[vsnet-grb-info 1813] GRB 050709: Refined Spectral and Temporal Properties

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3653
SUBJECT: GRB 050709: Refined Spectral and Temporal Properties
DATE:    05/07/21 02:47:04 GMT
FROM:    Nat Butler at MIT/CSR  <nrbutler at space.mit.edu>

M. Boer, G. Ricker, J-L. Atteia, N. Kawai, D. Lamb, and S. Woosley, on
behalf of the HETE Science Team;

M. Arimoto, T. Donaghy, E. Fenimore, M. Galassi, C. Graziani,
J. Kotoku, M. Maetou, M. Matsuoka, Y. Nakagawa, T. Sakamoto, R. Sato,
Y. Shirasaki, M. Suzuki, T. Tamagawa, K. Tanaka, Y. Yamamoto,
and A. Yoshida, on behalf of the HETE WXM Team;

N. Butler, G. Crew, J. Doty, G. Prigozhin, R. Vanderspek,
J. Villasenor, J. G. Jernigan, A. Levine, G. Azzibrouck, J. Braga,
R. Manchanda, and G. Pizzichini, on behalf of the HETE Operations and
HETE Optical-SXC Teams;

J-F Olive, J-P Dezalay, and K. Hurley, on behalf of the HETE FREGATE
Team;

report:

We have performed a joint fit to the WXM and FREGATE spectral data
for GRB 050709, in order to refine the numbers initially reported in
GCN 3570 (Butler et al. 2005) for the short and long pulse components.

The short pulse is well-fit (chi2/DOF= 35.37/48) by a power-law times
exponential model with photon index alpha = -0.7 +/- 0.2 and nu-F_nu peak
energy Epeak = 83 +18/-12 keV.  The energy fluences of the short pulse
are

S_E(2-30 keV)   = (9.3 +/- 0.9) x 10^(-8) erg cm^-2 and
S_E(30-400 keV) = (2.9 +/- 0.4) x 10^(-7) erg cm^-2.

The X-ray to gamma-ray fluence ratio is thus 0.32.  The t90 duration of
the short pulse is 220 +/- 50 ms in the 2-25 keV energy band and 70 +/- 10
ms in the 30-400 keV band.  The t90 duration and Epeak of the short
pulse are consistent with those found for short/hard GRBs (see, e.g.,
Ghirlanda, Ghisellini, & Celotti 2004, A&A, 422, L55 for plots of the
short/hard GRB Epeak distribution).

The long pulse is well fit (chi2/DOF = 108.86/138) by a power-law model
with photon index beta = -2.2 +0.2/-0.4.  The energy fluences of the
long pulse are

S_E(2-30 keV)   = (7.1 +/- 1.5) x 10^(-7) erg cm^-2 and
S_E(30-400 keV) = (3.9 +4.1/-2.7) x 10^(-7) erg cm^-2.

These spectral results are preliminary because there is a linear trend
in the background that affects the spectral analysis of the long pulse
and has not yet been taken into account.  The t90 of the long pulse is
130 +/- 7 s in the 2-25 keV energy band.

Using the redshift of z = 0.16 measured by Price et al. (2005; GCN 3605),
the isotropic-equivalent energy of the short pulse in the 1-10,000 keV
energy band in the rest frame of the source is

Eiso (short pulse) = (2.7 +1.1/-0.3) x 10^(49) erg,

using Omega_M=0.3, Omega_Lambda=0.7, and h=0.65.  The Eiso and Epeak
values for the short pulse place it well off of the Amati et al. (2002;
A&A, 390, 81) relation for long duration GRBs.

Taking a time interval 0.060 s in the rest frame of the source
(corresponding to t90 = 0.07 s in the 30-400 keV energy band in the
observer frame) and assuming the same cosmology and energy band as
used above for the Eiso determination, the luminosity of the short
pulse in the 1-10,000 keV energy band in the rest frame of the source
is

Liso (short pulse) = (5.2 +/- 1.4) x 10^(50) erg s^-1.

Taking even this short time interval, the Liso and Epeak values for the
short pulse place it off the Yonetoku et al. (2004; ApJ, 609, 935)
correlation for long duration GRBs.

A composite lightcurve showing the short and long pulses in both the
2-25 keV and 30-400 keV bands can be found at

http://space.mit.edu/HETE/Bursts/GRB050709 .