arXiv subjects

C. Graziani

Publications and source records attributed to C. Graziani.

At least 37 records · Page 2Linked to original sources

arXiv2005

Likelihood Analysis of GRB Evolution with Redshift

We present a likelihood approach to modeling multi-dimensional GRB Epeak--fluence--redshift data that naturally incorporates instrument detection thresholds. The treatment of instrument thresholds is essential for analyzing evidence for GRB evolution. The method described here compares the data to a uniform jet model, in which the jet parameters are allowed to vary with redshift. Data from different experiments may be modeled jointly. In addition, BATSE data (for which no redshift information is available) may be incorporated by ascribing to each event a likelihood derived from the full model by integrating the probability density over the unknown redshift. The loss of redshift information is mitigated by the large number of available bursts. We discuss the implementation of the method, and validation of it using simulated data.

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arXiv2005

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.

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arXiv2005

High Energy Observations of XRF 030723: Evidence for an Off-axis Gamma-Ray Burst?

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.

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arXiv2004

HETE Observations of the Gamma-Ray Burst GRB030329: Evidence for an Underlying Soft X-ray Component

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.

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arXiv2004

Global Characteristics of X-Ray Flashes and X-Ray-Rich GRBs Observed by HETE-2

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 $νF_ν$, 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.

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arXiv2004

A Unified Jet Model of X-Ray Flashes, X-ray Rich GRBs and GRBs

HETE-2 has provided strong evidence that the properties of X-Ray Flashes (XRFs) and GRBs form a continuum, and therefore that these two types of bursts are the same phenomenon. We show that both the structured jet and the uniform jet models can explain the observed properties of GRBs reasonably well. However, if one tries to account for the properties of both XRFs and GRBs in a unified picture, the uniform jet model works reasonably well while the structured jet model fails utterly. The uniform jet model of XRFs and GRBs implies that most GRBs have very small jet opening angles (~ half a degree). This suggests that magnetic fields play a crucial role in GRB jets. The model also implies that the energy radiated in gamma rays is ~100 times smaller than has been thought. Most importantly, the model implies that there are ~10^4 -10^5 more bursts with very small jet opening angles for every such burst we see. Thus the rate of GRBs could be comparable to the rate of Type Ic core collapse supernovae. Accurate, rapid localizations of many XRFs, leading to identification of their X-ray and optical afterglows and the determination of their redshifts, will be required in order to confirm or rule out these profound implications.

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arXiv2004

Optical and X-ray Observations of the Afterglow to XRF030723

The X-ray-flash XRF030723 was detected by the HETE satellite and rapidly disseminated, allowing for an optical transient to be detected ~1 day after the burst. We discuss observations in the optical with Magellan, which confirmed the fade of the optical transient. In a 2-epoch ToO observation with Chandra, we discovered a fading X-ray source spatially coincident with the optical transient. We present spectral fits to the X-ray data. We also discuss the possibility that the source underwent a rebrightening in the X-rays, as was observed in the optical. We find that the significance of a possible rebrightening is very low (~1 sigma).

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arXiv2003

GRB 020531: A Short, Hard Gamma-Ray Burst Localized and Observed by HETE-2

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.

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arXiv2003

A Unified Jet Model of X-Ray Flashes and Gamma-Ray Bursts

HETE-2 has provided strong evidence that the properties of X-Ray Flashes (XRFs) and GRBs form a continuum, and therefore that these two types of bursts are the same phenomenon. We show that both the structured jet and the uniform jet models can explain the observed properties of GRBs reasonably well. However, if one tries to account for the properties of both XRFs and GRBs in a unified picture, the uniform jet model works reasonably well while the structured jet model does not. The uniform jet model of XRFs and GRBs implies that most GRBs have very small jet opening angles (~ half a degree). This suggests that magnetic fields play a crucial role in GRB jets. The model also implies that the energy radiated in gamma rays is ~100 times smaller than has been thought. Most importantly, the model implies that there are ~10^4 -10^5 more bursts with very small jet opening angles for every such burst we see. Thus the rate of GRBs could be comparable to the rate of Type Ic core collapse supernovae. Determination of the spectral parameters and redshifts of many more XRFs will be required in order to confirm or rule out the uniform jet model and its implications. HETE-2 is ideally suited to do this (it has localized 16 XRFs in ~2 years), whereas Swift is less so. The unique insights into the structure of GRBs jets, the rate of GRBs, and the nature of Type Ic supernovae that XRFs may provide therefore constitute a compelling scientific case for continuing HETE-2 during the Swift mission.

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arXiv2003

Scientific Highlights of the HETE-2 Mission

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.

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arXiv2003

Evidence From HETE-2 For GRB Evolution With Redshift

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.

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arXiv2003

GRB Redshift Evolution Within the Unified Jet Model

HETE-2 has provided new evidence that gamma-ray bursts may evolve with redshift. We investigate the consequences of this possibility for the unified jet model of XRFs and GRBs. We find that burst evolution with redshift can be naturally explained within the unified jet model, and the resulting model provides excellent agreement with existing HETE-2 and BeppoSAX data sets. In addition, this evolution model produces reasonable fits to the BATSE peak photon number flux distribution -- something that cannot be easily done without redshift evolution.

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arXiv2003

Observation and implications of the Epeak - Eiso correlation in Gamma-Ray Bursts

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 $νf ν$ 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.

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arXiv2003

HETE-2 Observations of the Extremely Soft X-Ray Flash XRF 020903

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.

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arXiv2003

Design and Performance of the Wide-Field X-Ray Monitor on Board the High-Energy Transient Explorer 2

The Wide-field X-ray Monitor (WXM) is one of the scientific instruments carried on the High Energy Transient Explorer 2 (HETE-2) satellite launched on 2000 October 9. HETE-2 is an international mission consisting of a small satellite dedicated to provide broad-band observations and accurate localizations of gamma-ray bursts (GRBs). A unique feature of this mission is its capability to determine and transmit GRB coordinates in almost real-time through the burst alert network. The WXM consists of three elements: four identical Xe-filled one-dimensional position-sensitive proportional counters, two sets of one-dimensional coded apertures, and the main electronics. The WXM counters are sensitive to X-rays between 2 keV and 25 keV within a field-of-view of about 1.5 sr, with a total detector area of about 350 cm$^2$. The in-flight triggering and localization capability can produce a real-time GRB location of several to 30 arcmin accuracy, with a limiting sensitivity of $10^{-7}$ erg cm$^{-2}$. In this report, the details of the mechanical structure, electronics, on-board software, ground and in-flight calibration, and in-flight performance of the WXM are discussed.

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arXiv2003

Gamma-Ray Bursts as a Laboratory for the Study of Type Ic Supernovae

HETE-2 has confirmed the connection between GRBs and Type Ic supernovae. Thus we now know that the progenitors of long GRBs are massive stars. HETE-2 has also provided strong evidence that the properties of X-Ray Flashes (XRFs) and GRBs form a continuum, and therefore that these two types of bursts are the same phenomenon. We show that both the structured jet and the uniform jet models can explain the observed properties of GRBs reasonably well. However, if one tries to account for the properties of both XRFs and GRBs in a unified picture, the uniform jet model works reasonably well while the structured jet model fails utterly. The uniform jet model of XRFs and GRBs implies that most GRBs have very small jet opening angles (~ half a degree). This suggests that magnetic fields play a crucial role in GRB jets. The model also implies that the energy radiated in gamma rays is ~100 times smaller than has been thought. Most importantly, the model implies that there are ~10^4 -10^5 more bursts with very small jet opening angles for every such burst we see. Thus the rate of GRBs could be comparable to the rate of Type Ic core collapse supernovae. Accurate, rapid localizations of many XRFs, leading to identification of their X-ray and optical afterglows and the determination of their redshifts, will be required in order to confirm or rule out these profound implications. HETE-2 is ideally suited to do this (it has localized 16 XRFs in ~2 years), whereas Swift is not. The unique insights into the structure of GRB jets, the rate of GRBs, and the nature of Type Ic supernovae that XRFs may provide therefore constitute a compelling scientific case for continuing HETE-2 during the Swift mission.

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arXiv2003

HETE-2 Localization and Observation of the Bright, X-Ray-Rich Gamma-Ray Burst GRB021211

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.

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arXiv2003

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 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.

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