GCN Circular 7176

Weidong Li, Ryan Chornock, Ryan J. Foley, Alexei V. Filippenko, 
Maryam Modjaz, Dovi Poznanski, and Joshua S. Bloom (UC Berkeley) 
report: 

We analyzed the Swift/UVOT images of XRB 080109/SN 2008D in the 
U, B, and V filters taken on Jan 11 and 12 UT. We constructed 
template images for the field from follow-up data taken for 
SN 2007uy, and attempted image subtraction to remove the heavy
galaxy contamination at the site of SN 2008D. We were able to
obtain clean subtracted images as demonstrated at
http://astro.berkeley.edu/~bait/sn2008d.uvot-sub.gif

Using the calibration and photometry recipe of the UVOT 
U, B, V filters as described by Li et al. (2006, PASP 118, 37), 
we obtained the following photometry for SN 2008D. We used
t_0 as determined by Modjaz et al. (GCN 7175
Not found
 
 
). 

UT date       t(days)   Total exp. time       Mag and err 

Jan 11.569     2.0047      2771.32s           U = 18.65 +/- 0.04   
Jan 11.591     2.0265      2579.13s           B = 19.05 +/- 0.04   
Jan 11.585     2.0203      2349.89s           V = 18.38 +/- 0.05   

Jan 12.408     2.9434       730.82s           U = 19.02 +/- 0.07  
Jan 12.409     2.8443       668.02s           B = 19.34 +/- 0.07  
Jan 12.445     2.8802       605.26s           V = 18.56 +/- 0.08  

The SN is seen to decline in the U (by 0.37 mag), B (by 0.29 mag),
and V (by 0.18 mag) filters between t = 2.0-2.9 d after the XRB. 
As the SN declined faster in U than in V, it became progressively
redder during this period. 

Possible explanations for the decline in the brightness at such early 
times for SN 2008D in the context of a SN Ib/c include the following.
(a) The SN had a bright initial optical flash from shock breakout,
and is now declining due to adiabatic expansion, similar to what
was observed for SN 1999ex (Stritzinger et al. 2002, AJ 124, 2100).
If this is the case, the SN will brighten again when the energy
input from radioactive decay starts becoming visible.
(b) The object is still dominated by the afterglow emission
from the XRB, similar to SN 2003dh/GRB 030329. However, spectroscopy
by Malesani et al. (GCN 7169
Not found
 
 
), Blondin et al. and Valenti et al. (CBET 
1205) indicate rather strong SN features with no prominent power-law 
continuum.
(c) The SN has a similar behavior to the peculiar SN 2005bf
(Folatelli et al. 2006, ApJ 641, 1039), with two broad peaks in 
the light curves. However, the first peak of SN 2005bf was reached
more than 20 d after its discovery, and SN 2008D may have a 
very early and sharp first peak at t < 2 d. 

[GCN OPS NOTE(14jan08): Per author's request, the Malesani reference
was added to the "(b)" paragraph.]