Engineering topics

Kouveliotou, Chryssa

Publications and source records attributed to Kouveliotou, Chryssa.

At least 127 records · Page 7

NASA NTRS1998

SGR 1627-41

BATSE has detected repeated soft gamma-ray bursts consistent with the same (previously unknown) location. We recorded three bursts on June 15.109, 15.296, 15.411 UT (BATSE triggers 6825, 6826, 6827, respectively) with an average duration of about 200 milliseconds. Preliminary spectral analysis of the data indicates that the events are very soft, with power law spectral indices varying between -3.0 and -7.0. BATSE triggered 5 more times, on June 17.873,17.901, 18.004, 18.035, and 18.071 UT (BATSE trigger numbers 6833, 6834, 6835, 6836 and 6837, respectively). The last trigger had a peak count rate of over 300000 counts/sec above background (integrated over 4 detectors between 20-1000 keV, in 1 second interval) and a duration of about 3 seconds. Due to Rs very high peak intensity, deadtime effects prevent us from using it for localization, until we receive more data types. The weighted location of the 7 remaining triggers is centered on R.A. = 16h27ml2.0s and dec = -41 d06.0' with an error radius of about 2 degrees. This location does not correspond to any previously known SGR source; we conclude that we have discovered a new source, SGR1627-41. We have initiated an RXTE ToO and we strongly encourage wide field observations at other wavelengths.

Kouveliotou, Chryssa↗

NASA NTRS1997

A Search for Nontriggered Gamma-Ray Bursts in the BATSE Database

We describe a search of archival data from the Burst and Transient Source Experiment (BATSE). The purpose of the search is to find astronomically interesting transients that did not activate the burst-detection (or "trigger") system on board the spacecraft. Our search is sensitive to events with peak fluxes (on the 1.024 s timescale) that are lower by a factor of approximately 2 than can be detected with the on-board burst trigger. In a search of 345 days of archival data, we detected 91 events in the 50-300 keV range that resemble classical gamma-ray bursts but that did not activate the on-board burst trigger. We also detected 110 low-energy (25-50 keV) events of unknown origin that may include activity from' soft gamma repeater (SGR) 1806-20 and bursts and flares from X-ray binaries. This paper gives the occurrence times, estimated source directions, durations, peak fluxes, and fluences for the 91 gamma-ray burst candidates. The direction and intensity distributions of these bursts imply that the biases inherent in the on-board trigger mechanism have not significantly affected the completeness of the published BATSE gamma-ray burst catalogs.

Kommers, Jefferson M.↗

NASA NTRS1997

Gamma-Ray Bursts

Gamma-ray burst (GRB) have been an unsolved mystery in high-energy astrophysics for the last 30 years. Immediately after GRB were discovered, scientists tried to understand the mechanism that causes these events and where they come from. Since than, many theories have been suggested to explain GRB which have durations spanning five orders of magnitude (ranging between a few milliseconds and minutes) and spectrals that peak generally in the range of 0.1 to 1 MeV. Given this numbers, most theorists would think of processes occurring near neutron stars in our galaxy, many of which are known sources of rapidly varying, high-energy photon emission.

Kouveliotou, Chryssa↗

NASA NTRS1997

IAUC 6530: GRO J1744-28; C/1995 01

We have recorded a series of outbursts starting on 1996 Dec. 2 that are consistent with the position of GRO J1744-28, the bursting pulsar. The bursts are detected between 20 and 100 keV, and on Dec. 2 they came in variable rates ranging from every 300 to every 500 s, with a preliminary total number estimated between 80 and 1 00 events. After Dec. 3, the burst rate dropped to 15-20 bursts/day, where it has remained. The current average error radius of the burst locations is about 4 deg. The burst fluence is currently about 2 x 10E-7 erg cmE-2. This indicates that the source is in outburst again. Although the source location is currently very close to the sun, we encourage observations in other wavelengths whenever possible. Persistent, hard x-ray emission from GRO J1744-28 (IAUC 6272, 6284, 6285) is currently being detected through both pulsation and earth occultation. Pulsations with a period near 0.467 s became detectable on Dec. 17, rising to a 20- to 40-keV-rms-pulsed flux of 1.3(1) x 10E-9 erg cmE-1 sE-1 (170 mCrab) by 1997 Jan. 3. On Jan. 1.0, the intrinsic pulse frequency was 2.1408983(4) Hz, and the spin-up rate was 2.8(5) x l0E-12 HzsE-1. This assumes the following circular orbit parameters: P = 11.83665(14) days; epoch of longitude 90 deg = JD 2450126.9977(4) TDB; a sin i = 2.6371(5) light-s (determined using data from the 1995 Dec.-l996 Apr. outburst). Earth-occultation monitoring of flux from GRO J1744-28 shows its intensity increasing by a factor of about two during 1996 Dec. 25-1997 Jan. 6. Although source confusion prevents a precise flux estimate, subtracting a 25-percent background for unresolved galactic-center sources yields a persistent flux for Jan. 3-6 in the energy bands 20-30, 30-40, and 40-50 keV of 590, 470, and 290 mcrab, respectively."

Kouveliotou, Chryssa↗

NASA NTRS1996

The Third BATSE Gamma-Ray Burst Catalog

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) has triggered on 1122 cosmic gamma-ray bursts between 1991 April 19 and 1994 September 19. These events constitute the Third BATSE (3B) burst catalog. This catalog includes the events previously reported in the 2B catalog, which covered the time interval 1991 April 19 to 1993 March 9. We present tables of the burst occurrence times, locations, peak fluxes, fluences, and durations. In general, results from previous BATSE catalogs are confirmed here with greater statistical significance. The angular distribution is consistent with isotropy. The mean galactic dipole and quadrupole moments are within 0.6 a and 0.3 a, respectively, of the values expected for isotropy. The intensity distribution is not consistent with a homogeneous distribution of burst sources, with V/V(sub max) = 0.33 +/- 0.01. The duration distribution (T(sub 90)) exhibits bimodality, with peaks at approx. 0.5 and approx. 30 s. There is no compelling evidence for burst repetition, but only weak limits can be placed on the repetition rate.

Meegan, Charles A.↗

NASA NTRS1996

Systematic Effects on Duration Measurements of Gamma-Ray Bursts

The parameters T(sub 90) and T(sub 50) have recently been introduced as a measurement of the duration of gamma-ray bursts. We present here a description of the method of measuring T(sub 90) and T(sub 50) and its application to gamma-ray bursts observed with the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma-Ray Observatory (CGRO). We use simulated as well as observed time profiles to address some of the possible systematic effects affecting individual T(sub 90) (T(sub 50)) measurements. We show that these systematic effects do not mimic those effects that would result from time dilation if the burst sources are at distances of several Gpc. We discuss the impact of these systematic effects on the T(sub 90) (T(sub 50)) distributions for the gamma-ray bursts observed with BATSE. We distinguish between various types of T(sub 90) (T(sub 50)) distributions, and discuss the ways in which distributions observed with different experiments can vary, even though the measurements for commonly observed bursts may be the same. We then discuss the distributions observed with BATSE and compare them to those observed with other experiments.

Koshut, Thomas M.↗

NASA NTRS1996

BATSE Observations of the Large-Scale Isotropy of Gamma-Ray Bursts

We use dipole and quadrupole statistics to test the large-scale isotropy of the first 1005 gamma-ray bursts observed by the Burst and Transient Source Experiment (BATSE). In addition to the entire sample of 1005 gamma-ray bursts, many subsets are examined. We use a variety of dipole and quadrupole statistics to search for Galactic and other predicted anisotropies and for anisotropies in a coordinate-system independent manner. We find the gamma-ray burst locations to be consistent with isotropy, e.g., for the total sample the observed Galactic dipole moment (cos theta) differs from the value predicted for isotropy by 0.9 sigma and the observed Galactic quadrupole moment (sin(exp 2) b - 1/3) by 0.3 sigma. We estimate for various models the anisotropies that could have been detected. If one-half of the locations were within 86 deg of the Galactic center, or within 28 deg of the Galactic plane, the ensuing dipole or quadrupole moment would have typically been detected at the 99% confidence level. We compare the observations with the dipole and quadrupole moments of various Galactic models. Several Galactic gamma-ray bursts models have moments within 2 sigma of the observations; most of the Galactic models proposed to date are no longer in acceptable agreement with the data. Although a spherical dark matter halo distribution could be consistent with the data, the required core radius is larger than the core radius of the dark matter halo used to explain the Galaxy's rotation curve. Gamma-ray bursts are much more isotropic than any observed Galactic population, strongly favoring but not requiring an origin at cosmological distances.

Briggs, Michael S.↗

NASA NTRS1995

Gamma-Ray Burst Precursor Activity as Observed with BATSE

Gamma-ray burst time histories often consist of multiple episodes of emission with the count rate dropping to the background level between adjacent episodes. We define precursor activity as any case in which the first episode (referred to as the precursor episode) has a lower peak intensity than that of the remaining emission (referred to as the main episode) and is separated from the remaining burst emission by a background interval that is at least as long as the remaining emission. We find that approx. 3% of the bursts observed with the Burst and Transient Source Experiment (BATSE) on Compton Gamma Ray Observatory (CGRO) satisfy this definition. We present the results of a study of the properties of these events. The spatial distribution of these sources is consistent with that of the larger set of all BATSE gamma-ray bursts: inhomogeneous and isotropic. A correlation between the duration of the precursor emission and the duration of the main episode emission is observed at about the 3 sigma confidence level. We find no meaningful significant correlations between or among any of the other characteristics of the precursor or main episode emission. It appears that the characteristics of the main episode emission are independent of the existence of the precursor emission.

Koshut, Thomas M.↗

NASA NTRS1995

Do gamma-ray burst sources repeat?

The demonstration of repeated gamma-ray bursts from an individual source would severely constrain burst source models. Recent reports (Quashnock and Lamb, 1993; Wang and Lingenfelter, 1993) of evidence for repetition in the first BATSE burst catalog have generated renewed interest in this issue. Here, we analyze the angular distribution of 585 bursts of the second BATSE catalog (Meegan et al., 1994). We search for evidence of burst recurrence using the nearest and farthest neighbor statistic and the two-point angular correlation function. We find the data to be consistent with the hypothesis that burst sources do not repeat; however, a repeater fraction of up to about 20% of the observed bursts cannot be excluded.

Meegan, Charles A.↗

NASA NTRS1994

The continuum spectral characteristics of gamma-ray bursts observed by BATSE

Distributions of the continuum spectral characteristics of 260 bursts in the first Burst And Transient Source Experiement (BATSE) catalog are presented. The data are derived from flux calculated from BATSE Large Area Detector (LAD) four-channel discriminator data. The data are converted from counts to protons using a direct spectral inversion technique to remove the effects of atmospheric scattering and the energy dependence of the detector angular response. Although there are intriguing clusters of bursts in the spectral hardness ratio distributions, no evidence for the presence of distinct burst classes based in spectral hardness ratios alone is found. All subsets of bursts selected for their spectral characteristics in this analysis exhibit spatial distributions consistent with isotropy. The spectral diversity of the burst population appears to be caused largely by the highly variable nature of the burst production mechanisms themselves.

Pendleton, Geoffrey N.↗

NASA NTRS1994

BATSE results on observational properties of gamma-ray bursts

The Burst and Transient Source Experiment (BATSE) has observed over 600 gamma-ray bursts since its activation on 1991 April 21. We present here results on the global properties of the first 542 events. Their angular distribution is consistent with isotropy; their peak intensity distribution shows a depletion at low intensities, consistent with inhomogeneity in Euclidean space.

Kouveliotou, Chryssa↗

NASA NTRS1994

Spectral evolution of a subclass of gamma-ray bursts observed by batse

Among the gamma-ray bursts (GRBs) observed by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory we define a subclass of bursts based on similar morphology: a sharp rise followed by a longer decay time. About 7% of all the gamma-ray bursts observed by BATSE fall into this subclass. We study the spectral evolution of these bursts by fitting models to time-segmented burst spectra and find no clear distinction between the spectral evolutionary properties of this subclass and those of other bursts. Further, we study the high time resolution spectral evolution of this subclass of GRBs using their spectral hardness ratios. A majority of the bursts show hardness ratio leading the counting rate and also display a continuous hard to soft evolution. The time lag between the counting rate and the hardness ratio is found to be directly correlated with the rise time of the counting rate profile. We also find, for the first time, evidence for spectral variation in a timescale of 64 ms.

Bhat, P. N.↗

NASA NTRS1994

The first BATSE gamma-ray burst catalog

The Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory detected 260 cosmic gamma-ray bursts during the period 19 Apr 1991 to 5 Mar 1992. This paper presents the occurrence times, locations, peak count rates, peak fluxes, fluences, durations, and plots of time histories for these bursts. The angular distribution is consistent with isotropy. The intensity distribution shows a deficit in the number of weak bursts, which is not consistent with a homogeneous distribution of burst sources in Euclidean space. The duration distribution shows evidence for a separate class of bursts with durations less than about 2 seconds.

Fishman, Gerald J.↗

NASA NTRS1994

BATSE observations of the very intense gamma-ray burst GRB 930131

Burst and Transient Source Experiment (BATSE) observed its most intense gamma-ray burst on 1993 January 31. The event reached count rates is approximately greater than 2 x 10(exp 6) counts/s with most of the flux emitted in an extremely short (is approximately less than 0.1 s) interval followed by a long tail, lasting about 50 s. Most of this initial pulse was recorded by our instrument with unique, very high temporal resolution (1 ms). We were thus able to show large changes in spectral hardness on 2 ms timescales throughout this initial complex. Photons as low as 25 keV and extending up to greater than 4 MeV in energy were recorded by BATSE during this first interval. The burst spectrum is best fitted by a broken power law with a break energy of 170 +/- 27 keV. The low-energy spectral index is -1.30 +/- 0.05, while a softer spectral index of -1.9 fits the spectrum between 170 keV and 2 MeV. Our data provide the only low-energy spectrum for this event; the combination of our spectrum with the one reported for GRB 930131 by the Energetic Gamma Ray Experiment Telescope (EGRET) group extends the total energy spectrum of a GRB for the first time over five decades, up to the GeV range.

Kouveliotou, Chryssa↗

NASA NTRS1994

Rapid searches for counterparts of GRB 930131

A fading counterpart to a gamma-ray burst (GRB) would appear as a point source inside a GRB error region soon after the burst which dims on a timescale from minutes to days. The favorable circumstances of the burst GRB 930131 allowed for an international campaign to search for fading counterparts starting 6.8 hr after the burst. We report observations from many optical sites, two radio telescopes, and archival ROSAT data, including deep Schmidt exposures 35, 44, and 64 hr after the burst. No fading counterparts were detected with our observations.

Schaefer, Bradley E.↗

NASA NTRS1994

The continuum spectral characteristics of gamma ray bursts observed by BATSE

Distributions of the continuum spectral characteristics of 260 bursts in the first Burst and Transient Source Experiment (BATSE) catalog are presented. The data are derived from flux ratios calculated from the BATSE Large Area Detector (LAD) four channel discriminator data. The data are converted from counts to photons using a direct spectral inversion technique to remove the effects of atmospheric scattering and the energy dependence of the detector angular response. Although there are intriguing clusterings of bursts in the spectral hardness ratio distributions, no evidence for the presence of distinct burst classes based on spectral hardness ratios alone is found. All subsets of bursts selected for their spectral characteristics in this analysis exhibit spatial distributions consistent with isotropy. The spectral diversity of the burst population appears to be caused largely by the highly variable nature of the burst production mechanisms themselves.

Pendleton, Geoffrey N.↗

NASA NTRS1994

Continuum spectral characteristics of bursts measured with BATSE large area detectors

The continuum spectra of bursts are studied with particular emphasis placed on the search for spectral subcomponents. The spectra observed in individual bursts show significant evolution throughout their durations. The degree to which hard and soft spectra are combined in bursts and the effect this has on the complexity of the observed spectra are addressed. In particular the presence of broad cusps in spectra in the 50-100 keV range are studied to determine both the degree to which these features are present in the LAD data and whether superposition of hard and soft spectra simultaneously can account for them. Also the time histories of the power output (ergs/sec) from bursts are studied to investigate the dynamic coupling between hard and soft spectra in bursts as a probe of the autonomy of the hard and soft emission mechanisms.

Pendleton, Geoffrey N.↗

NASA NTRS1993

Identification of two classes of gamma-ray bursts

We have studied the duration distribution of the gamma-ray bursts of the first BATSE catalog. We find a bimodality in the distribution, which separates GRBs into two classes: short events (less than 2 s) and longer ones (more than 2 s). Both sets are distributed isotropically and inhomogeneously in the sky. We find that their durations are anticorrelated with their spectral hardness ratios: short GRBs are predominantly harder, and longer ones tend to be softer. Our results provide a first GRB classification scheme based on a combination of the GRB temporal and spectral properties.

Kouveliotou, Chryssa↗