The symbiotic star AG Dra is undergoing a hot-type outburst, the first of a new train ?
ATel #17406; U. Munari (INAF Padova), A. Vagnozzi, A. Maitan, S. Moretti, G. L. Righetti, S. Dallaporta, P. Valisa (ANS Collaboration)
on 21 Sep 2025; 09:00 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
Subjects: Optical, Cataclysmic Variable
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, 1994-1996, 2002-2008, 2015-2018, eg. Leedjarv et al. 2016 MNRAS 456, 2558, Galis et al. 2017, OEJV 180, 24, Merc et al. 2018, RNAAS, 2, 142). The less frequent, longer lasting, and larger amplitude eruptions (peaking around V~8.0) are usually of the "cool" type, while the faster-evolving and lower amplitude outbursts (peaking around V~8.8) normally come in trains of 3-4 individual events and are of the "hot" type (Gonzalez-Riestra et al.
1999, A&A 1999, 347, 478). The hot-type outbursts are characterized by a flux ratio HeII/Hbeta rising well above 1 and by the OVI Raman-scattered bands (6825, 7088 Ang) turning stronger compared to quiescence, while during a cool-type event the HeII/Hbeta flux ratio reduces to 0.5 or less and the OVI Raman-scattered bands disappear (Munari et a. 2009, PASP 121, 1070; Shore et al. 2010, A&A 510, A70).
Since the 2008 return to quiescence following the major, cool-type outburst of 2006, AG Dra has displayed just one train of four hot-type eruptions peaking in 2015, 2016, 2017, and 2018 (ATel #8975, #10390, #11559). Now, after seven years of further peaceful quiesence, AG Dra is again undergoing a hot-type outburst, possibly the first of a new train.
We are constantly collecting spectroscopic and photometric observations of AG Dra with Asiago 1.82m, 1.22m, and 67/92cm telescopes, and an array of other instruments operated by ANS Collaboration, including Stroncone 50cm and Varese 84cm. Our data show that AG Dra left quiescence toward the end of July 2025 (when it was still around B~11.1), and spent the last month and a half at nearly constant brightness around B~10.3. Our last photometric observations read:
| UT | B | V | R | I |
|---|
| Sept. 06.850 | 10.258 | 9.297 | 8.410 | 7.929 |
| Sept. 11.861 | 10.278 | 9.274 | 8.413 | 7.916 |
| Sept. 16.811 | 10.340 | 9.319 | 8.455 | 7.982 |
| Sept. 20.791 | 10.399 | 9.371 | 8.529 | 8.038 |
Spectroscopically, the peak activity was probably reached around mid-August 2025. On our Asiago 1.82m + Echelle spectrum for Aug 13.832 UT, the HeII/Hbeta flux ratio reached 1.32, and OVI Raman-scattered bands peaked at their highest value in the last eight years. On our latest spectra obtained with the Stroncone 50cm (on Sept. 20.792 UT) and Varese 84cm (on Sept 20.855 UT), the HeII/Hbeta flux ratio lowered to 1.07 in agreement with the post-maximum brightness decline.
