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G.E. Romero
Instituto Aregntino de Radioastronomía (IAR),
Facultad de Ciencias Astronómicas y Geofísicas, University of La Plata, Argentina.
ContentsContents• Introduction
• Observational Features and Their Implications
• Standard Radiation Models – Fireball Models
• Central Engines
– Popular Models
– Alternative Models
• Speculations
(1)(1) IntroductionIntroduction
• Most intensive transient gamma-ray sources ~ 10-5 erg cm-2 s-1, lasting about ~ seconds.
(Pulsars ~ 10-8 erg cm-2 s-1)
(AGN ~ 10-9 erg cm-2 s-1)
and randomly occur in time and space.
Discovery HistoryDiscovery History• Discovered in 1967
( Klebesadel, Strong and Olsen 1973)
• Pre-BASTE phase (before 1990)
– Rate ~ tens per year
– Cyclotron line features and galactic plane concentration Galactic neutron stars.
• BASTE phase (after 1991)– Rate ~ 300 per year– No cyclotron lines– Isotropic Distribution (Fig.1)– Deficiency of weak sources (Fig. 2)
Cosmological Origin.However, no counter parts were found !
• BeppoSAX phase (after 1997)– Afterglows– Identified X-ray counter parts– Later optical and radio counter parts– Host galaxies – with red shift > 4
Cosmological origins.– SN and Star Formation Region associations
Strongly constraint the theoretical models.
However, BeppoSAX only sensitively to long bursts (>10 s)
(2) (2) Observational Features and Observational Features and Their ImplicationsTheir Implications
• Spatial Features – Cosmological Origins
• Temporal Features
• Spectral Features
• Afterglows
Temporal FeaturesTemporal Features• Profiles
– Complicated and irregular– Multi-peaked or single-peaked
• Durations (T)
~ 5 ms to ~ 5 103 s, Typically ~ a few seconds
• Variabilities (T)
~ 1 ms, even ~ 0.1 ms, Typically ~ 10-2 T
Stellar Events ?Stellar Events ?
Even for black hole, combined with R = 2GM/c2
M 100 M
T ~ ms Ri cT = 300 km
( Ri : scale of initial region)
-ray bursts : Stellar Objects (Compact)
Spectral FeaturesSpectral Features
• Photon Energy Range
– ~10 keV to ~ 10 GeV
– Typically: ~ 0.1 to 1 MeV
• Non-thermal: N(E)dEE-dE, 1.8 – 2
• High Energy Tail: no sharp cutoff above 1 MeV
• Fluence:
– (0.1 to 100) 10-6 ergs/cm2
Afterglows of GRBsAfterglows of GRBs(other wavebands)(other wavebands)
• Time scales:
– X-ray: days; Optical: weeks; Radio: months
• General spectral features
– Multi-wave bands, Non-thermal spectrum, Decay power law: Fv t-
(x = 1.1 to 1.6, optical = 1.1 to 2.1 and broken power law
suggests jet-like behavior in GRBs)
• Associations SNs and star formation regions
• Host galaxies: Red-shifts : up to 3.4 even 5
FireballFireball
fp : fraction of photon pairs satisfying the pair condition,
F: fluence of GRB, D: distance of GRB
Optical depth ( -> e+e-):
Original Fireball Initial energy
E0 > 1051 ergs
1ms103ergs/cm10
10822
2713
22
2
T
Gpc
DFf
cmR
FDfp
ei
Tp
Optically thick Space scale
Ri cT = 300 km
Solution
(Original fireball, under such high pressure, should expand to ultra-relativistic speed, and become optically thin, leading to non-thermal gamma-ray radiation.)
Non-thermal
optically thin
Ri cT
optically thick
Ultra-relativistic Expansion with
Lorentz factor: >> 1
Expanding FireballExpanding Fireball
Baryon Contamination ProblemBaryon Contamination Problem• Expanding with Lorentz factor
Ri cT Re 2cTfp fp/ 2
22
2724
13
22
2
ms103ergs/cm10
108
T
Gpc
DFf
cmR
FDfp
ee
Tp
1 (optically thin)
> 102
M ~ E/ < 10-5 (E/21051 ergs) M
ShocksShocks
Internal shocks External shocks(between shells) (colliding with ISM)
Expanding fireball Relativistic ejecta slowed down
Shocks
(electrons accelerated in the shocks emit radiation via synchrotron emission)
-ray burst
afterglow
(4)(4) Central Engines :Energy Central Engines :Energy Source ModelsSource Models
• Isotropic emission:– 1051 – 1054 ergs in -rays only– Example: GRB990123: z = 1.61 and F ~ 5 10-4 erg
cm-2
– Eiso, = 4DL2F 3.4 1054 ergs 1.9 Mc2
– (H0 = 65 km s-1 Mpc-1, 0 = 0.2, 0 = 0 used, DL = 3.7 1028 cm)
• If the disk carries strong magnetic field, the rotation energy of the BH can be taken out via BZ process.
Key problem for the merger model
An NS has the proper velocity ~ 450 km/s and the life time is ~ 108 yr ( time scale for orbital decay), so the merger of compact objects will take place at ~ 30kpc outside their birthplaces. This model is inconsistent with the observational evidence for the association of several GRBs with star forming regions.
Advantages and Problems of the Hypernova Advantages and Problems of the Hypernova ModelsModels
Advantages :
Associations with SNs and Star Formation Regions
Major Problem:
How to avoid baryon contamination?
This Model suggests a two-step energy release process for GRBs associated with supernovae to avoid the baryon contamination.
- The first jet produced by a super-Eddington accreting neutron star pushes its front baryons and then forms a large bubble.
- The second jet produced by a super-Eddington accreting black hole has larger energy and fewer loading mass
(B) Alternative Models(B) Alternative ModelsTwo-step model Two-step model
(5) Speculations(5) Speculations
• GRBs resulting from phase transition of
Neutron Stars to Strange Stars ?
• GRBs causing Dinosaur Extinction ?
This model provides a natural way to avoid baryon contamination because the baryon of strange star only in thin Crust ~ 10-5 M
• Energy: (Phase Transition Energy per baryon ~ 20
MeV and 1058 baryons in a neutron star) ~ 21052
ergs
• Rate of Accreting NS in LMXB to SS ~10-6 / yr
per galaxy