KPED Photometric Observations of AT2018cow

ATel #11752; Tomas Ahumada (UMD), Michael W. Coughlin, Kevin B. Burdge, Shri R. Kulkarni (Caltech)
on 21 Jun 2018; 15:20 UT
Credential Certification: Michael Coughlin (mcoughli@caltech.edu)

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We obtained photometry of AT2018cow (ATLAS18qqn; Smartt et al. ATel #11727) using the KPED camera on the Kitt Peak 84 inch telescope. Photometry began at June 20 04:19 UT on 2018-06-20. KPED is an EMCCD sensor, with the ability to run as a normal CCD, and photometry was run in both modes. The first was performed in the normal CCD mode, and we derive the following magnitudes (all in the AB system) using aperture photometry and field stars to set the zeropoint: U = 13.9+/- 0.3 mag g' = 14.4 +/- 0.2 mag, r' = 14.2 +/- 0.1 mag, I = 14.5 +/- 0.2 mag, Our photometry has not been corrected for host galaxy background contamination or Galactic extinction. The magnitudes are consistent with the blue, power-law reported by others (ATel #11732) and show fading of r=0.1 mags and g=0.7 mags compared to GROND measurements from 2018-06-19 (ATel #11734). A second set of photometry was obtained in g-band in the EMCCD mode at a cadence of 1 Hz. Observations began at June 20 04:58 UT on 2018-06-20 and lasted approximately 4.5 hrs. The g-band observations show a predominantly monotonically decreasing brightness, fading approximately 0.1 mag over 4.5 hrs, consistent with the fading reported by GROND (ATel #11734). We attach a link to a finding chart based on KPED observations below. Further observations with KPED are planned.

AT2018cow Finding Chart