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Detection of a Large Flare in FR Cnc (=1RXS J083230.9+154940)

We report detection of an optical flare in the BY Draconis type star FR Cnc. The flare duration is 41 min, the amplitude is in the B band 1.02 m. It is the first flare reported for this object.

It was classified as BY Dra type star (i.e. its variability is caused by rotational modulation of starspots) and given the name FR Cnc by Kazarovets et al. (1999). The presence of Ca II H, K and H α emission lines in the spectra indicates high chromospheric activity in FR Cnc (Pandey et al., 2002;Pandey, 2003). The other details concerning history of investigation of this object can be found in Pandey et al. (2005) Flares in FR Cnc were not previously reported. FR Cnc was observed on 23 November, 2006 quasi-simultaneously in B, V, R j , I j bands at Crimean Astrophysical Observatory (Ukraine) by Alex Golovin, using 38-cm Cassegrain telescope, which is equipped with SBIG ST-9 CCD camera, cooled by a Peltier system to about −30 • C. The exposure times were 20 s, 13 s, 8 s and 17 s for B, V, R j , I j bands respectively. Data reduction was done using "Maxim DL" package. Reduction included bias, dark-frame subtraction and flat field correction using twilight sky exposures. Since the field of FR Cnc is not crowded, the technique of aperture photometry was applied to extract the differential magnitudes. The total number of useful frames was 89 for each band.  The flare of FR Cnc was detected on 23 November, 2006 with the maximum at 00:19 (UT). After the initial rapid flaring, the brightness of FR Cnc decreased slowly. The time between the flare began and reached its maximum was about 4 minutes, while the total duration of the flare was about 41 minutes.
The flare had a maximum amplitude (1. m 02) in the B band. In other bands the amplitudes were 0. m 49, 0. m 21 and 0. m 14 for V, R j and I j bands respectively.
Noteworthy, in 8 minutes after the flare's maximum a notable "spike" was observed in B and V bands (in other bands the amplitude was probably too low) during the brightness decline. Remarkable, that FR Cnc remained to be about 0. m 05 brighter for at least an hour after the flare began comparing with brightness before flare.
Following the idea, described at Kozhevnikova et al. (2006), we calculated the intensity of the flare and the absolute energy output. The relative intensity of the flare was determined via the following relation: where I 0 + I f is the intensity of the object, integrated over the duration of the flare, I 0 is the intensity of the star in quiescent level in one of the bands (corrected to the flare duration). For calculation of the absolute energy output, we assume for FR Cnc's quiescent level the following magnitude and colour indices: V = 10.43, B − V = 1.35, V − R = 1.15, V − I = 1.93. We used 30.24 ± 2.03 mas parallax (Perryman et al., 1997) that imply distance 33 ± 2 pc.
Similar calculations of the flare intensity and energy output were also done by Moffett (1973) and by Panov et al. (2000).
So, we get the values listed in Table 1. Fig. 1 shows differential lightcurves  unusually short for such type of stars, which implies that this star should manifest strong flaring activity (see Dorren et al., 1994). We detected a flare of FR Cnc for the first time. Further monitoring of this object is highly desirable.