Gamma Ray Bursts: open issues

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Gamma Ray Bursts open issues
Gabriele Ghisellini Osservatorio di Brera

• Brief history
• Power
• Short history of the paradigm internal vs
  external shocks
• Afterglows external shocks
• The spectral-energy relations
• GRBs for cosmology

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Gamma-Ray Bursts The story begins
Treates banning nuclear tests between USA and
USSR in early 60s VELA Satellites X and soft
g-ray detectors
Brief, intense flashes of g-rays
Klebesadel R.W., Strong I.B., Olson R., 1973,
Astrophysical Journal, 182, L85
Observations of Gamma-Ray Bursts of Cosmic
Origin
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Two flavours, long and short
SHORT
LONG
Short Hard Long - Soft
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Spectra
Non thermal spectra
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1997 The BeppoSAX satellite
Slewing in several hours
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Discovery of first afterglow!
3 March
28 February
GRB 28
97
02
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Optical id. host galaxy redshifts
Cosmological origin !
120 / 3000 with z lt0.1 6.3 (Batse, SAX,
HETE-II, Integral, Swift, )
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Energy and Power
Huge isotropic equivalent energy!
Assume Isotropy
GRB typical Fluence (i.e. time int. flux) is 10-8
10-4 erg/cm2 (1keV 10 MeV)
119 GRBs with z
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GRB are powerful

• AGN L lt 1048 erg/s
• SN L lt 1045 erg/s (in photons)
• GRB L lt 1053 erg/s

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• first light PopIII
• chemical evolution
• large scale structures
• cover the epoch of re-ionization
• Star Formation Rate

• Probes of far universe

SNIa
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Huge energySmall VolumeFireball
Invented even before knowing that GRBs are
cosmological.
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A short history of fireballs
1978 Cavallo Rees fireball photons trapped
by their own pairs 1978 Rees internal
shocks in M87 to transport energy along the jet
1986 Paczynski Cosmological GRB? L1051 erg/s
and T1 MeV 1986 Goodman Tobs remains T during
expansion. Doppler balances adiabatic cooling
1992 Pure fireball made by n n ? ee- .
Focussing by gravitation
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ee-
n
NS
n
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A short history of fireballs
1978 Cavallo Rees fireball photons trapped
by their own pairs 1978 Rees internal
shocks in M87 to transport energy along the jet
1986 Paczynski Cosmological GRB? L1051 erg/s
and T1 MeV 1986 Goodman Tobs remains T during
expansion. Doppler balances adiabatic cooling
1992 Pure fireball made by n n ? ee- .
Focussing by gravitation 1992 Dirty fireball
polluted by baryons. Re-conversion of bulk
kinetic into radiation through shocks with
external medium 1994 Internal shocks due to
shells moving with different G
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Why internal shocks?
A process that repeats itself
Spikes have same duration
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The modelInternal/External Shocks
Rees-Meszaros-Piran
Shell still opaque
Relativ. e- B synchrotron??
Relativ. e- B synchrotron
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Progenitors
GRBs associated with SN (Ib,c)
A few spectroscopic ident. (underluminous?)
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Progenitors
core collapse of massive stars (M gt 30 Msun)
long GRBs Collapsar or Hypernova
(MacFadyen Woosley 1999)
GRB simultaneous with SN
Supranova two-step collapse
(Vietri Stella 1998)
GRB delayed by few months-years
?
Discriminants host galaxies, location within
host, duration, environment, redshift
distribution, ...
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The engine
Formation of a spinning BH dense torus,
sustaining B 1014-1015 G Extraction BH spin
energy (0.29 MBHc2)
Extract E gt 1052 erg tGRB 104 tdyn
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Jets
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Jet effect
? gtgt 1/?
? ? 1/?
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GRB Jet measure
Jet break
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True energetics
Isotropic equivalent energy
Epeak was not considered
Etrue Eiso (1 cos ?)
Frail et al. 2001
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Peak energy Isotropic energy Correlation
92 BeppoSAX GRBs
Epeak ? Eiso0.5
Amati et al. 2002
Epeak(1z)
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1- cos qjet
Ghirlanda (25)
Amati (62)
Nava et al. 2006 Ghirlanda et al. 2007
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GRBs
GRBs can be used as cosmological RULERS !
Ghirlanda, Ghisellini, Lazzati Firmani 2004
Luminosity distance
Supernovae
redshift
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Problems 1 Efficiency
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EfficiencyRadiated/total energy
Only the RELATIVE kinetic energy can be used!
Shells of equal masses
Dynamical efficiency ()
Shells of equal energies
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Gfinal (G1G2)1/2
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A lot of kinetic energy should remain to power
the afterglow
Prompt
SAX X-ray afterglow light curve
Piro astro-ph/0001436
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SWIFT
Willingale et al. 2007
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Eafterglow lt Eprompt
Eafterglow 0.1 Eprompt
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Problems2 Early afterglow
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Good old times
Piro astro-ph/0001436
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Now a mess
GRB 050904 z6.29
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X
Opt.
Panaitescu 2006
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X-ray and optical behave differently
Is this real afterglow? i.e. external shock?
X-rays steep-flat-steep
TA
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Ghisellini et al. 2007
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Long lasting engine??

• Rs/c 10-4 s (for a 10 solar mass BH)
• Even 10 s are 105 dynamical times
• Two-phase accretion?

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Conclusions

• Paradigm internalexternal shocks, synchrotron
  for both it helps, but it is limiting
• Efficiency is an issue
• Progenitors for long done. For short not yet
• Central engine? How long does it live?
• GRBs as probes of the far universe (continue)

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There can be a Black Body but
Time integrated spectrum
Time resolved spectra
Ghirlanda et al. 2007b
The same occurs for ALL GRBs detected by BATSE
and with WFC
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Memory
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Ghirlanda PhD thesis
cts/sec
EF(E)
Time sec
EF(E)
EF(E)
GRB spectrum evolves with time within single
bursts
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Hard to Soft evolution
phot /cm2 sec
Epeak, a(t), b(t)
Epeak
Decrease independent of the rise and decay of the
flux
a
b
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Tracking evolution
Photon flux
Correlated with
Epeak
Epeak(t), a(t) , b(t)
a
b
time
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By construction, internal shocks should all be
equal. Then, why does the spectrum evolve?
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Spectra
Spectra
na
nb
Fishman Meegan 1995
Epeak
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Prompt radiation Synchrotron?
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Energy spectrum of a cooling electron
Fast cooling synchro
E(n) n-1/2 N(n) n-3/2
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Typical synchrotron frequency
nsyn 3.6x106Bg2G/(1z) Hz Magnetic field
from LB eBLkin R2G2B2c
eBLshell Size from R R0G2 (internal shock)
Electron energy from gmec2eempc2(G-1)
eempc2
1/2
1/2
eB Lshell
B
G3 R0
g eemp/me
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Synchrotron g-ray emission?

• Extremely short - No way to make it longer
• tcool ltlt tdynamical 10-2 sec
• It must be short, if not, how can the flux vary?

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0.2 ms
More than exponential
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Fn n1/3
Fn n-1/2
Preece et al.
-2 -1
0 1 Low energy
photon power law index N(E) kE-a
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Can it be rescued by

• Reacceleration? No, in ISS e- are accelerated
  only once
• Adiabatic losses? No, too small regions would be
  involved, too much IC
• Self absorption? No, lots of e- needed, too much
  IC
• Self Compton? No, tcool too small even in this
  case

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Clustering of the optical luminosities
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Flux vs observed time
s0.48
Nardini et al. 2006
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Luminosity vs rest frame time
s0.28
Nardini et al. 2006
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Swift GRBs
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Pre-Swift
21
3
Swift
27
5
Dark??
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Lx _at_ 12 hours
pre-Swift (19)
(30)
Including Swift
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G1100, G2200
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Thompson, Meszaros Rees 2007
At R Rstar the fireball dissipates part of its
energy ? BB L q2 Liso ? q2 L/Liso Liso
R2 s G2 T4 (R/G)2 s T4
G 1/q
Liso R2 (L /Liso)T4 ?
Epeak Liso1/2L-1/4
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A short history of fireballs
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Short Bursts
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The spectrum of short bursts
harder
softer
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harder because a is harder, Epeak is the same or
even smaller
long
short
Ghirlanda et al. 2004
Log Epeak Log a
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Density
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Star forming regions are dense
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GRB Afterglow Temporal Properties
GRB emission in X, Optical
Panaitescu Kumar
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Why densities are so small?
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L-1.5-0.05
Firmani et al. 2004
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No corr.
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Lx gt Lopt Lopt more clustered than Lx vc between
optical and X-rays
same values of eB, ee, Ek,iso moderate cooling
(small eB) large Comptonization y
parameter different p
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Universal energy reservoir?
Best n from fits
Bloom et al. 2003
Frail et al. 2001
Frail et al. 2001
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Same energy, different angles?
Frail et al. 2001
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Structured jets
qjet
E(q)const
f(L) kL-2
E(q)E0q-2
qview
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Universal Epeak?
200 keV, observer frame
BATSE
Preece et al.
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HETE II
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The Amati et al. relation
1/2
Epeak 100 keV Eiso,52
Eiso Etrue/Q2
Epeak 1/Q
Amati et al. Lamb et al.
X-ray flashes