GRB 080319B

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GRB 080319B the Naked Eye Burst
Stefano Covino, on behalf of the MISTICI
collaboration INAF / Brera Astronomical
Observatory
5th Italian-Sino Workshop on Relativistic
Astrophysics
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• In the last few years our understanding of GRB
  phenomena has increased considerably.
• At least, observationally, we now know GRBs are
  much more complex than previously thought.

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• There is general consensus about a few firmly
  established issues (Kumar Panaitescu 2008)
• GRBs arise from highly relativistic collimated
  outflows
• At least a few long-duration GRBs are associated
  with the collapse of massive stars
• Less than a few percent SN Ib/c give rise to
  GRBs
• Some short-duration GRBs are associated to old
  stellar populations.
• And there are a number of unanswered questions.
  In particular
• How ?-rays are produced?
• What is the composition of the relativistic jet
  (baryonic, e and/or magnetic)

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• Observationally, there is still a regime not yet
  probed, and accessible thanks to somehow
  fortuitous circumstances (and strongly related to
  the unaswered questions reported before).
• GRB 080319B, with a very bright optical flash
  (5th visual mag), offers a magnificent test bed.

Credit Pi of the Sky
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• Lets try to summarize what we have
• Bright optical flash roughly coincident in time
  with the prompt emission
• wide field optical telescopes were monitoring the
  field before the high-energy alert
• high-energy prompt emission observed by
  Konus-Wind and Swift-BAT
• early afterglow observed by wide-field telescopes
  and narrow-field robotic telescopes (REM, UVOT)
• Swift-XRT monitoring of the X-ray light-curve
• multi-band optical/NIR early/intermediate/late
  time monitoring with a wealth of ground-based
  telescopes
• low and high-resolution, time-resolved,
  spectroscopy of the afterglow
• HST observations of the host galaxy.

An unprecedented high-quality dataset deserving
the best modeling efforts!
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• How unusual is GRB 080319B?

• GRB 080319B compared to other GRBs or
  cosmological sources (Bloom et al.
  astro-ph/0803.3215)

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• Many possible interpretations have been delivered
  so far (Dado et al. astro-ph/0804.0621 Kumar
  Panaitescu astro-ph/0805.0144).
• Now, in the context of the standard fireball
  model, we (Racusin et al.) propose that
• the prompt optical and ?-ray emissions from this
  event likely arise from different spectral
  components within the same physical region
  located at a large distance from the source
• this in turn implies an extremely relativistic
  outflow
• there are good evidence for a bright reverse
  shock component, implying a near-equipartition
  magnetic field in the GRB outflow
• the chromatic behavior of the broadband afterglow
  is consistent with viewing the GRB down the very
  narrow inner core of a two-component jet that is
  expanding into a wind-like environment.

Racusin et al. submitted to Nature
(astro-ph/0805.1557)
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Just a simple reminder of the standard fireball
model

• Inner engine releases unsteady flow of 1052 erg,
  shells with different ? catch up at R 1013 cm,
  shock waves (internal) accelerate electrons and
  generate magnetic fields causing synchrotron
  emission, at R 1016 cm an (external) shock is
  driven in the interstellar medium, again
  synchrotron emission but at lower and decreasing
  frequencies.

Zhang Meszaros IJMP A 19, 15 (2004) and
references therein
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Z 0.937
Vreeswijk et al. GCN 7444 Cucchiara Fox GCN
7456 Bloom et al. astro-ph/0803.3215 DElia et
al. astro-ph/08042141)
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Swift-BAT

• T90 57s
• Epeak 675 keV
• Fpeak 2.3 x10-5 erg cm2 s1
• Lpeak,iso 1.0 x 1053 erg s-1
• strong spectral evolution

Konus-Wind

• Fluence? (20 keV 7 MeV) 6.1 x 104 erg cm2
• E?,iso 1.3 x 1054 erg (20 keV 7 MeV)
• No clear precursor, field in BAT field of view
  before GRB discovery

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• GRB 080319B was discovered by Swift and its
  location in the sky was just 10 away from the
  previously discovered (about 30min earlier) GRB
  080319A.
• This means that wide-field telescopes already
  observing the field of GRB 080319A had GRB
  080319B in their field of view before the
  delivery of the high-energy alert

Racusin et al.
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• The synergy between the TORTORA wide-field camera
  and the REM robotic optical/NIR telescope allowed
  an unprecedented coverage of GRB080319B.
• TORTORA was getting data in the field well before
  the Swift alert was delivered. REM pointed the
  field after 40s and then followed in the optical
  and NIR the afterglow evolution until bigger and
  slower telescopes could begin data acquisition.

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• optical emission starts within a few seconds from
  the high-energy emission and ends about at the
  same time optical and ?-ray comes from the same
  emitting region
• only a general correlation, not a detailed one,
  between optical and ?-ray
• lack of detailed correlation with optical peaking
  later and with broader peaks optical just below
  self-absorption frequency.

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?-ray and optical spectral energy distribution

• optical data 104 times brighter than
  Band-function ?-ray extrapolation
• optical and high-energy come from two distinct
  components
• possibly synchrotron for optical and SSC for
  ?-ray, but other solutions are viable
• Compton Y parameter needs to be 10
• If true a second-order IC component at 20 GeV
  carries more energy than at the observerved
  ?-rays

3s 17s 32s
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• Compton Y parameter (ratio of IC to synchrotron
  energy losses)
• Y ?F(Ep) / ?F(Ep,syn) ?10
• If true, second order IC
• Ep,2 Ep2/Ep,syn 23(Ep,syn/20 eV)1 GeV
• Klein-Nishina suppression important only at
  higher energies
• E gt 94(Ep,syn / 20 eV) 1/2 G3 GeV, where G 103
  G3
• GeV emission easily detectable by AGILE (if not
  occulted by Earth) and GLAST.

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• An exciting opportunity for MAGIC which
  unfortunately could not observe because the alert
  came at the La Palma twilight.

• MAGIC, and MAGIC II, offer undoubtedly the best
  perspectives for a future detection of GRB
  emission at very high energies.

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• So, why did we detect such a bright optical
  flash?
• As already mentioned, in this picture, the
  optical brightness means synchrotron
  self-absorption frequency, ?a, should be much
  above the optical band.
• If tv (z1)RG2(2c)1 is the variability
  time-scale for internal shocks, the optical
  brightness imply that 300 G (tv/3 s)2/3 1400.
• And therefore G 103, where of course G is the
  bulk Lorentz factor.
• High G also means internal shocks happen at a
  large radius, R 1016 cm or larger.
• Therefore the paucity of bright optical flashes
  can be attributed to the low frequency of very
  high (103) Lorentz factors.

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• Putting together Swift-BAT, Konus-Wind,
  Swift-XRT, Pi of the Sky, TORTORA, REM,
  Swift-UVOT, VLT, LT, FTN, Gemini-N, Kait, Nichel,
  GEMINI-S, VLA, we get

Racusin et al.
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• To model the afterglow we assume emission is due
  to synchrotron.
• Phenomenologically, after the initial optical
  flash, the optical decay can be described as the
  sum of three different power-laws, with decay
  indices (tlt50s) a16.5, (50slttlt800s) a22.5, and
  (tgt800s) a31.3.
• The X-ray afterglow follows a different
  behaviour.
• Again three power-laws, after an initial flat
  phase, can describe the data, with decay indices
  (80slttlt2000s) a11.4, (200slttlt4x104s) a21.9, and
  (tgt4x104s) a32.6.

the optical afterglow
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• Different temporal behaviours for optical and
  X-rays means we have a chromatic evolution, as
  can clearly be seen plotting SEDs at various
  epochs (assuming rest frame EB-V 0.05)

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• The colour evolution, and the various temporal
  behaviours, can not basically be interpreted in
  the context of the standard fireball
  single-component model.
• Things can be different if we allow the outflow
  be structured, i.e. multi-component.
• This is, by the way, a natural output of
  simulations of jet formation (e.g. Ramirez-Ruiz
  et al. MNRAS 337, 1349, 2002 Zhang et al. ApJ
  586, 356, 2003 Peng et al. ApJ 626, 966, 2005).
• A simple proxy can be a two-component jet as
  shown in the figure

Courtesy by J.D. Myers (NASA)
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• Schematic jet formation in the collapsar scenario
  (from Ramirez-Ruiz et al. 2002)

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• Therefore we model the outflow as composed by a
  narrow jet (NJ) with high Lorentz factor and
  half-opening angle ? 0.2 and a wider one (WJ,
  ? 4) and lower Lorentz factor.
• The two jets carry about the same amount of
  energy ( 2 x 1050 erg)

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• Optical initially dominated by the WJ RS, which
  is hidden at very early time by the powerful
  prompt optical emission.
• Powerful optical RS means the outflow is not
  strongly magnetized.
• X-ray is mainly due to the NJ FS propagating in
  an environment with wind-shaped density profile.
• The complex time-evolution of the various
  synchrotron frequencies generates the different
  observed regimes.

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• At 50s lt t lt 800s for the optical we have a2.5
  and ß0.5, consistent with high-latitude emission
  from the WJ RS (a2ß) with cooling frequency,
  ?c, below optical band and injection frequency,
  ?m, above 1016 Hz.
• Outflow magnetization, s ( electromagnetic to
  magnetic energy flux ratio), is in the range
  0.1-1.
• Much lower magnetization allows bright RS but not
  in the optical, much higher supresses RS (Kumar
  Panaitescu MNRAS 346, 905, 2003 Zhang et al. ApJ
  595, 950, 2003 Zhang et al. ApJ 628, 315, 2005).
• At 50s lt t lt 40ks X-ray is dominated by NJ FS
  with ?m lt ?x lt ?c (slow cooling).
• Break in the X-ray light-curve at t2800s is
  considered the NJ component jet-break.
• Optical after t800s shows a1.3 and ß0.5,
  consistent with WJ FS with ?m lt vO lt vc.
• After t40ks X-ray too is dominated by the WJ FS.
• WJ component jet-break should be at a few days
  after the burst.

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• Acceptable agreement with observations but still
  many open questions
• Jet breaks steeper and sharper than expected,
  SEDs only qualitatively modeled, many
  small-scales phenomena neither predicted nor
  modeled (i.e. early time NIR flare).
• Dataset rich enough to push to the limits our
  present understanding of GRB physics.

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• Among the various alternatives we also mention
• Other possible explanations, within the fireball
  model, require circumburst medium with complex
  density profile (Racusin et al. 2008) or
  time-varying microphysical parameters (Racusin et
  al. 2008 Kumar Panaitescu 2008).
• Optical from regular forward shock emission,
  X-ray from reprocessing of the forward-shock
  emission by scattering off a lagged part of the
  relativistic outflow (Panaitescu A., MNRAS 383,
  1143, 2008).
• A different scenario is envisaged in the context
  of the so-called cannonball model
• Following Dado et al. (2008), GRB 080319B is a
  regular GRB simply seen peculiarly on-axis. The
  prompt high-energy emission is due to IC of the
  photons of the early SN light scattered away from
  the radial direction by the pre-SN ejecta.
• The optical prompt and the afterglow are due to
  synchrotron emission as soon as the CBs
  decelerate by gathering and scattering ISM
  particles along the way.
• However, a detailed multi-band time-resolved
  analysis of this burst is still lacking.

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• Is there a SN associated to GRB 080319B?
• Late time observations show an almost
  uninterrupted decay.
• However, the colour of the afterglow became
  definitely redder (Tanvir et al. GCN 7621).
• Possible explanation a SN component is rising
  (redder colour) also masking a jet-break at a few
  days.
• Late-time afterglow too faint for a spectrum.

Tanvir et al. GCN 7569
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• Together with photometric observations GRB
  080319B allowed us to derive the best S/N
  high-resolution spectrum for a GRB afterglow ever
  obtained (DElia et al. 2008).
• Detailed analysis of these data will last for
  years
• Preliminary results are already exciting direct
  evidence for UV pumping (Prochaska et al. ApJ
  648, 95, 2006 Vreeswijk et al. AA 468, 83,
  2007) by GRB photons due to FeII fine structure
  lines reducing optical depth by a factor 4-20 in
  a few hours.
• Three sets of observations 8.5m, 2hr and 3hr
  after the GRB.

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• The host galaxy ISM is complex, showing multiple
  components spanning a total velocity range of 120
  km s-1.

• Absorbers should be relatively far (18-34 kpc)
  from the GRB site, as derived by consideration on
  the number of absorbed photons.

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• And the future?
• A GRB comparable to GRB 080319B could have been
  detected up to z5
• For the future EXIST mission (Grindlay AIP Conf.
  Ser. 921, 211, 2007) up to z12 (Bloom et al.
  2008).