Search

We present the time-resolved optical emission of gamma-ray bursts GRB 060904B and GRB 070420 during their prompt and early afterglow phases. We used time resolved photometry from optical data taken by the TAROT telescope and time resolved spectroscopy at high energies from the Swift spacecraft instrument. The optical emissions of both GRBs are found to increase from the end of the prompt phase, passing to a maximum of brightness at t_{peak}=9.2 min and 3.3 min for GRB 060904B and GRB 070420 respectively and then decrease. GRB 060904B presents a large optical plateau and a very large X-ray flare. We argue that the very large X-flare occurring near t_{peak} is produced by an extended internal engine activity and is only a coincidence with the optical emission. GRB 070420 observations would support this idea because there was no X-flare during the optical peak. The nature of the optical plateau of GRB 060904B is less clear and might be related to the late energy injection., Comment: 11 pages, 5 color figues, 2 b&w figures, accepted for publication by Astronomy and Astrophysics

The study of time lags between spikes in Gamma-Ray Bursts light curves in different energy bands as a function of redshift may lead to the detection of effects due to Quantum Gravity. We present an analysis of 15 Gamma-Ray Bursts with measured redshift, detected by the HETE-2 mission between 2001 and 2006 in order to measure time lags related to astrophysical effects and search for Quantum Gravity signature in the framework of an extra-dimension string model. The use of photon-tagged data allows us to consider various energy ranges. Systematic effects due to selection and cuts are evaluated. No significant Quantum Gravity effect is detected from the study of the maxima of the light curves and a lower limit at 95% Confidence Level on the Quantum Gravity scale parameter of 3.2x10**15 GeV is set., Comment: 4 pages, 5 figures. v3: Error corrected in Eq. 1. Results updated. Proceedings of the 30th ICRC, Merida, Mexico (2007)

In this work, we present a multi-wavelength study of XRF 040912, aimed at measuring its distance scale and the intrinsic burst properties. We performed a detailed spectral and temporal analysis of both the prompt and the afterglow emission and we estimated the distance scale of the likely host galaxy. We then used the currently available sample of XRFs with known distance to discuss the connection between XRFs and classical Gamma-ray Bursts (GRBs). We found that the prompt emission properties unambiguously identify this burst as an XRF, with an observed peak energy of E_p=17+/-13 keV and a burst fluence ratio S(2-30keV)/S(30-400keV)>1. A non-fading optical source with R~24 mag and with an apparently extended morphology is spatially consistent with the X-ray afterglow, likely the host galaxy. XRF 040912 is a very dark burst since no afterglow optical counterpart is detected down to R>25 mag (3 sigma limiting magnitude) at 13.6 hours after the burst. The host galaxy spectrum detected from 3800A to 10000A, shows a single emission line at 9552A. The lack of any other strong emission lines blue-ward of the detected one and the absence of the Ly alpha cut-off down to 3800A are consistent with the hypothesis of the [OII] line at redshift z=1.563+/-0.001. The intrinsic spectral properties rank this XRF among the soft GRBs in the E_peak-E_iso diagram. Similar results were obtained for most XRFs at known redshift. Only XRF 060218 and XRF 020903 represent a good example of instrinsic XRF(i-XRF) and are possibly associated with a different progenitor population. This scenario may calls for a new definition of XRFs., Comment: 10 pages, 7 figures, accepted for publication in Astronomy & Astrophysics

Thanks to the rapid position notice and response by HETE-2 and Swift, the X-ray afterglow emissions have been found for four recent short gamma-ray bursts (GRBs; GRB 050509b, GRB 050709, GRB 050724, and GRB 050813). The positions of three out of four short GRBs are coincident with galaxies with no current or recent star formation. This discovery tightens the case for a different origin for short and long GRBs. On the other hand, from the prompt emission point of view, a short GRB shows a harder spectrum comparing to that of the long duration GRBs according to the BATSE observations. We investigate the prompt emission properties of four short GRBs observed by Swift/BAT. We found that the hardness of all four BAT short GRBs is in between the BATSE range for short and long GRBs. We will discuss the spectral properties of short GRBs including the short GRB sample of Konus-Wind and HETE-2 to understand the hard nature of the BATSE short GRBs., Comment: 5 pages, 8 figures. To appear in the proceedings of "Gamma Ray Bursts in the Swift Era", eds. S. Holt, N. Gehrels & J. Nousek

Here we report the localizations and properties of four short-duration GRBs localized by the High Energy Transient Explorer 2 satellite (HETE-2): GRBs 010326B, 040802, 051211 and 060121, all of which were detected by the French Gamma Telescope (Fregate) and localized with the Wide-field X-ray Monitor (WXM) and/or Soft X-ray Camera (SXC) instruments. We discuss eight possible criteria for determining whether these GRBs are "short population bursts" (SPBs) or "long population bursts" (LPBs). These criteria are (1) duration, (2) pulse widths, (3) spectral hardness, (4) spectral lag, (5) energy Egamma radiated in gamma rays (or equivalently, the kinetic energy E_KE of the GRB jet), (6) existence of a long, soft bump following the burst, (7) location of the burst in the host galaxy, and (8) type of host galaxy. In particular, we have developed a likelihood method for determining the probability that a burst is an SPB or a LPB on the basis of its T90 duration alone. A striking feature of the resulting probability distribution is that the T90 duration at which a burst has an equal probability of being a SPB or a LPB is T90 = 5 s, not T90 = 2 s, as is often used. All four short-duration bursts discussed in detail in this paper have T90 durations in the Fregate 30-400 keV energy band of 1.90, 2.31, 4.25, and 1.97 sec, respectively, yielding probabilities P(S|T90) = 0.97, 0.91, 0.60, and 0.95 that these bursts are SPBs on the basis of their T90 durations alone. All four bursts also have spectral lags consistent with zero. These results provide strong evidence that all four GRBs are SPBs (abstract continues)., Comment: 60 pages, 19 figures, submitted to ApJ; added new references and corrected typos

The study of time lags between spikes in Gamma-Ray Burst light curves in different energy bands as a function of redshift may lead to the detection of effects due to Quantum Gravity. We present an analysis of 15 Gamma-Ray Bursts with measured redshift, detected by the HETE-2 mission in order to measure time lags related to astrophysical effects and search for Quantum Gravity signature in the framework of an extra-dimension string model. The wavelet transform method is used both for de-noising the light curves and for the detection of sharp transitions. The use of photon tagged data allows us to consider various energy ranges and to evaluate systematic effects due to selections and cuts. The analysis of maxima and minima of the light curves leads to no significant Quantum Gravity effect. A lower limit at 95% Confidence Level on the Quantum Gravity scale parameter of 2x10**15 GeV is set., Comment: 34 pages, 11 figures. Submitted to ApJ

Gamma-ray bursts (GRB), at least those with a duration longer than a few seconds are the most energetic events in the Universe and occur at cosmological distances. The ECLAIRs micro-satellite, to be launched in 2009, will provide multi-wavelength observations of GRB, to study their astrophysics and to use them as cosmological probes. Furthermore in 2009 ECLAIRs is expected to be the only space borne instrument capable of providing a GRB trigger in near real-time with sufficient localization accuracy for GRB follow-up observations with the powerful ground based spectroscopic telescopes available by then. A "Phase A study" of the ECLAIRs project has recently been launched by the French Space Agency CNES, aiming at a detailed mission design and selection for flight in 2006. The ECLAIRs mission is based on a CNES micro-satellite of the "Myriade" family and dedicated ground-based optical telescopes. The satellite payload combines a 2 sr field-of-view coded aperture mask gamma-camera using 6400 CdTe pixels for GRB detection and localization with 10 arcmin precision in the 4 to 50 keV energy band, together with a soft X-ray camera for onboard position refinement to 1 arcmin. The ground-based optical robotic telescopes will detect the GRB prompt/early afterglow emission and localize the event to arcsec accuracy, for spectroscopic follow-up observations., Comment: 7 pages, 1 figure, proceedings of the conference "New Developments in Photodetection", Beaune (France), June 25005. Submitted to NIM-A (Elsevier Science)

In this letter we discuss the flux and the behavior of the bright optical flare emission detected by the 25 cm TAROT robotic telescope during the prompt high-energy emission and the early afterglow. We combine our data with simultaneous observations performed in X-rays and we analyze the broad-band spectrum. These observations lead us to emphasize the similarity of GRB 050904 with GRB 990123, a remarkable gamma-ray burst whose optical emission reached 9th magnitude. While GRB 990123 was, until now, considered as a unique event, this observation suggests the existence of a population of GRBs which have very large isotropic equivalent energies and extremely bright optical counterparts. The luminosity of these GRBs is such that they are easily detectable through the entire universe. Since we can detect them to very high redshift even with small aperture telescopes like TAROT, they will constitute powerful tools for the exploration of the high-redshift Universe and might be used to probe the first generation of stars., Comment: 9 pages, 3 figures. Accepted in ApJL

Gamma-Ray Bursts (GRBs) fall into two classes: short-hard and long-soft bursts. The latter are now known to have X-ray and optical afterglows, to occur at cosmological distances in star-forming galaxies, and to be associated with the explosion of massive stars. In contrast, the distance scale, the energy scale, and the progenitors of short bursts have remained a mystery. Here we report the discovery of a short-hard burst whose accurate localization has led to follow-up observations that have identified the X-ray afterglow and (for the first time) the optical afterglow of a short-hard burst. These, in turn, have led to identification of the host galaxy of the burst as a late-type galaxy at z=0.16 showing that at least some short-hard bursts occur at cosmological distances in the outskirts of galaxies, and are likely to be due to the merging of compact binaries., Comment: 19 pages, 4 figures. Submitted to Nature

An astronomical complex intended to detect optical transients (OTs) in a wide field and follow them up with high time resolution investigation is described., Comment: 4 pages, 3 figures. To be published in "Il Nuovo Cimento", Proceedings of the 4th Rome Workshop on Gamma-Ray Bursts in the Afterglow Era, eds. L. Piro, L. Amati, S. Covino, B. Gendre

[Abridged] Very recently, relations between the peak energy of Gamma-Ray Burst burst spectra, the isotropic-equivalent energy of the burst, and the radiated energy of the burst have been found. In a way that is exactly analogous to the way in which the relation between the peak luminosity and the rate of decline of the light curve of Type Ia supernovae can be used to make Type Ia supernovae excellent standard candles for cosmology, so too, the relations between Epk, Eiso, and Egamma point toward a methodology for using GRBs as excellent standard candles for cosmology. In addition, GRBs occur over the broad redshift range from z=0.1 to at least z=4.5, and both they and their afterglows are easily detectable out to z > 8. Thus GRBs show great promise as cosmological ``yardsticks'' to measure the rate of expansion of the universe over time, and therefore the properties of dark energy (i.e., Omega_M, Omega_Lambda, w_0, and w_a). We describe a concept for a possible MIDEX-class mission dedicated to using GRBs to constrain the properties of dark energy that would obtain these quantities for > 800 bursts in the redshift range 0.1 >~ z <~ 10 during a 2-year mission. This burst sample would enable both Omega_M and w_0 to be determined to +/- 0.07 and +/- 0.06 (68% CL), respectively, and w_a to be significantly constrained., Comment: White paper submitted to the Dark Energy Task Force. 37 pages

We discuss the origin of X-Ray Flashes (XRFs), a recently discovered class of Gamma-Ray Bursts (GRBs). Using a simplified model for internal shocks we check if XRFs can be intrinsically soft due to some specific values of the parameters describing the relativistic outflow emerging from the central engine. We generate a large number of synthetic events and find that XRFs are obtained when the contrast Gamma_max/Gamma_min of the Lorentz factor distribution is small while the average Lorentz factor Gamma is large. A few XRFs may be GRBs at large redshifts but we exclude this possibility for the bulk of the population. If outflows with a small contrast are commonly produced, even a large population of XRFs could be explained. If conversely the Lorentz factor distribution within the wind is broad, one should then rely on extrinsic causes, such as viewing angle effects or high redshift., Comment: 9 pages, 8 figures, to appear in A&A

We present time resolved optical data acquired by the TAROT automated observatory on the afterglow of GRB 050525a from 6 to 136 minutes after the GRB. We evidence a rapid re-brightening of 0.65 magnitude of the afterglow at $\sim$ 33 min after the GRB. The decay slope $\alpha$ is $1.14\pm 0.07$ in the first part and is $1.23\pm 0.27$ after the re-brightening event. The afterglow of GRB 050525a is the third known afterglow that exhibits a re-brightening event begining at 0.01--0.02 day in the rest time frame., Comment: 4 pages, 2 figures, submitted to A&A

The measure of the distances and luminosities of gamma-ray bursts (GRBs) led to the discovery that many GRB properties are strongly correlated with their intrinsic luminosity, leading to the construction of reliable luminosity indicators. These GRB luminosity indicators have quickly found applications, like the construction of 'pseudo-redshifts', or the measure of luminosity distances, which can be computed independently of the measure of the redshift. In this contribution I discuss various issues connected with the construction of luminosity-redshift indicators for gamma-ray bursts., Comment: 6 pages, 1 figure, Sollicited contribution at the 4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome,18-22 October 2004. Editors: L. Piro, L. Amati, S. Covino, and B. Gendre. Il Nuovo Cimento, in press

GRB 020124 and GRB 030323 constitute half the sample of gamma-ray bursts with a measured redshift greater than 3. This paper presents the temporal and spectral properties of these two gamma-ray bursts detected and localized with HETE-2. While they have nearly identical redshifts (z=3.20 for GRB 020124, and z=3.37 for GRB 030323), these two GRBs span about an order of magnitude in fluence, thus sampling distinct regions of the GRB luminosity function. The properties of these two bursts are compared with those of the bulk of the GRB population detected by HETE-2. We also discuss the energetics of GRB 020124 and GRB 030323 and show that they are compatible with the Epeak - Eiso relation discovered by Amati et al. (2002). Finally, we compute the maximum redshifts at which these bursts could have been detected by HETE-2 and we address various issues connected with the detection and localization of high-z GRBs., Comment: 19 pages, 4 figures, submitted to ApJ

The cosmological revolution of 1997 has established that (at least long duration) gamma-ray bursts (GRB) are among the most energetic events in the Universe and occur at cosmological distances. The ECLAIRs micro-satellite, to be launched in 2009, will provide multi-wavelength observations for astrophysical studies of GRB and for their possible use as cosmological probes. It is expected to be the only space borne GRB trigger available for ground based robotic telescopes operational at that time. This paper presents the ECLAIRs project and its status. An X/gamma-ray camera onboard ECLAIRs with a wide field of view of 2 sr, will detect ~100 GRB/yr in the 4-50 keV energy range, localize the GRB with a precision of ~10 arcmin on the sky, and transmit this information to the ground in near real-time, as a GRB trigger for ground based optical telescopes. Inspired by the INTEGRAL imager IBIS, it is based on a CdTe detection plane covering 1000 cm^2, placed 35 cm below a coded mask. An optical camera, sensitive to magnitude-15 stars, covering up to 1/4th of the X/gamma-ray camera's field of view, will observe the prompt emission and a possible precursor of ~10 GRB/yr in the visible-band. Used in a continuous acquisition mode at a rate of ~5 images/s dumped into an on-board memory, a GRB event sent by the X/gamma-ray camera triggers a seek-back in memory for the GRB optical precursor. The full X/gamma-ray and visible-band data of a GRB are sent to ground when a high data-rate telemetry ground receiver is reachable., Comment: 8 pages, 11 figures, IEEE NSS 2004 conference, submitted to IEEE Transact. Nucl. Sci

We describe and discuss the global properties of 45 gamma-ray bursts (GRBs) observed by HETE-2 during the first three years of its mission, focusing on the properties of X-Ray Flashes (XRFs) and X-ray-rich GRBs (XRRs). We find that the numbers of XRFs, XRRs, and GRBs are comparable. We find that the durations and the sky distributions of XRFs and XRRs are similar to those of GRBs. We also find that the spectral properties of XRFs and XRRs are similar to those of GRBs, except that the values of the peak energy $E^{\rm obs}_{\rm peak}$ of the burst spectrum in $\nu F_\nu$, the peak energy flux $\Fp$, and the energy fluence $S_E$ of XRFs are much smaller -- and those of XRRs are smaller -- than those of GRBs. Finally, we find that the distributions of all three kinds of bursts form a continuum in the [$S_E$(2-30 keV),$S_E$(30-400) keV]-plane, the [$S_E$(2-400 keV), $E_{\rm peak}$]-plane, and the [$F_{\rm peak}$(50-300 keV), $E_{\rm peak}$]-plane. These results provide strong evidence that all three kinds of bursts arise from the same phenomenon., Comment: 33 pages, 15 figures, submitted to ApJ

We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-ray Monitor/French Gamma Telescope observations of XRF030723 along with observations of the XRF afterglow made using the 6.5m Magellan Clay telescope and the Chandra X-ray Observatory. The observed peak energy E_pk_obs of the nu F_nu burst spectrum is found to lie within (or below) the WXM 2-25 keV passband at 98.5% confidence, and no counts are detected above 30 keV. Our best fit value is E_pk_obs=8.4+3.5/-3.4 keV. The ratio of X-ray to Gamma-ray flux for the burst follows a correlation found for GRBs observed with HETE-2, and the duration of the burst is similar to that typical of long-duration GRBs. If we require that the burst isotropic equivalent energy E_iso and E_pk_rest satisfy the relation discovered by Amati et al. (2002), a redshift of z=0.38+0.36/-0.18 can be determined, in agreement with constraints determined from optical observations. We are able to fit the X-ray afterglow spectrum and to measure its temporal fade. Although the best-fit fade is shallower than the concurrent fade in the optical, the spectral similarity between the two bands indicates that the X-ray fade may actually trace the optical fade. If this is the case, the late time rebrightening observed in the optical cannot be due to a supernova bump. We interpret the prompt and afterglow X-ray emission as arising from a jetted GRB observed off-axis and possibly viewed through a complex circumburst medium due to a progenitor wind., Comment: 14 pages, 9 figures, to appear in ApJ

We present the results obtained by INTEGRAL on the Soft-Gamma Ray Repeater SGR 1806-20. In particular we report on the temporal and spectral properties of the bursts detected during a moderately active period of the source in September and October 2003 and on the search for quiescent emission., Comment: To appear in the proceedings (ESA-SP) of the 5th INTEGRAL Workshop, "The INTEGRAL UNIVERSE", Munich, 16-20 February 2004

We present a detailed analysis of a 3.5 s long burst from SGR1900+14 which occurred on 2001 July 2. The 2-150 keV time-integrated energy spectrum is well described by the sum of two blackbodies whose temperatures are approximately 4.3 and 9.8 keV. The time-resolved energy spectra are similarly well fit by the sum of two blackbodies. The higher temperature blackbody evolves with time in a manner consistent with a shrinking emitting surface. The interpretation of these results in the context of the magnetar model suggests that the two blackbody fit is an approximation of an absorbed, multi-temperature spectrum expected on theoretical grounds rather than a physical description of the emission. If this is indeed the case, our data provide further evidence for a strong magnetic field, and indicate that the entire neutron was radiating during most of the burst duration., Comment: Accepted for publication in the Astrophysical Journal

An exceptionally intense gamma-ray burst, GRB030329, was detected and localized by the instruments on board the High Energy Transient Explorer satellite (HETE) at 11:37:14 UT on 29 March 2003. The burst consisted of two \~10s pulses of roughly equal brightness and an X-ray tail lasting >100s. The energy fluence in the 30-400 keV energy band was 1.08e-4 erg/cm2, making GRB030329 one of the brightest GRBs ever detected. Communication of a 2 arcmin error box 73 minutes after the burst allowed the rapid detection of a counterpart in the optical, X-ray, radio and the ensuing discovery of a supernova with most unusual characteristics. Analyses of the burst lightcurves reveal the presence of a distinct, bright, soft X-ray component underlying the main GRB: the 2-10 keV fluence of this component is ~7e-6 erg/cm2. The main pulses of GRB030329 were preceded by two soft, faint, non-thermal bumps. We present details of the HETE observations of GRB030329., Comment: 22 pages, 5 figures, to be published in ApJ 617, no. 2 (10 December 2004). Referee comments have been incorporated; results of improved spectral analysis are included

After taking into account threshold effects, we find that the isotropic-equivalent energies E_iso and luminosities L_iso of gamma-ray bursts (GRBs) are correlated with redshift at the 5% and 0.9% signficance levels, respectively. Our results are based on 10 BeppoSAX GRBs and 11 HETE-2 GRBs with known redshifts. Our results suggest that the isotropic-equivalent energies and luminosities of GRBs increase with redshift. They strengthen earlier clues to this effect from analyses of the BATSE catalog of GRBs, using the variability of burst time histories as an estimator of burst luminosities (and therefore redshifts), and from an analysis of BeppoSAX bursts only. If the isotropic-equivalent energies and luminosities of GRBs really do increase with redshift, it suggests that GRB jets at high redshifts may be narrower and thus the cores of GRB progenitor stars at high redshifts may be rotating more rapidly. It also suggests that GRBs at very high redshifts may be more luminous -- and therefore easier to detect -- than has been thought, which would make GRBs a more powerful probe of cosmology and the early universe than has been thought., Comment: 5 pages, 3 figures, to appear in proc. 2003 GRB Conference, Santa Fe, NM

The HETE-2 FREGATE and WXM instruments detected a short, hard GRB at 00:26:18.72 UT on 31 May 2002. A preliminary localization was reported as a GCN Position Notice 88 min after the burst, and a refined localization was disseminated 123 minutes later. An IPN localization of the burst was reported 18 hours after the GRB, and a refined IPN localization was disseminated ~5 days after the burst. The final IPN localization, disseminated on 25 July 2002, is a diamond-shaped region centered on RA=15h 15m 11.18s, Dec=-19o 24' 27.08" (J2000), and has an area of ~9 square arcminutes (99.7% confidence region). The prompt localization of the burst by HETE-2, coupled with the refinement of the localization by the IPN, made possible the most sensitive follow-up observations to date of a short, hard GRB at radio, optical, and X-ray wavelengths. The time history of GRB020531 at high (>30 keV) energies consists of a short, intense spike followed by a much less intense secondary peak, which is characteristic of many short, hard bursts. The duration of the burst increases with decreasing energy and the spectrum of the burst evolves from hard to soft, behaviors which are similar to those of long GRBs. This suggests that short, hard GRBs are closely related to long GRBs., Comment: 4 pages, 3 figures, to appear in proc. 3rd Rome Workshop on Gamma-Ray Bursts in the Afterglow Era, 2002

The availability of a few dozen GRB redshifts now allows studies of the intrinsic properties of these high energy transients. Amati et al. recently discovered a correlation between Epeak, the intrinsic peak energy of the $\nu f \nu$ spectrum, and Eiso, the isotropic equivalent energy radiated by the source. Lamb et al. have shown that HETE-2 data confirm and extend this correlation. We discuss here one of the consequences of this correlation: the existence of a 'spectral standard candle', which can be used to construct a simple redshift indicator for GRBs., Comment: Proceedings of the GRB 2003 Conference in SantaFe, 5 pages, 4 figures

The HETE-2 mission has been highly productive. It has observed more than 250 GRBs so far. It is currently localizing 25 - 30 GRBs per year, and has localized 43 GRBs to date. Twenty-one of these localizations have led to the detection of X-ray, optical, or radio afterglows, and as of now, 11 of the bursts with afterglows have redshift determinations. HETE-2 has also observed more than 45 bursts from soft gamma-ray repeaters, and more than 700 X-ray bursts. HETE-2 has confirmed the connection between GRBs and Type Ic supernovae, a singular achievement and certainly one of the scientific highlights of the mission so far. It has provided evidence that the isotropic-equivalent energies and luminosities of GRBs may be correlated with redshift; such a correlation would imply that GRBs and their progenitors evolve strongly with redshift. Both of these results have profound implications for the nature of GRB progenitors and for the use of GRBs as a probe of cosmology and the early universe. HETE-2 has placed severe constraints on any X-ray or optical afterglow of a short GRB. It has made it possible to explore the previously unknown behavior optical afterglows at very early times, and has opened up the era of high-resolution spectroscopy of GRB optical afterglows. It is also solving the mystery of "optically dark" GRBs, and revealing the nature of X-ray flashes (XRFs)., Comment: 12 pages, 6 figures, to appear in proc. 2nd VERITAS Symposium on TeV Astrophysics, Chicago, Illinois; revised text, added refs

The HETE-2 mission has been highly productive. It has observed more than 250 GRBs so far. It is currently localizing 25 - 30 GRBs per year, and has localized 43 GRBs to date. Twenty-one of these localizations have led to the detection of X-ray, optical, or radio afterglows, and as of now, 11 of the bursts with afterglows have known redshifts. HETE-2 has confirmed the connection between GRBs and Type Ic supernovae, a singular achievement and certainly one of the scientific highlights of the mission so far. It has provided evidence that the isotropic-equivalent energies and luminosities of GRBs are correlated with redshift, implying that GRBs and their progenitors evolve strongly with redshift. Both of these results have profound implications for the nature of GRB progenitors and for the use of GRBs as a probe of cosmology and the early universe. HETE-2 has placed severe constraints on any X-ray or optical afterglow of a short GRB. It is also solving the mystery of "optically dark' GRBs, and revealing the nature of X-ray flashes., Comment: 10 pages, 9 figures, to appear in proc. "The Restless High-Energy Universe", Royal Tropical Institute, Amsterdam; revised text, added refs

We report HETE-2 WXM/FREGATE observations of the X-ray flash, XRF 020903. This event was extremely soft: the ratio log(S_X/S_gamma) = 0.7, where S_X and S_gamma are the fluences in the 2-30 and 30-400 keV energy bands, is the most extreme value observed so far by HETE-2. In addition, the spectrum has an observed peak energy E^{obs}_{peak} < 5.0 keV (99.7 % probability upper limit) and no photons were detected above ~10 keV. The burst is shorter at higher energies, which is similar to the behavior of long GRBs. We consider the possibility that the burst lies at very high redshift and that the low value of E^{obs}_{peak} is due to the cosmological redshift, and show that this is very unlikely. We find that the properties of XRF 020903 are consistent with the relation between the fluences S(7-30 keV) and S(30-400 keV) found by Barraud et al. for GRBs and X-ray-rich GRBs, and are consistent with the extension by a decade of the hardness-intensity correlation (Mallozzi et al. 1995) found by the same authors. Assuming that XRF 020903 lies at a redshift z = 0.25 as implied by the host galaxy of the candidate optical and radio afterglows of this burst, we find that the properties of XRF 020903 are consistent with an extension by a factor ~300 of the relation between the isotropic-equivalent energy E_iso and the peak E_peak of the nu F_nu spectrum (in the source frame of the burst) found by Amati et al. for GRBs. The results presented in this paper therefore provide evidence that XRFs, X-ray-rich GRBs, and GRBs form a continuum and are a single phenomenon. The results also impose strong constraints on models of XRFs and X-ray-rich GRBs., Comment: 32 pages, 12 figures, revised, accepted by ApJ

We propose a new empirical redshift indicator for gamma-ray bursts. This indicator is easily computed from the gamma-ray burst spectral parameters, and its duration, and it provides ``pseudo-redshifts'' accurate to a factor two. Possible applications of this redshift indicator are briefly discussed., Comment: Final version, 4 pages, 3 figures, A&A, vol. 407, L1

A bright, x-ray-rich GRB was detected by HETE-2 at 11:18:34.03 UT on 11 Dec 2002. The WXM localization was to 14' and relayed to the GCN 22 s after the start of the burst. The ground SXC localization was within 2' of R.A. 08h 09m 00s, Dec 06d 44' 20" (J2000). GRB021211 consists of a single, FRED-like pulse with t90s of 2.3 s (85-400 keV) and 8.5 s (2-10 keV). The peak photon number and photon energy fluxes in the 2-400 keV band, are 34.0 +/- 1.8 ph cm^-2 s^-1 and 1.68 +/- 0.11 x 10^-6 erg cm^-2 s^-1, respectively. The energy fluences in the 2-30 keV and 30-400 kev energy bands are S_X = 1.36 +/- 0.05 x 10^-6 erg cm^-2 and S_gamma 2.17 +/- 0.15 x 10^-6 erg cm^-2, respectively. Thus GRB021211 is an X-ray-rich GRB (S_X/S_gamma = 0.63 > 0.32). The spectrum is well-fit by a Band function (alpha = -0.805, beta = -2.37, E_peak = 46.8 keV). The prompt localization allowed the detection of an optical afterglow for what would otherwise have been an ``optically dark'' GRB. GRB 021211 demonstrates that some fraction of burst afterglows are ``optically dark'' because their optical afterglows at times > 1 hr after the burst are very faint, and thus have often escaped detection. GRB 021211 shows that such ``optically dim'' bursts can have very bright afterglows at times < 20 min after the burst., Comment: 21 pages, 4 figures, accepted by ApJ, abstract. condensed; revised

We present a spectral analysis of 35 GRBs detected with the HETE-2 gamma-ray detectors (the FREGATE instrument) in the energy range 7-400 keV. The GRB sample analyzed is made of GRBs localized with the Wide Field X-ray Monitor onboard HETE-2 or with the GRB Interplanetary Network. We derive the spectral parameters of the time-integrated spectra, and present the distribution of the low-energy photon index, alpha, and of the peak energy, e_peak . We then discuss the existence and nature of the recently discovered X-Ray Flashes and their relationship with classical GRBs., Comment: 14 pages, 43 figures, accepted for publication in Astronomy and Astrophysics

The {\it HETE-2} (hereafter \HETE) French Gamma Telescope (FREGATE) and the Wide-field X-ray Monitor (WXM) instruments detected a short ($t_{50} = 360$ msec in the FREGATE 85-300 keV energy band), hard gamma-ray burst (GRB) that occurred at 1578.72 SOD (00:26:18.72 UT) on 31 May 2002. The WXM flight localization software produced a valid location in spacecraft (relative) coordinates. However, since no on-board real-time star camera aspect was available, an absolute localization could not be disseminated. A preliminary localization was reported as a GCN Position Notice at 01:54:22 UT, 88 min after the burst. Further ground analysis produced a refined localization, which can be expressed as a 90% confidence rectangle that is 67 arcminutes in RA and 43 arcminutes in Dec (90% confidence region), centered at RA = +15$^{\rm h}$ 14$^{\rm m}$ 45$^{\rm s}$, Dec = -19$^\circ$ 21\arcmin 35\arcsec (J2000). An IPN localization of the burst was disseminated 18 hours after the GRB (Hurley et al. 2002b). A refined IPN localization was disseminated $\approx$ 5 days after the burst. This hexagonal-shaped localization error region is centered on RA = 15$^{\rm h}$ 15$^{\rm m}$ 03.57$^{\rm s}$, -19$^\circ$ 24\arcmin 51.00\arcsec (J2000), and has an area of $\approx$ 22 square arcminutes (99.7% confidence region). The prompt localization of this short, hard GRB by \HETE and the anti-Sun pointing of the \HETE instruments, coupled with the refinement of the localization by the IPN, has made possible rapid follow-up observations of the burst at radio, optical, and X-ray wavelengths., Comment: 13 pages, 7 figures. Submitted to ApJ

The FREGATE gamma ray detector of HETE-2 is sensitive to photons between 6 and 400 keV. This sensitivity range, extended towards low energies, allows us to explore the emission of GRBs in hard X-rays. We fit the spectra of 23 GRBs with Band's spectral function in order to derive the distribution of their peak energies (E-peak). This distribution is then compared with the E-peak distributions measured by BATSE and GINGA., Comment: 3 pages, Woods Hole Proceedings

We present the first results of the in-flight validation of the spectral response of the FREGATE X/gamma detectors on-board the HETE-2 satellite. This validation uses the Crab pulsar and nebula as reference spectra., Comment: 3 pages, Latex, Woods Hole 2001 GRB conference

After a long period of quiescence, the soft gamma repeater SGR1900+14 was suddenly reactivated on April 2001. On July 2, 2001, a bright flare emitted by this source triggerred the WXM and FREGATE instruments onboard the HETE-2 satellite. Unlike typical short (0.1 s) and spiky SGRs recurrent bursts, this event features a 4.1 s long main peak, with a sharp rise (50 ms) and a slower cutoff (250 ms). This main peak is followed by a 2 sec decreasing tail. We found no evidence of any precursor or any extended `afterglow' tail to this burst. We present the preliminary spectral fits of the total emission of this flare as observed by the FREGATE instrument between 7 and 150 keV. The best fit is obtained with a model consisting of two blackbody components of temperatures 4.15 keV and 10.4 keV. A thermal bremsstrahlung can not be fitted to this spectrum. We compare these features and the burst energetics with the other strong or giant flares from SGR1900+14., Comment: 5 pages, Latex, Woods Hole 2001 GRB conference

The gamma-ray detector of HETE-2, called FREGATE, has been designed to detect gamma-ray bursts in the energy range [6-400] keV. Its main task is to alert the other instruments of the occurrence of a gamma-ray burst (GRB) and to provide the spectral coverage of the GRB prompt emission in hard X-rays and soft gamma-rays. FREGATE was switched on on October 16, 2000, one week after the successful launch of HETE-2, and has been continuously working since then. We describe here the main characteristics of the instrument, its in-flight performance and we briefly discuss the first GRB observations., Comment: Invited lecture at the Woods Hole 2001 GRB Conference, 8 pages, 15 figures

We show that the source population of long gamma-ray bursts (GRBs) has declined by at least a factor of 12 (at the 90% confidence level) since the early stages of the Universe ($z \sim 2 - 3$). This result has been obtained using the combined BATSE and \it Ulysses \rm GRB brightness distribution and the detection of four GRBs with known redshifts brighter than 10$^{52}$ erg s$^{-1}$ in the 50 - 300 keV range at their peak. The data indicate that the decline of the GRB source population is as fast as, or even faster than, the measured decline of the star formation rate. Models for the evolution of neutron star binaries predict a significantly larger number of apparently bright GRBs than observed. Thus our results give independent support to the hypernova model, which naturally explains the fast decline in the progenitor population., Comment: 7 pages, 6 figures, submitted to ApJ, added references

On September 21 at 18950.56 SOD (05:15:50.56) UT the FREGATE gamma-ray instrument on the High Energy Transient Explorer (HETE) detected a bright gamma-ray burst (GRB). The burst was also seen by the X-detector on the WXM X-ray instrument and was therefore well-localized in the X direction; however, the burst was outside the fully-coded field-of-view of the WXM Y-detector, and therefore information on the Y direction of the burst was limited. Cross-correlation of the HETE and Ulysses time histories yielded an Interplanetary Network (IPN) annulus that crosses the HETE error strip at a ~45 degree angle. The intersection of the HETE error strip and the IPN annulus produces a diamond-shaped error region for the location of the burst having an area of 310 square arcminutes. Based on the FREGATE and WXM light curves, the duration of the burst is characterized by a t90 = 18.4 s in the WXM 4 - 25 keV energy range, and 23.8 s and 21.8 s in the FREGATE 6 - 40 and 32 - 400 keV energy ranges, respectively. The fluence of the burst in these same energy ranges is 4.8 10^{-6}, 5.5 10^{-6}, and 11.4 10^{-6} erg cm^{-2}, respectively. Subsequent optical and radio observations by ground-based observers have identified the afterglow of GRB010921 and determined an apparent redshift of z = 0.450., Comment: 5 pages, 2 figures. To be submitted to Astrophysical Journal Letters

We report the discovery of the optical and radio afterglow of GRB 010921, the first gamma-ray burst afterglow to be found from a localization by the High Energy Transient Explorer (HETE) satellite. We present optical spectroscopy of the host galaxy which we find to be a dusty and apparently normal star-forming galaxy at z = 0.451. The unusually steep optical spectral slope of the afterglow can be explained by heavy extinction, A_V > 0.5 mag, along the line of sight to the GRB. Dust with similar A_V for the the host galaxy as a whole appears to be required by the measurement of a Balmer decrement in the spectrum of the host galaxy. Thanks to the low redshift, continued observations of the afterglow will enable the strongest constraints, to date, on the existence of a possible underlying supernova., Comment: 16 pages, 3 figures. Submitted to the Astrophsical Journal Letters

The FREGATE experiment aboard HETE-II has been successfully integrated into the Third Interplanetary Network (IPN) of gamma-ray burst detectors. We show how HETE's timing has been verified in flight, and discuss what HETE can do for the IPN and vice-versa., Comment: To appear in the proceedings of the conference on Gamma-Ray Burst and Afterglow Astronomy 2001: A Workshop Celebrating the First Year of the HETE Mission, to be published by AIP. Figures must be downloaded and printed separately

The T\'elescope \`a Action Rapide pour les Objets Transitoires (TAROT-1) has as prime objective the observation of the prompt and delayed emission of cosmic gamma-ray bursts (GRBs). We have performed a search for optical emission from 6 GRBs detected by BATSE. The positioning error circle was fully covered within typically thirty minutes after the trigger. No detection of the early afterglow phase was made, and magnitude limits in the range of $ \mathrm{m}_{\mathrm{R}} = 13-15 $ were estimated using 20s exposures. These limits are compared to optical afterglow data obtained in later phases and the results are interpreted in terms of source distances. They correspond to a median redshift of z = 0.5. With HETE-2 and the planned instrument upgrade, TAROT-1 will be able to detect the early optical emission of GRBs up to a redshift of the order of 5., Comment: 7 pages, accepted in Astronomy and Astrophysics

We conjecture that bright gamma-ray bursts (GRBs) are bright because they come from sources which are intrinsically over luminous and not because they come from nearby sources. We show that this hypothesis is supported by theoretical and observational arguments and that it explains some well-known properties of GRBs such as their Hardness-Intensity Correlation or the No-Host problem. We discuss the consequences of this hypothesis on our understanding of the properties of the GRB population., Comment: Accepted in Astronomy & Astrophysics Letters

Studies using the GRB brightness as a distance indicator require a measure of brightness with a small intrinsic dispersion (close to a standard candle). There is unfortunately no general agreement on the definition of such a quantity. We show here that the comparison of the size-frequency curves obtained with various measures of brightness can be used to select the quantity which is closer to a standard candle. Our method relies on a few general assumptions on the burster spatial distribution, namely that nearby bursters are homogeneously distributed in an Euclidean space with no density or luminosity evolution. We apply it to 5 measures of GRB brightness in the Current BATSE Catalog and we find that the GRB size-frequency distribution depends significantly on the energy window used to measure the GRB brightness. The influence of the time window being, in comparison, negligible. Our method suggests that the best distance indicator in this Catalog is the fluence measured below 100 keV, indicating that GRB luminosities have a smaller intrinsic dispersion below 100 keV than above., Comment: 5 pages (LateX), 2 Postscript figures, Proceedings of the 4th Huntsville GRB Workshop

TAROT-1 is an automatic, autonomous ground based observatory whose primary goal is the rapid detection of the optical counterparts of cosmic gamma-ray burst sources. It will be able to begin imaging any GRB localization 8 seconds after receipt of an alert from CGRO/BATSE or HETE-2. TAROT-1 will reach the 17th V magnitude in 10 seconds, at a 10$\sigma$ confidence level. TAROT will be able to observe GRB positions given by Beppo-SAX or RXTE, EUV transients from ALEXIS alerts, etc. TAROT will also study a wide range of secondary objectives and will feature a complete automatic data analysis system, and a powerful scheduling software. TAROT will be installed this fall on the Plateau du Calern, 1200m above sea level. We report on the status of the project., Comment: 5 pages. To appear in the proceedings of the 4th Huntsville Gamma-Ray Burst Symposium, eds C.A. Meegan and P. Cushman

It is usual, in gamma-ray burst (GRB) studies, to compare the average properties of bright and faint GRBs, with the assumption that brightness classes reflect distance classes. When brightness is intented to reflect the distance to the sources, it is nevertheless important to use a quantity with a small intrinsic dispersion. We propose here a method to compare the intrinsic dispersion of various measures of GRB brightness. This method assumes that nearby bursters are homogeneously distributed in an Euclidean space with no density or luminosity evolution. We then use it to compare 5 measures of GRB brightness in the BATSE Catalog. Our analysis reveals that better (i.e. less dispersed) measures of brightness are obtained at low energy and that GRBs are much closer to standard candles below 100 keV than above. We suggest that a beaming of the emission above 100 keV could explain this behaviour., Comment: 4 pages (Latex), 2 Postscript figures, final version to be published in A&A Letters