MEASURING THE BULK LORENTZ FACTORS OF GAMMA-RAY BURSTS WITHFERMI

Abstract

Gamma-ray bursts (GRBs) are powered by ultra-relativistic jets. Usually a minimum value of the Lorentz factor of the relativistic bulk motion is obtained based on the argument that the observed high energy photons (≫ MeV) can escape without suffering from absorption due to pair production. The exact value, rather than a lower limit, of the Lorentz factor can be obtained if the spectral cutoff due to such absorption is detected. With the good spectral coverage of the Large Area Telescope (LAT) on Fermi, measurements of such cutoff become possible, and two cases (GRB 090926A and GRB 100724B) have been reported to have high-energy cutoffs or breaks. We systematically search for such high energy spectral cutoffs/breaks from the LAT and the Gamma-ray burst monitor (GBM) observations of the prompt emission of GRBs detected since August 2011. Six more GRBs are found to have cutoff-like spectral feature at energies of ∼ 10 − 500 MeV. Assuming that these cutoffs are caused by pair-production absorption within the source, the bulk Lorentz factors of these GRBs are obtained. We further find that the Lorentz factors are correlated with the isotropic gamma-ray luminosity of the bursts, indicating that more powerful GRB jets move faster.

Low-Γ Jets From Compact Stellar Mergers: Candidate Electromagnetic Counterparts to Gravitational Wave Sources

“…Liang et al (2010), Ghirlanda et al (2012) and Tang et al (2015) report correlations between Lorentz factor Γ and the isotropic luminosity L γ (or the isotropic energy E γ ) for long GRBs:…”

Evidence of an Internal Dissipation Origin for the High-energy Prompt Emission of GRB 170214A

“…Thus, we test a BandCut model, which is described as the Band with a high energy cutoff, i.e., e −E/E c , where E c is the exponential cutoff energy. With one more parameter, the BandCut will be regarded as a more preferred model than a single Band if ∆(CSTAT) is larger than 28 (Ackermann et al 2013;Tang et al 2015). However, only ∆(CSTAT)∼25 is found for the BandCut model with a cutoff energy of 224 ± 58 MeV.…”

The effect of pair cascades on the high-energy spectral cut-off in gamma-ray bursts

“…In many works (e.g. Tang et al 2015) that find the spectral cutoff to lie in the Thomson-thick regime, Γ is estimated using Γ max . It is clear from Fig.…”

“…So far, a high-energy cut-off has only been observed in a handful of sources, e.g. GRB 090926A (Ackermann et al 2011), and GRBs 100724B & 160409A (Vianello et al 2017, for a more complete list see Tang et al 2015). Most analytic works employ a simple one-zone model with an isotropic (comoving) radiation field (e.g.…”