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A relation to estimate the redshift from the X-ray afterglow light curve

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. GRB s can be good event s for cosmological purposes See e.g. D. Lamb talk this afternoon.

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A relation to estimate the redshift from the X-ray afterglow light curve

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

  2. 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 : 15-300 keV) Another estimator based on the afterglow may be useful

  3. An empirical relationship between flux and decay index • Comparison of X-ray afterglow light curves with known redshift rescaled to a common distance and energy band • Band is 2.0-10.0 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

  4. The relation • Group I : bright fast fading afterglows • decay index ~ 1.6 • Flux ~ 7 10-12 erg s-1 cm-2 (1 day) • Group II : fainter slow fading afterglows • decay index ~ 1.2 • Flux ~ 6 10-13 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) Probability of chance clustering : 1.2 10-6

  5. Validity of the relation as a redshift estimator Nardini et al. 2005 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 050904 make conclusions difficult) Is the relationship valid at low redshift Seems no (up to z ~ 0.1)

  6. Accuracy of the relation as a redshift estimator • 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 The method is not valid for nearby bursts (z<0.5) The redshift uncertainty is ~ 30 %, but can be higher

  7. BeppoSAX-era bursts without known redshift • GRB 020410 : z = 0.5  0.4 • reach the low redshift limitation : rejected value • GRB 001025A : z = 5.8 ± 0.8 • This burst is classified as dark (Pedersen et al. 2005). • Maybe due to Ly absorption

  8. Comments on that numbers • GRB 980519 (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…)

  9. 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

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