Engineering topics

Kouveliotou, C.

Publications and source records attributed to Kouveliotou, C..

At least 145 records · Page 8

NASA NTRS1994

A search for the radio counterpart to the 1994 March 1 gamma-ray burst

We report on the results of a search for the radio counterpart to the bright gamma-ray burst of 1994 March 1. Using the Dominion Radio Astrophysical Observatory Synthesis Telescope sensitive, wide-field radio images at 1.4 GHz and 0.4 GHz were made of a region around GRB 940301. A total of 15 separate radio images were obtained at each frequency, sampling a near-continuous range of post-burst timescales between 3 and 15 days, as well as 26, 47, and 99 days. We place an upper limit of 3.5 mJy on a fading/flaring radio counterpart at 1.4 GHz and 55 mJy at 0.4 GHz. Unlike past efforts our counterpart search maintains high sensitivity over two decades of post-burst time durations. Time-variable radio emission after the initial gamma-ray burst is a prediction of all fireball models, currently the most popular model for gamma-ray bursts. Our observations allow us to put significant constraints on the fireball parameters for cosmological models of gamma-ray bursts.

Frail, D. A.↗

NASA NTRS1994

Gross spectral differences between bright and dim gamma-ray bursts

We find that dim gamma-ray bursts (GRBs) are softer than bright GRBs, as indicated on average by data from the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO). We show that this correlation is statistically significant with respect to variations due to random differences between GRBs. This effect is discernible using a variety of methods and data sets, including public domain data. We analyze several types of systematic errors and selection effects in the BATSE data and conclude that the observed effect is not dominated by any of them. We therefore assert that this dim/soft effect is a real property of GRBs. It is possible that this correlation is a consequence of the time dilation and that the burst sources are located at cosmological distances.

Nemiroff, R. J.↗

NASA NTRS1994

Null Result in gamma-ray burst lensed echo search

We have searched for gravitational-lens-induced echoes between gamma-ray bursts (GRBs) in Burst and Transient Source Experiment (BATSE) data. The search was conducted in two phases. In the first phase we compared all GRBs in a brightness-complete sample of the first 260 GRBs with recorded angular positions having at least a 5% chance of being coincident from their combined positional error. In the second phase, we compared all GRB light curves of the first 611 GRBs with recorded angular positions having at least a 55% chance of being coincident from their combined positional error. No unambiguous gravitational lens candidate pairs were found in either phase, although a 'library of close calls' was accumulated for future reference. This result neither excludes nor significantly constrains a cosmological origin for GRBs.

Nemiroff, R. J.↗

NASA NTRS1994

Network synthesis localization of two soft gamma repeaters

We introduce the method of 'network synthesis,' which allows the detection of very weak gamma-ray transient signals in the data of the Ulysses gamma-ray burst (GRB) experiment from repeating sources. It consists of defining a grid of alpha, delta values, and for each BATSE detection of a burst from a soft gamma repeater, predicting the arrival time of the burst at Ulysses and co-adding the Ulysses data rephased so that the burst signals are aligned in time and produce a detectable pulse. We demonstrate that this method identifies the position of the soft repeater SGR 1806-20, and apply it to the repeater B1900+14. We show that the counterpart to this burst source is probably in or in the vicinity of the Galactic supernova remnant G42.8+0.6.

Hurley, Kevin↗

NASA NTRS1994

10 micron detection of the hard X-ray transient GRO J0422+32: Free-free emission from an X-ray-driven accretion disk wind?

We report the detection of 10 micrometer emission from the transient low-mass X-ray binary (LMXB) and optical nova GRO J0422+32 near the maximum of its outburst. We discuss this result in terms of (1) a 'standard' model according to which low-energy radiation of LMXB is caused by reprocessing of X-rays in an accretion disk; (2) emission from a cool secondary star; (3) emission from dust grains heated by the transient X-rays, and (4) free-free emission from an X-ray-driven wind from the accretion disk. Only the fourth alternative provides a viable explanation for the observed 10 micrometer emission, with a mass-loss rate in the disk wind that may be substantially higher than the rate of accretion onto the compact star. The presence of such a wind may have a profound effect on the evolution of the binary, and contribute to the solution of the 'birthrate problem' of millisecond ratio pulsars.

Paradijs, Van J.↗

NASA NTRS1994

The optical and X-ray content of the 1992 May 1 gamma-ray burst error box

A gamma-ray burst which occurred on 1992 May 1 was observed by three spacecraft in the third interplanetary network, and rapidly localized to a small error box. The coordinates were promptly circulated to a wide astronomical community, and radio, optical, and X-ray counterpart searches were carried out. A weak X-ray source was found in the error box, and two radio sources are discovered outside the error box, but in alignment with the X-ray source. The X-ray source position contains approximately 25 optical objects down to 23d magnitude. We discuss the prospects for identifying the burster counterpart.

Hurley, K.↗

NASA NTRS1994

Discovery of Intense Gamma-Ray Flashes of Atmospheric Origin

Detectors aboard the Compton Gamma Ray Observatory have observed an unexplained terrestrial phenomenon: brief, intense flashes of gamma rays. These flashes must originate in the atmosphere at altitudes above at least 30 kilometers in order to escape atmospheric absorption and reach the orbiting detectors. At least a dozen such events have been detected over the past 2 years. The photon spectra from the events are very hard (peaking in the high-energy portion of the spectrum) and are consistent with bremsstrahlung emission from energetic (million-electron volt) electrons. The most likely origin of these high- energy electrons, although speculative at this time, is a rare type of high-altitude electrical discharge above thunderstorm regions.

Fishman, G. J.↗

NASA NTRS1994

Detection of signature consistent with cosmological time dilation in gamma-ray bursts

If gamma ray bursts are at cosmological distances-as suggested by their isotropic distribution on the sky and by their number-intensity relation-then the burst profiles will be stretched in time, by an amount proportional to the redshift, 1 + Z. We have tested data from the Compton Gamma Ray Observatory's (CGRO's) Burst and Transient Source Experiment (BATSE) for such time dilation. Out of 590 bursts observed by BATSE, 131 bursts were analyzed; bursts with durations shorter than 1.5 s were excluded. We used three tests to compare the timescales of bright and dim bursts, the latter, on average, being more distant than the former. Our measures of timescale are constructed to avoid selection effects arising from intensity differences by rescaling all bursts to fiducial levels of peak intensity and noise bias. (1) We found that the total rescaled count above background for the dim burst ensemble is approximately twice that for the brightest bursts-translating into longer durations for the dim bursts. (2) Wavelet-transform decompositions of the burst profiles confirmed that this dilation operates over a broad range of timescales. (3) Structure on the shortest timescales was examined using a procedure which aligns the highest peaks of profiles from which the noise has been optimally removed using a wavelet threshold technique. In all three tests, the dim bursts are stretched by a factor of approximately 2 relative to the bright ones, over seven octaves of timescale. We calibrated the measurements by dilating synthetic bursts that approximate the temporal characteristics of bright BATSE bursts. Results are consistent with bursts at BATSE's peak-flux completeness limit being at cosmological distances corresponding to Z approximately equal to 1, and thus with independent cosmological interpretations of the BATSE number-intensity relation. Alternative explanations of our results, arising from the nature of physical processes in bursts, are still possible.

Norris, J. P.↗

NASA NTRS1994

The rarity of soft gamma-ray repeaters deduced from reactivation of SGR1806-20

Only two different types of gamma-ray transient sources are presently known: over one thousand Gamma-Ray Bursters (GRBs) and only three Soft Gamma-Ray repeaters (SGRs). The latter are distinguished by their propensity for recurrent burst behaviour, in contrast to the nonrepeating GRB sources. Recurrent emission from one of the repeaters, SGR1900 + 14, has been detected earlier by the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory. Here we report renewed burst activity from SGR1806 - 20, the most prolific of the three known SGRs. This detection of reactivation of this source has been rapidly followed by identification of an X-ray counterpart, which also coincides with a compact radio source now identified as a plerionic (pulsar-powered) supernova remnant. In combination, these results are leading to a convergence of ideas about the nature of SGRs, which can now be firmly identified as neutron stars. That BATSE has detected no new sources in its two and a half years of operation indicates that SGRs are rare in our Galaxy.

Kouveliotou, C.↗

NASA NTRS1994

Gamma-ray bursts are time-asymmetric

A simple test for time asymmetry is devised and carried out on the brightest gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory. We show evidence that individual bursts are time-asymmetric on all timescales tested, from a timescale shorter than that of pulses which compose GRBs to a timescale similar to a greater envelope that contains these pulses. We also find bursts which manifest significant asymmetry only on timescales comparable to the duration of burst, and bursts for which no clear asymmetry on any timescale is present. The sense of the asymmetry is that bursts and/or component structures rise in a shorter time than they decay. We also find that our whole sample of bursts taken together is time-asymmetric, in that there are sifnificantly more bursts and pulses where the rise is more rapid than the decay, on all timescales tested and for all energy bands tested. When our whole GRB sample is binned at 64 ms and integrated over all BATSE energies, the statistical significance is at the 6 sigma level. Models that predict time symmetry are therefore excluded.

Nemiroff, R. J.↗

NASA NTRS1994

Comptel measurements of the gamma-ray burst GRB 930131

On 1993 January 31 at 1857:12 Universal Time (UT), the Imaging Compton Telescope COMPTEL onboard the Compton Gamma Ray Observatory (CGRO) detected the cosmic gamma-ray burst GRB 930131. COMPTEL's MeV imaging capability was employed to locate the source to better than 2 deg (1 sigma error radius) within 7 hr of the event, initiating a world-wide search for an optical and radio counterpart. The maximum likelihood position of the burst from the COMPTEL data is alpha(sub 2000) = 12h 18m, delta(sub 2000) = -9 deg 42 min, consistent with independent CGRO-Burst and Transient Source Experiment (CGRO-BATSE) and Energetic Gamma Ray Experiment Telescope (EGRET) locations as well as with the triangulation annulus constructed using BATSE and Ulysses timing data. The combined COMPTEL and EGRET burst data yield a better estimate of the burst location: alpha(sub 2000) = 12h 18m and delta(sub 2000) = -10 deg 21 min, with a 1 sigma error radius of 32 min. In COMPTEL's energy range, this burst was short, consisting of two separate spikes occurring within a approximately 1 s interval with a low intensity tail for approximately 1 s after the second spike. No statistically significant flux is present for a 30 s period after the main part of the burst. This is consistent with the EGRET data. The COMPTEL telescope events indicate a hard, power-law emission extending to beyond 10 MeV with a spectral index of -1.8 +/- 0.4. The rapid fluctuations and high intensities of the gamma-ray flux greater than 10 MeV place the burst object no farther than 250 pc if the burst emission is not beamed.

Ryan, J.↗

NASA NTRS1994

Discovery of intense gamma-ray flashes of atmospheric origin

Observations have been made of a new terrestrial phenomenon: brief (approx. millisecond), intense flashes of gamma rays, observed with space-borne detectors. These flashes must originate at altitudes in the atmosphere above at least 30 km in order to be observable by orbiting detectors aboard the Compton Gamma-Ray Observatory (CGRO). At least a dozen events have been detected over the past 2 years. The photon spectra from the events are very hard and are consistent with bremsstrahlung emission from energetic (MeV) electrons. The most likely origin of these high energy electrons, while speculative at this time, is a rare type of high altitude electrical discharge above thunderstorm regions.

Fishman, G. J.↗

NASA NTRS1994

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 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 ad 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 bursts cannot be excluded.

Meegan, C. A.↗

NASA NTRS1993

Rosat and BATSE observations of GRS 1915+105

The determination of an accurate X-ray position of GRS 1915+1405, using a target of opportunity Rosat HRI (High Resolution Imager) observation in Oct. 1992, which allowed deep optical follow up studies, is reported. A spectral fit to Rosat PSPC (Position Sensitive Proportional Counter) and BATSE (Gamma Ray Burst (GRB) instrument team for the Compton Gamma Ray Observatory) observations in Oct. 1992 reveal a steep spectrum with power law index alpha = -3 between 0.5 and 300 keV. The soft X-rays are strongly absorbed (N(sub H) = 5 x 10(exp 22)/sq cm) suggesting a distance to GRS 1915+105 of the order of 8 kpc. Possible explanations of the source nature are discussed together with the peculiar light curve at hard X-rays.

Greiner, J.↗

NASA NTRS1993

Searching gamma-ray bursts for gravitational lensing echoes - Implications for compact dark matter

The first available 44 gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment on board the Compton Gamma-Ray Observatory have been inspected for echo signals following shortly after the main signal. No significant echoes have been found. Echoes would have been expected were the GRBs distant enough and the universe populated with a sufficient density of compact objects composing the dark matter. Constraints on dark matter abundance and GRB redshifts from the present data are presented and discussed. Based on these preliminary results, a universe filled to critical density of compact objects between 10 exp 6.5 and 10 exp 8.1 solar masses are now marginally excluded, or the most likely cosmological distance paradigm for GRBs is not correct. We expect future constraints to be able either to test currently popular cosmological dark matter paradigms or to indicate that GRBs do not lie at cosmological distances.

Nemiroff, R. J.↗

NASA NTRS1993

The consistency of standard cosmology and the BATSE number versus brightness relation

The integrated number-peak-flux relation measured by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory is compared with several standard cosmological distributions for gamma-ray bursts (GRBs). Friedmann-Robertson-Walker models were used along with the assumption that the bursts are standard candles and have no number or luminosity evolution. For a given Omega spectral shape, we used a free parameter, essentially the comoving number density of bursts, to generate a best fit between the cosmology and the measured relation. Our results are shown for a subsample of the first 260 GRBs recorded by BATSE. We find acceptable fits between simple cosmological models and the brightness distribution data, as determined by the Kolmogorov-Smirnov one-distribution statistical test. One cannot distinguish a single best cosmological model from the goodness of the fits. The best fit implies that BATSE GRBs are complete out to a redshift of about unity. However, significantly higher and lower redshifts, by as much as a factor of 2, are possible for other marginally acceptable fits.

Wickramasinghe, W. A. D. T.↗

NASA NTRS1993

Recurrent burst activity from the soft gamma-ray repeater SGR 1900 + 14

Three short very soft gamma-ray (SGR) transient events from a location consistent with that of the SGR 1900 + 14, first described by Mazets et al. (1979), were detected by the Burst and Transient Source Experiment. The results of observations of the temporal and spectral properties of the SGR 1900 + 14 suggest that the SGR phase lasts at least 13 years, lending support to the suggestion by Kouveliotou et al. (1987) and Fishman et al. (1989) that SGRs are related to galactic (possibly population I) objects, perhaps neutron stars.

Kouveliotou, C.↗

NASA NTRS1993

Gamma-ray burst color-color diagrams

We calculate the spectral hardness ratios for several intense gamma-ray bursts (GRBs) which have sufficient statistics in four energy channels. We study the evolution of these hardness ratios during the events using color-color diagrams (CCDs) and we attempt a preliminary classification of GRBs based on their CCD evolution.

Kouveliotou, C.↗