Low‐Luminosity Gamma‐Ray Bursts as a Unique Population: Luminosity Function, Local Rate, and Beaming Factor

Abstract

Swift BAT has detected $200 long-duration GRBs, with redshift measurements for $50 of them. We derive the luminosity function (È HL ) and the local event rate ( HL 0 ) of the conventional high-luminosity (HL) GRBs by using the z-known Swift GRBs. Our results are generally consistent with that derived from the CGRO BATSE data. However, the fact that Swift detected a low-luminosity (LL) GRB, GRB 060218, at z ¼ 0:033 within $2 years of operation, together with the previous detection of the nearby GRB 980425, suggests a much higher local rate for these LL-GRBs. We explore the possibility that LL-GRBs are a distinct GRB population from the HL-GRBs. We find that LL 0 is $325 þ352 À177 Gpc À3 yr À1 , which is much higher than HL 0 (1:12 þ0:43 À0:20 Gpc À3 yr À1 ). This rate is $0.7% of the local Type Ib/c SNe. Our results, together with the finding that less than 10% of Type Ib/c SNe are associated with off-beam GRBs, suggest that LL-GRBs have a beaming factor typically less than 14, or a jet angle typically wider than 31 . The high local GRB rate, small beaming factor, and low-luminosity make the LL-GRBs distinct from the HL-GRBs. Although the current data could not fully rule out the possibility that both HL-and LL-GRBs are the same population, our results suggest that LL-GRBs are likely a unique GRB population and that the observed low-redshift GRB sample is dominated by the LL-GRBs.

The Arcminute Microkelvin Imager catalogue of gamma-ray burst afterglows at 15.7 GHz

“…However, compared to a similar luminosity plot constructed by Chandra & Frail (2012, see their Figure 6), we note that AMI has not detected the much fainter GRBs with radio spectral luminosities between 10 27 − 10 29 erg s −1 Hz −1 . Such events are known as sub-energetic or low luminosity GRBs (Soderberg et al 2004) and are extremely rare with only a handful known (see Margutti et al 2013, and references therein), possibly representing a different GRB population (for example see Liang et al 2007). …”

Failed Collapsar Jets to Explain Low Luminosity GRB Properties

“…Angular and energetic properties suggest that brief engines (with T inj < T breakout , either due to short T inj or large θ 0 ) represent excellent models to explain the debated llGRBs. In particular, brief engines' jets display two of llGRBs peculiar, and hard to explain, features: 1) an estimated llGRBs rate ~100 times higher than that of GRBs [3,4,5] and, 2) a potentially energetic SN emission ~ 10 51-52 erg [6] (as most of these failed jets' are not well collimated and expands with sub-relativistic velocities). These two features only arise from brief engines.…”

“…For instance, in terms of the isotropic equivalent energy, GRB energies are spread over 6 orders of magnitude [7]. Among the newly discovered GRBs, are low luminosity GRBs (llGRBs), a class of faint GRBs with an isotropic equivalent energy E iso < 10 49 erg [5,8]. llGRBs and their contrast to classical GRBs, present a challenge to the traditional collapsar model.…”

“…llGRBs and their contrast to classical GRBs, present a challenge to the traditional collapsar model. Apart from being several orders of magnitude softer than typical GRBs, llGRBs show: 1) Huge rates, ~100 times than typical GRBs: ~220 Gpc -3 yr -1 [9], ~110 Gpc -3 yr -1 [10], ~260 Gpc -3 yr -1 [11], a rate of llGRBs/GRBs ~ 300 -1000 [12], ~325 Gpc -3 yr -1 [5] and, ~380 Gpc -3 yr -1 [13] (the rate of GRB is ~1 Gpc-3 yr-1). 2) Strong and clear SN connection, in contrast with the typically SN-less GRBs.…”

“…Our results show that the expanding jet's hydrodynamical properties significantly differ, in particular outflow collimation and relativistic acceleration. The implication of this is that brief engines (with T inj < T breakout , either due to a short T inj or to a large θ 0 ) represent excellent systems to explain the debated low-luminosity GRBs (llGRBs), displaying two of llGRBs peculiar features: i) the estimated llGRBs rate at least about 100 times higher than that of GRBs [3,4,5], and ii) potentially energetic SN emission [6]. We find that these two features only arise from brief engines.…”