Low-Resolution Spectroscopy of Gamma-Ray Burst Optical Afterglows: Biases in the Swift

Sample(ApJ Supplement, 2009, 185, 526-573)

Palli JakobssonCentre for Astrophysics & Cosmology

University of Iceland

Low-Resolution Spectroscopy of Gamma-Ray Burst Optical Afterglows: Biases in the Swift

Sample(ApJ Supplement, 2009, 185, 526-573)

Palli JakobssonCentre for Astrophysics & Cosmology

University of Iceland

The dark burst diagram (βOX)

Jakobsson et al. (2004)

Definition of βOX

Sari et al. (1998)

X-ray

Optical

Definition of βOX

Sari et al. (1998)

X-ray

2.0 < p < 2.5Optical

The dark burst diagram (βOX)

Swift Identification of Dark Gamma-Ray Bursts: Jakobsson et al. (2004)

The dark burst diagram (βOX)

The sample1. A Swift-detected GRB with t90 > 2 s.2. XRT afterglow position distributed within 12 hours.3. Small foreground Galactic extinction: AV < 0.5 mag.4. Favorable declination: -70° < δ < 70°.

5. Sun-to-field distance larger than 55°.

March 2005 – September 2008: 146 GRBs75% have an optical/NIR afterglow

The 3 sub-samples1. GRBs with redshifts measured from OA spectroscopy.

2. GRBs with detected optical/NIR afterglow, but no afterglow-based redshift.

3. GRBs with no detection of the OA.

72

35

39

The dark burst diagram (βOX)[z from OA][OA but no z][no OA]

14%38%>39

%

050904 (z = 6.3)

080913 (z = 6.7)

XRT excess absorption distribution

KS test probability(1 and 2): 2 x 10-3

KS test probability(1 and 3): 7 x 10-7

βOX vs. X-ray excess absorption

dark bursts

Redshift distribution: 146 GRBs

< 18% @ z > 7<z> =

2.2

Grupe et al. (2007)

Cumulative redshift distribution