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

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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

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• 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!

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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

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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:

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Isotropically distributed

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Very likely to be extragalactic

How would Galactic and extragalactic sources be distributed?

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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!

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Burst lightcurves

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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.

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X-ray afterglows

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Gamma-ray burst

X-ray Afterglow

Big data gap!!

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

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Optical afterglowsSlide 17

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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.

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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.

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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

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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

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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

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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.

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

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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.

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• 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!

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X-ray Telescope

UV and Optical Telescope

Spacecraft andinstrumentation

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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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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)

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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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UVOT hardware

UVOT Telescope Optics: Primary and Secondary Mirrors.

Filter Wheel and Detector Assembly

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UVOT finds the afterglow: GRB 050525a z = 0.606

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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

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Little galaxies and GRBs

First UV spectrum of a gamma ray burst, GRB081203a, taken with the Swift UVOT

grism built at MSSL.

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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.

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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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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

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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

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