COSMIC GAMMA-RAY BURSTSThe Current Status

Kevin HurleyUC 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

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SULYSSES

GRB00060725-150 keV

SHORT BURST

LONG BURST

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THE GRB DURATION DISTRIBUTION

SHORTBURSTS(~25%)

LONGBURSTS

~75%

HARDERENERGYSPECTRA

SOFTERENERGYSPECTRA

WE ONLYKNOW ABOUT

THE ORIGINOF THE LONG

BURSTS

ENERGY SPECTRA OF THE LONG BURSTS

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PO W ER LAW W ITHEXPO N E N TIAL C U TO FF

SM O O TH BR E AK

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B AN D SPEC TR U M

…OBSERVED UP TO 18 GeV

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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)

X-RAY FLASH

GAMMA-RAYBURST

Epeak~keV

Epeak~200 keV

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GAMMA-RAY BURSTS ARE FOLLOWED BY X-RAY AFTERGLOWS…

BeppoSAX: Costa et al. 1997

T0+8h T0+2d

1-10 keV

1’

…OPTICAL AFTERGLOWS…

Pandey et al. 2004

…AND RADIO AFTERGLOWS

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Frail et al. 2003

FIREBALL MODEL

ISM

INTERNALSHOCK

RAYS

EXTERNALSHOCK

X-RAYS

OPTICALRADIO

20 km

1-6 AU

1000-2000 AU

SIMULTANEOUS OPTICAL/GAMMA-RAY EMISSION HAS NOW BEEN DETECTED TWICE

GRB990123(BATSE)

ROTSE (www.rotse.net)

GRB041219(INTEGRAL)

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

990705 (z=0.8424)990506

980613 (z=1.0964)

980519 980329000301(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

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GRB ENERGETICS

• Isotropic gamma-ray energies range from >1051 to >1054 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

AFTERGLOWINTENSITY

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 ~ 1.3x1051 erg

Isotropic energies,no beaming

Correctedfor beaming

Frail et al. 2001

HOW IS THE ENERGY DISTRIBUTED?

keV rays: 65%

2 1-10 keV X-rays: 7%

3 Optical: 0.1%

4 Radio ?

5 MeV/GeV/TeV ? >10%?

6 Gravitational radiation ?

keV rays: 7%

2 1-10 keV X-rays: 9%

3 Optical: 2%

4 Radio: 0.05%

DURING THE BURST AFTERGLOW

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 onposters to raise money for charitable purposes

Matheson et al. 2004

• 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

DARKBURSTS

Fox et al. 2003Fox 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 byobservation? Not so far

OBSERVATIONS OF SWIFT BURSTS

DARKBURSTS

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 SGR1806-20RHESSI DATA

•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

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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?

• 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

UHEcosmic ray

acceleration;

Quantumgravity Mass extinctions,

morbid curiosityof the

general public

Earlyuniverse,

reionization

Merging neutronstars, GW

Stellarcollapse

khurley@ssl.berkeley.edu

Oh oh…

THREE INTERESTING GAMMA-RAY BURST/SUPERNOVA PARAMETERS

GRBs SUPERNOVAE

UNIVERSE-WIDE RATE 100’s-1000/day 100000/day

RATE PER GALAXY 1/105 years 1/50-100 years

ENERGY 1051-52 erg 1051-52 erg

THE Epeak-Eisotropic energy RELATION

• Amati (2002) found that the peak energy in a GRB spectrum is related to the isotropic equivalent energy: EpeakEiso

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

LOCALIZATIONACCURACY

IPN 4π 100 5’

HETE 1.6 25 1’

INTEGRAL 0.02 8 1.5’

Swift 1.4 84 3’

NO NO

NO NO

ONBOARD FOLLOWUPX-RAYS? OPTICAL?

NO NO

YES YES