{
  "subject": "GRB060807: Possible Highly Significant X-ray Emission Lines",
  "eventId": "GRB 060807",
  "bibcode": "2006GCN..5435....1B",
  "createdOn": 1155240475000,
  "circularId": 5435,
  "submitter": "Nat Butler at MIT/CSR  <nrbutler@space.mit.edu>",
  "email": "nrbutler@space.mit.edu",
  "body": "N. Butler (UC Berkeley) reports:\n\nWe have analyzed the XRT PC mode data for GRB060807 (De Pasquale et al. 2006;\nGCN5409), also analyzed carefully by Page et al. (2006; GCN5423).  We find \nidentical spectral parameters (within errors) for the absorbed powerlaw model.\nHowever, we note that the fit goodness for the data in the 0.3-10 keV band from \n0.1-250 ksec after the BAT trigger is marginal (chi^2/nu=138.8/103), rejectable \nat 99% confidence.\n\nThe spectrum exhibits rare (see, e.g., Butler 2006, astro-ph/0604083, for a \nfew additional possible cases) and narrow modulations about the best fit \nmodel.  We can fit these modulations with 3 unresolved emission lines (giving \na delta_chi^2 fit improvement of 39.0, for 6 additional degrees of freedom).  \nWe estimate a significance for the 3 line set of 5.0 sigma, including a \ndecrease in the estimated significance due to the search for lines at many \npossible centroid energies in the 0.3-5.0 keV band.  Plots of the spectrum \nwith and without the 3 most significant lines can be found at:\n\nhttp://astro.berkeley.edu/~nat/GRB060807/xrt_060807_nolines.jpg , and\nhttp://astro.berkeley.edu/~nat/GRB060807/xrt_060807_lines.jpg .\n\nThe centroid energies of the lines are 0.73, 0.90, and 1.24 keV, \nwith uncertainties ~0.05 keV.  The brightest line at 0.73 keV has a flux \n~5 x 10^-14 erg cm^-2 s^-1 for the time period 0.1-250 ksec after the\nBAT trigger, or 3% of the 0.3-10 keV afterglow flux.\n\nAllowing associations for the lines with K-alpha transitions in H- or He-like \nspecies of common light metals or any ionization state in the Fe group \nelements, the 3-line set allows several redshift solutions.  The highest \nprobability peaks have dz/z~0.2 and occur at z=0.4, 1.0, 1.6, and 4.5.  \nThe large X-ray absorbing column (e.g., GCN5423) suggests that the source is \nnot at high-z.  An optical spectrum of the host galaxy or optical transient\ncould resolve the redshift degeneracy and would provide important insight \ninto the nature of the X-ray emission."
}