Is the Amati relation due to selection effects?

1. INAF - Osservatorio Astronomico di Brera, Italy Universita’ degli Studi dell’Insubria, Como, Italy Is the Amati relation due to selection effects? Lara Nava In collaboration with G. Ghirlanda, G.Ghisellini, C. Firmani Egypt, March 30-April 4, 2009 NeutronStars & GammaRayBursts

2. Amati and Yonetoku relations Eiso [erg] Lpeak,iso [erg/s] scatter = 0.22 scatter = 0.26

3. Possible instr. selection effects Possible instr. selection effects No instr. selection effects No instr. selection effects Observational planesAmati Yonetoku Epeak,obs –Fluence Epeak,obs – Peak Flux Epeak,obs [keV] Fluence [erg] Peak Flux [erg/cm2 s]

4. Instrumental Selection Effects Epeak TRIGGER THRESHOLD Allowed region Minimum Energy Peak Flux: Plim (Epeak,obs) Peak flux SPECTRAL ANALYSIS THRESHOLD Epeak Minimum Fluence: Flim (Epeak,obs) Fluence

5. Amati relation in the observational plane All GRBs with published Epeak and Fluence All GRBs with redshift HETE: Sakamoto et al. 2005 Swift/BAT: Butler et al. 2007 (freq) BATSE: Kaneko et al. 2006 Konus/Wind: Golenetskii et al. (GCNs)

6. Extend the Bright BATSE GRB sample (Kaneko et al. 2006) to lower fluences Build a complete spectral sample of BATSE bursts down to ~ 10-6 erg/cm2 Nava et al., 2008, MNRAS Density contours

7. Epeak,obs distribution of BATSE bursts Lower fluences correspond to lower Epeak bright Considering the whole sample we found: ‹Ep› = 160 keV faint

8. Yonetoku relation in the observational plane Epeak,obs [keV] HETE: Sakamoto et al. 2005 Swift/BAT: Butler et al. 2007 (freq) BATSE: Kaneko et al. 2006 Konus/Wind: Golenetskii et al. (GCNs)

9. Epeak-Liso Bosnjak et al. 2009 (submitted) Ep-Liso correlation found with time integrated spectra holds also within a burst!!

10. FERMI(preliminary results from GCN) Fluence [erg] Peak Flux [erg/cm2 s]

11. Summary AMATI YONETOKU Correlation in the observer frame YES YES Determined by selection effects NO NO Without z same region that with z NO (larger) YES Within a single burst NO YES Short bursts See talk tomorrow by Ghirlanda The Yonetoku correlation is more fundamental and requires more attention and theoretical efforts to explain its physical origin

13. Summary Epeak-Fluence or Epeak-Peak Flux show strong correlations Add bursts without redshifts (+ a complete BATSE sample) Strong Epeak-Fluence correlation Strong Epeak-Peak Flux correlation In the future Epeak-Eiso correlation will have a different slope & larger scatter but maybe not for the Epeak-Liso correlation 6% of outliers of the Epeak-Eiso 0.3% of outliers of the Epeak-Liso

14. What about short bursts? Short bursts populate the same region of long bursts in the Epeak,obs-Peak Flux plane. They can be consistent with the Epeak-Liso correlation Short bursts have similar Epeak but lower fluences in respect to Long bursts. They cannot be consistent with the Epeak-Eiso correlation

15. Summary: The existence of a correlation in the plane Epeak,obs-Fluence is not determined by instrumental biases However, they affect the slope and the dispersion of the Epeak-Eiso correlation Swift/BAT bursts are strongly biased by the spectral and trigger threshold and by the very small energy range → flat correlation in the observational plane → flat correlation in the rest frame plane Epeak-Eiso correlation: 6% of BATSE bursts are outliers Epeak-Eiso Epeak-Liso

16. Rest frame plane No redshift evolution Observational plane

17. Epeak-Eiso correlation: 6% of BATSE bursts are outliers

18. Extend the Bright Batse GRB sample (Kaneko et al. 2005) to lower fluences Red points: bright BATSE bursts from Kaneko et al. 2006 Green points: 100 lower fluence bursts representatives of around 1000 bursts (our analysis) Build a complete spectral sample of BATSE bursts down to ~ 10-6 erg/cm2