W. S. Paciesas

The 1996 Soft State Transition of Cygnus X-1

We report continuous monitoring of Cygnus X-1 in the 1.3-200 keV band using All-Sky Monitor/Rossi X-Ray Timing Explorer and BATSE/Compton Gamma Ray Observatory for about 200 days from 1996 February 21 to 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 anticorrelated 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 ~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 are probably caused by a combination of the optically thick disk mass accretion rate increase and a decrease of the inner disk radius.

Broadband 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. Because it is a relatively close-by system (d ~ 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 broadband 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☉, above the well-established mass upper limit of 3.2 M☉ for a neutron star (the optical mass function for GRO J1655-40 is 3.16 ± 0.2 M☉). 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°, consistent with estimates obtained from optical light curves and radio jet kinematics.

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 the Rossi X-Ray Timing Explorer. 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% over the full Proportional Counter Array energy band. At high count rates, two high-frequency QPOs are detected simultaneously. The difference in their frequency centroids is consistent with a constant value of 323 ± 4 Hz in all observations. During one interval, a third signal is detected at 328 ± 2 Hz. This suggests that the system has a stable clock that is most likely the neutron star with spin period 3.1 ms. Thus, our observations of 4U 0614+091, and those of 4U 1728-34 and KS 1731-260, 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. The constant difference in the high-frequency QPOs suggests a beat-frequency interpretation. In this model, the high-frequency QPO is associated with the Keplerian frequency of the inner accretion disk, and the lower frequency QPO is a beat between 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 M☉ and a maximum radius of 17 km for the neutron star.

Energy Spectra and High Frequency Oscillations in 4U 0614+091

We investigate the behavior of the high-frequency quasi-periodic oscillations (QPOs) in 4U 0614+091, combining timing and spectral analyses of RXTE observations. The energy spectra of the source can be described by a power law (α ~ 2.8) and a blackbody (kT ~ 1.5 keV), with the blackbody accounting for 10%-20% of the total energy flux. We find a robust correlation of the frequency, ν, of the higher frequency QPO near 1 kHz with the flux of the blackbody, FBB. The slope of this correlation, d log ν/d log FBB, is 0.27-0.37. The source follows the same relation even in observations separated by several months. The QPO frequency does not have a similarly unique correlation with the total flux or the flux of the power-law component. The rms fraction of the higher frequency QPO rises with energy from 6.8% ± 1.5% (3-5 keV) to 21.3% ± 4.0% (10-12 keV). For the lower frequency QPO, however, it is consistent with a constant value of 5.4% ± 0.9%. The results may be interpreted in terms of a beat-frequency model for the production of the high-frequency QPOs.

KiloHertz Quasi-Periodic oscillations in Island State of 4U 1608-52 as observed with RXTE/PCA

We report RXTE/PCA observations of 4U 1608-52 on March 15, 18, and 22 immediately after the outburst in early 1996. The persistent count rates ranged from 190 to 450 counts s-1 (1-60 keV). During this period of time, 4U 1608-52 was in the island state. We detected quasi-periodic oscillation (QPO) features in the power density spectra (PDS) at 567-800 Hz on March 15 and 22, with source fractional root mean square (rms) amplitude of 13%-17% and widths of 78-180 Hz. The average rms amplitude of these QPO features is positively correlated with the energy. Our results imply that the neutron star spin frequency is possibly between 300 and 365 Hz.

A Sequence of Declining Outbursts from GX 339–4

The flux and spectrum of the black hole candidate GX 339-4 has been monitored by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory since the observatory became operational in 1991 May. Between the summer of 1991 and the fall of 1996, eight outbursts from GX 339-4 were observed. The history of these outbursts is one of declining fluence or total energy release, as well as a shortening of the time between outbursts. A rough linear correlation exists between the fluence emitted during an outburst and the time elapsed between the end of the previous outburst and the beginning of the current one. The peak flux is also roughly linearly correlated with outburst fluence. The light curves of the earlier, more intense, outbursts (except for the second one) can be modeled by a fast exponential (time constant ~10 days) followed by a slower exponential (~100 days) on the rise and a fast exponential decay (~5 days) on the fall. The later, weaker, outbursts are modeled with a single, rising time constant (~20 days) and a longer decay on the fall (~50 days). An exponential model gives a marginally better fit than a power law to the rise/decay profiles. GX 339-4 is a unique source in having more frequent outbursts than other low-mass X-ray binary black hole candidates. These observations can be used to constrain models of the behavior of the accretion disk surrounding the compact object.

BATSE Observations and Orbit Determination of the Be/X-Ray Transient EXO 2030+375

The Be/X-ray binary transient pulsar EXO 2030+375 (Ps≈42 s) has been observed with the large-area detectors (LADs) of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO). Beginning in 1991 May, 22 outbursts were observed over 4 years. Thirteen outbursts between 1992 February and 1993 August occurred consecutively at intervals of ≈46 days, close to the orbital period determined by Parmar and colleagues using EXOSAT data. The pulse profiles from the BATSE data are double peaked and show no significant energy or luminosity dependence, unlike the EXOSAT observations of 1985 May-August. An exponential model was used to fit the observed hard X-ray energy spectra from the 13 consecutive outbursts. When EXOSAT discovered this pulsar during a giant outburst in 1985 May, the X-ray luminosity peaked at LX=1.0×1038 ergs s-1 (1-20 keV), assuming a 5 kpc distance to the source. The BATSE outbursts are found to be weaker, 0.3×1037 ≤ LX(1-20 keV) ≤ 3.0×1037 ergs s-1 after extrapolating the observed flux (20-50 keV) to the EXOSAT energy band. Pulse phases derived from the 13 outbursts were fitted to two different models to determine a binary orbit. The new orbit is used to estimate 95% confidence limits for the mean peak spin frequency change during the outbursts observed with BATSE. This and the mean peak flux are compared to the spin-up rates and fluxes determined by EXOSAT from the 1985 giant outburst, where disk accretion was thought to have occurred. It is unclear whether these normal outbursts were driven by wind or disk accretion.

Pulse width distributions and total counts as indicators of cosmological time dilation in gamma‐ray bursts

The spatial distribution of bursts observed by BATSE is isotropic and inhomogeneous, implying that their sources may be at cosmological distances. We describe three tests that we have applied to search for the signature of time dilation. For both tests selection effects arising from intensity differences are avoided by rescaling all burst time profiles to a canonical dim peak intensity and rendering their backgrounds and noise biases uniform. These tests are (1) the distribution of pulse widths for these groups; a measure of the average rescaled integrated counts (2) within fitted pulse structures and (3) above fitted background for the burst ensembles. All tests use pulse shapes obtained from fitting major structures in temporal profiles. Results from 135 BATSE bursts indicate that the relative time dilation factor, dim to bright bursts, would be of order two, consistent with three previous tests, and with cosmological interpretations of the BATSE number‐intensity relation, all of which place the more di...

The average temporal profile of BATSE gamma‐ray bursts: Comparison between strong and weak events

First results are presented from the analysis of 260 BATSE gamma‐ray bursts (GRBs) using a method whereby all events are synchronized and averaged around the bins which are the brightest time intervals of each of them. For the averaged time history, a difference is found between the rise front and the back slope, and good evidence is found for the presence of hard‐to‐soft spectral evolution. We compare sub‐sets of ‘‘strong’’ and ‘‘weak’’ events and find no evidence for time dilation in weak GRBs, as would be expected in cosmological GRB models. On the other hand, for the strong events the averaged hardness ratio is found to be larger than for the weak GRBs.

Simultaneous optical/gamma-ray observations of GRBs

We report on the on-going project to search for serendipitous time-correlated optical photographic observations of γ-ray bursts (GRBs). The regular photography at 12 observatories is used to look for plates which have been exposed simultaneously with a GRB detected by BATSE and contain its position. The results of this search for GRBs detected during the previous 4 yrs of BATSE operation are presented.