UV-NIR Observations of the Probable Luminous Supernova CSS 081009:002151-163204

ATel #1842; A. A. Miller, R. Chornock, N. R. Butler, D. A. Perley, W. Li, N. Smith, J. S. Bloom, A. V. Filippenko (UC Berkeley)
on 13 Nov 2008; 04:33 UT
Credential Certification: Weidong Li (weidong@astron.berkeley.edu)

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We report on further observations of the SN candidate CSS081009:002151-163204 discovered by the Catalina Sky Survey (Drake et al., ATel #1823) which was determined by Silverman et al. (ATel #1833) to be at redshift 0.285.

We triggered ToO observations with the Swift satellite starting on 2008 Nov. 11 at 04:10 UT. The transient was observed in all six ultraviolet and optical filters on the UVOT. We reduced the observations following the calibration methods described by Li et al. (2006, PASP, 118, 37) and Poole et al. (2008, MNRAS, 383, 627). Our preliminary photometry is:

UVW2 = 19.60 ± 0.11

UVM2 = 19.20 ± 0.11

UVW1 = 19.02 ± 0.08

U = 18.67 ± 0.05

B = 19.25 ± 0.06

V = 18.58 ± 0.07

In addition, we observed the transient using the Peters Automated Infrared Imaging Telescope (PAIRITEL) in J, H, and K_s nearly simultaneously with the Swift observations. The transient is undetected in the K_s band, marginally detected in the H band, but securely detected in the J band at a magnitude (calibrated against 2MASS) of:

J = 18.5 ± 0.2

These magnitudes have not been corrected for the small component of Galactic extinction along the line of sight or for the small contribution from a faint presumed host galaxy seen in the DSS images.

The Swift observations also totaled 6.15 ks of exposure time with the XRT. The source is not detected, with a 90% upper limit of F_x(0.3-10 keV) < 4e-14 erg cm^-2 s^-1 assuming a power-law spectrum with Gamma=2 and N_H=1e21 cm^-2. At the redshift of the source, this is an upper limit to the X-ray luminosity of 1e43 erg s^-1. We note that the lack of an X-ray detection is consistent with the interpretation of these super-luminous SNe as optically thick, shocked shells (Smith & McCray 2007, ApJ, 671, L17).

The downturn in the UV continuum shape at short wavelengths, coupled with the X-ray non-detection, is inconsistent with an AGN hypothesis for this transient. Instead, the full SED can be approximately fit by a blackbody spectral shape, with a characteristic temperature of ~10,500 K (in the rest frame). A direct trapezoidal integration under the observed fluxes and an integration of the blackbody fit give similar estimates for the bolometric luminosity of the source of ~2e44 erg s-1. This extraordinary luminosity and blackbody temperature are very similar to those of SN 2008es (Miller et al. 2008, arXiv:0808.2193; Gezari et al. 2008, arXiv:0808.2812) at a similar age of about 30 rest-frame days after discovery. Further spectroscopic observations are encouraged in order to confirm the SN nature of this transient.

We thank the Swift PI Neil Gehrels for approving the requested ToO, and the Swift staff for planning and executing the observations.