GROND followup observations of ATLAS18bcde (AT2018kpo): blue and hot object

ATel #12329; Ting-Wan Chen (MPE Garching)
on 25 Dec 2018; 13:00 UT
Credential Certification: Ting-Wan Chen (jchen@mpe.mpg.de)

Tweet

We observed the field of ATLAS18bcde/AT2018kpo (Smartt et al. ATel #12326) as part of the GREAT survey (Chen et al. 2018, ApJ, 867L, 31), simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the MPG 2.2m telescope at the ESO La Silla Observatory (Chile),

Observations started at 02:01 UT on 2018-12-25 (MJD = 58477.084), 0.71 days after the ATLAS discovery. We derive the following magnitudes (all in the AB system) using aperture photometry with a aperture of the mean FWHM of the field stars:

g' = 17.36 +/- 0.01 mag,
r' = 17.74 +/- 0.01 mag,
i' = 18.01 +/- 0.01 mag,
z' = 18.12 +/- 0.01 mag,
J = 18.62 +/- 0.04 mag,
H = 19.05 +/- 0.11 mag, and
K = 19.16 +/- 0.33 mag.

We also observed the other epoch 3 hours later, at 05:16 UT on 2018-12-25 (MJD = 58477.220),

g' = 17.36 +/- 0.01 mag,
r' = 17.71 +/- 0.01 mag,
i' = 17.96 +/- 0.01 mag,
z' = 18.06 +/- 0.02 mag,
J = 18.48 +/- 0.05 mag,
H = 19.20 +/- 0.16 mag, and
K = 19.07 +/- 0.33 mag.
No significant variation in all bands.

Given magnitudes are calibrated against SDSS 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) = 0.05 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

After correcting for Milky Way extinction, we fitted SED assuming a black body, which indicates a temperature of >20000 K. This object may have risen 0.5 mag/day roughly compared between GROND-r and ATLAS-o bands.

We acknowledge excellent help in obtaining these data from our Chilean colleagues on La Silla at Christmas Eve.