SOLVED AND UNSOLVED MYSTERIES IN GAMMA-RAY BURSTS

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COSMIC GAMMA-RAY BURSTS The Current Status
Kevin Hurley UC Berkeley Space Sciences Laboratory
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SOME ABSOLUTELY INCONTROVERTIBLE GRB PROPERTIES
THAT NO REASONABLE PERSON COULD POSSIBLY DISAGREE
WITH

1. There are two morphological classes of GRBs, long
   bursts (20 s duration) and short bursts (0.2 s
   duration)
2. Counterparts and redshifts have been found for
   many long bursts
3. No counterpart or redshift has been found for any
   short burst
4. Most of the long bursts display long-wavelength
   (radio and optical) afterglows but some of
   them have no detectable optical or radio
   counterparts (dark bursts)
5. There is good evidence which links some long
   bursts to the deaths of massive stars

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1. The energy spectra of the long bursts form a
   continuum, from X-ray flashes (with few or no
   ?-rays), X-ray rich bursts, and GRBs
2. There is no experimental evidence to suggest that
   any class of burst (long/short, X-ray rich, dark)
   has a different origin, or a different spatial
   distribution, from any other class but there
   are many theories which do suggest different
   origins

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SHORT BURST
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LONG BURST
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THE GRB DURATION DISTRIBUTION
WE ONLY KNOW ABOUT THE ORIGIN OF THE LONG BURSTS
SOFTER ENERGY SPECTRA
HARDER ENERGY SPECTRA
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ENERGY SPECTRA OF THE LONG BURSTS
????
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THE ENERGY SPECTRA OF THE LONG BURSTS FORM A
CONTINUUM, FROM SOFT-SPECTRUM X-RAY FLASHES TO
HARD-SPECTRUM GAMMA-RAY BURSTS (BeppoSAX, HETE)
GAMMA-RAY BURST
? Epeak200 keV
EpeakkeV ?
X-RAY FLASH
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GAMMA-RAY BURSTS ARE FOLLOWED BY X-RAY AFTERGLOWS
1-10 keV
1
T08h T02d
BeppoSAX Costa et al. 1997
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OPTICAL AFTERGLOWS
Pandey et al. 2004
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AND RADIO AFTERGLOWS
100
Flux density, µJy
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Frail et al. 2003
1
1 10 100
1000 Time after
GRB970508, days
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FIREBALL MODEL
1000-2000 AU
1-6 AU
G2
G1
ISM
20 km
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SIMULTANEOUS OPTICAL/GAMMA-RAY EMISSION HAS NOW
BEEN DETECTED TWICE
ROTSE (www.rotse.net)
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RAPTOR (http//www.raptor.lanl.gov/index.htm)
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• GRB HOST GALAXIES
• Arent pretty but they are normal
• Not active galaxies
• Indistinguishable from field galaxies with
  similar ages

990506
990705 (z0.8424)
980613 (z1.0964)
980519
980329
000301(z2.0335)
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REDSHIFT DISTRIBUTION OF 34 LONG GAMMA-RAY BURSTS
LOWEST REDSHIFT0.104 (INTEGRAL, GRB031203)
HIGHEST4.5 (IPN, GRB000131) AVERAGE1.4
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GRB ENERGETICS

• Isotropic gamma-ray energies range from gt1051 to
  gt1054 erg
• Two possibilities for liberating large amounts of
  energy
• Merging neutron stars (short bursts?)
• Collapsars (also called hypernovae, or energetic
  supernovae long bursts)
• In either case, beaming is also required there
  is observational evidence in afterglow light
  curves that it occurs in some cases

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THE OPTICAL AFTERGLOW CAN GIVE INFORMATION ABOUT
BEAMING
OBSERVER
AFTERGLOW INTENSITY
TIME
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BEAMING CAN TURN GRBs INTO (MODEL-DEPENDENT)
STANDARD CANDLES

• Beaming angles range from 1º to 25º average
  4º
• Distribution of energy assumed uniform within the
  beam
• Energy 1.3x1051 erg

Isotropic energies, no beaming
Corrected for beaming
Frail et al. 2001
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HOW IS THE ENERGY DISTRIBUTED?
DURING THE BURST
AFTERGLOW

• gt25 keV ? rays 65
• 1-10 keV X-rays 7
• Optical 0.1
• Radio ?
• MeV/GeV/TeV ? ? gt10?
• Gravitational radiation ?

• gt25 keV ? rays 7
• 1-10 keV X-rays 9
• Optical 2
• Radio 0.05

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GRB030329 THE POSTER CHILD FOR THE
GRB-SUPERNOVA CONNECTION

• GRB030329 was a bright (top 1) nearby (z0.17)
  burst, discovered by HETE
• It is the best-studied GRB to date (gtgt100
  observations)
• Its optical afterglow light curve and spectrum
  point to an underlying supernova component
  (SN2003dh)
• These signatures have been observed before in
  numerous GRBs, starting with GRB980425
  (SN1998bw, peculiar Type Ic the previous
  poster child), but GRB030329 is the most
  convincing case

Poster child n. A child afflicted by some
disease or deformity whose picture is used
on posters to raise money for charitable purposes
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• Optical afterglow spectrum resembles that of
  SN1998bw
• Broad, shallow absorption lines imply large
  expansion velocities

• Afterglow light curve can be decomposed into two
  components power law decay supernova

? Some long GRBs are associated with the deaths
of massive stars (gt30M?)
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MYSTERY OF THE OPTICALLY DARK BURSTS
Fox et al. 2003
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THE MYSTERY OF THE OPTICALLY DARK BURSTS IS BEING
SOLVED

• 35 of the GRBs detected by BeppoSAX and the IPN
  had no detectable optical counterparts why?
• Absorbed by dust within the host galaxy?
• Intrinsically faint and/or rapidly fading?
• High redshift?
• Only 10 of the bursts detected by HETE are
  optically dark
• HETE gets positions out to the astronomers faster
  than BeppoSAX and the IPN did
• Swift is now doing the same, and carrying out
  optical observations within minutes
• Some Swift bursts do appear to be optically dark

Confirmed by observation? ? ? Not
so far
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OBSERVATIONS OF SWIFT BURSTS
?
?
?
?
?
?
?
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WHAT ARE X-RAY FLASHES?

• GRBs observed away from the jet axis?
• Explosions with less relativistic ejecta?
• GRBs at high redshift?
• We have only one XRF redshift (XRF020903,
  z0.251) in this case, the answer is clearly 2
  (Soderberg et al. 2004)

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ARE THE SHORT GRBS NEARBY MAGNETAR FLARES?

• Giant flares begin with 0.2 s long, hard
  spectrum spikes
• Their energy can be 1047 erg
• The spike is followed by a pulsating tail with
  1/1000th of the energy
• Viewed from a large distance, only the initial
  spikes would be visible
• They would resemble the short GRBs
• Swift can detect them out to 100 Mpc
• Are all short GRBs magnetar flares?
• Uncertainties are the progenitors of magnetars
  and the number-intensity relation for giant flares

GIANT FLARE FROM SGR1806-20 RHESSI DATA
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CONCLUSIONS

• Good evidence now links some of the long GRBs to
  Type Ic supernovae and the deaths of massive
  stars
• The origin of one X-ray flash has been determined
  but does this explain all of them?
• The origin of the short bursts is probably the
  most outstanding mystery neutron star/neutron
  star mergers, magnetar flares in nearby galaxies,
  both, something else?
• The mystery of the dark bursts is being solved
  but are some at high redshift?
• GRBs are bright enough to be detected out to
  zgt10 but are they actually generated there?
• HETE, INTEGRAL, and Swift may solve these
  mysteries

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GRB
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(No Transcript)
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Oh oh
khurley_at_ssl.berkeley.edu
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THREE INTERESTING GAMMA-RAY BURST/SUPERNOVA
PARAMETERS
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THE Epeak-Eisotropic energy RELATION

• Amati (2002) found that the peak energy in a GRB
  spectrum is related to the isotropic equivalent
  energy Epeak?Eiso0.52 (BeppoSAX results)
• Lamb (2004) has begun to extend this relation
  down to the XRFs using HETE results the
  relation holds also for XRFs
• There are still several possible explanations for
  this, but in any case it strongly suggests that
  XRFs and GRBs are related

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COMPARISON OF CURRENT MISSIONS