arXiv subjects

C. Graziani

Publications and source records attributed to C. Graziani.

44 records · Page 3Linked to original sources

arXiv2002

Astrometric Calibration and Estimate of the Systematic Error in WXM Localizations Obtained by the Chicago Bayesian Method

WXM gives GRB localizations in instrument coordinates. WXM localizations must be converted to celestial coordinates using spacecraft aspect information obtained by the optical cameras on HETE. We must therefore accurately determine the alignment of the WXM boresight with respect to that of the optical cameras, in order to accurately determine the celestial coordinates of WXM burst locations. We use a seven-parameter model that treats as free parameters the three Euler angles of a pure rotation, two horizontal shifts of the coded-aperture masks with respect to the detectors, and the heights of the masks above the two detectors. We determine the alignment by fitting the model to a set of 252 WXM localizations of Sco X-1 obtained between 23 April and 28 June 2001. We estimate the systematic error in WXM GRB locations by comparing the actual and the calculated locations of Sco X-1. We find that the systematic error corresponding to a 68.3% confidence region is 1.7$'$, and the systematic error corresponding to a 90% confidence region is 2.4$'$. We find that this astrometric solution also provides a satisfactory fit to an independent sample of SGR and XRB events. These results are consistent with the astrometric calibration and the systematic error in WXM localizations derived independently using the RIKEN localization method.

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arXiv2002

HETE-2 Localization and Observations of the Short, Hard Gamma-Ray Burst GRB020531

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.

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arXiv2002

GRB010921: Localization and Observations by the HETE Satellite

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.

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arXiv2002

GRB 010921: Discovery of the First HETE Afterglow

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.

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arXiv1999

Evidence Against an Association Between Gamma-Ray Bursts and Type I Supernovae

We use a rigorous method, based on Bayesian inference, for calculating the odds favoring the hypothesis that any particular class of astronomical transients produce gamma-ray bursts over the hypothesis that they do not. We then apply this method to a sample of 83 Type Ia supernovae and a sample of 20 Type Ib-Ic supernovae. We find overwhelming odds against the hypothesis that all Type Ia supernovae produce gamma-ray bursts, whether at low redshift ($10^{9}:1$) or high-redshift ($10^{12}:1$), and very large odds ($6000:1$) against the hypothesis that all Type Ib, Ib/c, and Ic supernovae produce observable gamma-ray bursts. We find large odds ($34:1$) against the hypothesis that a fraction of Type Ia supernovae produce observable gamma-ray bursts, and moderate odds ($6:1$) against the hypothesis that a fraction of Type Ib-Ic supernovae produce observable bursts.

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arXiv1999

Statistical Analysis of Spectral Line Candidates in Gamma-Ray Burst GRB870303

The Ginga data for the gamma-ray burst GRB870303 exhibit low-energy dips in two temporally distinct spectra, denoted S1 and S2. S1, spanning 4 s, exhibits a single line candidate at ~ 20 keV, while S2, spanning 9 s, exhibits apparently harmonically spaced line candidates at ~ 20 and 40 keV. We evaluate the statistical evidence for these lines, using phenomenological continuum and line models which in their details are independent of the distance scale to gamma-ray bursts. We employ the methodologies based on both frequentist and Bayesian statistical inference that we develop in Freeman et al. (1999b). These methodologies utilize the information present in the data to select the simplest model that adequately describes the data from among a wide range of continuum and continuum-plus-line(s) models. This ensures that the chosen model does not include free parameters that the data deem unnecessary and that would act to reduce the frequentist significance and Bayesian odds of the continuum-plus-line(s) model. We calculate the significance of the continuum-plus-line(s) models using the Chi-Square Maximum Likelihood Ratio test. We describe a parametrization of the exponentiated Gaussian absorption line shape that makes the probability surface in parameter space better-behaved, allowing us to estimate analytically the Bayesian odds. The significance of the continuum-plus-line models requested by the S1 and S2 data are 3.6 x 10^-5 and 1.7 x 10^-4 respectively, with the odds favoring them being 114:1 and 7:1. We also apply our methodology to the combined (S1+S2) data. The significance of the continuum-plus-lines model requested by the combined data is 4.2 x 10^-8, with the odds favoring it being 40,300:1.

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arXiv1998

Evidence Against an Association Between Gamma-Ray Bursts and Type I Supernovae

We present a rigorous method, based on Bayesian inference, for calculating the odds favoring the hypothesis that any particular class of astronomical transients produce gamma-ray bursts over the hypothesis that they do not. We then apply this method to a sample of 83 Type Ia supernovae and a sample of 20 Type Ib-Ic supernovae. We find overwhelming odds against the hypothesis that all Type Ia supernovae produce gamma-ray bursts, whether at low redshift ($10^{9}:1$) or high-redshift ($10^{12}:1$), and very large odds ($6000:1$) against the hypothesis that all Type Ib, Ib/c, and Ic supernovae produce observable gamma-ray bursts. We find large odds ($34:1$) against the hypothesis that a fraction of Type Ia supernovae produce observable gamma-ray bursts, and moderate odds ($6:1$) against the hypothesis that a fraction of Type Ib-Ic supernovae produce observable bursts. We have also re-analyzed both a corrected version of the Wang & Wheeler sample of Type Ib-Ic SNe and our larger sample of 20 Type Ib-Ic SNe, using a generalization of their frequentist method. We find no significant evidence in either case of a correlation between Type Ib-Ic SNe and GRBs, consistent with the very strong evidence against such a correlation that we find from our Bayesian analysis.

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arXiv1993

Evidence for Two Distinct Morphological Classes of Gamma-Ray Bursts from their Short Timescale Variability

We have analyzed the 241 bursts for which peak counts $\C$ exist in the publicly available Burst and Transient Source Experiment (BATSE) catalog. Introducing peak counts in 1024 ms as a measure of burst brightness $\B$ and the ratio of peak counts in 64 and 1024 ms as a measure of short timescale variability $\V$, we find a statistically significant correlation between the brightness and the short timescale variability of \g-ray bursts. The bursts which are smoother on short timescales are both faint and bright, while the bursts which are variable on short timescales are faint only, suggesting the existence of two distinct morphological classes of bursts.

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