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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. Outline of This Talk In this talk, I will discuss the implications of HETE-2 and follow-up observations for: • Short, hard GRBs • GRB afterglows as a probe of the circumburst environment • “Optically dark GRBs” • X-ray-rich GRBs and X-Ray Flashes (XRFs)

  4. HETE-2 Observations of GRB 020531 • BeppoSAX did not detect any short GRBs in its 6-year mission lifetime, despite extensive efforts • GRB 020531 is first observation of short GRB that has allowed rapid follow-up observations (previous best was > 2 weeks).

  5. GRB020531: Light Curves in 4 Energy Bands Lamb et al. (2002) 6-13 keV 30-85 keV 13-30 keV 85-300 keV (t50= 360ms)

  6. Spectrum of GRB020531 Lamb et al. (2002) Spectrum is relatively soft for a short, hard GRB (alpha = 1.26 +/- 0.05, E_peak = 170 +/- 20 keV)

  7. GRB020531: Strong Spectral Evolution • Power law x exponential adequately fits spectrum of both of the time intervals t_1 = 0 - 0.8 s and t_2 = 0.8 – 1.3 s • Solid curves are 1-, 2-, and 3-sigma contours for t_1 • Dashed and dash-dotted curves are same for t_2

  8. GRB020531: Strong Spectral Evolution Probability that there is nochange in spectral slope = 1.4 x 10^-3

  9. GRB020531: Duration Increases as Energy Decreases Lamb et al. (2002)

  10. GRB020531: Implications • HETE-2 observations show that this short, hard burst is similar to long bursts in several ways: • Exhibits strong spectral evolution from hard to soft • Duration increases toward lower energies as E^-alpha with alpha ~ 0.4 • Has a soft tail, lasting ~ 6 seconds • Similarity of these properties to those of long bursts suggests similar or same emission mechanism, and possibly same central engine

  11. Rapid Follow-Up Observations of GRB 020531

  12. GRB020531: Implications • Rapid HETE-2 and IPN localizations made possible rapid optical (t = 2 - 3 hrs) follow-up observations (previous best was t > 2 weeks) • Chandra observations at t = 5 days => this short GRB • not associated with a fading X-ray source (C00 or C48) • not associated with a slow-fading optical source (C05) • not associated with high-redshift galaxy (C48) • L_x (short)/L_x (long) < 0.01 - 0.03 @ t = 5 days • Suggests there may be great premium on real time or near-real time X-ray follow-up observations of short GRBs

  13. HETE-2 Observations of GRB 020813 • Long, bright GRB. • HETE-2 localized GRB in near-real time.

  14. HETE-2 Observations of GRB 020819 • HETE-2 localized GRB in near-real time. • Example of “optically dark” GRB.

  15. GRB021004: Structure of Circumburst Medium

  16. GRB021004: T + 43 sec

  17. GRB021004: T + 73 min

  18. GRB021004: T + 154 min

  19. GRB021004: T + 180 min(Optical Transient Detected at T + 9 min)

  20. Discovery of GRB 021004 Optical Afterglow • Observations using 48” Schmidt telescope at Palomar Observatory began 9 minutes after start of GRB. • Spectroscopic observations < 3 hours later gave first redshift info (z > 1.6) on burst. • Optical afterglow very bright (R = 15.4) at discovery.

  21. GRB 021004 Optical Afterglow Light Curve • Follow-up observations made by > 100 telescopes worldwide. • Early observations may show effect of “forward shock” encountering variations (“clumping”) of medium immediately surrounding GRB • GRB afterglows provide info on last years - days of life of (massive) pre-supernova star • Many unexplained bumps in optical afterglow light curve. • Radio afterglow has strange spectrum.

  22. GRB 021004 Optical Afterglow Spectrum • Redshift of GRB (and its host galaxy) are z = 2.32 • Spectrum reveals cloud structure in host galaxy • Many absorption lines produced by metals in galaxies and gas clouds along line-of-sight between GRB and us • Provides info on metallicity history of universe out to redshift of GRB

  23. Change in GRB 021004 Afterglow Spectrum Spectrum of optical afterglow reddened significantly in < 1 day (expected in most popular model, due to electron cooling, but this is1st time such a change has been directly observed).

  24. “Optically Dark” GRBs • HETE-2 is solving the mystery of “optically dark” GRBs • Two explanations have been widely discussed: • GRBs lie at very high redshifts (Lamb and Reichart 2000) • Optical afterglows are extinguished by dust in the host galaxy (see, e.g., Reichart and Price 2001) • Rapid follow-up observations of HETE-2—localized burst GRB030115 show that this burst is best case to date of extinction by dust

  25. HETE-2 Observations of GRB030115

  26. Follow-Up Network for GRBs Consortium • Currently involves 9 institutions: • University of North Carolina (Dan Reichart, Chris Clemens, Mellisa Nysewander, Jane Moran, et al.) • University of Chicago (Don Lamb, Don York, Russet McMillan, et al.) • USNO (Arne Henden, et al.) • MMT/SAO (Grant Williams, et al.) • University of Wyoming (Ron Canterna, Ray Martin, et al.) • Thueringer Landessternwarte Tautenburg (Sylvio Close, et al.) • Clemson University (Dieter Hartmann, et al.) • Elon University (Anthony Crider, et al.) • Tenagra Observatories (Michael Schwartz) • Telescopes include ARC 3.5-m @ APO, SAO 90-inch Bok @KPNO, UW 2.3-m @WIRO,1.5-m Kaj Strand @ USNO, 61-inch UA Kuiper @ Steward, TLS 1.34-m Schmidt @ Tautenburg, 1.3-m and 1.0-m @ USNO, 24-inch CU Super-LOTIS @ KPNO

  27. APO Observations of GRB030115 Lamb et al. (2003) Shown above is an APO i* image taken 3.5 hours after the burst (right panel) and another taken two weeks later (left panel)

  28. APO and USNO Observations of GRB030115 (Lamb et al. 2003) • APO observations on 15 Jan 2002 UT began about 4 hours after the burst, measured i* = 22.1 +/- 0.04 (left panel) and r* = 23.40 +/- 0.11 (right panel) • USNO observations also began about 3.5 hours after the burst; measured J = 19.78 +/-0.20, H = 18.28 +/- 0.13 (Henden et al. 2003)

  29. GRB030115: Evidence for Extinction by Dust (Lamb et al. 2003) • Chandra follow-up observations crucial: needed to fix slope of afterglow spectrum

  30. GRB030115: Host Galaxy

  31. HETE-2 Observations of GRB021211 Crew et al. (2003)

  32. APO Observations of GRB021211

  33. GRB021211: Host Galaxy

  34. GRB021211: Near Real-Time Afterglow Observations Li et al. (2003)

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

  36. GRB021211: Implications for “Optically Dark” GRBs • Nature of “optically dark” bursts is largely unknown • Two explanations have been widely discussed: • GRBs lie a very high redshifts (Lamb and Reichart 2000) • Optical afterglows are extinguished by dust in the host galaxy (see, e.g., Reichart and Price 2001) • Rapid follow-up observations of HETE-2—localized burst GRB021211 shows that there is a third explanation in the case of some bursts: their optical afterglow is much, much fainter (> mag) 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< 10 min

  37. “X-Ray Flashes” Heise et al. (2000) discovered bursts in BeppoSAX WFCs that the BeppoSAX GRBM did not see.

  38. “X-Ray Rich” F_x/F_g>0.32 [17 GRBs] “Normal” F_x/F_g < 0.32 [20 GRBs] “X-Ray Flashes” • Defining “X-ray flashes” as burst for which log (S_x/S_gamma) > 0 (i.e., > 100 times that for “normal” GRBs), ~25% of bursts localized by HETE-2 are XRFs • Nature of XRFs is largely unknown

  39. X-Ray-Rich GRBs and X-Ray Flashes • There is increasing evidence that GRBs, X-ray-rich GRBs, and XRFs form a continuum: • Heise et al. (2000) showed that durations and time histories of XRFs are similar to those of GRBs; Kippen et al. (2002) showed that E_peak’s and S’s of XRFs lie near the smallest values of these seen for GRBs • Reichart et al. (2002) successfully applied their variability measure to XRFs, and showed that, if the variability measure is an estimator of the isotropic-equivalent L of the XRF (and therefore the redshift), the L’s and z’s of XRFs are similar to those of GRBs • Barraud et al. (2003) showed that the values of (S_gamma, E_peak) and (S_x,S_gamma) for X-ray-rich GRBs form an extension of those for GRBs, using HETE-2 FREGATE data

  40. HETE-2 Observations of GRB020903 Sakamoto et al. (2003)

  41. GRB020903: Spectrum Sakamoto et al. (2003) E_peak = 3.4 kev!

  42. 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)

  43. GRB020903: S_gamma vs. S_x Sakamoto et al. (2003) Barraud et al. (2003)

  44. GRB020903: S_gamma vs. E_peak Sakamoto et al. (2003) Barraud et al. (2003)

  45. Amati et al. (2002) Relation: BeppoSAX GRBs

  46. Amati et al. (2002) Relation: HETE-2 GRBs Sakamoto et al. (2003)

  47. Amati et al. (2003) Relation: BeppoSAX + HETE-2 GRBs Sakamoto et al. (2003)

  48. GRB020903: Implications • HETE-2 and optical follow-up observations of GRB020903 show that in the case of this XRF: • It lies on the extensions of the above distributions • It lies on an extension of the Amati et al. (2002) relation • It’s host galaxy is copiously producing stars, similar to those of GRBs • It’s 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 X-Ray Flashes are the same phenomenon

  49. Conclusions • HETE-2 has provided important new information about the nature of short, hard GRBs • HETE-2 is solving the mystery of “optically dark” GRBs: • Some are expected to lie at very high redshifts (Lamb and Reichart 2000) • HETE-2 and follow-up observations of GRB030115 have provided a”gold-plated” example of a GRB which is “optically” dark because of extinction by dust • HETE-2 and follow-up observations of GRB021211 have shown that the afterglows of some GRBs can be much fainter than those observed previously (i.e., they are “optically dim,” rather than “optically dark”); yet these afterglows can be very bright at t < 10 minutes after the burst • HETE-2 and follow-up observations of GRB020903 have provided strong new evidence that GRBs, X-ray-rich GRBs, and “X-Ray Flashes” are all the same phenomenon

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