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

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

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

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

GRB020531: Light Curves in 4 Energy Bands 6-13 keV30-85 keV keV (t 50 = 360ms) keV Lamb et al. (2002)

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

GRB020531: Strong Spectral Evolution qPower law x exponential adequately fits spectrum of both of the time intervals t_1 = s and t_2 = 0.8 – 1.3 s qSolid curves are 1-, 2-, and 3-sigma contours for t_1 qDashed and dash-dotted curves are same for t_2

GRB020531: Strong Spectral Evolution Probability that there is no change in spectral slope = 1.4 x 10^-3

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

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

Rapid Follow-Up Observations of GRB

GRB020531: Implications qRapid HETE-2 and IPN localizations made possible rapid optical (t = hrs) follow-up observations (previous best was t > 2 weeks) qChandra 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) < t = 5 days  Suggests there may be great premium on real time or near-real time X-ray follow-up observations of short GRBs

HETE-2 Observations of GRB  Long, bright GRB.  HETE-2 localized GRB in near-real time.

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

GRB021004: Structure of Circumburst Medium

GRB021004: T + 43 sec

GRB021004: T + 73 min

GRB021004: T min

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

Discovery of GRB Optical Afterglow  Observations using 48” Schmidt telescope at Palomar Observatory began 9 minutes after start of GRB.  Spectroscopic observations 1.6) on burst.  Optical afterglow very bright (R = 15.4) at discovery.

GRB 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.

GRB Optical Afterglow Spectrum qRedshift of GRB (and its host galaxy) are z = 2.32 qSpectrum reveals cloud structure in host galaxy qMany absorption lines produced by metals in galaxies and gas clouds along line-of-sight between GRB and us qProvides info on metallicity history of universe out to redshift of GRB

Change in GRB Afterglow Spectrum Spectrum of optical afterglow reddened significantly in < 1 day (expected in most popular model, due to electron cooling, but this is1 st time such a change has been directly observed).

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

HETE-2 Observations of GRB030115

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

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

APO and USNO Observations of GRB qAPO observations on 15 Jan 2002 UT began about 4 hours after the burst, measured i* = / (left panel) and r* = / (right panel) qUSNO observations also began about 3.5 hours after the burst; measured J = /-0.20, H = / (Henden et al. 2003) (Lamb et al. 2003)

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

GRB030115: Host Galaxy

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

APO Observations of GRB021211

GRB021211: Host Galaxy

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

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

GRB021211: Implications for “Optically Dark” GRBs qNature of “optically dark” bursts is largely unknown qTwo explanations have been widely discussed: q GRBs lie a very high redshifts (Lamb and Reichart 2000) q Optical afterglows are extinguished by dust in the host galaxy (see, e.g., Reichart and Price 2001) qRapid follow-up observations of HETE-2—localized burst GRB 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” qEven GRBs whose optical afterglows are dim may have very bright optical afterglows at t< 10 min

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

“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 “Normal” F_x/F_g < 0.32 [20 GRBs] “X-Ray Rich” F_x/F_g>0.32 [17 GRBs]

X-Ray-Rich GRBs and X-Ray Flashes qThere is increasing evidence that GRBs, X-ray-rich GRBs, and XRFs form a continuum: q 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 q 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 q 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

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

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

GRB020903: Discovery of Optical Afterglow Palomar 48-inch Schmidt images: 2002 Sep 6 (left image), 2002 Sep 28 (middle image; subtracted image (right image) Soderberg et al. (2002)

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

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

Amati et al. (2002) Relation: BeppoSAX GRBs

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

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

GRB020903: Implications qHETE-2 and optical follow-up observations of GRB show that in the case of this XRF: q It lies on the extensions of the above distributions q It lies on an extension of the Amati et al. (2002) relation q It’s host galaxy is copiously producing stars, similar to those of GRBs q It’s host galaxy has a redshift z = 0.25, simlar to those of GRBs qThese results provide strong evidence that GRBs, X-ray-rich GRBs, and X-Ray Flashes are the same phenomenon

Conclusions qHETE-2 has provided important new information about the nature of short, hard GRBs qHETE-2 is solving the mystery of “optically dark” GRBs: q Some are expected to lie at very high redshifts (Lamb and Reichart 2000) q HETE-2 and follow-up observations of GRB have provided a”gold-plated” example of a GRB which is “optically” dark because of extinction by dust q HETE-2 and follow-up observations of GRB 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 qHETE-2 and follow-up observations of GRB have provided strong new evidence that GRBs, X-ray-rich GRBs, and “X-Ray Flashes” are all the same phenomenon