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 emission, with the goal of determining the physical conditions leading to a prominent reverse-shock emission. We identify 10 GRBs with reverse shock signatures - GRBs 990123, 021004, 021211, 060908, 061126, 080319B, 081007, 090102, 090424 and 130427A. Modeling these 10 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. Even though we can only loosely constrain the parameters, we find that physical properties cover a wide parameter space and don’t 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 (< 1ks) optical spectral luminosity, X-ray afterglow luminosity and -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 forward-shock emission at late time (> 10 ks). We find that GRBs with an identifiable reverse shock component are linked with high magnetization parameters of R_B = epsilon_(B,r)/epsilon_(B,f) lies in the range 2-10^4. We conclude that our results confirm the mildly magnetized baryonic jet model of GRBs.