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 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

6.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 7.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

SHORT BURST

LONG BURST

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

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

THE ENERGY SPECTRA OF THE LONG BURSTS FORM A CONTINUUM, FROM SOFT-SPECTRUM X-RAY FLASHES TO HARD-SPECTRUM GAMMA-RAY BURSTS (BeppoSAX, HETE) X-RAY FLASH GAMMA-RAY BURST E peak ~keV   E peak ~200 keV

GAMMA-RAY BURSTS ARE FOLLOWED BY X-RAY AFTERGLOWS… BeppoSAX: Costa et al T 0 +8h T 0 +2d 1-10 keV 1’

…OPTICAL AFTERGLOWS… Pandey et al. 2004

…AND RADIO AFTERGLOWS Time after GRB970508, days Flux density, μJy Frail et al. 2003

FIREBALL MODEL   ISM INTERNAL SHOCK  RAYS EXTERNAL SHOCK X-RAYS OPTICAL RADIO 20 km 1-6 AU AU

SIMULTANEOUS OPTICAL/GAMMA-RAY EMISSION HAS NOW BEEN DETECTED TWICE GRB (BATSE) ROTSE (

GRB (INTEGRAL) RAPTOR (

(z=0.8424) (z=1.0964) (z=2.0335) GRB HOST GALAXIES Aren’t pretty; but they are normal Not active galaxies Indistinguishable from field galaxies with similar ages

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

GRB ENERGETICS Isotropic gamma-ray energies range from >10 51 to >10 54 erg Two possibilities for liberating large amounts of energy: 1.Merging neutron stars (short bursts?) 2.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

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

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 ~ 1.3x10 51 erg Isotropic energies, no beaming Corrected for beaming Frail et al. 2001

HOW IS THE ENERGY DISTRIBUTED?  keV  rays: 65% keV X-rays: 7% 3Optical: 0.1% 4Radio ? 5MeV/GeV/TeV ? >10%? 6Gravitational radiation ?  keV  rays: 7% keV X-rays: 9% 3Optical: 2% 4Radio: 0.05% DURING THE BURSTAFTERGLOW

GRB – THE “POSTER CHILD”* FOR THE GRB-SUPERNOVA CONNECTION GRB 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 GRB (=SN1998bw, peculiar Type Ic – the previous poster child), but GRB 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

Matheson et al 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  ) Stanek et al. 2003

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

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? 1.Absorbed by dust within the host galaxy? 2.Intrinsically faint and/or rapidly fading? 3.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

OBSERVATIONS OF SWIFT BURSTS DARK BURSTS       

WHAT ARE X-RAY FLASHES? 1.GRBs observed away from the jet axis? 2.Explosions with less relativistic ejecta? 3.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)

ARE THE SHORT GRBS NEARBY MAGNETAR FLARES? GIANT FLARE FROM SGR RHESSI DATA Giant flares begin with ~0.2 s long, hard spectrum spikes Their energy can be ~10 47 erg The spike is followed by a pulsating tail with ~1/1000 th 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

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

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

Oh oh…

THREE INTERESTING GAMMA-RAY BURST/SUPERNOVA PARAMETERS

THE E peak -E isotropic energy RELATION Amati (2002) found that the peak energy in a GRB spectrum is related to the isotropic equivalent energy: E peak  E iso 0.52 (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

COMPARISON OF CURRENT MISSIONS FOV, sr # BURSTS/ YEAR LOCALIZATION ACCURACY IPN4π4π1005’HETE ’ INTEGRAL ’ Swift ’ NO ONBOARD FOLLOWUP X-RAYS? OPTICAL? NO YES