English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Gamma-ray burst (GRB) emission is believed to originate in highlyrelativistic fireballs. Currently, only lower limits were securely set to theinitial fireball Lorentz factor Gamma_0. We aim to provide a direct measure ofGamma_0. The early-time afterglow light curve carries information aboutGamma_0, which determines the time of the afterglow peak. We have obtainedearly observations of the near-infrared afterglows of GRB 060418 and GRB060607A with the REM robotic telescope. For both events, the afterglow peakcould be clearly singled out, allowing a firm determination of the fireballLorentz of Gamma_0 ~ 400, fully confirming the highly relativistic nature ofGRB fireballs. The deceleration radius was inferred to be R_dec ~ 10^17 cm.This is much larger than the internal shocks radius (believed to power theprompt emission), thus providing further evidence for a different origin of theprompt and afterglow stages of the GRB.Comment: 6 pages, 3 color figures, 3 tables (including two online); discussion  significantly extended, results unchanged; accepted for publication in A&A  Letter

REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination