Towards a More Standardized Candle Using GRB Energetics and Spectra
  The use of GRB energetics for cosmography has long been advanced as a means to probe out to high redshifts, to the epoch of deceleration. However, the prompt energy release in GRBs, even when corrected for jetting geometry, is far from being a standard candle. We explore the cosmographic potential of a GRB standard candle based on the newly-discovered relation by Ghirlanda et al. between the apparent geometry-corrected energies (E_gamma) and the peak of the prompt burst spectrum (E_p). We present an explicit, self-consistent formalism for correcting GRB energies with a thorough accounting for observational uncertainties. In contrast to other work, we demonstrate that the current sample of 19 GRBs is not yet cosmographically competitive with results from other studies (e.g., SNe Ia). Although the E_p-E_gamma relation is a significant correlation across a range of cosmologies, the goodness of fit of the data to a power law, depends strongly on input assumptions -- especially the unknown density of the circumburst medium. Independent of assumptions, with very few low-z bursts, the current sample is most sensitive to Omega_M but essentially insensitive to Omega_Lambda. The proper use of the relation clearly brings GRBs an impressive step closer toward a standardizable candle, but until the physical origin of the E_p-E_gamma relation is understood, calibrated with a training set of low redshift bursts, and the major potential systematic uncertainties and selection effects are addressed, we urge caution concerning claims of the utility of GRBs for cosmography (abridged). 
Andrew S. Friedman (Harvard/CfA) and Joshua S. Bloom (Harvard/CfA; UC   Berkeley)