A relation to estimate the redshift from the X-ray afterglow light curve Bruce Gendre (IASF-Roma/INAF) & Michel Boër (OHP/CNRS)

GRBs as cosmological tools GRBs can be good events for cosmological purposes See e.g. D. Lamb talk this afternoon One (important) problem : the redshift determination is based on optical follow up, but some bursts do not feature optical afterglow optical follow up not always available Redshift estimators are available based on prompt spectral properties Use is not straightforward with SWIFT (BAT spectral range : keV) Another estimator based on the afterglow may be useful

Comparison of X-ray afterglow light curves with known redshift rescaled to a common distance and energy band Band is keV (free of absorption) Distance is chosen to be z=1 Time dilatation is corrected by calculating the time in the rest frame (important for flux accuracy : no flux extrapolation is made) Standard k-correction to take into account other cosmological effects Reference papers : Boër & Gendre 2000 for BeppoSAX bursts Gendre & Boër 2005 for Chandra & XMM-Newton updates Gendre et al. in preparation for SWIFT updates An empirical relationship between flux and decay index

The relation Probability of chance clustering : Group I : bright fast fading afterglows decay index ~ 1.6 Flux ~ erg s -1 cm -2 (1 day) Group II : fainter slow fading afterglows decay index ~ 1.2 Flux ~ erg s -1 cm -2 (1day) 2 outliers Decay index very low (~0.5) Results observed also by other groups on other samples in X-ray (Kouveliotou et al. 2004, Nousek et al. 2005) or optical (Kann et al. 2006, Nardini et al. 2006, Liang & Zhang 2006)

Validity of the relation as a redshift estimator Nardini et al indicated that Chandra Grating points did not fit the relationship Fixed : extrapolation effect SWIFT bursts may be different No obvious difference Is the relationship valid at high redshift ? Seems yes (but flares in the light curve of GRB make conclusions difficult) Is the relationship valid at low redshiftSeems no (up to z ~ 0.1)

Accuracy of the relation as a redshift estimator The method is not valid for nearby bursts (z<0.5) The redshift uncertainty is ~ 30 %, but can be higher Method tested by using the bursts of the relationship Maybe a possible bias (bursts defining a relationship used to test its application) Two outliers not included Good agreement between estimated and measured redshifts problems at low redshift large errors at high redshift

BeppoSAX-era bursts without known redshift GRB AGRB GRB GRB GRB      0.3 GRB GRB GRB GRB GRB GRB  0.2 (3.8  0.8) 3.8     0.7(0.5  0.4) GRB : z = 0.5  0.4 reach the low redshift limitation : rejected value GRB A : z = 5.8 ± 0.8 This burst is classified as dark (Pedersen et al. 2005). Maybe due to Ly  absorption

Comments on that numbers GRB (3.8  0.7): burst detected in U band, preferred redshift should be ~ 1.5 (Kann, Klose & Zeh, 2006) Decay very steep (2.18), classify this burst as group I burst. However, if we assume it to be a group II burst, we obtain 1.4  0.2 Mean estimated redshift is 2.5 Previous BeppoSAX and XMM-Newton mean known redshift was 1.1 Maybe some selection effect (distant bursts may be fainter in optical, thus not detected) Mean SWIFT redshift : 2.46 No strong differences with this estimated sample, but difficult to conclude (selection bias…)

Conclusions An empirical relationship links the luminosity and the decay index of X-ray afterglows. Clustering into groups well separated soon after the burst Few (and usual) outliers This relation can be used as distance estimator Estimator valid up to large distance (at least up to z = 6.3) However, not valid for nearby bursts (z < 0.5) The presence of two groups can induce false results The estimator is easy to use. One only needs : a spectral index the decay index estimate (for group guessing) the precise flux value 1 day (this will be updated to more natural SWIFT date soon) after the burst