GRB at z = 5.3 and a Mean Redshift of 2.8 for Swift GRBs A&A, in press (astro-ph/ ) Páll Jakobsson Robert Priddey Darach Watson Priya Natarajan Andrew Levan Jens Hjorth Brian L. Jensen Javier Gorosabel Johan P. U. Fynbo Nial Tanvir Jesper Sollerman Kristian Pedersen X 2.7

GRB t 90 = s (poorly constrained). XRT 138 s. 167 s: V > 20.5 mag. NOT observations of 14 hours: V > 25.2 mag; R = 23.4 mag; I = 20.6 mag. R – I  2.8 mag  β  11.7 UKIRT detection in the NIR (J and K).

Spectral Energy Distribution

GRB (z = 5.3)GRB (z = 6.3) Tagliaferri et al. (2005)

Spectral Energy Distribution

 2 (z) for GRB z = 5.3 ± 0.3 β = 1.0 (fixed) A V  0.9 mag

The Redshift Distribution of Swift LGRBs Is it consistent with models were GRBs trace the global star formation? Important to construct a “clean” sample as unbiased as possible. Metallicity dependence?

Selection Criteria Small error circles: XRT localization (68). Sufficiently small Galactic extinction: A V < 0.5 mag (46). Quick distribution of XRT error circles for a relatively rapid follow-up: after 1 March 2005 (39). Reject bursts with an unsuitable declination: above +70 º or below –70 º (36). A total of 83 long-duration Swift GRBs

The Sample (36 bursts) A B B A A B C A B A B < < < 8.0 < < < GRB zzz

Cumulative Fraction Natarajan et al. (2005)Gorosabel et al. (2004)

At least z > A B B A A B C A B A B < < < 8.0 < < < GRB zzz