High energy Astrophysics

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High energy Astrophysics
Mat Page
Mullard Space Science Lab, UCL
11. Gamma-ray bursts
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11. Gamma-ray bursts
Slide 2

• This lecture
• Discovery of g-ray bursts
• Burst properties
• Models for g-ray bursts
• Detection and follow up in other wavebands

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So what is a g-ray burst?
Slide 3

• Brief, intense burst of extraterrestrial g-rays
• Duration between 0.001 and 1000 seconds
• For this period they might be the brightest
  g-ray source in the sky
• Appears to be a once-only phenomena

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Discovery of gamma-ray bursts
Slide 4

• Discovered in the 1960s by military satellites.
• First announced in public in 1973.

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The big mystery
Slide 5

• Since their discovery, g-ray bursts were about
  the most mysterious objects ever discovered.
• Why?
• Only appear in g-rays
• Only last a tiny length of time
• Very difficult to investigate
• For the first 25 years, we didnt even know if
  they were from within or from outside our Galaxy!

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Speculation
Slide 6

• There have been more different models for g-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

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Big advances in the 1990s
Slide 7

• 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

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Isotropically distributed
Slide 8
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Very likely to be extragalactic
Slide 9
How would Galactic and extragalactic sources be
distributed?
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Luminosities
Slide 10

• g-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 g-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!

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Burst light curves
Slide 11
Short GRBs
Long GRBs
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Burst lightcurves
Slide 12

• 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
  g-ray bursts.
• Likely that long and short bursts are
  fundamentally different.
• Dividing line between long and short bursts at
  about 2s.

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X-ray afterglows
Slide 14
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Slide 15
Big data gap!!
Gamma-ray burst
X-ray Afterglow
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Slide 16

• X-ray afterglows decline over a much longer
  timescale than the g-ray bursts themselves
  still visible a week after the burst.

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Slide 17
Optical afterglows
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Slide 18
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Optical transients
Slide 19

• Bright optical transient seen in GRB990123
• 9th magnitude optical flash was observed while
  the g-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 z1
• Of course optical emission means that we can
  harness our biggest optical telescopes to get
  spectra and redshifts for the bursts.

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Slide 20
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Models for g-ray bursts
Slide 21

• 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.

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Pair dominated plasma
Slide 22

• Inverse Compton emission probably initial
  fundamental. However, balance of e e- pairs an
  important consideration because the g-ray energy
  density is extremely high
• e e- lt-gt g g
• Would expect the burst to be optically thick
  above 0.5 MeV
• The initial g-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

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The X-ray and optical afterglow
Slide 23

• 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
  g-ray burst because the velocity is so close to
  c.
• Remember its 1/(1-v2/c2)1/2 thats important

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Slide 24
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The progenitor
Slide 25

• Why does a g-ray burst take place?
• The bursts weve identified so far do NOT take
  place in the centres of their host galaxies, so
  they arent 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

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First direct Long GRB SN connection GRB980425 /
SN1998bw
SRON
VLT
Galama et al. 1998
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What about the short bursts?
Slide 26

• 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.

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Slide 27

• Whatever the progenitor, the result is almost
  certainly a black hole.

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Bursts as cosmological probes
Slide 28

• We know how that some g-ray bursts originate in
  distant galaxies, and have phenomenal
  luminosities.
• With current technology we could detect these
  bursts at redshifts of 10-15
• If g-ray bursts happened at these early epochs,
  we could use them to probe parts of the universe
  we have never seen before.

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Slide 29

• 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.

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The NASA Swift Satelite has made GRBs the fastest
moving area in astrophysics!
Slide 30
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Spacecraft and instrumentation
UV and Optical Telescope
X-ray Telescope
Slide 31
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Slide 32
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
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Slide 33
XRT hardware
Cooled X-ray CCD Detector 360,000 individual
pixel sensors (Leicester/E2V)
X-ray Mirror 12 Gold-coated Nickel Shells
(Brera)
Focal Plane Camera Assembly (Leicester)
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Slide 34
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
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Slide 35
UVOT hardware
Filter Wheel and Detector Assembly
UVOT Telescope Optics Primary and Secondary
Mirrors.
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Slide 36
UVOT finds the afterglow GRB 050525a
z 0.606
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Slide 37
XRT light curves
Nousek et al. 2006
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Slide 38
Long GRBs the canonical X-ray lightcurve
revealed by the Swift XRT
flares
slow decline
final steeper decline
flux
initial steep decline
time
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Little galaxies and GRBs
Slide 39
First UV spectrum of a gamma ray burst,
GRB081203a, taken with the Swift UVOT grism built
at MSSL.
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Slide 40
High-redshift GRBs
XRT lightcurve
Latest record breakers GRB 090423 at z8.2, and
090429b at z9.4 were the most distant objects
ever detected at the time.
GRB 050904 at z 6.29
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Slide 41
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

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Slide 42
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. No
Supernova detected down to deep limits In this
case, the short burst is more likely to have been
caused by a collision between two neutron stars.
NASA
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Some key points
Slide 43

• g-ray bursts are brief, intense bursts of g-rays
• They are the most luminous explosions we know
  about apart from the big bang.
• The g-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

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GRB Picture
Gehrels et al. 2002, Scientific American