Gamma-Ray Bursts and their optical counterparts

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Unveiling GRB hard X-ray afterglow emission with
Simbol-X
L. Amati, E. Maiorano, E. Palazzi, R. Landi, F.
Frontera, N. Masetti, L. Nicastro, M.
Orlandini INAF-IASF Bologna (Italy)
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The GRB phenomenon

• prompt emission

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• afterglow emission

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• prompt gt afterglow

• Swift era (2005-)

• BeppoSAX era (1996-2002)

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LONG (2 s 2000s)
SHORT (0.001-2s)

• 0.1 lt z lt 6.4 , isotropic radiated energy from
  1050 up to gt1054 erg
• possibly collimated emission
• evidence of dense metal rich circum-burst
  environment
• located in star forming region of high SFR host
  galaxies
• GRB-SN (hypernovae) connection
• origin death of peculiar high mass stars
  (collapsar scenario)

• still a few afterglow detections and z estimates
  (z lt 1)
• energy budget up to 5x1051
• host galaxies no clear distinction with those
  of long
• origin merging of compact objects (NS-NS,
  NS-BH, )

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but prompt and afterglow emission mechanisms
still to be settled !

• ms time variability huge energy detection of
  GeV photons -gt plasma occurring
  ultra-relativistic (G gt 100) expansion (fireball)
  
• non thermal spectra -gt shocks synchrotron
  emission (SSM)
• fireball internal shocks -gt prompt emission
• fireball external shock with ISM -gt afterglow
  emission

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• deviations form synchrotron predictions
  observed in prompt emission spectra of a fraction
  of GRBs
• physics of prompt emission still not settled,
  various scenarios SSM internal shocks,
  IC-dominated internal shocks, external shocks,
  photospheric emission dominated models, kinetic
  energy dominated fireball , poynting flux
  dominated fireball)

Frontera et al. (2000)
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Afterglow emission less complex
The multi-wavelength afterglow emission is
modeled as due to synchrotron. F(t,?) ? t-a
?-ß with a and ß depending on p, where N(E) ?
E-p is the electron energy distribution.
Sari et al. (1998, 1999)
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but standard model not always works !

• SED of GRB 970508 fit with standard synchrotron
  shock model in slow cooling regime is OK
• SED of GRB 000926 excess of X-ray emission with
  respect to synchrotron prediction IC component ?
  

Galama et al. (1997)
Harison et al. (2001)
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The puzzling case of GRB990123 - I

• Only one case of afterglow emission clear
  detection at energies gt 15 keV the bright GRB
  990123 by BeppoSAX/PDS
• The 15-60 keV flux is inconsistent with the lower
  energy spectrum and synchrotron emission models
  predictions

Maiorano et al. (2005)
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The puzzling case of GRB990123 - II

• the fit with a synchrotron IC component is
  more satisfactory, but still problems with the
  closure relationships between spectral and
  decay indices
• alternative explanations include peculiar
  circum-burst properties and/or peculiar shock
  physics
• this shows the relevance of sensitive
  measurements of GRB hard X-ray afterglow emission

Corsi et al. (2005)
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The ambiguous case of GRB 990806

• Hard X-ray afterglow emission might have been
  detected also for the BeppoSAX GRB990806
• but the presence of another X-ray source in the
  PDS field of view, as LECS and MECS images show
  (Fig. 5), make the detection quite uncertain
• relevance of hard X-ray imaging

From BeppoSAX ASDC archive
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Afterglow X-ray emission with Simbol-X - I

• Unprecedented sensitivity 15-60 keV less than 1
  mCrab (several hundreds times better than
  BeppoSAX/PDS)
• Imaging capability comparable to lower energy
  X-ray telescopes
• at 11hr from the GRB 1/3 of GRB afterglows show a
  flux gt 100 µcrab
• Critical issue time needed to be on-target for
  a TOO observation (12 hours ? 1-2 days ?)

Pareschi Ferrando (2006)
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Afterglow X-ray emission with Simbol-X - II

• 10 brightest BeppoSAX (and Swift) afterglows
  show a 2-10 keV flux gt 230 µCrab (5x10-12
  erg/cm2/s) at 11hr from the burst
• by assuming the average photon index of 2.2
  (Crab-like) and the average temporal decay index
  (1.3), the expected 15-60 keV flux at 48 hr is
  about 35 µCrab, and the average flux from 48hr
  to 76 hr (corresponding to a 100 ks long
  observation period) is 25 µCrab
• even a 100 ks Simbol-X observation starting at 2
  days after the GRB will allow a sensitive
  spectral measurement in 15-60 keV

De Pasquale et al. (2006)
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Afterglow X-ray emission with Simbol-X - III

• Simulation of Simbol-X image and spectrum of a
  bright afterglow (NgtF 10) with no IC component
  observed with a 100 ks TOO starting 2 days after
  the GRB (assumed decay index 1.3, the average
  flux is 25 mCrab)
• clear detection (about 18 s) in the image and
  well determined spectral shape in 10-60 keV
• this simulation is equivalent to a 100 ks
  observation of an afterglow of medium intensity
  observed after 12 hr from the GRB

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Afterglow X-ray emission with Simbol-X - IV

• Simulation of Simbol-X spectrum of GRB 990123
  (including IC component) with a 100 ks TOO
  starting 2 days after the GRB (decay index of 1.3
  assumed, the average flux is 25 mCrab)
• the IC excess in 20-60 keV is clearly visible in
  the residuals and has a 6.5 s significance
• this simulation is equivalent to a 100 ks
  observation of an afterglow of medium intensity,
  showing an IC component and observed after 12 hr
  from the GRB

BeppoSAX at 6-10 hr
Simbol-X at 48-72 hr
Maiorano et al. (2005)
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Who will provide GRB detection and localization ?

• These simulations are very conservative it will
  be likely possible to perform TOO observations of
  brigth afterglows at 12/24 hr from GRB onset
  (fluxes of 210-80 mCrab for the brightest 10
  assuming decay index 1.3)
• GRB detection and localization to a few arcmin is
  necessary who will provide it in the gt2012 time
  frame ?
• Swift (operating since December 2004, detection
  and arcsec localization), Chandra, XMM, Suzaku
  (afterglow localization to a few arcsec, flux,
  decay index), AGILE (GRB detection and a few
  arcmin localization) who knows ?
• GLAST (GRB detection and possibly few arcmin
  localization) likely
• SVOM/ECLAIRS (GRB detection and few arcmin
  localization) likely
• EDGE (GRB detection and localization, afterglow
  few arcsec localization, flux, decay index),
  Lobster (GRB detection and a few arcmin
  localization), maybe
• Optical telescopes (afterglow localization,
  brightness, decay index) always
• Information on afterglow brightness and decay
  slope is also important to decide to perform a
  TOO if X-ray information is lacking, these can
  be inferred from prompt emission intensity and
  optical afterglow intensity and decay slope

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Conclusions

• despite the enormous observational progress
  occurred in the last 10 years, the GRB phenomenon
  is still far to be fully understood
• one of the main open issues is the understanding
  of physical mechanisms at the basis of prompt
  and afterglow emission
• the case of GRB 990123 shows that measurements of
  the nearly unexplored GRB hard (gt 15 keV) X-ray
  afterglow emission can provide very stringent
  test to emission models
• thanks to its unprecedented sensitivity in the
  15-60 keV energy band, Simbol-X can provide a
  significant step forward in this field
• simulations based on observed distribution of
  X-ray afterglow fluxes and spectral and decay
  indices show that even a 100 ksTOO observation
  starting 2 days after the GRB can provide
  sensitive spectral measurements and allow to
  discriminate different emission components for a
  significant fraction of events
• it is likely that significantly lower TOO stat
  times (12/24 hr) will be possible for a few
  event/year
• the needed GRB detection and few arcmin
  localizations will be provided by space missions
  likely flying in the gt2012 time frame and optical
  telescopes

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The fireball model

• ms time variability huge energy detection of
  GeV
• photons -gt plasma occurring
  ultra-relativistic (G gt 100)
• expansion (fireball)
• non thermal spectra -gt shocks synchrotron
  emission
• fireball internal shocks -gt prompt emission
• fireball external shock with ISM -gt afterglow
  emission

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LONG
SHORT

• energy budget up to gt1054 erg
• long duration GRBs
• metal rich (Fe, Ni, Co) wind
• circum-burst environment
• GRBs occur in star forming
• regions
• GRBs are associated with SNe
• naturally explained collimated
• emission

• energy budget up to 1049
• 1050 erg
• short duration GRBs (lt 2 s)
• clean homogeneous circum-burst
• environment
• GRBs in the outer regions of the
• host galaxy

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The fireball model
Ultrarelativstically expanding source releases a
huge amount of energy (1051 erg) in a rather
small volume (R10-1000 km), in form of a
fireball expanding with relativistic velocity.
The central engine produces several shells with
different Lorentz factors that can overtake each
other and collide, causing a relativistic blast
wave (Blandford McKee 1976).
In the internal-external model, when shells with
different velocity collide each other (internal
shocks) produce the gamma-ray burst,while the
afterglow occurs when the fireball hits the
surrounding material (external shoks) (Sari 1997).
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Prompt and afterglow light curves
(Corsi et al. 2005)
WFC (top and central panel) and GRBM (bottom
panel) light curves. Hard-to-soft evolution is
present. Atmospheric absorption in the GRB tail
(80 sec) affected soft X-ray data.
Multiwavelength light curves from the prompt
event to the afterglow. t0 corresponds to the
time of GRB onset.
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The prompt event
t 7 s
t 32 s
t 58 s

(Corsi et al. 2005)
(Amati et al. 2002)
Simultaneous multiwavelength spectra derived at
three times during the burst (ROTSE V-band,X-ray
and ?-ray)
GRBMWFC average spectrum is well fit with the
Band function a -0.89 0.08 ß -2.45 0.97
Eb 703 32 keV
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The broadband spectrum of the afterglow
The dashed line is the best-fit power-law
describing optical and NIR data ßopt 0.60
0.04 The solid line is the power-law which best
fits the X-ray data ßx 0.94 0.07
The spectral turnover between optical and X-ray
bands is identified with the presence of the
synchrotron cooling frequency at ?c 0.47 keV
1.14 x 1017 Hz
.
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Closure Relations
aopt 1.10
ax 1.46

ßopt 0.60
?c 1.1 x 1017 Hz
ßx 0.94

ßxp/2
Fully consistent with the value we found
?o lt ?c lt ?x
p 2
ßopt (p-1)/2 0.5
We expect aX - aopt (3p-2)/4 3(p-1)/4
1/4 ßX - ßopt p/2 (p-1)/2 1/2 We
observed aX - aopt 0.36 0.05 ßX - ßopt 0.34
0.08
Assuming isotropically adiabatic expansion
within homogeneous medium
Both consistent with the value we found
ßopt/ aopt 2/3 0.54 0.04 (ßX 1/3) /aX
2/3 0.30 0.05
but

Both the measured ratios are statistically
inconsistent with the expectations
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Conclusions GRB990123

• One of the most energetic events and first case
  of prompt optical emission
• WFC detection of the afterglow already 20 min
  after the GRB
• Smooth GRB-afterglow light curve connection
• First X-ray afterglow detected up to 60 keV in
  the PDS
• During the BeppoSAX observation the X-ray
  afterglow decays faster than the optical one
• On 24.65 January 1999 UT the broadband afterglow
  of GRB990123 is consistent with a synchrotron
  spectrum with ?c located at the lower energy
  border of the X-ray range covered by BeppoSAX
• A self-consistent interpretations of the
  afterglow with pure synchrotron emission is not
  viable. Also the presence of IC component in the
  X-ray band (Corsi et al. 2005) does not overcome
  the inconsistency. A more complex model is
  required to solve the puzzle.

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The fireball model

• ms time variability huge energy detection of
  GeV
• photons -gt plasma occurring
  ultra-relativistic (G gt 100)
• expansion (fireball)
• non thermal spectra -gt shocks synchrotron
  emission
• fireball internal shocks -gt prompt emission
• fireball external shock with ISM -gt afterglow
  emission

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Headlines of this talk

• GRB afterglow emission
• Hard X-ray emission

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The puzzling case of GRB990123 - I
15-28 keV
2-10 keV
The dashed line is the best-fit decay obtained
from the X-ray afterglow data. The extrapolation
smoothly reconnects with the late time WFC data
points and upper limits suggesting that the X-ray
afterglow had already started 20 min after the
prompt event.
LECS,MECS,PDS first 20 ks spectrum an absorbed
power-law with photon index G 1.94 0.07 best
fits the data. NH (Gal) 1.98 x 1020 cm-2
SFD radio, IR and optical flux densities
together with the 2-10 keV flux observed on
24.65 January 1999 UT