G.E. Romero Instituto Aregntino de Radioastronomía (IAR), Facultad de Ciencias Astronómicas y Geofísicas, University of La Plata, Argentina.

Contents Introduction Observational Features and Their Implications Standard Radiation Models – Fireball Models Central Engines –Popular Models –Alternative Models Speculations

(1)Introduction Most intensive transient gamma-ray sources ~ erg cm -2 s -1, lasting about ~ seconds. (Pulsars ~ erg cm -2 s -1 ) (AGN ~10 -9 erg cm -2 s -1 ) and randomly occur in time and space.

Discovery 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) Observational Features and Their Implications Spatial Features – Cosmological Origins Temporal Features Spectral Features Afterglows

Temporal Features Profiles –Complicated and irregular –Multi-peaked or single-peaked Durations (T) ~ 5 ms to ~ 5  10 3 s, Typically ~ a few seconds Variabilities (  T) ~ 1 ms, even ~ 0.1 ms, Typically ~ T

Stellar Events ? Even for black hole, combined with R = 2GM/c 2  M  100 M   T ~ ms  Ri  c  T = 300 km ( Ri : scale of initial region)  -ray bursts : Stellar Objects (Compact)

Spectral Features Photon Energy Range –~10 keV to ~ 10 GeV –Typically: ~ 0.1 to 1 MeV Non-thermal: N(E)dE  E -  dE,   1.8 – 2 High Energy Tail: no sharp cutoff above 1 MeV Fluence: –(0.1 to 100)  ergs/cm 2

Afterglows of GRBs (other wavebands) Time scales: –X-ray: days; Optical: weeks; Radio: months General spectral features –Multi-wave bands, Non-thermal spectrum, Decay power law: F v  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 Models Fireball Internal-External Shocks

Fireball f p : fraction of photon pairs satisfying the pair condition, F: fluence of GRB, D: distance of GRB Optical depth   (  -> e + e - ):  Original Fireball  Initial energy E 0 > ergs  Optically thick  Space scale Ri  c  T = 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 R i  c  T optically thick Ultra-relativistic Expansion with Lorentz factor:  >> 1 Expanding Fireball

Baryon Contamination Problem Expanding with Lorentz factor  R i  c  T  R e   2 c  T fp fp/ 2fp fp/ 2    1 (optically thin)   > 10 2  M ~ E/  < (E/2  ergs) M 

Shocks 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)Central Engines :Energy Source Models Isotropic emission: –10 51 – ergs in  -rays only –Example: GRB990123: z = 1.61 and F  ~ 5  erg cm -2 –E iso,  = 4  D L 2 F   3.4  ergs  1.9 M  c 2 –(H 0 = 65 km s -1 Mpc -1,  0 = 0.2,  0 = 0 used, D L = 3.7  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 ~ 10 8 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 Models 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 Two-step model

Kick and Delay Phase Transition Model

The Guitar Nebula : A Pulsar Shock Front

(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 ~ M  Energy: (Phase Transition Energy per baryon ~ 20 MeV and baryons in a neutron star) ~ 2  ergs Rate of Accreting NS in LMXB to SS~10 -6 / yr per galaxy

Soft Gamma-ray Repeaters