PHENOMENOLOGY OF REVERSE-SHOCK EMISSION IN THE OPTICAL AFTERGLOWS OF GAMMA-RAY BURSTS

We use a parent sample of 118 gamma-ray burst (GRB) afterglows, with known redshift and host galaxy extinction, to separate afterglows with and without signatures of dominant reverse-shock (RS) emission and to determine which physical conditions lead to a prominent reverse-shock emission. We identify 10GRBs with reverse-shock signatures: 990123, 021004, 021211, 060908, 061126, 080319B, 081007, 090102, 090424, and 130427A. By modeling their optical afterglows with reverse-and forward-shock analytic light curves and using Monte Carlo simulations, we estimate the parameter space of the physical quantities describing the ejecta and circumburst medium. We find that physical properties cover a wide parameter space and do not seem to cluster around any preferential values. Comparing the rest-frame optical, X-ray, and high-energy properties of the larger sample of non-RS-dominated GRBs, we show that the early-time (<1 ks) optical spectral luminosity, X-ray afterglow luminosity, and gamma-ray energy output of our reverse-shock dominated sample do not differ significantly from the general population at early times. However, the GRBs with dominant reverse-shock emission have fainter than average optical forwardshock emission at late times (> 10 ks). We find that GRBs with an identifiable reverse-shock component show a high magnetization parameter R-B = epsilon(B,r)/epsilon(B,f) similar to 2-10(4). Our results are in agreement with the mildly magnetized baryonic jet model of GRBs.