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 BragaRavi ManchandaGraziella Pizzichini

Los Alamos National Laboratory

Los Alamos, NM USA

Edward E. FenimoreMark Galassi

Space Science LaboratoryUniversity 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 GrazianiTim 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, MDUSA

Center for Space Research Massachusetts Institute of TechnologyCambridge, MA USA

George R. Ricker (PI)Geoffrey B. Crew John P. Doty

Alan M. Levine Roland K. VanderspekJoel Villasenor

(Mission Scientist)

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)

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).

GRB020531: Light Curves in 4 Energy Bands

6-13 keV 30-85 keV

85-300 keV(t50= 360ms)

13-30 keV

Lamb et al. (2002)

Spectrum of GRB020531Lamb et al. (2002)

Spectrum is relatively soft for a short, hard GRB (alpha = 1.26 +/- 0.05, E_peak = 170 +/- 20 keV)

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

GRB020531: Strong Spectral Evolution

Probability that there isProbability that there is nono changechange in spectral in spectral slope = 1.4 x 10^-3slope = 1.4 x 10^-3

GRB020531: Duration Increases as Energy Decreases

Lamb et al. (2002)

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

Rapid Follow-Up Observations of GRB 020531

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

HETE-2 Observations of GRB 020813

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

HETE-2 Observations of GRB 020819

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 + 154 min

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

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.

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.

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

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).

“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

HETE-2 Observations of GRB030115

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

APO Observations of GRB030115

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 GRB030115

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)

(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 GRB021211

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

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

“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”Normal”F_x/F_g < 0.32F_x/F_g < 0.32[20 GRBs][20 GRBs]

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

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

HETE-2 Observations of GRB020903

Sakamoto et al. (2003)

GRB020903: SpectrumSakamoto 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_peakSakamoto 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

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

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