Transcript Slide 1

Agnieszka Janiuk
N. Copernicus Astronomical Center, Warsaw
Gamma Ray Bursts from
Collapsing Massive Stars
Collaborations:
R. Moderski (CAMK), D. Proga(UNLV), Y. Yuan (ChAS),
R. Perna (JILA), T. Di Matteo (CMU), B. Czerny (CAMK),
D. Cline (UCLA), S. Otwinowski (CERN), C. Matthey
(CERN)
To emit fireball, the engine must be
very energetic. To produce shocks,
the engine must be active and
variable for a long time
1014
cm
Gamma rays are believed to come
from the internal shocks, produced in
the relativistic (Γ>100) fireball.
Digression: what makes the jet?
• Jets are ubiquitous in nature: AGNs,
QSOs, XRBs, YSOs, GRBs...
• They are not required by any physical
law (such as energy conservation).
• The 3 proposed mechanisms of jet
acceleration:
– Radiation pressure
– Thermal expansion
– Magnetic fields and rotation
• Jet is domineted by
– Poynting flux (small scales)
– Matter (large scales)
• Jets are collimated by:
– Accretion disk/coronal walls
– Pressure gradients in the
environment
– Surrounding matter-dominated jet
– Poynting-jet is able to collimate
itself through the toroidal B field
Fragile, 2008 (arXiv:0810.0526)
The model of a central engine for GRB
must answer, which astrophysical
process produces the relativistic fireball
that emits gamma-rays.
Important constraints (Piran 2005):
Timescales and variability: dt/T
~ 10-3 – 10-4 for 80% of bursts
 Short and long GRB dichotomy

Short - hard GRBs (T90<2 s)
Long -soft GRBs (T
90 > 2s)
 Energy (significant fraction of the binding

energy for compact object)
o
o
 Collimation (1 < θ < 20 )
-5 per year per galaxy)
 Rates (about 3 ×10
To emit fireball, the engine
must be very energetic. To
produce shocks, the engine
must be active and variable
for a long time.
The most popular model invokes
the internal shocks
in the jet that produce gamma
rays and variability (Sari & Piran
1997).
Also, variability can be well
reproduced with a shot-gun
model (Heinz & Begelman
1999).
Janiuk, Czerny, Moderski et al. (2006)
Kinematic jet model: theoretical lightcurves of long
GRBs, depending on the observer’s viewing angle
Tested against observations: lightcurves, PDS
spectra; Prokopiuk & Janiuk, in prep.
Collapse of massive star favored for long GRBs:
- associacion with star forming galaxies (e.g. Fruchter et al. 2006)
- concurrent “SN-like” outbursts (Bloom et al. 1999; Stanek et al. 2003)
- redshift distribution follows the star formation rate (Coward 2007)
Supernovae
type I: rapid lightcurve evolution
- type Ia: standard;
- type Ib: He lines produced in
the massive ejecta, by non-thermal
excitation by fast particles emitted by the
(56)Ni -> (56)Co-> (56)Fe decay.
- type Ic: progenitor must be
either an extreme WR star, or a binary
(Nomoto 1995)
- type II: progenitor is a massive red
giant
Supernovae observed in
associacion with GRBs
- SN1998 bw: GRB 980425
- SN 2003 dh: GRB 030329
- SN 2003 lw: GRB 031203
- SN 2006 aj: XRF 060218
All of these are Type Ic
All have broad line spectra ->
ejection velocities ~ 50,000 km/s
They account for 20% of the
BL SN Ic = 2% of all SN Ic
Hypernova:
- very high expansion velocity
- bright luminosity
- postulated to be an energetic outburst
produced by a collapsar (Woosley 1993;
Paczyński 1998)
- very strong explosion energy (> few x 1051
ergs)
- strong evidence for assymetry (Nomoto et al.
2005)
- massive star models fit well the observed
hypernovae ( Mazzali et al. 2006)
- large uncertainty in modeling due to the initial
mass function of massive stars (5-40% core
collapse SN form the black hole; Fryer &
Kalogera 2001)
Eta Carinae: future
candidate for hypernova
Rates of Supernova vs. Hypernova
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Hypernovae are rare (about 1000 times less frequent than normal
SN; (Soderberg et al. 2006)
All hypernovae have been classified as Ib/c SN (no H lines, nor He
lines in the spectra); probably a subset of them
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Rate of all core-collapse SN: 6x10-3 /yr/galaxy ( Fryer et al. 2007)
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Type Ib/c are 15% of all core collapse
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Hypernovae are 5-10% of observed type I b/c
1-10% of SN Ib/c can be associated with GRBs; this coincides with
that of hypernovae
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Rate of all core-collapse increases with redshift (no specific data
for I b/c or hypernova)
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The collapsar
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Woosley (1993): SN Type Ib 'failed' because of
a fast rotation of the Wolf-Rayet star
Paczyński (1998): some GRBs must be linked
to the cataclysmic deaths of massive stars ->
hypernovae
MacFadyen & Woosley (1999) and follow up
works: hydrodynamical computations of the
relativistic jet propagation through the stellar
envelope
Two reasosns for SN to fail (Fryer 1999):
 Large ram pressure at the top of the convective
zone
 Large binding energy for the most massive stars
GRB progenitors: the most massive stars,
that fail to produce an explosion under
the standard core-collapse supernova
MacFadyen & Woosley (1999)
The collapsar engine of a GRB
Must form a black
hole in the center of
the star
Must produce
sufficient angular
momentum to form a
disk around black
hole
Must eject the
hydrogen envelope,
so that the jet can
punch out of the star
Progenitors of the I b/c Supernovae: single or binary stars?
Most massive stars (Mass > 20 solar
masses; Hirshi et al. 2004)
Wolf-Rayet stars: have lost the H envelope
due to strong winds
Single stars: only fast rotating stars above
solar metallicity produce strong shocks and
eject lots of nickel (Heger at al. 2003)
Fast rotating stars can mix their envelopes,
burning effectively H into He (Yoon &
Langer 2005)
Binary stars: mass transfer can eject matter
and lead to He star formation.
Possibly, >75% of all massive stars are in
close binaries (Kobulnicky et al. 2006)
SN 2008D
Single stars: only fast
rotating stars above
solar metallicity
produce strong
shocks and eject lots
of nickel (Heger at al.
2003)
This GRB rate
must
be lower by a
factor
Indicating the
fraction
of stars that
retain large
angular
momentum
Metallicity measurements
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Wind mass loss sensitive to
metallicity
At lower metallicities, weaker winds
allow more massive cores => GRBs
probably will not occur above solar
metallicity
Nebula NGC 2359
Metallicity measurements:
 Absorption lines in the GRB
afterglows
 Emission lines of HII regions in the
GRB host galaxy
 Interstellar extinction in the host
galaxy
 Morphology of the host galaxy, e.g.
Compared to SMC/LMC
There is no consistent picture: direct measurements argue for higher Z,
while indirect measurements indicate lower Z.
Progenitors of Hypernovae
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Most of the currently discussed progenitors do not
distinguish between fallback and direct collapse
black holes
GRBs probably will not occur at solar metallicity, if
we need a direct collapse to black hole. At lower
metallicities, weaker winds allow more massive
cores.
Below ~0.4 ZSun, the stars cannot loose the He
envelope (Heger 2003).
Star is either born rotating rapidly, or is spun up by
interaction (tidal forces, merger). In binaries, the
companion is used to strip off the hydrogen
envelope without the angular momentum loss.
WR124
Single stars can also loose the H envelope because
of mixing and burning to He (Yoon & Langer 2002).
But if the He envelope is also lost, these models are
ruled out.
„Constraints are more restrictive for single-star models, but without better
understanding of winds we cannot say more” (Fryer et al. 2007).
(Janiuk, Proga, Moderski. 2008a, 2008b)
How long is a long GRB?
Chemical composition and density distribution in the pre-SN star (Woosley &
Weaver 1995)
How the pre-collapse star rotates?
The distribution of specific angular momentum in the pre-SN star
unknown.
Stellar evolution models:
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Neglect centrifugal forces
Do not accurately treat the angular momentum transport through
magnetic fields
Sensitive to the loss of ang. momentum through wind
Some assumptions we have made:
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Polar angle dependence (differential rotation)

Radius dependence (rigid rotation, with a possible cut-off on lspec)
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Constant ratio of centrifugal to gravitational forces
lspec = l0 (1-cos θ)
lspec = l0 (r/rin)sin2θ
(Janiuk et al. 2008a, 2008b)
Conditions for torus existence
The rotation must prevent the envelope material from the
radial infall onto BH.
Specific
angular
momentum
lspec > lcrit = 2GMBH/c (2-A+2(1-A)1/2)1/2
BUT:
–
Black hole mass is growing fast (accretion rate of 0.01-1 Msun/s)
–
Spin can be changing
=> the GRB is emitted only until l>lcrit is satisfied.
The black hole grows due to accretion
The time evolution of the collapsar => iterative procedure
1. BH mass = iron core mass
2. Envelope schells accrete
3. Check for conditions given by the changing BH mass and
spin
Various possible accretion scenarios
GRB requires: large accretion
rate and spinning BH
Schwarzschild and Kerr BH case:
Janiuk & Proga, 2008, ApJ, 675, 519;
Janiuk, Moderski & Proga, 2008, ApJ, in
press;
Hyperaccretion: neutrino-cooled
disk
Electron-positron capture and
beta-decay
p + e- → n + ν-e
n + e+ → p + νe
n → p + e- + νe
- emission
Thermal
e+ + e- → νi + νi
n -+ n → n + n + νi + νi
γ →νe + νe
- Cooling mechanisms: neutrino emission,
advection, Helium photodisintegration,
radiation
- Neutrinos can be absorbed and scattered
- Equation
of state should treat the species
under the condition of reactions equilibrium
and supplemented by the charge neyutrality
condition
ν
(Popham, Woosley & Fryer 1999; Di
Matteo, Perna & Narayan 2002; Kohri &
Mineshige 2002; Janiuk et al. 2004; Kohri,
Narayan & Piran 2005; Janiuk et al. 2007;
Chen & Beloborodov 2007)
p
e- n
+
α e
 The disk accreting at rates > 0.1 MSun
s-1 is so hot and dense (T~1010-1011
K, ρ~1010-1012 g cm-3) that the
plasma is totally opaque to photons,
and neutrinos can also be trapped
 Energy from the disk can be
extracted by neutrino annihilation
 Alternatively, energy can be extracted
by the magnetic field and spinning
black hole (the Blandford-Znajek
mechanism)
Chen & Beloborodov (2007)
Efficiency of neutrinos depends on initial accretion
rate, and decreases in time
Janiuk et al. (2004)
Chen & Beloborodov (2007)
Neutrino annihilation inefficient when accretion rate < ~0.01 Msun/s.
This will slightly depend on viscosity and black hole spin.
We end up with three kinds of jets from
the collapsar:
0-1.5 s
0-130 s
Precursor jet,
powered by v-v
large m, small A
0-430 s
First jet,
powered by
both v-v- and BH
rotation
large m, large A
Second jet, powered
by BH rotation
.
small m, large A
Lazzati (2005)
Precursors found in ~20% of
BATSE sample
GRB 0803319B
Brightest optical
counterpart: mv = 5.3
Empirical model of a 2-component jet fitted
with two opening angles (Racusin et al. 2008)
Image from “Pi of the sky”,
http://grb.fuw.edu.pl
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Instabilities in the accreting torus: possible mechanism of causing a long time gap
between the precursor and the burst, or the short-term variability seen in the
prompt phase (Wang & Meszaros 2007).
Precursor phase in the prompt emission seen in some GRBs, might be produced
by the jet breaking through envelope (Paczyński 1998; Ramirez-Ruiz et al. 2002)
Recent hydro Simulations by Morsony, Lazzati & Begelman (2007) found three
distinct phases during the jet propagation: precurosr jet, shocked phase and
unshocked phase.
Density, pressure and
gamma_inf at time 30 s.
Cocoon: high ρ, high P,
low Γ;
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Precursor: high Γ, offaxis;
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Shocked jet: low ρ, high
P, high ρ;
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Unshocked jet: low ρ,
low P, high Γ
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Instabilities in the accretion disk:
- May be related to the late-time activity of the GRB (such as X-ray flares; e.g. Perna
et al. 2006)
- proposed as the sources of gravitational waves, that may probe the angular
momentum of the collapsing star (Fryer et al. 2002)
- Black hole spin can be coupled to the disk, enhancing the strength of the instability,
then possibly detectable by LIGO (van Putten 2005)
Thermal instability: the local density and pressure drops, while the temperature increases.
Our solution is based on the detailed treatment of the EOS, coupling the beta-equilibrium
and the neutrino trapping effects, as well as including the information of the chemical
composition in the process of Helium photodisintegration.
Janiuk et al. (2007)
Summary
GRB long durations may provide constraints for the rotation law in the pre-SN star.
The minimum accretion rate limit for the neutrino-powered jets, in the Schwarzschild
black hole models, results in GRB durations up to 40-100 s.
The minimum accretion rate and BH spin limit, for jets powered by both neutrinos and
black hole spin, results in GRB durations up to 50-130 s.
The above values will be smaller if the H/He envelope was already stripped
In the Kerr black hole models, we find the solutions corresponding to three kinds of jets:
precursor jet, early jet and late jet, powered by different mechanisms. Possibly, the
opening angle of these jets is changing, which would have some observational
consequences.
The instabilities in the accreting torus play important role for the observed emission
Constraints on the GRB progenitor
from observations and SN models
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Type Ic SN => progenitor must loose the H and
most of He envelope
Occur in the brightest parts of galaxies => come
from the most massive stars
Occur in metallicities from 0.01 to 1 => single star
models strongly constrained
Single star models may require mixing to burn H
into He effectively
Binary star models fit better to the observational
constraints
Thank you