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New burst of the active symbiotic star BF Cyg at the beginning of 2017.

ATel #10086; A. Skopal, M. Sekeras, S. Shugarov, N. Shagatova (Astronomical Institute, Tatranska Lomnica)
on 17 Feb 2017; 08:33 UT
Credential Certification: Augustin Skopal (skopal@ta3.sk)

Subjects: Optical, Binary, Cataclysmic Variable

BF Cyg is an eclipsing symbiotic binary with the orbital period of 757 d. Historical light curve of BF Cyg shows a slow symbiotic-nova-like outburst (1895-1960) with superposed eruptions of the Z And type and bursts on the time scale of, or less than, one year (e.g. Leibowitz and Formiggini 2006, MNRAS, 366, 675). On August 2006, BF Cyg entered a large active phase (Munari et al. 2006, CBET No. 596) keeping a high level of its brightness to the present. Between Nov. 22, 2014 and Jan. 29, 2015, Munari et al. 2015 (ATel #7013) reported a large amplitude flare from BF Cyg. During the burst, the blueward deep and broad absorptions extended to ~ -550 km/s accompanied with redward emission bump extended to ~ +550 km/s developed in the H-alpha and H-beta line profiles (see ATel #7258).

Here we report on a new burst of the active symbiotic star BF Cyg. According to our recent (U)BVRcIc CCD photometry, the burst started between Nov. 23.688, 2016, when we measured BF Cyg at B=11.089, V=10.501, Rc=9.574, Ic=8.560 and Dec. 16.697, 2016, when its brightness increased to B=10.898, V=10.373, Rc=9.475, Ic=8.502. Our latest photometric observations made on Feb. 13.173, 2017 (U=9.702, B=10.052, V=9.608, Rc=9.121, Ic=8.493) and on Feb. 15.158, 2017 (B=10.042, V=9.595, Rc=9.065, Ic=8.459) suggests that BF Cyg is around its peak brightness of the current burst. Our UBVRcIc photometry was obtained with a 60 cm telescope using the FLI ML3041 CCD camera (Pavilion G2) and additional BVRcIc measurements were made with an 18 cm Maksutov telescope using the SBIG CCD camera mounted at a montage of another 60 cm telescope for spectroscopic observations (Pavilion G1).

Just prior to, and during the current burst, on Nov. 23.673, Dec. 16.688, Dec. 22.652, 2016, and Feb. 11.168, 14.143, 2017, we obtained Echelle medium-resolution spectra of BF Cyg (420 - 715 nm, R~11000) with our 0.6 m telescope in the G1 pavilion, simultaneously with the BVRcIc photometry. At the maximum, the UBV continuum increased by a factor of ~2.5 with respect to the pre-burst level. Prior to the burst, during the burst rise and around its maximum, the line spectrum was dominated by hydrogen lines with a sharp absorption at ~ -70 km/s and weak emissions of HeI, FeII, and [OI] 6300 lines. Before the burst maximum, to Dec. 22, 2016, the hydrogen Balmer lines were accompanied with satellite emissions at around -400 and +200 km/s. At the optical maximum, a complex absorption extended to around -400 km/s developed. This dramatic change is best recognizable in the H-beta profile (see http://www.ta3.sk/~astrskop/atel_feb2017/bf16022017.png ). In addition, HeI lines nearly disappeared, whereas TiII lines and FeII, 42 (4924 A) developed a strong P-Cyg absorption component. All times here are in UT. This work was supported by the Slovak Research and Development Agency under the contract No. APVV-15-0458.