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. FenimoreMark 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 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, 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. VanderspekJoel Villasenor

(Mission Scientist)

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

Outline of This Talk

In this talk, I will discuss the implications of HETE-2 and follow-up observations for:GRB-SN connectionShort, hard GRBs “Optically dark” GRBsPreviously unknown behavior of optical afterglows

in the 3-20 hr “gap” immediately following the burst that existed in the BeppoSAX era

CosmologyX-Ray Flashes (XRFs) and X-ray-rich GRBsStructure of GRB jets, GRB rate, and nature of

Type Ib/Ic core collapse SNe

GRB030329: HETE “Hits a Home Run”

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

Vanderspek et al. (2003)

GRB030329:Structure of Circumburst Medium

GRB030329:Spectrum of SN 2003dh

Stanek et al. (2003)

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

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

HETE-2 Observations of GRB 020531

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

Lamb et al. (2002)

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

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

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

GRB021211: Afterglow Light Curve Relative to Those of Other GRBs

Fox et al. (2003)

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

Gamma-Ray Bursts as a Probe of Cosmology: GRBs Are Easily Detectable at z ~ 20

Lamb & Reichart (2000; see also Ciardi and Loeb 2000)

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

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.

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

“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

Density of HETE-2 Bursts in (S, E_peak)-Plane

Sakamoto et al. (2003)

E_iso (L_iso) – E_peak Relation

Lamb et al. (2003)

Relation spans > five decades in E_iso and L_iso

Lloyd-Ronning, Petrosian& Mallozzi (200); Amati et al.(2002); Lamb et al. (2003)

Lamb et al. (2003)

GRB020903: SpectrumSakamoto et al. (2003)

E_peak < 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: 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

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 continuumThis provides strong evidence that these three

kinds of bursts are all the same phenomenon

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)

GRBs Have “Standard” Energies

Frail et al. (2001); Kumar and Panaitescu (2001)

Bloom et al.(2003)

“Universal Jet” vs. Unified Jet Models

“Universal Jet” Model Unified Jet Model

(Diagram from Lloyd-Ronning and Ramirez-Ruiz 2002)

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.

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.)!

Determining If Bursts are Detected

Comparison of Uniform Jet and Universal Jet Models

Lamb, Donaghy, and Graziani (2003)

Unified Jet Model Universal Jet Model

E_iso – E_peak Relation

BeppoSAX and HETE-2 GRBs

Lloyd-Ronning, Petrosian & Mallozzi (2000); Amati et al. (2002); Lamb et al. (2003)

Comparison of Universal and Uniform Jet Models

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

Comparison of Predicted and Observed E_rad and E_peak Distributions

Lamb , Donaghy, and Graziani (2003)

Universal jet model

Comparison of Predicted and Observed E_rad and E_peak Distributions

Lamb , Donaghy, and Graziani (2003)

Uniform jet model

Density of HETE-2 Bursts in (S, E_peak)-Plane

Sakamoto et al. (2003)

Comparison of Predicted and Observed HETE-2 Fluence and E_peak Distributions

Lamb, Donaghy & Graziani (2003)

Universal jet model

Comparison of Predicted and Observed HETE-2 Fluence and E_peak Distributions

Lamb, Donaghy & Graziani (2003)

Uniform jet model

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

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

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)

HETE-2 Bursts in (S_E, E^obs_peak)-Plane

Sakamoto et al. (2003)

X-Ray and Optical Afterglows of XRFs Are Also Faint

Lamb, Donaghy & Graziani (2003)

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

Conclusions

Scientific highlights of HETE-2 mission include:GRB-SN connectionShort, hard GRBs “Optically dark” GRBsPreviously unknown behavior of optical afterglows in

the 3-20 hr “gap” immediately following the burst that existed in the BeppoSAX era

CosmologyX-Ray Flashes (XRFs) and X-ray-rich GRBsStructure of GRB jets, GRB rate, and nature of Type

Ib/Ic core collapse SNe