After the aborted 2015 attempt, the supersoft X-ray symbiotic star AG Dra is finally entering a bright outburst

ATel #8975; U. Munari (INAF Padova-Asiago), G. L. Righetti (ANS Collaboration)
on 20 Apr 2016; 19:52 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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After seven years of flat quiescence following the 2006-08 major outburst, in the late spring of 2015, AG Dra begun rising again in brightness toward what appeared to be a new outburst (#Atel 7582). However, in a few weeks the trend reversed and the object returned to flat quiescence. The peak amplitude of that attempt was 0.3 mag in the B band.

This time AG Dra seems doing it right. In a couple of weeks, the brightness in the B band has increased by a whole magnitude, from 11.3 on April 5 to 10.3 on April 19, already covering almost half of the way to the peak B=9.0 reached in September 2006 during the last 2006-08 major outburst (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 April 18.869 UT we measured B=10.351, V=9.360, Rc=8.610, and Ic=8.013, and on April 19.854 UT its was U=9.832, B=10.322, V=9.355, Rc=8.560, and Ic=8.003.

Large spectroscopic changes are accompanying the rapid increase in optical brightness. A low resolution spectrum (range 3450-8150 Ang, dispersion 2.31 Ang/pix), obtained on April 19.91 UT with the Asiago 1.22m telescope, shows a strong blue continuum veiling the redder quiescence spectrum, and the Balmer continuum has turned into outstanding emission. Compared to a similar spectrum we obtained a month ago for AG Dra in quiescence (March 19.97 UT), all emission lines have increased their integrated absolute flux. In units of 10(-12) erg cm(-2) sec(-1), Halpha increased from 54 to 70, HeI 5876 from 1.4 to 2.4, Raman 6825 band from 6.2 to 8.1, and HeII 4686 truly jumped up, from 6.8 to 31. Particularly noteworthy is the increase in the HeII/Hbeta ratio from 0.94 to 1.84. Following the nomenclature introduced by Gonzalez-Riestra et al. (1999, A&A 347, 478) the current outburst is of the "hot" type. An Echelle spectrum (range 3600-7400 Ang, resolving power 23000) obtained with the Asiago 1.82m telescope on April 19.95 UT, shows the emission lines to be sharp and with simple profiles, and no obvious P-Cyg absorptions. The FWHM of HeI lines is 75 km/s, 120 km/s is that of HeII 4686 and 5412, and 190 km/s for Balmer lines.

AG Draconis is the brightest symbiotic star in X-rays and one of the prototypes of the supersoft X-ray source class. During the 2006-08 outburst, XMM observations by Gonzalez-Riestra et al. (2008, A&A 481, 725) found a marked anti-correlation between X-ray flux and optical brightness, suggesting that during outburst the WD radiation increases, but is strongly absorbed by the circumstellar ionized gas. New X-ray observations during the current outburst would be relevant to test and refine the picture.