GRB030328: Full Observation, X-ray Fading and Spectrum Measured with Chandra
N. R. Butler, H. L. Marshall, P. G. Ford, R. K. Vanderspek, G. R.
Ricker (MIT), J. G. Jernigan (U.C. Berkeley), and D. Q. Lamb (U. Chicago)
We have analyzed the full 94.0 ksec (livetime) Chandra LETGS
observation of GRB030328 (Villasenor et al, GCN1978), lasting from
March 29.112 (t[burst] + 15.33 hr) until March 30.278 (t[burst] +
43.32 hr). The mean counting rate for the X-ray afterglow (Butler et
al. GCN2007) is 0.012 counts/s (summed over the dispersed signal from
the LETGS, and including the 0th order flux). We observe that the
brightness over the full observation decays with a slope -1.5 +/-
0.1, consistent with the value reported by Ford et al. (GCN2027).
(The chi^2 is 34.27 for 30 degrees of freedom, and the fit is
rejectable at only 73% confidence.) We see no evidence for a
temporal break. We fit the dispersed LETGS counts (+/-1 orders
summed) and the counts in 0th order jointly by minimizing chi^2,
requiring 12 or more counts per spectral bin. In the 0.5 to 3.0 keV
band, the data are well fit (chi^2/nu=48.45/48, rejectable at 54%
confidence) by an absorbed power-law:
dN/dE = A * E^ (-gamma) * exp(-NH * sigma[E]) ,
where NH = ( 5 +/- 3 ) x 10^20 cm^(-2) is the line of sight column
density, which is consistent with the anticipated galactic absorption
in the source direction; A = ( 6.8 +/- 0.9 ) x 10^(-5) ph cm^(-2)
s^(-1) keV^(-1); and gamma = 1.7 +/- 0.2 . These are 1 parameter
1-sigma confidence intervals.
Integrating the above model over the 0.5 to 3.0 keV band, the mean
flux for the duration of the Chandra observation is ~1.9 x 10^(-13)
ergs cm^(-2) s^(-1).
Our analyses are continuing, and more detailed results will be posted at:
We thank Harvey Tananbaum for his generous allocation of Director's
Discretion Time to this observation, and the Chandra X-ray
Observatory Operations personnel for the impressive promptness with
which this observation was planned and carried out.
The preliminary results reported here may be cited.