GCN Circular 7627
Subject
GRB 080319B: Jet Break, Energetics, Supernova
Date
2008-04-16T18:58:13Z (17 years ago)
From
Alexander Kann at TLS Tautenburg <kann@tls-tautenburg.de>
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) 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) 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) and Tanvir et
al. (GCN 7621) 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), 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), 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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