Stringent limits to the absence of flickering in V694 Mon = MWC 560 that is passing through record brightness

ATel #15066; U. Munari (INAF Padova) and S. Dallaporta (ANS Collaboration)
on 27 Nov 2021; 15:26 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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The symbiotic binary V694 Mon raised from obscurity to headlines in 1990 when it went into outburst and its spectra showed the daily launching of discrete blobs of optically thick material up to record velocities of 6,000 km/s (Tomov 1990, Nature 346, 637). The reckless spectroscopic variability that followed was always paralleled by large-amplitude photometric flickering. V694 Mon never returned to quiescence and entered instead a continued increase in brightness modulated by various periodicities (Munari et al. 2016, NewA 49, 43), primarily 331 and 1860 days, and a possible 9570 days. The flickering stopped during the rise of V694 Mon to the 2016 peak brightness, but soon reappeared as the object begun declining.

The V694 Mon that in 2018 emerged from solar conjunction was radically different from that observed during the previous three decades: persistent disappearance of flickering (Goranskij et al. 2018, ATel #12227; Zamanov et al. 2020, ATel #14239; Zamanov et al. 2021, ATel #14988) and a monotonic increase in brightness, continuing ever since and defying all previously observed periodicities.

Goranskij et al. (2021, ATel #15061) have recently called attention to the near disappearance of emission lines when they resumed spectroscopic observation of V694 Mon after the 2021 solar conjunction. As part of ANS Collaboration continued monitoring, we too resume photometric observations of V694 Mon as soon the object returned visible in the pre-dawn sky, and obtained the results listed in the table: they indicate a very smooth and constant-colors passage through peak brightness reached around Oct 27, 2021, which marks also a record for this star.

Searches for flickering in recent years have placed a limit of about 0.05 mag to its presence (ATel #12227, #14239, #14988). We have observed V694 Mon on 2021 Nov 25 with ANS Collaboration telescope ID 0310 to set more stringent limits to the presence of any flickering. We observed in Landolt's B and V bands, continuously flipping between the two for an hour and forty minutes between UT 1.23 and UT 3.02. The search for flickering was carried out in the B-band data, while V-band observations served primarily to solve the color equations to perform transformation from the instantaneous local photometric system to the standard Landolt one, adopting the local photometric sequence calibrated by Henden and Munari (2001, A&A 372, 145). The 60 B-band measurements (each lasting 30sec) distribute in a fine Gaussian shape around the mean B=8.787 mag value, with a sigma of 0.0067 mag. Considering that observational errors are contributing their share to such sigma, we may confidently conclude that if any flickering was present during the time interval of our observations, its amplitude does not exceed 0.005 mag.