"Large amplitude flare from symbiotic nova BF Cyg in outburst"

ATel #7013; U. Munari (INAF Padova), A. Siviero (Univ. Padova), S.
Dallaporta, L. Buzzi, A. Valisa, G. Cherini, S. Moretti, S. Tomaselli, A.
Maitan (ANS Collaboration)
on 31 Jan 2015; 11:54 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

Subjects: Optical, Cataclysmic Variable, Nova, Variables

The symbiotic nova BF Cyg was discovered when it erupted in 1894 peaking at
B=9.7. It remained close to maximum for 30 years, and starting with 1924 it
begun a slow and gradual descent toward quiescence (Leibowitz and Formiggini
2006, MNRAS 366, 675) when, in August 2006 while at passing through B=13.0,
it suddenly erupted again (Munari et al. 2006, CBET 596), reaching initially
B=10.3 and later peaking at B=9.7 in October 2008. It has remained around
maximum ever since, with the light-curve nicely modulated by the 757 day
orbital period.

We are intensively monitoring BF Cyg since 2005, both spectroscopically and
photometrically. On top the protracted maximum, now lasting for more than 8
years, a unexpected large amplitude and fast evolving flare has recently
developed. It started between 22 Nov 2014, when we measured BF Cyg at
B=11.15, V=10.47, Rc=9.64, Ic=8.82, and 21 Dec 2014 when BF was already at
B=10.72, V=9.98, Rc=9.30, Ic=8.55. According to our observations, the peak
brightness occurred around 26 January 2015 at B=9.95, V=9.44, Rc=9.03,
Ic=8.45.

The spectra largely changed in response to the flare. We obtained Echelle
spectra of BF Cyg with the Varese 0.61m telescope on 7 Aug 2014 (pre-flare)
and 28 Jan 2015 (flare peak), and with the Asiago 1.82m telescope of 6 Jan
2015 (flare rise). Low resolution spectra were obtained with the Asiago
1.22m on 18 Nov 2014 (pre-flare), 26 Dec 2014 (flare rise) and 31 Jan 2015
(flare peak). Pre-flare spectra were dominated by very strong Balmer
continuum and lines in emission, and strong and sharp HeI lines with no
P-Cyg associated absorption. During the flare rise, HeI and NaI lines
broadened and developed strong P-Cyg absorptions with terminal velocities
around -250 km/sec. At flare maximum, HeI lines and higher Balmer lines
turned into pure absorptions, with only Halpha and Hbeta remaining in
emission, and among the great number of weak FeII and other low excitation
emission lines, only FeII multiplet 42 lines retained a weak P-Cyg
absorption component. No high velocity jet feature has appeared in the
emission line profiles.