Transcript ppt
The prompt phase of GRBs
Dimitrios Giannios
Lyman Spitzer, Jr. Fellow
Princeton, Department of Astrophysical Sciences
Raleigh, 3/7/2011
Structure of the talk
Main properties of the prompt emission
Models for the GRB flow
Fireballs
Poynting-flux dominated flows
Internal shocks
Magnetic Reconnection
Radiation region
Thomson thin vs photospheric emission for the GRB
Fermi LAT bursts
Correlations: what can we learn for the central engine?
νfν
Nph (t)
Gamma-ray bursts: spectra and variability
E (MeV)
t (sec)
GRBs: ultrarelativistic jets
Clues
The prompt emission has
non-thermal spectral appearance Band et al. 1993; Preece et al. 1998
Rapid variability
The GRB-emitting flow is ultrarelativistic (γ>100, 300,
1000?) e.g. Piran 1999…
Big questions
Type of central-engine/Jet composition
How is the flow accelerated?
Which processes result in the observed GRB emission?
My focus: why and how do jets radiate?
?
Central
Engine
Acceleration Internal
dissipation
External
interactions
(How to tell a millisecond magnetar)
A millisecond neutron star has
rotational energy
DG 2010
E rot 3 10 52 Pms-2 erg
Extracted on a timescale of ~30 sec
for Bs 3 1015 G
Usov 1992; Thompson 1994; Uzdensky &
MacFadyen 2006; Metzger et al. 2007; 2011
After the GRB we are left with a
supermagnetar!
Contains E 1049 R 3 B 2 erg
B
6 16
The magnetic field decays fast
(100-1000yr; Thompson &
Duncan 1996)
May power SGR superflares ~100
times more powerful than that of
SGR 1806-20 in December 2004!
kouveliotou et al. 1998
GRBmagnetar flare
~10-3 yr-1
~10-5.5 yr-1
SGRs
galactic rate
Guetta et al. 2005
B Field
~1015 G
~1016 G
flaring @
~104 yr
~102-103 yr
Peak
luminosity
~1047 erg s-1
~1049 erg s-1
detectability
~25 Mpc
with BATSE, …
~250 Mpc
The driving mechanism:
MHD Energy Extraction and/or neutrino annihilation
B-fields extract rotational energy
from the compact object/inner
accretion disk at a rate
BR
EÝEM
B2R2
3
c
2
6
4
Neutrino annihilation energy
deposition rate (erg cm –3 s-1)
Blandford & Znajek 1977
Koide et al. 2001
van Putten 2001
Lee et al. 2001
Barkov & Komissarov 2008
Usov 1992
Uzdensky & McFadyen 2007
Bucciantini et al. 2007
Metzger et al. 2010
Ruffert & Janka 1999; Popham et al. 1999;
Aloy et al. 2000; Chen & Beloborodov
2007; Zalamea & Beloborodov 2011
General considerations: Acceleration
Important quantities of the flow:
luminosity L
L
Baryon
loading
1
2
mass flux M
Mc
Efficient acceleration can lead to γsr~η
Depending on the energy extraction mechanism, the flow can
be dominated by
Thermal energy thermal acceleration (Fireball)
Paczynski 1986; Goodman 1986; Sari & Piran 1991
Magnetic energy MHD acceleration (Poynting-flux dominated flow)
Usov 1992; Thompson 1994; Mészáros & Rees 1997; Drenkhahn & Spruit 2002; Lyutikov &
Blandford 2003
Fireballs
Go through fast acceleration r
Converting thermal energy into kinetic
Saturation takes place when
1.
2.
internal shocks
Parameters:
L, η, initial radius ro
thermal
component
photospheric
emission
almost all thermal
energy is used: γsr η
at the photospheric crossing γsr < η
Radiation and matter decouple when
τ~1
energy content
Photospheric emission takes place
kinetic
component
τ~1
distance r
Strongly magnetized jets
Recent progress in 2D axisymmetric
relativistic MHD simulations & theory
Vlahakis & Koenigl 2003; Komissarov et al. 2009; 2010; Tchekhovskoy
et al. 2009; 2010; Lyubarsky 2009; 2010
High magnetization flows accelerate to Γ>>1,
But most of the energy remains in the B field
Shocks are inefficient
Dissipative MHD processes are key to jet
emission (and acceleration)
Non-axisymmetric instabilities may
develop a large distance leading to
dissipation and emission e.g., Lyutikov & Blandford
2003; Narayan & Kumar 2008; Zhang & Yan 2011
The field is in general not
axisymmetric at the central engine
Model for GRBs: Magnetic field
changes polarity on small scales
and reconnects vrec=εc
Drenkhahn 2002 and Denkhahn & Spruit 2002;
see also McKinney & Uzdensky 2011
Dissipation is gradual and leads to
acceleration of the flow and
heating of plasma
The model predicts a strong
photospheric component and
optically thin dissipation
energy content
The reconnection model for GRBs
thermal
magnetic
component
×
× ×
r
B
photospheric
emission
thin emission
r
kinetic
component
×
τ~1
distance r
×
×
×
The prompt GRB
Prompt GRB
Central
engine
~106cm
Internal
dissipation
~1011-1017cm
Afterglow
External
interactions
~1017-1018cm
Where is the prompt emission produced?
in principle anywhere between the Thomson photosphere rph
(or slightly below) and the deceleration radius rd
Typically rph~1011cm and rd ~1017cm; in this range of radii:
density ~12 orders of magnitude
optical depth ~6 orders of magnitude
Different radiative mechanisms depending
on the location of the energy dissipation
Case 1: Thomson
thin dissipation
Case 2: Photospheric
dissipation
Dissipation in the Thomson thin regime
Big variety of spectra depending
on the various parameters:
єdiss -fraction of dissipated energy
єB, єe -fraction that goes to Bfields, fast electrons
Fraction ζ of accelerated electrons
Electron power-law index p
Distance of collision
Dominant processes: Synchrotron;
synchrotron-self-Compton
Similar for magnetic reconnection
at optically thin conditions!
Bosnjak, Daigne &
Dubus 2008
E*f(E)
Shocks accelerate particles and
amplify magnetic fields
E*f(E)
Photospheric emission
In the fireball the photospheric luminosity is e.g. Mészáros & Rees 2000
Lph
L
0.05
8 / 3 2/3
2.5 o,7
2/3
52
r
L
1/ 4
L52
, 1000
r
o,7
Spectrum quasi thermal Goodman 1986 (but not exactly black-body
Beloborodov 2011)
Energy dissipation (shocks, collisional heating) at τ ≥ 1 distorts the
spectra Mészáros & Rees 2005; Pe’er et al. 2006
In the reconnection model DG 2006; DG & Spruit 2007
Lph
L
0.16
L1/525 ( )1/3 5
2.5
, 150 L1/525 ( )1/3 5
Photospheric emission from the reconnection model
If fraction fe ~ 1 of the energy goes into heating the electrons then
Resulting emission spectrum with DG 2006; DG & Spruit 2007; DG 2008
heating-cooling balance gives the electron temperature everywhere in
the flow
Peak in the sub-MeV range
Flat high-energy emission
observed low-energy slope
Rather high efficiency Lph ~ 0.03…0.5L, for 100 ~
<η<
~ 1500
Dissipative photospheres: reconnection model
synchrotron emission
Fermi
Swift
η=1000
τ<<1
η=590
Robotic
telescopes
η=460
typically
observed
η=350
L
Ý 2
Mc
η=250
E (MeV)
τ~1
Compton scattering
DG 2006; DG & Spruit 2007; DG 2008
more models: Pe’er et al. 2006; Ioka 2010; Lazzati & Begelman 2010; Beloborodov 2010; Ryde et al. 2011
From the central engine to radiation
Millisecond magnetar
Spectrum
η
typical GRB
EÝ η
Metzger, Giannios, Thompson, Bucciantini & Quataert 2011
More dissipative photospheres
collisional heating; Beloborodov 2010;
f(E)
E*f(E)
Vurm et al.
f(E)
Pe’er et al. 2006
weak shocks; Lazzati & Begelman 2010
Recent Developments: GeV emission
counts
LAT emission: peaking with (late) MeV but lasts longer!
time
GRB 080916C; Abdo et al. 2009
Ghiselini et al. 2009
Physical origin of GeV emission is (in part?) different from the MeV
What to make of Fermi observations?
LAT ‘sees’ two components
(physically separated)
1. prompt
2. slow declining
Need to disentangle them
before constraining for the
prompt emission cite!
cannot assume a single
emission cite for MeV
and GeV (e.g. Zhang &
Pe’er 2009)
GBM
LAT
L
time
Correlations: what do we see?
Involve both time integrated and
instantaneous quantities (e.g.,
0.5
E peak E iso
, E peak (t) L0.5
)
iso(t)
Yonetoku et al. 2004
also Borgonovo & Ryde 2001; Liang et al. 2004;
Ghirlanda et al. 2004; Liang & Zhang 2006;
Firmani et al. 2006; Collazzi & Schaefer 2008…
Amati 2010
Firmani et al. 2009
Correlations: what can we learn?
Tendency for brighter
bursts to be cleaner?
Transparency of fireball
emerging from a collapsar?
Ave Peak Energy Epeak
Metzger et al. 2011; see also
DG & Spruit 2007
Peak Isotropic Jet Luminosity (erg s-1)
Morsony et al. 2011; Lazzati
et al. 2011; see also
Thompson et al. 2007
Interpretations within photospheric models for Epeak
Summary
The prompt emission most likely comes from internal dissipation
of energy in the fast flow
Internal shocks or Magnetic dissipation or …
Dissipation may take place in Thomson thin or thick conditions
Thin case: particle acceleration uncertainties єe, ζe, p, єB
The photospheric interpretation for MeVs is robust
Magnetic reconnection provides a promising process to power a
dissipative photosphere