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Title: Estimates for Lorentz factors of gamma-ray bursts from early optical afterglow observations

The peak time of optical afterglow may be used as a proxy to constrain the Lorentz factor Γ of the gamma-ray burst (GRB) ejecta. We revisit this method by including bursts with optical observations that started when the afterglow flux was already decaying; these bursts can provide useful lower limits on Γ. Combining all analyzed bursts in our sample, we find that the previously reported correlation between Γ and the burst luminosity L {sub γ} does not hold. However, the data clearly show a lower bound Γ{sub min} that increases with L {sub γ}. We suggest an explanation for this feature: explosions with large jet luminosities and Γ < Γ{sub min} suffer strong adiabatic cooling before their radiation is released at the photosphere; they produce weak bursts, barely detectable with present instruments. To test this explanation, we examine the effect of adiabatic cooling on the GRB location in the L {sub γ} – Γ plane using a Monte Carlo simulation of the GRB population. Our results predict detectable on-axis 'orphan' afterglows. We also derive upper limits on the density of the ambient medium that decelerates the explosion ejecta. We find that the density in many cases is smaller than expectedmore » for stellar winds from normal Wolf-Rayet progenitors. The burst progenitors may be peculiar massive stars with weaker winds, or there might exist a mechanism that reduces the stellar wind a few years before the explosion.« less
Authors:
;  [1] ; ;  [2]
  1. Physics Department and Columbia Astrophysics Laboratory, Columbia University, 538 West 120th Street, New York, NY 10027 (United States)
  2. Institut d'Astrophysique de Paris, UMR 7095 Université Pierre et Marie Curie-CNRS, 98 bis boulevard Arago, F-75014 Paris (France)
Publication Date:
OSTI Identifier:
22348067
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 782; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; AFTERGLOW; COMPUTERIZED SIMULATION; COOLING; CORRELATIONS; COSMIC GAMMA BURSTS; DECAY; DENSITY; EXPLOSIONS; LUMINOSITY; MONTE CARLO METHOD; PHOTOSPHERE; STARS; STELLAR WINDS