J-P. Dezalay

Measurement of the Crab Flux above 60 GeV with the CELESTE Cerenkov Telescope

We have converted the former solar electrical plant THEMIS (French Pyrenees) into an atmospheric Cerenkov detector called CELESTE, which records gamma rays above 30 GeV (7 × 1024 Hz). Here we present the first sub-100 GeV detection by a ground-based telescope of a gamma-ray source, the Crab Nebula, in the energy region between satellite measurements and imaging atmospheric Cerenkov telescopes. At our analysis threshold energy of 60 ± 20 GeV we measure a gamma-ray rate of 6.1 ± 0.8 minute-1. Allowing for 30% systematic uncertainties and a 30% error on the energy scale yields an integral gamma-ray flux of I(E > 60GeV) = 6.2 × 10-6 photons m-2 s-1. The analysis methods used to obtain the gamma-ray signal from the raw data are detailed. In addition, we determine the upper limit for pulsed emission to be less than 12% of the Crab flux at the 99% confidence level, in the same energy range. Our result indicates that if the power law observed by EGRET is attenuated by a cutoff of form e, then E0 < 26 GeV. This is the lowest energy probed by a Cerenkov detector and leaves only a narrow range unexplored beyond the energy range studied by EGRET.

Spectral analysis of 35 GRBs/XRFs observed with HETE-2/FREGATE

We present a spectral analysis of 35 GRBs detected with the HETE-2 gamma-ray detectors (the FREGATE instru- ment) 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, Ep. We then discuss the existence and nature of the recently discovered X-Ray Flashes and their relationship with classical GRBs.

HETE Observations of the Gamma-Ray Burst GRB 030329: Evidence for an Underlying Soft X-Ray Component

An exceptionally intense gamma-ray burst, GRB 030329, was detected and localized by the instruments on board the High Energy Transient Explorer satellite (HETE) at 11:37:14 UT on 2003 March 29. The burst consisted of two ~10 s pulses of roughly equal brightness and an X-ray tail lasting more than 100 s. The energy fluence in the 30-400 keV energy band was Sγ = 1.2 × 10-4 ergs cm-2, making GRB 030329 one of the brightest GRBs ever detected. Communication of a 2 error box 73 minutes after the burst allowed the rapid detection of a counterpart in the optical, X-ray, and radio and the ensuing discovery of a supernova with most unusual characteristics. Analyses of the burst light curves reveal the presence of a distinct, bright, soft X-ray component underlying the main GRB; the 2-10 keV fluence of this component is ~7 × 10-6 ergs cm-2. The main pulses of GRB 030329 were preceded by two soft, faint, nonthermal bumps. We present details of the HETE observations of GRB 030329.

Scientific highlights of the HETE-2 mission

Abstract The High Energy Explorer Satellite 2 (HETE-2) mission has been highly productive. It has observed more than 250 γ-ray bursts (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 γ-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 of 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.

HETE-2 localization and observation of the bright, X-ray-rich gamma-ray burst GRB 021211

A bright, X-ray-rich gamma-ray burst (GRB) was detected by the French Gamma Telescope (FREGATE) and localized with the Wide Field X-ray Monitor (WXM) and Soft X-ray Camera (SXC) instruments on the High Energy Transient Explorer 2 satellite (HETE-2) at 11:18:34.03 UT (40714.03 SOD) on 2002 December 11. The WXM flight software localized the burst to a 14 radius; this was relayed to the astronomical community 22 s after the start of the burst. Ground analysis of WXM and SXC data provided refined localizations; the latter can be described as a circle with a radius of 2 centered at R.A. 08h09m00s, decl. 06°4420 (J2000.0). GRB 021211 consists of a single, FRED-like pulse with a duration t90 ≈ 2.3 s at high energies (85-400 keV), which increases to t90 ≈ 8.5 s at low energies (2-10 keV). The peak photon number and photon energy fluxes in the 2-400 keV band are (34.0 ± 1.8) photons cm-2 s-1 and (1.68 ± 0.11) × 10-6 ergs cm-2 s-1, respectively. The energy fluences in the 2-30 and 30-400 keV energy bands are SX = (1.36 ± 0.05) × 10-6 ergs cm-2 and Sγ = (2.17 ± 0.15) × 10-6 ergs cm-2, respectively. Thus, GRB 021211 is an X-ray-rich GRB (SX/Sγ = 0.63 > 0.32). The average spectrum of the burst is well fitted by a Band function (low-energy power-law index α = -0.805; high-energy power-law index β = -2.37; and energy of the peak of the spectrum in νFν, E = 46.8 keV). The near-real-time optical follow-up of GRB 021211 made possible by HETE-2 led to the detection of an optical afterglow for what otherwise would quite likely have been classified as an optically dark GRB, since the optical transient faded rapidly (from R < 14 to R ≈ 19) within the first 20 minutes, and was fainter than R ≈ 23 within 24 hr after the burst. GRB 021211 demonstrates that some fraction of burst afterglows are optically dark because their optical afterglows at times greater than 1 hr after the burst are very faint, and previously have often escaped detection. Such bursts are optically dim rather than truly optically dark. GRB 021211 also shows that even such optically dim bursts can have very bright optical afterglows at times less than 20 minutes after the burst.

HETE-2 observation of two gamma-ray bursts at z > 3

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.

In flight performance and first results of FREGATE

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.

X-ray flashes or soft gamma-ray bursts? : The case of the likely distant XRF 040912

Context: The origin of X-ray Flashes (XRFs) is still a mystery and several models have been proposed. To disentangle among these models, an important observational tool is the measure of the XRF distance scale, so far available only for a few of them. Aims: In this work, we present a multi-wavelength study of XRF 040912, aimed at measuring its distance scale and the intrinsic burst properties. Methods: 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). Results: 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 S2{-30 keV}/S30{-400 keV}>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? limiting magnitude) at 13.6 h after the burst. The host galaxy spectrum detected from 3800 A to 10 000 A, shows a single emission line at 9552 A. The lack of any other strong emission lines blue-ward of the detected one and the absence of the Ly? cut-off down to 3800 A 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 call for a new definition of XRFs.

First detection of gamma rays from the crab nebula with the CELESTE “solar farm” Cherenkov detector

Abstract We have converted the THEMIS solar array (French Pyrenees) into an atmospheric Cherenkov telescope, called CELESTE, sensitive to astrophysical gamma rays above 30 GeV (7×10 24 Hz). In early 1998 the Crab nebula was detected at 80 GeV with a preliminary 18 heliostat setup. The full 40 heliostat array has since been commissioned. The STACEE experiment using the same technique in New Mexico is also analysing their first data. Thus, the window between the EGRET instrument and the Cherenkov imagers has been opened. We describe the CELESTE detector and the data analysis, and discuss the prospects for studying AGN (specifically, blazars) and galactic sources in this energy range.

HETE-2 Observation of the Extremely Soft X-Ray Flashes, XRF010213 and XRF020903

We report HETE‐2 WXM and FREGATE observations of two X‐ray flashes (XRFs), XRF010213 and XRF020903. The signal is only seen in 10 seconds, and this feature is similar to that of the “long” GRBs. According to the time‐averaged spectral analysis using both WXM and FREGATE data, the fluence ratio of 2–30 keV to 30–400 keV energy band is 11.4 and 5.6 for XRF010213 and XRF020903 respectively. The Epeak energy in the Band function is < 10 keV. They are likely to belong to the same class as the X‐ray flash events detected with GINGA and BeppoSAX. In this paper, we will present the detail study of the prompt emission of XRF010213 and XRF020903, and compare with the characteristics of classic GRBs.