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

(3) Standard Radiation (3) Standard Radiation ModelsModels

• Fireball

• Internal-External Shocks

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)

(A) Popular Models Merger of NS-NS, NS-BH

• 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.

Hypernova Models

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

Kick and Delay Phase Transition ModelKick and Delay Phase Transition Model

The Guitar Nebula : A Pulsar Shock Front

(5) Speculations(5) Speculations

• GRBs resulting from phase transition of

Neutron Stars to Strange Stars ?

• GRBs causing Dinosaur Extinction ?

● In LMXB, Phase Transition of Neutron Stars Strange Stars

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

Soft Gamma-ray Repeaters