High energy Astrophysics Mat Page Mullard Space Science Lab, UCL 11. Gamma-ray bursts.
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Transcript of High energy Astrophysics Mat Page Mullard Space Science Lab, UCL 11. Gamma-ray bursts.
High energy Astrophysics
Mat Page
Mullard Space Science Lab, UCL
11. Gamma-ray bursts
11. Gamma-ray bursts
• This lecture:• Discovery of -ray bursts
• Burst properties
• Models for -ray bursts
• Detection and follow up in other wavebands
Slide 2
So what is a -ray burst?
• Brief, intense burst of extraterrestrial -rays
• Duration between 0.001 and 1000 seconds
• For this period they might be the brightest -ray source in the sky
• Appears to be a once-only phenomena
Slide 3
• Discovered in the 1960s by military satellites.
• First announced in public in 1973.
Discovery of gamma-ray burstsSlide 4
The big mystery
• Since their discovery, -ray bursts were about the most mysterious objects ever discovered.
• Why?– Only appear in -rays– Only last a tiny length of time– Very difficult to investigate
• For the first 20 years, we didn’t even know if they were from within or from outside our Galaxy!
Slide 5
Speculation
• There have been more different models for -ray bursts than there are people in this room.– Giant supernovae– Jets or cannon-balls from supernovae– Exhaust from alien spaceships– Massive outbursts from AGN– Jets from pulsars– Neutron stars collapsing– Merging of neutron stars– Evaporating black holes
Slide 6
Big advances in the 1990s
• We learned a lot in the 1990s, primarily because of novel space observatories
• Compton Gamma-ray observatory– All-sky burst survey with BATSE
• BeppoSAX– Good positions and X-ray follow-up
• Here are some of the things that have been learned:
Slide 7
Isotropically distributed
Slide 8
Very likely to be extragalactic
How would Galactic and extragalactic sources be distributed?
Slide 9
Luminosities• -ray bursts must be very luminous if they are
extragalactic.– Instantaneously the most luminous sources of radiation in
the sky.
• The total energy radiated in -rays during the burst is between 1044-1047 J assuming the bursts are isotropic.
• The energy is emitted within a very short time– energy densities not seen since the big bang
• If the radiation is beamed, energy emitted per burst is reduced to ~1044 J – but the number of bursts increases accordingly!
Slide 10
Burst lightcurves
Slide 11
Burst lightcurves• The lightcurves or ‘profiles’ of bursts show a
variety of shapes, ranging from a smooth pulse to complicated flickering.
• With such a range of duration and pulse profiles, there must be a variety of things happening in -ray bursts.
• Likely that long and short bursts are fundamentally different.
• Dividing line between long and short bursts at about 2s.
Slide 12
X-ray afterglows
Slide 14
Gamma-ray burst
X-ray Afterglow
Big data gap!!
Slide 15
• X-ray afterglows decline over a much longer timescale than the -ray bursts themselves – still visible a week after the burst.
Slide 16
Optical afterglowsSlide 17
Slide 18
Optical transients
• Bright optical transient seen in GRB990123:– 9th magnitude optical flash was observed while
the -ray burst was going off.
• Current record holder: GRB080319b– V magnitude ~ 5.5 during the burst– You could have seen it with the naked eye!
• Both the record breakers were at z~1
• Of course optical emission means that we can harness our biggest optical telescopes to get spectra and redshifts for the bursts.
Slide 19
Slide 20
Models for -ray bursts• Whatever the progenitor, the leading model to
describe what actually happens during the burst is called the relativistic fireball
• A shell of material is expanding at highly relativistic speeds. – Almost inevitable – the photon pressure alone
would force a rapid expansion
• Obvious similarities with the relativistic jets observed in radio galaxies and quasars
• Material moving towards us dominates the observed emission – so time dilation effects important.
• Likely to be beamed.
Slide 21
Pair dominated plasma
• Inverse Compton emission probably initial fundamental. However, balance of e+ e- pairs an important consideration because the -ray energy density is extremely high:
• e+ + e- <-> + • Would expect the burst to be optically thick
above 0.5 MeV• The initial -ray burst must be caused by
internal shocks: collisions between successive waves of ejecta reduces their relative velocities to smaller fractions of c – and reduces the pair opacity.
• Complex lightcurves fit with repeated waves of ejecta
Slide 22
The X-ray and optical afterglow• The X-ray afterglow comes from
external shocks, as the ejecta ploughs into the surrounding interstellar medium.
• As the ejecta sweeps up material, it has to slow down, just like in a supernova remnant.
• ‘Appreciable’ slowing happens much faster in a -ray burst because the velocity is so close to c.– Remember its 1/(1-v2/c2)1/2 that’s important
Slide 23
Slide 24
The progenitor
• Why does a -ray burst take place?• The bursts we’ve identified so far do NOT take place in the
centres of their host galaxies, so they aren’t AGN.• Long bursts appear to be associated with star forming
regions in star-forming galaxies, which are typically irregular dwarf galaxies similar to the Magellanic Clouds.
• This suggests that their progenitors are massive stars – the ‘hypernova’ scenario. Could be core collapse in an extremely massive, low-metallicity star, or a massive star that is merging with a companion.
• Theoretically, this is a good mechanism to produce long bursts
Slide 25
What about the short bursts?• Afterglows from Short bursts had not
been detected until launch of Swift. • For the short bursts, neutron star -
neutron star mergers are the current leading model.
• When the neutron stars get close, their orbits decay rapidly due to gravitational radiation. Simulations suggest that as they collide about half a solar mass ends up as a toroidal structure which then collapses onto the merged star.
Slide 26
• Whatever the progenitor, the result is almost certainly a black hole.
Slide 27
Bursts as cosmological probes
• We know how that some -ray bursts originate in distant galaxies, and have phenomenal luminosities.
• With current technology we could detect these bursts at redshifts of 10-15
• If -ray bursts happened at these early epochs, we could use them to probe parts of the universe we have never seen before.
Slide 28
• They might tell us about star formation before the first galaxies had even formed!
• Their radiation has to pass through the early intergalactic medium – the passage will leave its mark on the radiation. – For example by absorption line spectroscopy
we could work out the composition, ionization state of the primordial gas, presence of dust etc.
Slide 29
The NASA Swift Satelite has made GRBs the fastest moving area in astrophysics!
Slide 30
X-ray Telescope
UV and Optical Telescope
Spacecraft andinstrumentation
Slide 31
The Burst Alert Telescope (BAT)
• Coded mask telescope; detector measures ‘shadow’ of random mask, which allows direction of incidence to be reconstructed.
• 1.4 Steradian field of view
• Measures GRB positions correct to 4 arcminutes
Built by GSFC
NASA
Slide 32
XRT hardware
X-ray Mirror
12 Gold-coated Nickel Shells
(Brera)
Cooled X-ray CCD Detector
360,000 individual pixel sensors
(Leicester/E2V)
Focal Plane Camera Assembly (Leicester)
Slide 33
The UV/Optical Telescope (UVOT)
30 cm Ritchie-Chretien UV/Optical telescope.
0.3 arcsecond positional accuracy; optical and UV filter photometry and grism spectroscopy.
Built at MSSL
Slide 34
UVOT hardware
UVOT Telescope Optics: Primary and Secondary Mirrors.
Filter Wheel and Detector Assembly
Slide 35
UVOT finds the afterglow: GRB 050525a z = 0.606
Slide 36
XRT lightcurve: GRB 050820a z = 2.612Slide 37
Long GRBs: the ‘canonical’ X-ray lightcurve revealed by the Swift XRT
time
flux
initial steep decline
flares
slow decline
final steeper decline
Slide 38
Little galaxies and GRBs
First UV spectrum of a gamma ray burst, GRB081203a, taken with the Swift UVOT
grism built at MSSL.
Slide 39
High-redshift GRBs
GRB 050904 at z = 6.29
XRT lightcurve
Latest record breakers: GRB 090423 at z=8.2, and 090429b at z=9.4 were the most distant objects ever detected at the time.
Slide 40
Short bursts: GRB 050509b• ~0.05 s burst of gamma-rays
• ~ 5 min detection of X-rays
• No UVOT counterpart – but potential host galaxy observed
Slide 41
Short bursts: GRB 050509b
• Probable host galaxy is populated by old red stars
An unlikely site for a hypernova explosion, as these happen to young, massive stars.
In this case, the short burst is more likely to have been caused by a collision between two neutron stars.
NASA
Slide 42
Some key points:
• -ray bursts are brief, intense bursts of -rays• They are the most luminous explosions we know
about apart from the big bang.• The -rays are thought to be produced as waves of
ejecta collide with each other• X-ray and optical afterglows come as the ejecta
collide with the surrounding medium• Short bursts thought to be merging neutron stars• Long bursts thought to be hypernovae• Could be valuable probes of early universe
Slide 43