GROND followup of ATLAS17gqa/AT2017ens and ATLAS17gqb/AT2017ent

ATel #10478; T.-W. Chen (MPE), C. Inserra (Southampton), S. J. Smartt (QUB), K. W. Smith (QUB), J. Tonry, B. Stalder, L. Denneau., A. Heinze, H. Weiland (IfA, Univ. of Hawaii), A. Rest (STScI)
on 9 Jun 2017; 22:30 UT
Credential Certification: Ting-Wan Chen (jchen@mpe.mpg.de)

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We observed the field of ATLAS17gqa/AT2017ens (Tonry et al. AT #11905) and ATLAS17gqb/AT2017ent (Tonry et al. AT #11906) 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). Targets were supplied by the ATLAS survey, see Tonry et al. (2011, PASP, 123, 58) and Tonry et al. (ATel #8680), and additional details can be obtained from the IAU Transient Name Server.

Observations started at 23:48 UT on 2017-06-08 (MJD = 57912.992), 3.7 days after the ATLAS discoveries. We derive the following magnitudes and 3-sigma limits (all in the AB system):

ATLAS17gqa/AT2017ens:
g' = 17.57 +/- 0.02 mag,
r' = 17.96 +/- 0.02 mag,
i' = 18.21 +/- 0.02 mag,
z' = 18.35 +/- 0.02 mag,
J = 18.80 +/- 0.08 mag,
H = 19.14 +/- 0.15 mag, and
K > 19.40 mag.

and

ATLAS17gqb/AT2017ent:
g' = 17.67 +/- 0.03 mag,
r' = 18.04 +/- 0.01 mag,
i' = 18.43 +/- 0.02 mag,
z' = 18.53 +/- 0.03 mag,
J = 19.12 +/- 0.10 mag,
H = 19.70 +/- 0.27 mag, and
K > 19.28 mag.

Given magnitudes are calibrated against SDSS (for ATLAS17gqa/AT2017ens), PS1 (for ATLAS17gqb/AT2017ent) field stars as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.02 mag in the direction of the transients (Schlafly & Finkbeiner 2011).

We fitted our colour SED assuming a black body, which indicates a temperature of 21000 +/- 3000 K for ATLAS17gqa/AT2017ens and 23000 +/- 2800 K for ATLAS17gqb/AT2017ent.

We acknowledge excellent help in obtaining these data from Simona Ciceri on La Silla.