arXiv subjects

E. E. Fenimore

Publications and source records attributed to E. E. Fenimore.

At least 55 records · Page 3Linked to original sources

arXiv2000

Pulse Width Evolution in GRBs: Evidence for Internal Shocks

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 Γ_{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.

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arXiv2000

Redshifts For 220 BATSE Gamma-Ray Bursts Determined by Variability and the Cosmological Consequences

We show that the time variability of a gamma-ray burst (GRB) appears to be correlated with the absolute luminosity of the burst: smooth bursts are intrinsically less luminous. This Cepheid-like relationship can be used to determine the redshift of a GRB from parameters measured solely at gamma-ray energies. The relationship is based on only seven events at present and needs to be further confirmed with more events. We present the details of converting GRB observables to luminosities and redshifts for 220 bright, long GRBs from the Burst and Transient Source Experiment (BATSE) and explore the cosmological consequences. In particular, we derive the GRB rate as a function of $z$ without assuming either a luminosity function or that the GRB rate follows the star formation rate (SFR). We find that the GRB formation rate scales as (1+z)^{3.3 \pm 0.3}. The observations used to derive the SFR can be strongly affected by dust for z > 2 whereas GRB observations are not. If GRBs trace star formation, then our results indicate that the SFR does not peak at z\sim 2 but instead continues to increase until z \sim 10. We have used the burst formation rate to correct the observed GRB luminosity function for the incompleteness due to the detection threshold, resulting in a luminosity function with a power law index of \sim -2.3 that slightly rolls over at low luminosities. The reality of our variability--luminosity relationship requires confirmation but, if valid, will provide a powerful tool for studying both GRBs and the early universe.

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arXiv1999

Gamma-Ray Bursts as Internal Shocks Caused By Deceleration

Gamma-ray bursts (GRBs) have been thought to originate from internal shocks that occur about 10^{15} cm from a central site. The shells responsible for these shocks merge together and undergo an external shock at about 10^{17} cm, producing the afterglows. We include deceleration in our model of internal shocks and find that, for values of the Lorentz factor greater than 10^3, deceleration is an effective catalyst for converting the bulk motion energy into radiation during the GRB phase. Substantial internal energy occurs because other shells run into the back of the first shell which has decelerated and because the first shell must energize the interstellar medium. Whereas internal shocks without deceleration are about 25% efficient, we can convert up to 85% of the bulk motion energy during the GRB phase. We demonstrate that the resulting time history can have three components. The first is due to internal shocks, excluding those that involve the first shell. This component produces narrow peaks throughout the time history. The second is due to internal shocks involving the first shell, and it produces progressively wider and wider peaks but they tend to be hidden in a slowly varying background in the event. The third component is from energizing the interstellar medium. It is very smooth and may contribute mostly to a lower energy bandpass than the BATSE experiment. There have been claims of upper limits on the possible Lorentz factor because the deceleration must occur at greater radii than the internal shocks to avoid making progressively wider peaks. We do not find this to be the case, and the Lorentz factor (and efficiency) can be much larger.

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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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arXiv1999

Resonant Cyclotron Radiation Transfer Model Fits to Spectra from Gamma-Ray Burst GRB870303

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.

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arXiv1999

The Observational Basis for Central Engines in Gamma Ray Bursts

We review the observational differences between gamma-ray bursts occurring on a single shell (such as in the external shock model) and multiple shells (such as in the internal shock model). We do not argue against external shocks or for internal shocks, as such, but rather against a single shell and for a central engine that is small enough to produce the typical GRB time history without violating causality or other kinematic limits. The expected profile and average spectral evolution from a single shell is compared to the average of many bursts and found to be different. The presence of gaps in many gamma-ray bursts is a strong argument against a single shell because an observer should see many causally disconnected regions at any one time.The rapid variability is also difficult to explain from a single shell because of the large number of causally disconnected regions. The pulse width as a function of time in a burst should increase because there is a one-to-one relationship between arrival time and the off-axis angle of emission. The observations show that the pulse width does not increase with time. Finally, in GRB990123 there is evidence for deceleration from the simultaneous optical observations, yet the gamma-ray pulses show no lengthening of their pulse structure. We conclude that gamma-ray bursts are caused by a relatively small central engine.

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arXiv1999

GRB990123: Evidence that the Gamma Rays Come from a Central Engine

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.

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arXiv1998

Temporal Evolution of the Pulse Width in GRBs

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. Peaks at the end of a burst have the same average duration to within a few percent as the peaks at the start of the burst. For emission sites that lie on a relativistically expanding shell, peaks should grow in duration because of deceleration. We find no deceleration over at least 2/3 of the burst duration. For emission sites that occupy a spread of angles on a shell, the curvature should cause the later peaks to grow in duration. Since we see no such growth, we can limit the total angular size of the shell to be substantially smaller than Γ^{-1} where Γis the bulk Lorentz factor. This lack of temporal evolution of the pulse width should be explained by any fireball shock scenario.

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arXiv1998

Gamma-Ray Burst and Relativistic Shells: The Surface Filling Factor

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, the symmetry of the shell must be broken on an angular scale much smaller than Gamma^{-1} where Gamma 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 0.005 although some values are near unity. We find that the surface filling factor is about 0.1 Delta T/T in both the constant Gamma phase (which probably produces the GRB) and the deaccelerating phase (which probably produces the x-ray afterglows). Here, ΔT is a typical time scale 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 (0.001) in the x-ray afterglow phase as well. Explanations for low surface filling factor can either require more or less energy (by a factor of about 1000) compared to that expected for a symmetric shell.

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arXiv1998

Gamma Ray Bursts Have Millisecond Variability

We have performed searches for isolated flares and for steady flickering in Gamma Ray Burst light curves on the microsecond to millisecond time scales. Two bursts out of our sample of 20 revealed four isolated flares with time scales from $256 μs$ to $2048 μs$. A wavelet analysis for our sample showed low level flickering for all bursts on time scales from $256 μs$ to 33 ms, with the majority of bursts containing rise times faster than four milliseconds and 30% having rise times faster than one millisecond. These results show that millisecond variability is common in classical bursts and not some exceptional activity by a possibly separate class of bursts. These fast rise times can be used to place severe limits on burst models: (1) The characteristic thickness of the energy generation region must be less than 1200 km along the line of sight. (2) The angular size of the gamma ray emission region as subtended from the central source must be less than 42 arc-seconds. (3) The expanding ejecta must have a range of Lorentz factors along a radius line with a dispersion of less than roughly 2%. (4) Within the external shock scenario, the characteristic dimension of the impacted cloud must be smaller than 16 Astronomical Units on average. (5) Within the collimated jet scenario, the collimation angle must be smaller than 42 arc-seconds.

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arXiv1998

Gamma-Ray Burst Spectra and Time Histories from 2 to 400 keV

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 ratio 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.

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arXiv1997

The Average Temporal and Spectral Evolution of Gamma-Ray Bursts

We have averaged bright BATSE bursts to uncover the average overall temporal and spectral evolution of gamma-ray bursts (GRBs). We align the temporal structure of each burst by setting its duration to a standard duration, which we call T_ . The observed average ``aligned $T_ '' profile for 32 bright bursts with intermediate durations (16 to 40 s) has a sharp rise (within the first 20% of T_ ) and then a linear decay. Exponentials and power laws do not fit this decay. In particular, the power law seen in the x-ray afterglow (T^{-1.4}) is not observed during the bursts, implying that the x-ray afterglow is not just an extension of the x-ray evolution seen during the gamma-ray phase. The average burst spectrum has a low energy slope of -1.03, a high energy slope of -3.31, and a peak in the νF_ν distribution at 390 keV. We determine the average spectral evolution. Remarkably, it is also a linear function, with the peak of the νF_ν distribution given by 680 - 600 (T/T_ ) keV. Since both the temporal profile and the peak energy are linear functions, the peak energy is linearly proportional to the intensity. This behavior is inconsistent with the external shock model. Previously, trends have been reported for GRB evolution, but our results are quantitative relationships that models should attempt to explain.

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arXiv1997

X-ray Spectral Characteristics of Ginga Gamma-Ray Bursts

The proportional counter and scintillator on Ginga provided unprecented spectral coverage of GRBs, from 2 - 400 keV. Below 20 keV we find evidence for a positive spectral number index in approximately 40% of our burst sample, with some evidence for a strong rolloff at lower energies in a few events. There is a correlation (Pearson's r = -0.62) between the low energy slope and the bend energy. We find that the distribution of spectral bend energies extends below 10 keV. There has been some concern in cosmological models of GRBs that the bend energy covers only a small dynamic range. Our result extends the observed dynamic range and, since we observe bend energies down to the limit of our instrument, perhaps observations have not yet limited the range. The Ginga trigger range was virtually the same as BATSE's, yet we find a different range of fit parameters. One possible explanation might be that GRBs have two break energies, one often in the 50 to 500 keV range and the other near 5 keV. Both BATSE and Ginga fit with only a single break energy so BATSE tends to find breaks near the center of its energy range and we tend to find breaks in our energy range. The observed ratio of energy emitted in the x-rays relative to the gamma-rays can be much larger than a few percent and, in fact, is sometimes larger than unity. The average for our 22 bursts is 24%. There is one example of a strong x-ray excess at low energy.

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arXiv1997

Variability in Shell Models of Gamma-Ray Bursts

Many cosmological models of gamma-ray bursts (GRBs) assume that a single relativistic shell carries kinetic energy away from the source and later converts it into gamma rays, perhaps by interactions with the interstellar medium or by shocks within the shell. We investigate methods of achieving the observed variability using symmetry. Since our model emphasizes geometric and statistical considerations, rather than the detailed physics of the shell, it is applicable to any theory that relies on relativistic shells. In order to achieve the observed variability in GRBs using a single-shell model, we have found that the gamma-ray emission must occur from small patches on the shell (entities). We have derived two significant characteristics of the time histories expected from such a model. First, the average number of entities contributing to the signal remains constant throughout much of the time history, although the overall photon flux decreases due to relativistic effects. Second, the "peaks" in a time history can be ascribed to Poisson variations in the actual number of entities contributing to the signal at any given time. Taken together, these properties imply that the relative variations in a GRB (i.e. the heights of the peaks relative to the average envelope of the signal) should remain constant throughout a time history.

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arXiv1997

Kinematic Arguments Against Single Relativistic Shell Models for Gamma-Ray Bursts

Two main types of models have been suggested to explain the long durations and multiple peaks of Gamma-Ray Bursts (GRBs). In one, there is a very quick release of energy at a central site resulting in a single relativistic shell that produces peaks in the time history through its interactions with the ambient material. In the other, the central site sporadically releases energy over hundreds of seconds forming a peak with each burst of energy. We present three kinematic arguments against a single relativistic shell. These arguments are based only on symmetry. We show that the average envelope of emission of GRBs is a linear function rather than the power law expected for a single relativistic shell. We show that presence of gaps in GRBs is the strongest argument against a single relativistic shell. We estimate that the fraction of a single shell that can produce gamma-rays in a GRB with multiple peaks is about 0.001, implying that single relativistic shells require 1000 times more energy than previously thought. We conclude that either the central site of a GRB must produce 10^{51} erg/s for hundreds of seconds, or the relativistic shell must have structure on a scales the order of sqrt(epsilon)/Gamma, where Gamma is the bulk Lorentz factor and epsilon is the efficiency.

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arXiv1997

X-ray Spectral Properties of Gamma-Ray Bursts

We summarize the spectral characteristics of a sample of 22 bright gamma-ray bursts detected with the gamma-ray burst sensors aboard the satellite Ginga. This instrument employed a proportional and scintillation counter to provide sensitivity to photons in the 2 - 400 keV range, providing a unique opportunity to characterize the largely unexplored X-ray properties of gamma-ray bursts. The photon spectra of the Ginga bursts are well described by a low energy slope, a bend energy, and a high energy slope. In the energy range where they can be compared, this result is consistent with burst spectral analyses obtained from the BATSE experiment aboard the Compton Observatory. However, below 20 keV we find evidence for a positive spectral number index in approximately 40% of our burst sample, with some evidence for a strong rolloff at lower energies in a few events. There is a correlation (Pearson's r = -0.62) between the low energy slope and the bend energy. We find that the distribution of spectral bend energies extends below 10 keV. The observed ratio of energy emitted in the X-rays relative to the gamma-rays can be much larger than a few percent and, in fact, is sometimes larger than unity. The average for our sample is 24%.

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arXiv1997

Relativistic Shells and Gamma-Ray Bursts

In many models of Gamma-Ray Bursts (GRBs) relativistic shells are responsible for the overall envelope of emission. We use kinematics and symmetry to calculate the time history and spectral evolution expected from a relativistic shell including effects from intrinsic variations in the shell's intensity and spectra. We find that the decay phase of an envelope is produced by photons delayed by the shell's curvature. These delayed photons are produced by regions that are off-axis such that the spectra evolve according to a universal function (T^{-1}) regardless of intrinsic variations in the rest frame of the shell. We compare our predictions to the overall envelope of emission of GRBs. The observed spectra evolve faster. Intrinsic variations cannot make the spectra evolve that fast, which adds strength to the ``shell symmetry'' problem: models, in particular, the external shock model, that involve relativistic shells must either confine the material to narrow pencil beams, be very inefficient, or break the local spherical symmetry so that the shell acts like a parallel slab. In the case of the internal shock models involving winds (i.e., central engines), it will probably be easier to break the local spherical symmetry, but then one must postulate nearly continuous energy generation at 10^{51} erg s^{-1} lasting up to hundreds of seconds at the central site.

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arXiv1997

Limits on Expanding Relativistic Shells from Gamma-Ray Burst Temporal Structure

We calculate the expected envelope of emission for relativistic shells under the assumption of local spherical symmetry. Gamma-Ray Burst envelopes rarely conform to the expected shape, which is similar to a FRED; a fast rise and exponential decay. The fast rise is determined by the time that the relativistic shell prodcues gamma rays. The decay has the form of a power law and arises from the curvature of the shell. The amount of curvature comes from the overall size of the shell so the duration of the decay phase is related to the time the shell expands before converting its energy to gamma rays. From the envelope of emission, one can estimate when the central explosion occurred and, thus, the energy required for the shell to sweep up the ISM. The energy greatly exceeds 10^{53} erg unless the bulk Lorentz factor is less than 75. This puts extreme limits on the "external" shock models. However, the alternative, "internal" shocks from a central engine, has a problem: the entire long complex time history lasting hundreds of seconds must be postulated at the central site.

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