Showing papers by "E. E. Fenimore published in 1999"
TL;DR: In this article, the width of the large amplitude pulses in GRB time histories remains remarkably constant throughout the classic GRB phase, and the relationship between pulse amplitude and pulse width is shown to be a power law with an index of about -2.8.
Abstract: Many cosmological models of GRBs envision the energy source to be a cataclysmic stellar event leading to a relativistically expanding fireball. Particles are thought to be accelerated at shocks and produce nonthermal radiation. The highly variable temporal structure observed in most GRBs has significantly constrained models. By using different methods of statistical analysis in the time domain we find that the width of the large amplitude pulses in GRB time histories remain remarkably constant throughout the classic GRB phase. This is also true for small amplitude pulses. However, small and large pulses do not have the same pulse width within a single time history. We find a quantitative relationship between pulse amplitude and pulse width: the smaller amplitude peaks tend to be wider, with amplitude following a power law with an index of about -2.8. Internal shocks simulated by randomly selecting the Lorentz factor and energy per shell are consistent with a power law relationship. This is strong quantitative evidence that GRBs are, indeed, caused by internal shocks. The dependency of the width-vs.-intensity relationship on the maximum Lorentz factor provides a way to estimate that elusive parameter. Our observed power law index indicates that \Gamma_{max} is less or equal than 10^3. We also interpret the narrowness of the pulse width distribution as indicating that the emission, that occurs when one shell over takes another, is produced over a small range of distances from the central site.
179 citations
TL;DR: The analysis of spectral lag between energy bands, which combines temporal and spectral analyses, can add strict constraints to gamma-ray burst (GRB) models as mentioned in this paper, and it has been shown that the spectral lags are small.
Abstract: The analysis of spectral lag between energy bands, which combines temporal and spectral analyses, can add strict constraints to gamma-ray burst (GRB) models. In previous studies, the lag analysis focused on the lags between channel 1 (25-57 keV) and channel 3 (115-320 keV) from the Burst and Transient Source Experiment (BATSE). In this paper, we analyzed the cross-correlation average lags (including approximate uncertainties) between energy bands for two GRB samples: 19 events detected by Ginga, and 109 events detected by BATSE. We paid special attention to the BATSE GRBs with known redshifts, because there has been a reported connection between lag and luminosity. This extends our knowledge of spectral lags to lower energy ($\sim 2$ keV). We found that lags between energy bands are small. The lag between the peak of $\sim 50$ keV photons and $\sim 200$ keV photons is $\sim 0.08$ sec. The upper limit in the lag between $\sim 9$ keV photons and $\sim 90$ keV photons is $\sim 0.5$ sec. Thus, there are not large shifts at low energy. We found that about 20% of GRBs have detectable lags between energy bands in the Ginga and BATSE samples. From the internal shock model there are three sources of time structure in GRB pulses: cooling, hydrodynamics, and angular effects. We argue that cooling is much too fast to account for our observed lags and angular effects are energy independent. Thus, only hydrodynamics can produce these lags. Perhaps the radiation process varies as the reverse shock moves through the shell.
50 citations
TL;DR: This paper analyzed the fine-time structure observed in the gamma-ray data from BATSE and determined that the width of the peaks does not increase as expected for a decelerating shell; the later pulses are only 1.034±0.035 longer than the earlier pulses.
Abstract: GRB 990123 was a long, complex gamma-ray burst with an optical transient that started early within the gamma-ray phase. The peak and power-law decay of the early optical emission strongly indicate the presence of a decelerating relativistic shell during that phase. Prior to this burst, it was not known if the shell decelerated during the burst, so an external shock origin for the gamma rays was still possible. If the gamma rays are produced in the external shock, then the pulse widths should reflect the observed deceleration of the shell and increase by a factor between 1.25 and 2.3, depending on the angular extent of the shell. We analyze the fine time structure observed in the gamma-ray data from BATSE and determine that the width of the peaks does not increase as expected for a decelerating shell; the later pulses are only 1.034±0.035 longer than the earlier pulses. The lack of pulse width evolution eliminates the only remaining kinematically acceptable external shock explanation for the gamma-ray phase and, thus, the gamma rays must originate at a central engine.
49 citations
TL;DR: In this article, the authors analyzed the fine-time structure observed in the gamma-ray data from BATSE and determined that the width of the peaks do not increase as expected for a decelerating shell; the later pulses are, at most, a factor of 1.15 longer than the earlier pulses.
Abstract: GRB990123 was a long complex gamma-ray burst with an optical transient that started early within the gamma-ray phase. The peak and power law decay of the early optical emission strongly indicates the presence of a decelerating relativistic shell during that phase. Prior to this burst, it was not known if the shell decelerated during the burst, so an external shock origin for the gamma rays was still possible. If the gamma-rays are produced in the external shock, then the pulse widths should reflect the observed deceleration of the shell and increase by about 2.3. We analyze the fine time structure observed in the gamma-ray data from BATSE and determine that the width of the peaks do not increase as expected for a decelerating shell; the later pulses are, at most, a factor of 1.15 longer than the earlier pulses. We also analyze the variability to determine what fraction of the shell's surface could be involved in the production of the gamma rays, the so-called surface filling factor. For GRB990123 we find a filling factor of 0.008. The lack of pulse width evolution eliminates the only remaining kinematically acceptable external shock explanation for the gamma-ray phase and, thus, the gamma rays must originate at a central engine.
42 citations
TL;DR: In this article, the surface filling factor is defined as the ratio of the area of causally connected regions that become active to the observable area of the shell, and it is shown that the average filling factor for a single, symmetric, relativistic shell interacting with the ambient material is typically 5×10.
Abstract: The variability observed in many complex gamma-ray bursts (GRBs) is inconsistent with causally connected variations in a single, symmetric, relativistic shell interacting with the ambient material ("external shocks") Rather, either the central site must produce ~1050 ergs s-1 for hundreds of seconds ("internal shocks"), or the local spherical symmetry of the shell must be broken on an angular scale much smaller than Γ-1, where Γ is the bulk Lorentz factor for the shell The observed variability in the external shock models arises from the number of causally connected regions that (randomly) become active We define the surface filling factor to be the ratio of the area of causally connected regions that become active to the observable area of the shell From the observed variability in 52 BATSE bursts, we estimate the surface filling factor to be typically ~5×10, although some values are near unity We find that the surface filling factor, f, is ~01ΔT/T in both the constant Γ phase (which probably produces the GRB) and the decelerating phase (which probably produces the X-ray afterglows) Here, ΔT is a typical timescale of variability, and T is the time since the initial signal We analyze the 2 hr flare seen by ASCA 36 hr after the GRB and conclude that the surface filling factor must be small (10-3) in the X-ray afterglow phase as well Compared with the energy required for an isotropic shell, Eiso, explanations for a low surface filling factor can either require more energy (fE~10 ergs) or less energy [(ΔT/4T)E~10 ergs] Thus, the low filling factor cannot be used as a strong argument that GRBs must be internal shocks
42 citations
TL;DR: In this paper, an analysis of the evolution of the X-ray spectrum in terms of a single power-law model is presented, showing that the photon index evolved from 22.0 to 21.1 to 22.4.
Abstract: The prompt X-ray emission from gamma-ray burst (GRB) 980519, as measured with the Wide Field Cameras on board BeppoSAX, is characterized by a strong soft-to-hard-to-soft evolution. An analysis of the evolution of the X-ray spectrum in terms of a single power-law model shows that the photon index evolved from 22.0 to 21.1 to 22.4. The onset of the burst has such a soft spectrum that the 2‐27 keV emission appears to precede the *107 keV emission of GRB 980519 (as measured with the Burst and Transient Source Experiment) by about 70 s. Nevertheless, we show that this early spectral variation is part of a smooth evolution over the whole burst and that there is no convincing evidence that the early X-rays originate from a physical process that is different from that giving rise to the remainder of the burst. Subject headings: gamma-rays: bursts — X-rays: general
29 citations
TL;DR: In this article, the authors employ the information present in the data to select the simplest model that adequately describes the data from among a wide range of continuum and line(s) models, which in their details are independent of the distance scale to gamma-ray bursts.
Abstract: The Ginga data for the gamma-ray burst GRB 870303 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. The centers of the time intervals corresponding to S1 and S2 are separated by 22.5 s. We rigorously 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 a forthcoming paper. 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 χ2 maximum likelihood ratio test. We describe a parameterization 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. We find that the significance of the continuum-plus-line model requested by the S1 data is 3.6 × 10-5, with the odds favoring it being 114:1. The significance of the continuum-plus-lines model requested by the S2 data is 1.7 × 10-4, with the odds favoring it being 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 × 10-8, with the odds favoring it being 40,300:1.
28 citations
TL;DR: In this paper, a triangulation of 15 gamma-ray bursts from Ulysses, PVO, and EURECA spacecraft is presented, which range in area from 0.9 to 530 arcminutes.
Abstract: Between the launch of the Ulysses spacecraft in 1990 October and the entry of Pioneer Venus Orbiter (PVO) into the atmosphere of Venus in 1992 October, concurrent coverage by Ulysses, PVO, the WATCH experiments aboard the Granat and EURECA spacecraft, and the SIGMA and PHEBUS experiments aboard the Granat spacecraft was obtained for numerous gamma-ray bursts. 15 of them were detected by 3 or more instruments on spacecraft separated by distances of several AU, and could therefore be accurately localized by triangulation. In some cases independent, accurate locations were obtained by SIGMA and/or WATCH. We present these localizations, which range in area from 0.9 to 530 arcminutes$^2$.
23 citations
TL;DR: In this article, the authors 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, and calculate the significance of the continuum-plus-line(s) models using the Chi-square Maximum Likelihood Ratio test.
Abstract: 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.
22 citations
TL;DR: In this paper, it was shown that the width of the pulses in GRBs time histories remain remarkably constant throughout the classic GRB phase, and that the lack of temporal evolution of the pulse width should be explained by any fireball shock scenario.
Abstract: Many cosmological models of GRBs envision the energy source to be a cataclysmic stellar event leading to a relativistically expanding fireball. Particles are thought to be accelerated at shocks and produce nonthermal radiation. The highly variable temporal structure observed in most GRBs has significantly constrained models. By using different methods of statistical analysis in the time domain we show that the width of the pulses in GRBs time histories remain remarkably constant throughout the classic GRB phase. If the emission sites lie on a relativistically expanding shell, we determine both the amount of deceleration and the angular spread of the emitting region from the time dependency of the pulse width. We find no deceleration over at least 2/3 of the burst duration and angular spreads of the complete emitting shell that are substancially smaller than . The lack of temporal evolution of the pulse width should be explained by any fireball shock scenario.
21 citations
Abstract: We demonstrate that models of resonant cyclotron radiation transfer in a strong field (i.e., cyclotron scattering) can account for spectral lines seen at two epochs, denoted S1 and S2, in the Ginga data for GRB 870303. S1, which extends 4 s, exhibits one line at ≈20 keV, while S2, which extends 9 s, exhibits harmonically spaced lines at ≈20 and 40 keV. The midpoints of S1 and S2 are separated by 22.5 s. Using a generalized version of the Monte Carlo code of Wang et al., we model line formation by injecting continuum photons into a static plane-parallel slab of electrons threaded by a strong neutron star magnetic field (~1012 G) that may be oriented at an arbitrary angle relative to the slab normal. We examine two source geometries, which we denote "1-0" and "1-1," with the numbers representing the relative electron column densities above and below the continuum photon source plane. The 1-0 geometry may represent, e.g., a line formation region levitating above the surface of the neutron star, or possibly a plasma-filled flux tube illuminated from below. The 1-1 geometry, on the other hand, corresponds to line formation in a semi-infinite atmosphere at the surface of a neutron star. We apply rigorous statistical inference to compare azimuthally symmetric models, i.e., models in which the magnetic field is parallel to the slab normal, with models having more general magnetic field orientations. If the bursting source has a simple dipole field, these two model classes represent line formation at the magnetic pole, or elsewhere on the stellar surface. We find that the data of S1 and S2, considered individually, are consistent with both geometries, and with all magnetic field orientations, with the exception that the S1 data clearly favors line formation away from a polar cap in the 1-1 geometry, with the best-fit model placing the line-forming region at the magnetic equator. Within both geometries, fits to the combined (S1 + S2) data marginally favor models that feature equatorial line formation, and in which the observer's orientation with respect to the slab changes between the two epochs. We interpret this change as being due to neutron star rotation, and we place limits on the rotation period.
TL;DR: In this article, a generalized version of the Monte Carlo code is used to model line formation at two epochs, denoted S1 and S2, in the Ginga data for GRB870303.
Abstract: We demonstrate that models of resonant cyclotron radiation transfer in a strong field (i.e. cyclotron scattering) can account for spectral lines seen at two epochs, denoted S1 and S2, in the Ginga data for GRB870303. Using a generalized version of the Monte Carlo code of Wang et al. (1988,1989b), we model line formation by injecting continuum photons into a static plane-parallel slab of electrons threaded by a strong neutron star magnetic field (~ 10^12 G) which may be oriented at an arbitrary angle relative to the slab normal. We examine two source geometries, which we denote "1-0" and "1-1," with the numbers representing the relative electron column densities above and below the continuum photon source plane. We compare azimuthally symmetric models, i.e. models in which the magnetic field is parallel to the slab normal, with models having more general magnetic field orientations. If the bursting source has a simple dipole field, these two model classes represent line formation at the magnetic pole, or elsewhere on the stellar surface. We find that the data of S1 and S2, considered individually, are consistent with both geometries, and with all magnetic field orientations, with the exception that the S1 data clearly favor line formation away from a polar cap in the 1-1 geometry, with the best-fit model placing the line-forming region at the magnetic equator. Within both geometries, fits to the combined (S1+S2) data marginally favor models which feature equatorial line formation, and in which the observer's orientation with respect to the slab changes between the two epochs. We interpret this change as being due to neutron star rotation, and we place limits on the rotation period.
TL;DR: The Wide-eld X-ray Monitor on HETE-2 con- sists of four Xe-lled 1-D position-sensitive proportional counters, and is sensitive to X-rays between 2 keV and 25 keV.
Abstract: The Wide-eld X-ray Monitor on HETE-2 con- sists of four Xe-lled 1-D position-sensitive proportional counters. It is sensitive to X-rays between 2 keV and 25 keV, and is designed to localize gamma-ray bursts in real time with 10 0 accuracy. The design and basic char- acteristics of the detectors are presented.
01 Jan 1999
TL;DR: The Gamma-Ray burst detector on Ginga consisted of a proportional counter to observe the x-rays and a scintillation counter to observing the gamma-rays as mentioned in this paper, which was ideally suited to study the xrays associated with gamma-ray bursts (GRBs).
Abstract: The Gamma-Ray burst detector on Ginga consisted of a proportional counter to observe the x-rays and a scintillation counter to observe the gamma-rays. It was ideally suited to study the x-rays associated with gamma-ray bursts (GRBs). Ginga detected ∼ 120 GRBs and 22 of them had sufficient statistics to determine spectra from 2 to 400 keV. Although the Ginga and BATSE trigger criteria were very similar, the distribution of spectral parameters was different. Ginga observed bend energies in the spectra down to 2 keV and had a larger fraction of bursts with low energy power law indexes greater than zero. The average ration of energy in the x-ray band (2 to 10 keV) compared to the gamma-ray band (50 to 300 keV) was 24%. Some events had more energy in the x-ray band than in the gamma-ray band. One Ginga event had a period of time preceding the gamma rays that was effectively pure x-ray emission. This x-ray “preactivity” might be due to the penchant for the GRB time structure to be broader at lower energy rather than a different physical process. The x-rays tend to rise and fall slower than the gamma rays but they both tend to peak at about the same time. This argues against models involving the injection of relativistic electrons that cool by synchrotron radiation.