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COSMIC GAMMA-RAY BURSTS The Current Status

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

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COSMIC GAMMA-RAY BURSTS The Current Status

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  1. COSMIC GAMMA-RAY BURSTS The Current Status Kevin Hurley UC Berkeley Space Sciences Laboratory

  2. SOME ABSOLUTELY INCONTROVERTIBLE GRB PROPERTIES THAT NO REASONABLE PERSON COULD POSSIBLY DISAGREE WITH • There are two morphological classes of GRBs, long bursts (~20 s duration) and short bursts (~0.2 s duration) • Counterparts and redshifts have been found for many long bursts • No counterpart or redshift has been found for any short burst • 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) • There is good evidence which links some long bursts to the deaths of massive stars

  3. The energy spectra of the long bursts form a continuum, from X-ray flashes (with few or no γ-rays), X-ray rich bursts, and GRBs • 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

  4. SHORT BURST

  5. LONG BURST

  6. LONG BURSTS ~75% SHORT BURSTS (~25%) THE GRB DURATION DISTRIBUTION WE ONLY KNOW ABOUT THE ORIGIN OF THE LONG BURSTS SOFTER ENERGY SPECTRA HARDER ENERGY SPECTRA

  7. Epeak~100’s of keV …OBSERVED UP TO 18 GeV ENERGY SPECTRA OF THE LONG BURSTS 

  8. 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  Epeak~200 keV Epeak~keV  X-RAY FLASH

  9. GAMMA-RAY BURSTS ARE FOLLOWED BY X-RAY AFTERGLOWS… 1-10 keV 1’ T0+8h T0+2d BeppoSAX: Costa et al. 1997

  10. …OPTICAL AFTERGLOWS… Pandey et al. 2004

  11. …AND RADIO AFTERGLOWS 100 Flux density, μJy 10 Frail et al. 2003 1 1 10 100 1000 Time after GRB970508, days

  12. X-RAYS g-RAYS OPTICAL INTERNAL SHOCK RADIO EXTERNAL SHOCK FIREBALL MODEL 1000-2000 AU 1-6 AU G2 G1 ISM 20 km

  13. GRB990123 (BATSE) SIMULTANEOUS OPTICAL/GAMMA-RAY EMISSION HAS NOW BEEN DETECTED TWICE ROTSE (www.rotse.net)

  14. GRB041219 (INTEGRAL) RAPTOR (http://www.raptor.lanl.gov/index.htm)

  15. GRB HOST GALAXIES • Aren’t pretty; but they are normal • Not active galaxies • Indistinguishable from field galaxies with similar ages 990506 990705 (z=0.8424) 980613(z=1.0964) 980519 980329 000301(z=2.0335)

  16. ONLY ONE REDSHIFT HAS BEEN MEASURED FOR AN X-RAY FLASH z=0.25 REDSHIFT DISTRIBUTION OF 34 LONG GAMMA-RAY BURSTS LOWEST REDSHIFT=0.104 (INTEGRAL, GRB031203); HIGHEST=4.5 (IPN, GRB000131); AVERAGE=1.4

  17. GRB ENERGETICS • Isotropic gamma-ray energies range from >1051 to >1054 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

  18. BREAK THE OPTICAL AFTERGLOW CAN GIVE INFORMATION ABOUT BEAMING OBSERVER AFTERGLOW INTENSITY TIME

  19. 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

  20. >25 keV  rays: 65% 1-10 keV X-rays: 7% Optical: 0.1% Radio ? MeV/GeV/TeV ? >10%? Gravitational radiation ? >25 keV  rays: 7% 1-10 keV X-rays: 9% Optical: 2% Radio: 0.05% HOW IS THE ENERGY DISTRIBUTED? DURING THE BURST AFTERGLOW

  21. GRB030329 – THE “POSTER CHILD”* FOR THE GRB-SUPERNOVA CONNECTION • GRB030329 was a bright (top 1%) nearby (z=0.17) burst, discovered by HETE • It is the best-studied GRB to date (>>100 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

  22. Matheson et al. 2004 Stanek et al. 2003 • 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 GRB’s are associated with the deaths of massive stars (>30M)

  23. DARK BURSTS MYSTERY OF THE OPTICALLY DARK BURSTS Fox et al. 2003

  24. THE MYSTERY OF THE OPTICALLY DARK BURSTS IS BEING SOLVED Confirmed by observation?   Not so far • 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

  25. DARK BURSTS OBSERVATIONS OF SWIFT BURSTS       

  26. 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, z=0.251); in this case, the answer is clearly 2 (Soderberg et al. 2004)

  27. 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

  28. 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? • GRB’s are bright enough to be detected out to z>10 – but are they actually generated there? • HETE, INTEGRAL, and Swift may solve these mysteries

  29. Stellar collapse UHE cosmic ray acceleration;  Merging neutron stars, GW Early universe, reionization Quantum gravity Mass extinctions, morbid curiosity of the general public GRB

  30. Oh oh… khurley@ssl.berkeley.edu

  31. THREE INTERESTING GAMMA-RAY BURST/SUPERNOVA PARAMETERS

  32. 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 XRF’s using HETE results: the relation holds also for XRF’s There are still several possible explanations for this, but in any case it strongly suggests that XRF’s and GRB’s are related THE Epeak-EisotropicenergyRELATION

  33. FOV, sr # BURSTS/ YEAR LOCALIZATION ACCURACY ONBOARD FOLLOWUP X-RAYS? OPTICAL? IPN 4π 100 5’ NO NO HETE NO NO 1.6 25 1’ INTEGRAL 0.02 8 1.5’ NO NO 1.4 Swift 84 3’ YES YES COMPARISON OF CURRENT MISSIONS

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