GCN Circular 7627

D. A. Kann, S. Schulze (TLS Tautenburg) & A. C. Updike (Clemson 
University) report:

In light of recent reports on the further evolution of GRB 080319B, we did 
a preliminary analysis of the multiwavelength data set.

Jet Break:
Racusin et al. (GCN 7567
Not found
 
 
) reported the existence of a potential jet break 
at 1.04 +/- 0.43 Ms in the X-ray band, with a hint of a steeper decay seen 
in the optical as well. Using the most up-to-date data from  the Swift XRT 
light curve repository (Evans et al. 2007), we confirm the  findings of 
the Swift team and derive (using data starting at 0.5 days after the GRB):

chi^2/d.o.f. = 39/58
alpha_1 = 1.01 +/- 0.05
alpha_2 = 2.40 +/- 0.39
t_b = 9.43 +/- 1.73 days (0.815 +/- 0.149 Ms)
n = 5 fixed, no "host galaxy"

These values are in agreement with those of the Swift team, with 
significantly reduced errors. We note that there seems to be a steep decay 
in the X-ray light curve from 0.35 to 0.5 days, and a similar evolution 
has been reported in the optical (Krugly et al., GCN 7519
Not found
 
 
) at about the 
same time. This is reminiscent of the X-ray light curve of GRB 070110 
(Troja et al. 2007).

In the optical, we add data from Perley et al. (GCN 7535
Not found
 
 
) and Tanvir et 
al. (GCN 7621
Not found
 
 
) to the data set from Bloom et al. 2008 (arXiv:0803.3215). 
Tanvir et al. report a significant reddening, which we confirm. 

At the moment, it is unclear how much the host galaxy and a potential 
supernova contribute in the g band, but a roughly achromatic steepening is 
seen in this band (Tanvir et al.) in comparison to the X-ray light curve.

Energetics:
Using the prompt emission data derived from Konus-Wind (Golenetskii et 
al., GCN 7482
Not found
 
 
), we find in the bolometric band (1 - 10000 keV, host 
frame):

E_iso = 1.32 +/- 0.03 x 10^54 erg (log E_iso = 54.12)

In the sample of Kann et al. 2007 (arXiv:0712.2186), only two GRBs (000131 
and 990123) exceed this value.

Using the jet break time derived above, as well as the redshift z=0.937 
(Vreeswijk et al., GCN 7444
Not found
 
 
), and assuming standard parameters 
(constant-density medium, circumburst density n = 10 cm^-3, efficiency eta 
= 0.2), we find:

theta_jet = 10.24 +/- 0.71 degrees
E_jet = 2.11 +/- 0.30 x 10^52 erg (log E_jet = 52.3)

This value is comparable to or higher than for GRB 050904 (Tagliaferri et 
al. 2005, Frail et al. 2006), GRB 050820A (Cenko et al. 2006) and GRB 
070125 (Updike et al. 2008, Chandra et al. 2008), implying that this is 
the fourth hyper-energetic GRB (cf. Chandra et al. 2008). We note that 
broadband modeling may refine the circumburst density; if it is 
significantly higher than 10 cm^-3, the colimation-corrected energy 
will also be significantly higher.

Supernova:
In the i' band, the afterglow does not show a late steep decay, indicating 
the possibility of a supernova that is by now contributing to the optical 
transient (Tanvir et al.). Using the composite light curve (Bloom et al.) 
shifted to the i' band (using the early Rc - i' color), and assuming an 
achromatic break (t_b, alpha_2 fixed from the X-ray fit) and no host 
galaxy, we find, using data after 0.7 days (after the steep decay + 
plateau phase, Krugly et al.):

chi^2/d.o.f. = 44/10 (scatter)
alpha_1 = 1.33 +/- 0.02
k = 2.40 +/- 0.15
s = 1 fixed (stretch factor)

k is the peak luminosity in units of the SN 1998bw peak luminosity. This 
is a high value (Ferrero et al. 2006) and represents an upper limit on the 
SN flux. Assuming m_host(i') = 25, we find k = 1.24 +/- 0.14, a more 
reasonable result, indicating the contribution of a host galaxy to 
the late afterglow.

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