AG Dra is going into outburst again, of the hot type and the third in the present series

ATel #10390; U. Munari (INAF Padova), G. L. Righetti, P. Valisa, L. Buzzi and S. Moretti (ANS Collaboration)
on 18 May 2017; 12:19 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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The outbursts of the Halo symbiotic binary and super-soft X-ray source AG Dra are known to come in series (1932-1939, 1949-1955, 1963-1966, 1980-1986, 1993-2008, Leedjarv et al. 2016 MNRAS 456, 2558, Galis et al. 2017, OEJV 180, 24), usually started by a large amplitude eruption of the "cool" type, followed by a train of lower amplitude events of the "hot" type (Gonzalez-Riestra et al. 1999, A&A 1999, 347, 478). The latter are characterized by a HeII/Hbeta flux ratio close to 1 and the OVI Raman-scattered bands that remain strong, while during a cool type event He II/Hbeta reduces to 0.5 or less and the O VI Raman-scattered bands disappear (Munari et a. 2009, PASP 121, 1070).

Two years ago, after a long and flat quiescence lasting from mid-2008 to early-2015, AG Dra has entered a new phase of activity, which did not begin with a major outburst of the cool type but instead with two lower amplitude events of the hot variant, in May 2015 (ATel #7582) and April 2016 (ATel #8975). Here we report about the onset of a new outburst that has just begun, again of the hot type and the third during the present active period.

On the tight photometric monitoring we are carrying out with ANS Collaboration telescopes, AG Dra has remained at photometric quiescence until a month ago, following the end of the 2016 eruption. On April 13.867 UT, we still obtained B=11.080, V=9.716, Rc=8.853, Ic=8.252, values typical of quiescence. Since then, AG Dra has begun a smooth rise in brightness, reaching U=10.29, B=10.723, V=9.528, Rc=8.728, and I=8.148 on May 16.939 UT (+/- 0.006 in BVRI, 0.03 in U). For reference, around the peak of the 2016 event we measured AG Dra at U=9.38, B=10.009, V=9.183, Rc=8.456, and Ic=7.942 on May 5.81.

As part of an assiduous spectroscopic monitoring, a low resolution spectrum of AG Dra (3200-8000 Ang, 2.31 Ang/pix) has been obtained with the Asiago 1.22m + B&C telescope on May 16.88 UT. In addition to a strong reinforcement of the Balmer continuum in emission, the HeI emission lines are now much more prominent than in quiescence, HeII/Hbeta=1.00 and OVI(6825)/HeI(6678)=2.62. A high resolution Echelle spectrum of AG Dra (resolving power 18000, range 3900-8600 Ang) has been obtained on May 17.93 with the Varese 0.6m telescope and mk.III multi-mode spectrograph. All emission lines remain sharp, with an average FWHM=55 km/s and no P-Cyg absorption, and at an average radial velocity of -151 km/s, close to the -147 km/s barycenter velocity derived for the binary orbit by Fekel et al. (2000, AJ 120, 3255).