Swift/UVOT Observations for SLSN-I Gaia16apd

ATel #9074; Nadejda Blagorodnova, Lin Yan (Caltech), Robert Quimby, Melanie Kae Olaes, (San Diego State University), Peter Brown (Texas A&M University), Jeffrey Cooke (Swinburne University of Technology)
on 22 May 2016; 22:14 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Nadejda Blagorodnova (nblago@ast.cam.ac.uk)

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The optical transient Gaia16apd was discovered by Gaia Photometric Science survey in the galaxy SDSS J120251.71+441527.4 at magnitude G=17.3. The NOT Unbiased Transient Survey (NUTS; ATel #8992) provided a classification spectrum with the 2.56 m Nordic Optical Telescope equipped with ALFOSC (range 320-960 nm; FWHM resolution 1.6 nm) on 2016-05-20.92 UT. The object showed a good match with a SLSN-I at -5 days before maximum brightness at redshift z=0.102 (Atel #9071).

The object was observed by Swift's Ultraviolet/Optical Telescope (UVOT; Gehrels et al. 2004, Roming et al. 2005). Observations began 2016-05-21 16:25:40 UT. The following UVOT magnitudes were measured:

 
  
JD          | EXPTIME      | FILTER | mag    | errstat | errsys 
57529.6953  |  78.5        | V      | 17.11  |  0.12   | 0.01 
57529.7206  |  157.0       | B      | 16.99  |  0.04   | 0.01 
57529.7198  |  157.0       | U      | 15.66  |  0.04   | 0.02 
57529.7172  |  314.5       | UVW1   | 15.40  |  0.03   | 0.03 
57529.6979  |  363.8       | UVM2   | 15.33  |  0.04   | 0.03 
57529.6929  |  314.7       | UVW2   | 15.81  |  0.04   | 0.03 
  

These magnitudes were obtained via the method outlined for the Swift Optical Ultraviolet Supernova Archive (SOUSA; Brown et al 2014, Ap&SS, 354, 89, arXiv:1407.3808) on the Swift/UVOT Vega-based photometric system (Poole et al. 2008, Breeveld et al. 2011). The magnitudes do not include host subtraction. However, the host galaxy magnitudes are faint: u=22.14=/-0.27,g=21.74=/-0.06,r=21.77=/-0.09,i=21.18=/-0.08,z=21.67=/-0.4, compared to the SN.

The apparent UV brightness of Gaia16apd is likely to be the most luminous in a SLSN-I detected up to date. As a comparison example, PTF12dam (Nicholl et. al 2013), had a similar redshift of z=0.107, but at -2.1 days before maximum, its apparent magnitudes were: UVW2=18.87=/-0.08, UVM2=18.36=/-0.08, UVW1=17.87=/-0.08.

We thank Neil Gehrels and the Swift team for rapid approval and scheduling of this ToO observation.