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ATLAS limits on explosion epoch of SN2016bau in NGC3631

ATel #8820; J. Tonry, L. Denneau, B. Stalder, A. Heinze, A. Sherstyuk (IfA, University of Hawaii), A. Rest (STScI), K. W. Smith, S. J. Smartt (Queen's University Belfast)
on 15 Mar 2016; 16:42 UT
Credential Certification: Stephen Smartt (s.smartt@qub.ac.uk)

Subjects: Optical, Supernovae

Referred to by ATel #: 8911

Following the discovery of SN2016bau in NGC3631 at 13Mpc by Ron Arbour (TNS Astronomical Transient Report No. 1487) and the spectroscopic confirmation of Granata et al. (ATel #8818, Asiago Transient Classification Program) as a young type Ib supernova, we report limits on the explosion epoch.

The ATLAS unit in Haleakala (see Tonry et al. ATel #8680) observed NGC3631 during its robotic sky operations with four 30 sec exposures on 2016-03-11 between 10:13 and 11:33 UT in the ATLAS cyan filter. (420-650nm, for more details see http://www.fallingstar.com ). Ron Arbour's discovery images are 2016-03-13 23:22 UT. SN2016bau is not visible in any of the four ATLAS images. Difference images were produced as part of standard ATLAS operations with respect to a reference sky. No object is detected and the stack of 4x30s also did not detect the SN.

We estimate the limiting 5-sigma magnitude in an aperture of radius equal to the measured FWHM. This is m_AB = 19.2 in the 4x30s stack, at the position of the SN. The background flux in the ATLAS pixels at this position (35" W 15" N of Host) is about 50% from the night sky and 50% from NGC3631.

Our limiting mag of 19.2 compares with Ron Arbour's discovery measurement of 17.8 approximately 2.5 days later. This confirms the conclusion of Granata et al. (ATel #8818) that this was indeed a supernova discovered very young. Given the early discovery and still faint absolute magnitude pointed out by Granata et al., further multi-wavelength observations are encouraged.