GRB optical afterglow and redshift selection effects: The learning curve effect at work

Abstract

We show how the observed gamma ray burst (GRB) optical afterglow (OA) and redshift distributions are changing in time from selection effects. For a subset of {\it Swift} triggered long duration bursts, we show that the mean time taken to acquire spectroscopic redshifts for a GRB OA has evolved to shorter times. We identify a strong correlation between the mean time taken to acquire a spectroscopic redshift and the measured redshift. This correlation reveals that shorter response times favour smaller redshift bursts. This is compelling evidence for a selection effect that biases longer response times with relatively brighter high redshift bursts. Conversely, for shorter response times, optically fainter bursts that are relatively closer are bright enough for spectroscopic redshifts to be acquired. This selection effect could explain why the average redshift, $<z>\approx 2.8$ measured in 2005, has evolved to $<z>\approx 2$, by mid 2008. Understanding these selection effects provides an important tool for separating the contributions of intrinsically faint bursts, those obscured by host galaxy dust and bursts not seen in the optical because their OAs are observed at late times. The study highlights the importance of rapid response telescopes capable of spectroscopy, and identifies a new redshift selection effect that has not been considered previously, namely the response time to measure the redshift.

5 pages, 4 figures, MNRAS Letter (accepted)

References in corpus (11)

• Swift Identification of Dark Gamma-Ray Bursts
• Dust and Gas in the Local Environments of Gamma-Ray Bursts
• Probing the cosmic star formation using long Gamma-Ray Bursts: New constraints from the Spitzer Space Telescope
• GRB 070306: A Highly Extinguished Afterglow
• Infrared and Optical Observations of GRB 030115 and its Extremely Red Host Galaxy: Implications for Dark Bursts
• The extreme, red afterglow of GRB 060923A: Distance or dust?
• Selection effects shaping the Gamma Ray Burst redshift distributions
• Observations of GRBs at High Redshift
• Open issues with the gamma-ray burst redshift distribution
• Where are the missing gamma ray burst redshifts?
• An improved method for estimating source densities using the temporal distribution of Cosmological Transients

Cited by in corpus (6)

• The luminosity function and the rate of Swift's Gamma Ray Bursts
• The Swift Gamma-Ray Burst redshift distribution: selection biases and optical brightness evolution at high-z?
• Improving sampling and calibration of GRBs as distance indicators
• Gamma Ray Bursts as Probes of the Distant Universe
• Are GRB optical afterglows relatively brighter at high z?
• The mystery of the missing GRB redshifts