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)