Jerry Fishman

NASA - Marshall Space Flight Center

National Space Science and Technology Center (NSSTC)

Huntsville, AL

AAVSO – HEA3 - Las Cruces, NM - March 21, 2005

GRBs – The Prompt Emission

GRBs – The Prompt Emission& Space-Borne Observations of GRBs

• A short history of observations

• Observable aspects of the prompt emission

– Temporal aspects (time profiles)

– Duration of prompt emission

– Spectral properties of the gamma radiation

• GRB energy source – “The Central Engine”

• Conversion of energy into gamma rays

• Future observations

Vela military satellites, 1963 Vela 4 – July 2, 1967: curious spike in gamma-ray detector readings First Gamma-Ray Burst was identified as an extra-terrestrial phenomenon 16 bursts in all between 1969 and 1972 Bursts coming from random directions in the sky GRBs baffled scientists, theorists began offering explanations

An Accidental Discovery

GRB Roadblock - during the 1970’s & 1980’s

• Numerous observations with small spacecraft

(primarily Russian & U.S.) • No new, fundamental observations

(except reports of gamma-ray line features)• A “Scientific Enigma”• Assumed to be Associated with Neutron Stars in

the Galaxy• Random nature frustrated observations

ComptonGRO

Compton Gamma Ray Observatory

April 5, 1991 – June 3, 2000 CGRO – four separate gamma-ray detection devices: EGRET, COMPTEL, OSSE, and BATSE 30 KeV – 30 GeV BATSE proved to be the most useful instrument for GRB detection

The Breakthrough Observations -1997:

Italian/Dutch Satellite Satellite - BeppoSAX

•Combined good GRB location with fast response

•Led to x-ray & optical afterglow observations

•The optical location, in turn, led to observations of host galaxies and redshift determinations

Beppo-SAX discovery of X-ray afterglow of a GRB

• Temporal aspects (time profiles)

• Duration of prompt emission

• Spectral properties of the gamma

radiation

The Prompt Gamma-Ray Emission

Diversity of GRB Profiles

Examples of Double-Peaked GRBs

Multiple-Episode Bursts

Distinct subclasses of –ray bursts: short/hard & long/soft

Hard Spectra Softer

Spectra

0.1 s 20 s

Prompt Optical Emission Observed with ROTSE:

GRB990123, an intense GRB

-An amazing feat that may not be repeated for some time

-Fully automated, robotic telescope

-Very wide FoV

ROTSE Cameras (4)

Simultaneous Optical & Optical Observations of a GRB

Peaked at

9th mag at

50 sec(z = 1.6)

ROTSE I

GRB 990123

Typical GRB Spectrum

– the Band function

Briggs, et al. 1999

Spectral Evolution of GRBs

-from Crider, et al. 1997

Launched November 2004

Localizations 1' – 4' at 20 sec 5" at 1 – 2 min 0.3" at 3 – 4 min

SWIFT

Swift Mission

• Burst Alert Telescope (BAT)– New CZT detectors– Detect ~300 GRBs per year– Most sensitive gamma-ray

imager ever

• X-Ray Telescope (XRT)– Arcsecond GRB positions– CCD spectroscopy

• UV/Optical Telescope (UVOT)– Sub-arcsecond imaging– Grism spectroscopy– 24th mag sensitivity (1000 sec)– Finding chart for other observers

• Autonomous re-pointing in 20 - 70 sec• Onboard and ground triggers

Instruments

The Burst Alert Telescope on Swift

Some GRB Theories

General Requirements:

-A Compact Object is needed; Black Hole formation is usually invoked

-Likely involves beamed emission from a highly relativistic jet

-Emission degrades in succession: gamma-ray > X-ray > Optical > Radio

-Role of magnetic field, its origin and its strength is under great debate

What is the trigger?

The duration of the burst is determined by the viscous timescale of the accreting gas

The duration of the burst is given by the fall-back time of the gas.

Collapsar Model Supermassive star burns off H, becomes Wolf-Rayet star with He, Fe core

Core burned, star collapses and forms black hole with matter accretion jets

Jets shatter outer shell of star, creates hypernova

Jets speed on and collide with other nearby material to create the subsequent gamma-ray burst

Gamma Ray Bursts

•

Merging Neutron Stars

Jets, Disks and Bursts from Coalescing Compact Binaries

(Neutron Stars)

Enrico Ramirez-Ruiz (IAS, Princeton)& Collaborators

The GLAST Spacecraft- to be Launched in 2007

GLAST Burst Monitor

• 12 NaI scintillation crystals- few keV to about one MeV

• 2 BGO scintillation crystals- about 150 keV to about 30 MeV

Image provided by Gamma Ray Astronomy Team at MSFC

Energetic X-ray Imaging Survey Telescope (EXIST)