G. Ghirlanda + G. Ghisellini, L. Nava,
Z. Bosnjak, C. Firmani, I. Cabrera, F.
Tavecchio & A. Celotti
[email protected]
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
Epeak  Eiso0.57
Q.
2red=7.2 (60 dof)
s=0.2
Epeak  Eg1.05
Liso  Ep1.62 T45-0.49
 =0.89 (23 dof)
 =0.7 (16 dof)
Spectral-energy
correlations:
s=0.08
s=0.06
true physics or selection effects?
2
2
red
Amati et al.
2002
Ghirlanda
et al. 2004
Physical interpretation
(e.g. Rees & Meszaros 2006,
Liso  t - 1.05& Rees
Thomson, Meszaros
2red=9.6 (34 dof)
2006)
Liso0.57  Epeak
(IV) Thermal component?
Norris et
al. 2000
red
Firmani et al.
2005
Study selection effects
Liso  V 3.0
2red=100 (46 dof)
1.85
(I) GRBs with z
(II) + GRBs without z
Yonetoku et
(III)
al. 2004
Still not
(V) news
Reichart et al. 2000
Ramirez-Ruiz &
convinced
?
Fenimore 2000
Epeak – Eiso
Amati et al. 2002, 2006, Nava et al.
2006, Ghirlanda et al. 2008 … etc
0.470.03
E peak  Eiso
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)
Slope
of the 76 with
GRBs
Evolution
redshift? NO
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)
No segregation in z
in the obs. frame
obs
Obs Frame  Epeak - F
low fluence –
intemediate/high
Epeak
High fluence –
intemediate/low
Epeak
Trigger threshold:
which flux to trigger?
Spectral threshold: which
fluence to measure Epeak?
(Band 2003, 2006)
Fmin  N min
Assume GRB spectrum
a=1
Epeak
b=2.3
Fbol
obs
E peak
Aeff ( E )
Background
Detector
response
 the error on Epeak(fit) < 100% in
97.7% of cases
 Fit with single powerlaw is
excluded at 2σ
(Band 2003, 2006)
BATSE could not
detect this burst
Spectral threshold: which
fluence to measure Epeak?
BATSE could
detected this burst
BATSE certainly
detected this burst
and measured Epeak
Ghirlanda et al. 2008
Trigger threshold:
which flux to trigger?
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
obs
0.39 0.05
E peak
 Fbol
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)??
HOW is populated the Ep-Fluence plane?

Add GRBs without redshift
From the literature
Sakamoto et al. 2005
Butler et al. 2007 (freq)
Kaneko et al. 2005
GCNs (Golenetski et al. …)
Nava et al. 2008 submitted
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
BATSE bursts
Outliers of the Epeak – Eiso correlation
Nava et al. 2008 submitted
6% of BATSE bursts are outliers
The Ep-Liso “Yonetoku” correlation
Isotropic luminosity
The Ep-Eiso “Amati” correlation
Isotropic energy
Nava et al. 2008 submitted
The 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)
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
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
Ep-Liso is equivalent to Ep(t)-Liso(t)
Ep-Liso correlation found with time
integrated spectra holds also within a burst!!
Bosnjak et al. 2008 (to be subm.)
(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 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
Thermal components in GRB spectra
Black Body +
powerlaw fits
Band model fits
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
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