Scientific Highlights of the HETE2 Mission

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Scientific Highlights of the HETE-2 Mission
D. Q. Lamb (U. Chicago)
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HETE-2 International Science Team
(Mission Scientist)
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HETE-2 is Going Great Guns

• HETE-2 is currently localizing 25 - 30 GRBs
  yr-1
  
• HETE-2 has localized 49 GRBs in 3 yrs of
  operation (compared to 52 GRBs localized by
  BeppoSAX in its 6-yr mission)
  
• 21 of these localizations have led to the
  detection of X-ray, optical, or radio afterglows
  (compared to 8 for BeppoSAX)
  
• As of the present time, redshift determinations
  have been reported for 12 of these afterglows
  
• HETE-2 has localized 16 XRFs so far (compared to
  17 for BeppoSAX)
  
• HETE-2 has observed 25 bursts from SGRs 1806-20
  and 190014 in the summer of 2001 2 in 2002 and
  18 in 2003 and has discovered a 6th SGR
  1808-20
• HETE-2 has observed 1000 XRBs so far

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Outline of This Talk

• In this talk, I will discuss the implications of
  HETE-2 and follow-up observations for
  
• GRB-SN connection
• Short, hard GRBs
• Optically dark GRBs
• Previously unknown behavior of optical afterglows
  in the 3-20 hr gap immediately following the
  burst that existed in the BeppoSAX era
  
• Cosmology
• X-Ray Flashes (XRFs) and X-ray-rich GRBs
• Structure of GRB jets, GRB rate, and nature of
  Type Ib/Ic core collapse SNe
  

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GRB030329 HETE Hits a Home Run
Vanderspek et al. (2003)
z 0.1675 ? probability of detecting a GRB this
close by is 1/3000 unlikely that HETE-2 or Sw
ift will see another such event
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GRB030329 Copious AAVSO Data
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GRB030329Structure of Circumburst Medium
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GRB030329Spectrum of SN 2003dh
Stanek et al. (2003)
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GRB030329 Implications

• HETE-2localized burst GRB030329/SN 2003dh
  confirms the GRB SN connection
  
• Implications
• We must understand Type Ib/Ic core collapse SNe
  in order to understand GRBs
  
• Conversely, we must understand GRBs in order to
  fully understand Type Ib/Ic core collapse SNe
  
• Result strengthens the expectation that GRBs
  occur out to z 20, and are therefore a
  powerful probe of cosmology and the early
  universe

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Chandra Follow-Up Observations of GRB 020813
Butler et al. (2003)

• Tantalizing evidence
• of X-ray emission lines from alpha-particle
  nuclei
  
• Would be from freshly-minted nuclei from SN
• If confirmed, lines would provide strong
  constraints on nature of GRBs and GRB jets

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HETE-2 Observations of GRB 020531

Lamb et al. (2002)

• BeppoSAX did not detect any short GRBs during its
  6-year mission lifetime, despite extensive
  efforts
  
• GRB 020531 is the first detection of a short,
  hard GRB that has allowed rapid optical
  and X-ray follow-up observations

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GRB020531 Implications

• Rapid HETE-2 and IPN localizations made possible
  rapid optical (t 2 - 3 hrs) follow-up
  observations no optical afterglow was
  detected
• Chandra follow-up observation (Butler et al.
  2002) L_x (short)/L_x (long) _at_ t 5 days
  
• Suggests real time or near-real time X-ray
  follow-up observations of short GRBs may be
  crucial ? Swift XRT and UVOT follow-up
  observations may be vital to unraveling the
  mystery of short GRBs

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HETE-2 is Solving Mystery of Optically Dark GRBs

• Three explanations of optically dark GRBs have
  been discussed
  
• Optical afterglows are extinguished by dust in
  the host galaxy (see, e.g., Reichart and Price
  2001)
  
• Some optical afterglows are intrinsically very
  faint (see, e.g., Fynbo et al. 2001 Berger et
  al. 2002)
  
• GRBs lie at very high redshifts (Lamb and
  Reichart 2000)
  
• Rapid follow-up observations of HETE-2localized
  burst GRB030115 show that this burst is best case
  to date of extinction by dust
  
• Rapid follow-up observations of HETE-2localized
  burst GRB021211 show that this burst is
  optically dim without rapid follow-up would
  have been classified as optically dark
• 13 of 14 HETE-2 WXM plus SXC localizations have
  led to the identification of an optical afterglow

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HETE-2 Observations of GRB030115
Kawai et al. (2003)
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GRB030115 Evidence for Extinction by Dust
(Lamb et al. 2003)
X-ray afterglow observations are crucial needed
to fix slope of afterglow
spectrum (would have determined A_v to /- 10)
Swift XRT will do this for most of the
GRBs that Swift detects
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HETE-2 Observations of GRB021211
Crew et al. (2003)
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GRB021211 Afterglow Light Curve Relative to
Those of Other GRBs
Fox et al. (2003)
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GRB021211 Implications for Optically Dark GRBs

• Rapid follow-up observations of HETE-2localized
  burst GRB021211 shows that, in the case of some
  bursts, their optical afterglow is much fainter
  ( 3 mag) at t 1/2 hour than those observed
  previously (i.e., they are optically dim
  rather than optically dark)
• Even GRBs whose optical afterglows are dim may
  have very bright optical afterglows at t (suggests Swift UVOT will detect many of them)
  
• Early bright phase of optical afterglows would
  last 3 hrs for GRBs at z 10, making
  very high redshift afterglows easier to detect
  and observe than thought

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Gamma-Ray Bursts as a Probe of Cosmology GRBs
Are Easily Detectable at z 20
Lamb Reichart (2000 see also Ciardi and Loeb
2000)
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Gamma-Ray Bursts as a Probe of CosmologyGRB
Afterglow Are Easily Detected

• Optical/infrared afterglow of GRBs contains a lot
  of information.
  
• Lamb and Reichart (2000) showed that the
  optical/infrared afterglows of GRBs can easily
  be detected out to redshifts z 20.
  
• Why?
• Proper distances stops increasing after z 3.
• Afterglow spectra are flat, so cosmological
  redshift does not reduce intensity.
  
• Cosmological time dilation more than compensates,
  since GRB afterglows are transients

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Gamma-Ray Bursts in Cosmological Context

• GRBs can address key questions in cosmology (Lamb
  and Reichart 2000)
  
• Info. on the moment of first light
• Info. on Pop III stars
• Info. on the metallicity history of the
  universe.
  
• Info. on the epoch of reionization.

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Evidence for Strong GRB Evolution With z
Lamb et al. (2003)

• Threshold-corrected significances are 9 x 10-3
  for
  
• L_iso and 5 x 10-2 for E_iso

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X-Ray Flashes

• Defining X-ray flashes (Heise et al. 2000) as
  bursts for which log (S_x/S_gamma) 0 (i.e.,
  30 times that for normal GRBs)
  
• 1/3 of bursts localized by HETE-2 are XRFs
• 1/3 are X-ray-rich GRBs
• Nature of XRFs is largely unknown
• In this talk, I will show that XRFs may provide
  unique insights into
  
• Structure of GRB jets
• GRB rate
• Nature of Type Ic supernovae

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Density of HETE-2 Bursts in (S, E_peak)-Plane
Sakamoto et al. (2003)
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E_iso (L_iso) E_peak Relation
Lamb et al. (2003)
Lloyd-Ronning, Petrosian Mallozzi (200) Amati
et al.
(2002) Lamb et al. (2003)
Lamb et al. (2003)

• Relation spans five decades in E_iso and L_iso

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GRB020903 Spectrum
Sakamoto et al. (2003)
E_peak 27
GRB020903 Discovery of Optical Afterglow
Soderberg et al. (2002)
Palomar 48-inch Schmidt images 2002 Sep 6 (left
image), 2002 Sep 28 (middle image subtracted
image (right image)
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GRB020903 Implications

• HETE-2 and optical follow-up observations of
  GRB020903 show that this XRF
  
• lies on the extension of (S, Eobs_peak)
  distribution
  
• lies on extension of the Amati et al. (2002)
  relation
  
• host galaxy is copiously producing stars, similar
  to those of GRBs
  
• host galaxy has a redshift z 0.25, simlar to
  those of GRBs
  
• These results provide strong evidence that GRBs,
  X-ray-rich GRBs, and XRFs form a continuum and
  are the same phenomenon
  

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Implications of HETE-2 Observations of XRFs and
X-Ray-Rich GRBs

• HETE-2 results, when combined with earlier
  results
  
• Show that properties of XRFs, X-ray-rich GRBs,
  and GRBs form a continuum
  
• This provides strong evidence that these three
  kinds of bursts are all the same phenomenon

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XRFs as a Probe of Structure of GRB Jets, GRB
Rate, and Nature of Type Ib/Ic SNe
D. Q. Lamb, T. Donaghy, and Carlo Graziani (U.
Chicago)
(Jet Simulation from Zhang and Woosley 2002)
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GRBs Have Standard Energies
Frail et al. (2001) Kumar and Panaitescu (2001)
Bloom et al.(2003)
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Universal Jet vs. Unified Jet Models
Universal Jet Model Unified Jet
Model
(Diagram from Lloyd-Ronning and Ramirez-Ruiz 2002)
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Universal Jet Model

• E_iso (theta_view) E_gamma (theta_view)-2
• Exponent -2 is necessary to recover the Frail
  et al. (2001) result (see, e.g., Rossi et al.
  2002, Zhang Meszaros 2002)
  
• Most viewing angles lie at 90 degrees to jet
  axis because that is where most of solid angle
  is
  
• This implies that most bursts (and most bursts
  that we see) have large theta_views, and
  therefore small E_isos, L_gammas, E_peaks,
  etc.

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Uniform Jet Model

• Frail et al. (2000) result E_iso
  E_gamma/Omega_jet
  
• Amati et al. (2002) relation
  
  
• E_peak (E_iso)(1/2) (E_gamma/Omega_jet)(
  1/2)
  
• HETE-2 results show that range in E_iso spans 5
  decades!
  
• HETE-2 results imply N(Omega_jet)
  Omega_jet(-2)
  
• there are many more bursts w. small Omega_jets
  than large however, we dont see most of them
  
• we see equal numbers of bursts per logarithmic
  decade in all properties (Omega_jet, E_iso,
  E_peak, L_gamma, L_x, L_R, etc.)!
  

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Simulations of Observed GRBs

• Our approach is the following
• We first model the bursts in the source frame
• We then progate the bursts from the source frame
  to the Earth, using the cosmology that we have
  adopted
  
• We determine which bursts are observed, using the
  properties of the instruments that observe them
  
• We execute our simulations as follows
• For each burst, we obtain a redshift z and a jet
  opening solid angle Omega_jet by drawing from
  specific distributions
  
• We introduce three Gaussian smearing functions to
  generate
  
• Spread in jet energy (E_gamma)
• Spread in E_peak around the Amati et al. (2002)
  relation
  
• Spread in the timescale T that converts fluence
  to flux
  
• Using these five quantities, we calculate various
  rest-frame quantities (E_iso, E_peak, etc.)
  
• Finally, we construct a Band function for each
  burst and transform it to the observer frame,
  which allows us to
  
• Calculate fluences and peak fluxes
• Determine if the burst would be detected by
  various instruments
  

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Gaussian Smearing Functions

• Observed distributions are well-fit by narrow
  Gaussians
  
• No evidence for evolution of any of Gaussians w.
  redshift z

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Determining If Bursts are Detected
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Comparison of Uniform Jet and Universal Jet Models
Lamb, Donaghy, and Graziani (2003)
Unified Jet Model
Universal Jet Model
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Comparison of Omega_jet (Omega_view) w.
Observations
Lamb, Donaghy, and Graziani (2003)
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E_iso E_peak Relation
BeppoSAX and HETE-2 GRBs
Lloyd-Ronning, Petrosian Mallozzi (2000) Amati
et al. (2002)
Lamb et al. (2003)
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Comparison of Universal and Uniform Jet Models

• Uniform jet model can account for both XRFs and
  GRBs
  
• Universal jet model can account for GRBs, but not
  both XRFs and GRBs

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Comparison of Predicted and Observed E_rad and
E_peak Distributions
Lamb , Donaghy, and Graziani (2003)
Universal jet model
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Comparison of Predicted and Observed E_rad and
E_peak Distributions
Lamb , Donaghy, and Graziani (2003)
Uniform jet model
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Density of HETE-2 Bursts in (S, E_peak)-Plane
Sakamoto et al. (2003)
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Comparison of Predicted and Observed HETE-2
Fluence and E_peak Distributions
Lamb, Donaghy Graziani (2003)
Universal jet model
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Comparison of Predicted and Observed HETE-2
Fluence and E_peak Distributions
Lamb, Donaghy Graziani (2003)
Uniform jet model
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Predicted z and Eobs_peak Distributions
Predicted z distributions for universal jet (red)
and uniform jet (blue) models
Predicted Eobs_peak distributions ? BATSE
results (blue histogram) Match Preece et al. (200
0)
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Implications of Uniform Jet Model

• Model provides unified picture of XRFs,
  X-ray-rich GRBs, and GRBs
  
• Model implies change of 90 degrees in
  polarization position angle at time of jet
  break this is seen in GRB 021004 (Rol et al.
  2003)
• Model implies most bursts have small Omega_jet
  (these bursts are the hardest and most luminous
  bursts) however, we see very few of these
  bursts
• Range in E_rad of five decades minimum range
  for Omega_jet is 6 x 10-4
• Unified jet model therefore implies that there
  are 105 more bursts with small Omega_jets for
  every such burst we see

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Implications of Uniform Jet Model

• Magic number R_Type Ic / R_GRB 105
  (Lamb 2000) unified jet model implies R_GRB
  may be comparable to R_Type Ic
  
• Model implies specific relation between Type Ic
  supernovae and high-energy transients
  
• spherically symmetric explosions produce XRFs
  while
  
• narrow jet-like explosions produce GRBs

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Implications of Uniform Jet Model

• Model implies that E_jet and E_gamma may be
  10 - 100 times smaller than has been
  thought
  
• Narrow jets implied by model suggest that GRB
  jets may be magnetic energy dominated (Vlahakis
  Konigl 2001 Konigl et al. 2003 Proga et al
  2003)
• GRB990123 (Zhang et al. 2003)
• GRB021211 (Kumar Panaitescu 2003)
• Strong (80 /- 20) polarization seen in
  GRB021206 (Coburn Boggs 2003)

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HETE-2 Bursts in (S_E, Eobs_peak)-Plane
Sakamoto et al. (2003)
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X-Ray and Optical Afterglows of XRFs Are Also
Faint
Lamb, Donaghy Graziani (2003)
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HETE-2 Is Ideally Suited to Localize and Study
XRFs

• HETE-2 instruments have
• Thresholds 1-6 keV
• Considerable effective area in X-ray energy
  range
  
• BAT on Swift has nominal threshold of 20 keV
• Relative rate of detection of XRFs by HETE-2 and
  Swift is
  
• 3 for E_peaks
• 10 for E_peaks
• Ability of HETE-2 to localize and study XRFs
  constitutes a compelling reason for continuing
  HETE-2 during the Swift mission

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Conclusions

• Scientific highlights of HETE-2 mission include
• GRB-SN connection
• Short, hard GRBs
• Optically dark GRBs
• Previously unknown behavior of optical afterglows
  in the 3-20 hr gap immediately following the
  burst that existed in the BeppoSAX era
  
• Cosmology
• X-Ray Flashes (XRFs) and X-ray-rich GRBs
• Structure of GRB jets, GRB rate, and nature of
  Type Ib/Ic core collapse SNe
  

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