GCN Circular 6486

J. Hill (LBTO/UAz), P. Garnavich (Notre Dame), O. Kuhn (LBTO/UAz),
N. Bouche, P. Buschkamp (MPE), X. Fan, (U Ariz), X. Dai, J. Prieto,
K. Z. Stanek (Ohio State), P. Milne, J. Bechtold (U Ariz),
R. M. Wagner (LBTO/OSU), J. Rhoads (Ariz State) report:

The Large Binocular Telescope (LBT) continued to image the position of the
GRB 070419A afterglow (Stamatikos et al, GCN 6302
Not found
 
 
; Chornock et al. GCN 6304
Not found
 
 
;
Garnavich et al. GCN 6406
Not found
 
 
) with the LBC-blue CCD camera
(http//lbc.mporzio.astro.it) and 8.4-m SX mirror. Data were obtained
2007 May 10.18 (UT) and May 20.22 (UT) which are 20.8 and 30.8
days after the burst. Using the calibration assumed by Garnavich et al.
(GCN 6406
Not found
 
 
), we estimate the brightness of the source at the
position of the afterglow using point-spread-function fitting
photometry as follows:

 UT Date   Age (days)  Exposures   Seeing    r mag  Error
-----------------------------------------------------------
Apr 23.15    3.7        10x200s     1.2"     24.75   0.18 (GCN 6406
Not found
 
 
)
May 10.18   20.8        25x200s     0.96"    25.29   0.05
May 20.22   30.8        15x200s     0.73"    25.71   0.13

The decay between 4 days (LBT observation from GCN 6406
Not found
 
 
) and 20 days
after the burst is very slow and corresponds to a power-law index of
0.3. This could indicate host galaxy contamination, but the
images remain unresolved. Extrapolating the decline to 30 days
post-burst predicts the afterglow should be 30% brighter than
the actual observed magnitude. This increased fading rate is
inconsistent with a host galaxy dominating the flux and
suggests light from a GRB progenitor supernova may be powering
the late-time light curve.

The Sloan-r filter has an effective wavelength of 623 nm which
maps to a rest-frame UV wavelength near 316 nm at a redshift
of 0.97 (Cenko et al. GCN 6322
Not found
 
 
). The U-band light curve
of SN 1998bw peaked 13 to 15 days after GRB 980425 (Galama
et al. 1998, Nature, 395, 670) and possibly earlier at short
wavelengths.  Some supernovae associated with GRB such as
SN 2001dh/011121 (Garnavich et al. 2003, ApJ, 582, 924) and
2006aj/060218 (Modjaz et al. 2006, ApJ, 645, 21) have peaked
earlier than SN 1998bw, so the decay seen at 16 rest-frame days
is consistent with a supernova interpretation.

Extrapolating the spectrum of SN 1998bw near maximum (Patat et al.
2001, ApJ, 555, 900) out to UV wavelengths, we estimate a peak
brightness of R=25.4 when viewed at z=0.97, which is also consistent
with the LBT observations.

A plot of the light curve can be found at:
http://www.nd.edu/~pgarnavi/grb070419a/grb070419a_lc.jpg

The LBT is an international collaboration among institutions in the
United States, Italy and Germany. The LBT Corporation partners are:
*  The University of Arizona on behalf of the Arizona university system
*  Istituto Nazionale di Astrofisica, Italy
*  LBT Beteiligungsgesellschaft, Germany, representing the Max Planck
Society, the Astrophysical Institute Potsdam, and Heidelberg University
*  The Ohio State University
*  The Research Corporation, on behalf of The University of Notre Dame,
University of Minnesota and University of Virginia


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