High energy Astrophysics Mat Page Mullard Space Science Lab, UCL 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 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 Discovery of gamma-ray bursts Slide 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 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 ~10 44 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 afterglows Slide 17

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Optical transients Bright optical transient seen in GRB990123: –9 th 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

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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. 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-v 2 /c 2 ) 1/2 that’s important Slide 23

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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 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 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 and instrumentation 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 a z = Slide 36

XRT lightcurve: GRB a z = Slide 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 at z = 6.29 XRT lightcurve Latest record breakers: GRB at z=8.2, and b at z=9.4 were the most distant objects ever detected at the time. Slide 40

Short bursts: GRB b ~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 b 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