1. Explosões Cósmicas de Raios Gama (Gamma-Ray Bursts)
Nova Física
no Espaço 2003
João Braga – INPE
breve história dos GRBs
BeppoSAX: afterglows
galáxias hospedeiras e redshifts
modelos para os progenitores
resultados recentes (HETE)
SWIFT, MIRAX e o futuro

2. History
July 1967: Vela satellites detect strong gamma ray signals coming from space
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:
almost no association with known objects
statistically poor distribution 
no clue

3. History 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)
8 s oscillations in ~200 s (softer emission) 
Nature of GRBs associated with Galactic neutron stars:
rapid variability  compact object (light-seconds)
cyclotron tens of keV  B ~ 1012 G :  = eB/mc
emission hundreds of KeV  redshifted 511 keV
zobs = z0 (1 – 2GM/c2 R)
periodicity  rotation of a NS : R3 < (GM/42) T2

4. 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 
Hundreds of models proposed

7. BATSE – COMPTON GRO Bimodal distribution
Euclidean
Bimodal distribution
— most are longer than 2 s
— ~1/3 are shorter than 2 s
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

8. Soft Gamma Ray Repeaters SGR
Burst of March 5th, 1979 (SGR )
SNR N49 in LMC (~10,000 ys) 
SOFT GAMMA RAY REPEATERS
bursts repeat in random timescales (normally hundreds of times) (4, maybe 5 objects known)
soft spectra (E  100 keV)
short duration (~100 ms)
Galactic “distribution”, associated with SNRs
possibly associated with magnetars and AXPs

9. Soft Gamma Ray Repeaters SGR

10. BeppoSAX and Afterglows
 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)
WFC

11. BeppoSAX and Afterglows
97 Feb 28: GRB
Discovered by GRBM and WFC
NFIs observe 1SAX J 
First clear evidence of a GRB X-ray tail
 Non-thermal spectra
 X-ray fluence is 40% of -ray fluence

12. BeppoSAX and Afterglows
BeppoSAX and RXTE discovered several other afterglows
Optical transients:
Observed in appr. ½ of the well localized bursts
GRB is the only one observed in the optical when the gamma-ray flash was still going on

13. GRB 990123 z=1.6 Keck OT spectrum HST image: host is an
irregular, possibly merging system

14. GRBs observed by BeppoSAX

15. GRB

16. GRB

17. Host galaxies Optical IDs  distant galaxies (low luminosity, blue)
~30 measured redshifts
All in the z = 0.3 – 4.5 range, with the exception of GRB , possibly associated with SN z = 0.008
OT is never far from center

18. redshifts
GRB
z=1.6
Keck OT spectrum

19. Energy (isotropy)
redshifts

20. redshifts & cosmology

21. Types of Bursts 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:
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.

22. Types of Bursts  25% 20 30% no 30 20% 0.3 ? Burst Class Long (normal)
Percentage of all
bursts
Typical duration
(sec)
Initial gamma-ray emission
Afterglow X-ray emission
Afterglow optical emission
Long
(normal)
25% 20 
(dark)
30% no
(X-ray rich or XRF) 30
Absent or weak
short
20%
0.3 ?

23. GRBs may be associated with rare types of supernovae
Progenitors
Long GRBs are probably associated with massive and short-lived progenitors 
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

24. Progenitors Short GRBs - ?? associated with mergers of compact objects
SGRs in external galaxies
phase transition to strange stars

25. Relativistic expanding fireball (e± , )
The fireball model
Observed fluxes require 1054 erg emitted in seconds in a small region (~km) 
Relativistic expanding fireball (e± , )
Problem: energy would be converted into Ek of accelerated baryons, spectrum would be quasi-thermal, 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.

26. 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

27. The fireball model

28. The cannonball model Bipolar 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 quasi-
thermal 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

29. HETE High Energy Transient Explorer
space.mit.edu/HETE
First dedicated GRB mission, X- and g-rays
Equatorial orbit, antisolar pointing
launched on Oct 9th, Pegasus
3 instruments, 1.5 sr common FOV
SXC ( keV) - < 30” localization
WXM (2 –25 keV) - < 10’ localization
FREGATE (6-400keV) -  sr localization
Rapid dissemination ( 1s) of GRB positions
(Internet and GCN)

30. HETE

31. HETE Investigator Team
RIKEN
Masaru Matsuoka Nobuyuki Kawai Atsumasa Yoshida
MIT
George R. Ricker (PI) Geoffrey Crew John P.Doty Al Levine Roland Vanderspek Joel Villasenor
UC Berkeley
Kevin Hurley J. Garrett Jernigan
UChicago
Donald Q. Lamb Carlo Graziani
CESR
Jean-Luc Atteia Gilbert Vedrenne Jean-Francois Olive Michel Boer
INPE
João Braga
LANL
Edward E. Fenimore Mark Galassi
CNR
Graziella Pizzichini
CNES
Jean-Luc Issler
UC Santa Cruz
Stanford Woosley
SUP’AERO
Christian Colongo
TIRF
Ravi Manchanda

32. Ground station network

33. IPN annulus (radius 60o ± 0.118o)
HETE results GRB
Bright (>80) burst detected on Sept 21, :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 
IPN annulus (radius 60o ± 0.118o)
intersection gives error region with
310 arcmin2 centered at
 ~ 22h55m30s,  ~ 40052’

34. GRB

35. 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 keV
Peak photon flux is ~4 times higher in the 4-25 keV

36. GRB
Long duration GRB
X-ray rich, but no XRF (high keV flux)
z =  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

37. GRB

38. GRB 020405 Highly significant polarization (9.9%) in the V band
measured 1.3 days after the burst
z = 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

39. 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

40. 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

41. 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

42. GRB

43. New missions 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.

44. 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 ) 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.

45. 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).

46. 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?