Is the Amati relation due to selection effects?

Lara Nava

In collaboration with

G. Ghirlanda, G.Ghisellini, C. Firmani

Egypt, March 30-April 4, 2009NeutronStars & GammaRayBursts

INAF - Osservatorio Astronomico di Brera, Italy

Universita’ degli Studi dell’Insubria, Como, Italy

Amati and Yonetoku relations

Eiso [erg] Lpeak,iso [erg/s]

scatter = 0.22 scatter = 0.26

No instr. selection effects

Possible instr.

selection effects

Observational planes

Amati Yonetoku

Epeak,obs – Fluence Epeak,obs – Peak Flux

No instr. selection effects

Possible instr.

selection effects

Fluence [erg] Peak Flux [erg/cm2 s]

Ep

eak

,ob

s [k

eV]

Instrumental Selection Effects

TRIGGER THRESHOLD

SPECTRAL ANALYSIS THRESHOLD

Minimum Energy Peak Flux:

Plim (Epeak,obs)

Peak flux

Epea

kAllowed region

Minimum Fluence:

Flim (Epeak,obs)

Epea

k

Fluence

HETE: Sakamoto et al. 2005 Swift/BAT: Butler et al. 2007 (freq)BATSE: Kaneko et al. 2006 Konus/Wind: Golenetskii et al. (GCNs)

Amati relation in the observational plane

All GRBs

with

redshift

All GRBs

with

published

Epeak and Fluence

Build a complete spectral sample of

BATSE bursts down to ~ 10-6 erg/cm2

Extend the Bright BATSE GRB sample (Kaneko et al. 2006)

to lower fluences

Density contours

Nava et al., 2008, MNRAS

Epeak,obs distribution of BATSE

bursts

bright

faint

Lower fluences

correspond to

lower Epeak

Considering the whole sample we

found:

‹Ep› = 160 keV

Yonetoku relation in the observational plane

HETE: Sakamoto et al. 2005 Swift/BAT: Butler et al. 2007 (freq)BATSE: Kaneko et al. 2006 Konus/Wind: Golenetskii et al. (GCNs)

Ep

eak

,ob

s [k

eV]

Bosnjak et al. 2009 (submitted)

Epeak-Liso

Ep-Liso correlation found with time Ep-Liso correlation found with time integrated spectra holds also within a integrated spectra holds also within a

burst!!burst!!

FERMI(preliminary results from GCN)

Fluence [erg] Peak Flux [erg/cm2 s]

Summary

Correlation in the observer frameDetermined by selection effectsWithout z same region that with z

Short bursts

AMATI YONETOKU

YES

YES

YES

NONO

NO

(larger)

YES

The Yonetoku correlation is more fundamental and requires more attention and theoretical efforts to explain

its physical origin

NOWithin a single burst

See talk tomorrow by Ghirlanda

Epeak-Fluence or Epeak-Peak Flux show strong correlations

Add bursts without redshifts (+ a complete BATSE sample)

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

Strong Epeak-Fluence correlation

Strong Epeak-Peak Flux correlation

Summary

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

Epeak-Eiso correlation: 6% of BATSE bursts

are outliers

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 Epeak-Liso

No redshift evolution

Rest frame plane

Observational plane

Epeak-Eiso correlation: 6% of BATSE bursts

are outliers

Build a complete spectral sample of BATSE bursts down to ~ 10-6 erg/cm2

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)