Gamma Ray Bursts

Gamma Ray Bursts

Discovered in the late 1960's by the VELA satellites intended to monitor the nuclear test ban treaty.

Flashes of High Energy Photons Non repeating

Isotropically distributed in the sky Seem to come in two categories:

"short" bursts ( < 1 sec)

"long" burts (few seconds)

Distance scale established in 1997

COSMOLOGICAL (a least for the long burts)

First GRB 2 july 1967

Vela satellites

BATSE on CGRO

8 detectors at the corner  of  the satellites observing the entire sky

Temporal Profiles 

Bimodal

time distribution time > 2 sec

time < 2 sec

Correlation between Duration 

 and

Hardness

Isotropic

distribution (Cosmological)

Energy Spectrum

E_peak~250 keV

Band function:

E^ exp(-E/E₀) E<E₀ E^ E>E₀

Distribution of the parameters 

BEPPO­SAX  X­ray Satellite (Italy/Netherland)

Named after Giuseppe  (Beppo) Occhialini

Launched 30 april 1996

Switched off 30 aprile 2002

BEPPO-SAX X-ray Satellite

Wide Field Camera

Narrow Field Instruments

WFC: Wide Field Camera Coded Mask Instrument

2-30 KeV

GRB970228

The first X­ray 

and Optical   AFTERGLOW

8 h  3 days

GRB970228 Discovery of the

AFTERGLOW

Opticals Afterglows

List of GRB's

localized and with known

Redshift

THE COMPACTNESS PROBLEM

Photon density at the source

Optical depth 

Consider Relativistic Motion Lorentz Factor  

Relativistic Motion

Rate of GRB's.

Observed by BATSE 300 GRB/year

efficiency approximately 0.5 (Earth obscuration) Rate = 600/year

­1

JET  EMISSION

The Ultrarelativistic  Flow of GRB is emitted not spherical, but in   narrow jets with opening of a few  degrees.

The strongest evidence comes from the observations of “Breaks”  in the time  evolution of  the AfterGlow  luminosity.

Emission from 

a sphere that is moving

with Lorentz Factor  Gamma

Observer sees only  a  region  1/Gamma of the sphere

The rest of the sphere could be absent 

When Gamma(t)  > 1

GRB970508

Radio Measurements

Diffractive Scintillation

Interference between light that takes different paths

 Quenching of oscillations

 Measurement of the increased size of the source  4 weeks  :  R > 10¹⁷ cm,  v_app > c

Mechanism for the AfterGlow Emission.

Synchrotron Emission produced

by accelerator electrons in a slowing down Flow (flow slowes down by interactions with the ISM

INNER ENGINE

of GAMMA RAY BURSTS

Collapsars

SupraNova   (Stella e Vietri)   

      Delayed  collapse to a black hole  of  a       fast rotating  supramassive neutron star Coalescence of Compact Objects

GRB980425 SN 1998bw

Additional Emission from GRB

TeV Gamma Rays Neutrinos

UHE (Ultra High Energy) Cosmic Rays

Gravitational Waves