Engineering topics

Paciesas, W. S.

Publications and source records attributed to Paciesas, W. S..

At least 37 records · Page 2

NASA NTRS2002

The Burst and Transient Source Experiment Earth Occultation Technique

An Earth orbiting detector sensitive to gamma-ray photons will see step-like occultation features in its count rate when a gamma-ray point source crosses the Earth's limb. This is due to the change in atmospheric attenuation of the gamma rays along the line of sight. In an uncollimated detector, these occultation features can be used to locate and monitor astrophysical sources provided their signals can be individually separated from the detector background. We show that the Earth occultation technique applied to the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) is a viable and flexible all-sky monitor in the low-energy gamma-ray and hard X-ray energy range (20 keV-1 MeV). The method is an alternative to more sophisticated photon imaging devices for astronomy and can serve well as a cost-effective science capability for monitoring the high energy sky. Here we describe the Earth occultation technique for locating new sources and for measuring source intensity and spectra without the use of complex background models. Examples of transform imaging, step searches, spectra, and light curves are presented. Systematic uncertainties due to source confusion, detector response, and contamination from rapid background fluctuations are discussed and analyzed for their effect on intensity measurements. A sky location-dependent average systematic error is derived as a function of Galactic coordinates. The sensitivity of the technique is derived as a function of incident photon energy and also as a function of angle between the source and the normal to the detector entrance window. Occultations of the Crab Nebula by the Moon are used to calibrate Earth occultation flux measurements independent of possible atmospheric scattering effects.

Harmon, B. A.↗

NASA NTRS2002

The BATSE Earth Occultation Catalog of Low Energy Gamma Ray Sources

The Burst and Transient Source Experiment (BATSE), aboard the Compton Gamma Ray Observatory (CGRO), provided a record of the hard X-ray/low energy gamma ray sky between April 1991 and June 2000. During that time, a catalog of known sources was derived from existing catalogs such as HEAO A-4 (Levine et al. 1984), as well as new transient sources discovered with BATSE and other X-ray monitors operating in the CGRO era. The Earth Occultation Technique (Harmon et al. 2001, astro-ph/0109069) was used to monitor a combination of these sources, mostly galactic, totaling to about 175 objects. The catalog will present the global properties of these sources and their probability of detection (greater than 10 mCrab, 20-100 keV) with BATSE. Systematic errors due to unknown sources or background components are included. Cursory analyses to search for new transients (35-80 mCrab in the 20-100 keV band) and super-orbital periods in known binary sources are also presented. Whole mission light curves and associated data production/analysis tools are being delivered to the High Energy Astrophysics Science Archive Research Center (HEASARC) for public use.

Harmon, B. A.↗

NASA NTRS2000

Gamma-Ray Burst Spectral Indices: Evidence for Deceleration of Synchrotron Shocks

The current scenario for gamma-ray bursts (GRBs) involves internal shocks for the prompt GRB emission phase and external shocks for the afterglow phase. Assuming synchrotron emission from energetic shocked electrons. GRB spectra observed with a low-energy power-law spectral index greater than -2/3 (for positive photon number indices E(sup alpha) indicate a problem with this model. The remaining spectra can test the synchrotron shock model prediction that the emission from a single distribution of electrons, cooling rapidly, is responsible for both the low-energy and high-energy power-low portions of the spectra. We find that the inferred relationship between the two spectral indices of observed GRB spectra is inconsistent with the constraints from the model, posing another problem for the synchrotron shock emission model. To overcome this problem, we describe a model where the average of -1, rather than the value of -3/2 predicted for cooling electrons. Situations where this might arise have been discussed in other contexts, and involve deceleration of the internal shocks during the GRB phase.

Preece, R. D.↗

NASA NTRS2000

General Differences between Early and Late Stages of BATSE Gamma-Ray Bursts

The early and late stages of gamma-ray bursts are studied in a statistical analysis of the large sample of long BATSE events. The primary peak is used as the boundary between the early and late stages of emission. Significant differences are found between the stages: the early stage is shorter, it has harder emission, and it becomes a smaller fraction of the total burst duration for burst groups of decreasing intensity.

Mitrofanov, I. G.↗

NASA NTRS2000

The Earth Occultation Technique with the Burst and Transient Source Experiment

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) is successfully being used as an all-sky hard X-ray monitor. The experiment consists of a set of eight uncollimated detectors sensitive to photons in the 20 keV to 2 MeV range. Since CGRO orbits the Earth at an altitude of about 450 km, about 33% of the sky, as viewed with BATSE, is covered by the Earth at any given time. The entire sky is subject to Earth occultation for some portion of CGRO's 52 day precession period. When a source sets below or rises above the Earth's limb, atmospheric attenuation produces step-like features in the BATSE data. The observed change in count rate in several energy bands provides a measurement of the source intensity and spectrum without sophisticated background models. These occultation features are used to locate and monitor astrophysical sources with BATSE when the source signal can be separated from the detector background. Examples of step searches, spectra, light curves, and transform imaging are presented.

Wilson, C. A.↗

NASA NTRS1999

Hard X-Ray Lags in GRO J1719--24

We have used the Fourier cross spectra of GRO J1719-24, obtained with BATSE, to estimate the phase lags between the X-ray flux variations in the 20 50 and 50-100 keV energy bands as a function of Fourier frequency in the interval 0.002-0.488 Hz. Our analysis covers the entire about 80 day X-ray outburst of this black hole candidate, following the first X-ray detection on 1993 September 25. The X-ray variations in the 50-100 keV band lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.072 + 0.010 rad in the frequency interval 0.02-0.20 Hz. The peak phase lags in the interval 0.02-0.20 Hz are about twice those of Cyg X-1 and GRO J0422+32. These results are consistent with models for Comptonization regions composed of extended nonuniform clouds around the central source.

VanDerHooft, F.↗

NASA NTRS1999

Hard X-Ray Variability of the Black Hole Candidate GRO J0422+32 During its 1992 Outburst

We have studied the hard X-ray variability of the soft X-ray transient GRO J0422+32 with BATSE (Burst and Transient Source Experiment) in the 20-100 keV energy band. Our analysis covers 180 days following the first X-ray detection of the source on August 5, 1992, fully covering its primary and secondary X-ray outbursts. We compute power density spectra (PDSs) in the 20-50, 50-100, and 20-100 keV energy bands, in the frequency interval 0.002-0.488 Hz. The PDSs of GRO J0422+32 are approximately flat up to a break frequency, and decay as a power law above it, with index approximately 1. During the first 70 days of the X-ray outburst, the PDSs of GRO J0422+32 show a significant quasi-periodic oscillation (QPO) peak near approximately 0.2 Hz, superposed on the power-law tail. The break frequency of the PDSs obtained during the primary X-ray outburst of GRO J0422+32 occurs at 0.041 +/- 0.006 Hz; during the secondary outburst, the break is at 0.081 +/- 0.015 Hz. The power density at the break ranged between 44% and 89% / square root of Hz (20-100 keV). The canonical anticorrelation between the break frequency and the power density at the break, observed in Cyg X-1 and other BHCs in the low state, is not observed in the PDSs of GRO J0422+32. We compare our results with those of similar variability studies of Cyg X-1. The relation between the spectral slope and the amplitude of the X-ray variations of GRO J0422+32 is similar to that of Cyg X-1; however, the relation between the hard X-ray flux and the amplitude of its variation is opposite to what has been found in Cyg X-1. Phase lags between the X-ray flux variations of GRO J0422+32 at high and low photon energies could only be derived during the first 30 days of its outburst. During this period, the variations in the 50-100 keV band lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.039(3) rad in the frequency interval 0.02-0.20 Hz. The time lags of GRO J0422+32 during the first 30 days of the outburst decrease with frequency as a power law with index 0.9 for v > 0.01 Hz.

vanderHooft, F.↗

NASA NTRS1999

The BATSE Gamma-Ray Burst Spectral Catalog: High Time Resolution Spectroscopy of Bright Bursts Using High Energy Resolution Data - 1

This is the first in a series of gamma-ray burst spectroscopy catalogs from the Burst And Transient Source Experiment (BATSE) on the Compton Gamma Ray Abstract: Observatory, each covering a different aspect of burst phenomenology. In this paper, we present time-sequences of spectral fit parameters for 156 bursts selected either for their high peak flux or fluence.

Preece, Robert D.↗

NASA NTRS1999

Analysis of Line Candidates in Gamma-Ray Bursts Observed by BATSE

A comprehensive search of BATSE Spectroscopy Detector data from 117 Gamma Ray Bursts (GRBs) has uncovered 13 statistically significant line candidates. The case of a candidate in GRB-930916 is discussed. In the data of SD-2 there appears to be a emission line at 46 keV, however the line is not seen in the data of SD-7. Simulations indicate that the lack of agreement between the results from SD-2 and SD-7 is implausible but not impossible.

Briggs, M. S.↗

NASA NTRS1999

XTE J1550-564; GRB 990123

The x-ray transient XTE Jl550-564 (IAUC 7008) was detected in Burst and Transient Source Experiment (BATSE) data beginning on Jan. 21. As of Jan. 23. the one-day averaged flux was 300 mCrab (+/- 20 percent, 20-100 keV), with a hard spectrum (power-law photon-number index -2.3 +/- 0.2). This is a reflare in hard x-rays, following the primary outburst in 1998 Sept.-Oct. (IAUC 7010). The source has been active in soft x-rays (2-12 keV) since about Dec. 18, according to observations by the Rossi X-ray Timing Explorer ASM.

Harmon, B. A.↗

NASA NTRS1999

Preview of the BATSE Earth Occultation Catalog of Low Energy Gamma Ray Sources

The Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory (CGRO) has been detecting and monitoring point sources in the high energy sky since 1991. Although BATSE is best known for gamma ray bursts, it also monitors the sky for longer-lived sources of radiation. Using the Earth occultation technique to extract flux information, a catalog is being prepared of about 150 sources potential emission in the large area detectors (20-1000 keV). The catalog will contain light curves, representative spectra, and parametric data for black hole and neutron star binaries, active galaxies, and super-nova remnants. In this preview, we present light curves for persistent and transient sources, and also show examples of what type of information can be obtained from the BATSE Earth occultation database. Options for making the data easily accessible as an "on line" WWW document are being explored.

Harmon, B. A.↗

NASA NTRS1999

The Earth Occultation Technique with the Burst and Transient Source Experiment

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) is an uncollimated all-sky instrument sensitive to photons in the 20 keV to 1 MeV range. Since CGRO orbits the Earth at an altitude of 450 km, about 33% of the sky, as viewed with BATSE, is covered by the Earth at any given time. The entire sky is subject to Earth Occultation for some portion of CGRO's 52 day precession period. When a source sets below (or rises above) the Earth's limb, atmospheric attenuation produces step-like features in the BATSE data. The observed change in count rate in several energy bands provides a measurement of the source intensity and spectrum without sophisticated background models. These occultation features are used to locate and monitor astrophysical sources with BATSE when the source signal can be separated from the detector background. Examples of step searches, spectra light curves, and transform imaging are presented. Systematic uncertainties due to source confusion, detector response, and rapid background variations are discussed.

Wilson, C. A.↗

NASA NTRS1998

Gamma-Ray Spectra and Variability of Cygnus Z-1 Observed by BATSE

We present new BATSE earth occultation observations of the 25 keV-1.8 MeV spectrum and variability of Cygnus X-1 made between August 1993 and May 1994. We observed that the normal soft gamma ray spectrum (gamma2) of Cygnus X-1 has two components: a Comptonized part seen below 300 keV, and a high-energy tail in the 0.3 - 2 MeV range. We interpret it in terms of a two-layer region, consisting of a high-energy core (with an equivalent electron temperature of approximately 210-250 keV) near the event horizon, embedded in an about 50 keV corona. In this scenario, the observed 25-300 keV photons were produced by Compton scattering of soft photons (about 0.5 keV) by the hot electrons in the outer corona. These same hard x rays were further up-scattered by a population of energetic electrons in the inner core, producing the spectral tail above 300 keV. Cygnus X-1 went through an extended sequence of transitions between August 1993 and May 1994, when the 45-140 keV flux first decreased steadily from approximately gamma2 to roughly one-quarter of its intensity over a period of about 140 days. The flux remained at this low level for about 40 days before returning, swiftly (approximately 20 days) to approximately the initial gamma2 level. During the transition, the spectrum evolved to a shape consistent with either a power law with photon index of about 2.6 or a single temperature Compton model with electron temperature kT = 110 +/- 11 keV, and optical depth t = 0.40 +/- 0.06, and then returned essentially to the original gamma2 spectrum at the end of the active period. The overall cooling of the system during the low flux period may be due to an increase in the soft photon population which effectively quenched the hot electrons in these regions through Compton scattering.

Ling, J. C.↗

NASA NTRS1998

X-Ray Spectral Behavior of the Relativistic Jet Source Cygnus X-3

Cyg X-3 is an unusual X-ray binary which shows remarkable correlative behavior between the hard X-ray, soft X-ray, and the radio. We present an analysis of these long term light curves in the context of spectral changes of the system. This analysis will also incorporate a set of pointed RXTE observations made during a period when Cyg X-3 made a transition from a quiescent radio state to a flaring state (including a major flare) and then returned to a quiescent radio state.

McCollough, M. L.↗

NASA NTRS1998

The Synchrotron Shock Model Confronts a "Line of Death" in the BATSE Gamma-Ray Burst Data

The synchrotron shock model (SSM) for gamma-ray burst emission makes a testable prediction: that the observed low-energy power-law photon number spectral index cannot exceed -2/3 (where the photon model is defined with a positive index: $dN/dE \propto E{alpha}$). We have collected time-resolved spectral fit parameters for over 100 bright bursts observed by the Burst And Transient Source Experiment on board the {\it Compton Gamma Ray Observatory}. Using this database, we find 23 bursts in which the spectral index limit of the SSM is violated, We discuss elements of the analysis methodology that affect the robustness of this result, as well as some of the escape hatches left for the SSM by theory.

Preece, Robert D.↗

NASA NTRS1998

Hard X-Ray Variability of the Black Hole Candidate GRO J0422+32 During its 1992 Outburst

We have studied the hard X-ray variability of the soft X-ray transient GRO J0422+32 with the Burst and Transient Source Experiment (BATSE) in the 20-100 keV energy band. Our analysis covers 180 days following the first X-ray detection of the source on 1992 August 3, fully covering its primary and secondary X-ray outburst. We computed power density spectra (PDSS) in the 20-50, 50-100, and 20-100 keV energy bands., in the frequency interval 0.002-0.488 Hz. The PDSs of GRO J0422+32 are approximately flat up to a break frequency and decay as a power law above, with index about 1. During the first 70 days of the X-ray outburst, the PDSs of GRO J0422+32 show a significant QPO Peak near about 0.2 Hz, superposed on the power-law tail. The break frequency of the PDSs obtained during the primary X-ray outburst of GRO J0422+32 occurs at 0.041+/-0.006 Hz; during the secondary outburst the break is at 0.081q0.01.5 Hz. The power density at the break ranged between 44 and 89% Hz(exp -1/2) 20-100 keV). The canonical anticorrelation between the break frequency and the power density at the break, observed in Cyg X-1 and other BHCs in the low state, is not observed in the PDSs of GRO J0422+32. We compare our results with those of similar variability studies of Cyg X-1. The relation between the spectral slope and the amplitude of the X-ray variations of GRO J0422+32 is similar to that of Cyg X-1.

VanderHooft, F.↗

NASA NTRS1998

Hard X-Ray Lags in GRO J1719-24

We have used the Fourier cross spectra of GRO J1719-24. as obtained with BATSE, to estimate the phase lags between the X-ray flux variations in the 20-,50 and 50-100 keV energy bands as a function of Fourier frequency in the interval 0.002-0.488 Hz. Our analysis covers the entire approximately 80 day X-ray outburst of this black-hole candidate, following the first X-ray detection on 1993 September 25. The X-ray variations in the 50-100 keV band, lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.072 +/- 0.010 rad in the frequency interval 0.02-0.20 Hz. The time lags of GRO J1719-24 decrease with frequency as a power law, with index 1.04 +/- 0.13 for frequencies >/= 0.01 Hz. These results are discussed together with those obtained in recent similar studies of the black-hole candidates Cyg X-1 and GRO J0422+32.

VanderHooft, Frank↗

NASA NTRS1998

Average Emissivity Curve of BATSE Gamma-Ray Bursts with Different Intensities

Six intensity groups with $/sim 150$ BATSE gamma-ray bursts each are compared using average emissivity curves. Time-stretch factors for each of the dimmer groups are estimated with respect to the brightest group. Which serves as the reference taking into account the systematics of counts-produced noise effects and choice statistics. The effect of stretching/intensity anti-correlation is found at the average back slopes of bursts with good statistical significance. A stretch factor $/sim 2$ is found between the 150 dimmest bursts with peak flux $less than 0.45$ ph cm$(exp -2)$ s$(exp -1)$, and the 147 brightest bursts with peak flux $greater than 4.1$ ph cm$(exp -2}$ s$(exp -1)$. On the other hand, only a marginally significant stretching effect $V(sub ec) 1.4$ is seen at the average rise fronts.

Mitrofanov, Igor G.↗