[vsnet-grb-info 3601] GRB060807: Possible Highly Significant X-ray Emission Lines

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5435
SUBJECT: GRB060807: Possible Highly Significant X-ray Emission Lines
DATE:    06/08/10 20:07:55 GMT
FROM:    Nat Butler at MIT/CSR  <nrbutler at space.mit.edu>

N. Butler (UC Berkeley) reports:

We have analyzed the XRT PC mode data for GRB060807 (De Pasquale et al. 2006;
GCN5409), also analyzed carefully by Page et al. (2006; GCN5423).  We find 
identical spectral parameters (within errors) for the absorbed powerlaw model.
However, we note that the fit goodness for the data in the 0.3-10 keV band from 
0.1-250 ksec after the BAT trigger is marginal (chi^2/nu=138.8/103), rejectable 
at 99% confidence.

The spectrum exhibits rare (see, e.g., Butler 2006, astro-ph/0604083, for a 
few additional possible cases) and narrow modulations about the best fit 
model.  We can fit these modulations with 3 unresolved emission lines (giving 
a delta_chi^2 fit improvement of 39.0, for 6 additional degrees of freedom).  
We estimate a significance for the 3 line set of 5.0 sigma, including a 
decrease in the estimated significance due to the search for lines at many 
possible centroid energies in the 0.3-5.0 keV band.  Plots of the spectrum 
with and without the 3 most significant lines can be found at:

http://astro.berkeley.edu/~nat/GRB060807/xrt_060807_nolines.jpg , and
http://astro.berkeley.edu/~nat/GRB060807/xrt_060807_lines.jpg .

The centroid energies of the lines are 0.73, 0.90, and 1.24 keV, 
with uncertainties ~0.05 keV.  The brightest line at 0.73 keV has a flux 
~5 x 10^-14 erg cm^-2 s^-1 for the time period 0.1-250 ksec after the
BAT trigger, or 3% of the 0.3-10 keV afterglow flux.

Allowing associations for the lines with K-alpha transitions in H- or He-like 
species of common light metals or any ionization state in the Fe group 
elements, the 3-line set allows several redshift solutions.  The highest 
probability peaks have dz/z~0.2 and occur at z=0.4, 1.0, 1.6, and 4.5.  
The large X-ray absorbing column (e.g., GCN5423) suggests that the source is 
not at high-z.  An optical spectrum of the host galaxy or optical transient
could resolve the redshift degeneracy and would provide important insight 
into the nature of the X-ray emission.