maxim barkov space research institute, russia university of leeds, uk, serguei komissarov
DESCRIPTION
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 PresentationTRANSCRIPT
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