High-energy Spectral Lag And Cosmological Use Of Gamma-ray Bursts

We first briefly summarize the observational and theoretical progress of gamma-ray bursts?GRBs?and their afterglows over the past nearly 50 years,and present our works on the high-energy spectral lag of GRBs and GRB cosmology.The spectral lag,referring to the arrival time delay of photons with different energies,is a common feature in GRB observations.In the first work,we investigate the spectral lag behaviors for a radiating jet shell with a high-energy cut-off radiation spectrum in detail.We find that,for the jet shell with a cut-off power-law?CPL?spectrum,the spectral lag monotonously increases with the photon energy and saturates at a certain photon energy,which is analogous to that for the Band cut-off spectrum?Bandcut?.It is very interesting to see that a turn-over from the positive lag to the negative lag presents at the high-energy range for the jet shell with Bandcut spectrum,which quite resembles that observed in GRB160625B by Fermi satellite.The dependences of the spectral lags on the spectral shape/evolution are studied in detail as well.In addition,the spectral lag behavior observed in GRB 160625B is naturally reproduced based on our theoretical outcome.Similar to type Ia supernovae?SNe Ia?,the luminosity correlations discovered in long-duration GRBs have made them standardizable as promising distance indicators to measure the high-redshift universe.In the second work,we simultaneously fit the cosmological parameters and calibrate the GRB luminosity correlations utilizing the Bayesian statistics and Markov Chain Monte Carlo?MCMC?techniques jointly with the Pantheon Sample.This approach allows us calibrating the GRB luminosity correlations in an arbotary cosmology,effectively alleviating the“circularity problem”for the cosmological use with GRBs.We find that the L?,iso(E_p,z,?₀)?L??correlation is well calibrated in?CDM cosmology within the Bayesian statistical framework,albeit with a large intrinsic scatter.Similarly,we also calibrate the new-constructed L?,iso(E_p,z,t_p,z)?LT?correlation,which is also well calibrated in the standard cosmology.Then,we treat these two calibrated correlations as GRB distance indicators to constrain the cosmological parameters and dark energy.The constraints on the cosmological parameters imposed by current GRBs data in several cosmic expansion scenarios provide an overall agreement on the standard cosmology.Moreover,the high-redshift GRBs data act to significantly narrow the distributions of cosmological parameters,thus showing more meaningful cosmological constraints.