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Very long GRBs. Maxim Barkov Space Research Institute, Russia University of Leeds, UK, Serguei Komissarov University of Leeds, UK. I. Gamma-Ray-Bursts. Discovery: Vela satellite (Klebesadel et al.1973); Konus satellite (Mazets et al. 1974); - PowerPoint PPT Presentation
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Maxim BarkovSpace Research Institute, Russia
University of Leeds, UK,
Serguei KomissarovUniversity of Leeds, UK
Very long GRBs
21.04.23 1HEA-2008, IKI
I. Gamma-Ray-Bursts
Discovery: Vela satellite (Klebesadel et al.1973);
Konus satellite (Mazets et al. 1974); Cosmological origin: 1. Beppo-SAX satellite – X-ray afterglows (arc-minute resolution) , optical afterglows – redshift measurements – identification of host galaxies (Kulkarni et al. 1996, Metzger et al. 1997, etc)
2. Compton observatory – isotropic distribution (Meegan et al. 1992);
Supernova connection of long duration GRBs: 1. Association with star-forming galaxies/regions of galaxies; 2. Few solid identifications with supernovae, SN 1998bw, SN 2003dh and others... 3. SN bumps in light curves of optical afterglows. 4. High-velocity supernovae ( 30,000km/s) or hypernovae (1052erg).
21.04.23 2HEA-2008, IKI
Bimodal distribution (two types of GRBs?):
Spectral properties: Non-thermal spectrum from 0.1MeV to GeV:
long durationGRBs
short durationGRBs
very long duration GRBs21.04.23 3HEA-2008, IKI
Relativistic jet/pancake model of GRBs and afterglows:
jet at birthpancake later
21.04.23 4HEA-2008, IKI
II. Models of central engines
(1) Potential of disk accretion onto stellar mass black holes:
disk binding energy:
thin disk life time:
Ultra-dense Hyper-Eddington neutrino cooled disks !!!
- gravitational radius
- disk outer radius
- accreted mass
Shakura–Sunyaev parameter
21.04.23 5HEA-2008, IKI
GRBs Jet magnetic acceleration:
•We get MHD acceleration of relativistic jet up to ≈300•Conversion of magnetic energy to kinetic one more than 50%•Acceleration have place on long distance req≈2rlc
•Jet is very narrow θ<2 (Astro-ph:0811.1467)
21.04.23 6HEA-2008, IKI
(???)1/ 2 cMEBZ
III. Magnetic Unloading
22
227
50106.3 MafEBZ
22
2
11 a
aaf
That is criteria ofexplosion start?How it happens?
21.04.23 7HEA-2008, IKI
The disk accretion makes easier the explosion conditions. The MF lines shape reduce local accretion rate.
10/1/ 2 cMEBZ
MNRAS (2008) 385L, 28 and arxiv:0809.1402 21.04.23 8HEA-2008, IKI
IV. Realistic initial conditions
• Strong magnetic field suppress differential rotation in the star (Spruit et. al., 2006).
• Magnetic dynamo can’t generate big magnetic flux (???), relict magnetic field is necessarily?
• In close binary systems we could expect fast solid body rotation.
• The most promising candidate for long GRBs is Wolf-Rayet stars.
21.04.23 9HEA-2008, IKI
)~(tR
BH
starR
)(rl
If l(r)<lcr then matter falling to BH directly
If l(r)>lcr then matter goes to disk and after that to BH
Agreement with model Shibata&Shapiro (2002) on level 1%
Simple model:
21.04.23 10HEA-2008, IKI
Power low density distribution model
3 r
21.04.23 11HEA-2008, IKI
Realistic model
M=35 Msun, MWR=13 Msun
Heger at el (2004)
21.04.23 12HEA-2008, IKI
Realistic model
M=35 Msun, MWR=13 MsunM=20 Msun, MWR=7 Msun
neutrino limit
BZ limit
Realistic model
Heger at el (2004)
21.04.23 13HEA-2008, IKI
Uniform magnetization R=150000km
B0= 1.4x107-8x107G
v
B
v
B
v
v
v
V. Numerical simulations: Collapsar model
GR MHD 2D
black holeM=10 Msun a=0.45-0.6
Setup
Bethe’s free fall model,T=17 s, C1=23
Initially solid body rotation
Dipolar magnetic field
21.04.23 14HEA-2008, IKI
Griped MF and angular momentum
3/42
00 1
sin,
fft
t
r
RlRl
mtffmm
mtffmr
rRifttRr
rRifttRBB 3/2
,13
3/4,
20
/1
/1cossin
mtffmm
mtffm
rRifttRr
rRifttRBB 3/5
,23
3/1,
20
/1
/12
2
sin
)(9
2
1
3
,
3/2
,
m
mffm
ffmmmt
rGM
rt
ttrr
21.04.23 15HEA-2008, IKI
a=0.6 Ψ=3x1028 a=0.45 Ψ=6x1028
Model a Ψ28 B0,7 L51 dMBH /dt η
A 0.6 1 1.4 - - -
B 0.6 3 4.2 0.44 0.017 0.0144
C 0.45 6 8.4 1.04 0.012 0.049
21.04.23 16HEA-2008, IKI
VI. Conclusions
+ Black holes of failed supernovae can drive very powerful GRB jets via Blandford-Znajek mechanism if the progenitor star has strong poloidal magnetic field;+ Blandford-Znajek mechanism of GRB has much lower limit on
accretion rate to BH then neutrino driven one (excellent for very long GRBs);
• The Collapsar is promising models for the central engines of GRBs.• Theoretical models are sketchy and numerical simulations are only now beginning to explore them. • Our results suggest that:
- Blandford-Znajek mechanism needs very hight magnetic flux or late explosion;
± All Collapsar model need high angular momentum, common envelop stage could help.
21.04.23 17HEA-2008, IKI
IV. Discussion
Magnetically-driven stellar explosions require combination of (i) fast rotation of stellar cores and (ii) strong magnetic fields.
Can this be achieved?
• Evolutionary models of solitary massive stars show that even much weaker magnetic fields (Taylor-Spruit dynamo) result in rotation being too slow for the collapsar model (Heger et al. 2005)
• Low metallicity may save the collapsar model with neutrino mechanism (Woosley & Heger 2006) but magnetic mechanism needs much strongermagnetic field.
• Solitary magnetic stars (Ap and WD) are slow rotators (solid body rotation).
21.04.23 18HEA-2008, IKI
• The Magnetar model seems OK as the required magnetic field can be generated after the collapse via dynamo inside the proto-NS (Thompson & Duncan 1995)
• The Collapsar model with magnetic mechanism. Can the required magnetic field be generated in the accretion disk?
- turbulent magnetic field (scale ~ H, disk height)
- turbulent velocity of -disk
Application to the neutrino-cooled disk (Popham et al. 1999):
21.04.23 19HEA-2008, IKI
Inverse-cascade above the disk (Tout & Pringle 1996) may give large-scale field (scale ~ R)
This is much smaller than needed to activate the BZ-mechanism!
• Possible ways out for the collapsar model with magnetic mechanism.
(i) strong relic magnetic field of progenitor, =1027-1028 Gcm-2;
(ii) fast rotation of helium in close binary or as the result of spiral-in of compact star (NS or BH) during the common envelope phase (e.g. Zhang & Fryer 2001 ). In both cases the hydrogen envelope is dispersed leaving a bare helium core.
21.04.23 20HEA-2008, IKI
Required magnetic flux , =1028 Gcm-2, still exceeds the highest value observed in magnetic stars.
• Accretion rate through the polar region can strongly decline several seconds after the collapse (Woosley & MacFadyen 1999), reducing the magnetic flux required for explosion (for solid rotation factor 3-10, not so effective as we want);
• Neutrino heating (excluded in the simulations) may also help to reduce the required magnetic flux;
• Magnetic field of massive stars is difficult to measure due to strong stellar winds – it can be higher than =1027 Gcm-2 ; • Strong relic magnetic field of massive stars may not have enough time to diffuse to the stellar surface, td ~ 109 yrs << tevol , (Braithwaite Spruit, 2005) 21.04.23 21HEA-2008, IKI
log10 (g/cm3), magnetic field lines, and velocity vectors 21.04.23 50HEA-2008, IKI
log10 21.04.23 51HEA-2008, IKI
log10 B/Bplog10 B21.04.23 52HEA-2008, IKI