Transcript GRBs: open issues

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
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’
Two flavours, long and short
Paciesas et al (2002)
Briggs et al (2002)
Koveliotou (2002)
SHORT
Shortest 6 ms
GRB 910711
LONG
Longest ~2000 s
GRB 971208
Short – Hard
Long - Soft
Spectra
Epeak
F ~ Ea
F ~ Eb
featureless continuum
power-laws - peak in F
Non thermal spectra
1997: The BeppoSAX satellite
NFI
Italian-Dutch
“Satellite per l’ Astronomia X”
Instruments
Wide Field Cameras:
5% of sky – positioning ~ 4’
+
Narrow Field Instruments
arcmin resolution
Slewing in several hours
Discovery of first afterglow!
28 February
GRB 97 0228
3 March
Optical id. host galaxy: redshifts
Cosmological origin !
~120 / 3000 with z: <0.1 – 6.3
(Batse, SAX, HETE-II, Integral, Swift, …)
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
GRB are powerful
Planck power:
Mc2
Rg/c
=
c5
G
= 3.6x1059 erg/s
• AGN: L < 1048 erg/s
• SN: L < 1045 erg/s (in photons)
• GRB: L < 1053 erg/s
“first light” & PopIII
Probes of
far universe
chemical evolution
large scale structures
cover the epoch of re-ionization
Star Formation Rate
SNIa
Huge energy
Small Volume
Fireball
Invented even before knowing that GRBs are cosmological….
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 L=1051 erg/s and T~1 MeV
1986 Goodman: Tobs remains T during expansion. Doppler balances
adiabatic cooling
1992 Pure fireball made by    e+e- . Focussing by gravitation
e+e-

NS

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 L=1051 erg/s and T~1 MeV
1986 Goodman: Tobs remains T during expansion. Doppler balances
adiabatic cooling
1992 Pure fireball made by    e+e- . 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
Why internal shocks?
Spikes have
same duration
A process
that repeats
itself
“The” model:
Internal/External
Shocks
Rees-Meszaros-Piran
Shell still opaque
Relativ. e- + B:
synchrotron??
Relativ. e- + B:
synchrotron
Progenitors
Matheson et al. 2003
GRBs associated with SN (Ib,c)
SN
afterglow
A few spectroscopic ident. (underluminous?) Afterglow re-brightening
Faint (mR ~ 25 ) galaxies
Sites of star formation
Low metallicities
Bloom et al. 2002
Host galaxies
Progenitors
core collapse of massive stars (M > 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
compact object mergers (NS-NS, NS-BH)
short GRBs
Discriminants: host galaxies, location within host, duration,
environment, redshift distribution, ...
The engine
Formation of a spinning BH + dense torus,
sustaining B ~ 1014-1015 G
Accreting
torus
Extraction BH spin energy (0.29 MBHc2)
Extract E > 1052 erg
tGRB ~ 104 tdyn
Jets
Jet effect
Jet half
opening angle
G >> 1/
Log(F)
G  1/
Jet
break
Log(t)
GRB Jet measure
“Jet break”
Jet break time tbreak
Jet opening angle 
Israel et al. 1999
“True” energetics
Etrue = Eiso (1 – cos )
Bloom et al. 2003
Frail et al. 2001
Isotropic equivalent energy
Peak energy – Isotropic energy Correlation
Amati et al. 2002
9+2 BeppoSAX GRBs
Nava et al. 2006; Ghirlanda et al. 2007
1- cos qjet
Luminosity distance
Ghirlanda, Ghisellini, Lazzati & Firmani 2004
redshift
Problems:
1: Efficiency
Efficiency=Radiated/total energy
Only the RELATIVE kinetic energy can be used!
Dynamical efficiency (%)
Shells of
equal masses
5%
Gfinal ~ (G1G2)1/2
Shells of
equal
energies
A lot of kinetic energy should remain to power the afterglow
Piro astro-ph/0001436
Prompt
SAX X-ray
afterglow
light curve
Willingale et al. 2007
SWIFT
Eafterglow < Eprompt
Problems:
2: Early “afterglow”
Piro astro-ph/0001436
Good old times
Now: a mess
GRB 050904
z=6.29
Panaitescu 2006
X
Opt.
X-ray and optical behave differently
X-rays:
steep-flat-steep
TA
Early (normal) prompt: G>>1/qj
Late prompt: G>1/qj
”real”
afterglow
Late prompt: G=1/qj
Late prompt: G<1/qj
Ghisellini et al. 2007
Long lasting engine??
• Rs/c ~ 10-4 s (for a 10 solar mass BH)
• Even 10 s are 105 dynamical times
• Two-phase accretion?
Conclusions
“Paradigm”: internal+external 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…)
There can be a Black Body … but
Ghirlanda et al. 2007b
Time integrated
spectrum
Time resolved
spectra
The same occurs
for ALL GRBs
detected by
BATSE and with
WFC
Memory
EF(E)
cts/sec
Ghirlanda PhD thesis
Epeak =509
keV
Epeak = 390
keV
EF(E)
EF(E)
Time [sec]
Epeak =503
keV
Epeak = 416 keV
GRB spectrum evolves with time within single bursts
a
b
k
Epea
phot /cm^2 sec
Hard to Soft
evolution
Epeak, a(t), b(t)
Decrease independent
of the rise and decay
of the flux
Tracking evolution
Photon flux
Correlated with
Epea
Epeak(t), a(t) , b(t)
k
a
b
time
By construction, internal shocks
should all be equal.
Then, why does the spectrum evolve?
a
b
Epeak
Fishman & Meegan 1995
Spectra
Spectra
Prompt radiation:
Synchrotron?
Energy spectrum of a cooling electron
Fast cooling
+ synchro:
E()
N()
 -1/2
 -3/2
Typical synchrotron frequency
syn
= 3.6x106Bg2G/(1+z) Hz
Magnetic field from:
LB =
eBLkin
R2G2B2c =
eBLshell
Size from:
R ~ R0G2 (internal shock)
1/2 1/2
B ~
eB Lshell
G3 R0
Electron energy from:
gmec2=eempc2(G’-1) ~ eempc2
g ~ eemp/me
Synchrotron g-ray emission?
2 1/2
e1/2
e
B
e Lkin,shell,50
hsyn ~ 400 keV
2
G3 R0,7 (1+z)
tcool ~ 10-7
ee
3
(G/100)
2
MeV
sec
Extremely short - No way to make it longer
tcool << tdynamical ~ 10-2 sec
It must be short, if not, how can the flux vary?
0.2 ms
More than exponential
F ~ 1/3
Preece et al.
F ~ -1/2
-2
-1
0
Low energy photon power law index
1
-a
N(E) = kE
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 eneeded, too much IC
Self Compton? No, tcool too small even
in this case
Clustering of the
optical luminosities
s=0.48
Nardini et al. 2006
Flux vs observed time
s=0.28
Nardini et al. 2006
Luminosity vs rest frame time
Swift GRBs
Pre-Swift
21
3
+Swift
5
Dark??
27
Lx @ 12 hours
pre-Swift (19)
Including Swift
(30)
G1=100, G2=200
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 T’4
~ (R/G)2 s T4
G = 1/q
Liso ~ R2 (L /Liso)T4  Epeak ~ Liso1/2L-1/4
A short history of fireballs
Short Bursts
The spectrum of short bursts
harder
softer
harder because a is
harder, Epeak is the
same or even smaller
Ghirlanda et al. 2004
long
short
Log Epeak
Log a
Density
Star forming regions are dense
GRB – Afterglow – Temporal
GRB emission
Properties
in X, Optical
Panaitescu & Kumar
Firmani et al. 2004
L-1.5+-0.05
No corr.
Lx > 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
Frail et al. 2001
Frail et al. 2001
Best n
from fits
Bloom et al. 2003
Universal energy reservoir?
Frail et al. 2001
Same energy, different angles?
Structured jets
qjet
E(q)=const
E(q)=E0q-2
f(L) = kL-2
qview
Universal Epeak?
Preece et al.
~200 keV, observer frame
BATSE
HETE II
1/2
Epeak = 100 keV Eiso,52
+
Eiso = Etrue/Q2
Epeak = 1/Q
Amati et al. +Lamb et al.
The “Amati et al.” relation