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Two blue optical transients: multicolor photometry

ATel #10349; A. Kurtenkov, K. Stoyanov, N. Petrov (Institute of Astronomy and NAO, Bulgaria)
on 4 May 2017; 20:52 UT
Credential Certification: Kiril Stoyanov (kstoyanov@astro.bas.bg)

Subjects: Optical, Cataclysmic Variable, Supernovae, Transient, Variables

Optical transients MASTER OT J224910.38+340850.6 and MASTER OT J231318.83+262628.2 were recently reported (ATel #10332) as possible outbursts of cataclysmic variables. On May 3 we conducted photometric observations of both objects with the Rozhen 2m telescope in Bulgaria. Two consecutive sets of 120 s exposures in Johnson-Cousins BVRI filters were obtained for each object at an airmass of 2 - 2.5. Stars from the SDSS DR9 catalog were used for comparison after applying the Sloan-to-BVRI color transformations by Lupton (2005, accessible on the SDSS-III website). We calculated the following magnitudes:

MASTER OT J224910.38+340850.6 (or J2249)
# date,UT filter mag
2017-05-03.048 V 17.01
2017-05-03.049 R 16.99
2017-05-03.051 I 16.91
2017-05-03.053 B 17.06
2017-05-03.055 V 16.92
2017-05-03.057 R 16.88
2017-05-03.058 I 16.79
2017-05-03.060 B 16.89
2017-05-03.090 R 16.99

MASTER OT J231318.83+262628.2 (or J2313)
# date,UT filter mag
2017-05-03.065 V 16.57
2017-05-03.067 R 16.61
2017-05-03.069 I 16.52
2017-05-03.071 B 16.75
2017-05-03.072 V 16.58
2017-05-03.074 R 16.59
2017-05-03.076 I 16.53
2017-05-03.078 B 16.75

All errors can be rounded to 0.02 mag.

Both transients present distinctly blue colors (J2249: B-V = 0.11, V-R = 0.02, V-I = 0.05 after accounting for variability; J2313: B-V = 0.17, V-R = -0.02, V-I = 0.05). The galactic reddening (maximum E(B-V) = 0.07 mag for both objects) is not taken into account.

J2249 is located 0.6" away from a 23 mag SDSS source. Our detection of short-term variability (a 0.1 mag brightness increase in 10 min, followed by a decrease) and the observed colors are consistent with a CV outburst. Assuming that the SDSS source is the progenitor, the outburst is currently > 6 mag brighter than the system in low state. It is probably past maximum.

J2313 is coincident with a 20 mag extended SDSS source, classified as a galaxy. Therefore it may also be considered a SN candidate (ATel #10332). Its brightness is comparable to the MASTER estimate 4 days prior, so the outburst probably is near maximum.

Further observations are strongly encouraged!

Based on data collected with the 2 m RCC telescope at the Rozhen National Astronomical Observatory.