white dwarf photosphere, with decline/disappearance of high ionization features) and of the "hot" type (probably caused by an enhancement in the wind from the white dwarf, with no reduction in the ionization degree; Gonzalez-Riestra et al. 1999, A&A 347, 478; Skopal et al. 2009, A&A 507, 1531; Shore et al. 2010, A&A 510, A70). During previous outbursts, the X-ray and the optical/UV brightness have been anti-correlated.