1. GRB Spectral-Energy correlations: perspectives and issues
G. Ghirlanda + G. Ghisellini, L. Nava, Z. Bosnjak, C. Firmani, I. Cabrera, F. Tavecchio & A. Celotti
GRB Spectral-Energy correlations: perspectives and issues
Why?
1) tools for understanding GRB physics
2) tools to standardize GRB energetics  cosmology
G. Ghirlanda – 2008 Nanjing Gamma Ray Burst Conference

2. true physics or selection effects?
Epeak  Eiso0.57
Epeak  Eg1.05
Liso  Ep1.62 T
2red=7.2 (60 dof)
s=0.2 Q.
Spectral-energy correlations:
true physics or selection effects?
2red=0.89 (23 dof)
s=0.08
2red=0.7 (16 dof)
s=0.06
Amati et al. 2002
Ghirlanda et al. 2004
Firmani et al. 2005
Physical interpretation
(e.g. Rees & Meszaros 2006, Thomson, Meszaros & Rees 2006)
Study selection effects
Liso  t
Liso  V 3.0
2red=9.6 (34 dof)
2red=100 (46 dof)
Liso0.57  Epeak 1.85
(I) GRBs with z
(II) + GRBs without z
(IV) Thermal component?
Reichart et al. 2000
Ramirez-Ruiz & Fenimore 2000
Norris et al. 2000
Yonetoku et al. 2004
(III) Still not convinced ?
(V) news

3. Epeak – Eiso 76 GRBs with z and Epeak 9 SAX (GRBM+WFC) 5 CGRO (Batse)
Amati et al. 2002, 2006, Nava et al. 2006, Ghirlanda et al … etc
76 GRBs with z and Epeak
35 before Nov. 2004
41 since Nov. 2004
9 SAX (GRBM+WFC)
5 CGRO (Batse)
15 Hete-II (Fr.+WXC)
20 Konus et al.
27(/41) Swift (BAT)
Passaggio: With this considerably large sample of busrsts we can also study of there is any evolution of this correlation with redshift. Indded, this sample of 76 events contains very low redshift bursts (at z<0.1) as well as very distant one (the record holder at z=6.3).

4. Evolution with redshift? NO
Slope of the 76 GRBs
1. No redshift evolution; 2. Except for very low z, redsfhits are distributed along the correlation

5. No segregation in z in the obs. frame
REST FRAME
GRBs with
F>Flim
Ep  [Emin, Emax]
Instrumental
selection effcts
OBSERVER FRAME
obs frame Ep-fluence correlation :
(Lloyd, Petrosian & Mallozzi 2000;
Lamb et al. 2005; Sakamoto et al. 2005)
1) No segregation in z means that the rest frame correlation cannot be due to the “stretching” of a blob of points due to the multiplication of strong functions of redshifts.
2) Any GRB instrument detected GRBs above some limiting flux and within a certain energy range
No segregation in z in the obs. frame

6. Selection effects Obs Frame  Epeak - F
low fluence – intemediate/high Epeak
Selection effects
High fluence – intemediate/low Epeak

7. a=1 b=2.3 Epeak Fbol Background Detector response
Trigger threshold: which flux to trigger?
(Band 2003, 2006)
Spectral threshold: which fluence to measure Epeak?
Assume GRB spectrum
Background
Detector response
a=1 b= Epeak Fbol
the error on Epeak(fit) < 100% in 97.7% of cases
Fit with single powerlaw is excluded at 2σ

8. Trigger threshold: which flux to trigger?
(Band 2003, 2006)
Spectral threshold: which fluence to measure Epeak?
Ghirlanda et al. 2008
BATSE could not detect this burst
BATSE could detected this burst
BATSE certainly detected this burst
and measured Epeak

9. 76 GRBs (updated to Oct. 2007) with z and spectrum
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
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

10. 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)??

11. HOW is populated the Ep-Fluence plane?  Add GRBs without redshift
From the literature
Passaggio: these are not complete samples!! Also the Batse bright bursts is not complete because it was cut in both fluence and peak flux. We need a complete sample of bursts.
Sakamoto et al. 2005
Butler et al (freq)
Kaneko et al. 2005
GCNs (Golenetski et al. …)
Nava et al submitted

12. Extend the Bright Batse GRB sample
(Kaneko et al. 2005) to lower fluences
Build a complete spectral sample of BATSE bursts down to ~2x10-6 erg/cm2
Peak energy distribution
Ep = 160 keV
Bright BATSE
Fainter BATSE

13. BATSE bursts

14. Outliers of the Epeak – Eiso correlation
6% of BATSE bursts are outliers
Nava et al submitted

15. Isotropic luminosity Isotropic energy
The Ep-Liso “Yonetoku” correlation
The Ep-Eiso “Amati” correlation
Isotropic luminosity
Isotropic energy

16. The Ep-Flux plane and the outliers of the Ep-Liso correlation
Nava et al submitted

17. Ep-Fluence or Ep-Peak flux show strong correlations
CONCLUSIONS (II)
Ep-Fluence or Ep-Peak flux show strong correlations
Add bursts without redshifts (+ a complete BATSE sample)
Strong Ep-Fluence correlation
Strong Ep-Peak flux correlation
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

18. 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!

19. 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

20. Physical origin for this !
Ep-Liso is equivalent to Ep(t)-Liso(t)
Ep-Liso correlation found with time integrated spectra holds also within a burst!!
Ep-Liso “Yonetoku”
Physical origin for this !
Bosnjak et al (to be subm.)

21. Interpretation of the <Epeak>  Eg,iso0.5
(4) Interpretation – Thermal BB
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  , , , , [Spectrum is thermal black body in the inital phase (~2 sec), later a non-thermal component dominates.]
Bosnjak, Celotti, Ghirlanda 
Ryde 2005, 2006  Fit with Black body + Powerlaw

22. Black Body + powerlaw fits
Thermal components in GRB spectra
Black Body + powerlaw fits
Band model fits

23. Thermal interpretation of the Amati relation
Time integrated spectrum = sum of time resolved
Time resolved spectra (BB+PL)
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

24. Last slide … more News: the Ep-Eg corrlation
Pre swift-era bursts
Swift era bursts (up to March 2007)
Swift era bursts of the last year
Jet Breaks from the Optical
Ghirlanda et al. 2007