arXiv subjects

W. S. Paciesas

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

53 records · Page 3Linked to original sources

arXiv1997

BATSE Gamma-Ray Burst Line Search: V. Probability of Detecting a Line in a Burst

The physical importance of the apparent discrepancy between the detections by pre-BATSE missions of absorption lines in gamma-ray burst spectra and the absence of a BATSE line detection necessitates a statistical analysis of this discrepancy. This analysis requires a calculation of the probability that a line, if present, will be detected in a given burst. However, the connection between the detectability of a line in a spectrum and in a burst requires a model for the occurrence of a line within a burst. We have developed the necessary weighting for the line detection probability for each spectrum spanning the burst. The resulting calculations require a description of each spectrum in the BATSE database. With these tools we identify the bursts in which lines are most likely to be detected. Also, by assuming a small frequency with which lines occur, we calculate the approximate number of BATSE bursts in which lines of various types could be detected. Lines similar to the Ginga detections can be detected in relatively few BATSE bursts; for example, in only ~20 bursts are lines similar to the GB 880205 pair of lines detectable. Ginga reported lines at ~20 and ~40 keV whereas the low energy cutoff of the BATSE spectra is typically above 20 keV; hence BATSE's sensitivity to lines is less than that of Ginga below 40 keV, and greater above. Therefore the probability that the GB 880205 lines would be detected in a Ginga burst rather than a BATSE burst is ~0.2. Finally, we adopted a more appropriate test of the significance of a line feature.

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arXiv1997

Long-Term Variability in Bright Hard X-ray Sources: 5+ Years of BATSE Data

The operation of CGRO/BATSE continues to produce, after more than 5 years, a valuable database for the study of long-term variability in bright hard X-ray sources. The all-sky capability of BATSE provides, using the Earth occultation technique, up to approximately 30 flux measurements per day for each source. The long BATSE baseline and the numerous rising and setting occultation flux measurements allow searches for periodic and quasi-periodic signals from hours to hundreds of days. We present initial results from our study of the hard X-ray variability in 24 of the brightest BATSE sources. Power density spectra are computed for each source. In addition, we present profiles of the hard X-ray orbital modulations in 8 X-ray binaries (Cen X-3, Cyg X-1, Cyg X-3, GX 301-2, Her X-1, OAO1657-415, Vela X-1 and 4U1700-37), several-hundred-day modulations in the amplitude and width of the main high state in the 35-day cycle in Her X-1, and variations in outburst durations and intensities in the recurrent X-ray transients.

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arXiv1997

Broad band high energy observations of the superluminal jet source GRO J1655-40 during an outburst

The X-ray/radio transient superluminal jet source GRO J1655-40 was recently suggested to contain a black hole from optical observations. Being a relatively close-by system (d \sim 3.2 kpc), it can likely provide us with rich information about the physics operating in both galactic and extragalactic jet sources. We present the first simultaneous broad band high energy observations of GRO J1655-40 during the 1995 July-August outburst by three instruments: ASCA, WATCH/GRANAT and BATSE/CGRO, in the energy band from 1 keV to 2 MeV. Our observations strengthen the interpretation that GRO J1655-40 contains a black hole. We detected a two-component energy spectrum, commonly seen from other galactic black hole binaries, but never detected from a neutron star system. Combining our results with the mass limits derived from optical radial velocity and orbital period measurements, we further constrain the mass of the central object to be between 3.3 and 5.8 M$_{\sun}$, above the well-established mass upper limit of 3.2 M$_{\sun}$ for a neutron star (the optical mass function for GRO J1655-40 is 3.16$\pm$0.2 M$_{\sun}$). This system is therefore the first galactic superluminal jet source for which there is strong evidence that the system contains a stellar mass black hole. The inclination angle of the binary system is constrained to be between 76 and 87 degrees, consistent with estimates obtained from optical light curves and radio jet kinematics.

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arXiv1997

The 1996 Soft State Transitions of Cygnus X-1

We report continuous monitoring of Cygnus X-1 in the 1.3 to 200 keV band using ASM/RXTE and BATSE/CGRO for about 200 days from 1996 February 21 to 1996 early September. During this period Cygnus X-1 experienced a hard-to-soft and then a soft-to-hard state transition. The low-energy X-ray (1.3-12 keV) and high-energy X-ray (20-200 keV) fluxes are strongly anti-correlated during this period. During the state transitions flux variations of about a factor of 5 and 15 were seen in the 1.3-3.0 keV and 100-200 keV bands, respectively, while the average 4.8-12 keV flux remains almost unchanged. The net effect of this pivoting is that the total 1.3-200 keV luminosity remained unchanged to within about 15%. The bolometric luminosity in the soft state may be as high as 50-70% above the hard state luminosity, after color corrections for the luminosity below 1.3 keV. The blackbody component flux and temperature increase in the soft state is probably caused by a combination of the optically thick disk mass accretion rate increase and a decrease of the inner disk radius.

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arXiv1996

Evolution of the Low-Energy Photon Spectra in Gamma-Ray Bursts

We report evidence that the asymptotic low-energy power law slope alpha (below the spectral break) of BATSE gamma-ray burst photon spectra evolves with time rather than remaining constant. We find a high degree of positive correlation exists between the time-resolved spectral break energy E_pk and alpha. In samples of 18 "hard-to-soft" and 12 "tracking" pulses, evolution of alpha was found to correlate with that of the spectral break energy E_pk at the 99.7% and 98% confidence levels respectively. We also find that in the flux rise phase of "hard-to-soft" pulses, the mean value of alpha is often positive and in some bursts the maximum value of alpha is consistent with a value > +1. BATSE burst 3B 910927, for example, has a alpha_max equal to 1.6 +/- 0.3. These findings challenge GRB spectral models in which alpha must be negative of remain constant.

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arXiv1996

Evidence from quasi-periodic oscillations for a millisecond pulsar in the low mass x-ray binary 4U 0614+091

We have detected quasi-periodic oscillations (QPOs) near 1 kHz from the low mass X-ray binary 4U 0614+091 in observations with XTE. The observations span several months and sample the source over a large range of X-ray luminosity. In every interval QPOs are present above 400 Hz with fractional RMS amplitudes from 3 to 12%. At high count rates, two high frequency QPOs are detected simultaneously. The difference of their frequency centroids is consistent with 323 Hz in all observations. During one interval a third signal is detected at 328 +/- 2 Hz. This suggests the system has a stable `clock' which is most likely the neutron star with spin period 3.1 msec. Thus, our observations and those of another neutron star system by Strohmayer et al. (1996) provide the first evidence for millisecond pulsars within low-mass X-ray binary systems and reveal the `missing-link' between millisecond radiopulsars and the late stages of binary evolution in low mass X-ray binaries (Alpar et al. 1982). We suggest that the kinematics of the magnetospheric beat-frequency model (Alpar and Shaham 1985) applies to these QPOs. In this interpretation the high frequency signal is associated with the Keplerian frequency of the inner accretion disk and the lower frequency `beat' signal arises from the differential rotation frequency of the inner disk and the spinning neutron star. Assuming the high frequency QPO is a Keplerian orbital frequency for the accretion disk, we find a maximum mass of 1.9 solar masses and a maximum radius of 17 km for the neutron star.

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arXiv1996

X-Ray Observations of GRS 1915+105

A comparison of ROSAT observations with simultaneous BATSE monitoring data shows a different intensity evolution in the soft and hard X-ray band of GRS 1915+105 since 1992. In addition, fits of simultaneous ROSAT/BATSE spectra require more than one spectral component. A short description of results of RXTE pointed observations of GRS 1915+105 since April 1996 is also presented.

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arXiv1996

Correlation Between BATSE Hard X-ray Spectral and Timing Properties of Cygnus X-1

We have analyzed approximately 1100 days of Cygnus X-1 hard X-ray data obtained with BATSE to study its rapid variability. We find for the first time correlations between the slope of the spectrum and the hard X-ray intensity, and between the spectral slope and the amplitude of the rapid variations of the hard X-ray flux. We compare our results with expectations from current theories of accretion onto black holes.

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arXiv1996

BATSE SD Observations of Hercules X-1

The cyclotron line in the spectrum of the accretion-powered pulsar Her X-1 offers an opportunity to assess the ability of the BATSE Spectroscopy Detectors (SDs) to detect lines like those seen in some GRBs. Preliminary analysis of an initial SD pulsar mode observation of Her X-1 indicated a cyclotron line at an energy of approximately 44 keV, rather than at the expected energy of approximately 36 keV. Our analysis of four SD pulsar mode observations of Her X-1 made during high-states of its 35 day cycle confirms this result. We consider a number of phenomenological models for the continuum spectrum and the cyclotron line. This ensures that we use the simplest models that adequately describe the data, and that our results are robust. We find modest evidence (significance Q ~ 10^-4-10^-2) for a line at approximately 44 keV in the data of the first observation. Joint fits to the four observations provide stronger evidence (Q ~ 10^-7-10^-4) for the line. Such a shift in the cyclotron line energy of an accretion-powered pulsar is unprecedented.

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arXiv1995

BATSE Gamma-Ray Burst Line Search: IV. Line Candidates from the Visual Search

We evaluate the significance of the line candidates identified by a visual search of burst spectra from BATSE's Spectroscopy Detectors. None of the candidates satisfy our detection criteria: an F-test probability less than 10^-4 for a feature in one detector and consistency among the detectors which viewed the burst. Most of the candidates are not very significant, and are likely to be fluctuations. Because of the expectation of finding absorption lines, the search was biased towards absorption features. We do not have a quantitative measure of the completeness of the search which would enable a comparison with previous missions. Therefore a more objective computerized search has begun.

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arXiv1995

BATSE Gamma-Ray Burst Line Search: III. Line Detectability

We evaluate the ability of the BATSE spectroscopy detectors to detect the absorption lines observed by Ginga in gamma ray burst spectra. We find that BATSE can detect the 20.6 keV line in the S1 segment of GB870303 with a detection probability of ~1/4 in nearly normally incident bursts, with the probability dropping off to nearly 0 at a burst angle of 50 degrees; the lines at 19.4 and 38.8 keV lines in GB880205 have a high detection probability in BATSE up to burst angles of 75 degrees. In addition, we calculate detection probabilities for these two line types as a function of signal-to-noise ratio and burst angle for use in detailed comparisons between BATSE and Ginga. Finally, we consider the probability averaged over the sky of detecting a line feature with the actual array of BATSE detectors on CGRO.

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arXiv1994

Duration Distributions of Bright and Dim BATSE Gamma-Ray Bursts

We have measured the T90 and T50 durations of bright and dim gamma-ray bursts (GRBs) detected by the Compton Gamma Ray Observatory's Burst and Transient Source Experiment (BATSE). The T90 (T50) duration is defined as the interval over which 5\% (25\%) to 95\% (75\%) of the burst counts accumulate. Out of 775 bursts observed by BATSE, 159 bursts were analyzed; bursts with durations shorter than 1.5 s were excluded. A Kolmogorov-Smirnov test yields a probability of 6 x $10^{-5}$ that the T50 durations of the dim and bright samples are drawn from the same parent population. We find that the centroid and extent of the duration distribution for the dim sample are scaled by approximately a factor of two relative to those of the bright sample. The measured time dilation factor is not sensitive to choice of energy band. These results are quantitatively consistent with previous tests for time dilation in a smaller sample of BATSE bursts. The dimmer bursts, if cosmological, would lie at redshifts of order two.

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arXiv1994

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. We show that this correlation is statistically significant with respect to variations due to random differences between GRBs. This effect is discernable 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 detected by Norris et al. (1994) and that this is additional evidence that burst sources are located at cosmological distances.

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arXiv1994

BATSE Observations of Gamma-Ray Burst Spectra. II. Peak Energy Evolution in Bright, Long Bursts -

We investigate spectral evolution in 37 bright, long gamma-ray bursts observed with the BATSE Spectroscopy Detectors. High resolution spectra are characterized by the energy of the peak of \nfn~and the evolution of this quantity is examined relative to the emission intensity. In most cases it is found that this peak energy either rises with or slightly precedes major intensity increases and softens for the remainder of the pulse. Inter-pulse emission is generally harder early in the burst. For bursts with multiple intensity pulses, later spikes tend to be softer than earlier ones indicating that the energy of the peak of \nfn~is bounded by an envelope which decays with time. Evidence is found that bursts in which the bulk of the flux comes well after the event which triggers the instrument tend to show less peak energy variability and are not as hard as several bursts in which the emission occurs promptly after the trigger. Several recently proposed burst models are examined in light of these results and no qualitative conflicts with the observations presented here are found.

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arXiv1994

Null Result in Gamma-Ray Burst Lensed Echo Search

We have searched for gravitational-lens induced echoes between gamma-ray bursts (GRBs) in NASA's orbiting {\it Compton} Gamma Ray Observatory's 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 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 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.

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arXiv1993

Detection of Signature Consistent with Cosmological Time Dilation in Gamma-Ray Bursts

If gamma-ray bursters 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 {\it Compton} Gamma Ray Observatory's Burst and Transient Source Experiment (BATSE) for such time dilation. Our measures of time scale are constructed to avoid selection effects arising from intensity differences by rescale all bursts to fiducial levels of peak intensity and noise bias. The three tests involved total count rate above background, wavelet decomposition, and alignment of the highest peaks. In all three tests, the dim bursts are stretched by a factor of about two relative to the bright ones, over seven octaves of time scale. We calibrated the measurements by dilating synthetic bursts that approximate the temporal characteristics of bright BATSE bursts. Results are consistent with bursts of BATSE's peak-flux completeness limit being at cosmological distances corresponding to $z \sim 1$, and thus with independent cosmological interpretations of the BATSE number-intensity relation.

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arXiv1993

Gamma-Ray Bursts are Time-Asymmetric

A simple test for time-asymmetry is devised and carried out on the brightest gamma-ray bursts detected by the Burst and Transient Source Experiment (BATSE) on board the {\it Compton} Gamma Ray Observatory. We show evidence that individual bursts are time-asymmetric on all time scales tested, from a time scale shorter than that of pulses which compose GRBs to a time scale similar to a greater envelope that contains these pulses. We also find bursts which manifest significant asymmetry only on time scales comparable to the duration of burst, and bursts for which no clear asymmetry on any time scale 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 significantly more bursts and pulses where the rise is more rapid than the decay, on all time scales 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 $σ$ level. Models that predict time-symmetry are therefore excluded.

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