The 'amplitude' parameter of gamma-ray bursts and its implications for GRB classification

Abstract

Traditionally, gamma-ray bursts (GRBs) are classified in the T90-hardness ratio twodimensional plane into long/soft and short/hard GRBs. In this paper, we suggest to add the 'amplitude' of GRB prompt emission as the third dimension as a complementary criterion to classify GRBs, especially those of short durations. We define three new parameters f, f<inf>eff</inf> and f<inf>eff,z</inf> as ratios between the measured/simulated peak flux of a GRB/pseudo-GRB and the flux background, and discuss the applications of these parameters to GRB classification. We systematically derive these parameters to find that most short GRBs are likely not 'tip-of-iceberg' of long GRBs. However, one needs to be cautious if a short GRB has a relatively small f (e.g. f < 1.5), since the chance for an intrinsically long GRB to appear as a 'disguised' short GRB is higher. Based on available data, we quantify the probability of a disguised short GRB below a certain f value is as P(< f) ∼ 0.78^+0.71<inf>-0.4</inf> f^-4.33±1.84. By progressively 'moving' a long GRB to higher redshifts through simulations, we also find that most long GRBs would show up as rest-frame short GRBs above a certain redshift. © 2014 The Authors.