GCN Circular 2601
Subject
GRB 030324: Detection and Color of Host Galaxy
Date
2004-05-18T18:40:54Z (20 years ago)
From
Melissa Nysewander at UNC,Chapel Hill <mnysewan@physics.unc.edu>
M. C. Nysewander, P. A. Price, D. E. Reichart, and D. Q. Lamb report on
behalf of the Follow-Up Network for Gamma-Ray Bursts (FUN GRB)
Collaboration:
On April 15, 2004, we observed the location of the candidate afterglow
(Lamb et al., GCNs 2139, 2140, 2239) of GRB 030324 (Donaghy et al., GCNs
1954, 1955) with the LRIS instrument on the Keck I telescope in
simultaneous imaging mode in G and R filters. We obtained stacked images
of 2130 sec in G and 1805 sec in R (cut short due to instrument error)
under conditions of moderate humidity and seeing.
Using the standard transformation equations of Smith et al. (2002) and the
field calibration of Henden (GCN 1950), we calibrated the stacked images
using the six unsaturated stars in our field. A source is present at the
location of the candidate afterglow in both stacked images, with magnitudes
g' = 25.29 +/- 0.09 mag and R = 25.16 +/- 0.24 mag.
First, we rule out the possibility of a variable star. Fitting a blackbody
+ Galactic extinction curve model to these data, we find that log T > 3.75
at the 3 sigma level. This rules out flare stars (UV Cet variables), which
are cooler than log T = 3.5.
The fitted temperature is consistent with RR Lyraes and shorter period
Cepheids, but these classes of variables stars are also ruled out, for two
reasons:
1. RR Lyraes and Cepheids vary by less than 1.3 mag. However, Lamb et al.
(GCN 2239) measured a fading from i* = 23.48 +/- 0.21 mag to i* > 25.4 mag
(2 sigma upper limit; GCN 2239).
2. To be this faint, an RR Lyrae would have to be >0.5 Mpc away and a
Cepheid would have to be >3.5 Mpc away, placing either well outside of the
Galaxy.
Consequently, we conclude that we have detected the host galaxy of GRB
030324. Detection of the galaxy in the G band implies that its redshift is
<2.7. The spectral flux distribution of the galaxy is flat, which makes
this a relatively blue galaxy, even for a GRB host galaxy.
Images and the spectral flux distribution are available at
www.physics.unc.edu/~mnysewan/grb030324.html
Smith et al. 2002, ApJ, 123, 2121