Chapter 5 Flashcards
Where are Gamma Ray Bursts positioned in sky distribution?
They are more likely to be confined to galactic plane than be at random
They are extra-galactic
Why are there more objects in the low latitude of the sky distribution?
The area is larger as the number of points per square cm is higher (spherical projection)
Why are galaxies randomly distributed?
They are at different distances in the sky which is seen from redshift
When gamma rays were first discovered, how did they appear on light curves?
A wide variety:
single < 1s spike
two widely spaced peaks
train of erratic sub-bursts
Fast Rise Exponential Decay (FREDs)
How are gamma rays categorised as bimodal?
Short duration = lasting less than 2 seconds
Long duration = lasting longer than 2 seconds
What is different about GRBs compared to black bodies?
They are non-thermal and isotropic and can be fitted with two power laws:
There is a hard “high energy tail” with a gradual downwards slope - the spectra do not fall exponentially at high energies as a black body spectrum would.
How were GRBs identified?
By detecting an afterglow which would come from radio to X -rays as a result of synchrotron radiation (meaning there is a magnetic field present).
Synchrotron emission is believed to be present if the highest energy emission is associated with shock accelerated particles
Where was the first GRB afterglow observed?
In a faint distant galaxy
GRB was redshifted as seen from spectrum of the host galaxy containing absorption lines
Showed they are the most distant objects in the universe
Where do long duration GRBs tend to occur?
In small irregular galaxies deficient in heavy elements (metals), in the brightest regions of their host galaxies
(suggests association with massive early stars DUE TO HIGHER REDSHIFT)
Some are related to hypernovae (exceptionally bright supernovae)
Where do short duration GRBs tend to occur?
Blue irregular galaxies (in the centre of galaxy)
Red elliptical galaxies containing an older stellar population (more metal rich)
Gamma rays are far detached from their galaxies and the galaxies are more regular than that of long duration GRBs.
Smaller redshifts so more recent
How do we see most GRBs in the sky?
By direction emission into fb (beaming factor) x that observed
if emission is directed into a fraction (1/fb) of the sky, we will miss seeing most GRBs
Where does the beaming factor come from?
If radiation is emitted into two conical jets of solid angle, the amount of energy required to give us the observed fluence is reduced as a factor of the beaming factor
Which GRBs are associated with supernovae?
LDGRBs (not all SDGRBs)
Which model is used to explain the behaviour of gamma rays?
FIREBALL MODEL
What happens in the fire ball model?
Stellar mass object undergoes a catastrophic event which releases energy in a compact region
Cooling occurs and the flow becomes optically thin and gamma rays can escape (produced by accelerated electrons (synchrotron or inverse Compton scattering)`)
What does the decelerating blast in the fireball model produce?
Lower frequency emission (the afterglow)
What is the engine of a LDGRB?
A massive star with a helium core of >Mo which has shed its outer layer and is now rapidly rotating (its core collapses)
What happens at the centre of the engine of LDGRB?
A few solar mass black hole forms surrounded by a thick accretion disc (collapsar)
As accretion goes on, energy is deposited along the star’s rotation axis leading to a jet
What causes an afterglow?
Outflow collimated by pressure from stellar mantle which breaks surface resulting in shock wave which destroys star and is blow out in stellar wind leading to an afterglow
When is synchrotron emission dominant at lower frequencies?
If the ejecta from supernova slows down, the electrons have lower energy.
Why are SDGRB not powered by a collapsar?
They are too fast (<2)
What can gamma ray bursts be used to do?
Trace star formation to much further than possible previously
What happens when a neutron star mergers with another neutron star?
Supermassive, hot central object with a belt of debris carrying excess angular momentum which collapses to a black hole
What does a NS-NS merger cause?
Accretion from the central object which provides sudden release of grav energy
Why does the orbit of the NS NS merger shrink?
Due to energy being lost from gravitational waves and the gravitational potential moves deeper and quicker
How does the fireball model relate to NS NS merger?
Fireball might be launched above poles of new central object (close to energy source but low density due to centrifugal effect)
How was the location of gamma rays established?
By the use of observations of the afterglow at longer wavelengths which established the extragalactic nature of the sources
Why is it believed LDGRBs come from supernovae?
Broad band optical spectra of LDGRBs show bumps similar to those seen in supernova spectra
They also appear in the same directions at the same times as supernovae
Why is large brightness of the gamma ray bursts at cosmological distances due to?
The relativistic beaming leading to the emission not being isotropic
