Transcript Document

High energy Astrophysics
Mat Page
Mullard Space Science Lab, UCL
11. Gamma-ray
bursts
Slide 2
11. Gamma-ray bursts
• This lecture:
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Discovery of g-ray bursts
Burst properties
Models for g-ray bursts
Detection and follow up in other
wavebands
Slide 3
So what is a g-ray burst?
• 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
Slide 4
Discovery of gamma-ray bursts
• Discovered in
the 1960s by
military
satellites.
• First
announced in
public in 1973.
Slide 5
The big mystery
• 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 20 years, we didn’t even know if
they were from within or from outside our
Galaxy!
Slide 6
Speculation
• There have been more different models for gray bursts than there are people in this room.
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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 7
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 8
Isotropically distributed
Slide 9
Very likely to be extragalactic
How would Galactic and extragalactic sources
be distributed?
Slide 10
Luminosities
• 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!
Slide 11
Burst lightcurves
Slide 12
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 gray bursts.
• Likely that long and short bursts are
fundamentally different.
• Dividing line between long and short bursts at
about 2s.
Slide 14
X-ray afterglows
Slide 15
Gammaray burst
Big data gap!!
X-ray
Afterglow
Slide 16
• X-ray afterglows decline over a much longer
timescale than the g-ray bursts themselves
– still visible a week after the burst.
Slide 17
Optical afterglows
Slide 18
Slide 19
Optical transients
• 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 z~1
• Of course optical emission means that we can
harness our biggest optical telescopes to get
spectra and redshifts for the bursts.
Slide 20
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.
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- <-> 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
Slide 23
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 g-ray burst because the
velocity is so close to c.
– Remember its 1/(1-v2/c2)1/2 that’s important
Slide 24
The progenitor
Slide 25
• Why does a g-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
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.
Slide 27
• Whatever the progenitor, the result
is almost certainly a black hole.
Slide 28
Bursts as cosmological probes
• 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.
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.
The NASA Swift Satelite has
made GRBs the fastest moving
area in astrophysics!
Slide 30
Spacecraft and
instrumentation
UV and Optical
Telescope
X-ray Telescope
Slide 31
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
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)
Slide 34
The UV/Optical Telescope (UVOT)
30 cm RitchieChretien
UV/Optical
telescope.
0.3 arcsecond
positional
accuracy;
optical and UV
filter photometry
and grism
spectroscopy.
Built at MSSL
Slide 35
UVOT hardware
Filter Wheel and Detector Assembly
UVOT Telescope Optics: Primary
and Secondary Mirrors.
Slide 36
UVOT finds the afterglow: GRB 050525a
z = 0.606
Slide 37
XRT lightcurve: GRB 050820a
z = 2.612
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
Little galaxies and GRBs
Slide 39
First UV spectrum of a gamma ray burst,
GRB081203a, taken with the Swift UVOT
grism built at MSSL.
Slide 40
High-redshift GRBs
XRT lightcurve
GRB 050904 at z = 6.29
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 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
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.
In this case, the short burst is more likely to
have been caused by a collision between
two neutron stars.
NASA
Slide 43
Some key points:
• 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