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Scientific Highlights of the HETE-2 Mission

Scientific Highlights of the HETE-2 Mission. D. Q. Lamb (U. Chicago). Los Alamos National Laboratory Los Alamos, NM USA. Center for Space Research Massachusetts Institute of Technology Cambridge, MA USA. Edward E. Fenimore Mark Galassi. George R. Ricker (PI) Geoffrey B. Crew

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Scientific Highlights of the HETE-2 Mission

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  1. Scientific Highlights of the HETE-2 Mission D. Q. Lamb (U. Chicago)

  2. Los Alamos National Laboratory Los Alamos, NM USA Center for Space Research Massachusetts Institute of Technology Cambridge, MA USA Edward E. Fenimore Mark Galassi George R. Ricker (PI) Geoffrey B. Crew John P. Doty Alan M. Levine Roland K. Vanderspek Joel Villasenor Space Science Laboratory University of California at Berkeley, CA USA Cosmic Radiation Laboratory Institute of Physical and Chemical Research (RIKEN) JAPAN Kevin Hurley J. Garrett Jernigan Masaru Matsuoka Nobuyuki Kawai Atsumasa Yoshida Astronomy and Astrophysics Department University of Chicago, IL USA Donald Q. Lamb Jr. Carlo Graziani Tim Donaghy Centre D’Etude Spatiale des Rayonnements (CESR) FRANCE Board of Astronomy and Astrophysics University of California at Santa Cruz, CA USA Jean-Luc Atteia Michel Boer Gilbert Vedrenne Stanford E. Woosley Goddard Space Flight Center Greenbelt, MD USA Brazil + India + Italy (Burst Alert Station Scientists) Joao Braga Ravi Manchanda Graziella Pizzichini Thomas L.Cline (NASA Project Scientist) HETE-2 International Science Team (Mission Scientist)

  3. 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 1900+14 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

  4. 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

  5. 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 Swift will see another such event

  6. GRB030329:Structure of Circumburst Medium

  7. GRB030329:Spectrum of SN 2003dh Stanek et al. (2003)

  8. GRB030329: Implications • HETE-2—localized 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

  9. 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

  10. 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

  11. 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) < 0.01 - 0.03 @ 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

  12. 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-2—localized burst GRB030115 show that this burst is best case to date of extinction by dust • Rapid follow-up observations of HETE-2—localized 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

  13. 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

  14. GRB021211: Afterglow Light Curve Relative to Those of Other GRBs Fox et al. (2003)

  15. GRB021211: Implications for “Optically Dark” GRBs • Rapid follow-up observations of HETE-2—localized 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 < 20 min (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

  16. Gamma-Ray Bursts as a Probe of Cosmology: GRBs Are Easily Detectable at z ~ 20 Lamb & Reichart (2000; see also Ciardi and Loeb 2000)

  17. Gamma-Ray Bursts as a Probe of Cosmology:GRB 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

  18. 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.

  19. 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

  20. “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

  21. Density of HETE-2 Bursts in (S, E_peak)-Plane Sakamoto et al. (2003)

  22. 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

  23. GRB020903: Spectrum Sakamoto et al. (2003) E_peak < 4 kev!

  24. 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)

  25. GRB020903: Implications • HETE-2 and optical follow-up observations of GRB020903 show that this XRF: • lies on the extension of (S, E^obs_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

  26. 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

  27. 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)

  28. GRBs Have “Standard” Energies Frail et al. (2001); Kumar and Panaitescu (2001) Bloom et al.(2003)

  29. “Universal Jet” vs. Unified Jet Models “Universal Jet” Model Unified Jet Model (Diagram fromLloyd-Ronning and Ramirez-Ruiz 2002)

  30. 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_view’s, and therefore small E_iso’s, L_gamma’s, E_peak’s, etc.

  31. 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_jet’s than large; however, we don’t 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.)!

  32. Determining If Bursts are Detected

  33. Comparison of Uniform Jet and Universal Jet Models Lamb, Donaghy, and Graziani (2003) Unified Jet Model Universal Jet Model

  34. E_iso – E_peak Relation BeppoSAX and HETE-2 GRBs Lloyd-Ronning, Petrosian & Mallozzi (2000); Amati et al. (2002); Lamb et al. (2003)

  35. 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

  36. Comparison of Predicted and Observed E_rad and E_peak Distributions Lamb , Donaghy, and Graziani (2003) Universal jet model

  37. Comparison of Predicted and Observed E_rad and E_peak Distributions Lamb , Donaghy, and Graziani (2003) Uniform jet model

  38. Density of HETE-2 Bursts in (S, E_peak)-Plane Sakamoto et al. (2003)

  39. Comparison of Predicted and Observed HETE-2 Fluence and E_peak Distributions Lamb, Donaghy & Graziani (2003) Universal jet model

  40. Comparison of Predicted and Observed HETE-2 Fluence and E_peak Distributions Lamb, Donaghy & Graziani (2003) Uniform jet model

  41. 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 < Omega_jet < 6 • Unified jet model therefore implies that there are ~ 10^5 more bursts with small Omega_jet’s for every such burst we see

  42. Implications of Uniform Jet Model • “Magic number:” R_Type Ic / R_GRB ~ 10^5 (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

  43. 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)

  44. HETE-2 Bursts in (S_E, E^obs_peak)-Plane Sakamoto et al. (2003)

  45. X-Ray and Optical Afterglows of XRFs Are Also Faint Lamb, Donaghy & Graziani (2003)

  46. 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_peak’s < 10 keV • 10 for E_peak’s < 5 keV • Ability of HETE-2 to localize and study XRFs constitutes a compelling reason for continuing HETE-2 during the Swift mission

  47. 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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