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GROND followup of Gaia17bay/AT2017dfe : a likely CV

ATel #10321; Ting-Wan Chen, Patricia Schady (MPE)
on 25 Apr 2017; 21:39 UT
Credential Certification: Ting-Wan Chen (jchen@mpe.mpg.de)

Subjects: Infra-Red, Optical, Cataclysmic Variable, Transient

Referred to by ATel #: 10510

We observed the field of Gaia17bay/AT2017dfe (Delgado et al. AT#10888) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG telescope at the ESO La Silla Observatory (Chile).

Observations started at 09:01 UT on 2017-04-25, 4 days after the blue featureless spectrum reported by Gutierrez et al. (ATel #10313). We derive the following preliminary magnitudes and upper limits (all in the AB system):

g' = 21.8 +- 0.1 mag,
r' = 22.4 +- 0.1 mag,
i' > 22.3 mag,
z' > 21.7 mag,
J > 20.1 mag,
H > 19.7 mag, and
K > 19.0 mag.

The light curve declines > 3.4 mag in one week (18.44 mag on 2017-04-18 in Gaia G band). Hence this source is likely a CV.

Given magnitudes are calibrated against GROND zeropoint as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.01 mag in the direction of the burst (Schlafly & Finkbeiner 2011).

We acknowledge excellent help in obtaining these data from the observer and supporting astronomer on La Silla, Helmut Steinle and Sam Kim.