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CEA
Explosões Cósmicas de
Raios Gama
(Gamma-Ray Bursts)
João Braga – INPE
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breve história dos GRBs
BeppoSAX: afterglows
galáxias hospedeiras e
redshifts
modelos para os progenitores
resultados recentes (HETE)
SWIFT, MIRAX e o futuro
Nova Física
no Espaço 2003
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History
July 1967: Vela satellites detect strong
gamma ray signals coming from space
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16 peculiar events of cosmic origin
short (~s) photon flashes with E > 100 MeV
publication only in 1973 (classified before that)
Phenomenology of bursts before the 90’s:
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almost no association with known objects
statistically poor distribution
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no clue
History
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Burst of March 5th, 1979
 intense -ray pulse (0.2 s), ~100 times as intense as any
previous burst
 SNR N49 in LMC (~10,000 ys)
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8 s oscillations in ~200 s (softer emission)
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Nature of GRBs associated with Galactic neutron stars:
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rapid variability  compact object (light-seconds)
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cyclotron lines @ tens of keV  B ~ 1012 G :  = eB/mc
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emission lines @ hundreds of KeV  redshifted 511 keV
zobs = z0 (1 – 2GM/c2 R)
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periodicity  rotation of a NS : R3 < (GM/42) T2
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BATSE – COMPTON GRO
launched on 1991 - ~10 years
• 2704 bursts (~1 each day)
• Isotropic distribution
- No concentration towards LMC, M31 or nearby
clusters
- No dipole and quadupole moments
• No spectral lines
• No periodicity
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Hundreds of models proposed
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BATSE – COMPTON GRO
• Bimodal distribution
— most are longer than 2 s
Euclidean
— ~1/3 are shorter than
2s
• Spectra: combination of two power-laws
- spectrum softens with time
- Ep decreases with time (in the E.f(E) x E plot)
• Fluence: ~ 10-6 — 10-4 erg cm-2
long duration and hard spectrum bursts deviate more from
a 3-D Euclidean brightness distribution
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Soft Gamma Ray Repeaters
SGR
Burst of March 5th, 1979 (SGR 0526-66)
 SNR N49 in LMC (~10,000 ys)
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SOFT GAMMA RAY REPEATERS
bursts repeat in random timescales (normally
hundreds of times) (4, maybe 5 objects known)
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soft spectra (E  100 keV)
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short duration (~100 ms)
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Galactic “distribution”, associated with SNRs
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possibly associated with magnetars and AXPs
Soft Gamma Ray Repeaters
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SGR
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BeppoSAX
and Afterglows
BeppoSAX:
WFC
 4 narrow field instruments
(.1 to 300 keV; ~arcminute res.)
 Wide Field Camera
(2 to 28 keV; 200 x 200 ; 5’; coded-mask)
 Gamma Ray Burst Monitor
(60 to 600 keV; side shield)
BeppoSAX and
Afterglows
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97 Feb 28: GRB 970228
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Discovered by GRBM and WFC
NFIs observe 1SAX J0501.7+1146
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First clear evidence of a GRB X-ray tail
 Non-thermal spectra
 X-ray fluence is 40% of -ray fluence
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BeppoSAX and
Afterglows
BeppoSAX and RXTE discovered several
other afterglows
Optical transients:
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Observed in appr. ½ of the well localized
bursts
GRB 990123 is the only one observed in the
optical when the gamma-ray flash was still
going on
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GRB 990123
z=1.6
Keck OT spectrum
HST image: host is an
irregular, possibly merging system
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GRBs observed by BeppoSAX
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GRB 011121
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GRB 011121
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Host galaxies
Optical IDs  distant galaxies
(low luminosity, blue)
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~30 measured redshifts
All in the z = 0.3 – 4.5 range, with the
exception of GRB 980425, possibly
associated with SN 1998bw @ z = 0.008
OT is never far from center
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redshifts
GRB 990123
z=1.6
Keck OT spectrum
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redshifts
Energy (isotropy)
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redshifts & cosmology
Types of Bursts
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 Long and short bursts: the normal ones.
Bimodal distribution; short bursts are harder
and have no counterparts; almost all long
bursts have X-ray afterglows.
 Dark bursts: long bursts with X-ray afterglows
but no optical or radio afterglows (½ of them).
Possible explanations:
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Absorption in the host galaxy
They are beamed away from the observer
X-ray flashes (XRF’s): little or no emission
above ~ 25 keV. Possibly related to X-ray rich
GRBs.
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Burst
Class
Types of Bursts
Percentage
of all
bursts
Typical
duration
(sec)
Initial
Afterglow
gammaX-ray
ray
emission
emission
Afterglow
optical
emission
Long
(normal)
25%
20
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Long
(dark)
30%
20
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no
Long
(X-ray rich
or XRF)
25%
30
Absent
or weak
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no
short
20%
0.3
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?
?
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Progenitors
Long GRBs are probably associated with
massive and short-lived progenitors
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GRBs may be associated with rare types of
supernovae
 Hypernovae: colapse of rotating massive
star  black hole accreting from a toroid
 Collapsar: coalescence with a compact
companion  GRBs and SN-type remnant
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Progenitors
Short GRBs - ??
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associated with mergers of compact objects
SGRs in external galaxies
phase transition to strange stars
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The fireball model
Observed fluxes require 1054 erg emitted in seconds
in a small region (~km)
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Relativistic expanding fireball (e± , )
Problem: energy would be converted into Ek of
accelerated baryons, spectrum would be quasithermal, and events wouldn’t be much longer
than ms.
Solution: fireball shock model: shock waves will
inevitably occur in the outflow (after fireball
becomes transparent)  reconvert Ek into
nonthermal particle and radiation energy.
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The fireball model
Complex light curves are due to internal shocks
caused by velocity variations.
Turbulent magnetic fields built up behind the
shocks  synchrotron power-law radiation
spectrum  Compton scattering to GeV range.
Jetted fireball: fireball can be significantly
collimated if progenitor is a massive star with
rapid rotation  escape route along the rotation
axis  jet formation  alleviate energy
requirements  higher burst rates
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The fireball model
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Bipolar
The cannonball model
jets of highly relativistic cannon balls
are launched axially in core-collapse SNe
The CB front surfaces are collisionally heated
to ~keV as they cross the SN shell and the
wind ejecta from the SN progenitor
A gamma-ray pulse in a GRB is the quasithermal radiation emitted when a CB
becomes visible, boosted and collimated by
its highly relativistic motion
The afterglow is mainly synchrotron radiation
from the electrons the CBs gather by going
through the ISM
HETE
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High Energy Transient Explorer
space.mit.edu/HETE
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First dedicated GRB mission, X- and -rays
Equatorial orbit, antisolar pointing
launched on Oct 9th, 2000 - Pegasus
3 instruments, 1.5 sr common FOV
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SXC (0.5-10 keV) - < 30” localization
WXM (2 –25 keV) - < 10’ localization
FREGATE (6-400keV) -  sr localization
Rapid dissemination ( 1s) of GRB positions
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(Internet and GCN)
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HETE
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HETE Investigator Team
MIT
George R. Ricker (PI)
Geoffrey Crew
John P.Doty
Al Levine
Roland Vanderspek
Joel Villasenor
LANL
Edward E. Fenimore
Mark Galassi
UC Santa Cruz
Stanford Woosley
RIKEN
UC Berkeley
Masaru Matsuoka Kevin Hurley
Nobuyuki Kawai J. Garrett Jernigan
Atsumasa Yoshida
UChicago
CESR
Donald Q. Lamb
Jean-Luc Atteia
Carlo Graziani
Gilbert Vedrenne
INPE
Jean-Francois Olive João Braga
Michel Boer
CNR
CNES
Graziella Pizzichini
Jean-Luc Issler
TIRF
SUP’AERO
Christian Colongo Ravi Manchanda
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Ground station network
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HETE results
GRB 010921
Bright (>80) burst detected on Sept 21, 2001
05:15:50.56 UT by FREGATE
First HETE-discovered GRB with counterpart
Detected by WXM, giving good X position
(10o x 20’ strip)
Cross-correlation with Ulysses time history
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IPN annulus (radius 60o ± 0.118o)
intersection gives error region with
310 arcmin2 centered at
 ~ 22h55m30s,  ~ 40052’
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GRB 010921
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GRB 010921
 Highly symmetric at high
energies
 Lower S/N for WXM due to
offset
 Durations increase by 65% at
lower energies
 Hard-to-soft spectral evolution
 Peak energy flux in the 4-25
keV band is 1/3 of 50-300 keV
 Peak photon flux is ~4 times
higher in the 4-25 keV
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GRB 010921
 Long duration GRB
 X-ray rich, but no XRF (high 50-300 keV flux)
 z = 0.450  isotropic energy of 7.8 x 1051 erg
(M=0.3, =0.7, H0=65 km s-1 Mpc-1) - less if
beamed
 Second lowest z  strong candidate for
extended searches for possible associated
supernova
 Final position available 15.2h after burst 
ground-based observations in the first night 
counterpart established well within HETE-IPN
error region
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GRB 011211
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GRB 020405
Highly
significant polarization (9.9%) in the V band
measured 1.3 days after the burst
z = 0.695 based on emission lines of
host galaxy
High polarization can be due to:
line of sight at the very edge of the jet if the
magnetic field is restricted to the plane of the shock
alignment of the magnetic field over causally connected
regions in the observed portion of the afterglow
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GRB 020531
 Short, hard GRB detected by FREGATE and WXM on 31
May 2002
 Short, intense peak followed by a marginal peak, which
is common on short, hard bursts
 T50 = 360 msec in the 85 – 300 keV band
 Preliminary localization 88min after burst,
refined IPN localization 5 days after burst
RA = +15h 15m 04s, Dec = -19o 24’ 51”
(22 square arcmin hexagonal region)
 Follow-up at radio, optical and X-rays
 Duration increases with decreasing energy
and spectrum evolves from hard to soft
► seem to indicate that short, hard bursts are
closed related to long GRBs
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GRB 021004
detected by Fregate, WXM and SXC
duration of ~100 sec (long GRB)
GCN position notice (WXM) given 49 s
after the beginning of the burst
SXC location given 154 min after burst
optical afterglow (R) detected in 9 min (15th mag)
HST and Chandra observed in the following day
best observed burst so far
absorption redshift of 2.3 (C IV, Si IV, Ly)
unusual brightenings seen in the light curve
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GRB 021211
Dark burst
Duration of ~2.5 sec (“ transitional” GRB)
GCN position notice (WXM) given 22 s
after the beginning of the burst
Raptor (LANL) observed 65 sec after burst
Optical afterglow extremely faint after 2 hours
GRB may have occurred on region with no
surrouding gas or dust, so the shock wave
had little material to smash into  may
support the binary merger theory for short GRB
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GRB 030115
New missions
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SWIFT (US): 3 instruments, large area, 250-300 bursts/yr,
coverage from optical to gamma-rays,
arcsecond positions, will detect bursts up
to z ~20. Will be launched in 2003.
INTEGRAL (Europe): launched last year. Several
instruments with high energy resolution.
EXIST (US): huge area hard X-ray mission for 2010.
GLAST (US): large area high energy gamma-ray
mission; will study high energy afterglows. To be
launched around 2007.
MIRAX (Brazil, US, Holland, Germany): broadband
imaging (6’) spectroscopy of a large source sample (1000
square degrees) in the central Galactic plane region.
Expected to detect ~1 GRB/month. Two hard X-ray
cameras and the flight model of the WFC. To be launched
in ~2007.
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What we do “know”
about GRBs so far
 Every GRB signals the birth of a sizable stellar-mass black
hole somewhere in the observable universe.
 Long GRBs occur in star forming galaxies at an average
redshift of ~1.
 There are now plausible or certain host galaxies found for all
but 1 or 2 GRBs with X-ray, optical or radio afterglows
positioned with arcsecond precision.
 ~30 redshifts have been measured for GRB hosts and/or
afterglows, ranging from 0.25 (or maybe 0.0085) to 4.5.
 BATSE results and current estimates for beaming imply that
GRBs occur at a rate of 1000/day in the universe.
 In a few cases, marginal evidence exist for transient X-ray
emission lines and absorption features in the prompt and
early afterglows.
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What to expect in
the coming years
 Early afterglows will be carefully studied  the missing link
between the prompt emission and the afterglow will be identified;
 The jet configuration will be identified  universal structured jet
model will be validated by future data;
 With accumulation of a large sampe of spectral information and
redshifts for GRB/XRF with Swift, we will know a lot more about
the site(s) and mechanism(s) for the prompt emission;
 Detection of GRB afterglows with z > 6 may provide a unique way
to probe the primordial star formation, massive IMF, early IGM,
and chemical enrichment at the end of the cosmic reionization era.
(Djorgovski et al. 2003);
 With Swift, we should get ~120 GRBs to produce Hubble diagrams
free of all effects of dust extinction and out to redshifts impossible
to reach by any other method (Schaefer 2003).
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Open questions
 What is the exact nature of the central engine?
 Why does it work so intermittently, ejecting blobs
with large contrast in their bulk Lorentz factors?
 What is the radiation mechanism of the prompt
emission?
 What is the jet angle? If between 2o and 20o, the
energy can vary by ~500 (~1050 – 1052 erg)
 What is the efficiency of converting bulk motion
into radiation?