Gamma-Ray Bursts and X-ray afterglows

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Gamma-Ray Bursts and X-ray afterglows
Luigi Piro Istituto Astrofisica Spaziale Fisica
Cosmica, INAF, Roma
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Outline

• Prompt vs afterglow observational signatures.
  Fireball model
• Environment and progenitor
• X-ray lines
• X-ray absorption,
• Wind vs ISM
• Late-time rebursting
• Precursors
• Deviations from standard fireball model
• Dark GRB
• X-Ray Flashes

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The deepest mystery

• Out of 40 GRB localized by BSAX, about 30 went
  off during Italian night time, week end, holydays
• One (GRB980703) during the penalties of the world
  championship soccer game ITALY vs FRANCE
• Terrestrial origin?

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GRB970228 the 1st X-ray and O afterglow

• Triggered by GRBM and localized by WFC

• Fast follow up with NFI in 8 hrs a bright
  unknown X-ray source

• A second pointing 3 days after the GRB fading
  X-ray counterpart (Costa et al 1997)

• Optical fading source (van Paradijs et al 1997)

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Power laws the hallmark of afterglows

• F t-d n-a dx 1.4 ax 1.0

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Prompt vs afterglow emission internal vs
external shock

• In contrast with the afterglow, the prompt
  emission is characterized by strong hard-to-soft
  spectral evolution from X- to Gamma rays (e.g.
  GRB960720 Piro et al 1997)

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Prompt vs afterglow emission
Prompt hard-to-soft
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Testing the fireball model
1016 cm
1013 cm
106 cm
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The progenitors of GRB
The nature of the progenitor can be inferred from
the environment

• NS-NS (BH-NS BH-WD) travel far from their
  formation sites before producing GRBs (Fryer et
  al 2000) gt clean environment no lines

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Iron features

• GB970508 (Piro et al 1999)

• GB000214 (Antonelli et al 2000)

• GB991216 (Piro et al 2000)

• GB990705 (Amati et al 2000)

• GB980828 (Yoshida et al 1999)

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Soft X-ray lines

• The GRB-SN connection furtherly confirmed by the
  detection of He/H-like Mg, Si, S, Ar metal lines
  blueshifted at v/c0.1 in the afterglow spectra
  of GRB011211 (by XMM, Reeves et al 02),
  GRB020813 (by Chandra, Butler et al 03) and
  GRB030227 (by XMM, Watson et al 03)

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Line models vs time behaviour

• In the distant reprocessor scenario (preSN
  explosion Supranova) line intensityconst gt
  line EW (vs continuum) increases with t lines
  detectable after tgtfew hours (not easily
  detectable by SWIFT)
• In the local reprocessor scenario (Meszaros
  Rees2000,2003) line EW should be roughly
  constant visible after few minutes (SWIFTOK)

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X-ray absorption

• X-ray absorption column densities in the
  afterglow NH1021-22 cm-2 (Stratta et al,
  ApJ03)
• Powerful probe of the environment of GRB and of
  the medium in the line of sight to the GRB

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GRB, star forming region standard fireball model

• Association of GRB with star-forming regions
• X-ray lines
• Distribution of OT location in their host
  galaxies (Bloom et al)
• SN-GRB connection
• X-ray absorption column densities consistent
  with NH1021-22 cm-2 in GMC
• Since the typical density in a GMC is n102-104
  cm-3 why the density derived from the standard
  fireball (e.g. Panaitescu, Kumar et al..) model
  is 3-4 orders of magnitude lower ? Wind ejection
  by progenitor.
• Wind environment is expected from progenitor
  (collapsar, in particular) but most afterglows
  are consistent with constant density profile ..

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Collapsar model
Woosley et al
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Wind vs ISM
ISM
Wind

• Telltale of wind X-ray decay SLOWER than optical
  (nolt nc lt nx ) by 0.25
• Distribution of dx-dO From BSAX Optical
• ISM preferred
• Wind cases 040106 (Gendre et al AA, 2004),
  GRB011121, XRR011211 (Piro et al ApJ05),
  XRF011030 (Galli tak)

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GRB011121

• X-ray precursor
• Hard prompt emission
• X-ray rebursting
• Late afterglow

Piro et al. 05, ApJ, in press.
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GRB011121

• Wind suggested e.g. by Price et al (OR)
• BSAX data (Piro et al, ApJ 05)

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GRB011121 late afterglow onset in a WIND

• Setting t0 at the beginnig of the rebursting
• Power law dX1.29 0.04 vs dO1.66 0.06 (Price
  et al 02), consistent only with WIND
• Interpretation other examples talk by A. Galli

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Deviations from standard model

• GRB010222 (int Zand 2002) and GRB990123
  (Maiorano et al 2005, Corsi et al 2005) do not
  fulfill closure relationship AND the X-ray data
  are above the extrapolation of the optical
  spectrum
• IC component? (Corsi et al 2005)

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Dark GRB

• With BeppoSAX
• 30 X-ray afterglow candidates out of 36 GRB
  follow up observations in X-rays (Piro et al
  2004, De Pasquale et al 2004)

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Ly-a forest by intergalactic H clouds
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Are there really dark GRB? Yes

• From the BSAX sample we find (De Pasquale, LP
  etal 03, ApJ) that
• Dark GRB are on average 6 times fainter in X-rays
  than OTGRB (explaining why HETE2 SXC localization
  lead to OTGRB)
• about 20 of dark GRB are truly dark, being
  their ratio of optical-to-X-ray fluxes smaller by
  a factor of about 6 compared to OTGRB
• Not consistent with the fireball model unless
• OT heavily absorbed by star forming region ?
• Or located at zgt5 (such that intragalactic gas
  will absorb photons below Lyman limit)

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OT GRB
Dark
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Dark GRB000210 BeppoSAX Chandra, ESO-VLT

• GRB localized by BeppoSAX.
• Simultaneous obs of the X-ray afterglow with
  Chandra.
• No OT gt23.5
• Deep VLT imaging and spectroscopy z0.86 (Piro
  et al ApJ 2002)

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X-ray flashes

• X-ray rich GRB/ X-Ray Flash a new class
  discovered by BSAX and confirmed by HETE2 about
  30 GRBs with no or very faint or gamma-ray
  emission (Sx/Sggt1).
• Several are dark
• A different type of GRBs or events at zgt5-10?
  (Few events at zlt3) or GRB seen off axis
  (unification scenario as in AGN)

Heise et al 2001
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XRF 031220

• Trigger by HETE 2, we carried out Chandra
  observation to pintpoint the location of the
  afterglow at 1
• No OT, Extremely red host galaxy R-K5.3 Note
  GRB000131_at_z4.5 R-K3.7 (zgt5 ?)
• Fitting with galaxy SED (vs stellar population,
  internal absorption, Lya absorption)
• z1.9 gt dark due to dust extinction (Melandri
  et al05)

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Origin of X-ray flashes

• We compiled a homogeneous BSAXHETE2 sample of
  XRF
• By comparing the properties of afterglows of GRB
  vs XRF gt strong implications on off-axis (and
  high z) models talk by Valeria DAlessio

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The quest for high-z GRB

• Why so much excitement? They can pinpoint
  obscured star-forming galaxies (X-rays and
  gamma-rays pierce through) and probe the region
  z10-20 where the first stars galaxies formed
  (current record holder is a qso at z6.7)
• If GRBSFR, about 20 of them at zgt5 (Bromm
  Loeb 03)
• Events at zgt5 will not be visible in the optical
  range, (Lyalpha forest absorption) they have to
  be dark
• Most of the redshift are now derived from optical
  obs gt strong bias against high-z GRB
• X-ray redshift or IR photometric/spectroscopy z

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Conclusions

• Massive star progenitor from the environment
  X-ray lines, X-ray absorption, SN association
• Prompt vs afterglow emission different spectral
  (and temporal) behaviour
• Wind vs ISM comparison with optical decay slopes
  shows ISM is slightly preferred. Way out wind
  termination shock produces a constant density
  region (Chevalier Li 99). Nonetheless we have
  found so far 3 cases where the wind is preferred
  (GRB011121, 040106, XRF011030
• New features X-ray precursors and rebursting
  (about 10 on BSAX bursts)
• Precursor is NOT thermal black body (power law)

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Conclusions (II)

• Late-time (200-1000 sec) rebursting (in 3 cases)
  is identified with the onset of the afterglow.
  Same spectral index and falls on the same power
  law decay when t0 is set at the onset of the
  reburst (talk by Galli)
• About 20 truly dark/optically faint GRB
• Origin of XRF (talk by DAlessio)
• GRB and cosmology

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Gamma-Ray Bursts from Astrophysics to Cosmology

• International Space Science School
• At LAquila (Italy) from Sept.12-16, 2005
• Advanced school for PhD, Post Doc and young
  researchers tutorials, lessons, and seminars in
  the different branches of Physics and
  Astrophysics relevant to the comprehension of the
  GRB phenomenon. GRB in Cosmology. Experiments and
  methods used in the field. A session devoted to
  data analysis of all instruments of SWIFT.