GRB Spectral-Energy correlations: perspectives and

issues

G. Ghirlanda + G. Ghisellini, L. Nava, Z. Bosnjak, C. Firmani, I. Cabrera, F. Tavecchio & A. Celotti

giancarlo.ghirlanda@brera.inaf.it

Why?

1) tools for understanding GRB physics

2) tools to standardize GRB energetics cosmology

G. Ghirlanda – 2008 Nanjing Gamma Ray Burst Conference

Epeak Eiso0.57

2red=7.2 (60 dof)

s=0.2

Epeak Eg1.05

2red=0.89 (23 dof)

s=0.08

Liso Ep1.62 T45

-0.49

2red=0.7 (16 dof)

s=0.06

Liso t - 1.05

2red=9.6 (34 dof) 2

red=100 (46 dof)

Liso V 3.0

Amati et al. 2002

Ghirlanda et al. 2004

Firmani et al. 2005

Norris et al. 2000

Reichart et al. 2000Ramirez-Ruiz & Fenimore 2000

Liso0.57 Epeak 1.85

Yonetoku et al. 2004

Spectral-energy correlations: true physics or selection effects?

(I) GRBs with z

(II) + GRBs without z

(III) Still not convinced ?

(V) news

Q.

Physical interpretation(e.g. Rees & Meszaros 2006, Thomson, Meszaros & Rees 2006)

Study selection effects

(IV) Thermal component?

Epeak – Eiso

9 SAX (GRBM+WFC)

5 CGRO (Batse)

15 Hete-II (Fr.+WXC)

20 Konus et al.

27(/41) Swift (BAT)

35 before Nov. 200441 since Nov. 2004

03.047.0 isopeak EE

76 GRBs with z and Epeak

Amati et al. 2002, 2006, Nava et al. 2006, Ghirlanda et al. 2008 … etc

Evolution with redshift?

Slope of the 76 GRBs

NO

GRBs with F>Flim

Ep [Emin, Emax]

REST FRAME

OBSERVER FRAME

No segregation in z in the obs. frame

obs frame Ep-fluence correlation : (Lloyd, Petrosian & Mallozzi 2000; Lamb et al. 2005; Sakamoto et al. 2005)

Instrumentalselection

effcts

obsObs Frame Epeak - F

High fluence – intemediate/low Epeak

low fluence – intemediate/high Epeak

Selection

effects

Trigger threshold: which flux to trigger?(Band 2003,

2006)

Spectral threshold: which fluence to measure Epeak?

)(minmin EA

ENF

eff

obspeak

Assume GRB spectrum

Background

Detector response

a=1 b=2.3 Epeak Fbol

the error on Epeak(fit) < 100% in 97.7% of cases Fit with single powerlaw is excluded at 2σ

Trigger threshold: which flux to trigger?(Band 2003,

2006)

Spectral threshold: which fluence to measure Epeak?

Gh

irla

nd

a e

t al.

2008

BATSE could not detect this burst

BATSE could detected this burst

BATSE certainly detected this burstand measured Epeak

CONCLUSIONS (I)

76 GRBs (updated to Oct. 2007) with z and spectrum

1) No evolution of the Epeak-Eiso correlation with redshift.

2) A correlation is found in the observer frame

05.039.0 bolobspeak FE

3.1) no z segregation

3.2) Instrumental selection effect: a) trigger threshold not biasing b) spectral analysis threshold yes on Swift no on Batse/Sax

HOW is populated the Ep-Fluence plane?

Q: are there intermediate/low fluence bursts (i.e. between those with z and the spectral analisys curves)??

Nava et al. 2008 submitted

HOW is populated the Ep-Fluence plane? Add GRBs without redshift

From the literature

Sakamoto et al. 2005Butler et al. 2007 (freq)Kaneko et al. 2005GCNs (Golenetski et al. …)

Build a complete spectral sample of BATSE bursts down to ~2x10-6 erg/cm2

Extend the Bright Batse GRB sample (Kaneko et al. 2005) to lower fluences

Bright BATSE

Peak energy distribution

Fainter BATSE

Ep = 160 keV

BATSE bursts

Nava e

t al.

20

08

su

bm

itte

dOutliers of the Epeak – Eiso correlation

6% of BATSE bursts are outliers

Isotropic luminosity Isotropic energy

The Ep-Liso “Yonetoku” correlation The Ep-Eiso “Amati” correlation

Nava e

t al.

20

08

su

bm

itte

dThe Ep-Flux plane and the outliers of the Ep-Liso correlation

CONCLUSIONS (II)

Ep-Fluence or Ep-Peak flux show strong correlations

Add bursts without redshifts (+ a complete BATSE sample)

The 20?? Ep-Eiso correlation will have a different slope & larger scatter but maybe not for the Ep-Liso correlation

6% of outliers of the Ep-Eiso

0.3% of outliers of the Ep-Liso

Strong Ep-Fluence correlation

Strong Ep-Peak flux correlation

Still not convinced ? Are the spectral-energy correlations revealing a

physical process or simply due to selection effects?

Liang, Dai & Wu 2004 noted that an Ep-Liso correlation holds WITHIN single bursts!

Still not convinced ? Are the spectral-energy correlations revealing a

physical process or simply due to selection effects?

We have studied the time resolved spectra of BATSE GRBs with measured redshifts

Bosnjak et al. 2008 (to be subm.)

Ep-Liso “Y

onetoku”Ep-Liso is equivalent to Ep(t)-Liso(t)

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

Physical o

rigin fo

r

this

!

Interpretation of the <Epeak> Eg,iso0.5

“Geometrical” models: Eichler & lenvinson 2005a,b; Toma et al. 2005

“Radiative” models: Rees & Meszaros 2005; Tompson 2006; Thompson, Meszaros & Rees 2006

If the spectrum of GRBs is dominated by a thermal balck body component then the luminosity is naturally LINKED to the peak energy.

Evidence of Black Body in GRBs:

Ghirlanda, Celotti, Ghisellini 2003 980326, 970111, 911118, 910807, 910927 [Spectrum is thermal black body in the inital phase (~2 sec), later a non-thermal component dominates.]

Bosnjak, Celotti, Ghirlanda 2005 990413

Ryde 2005, 2006 Fit with Black body + Powerlaw

(4) Interpretation – Thermal BB

Band model fits

Black Body + powerlaw fits

Thermal components in GRB spectra

Thermal interpretation of the Amati relation

Time resolved spectra (BB+PL)

Time integrated spectrum = sum of time resolved

5 GRBs detected by BATSE and with WFC data the BB+PL fit to the BATSE data is inconsistent with the X-ray (WFC) data. A single Band model is the best fit.

Ghirlanda et al. 2007

Last slide … more News: the Ep-Eg corrlation

Ghirlanda et al. 2007

Pre swift-era bursts Swift era bursts (up to March 2007)Swift era bursts of the last yearJet Breaks from the

Optical