B. C. Rubin

Observations of accreting pulsars

We discuss recent observations of accreting binary pulsars with the all-sky BATSE instrument on the Compton Gamma Ray Observatory. BATSE has detected and studied nearly half of the known accreting pulsar systems. Continuous timing studies over a two-year period have yielded accurate orbital parameters for 9 of these systems, as well as new insights into long-term accretion torque histories.

Rapid Spin-Up Episodes in the Wind-Fed Accreting Pulsar GX 301-2

The accreting pulsar GX 301-2 (P = 680 s) has been observed continuously by the large-area detectors of the Burst and Transient Source Experiment (BATSE) instrument on the Compton Gamma Ray Observatory since 1991 April 5. Orbital parameters determined from these data are consistent with previous measurements, with improved accuracy in the current orbital epoch. The most striking features in the pulsar frequency history are two steady and rapid spin-up episodes, with ν˙~(3-5)×10^(-12) Hz s^(-1), each lasting for about 30 days. They probably represent the formation of transient accretion disks in this wind-fed pulsar. Except for these spin-up episodes, there are virtually no net changes in the neutron star spin frequency on long timescales. We suggest that the long-term spin-up trend observed since 1984 (ν˙~2×10^(-13) Hz s^(-1)) may be due entirely to brief (~20 days) spin-up episodes similar to those we have discovered. We assess different accretion models and their ability to explain the orbital phase dependence of the observed flux. In addition to the previously observed preperiastron peak at orbital phase 0.956 +/- 0.022, we also find a smaller peak close to apastron at orbital phase 0.498 +/- 0.057. We show that if the companion stars effective temperature is less than 22,000 K, then it must have a mass M_c < 70 M_⊙ and a radius R_c < 85 R_⊙ so as not to overfill the tidal lobe at periastron. In order not to overflow the Roche lobe at periastron, the corresponding values are M_c < 55 M_⊙ and R_c < 68 R_⊙. These constraints are nearly at odds with the reclassification by Kaper et al. of the companion as a B1 Ia + hypergiant.

On the Dramatic Spin-up/Spin-down Torque Reversals in Accreting Pulsars

Dramatic torque reversals between spin-up and spin-down have been observed in half of the persistent X-ray pulsars monitored by the BATSE all-sky monitor on the Compton Gamma Ray Observatory. Theoretical models developed to explain early pulsar timing data can explain spin-down torques via a disk-magnetosphere interaction if the star nearly corotates with the inner accretion disk. To produce the observed BATSE torque reversals, however, these equilibrium models require the disk to alternate between two mass accretion rates, with ˙M5 producing accretion torques of similar magnitude but always of opposite sign. Moreover, in at least one pulsar (GX 114) undergoing secular spin-down, the neutron star spins down faster during brief (~20 day) hard X-ray flares—this is opposite the correlation expected from standard theory, assuming that BATSE pulsed flux increases with mass accretion rate. The 10 day to 10 yr intervals between torque reversals in these systems are much longer than any characteristic magnetic or viscous timescale near the inner disk boundary and are more suggestive of a global disk phenomenon. We discuss possible explanations of the observed torque behavior. Despite the preferred sense of rotation defined by the binary orbit, the BATSE observations are urprisingly consistent with an earlier suggestion for GX 1+4: the disks in these systems somehow alternate between episodes of prograde and retrograde rotation. We are unaware of any mechanism that could produce a stable retrograde disk in a binary undergoing Roche lobe overflow, but such flip-flop behavior does occur in numerical simulations of wind-fed systems. One possibility is that the disks in some of these binaries are fed by an X-ray–excited wind.

The Burst and Transient Source Experiment Earth Occultation Technique

An Earth orbiting detector sensitive to gamma-ray photons will see steplike occultation features in its count rate when a gamma-ray point source crosses the Earths 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.

Timescale invariance of rapid-x-ray variability of the black hole candidate GRO J1719-24

We present the results of an analysis of the time variability of the soft X-ray transient GRO J1719-24 (Nova Oph 1993), as observed with BATSE. Our analysis covers the entire ~80 day outburst, beginning with the first detection of this black hole candidate on 1993 September 25. We obtained power density spectra (PDSs) of the data in the 20-100 keV energy band, covering the frequency interval 0.002-0.488 Hz. The PDSs show a significant quasi-periodic oscillation peak, the centroid frequency of which increased from ~0.04 Hz at the onset of the outburst to ~0.3 Hz at the end. Additional noise is present in the PDSs, which we describe in terms of two components. We find that the evolution of the PDSs can be described as a gradual stretching by a factor of ~7.5 in frequency of the power spectrum, accompanied by a decrease of the power level by the same factor, such that the integrated power in a scaled frequency interval remains constant.

BATSE observations of transient hard X‐ray sources

Many transient sources have been detected and monitored by the Burst and Transient Source Experiment (BATSE) since launch using the Earth occultation technique. These include x‐ray novae, pulsars, and other galactic sources. We present preliminary results of the temporal and spectral characteristics for seven transients which are potentially black hole candidates due to their hard spectra, lack of pulsations, and the appearance of their light curves. The seven transients are: GRO J0422+32 (X‐Ray Nova Persei 1992), GS/GRS 1124−68 (Nova Muscae 1991), GX 339−4, 4U 1543−47, GRS 1915+105, GRO J1719−24=GRS 1716−249 (X‐Ray Nova Ophiuchi 1993) and GRS 1009−45 (X‐Ray Nova Velorum 1993). We search for signatures in their spectra and light curves which are found to be common to black hole candidates. Generally, these objects appear to divide into two classes based on their light curves: the x‐ray nova‐type transients with fast rise and approximately exponential decay, and the slow rising, long duration bright transi...

BATSE Observations of Galactic Black Hole Candidates

Various arguments indicate that a number of galactic X-ray sources may be binary systems containing a black hole. A dominant characteristic of these sources is their high degree of variability over a wide range of time scales. BATSE (the Compton Burst and Transient Source Experiment) functions as a sensitive all-sky monitor, affording an unprecented opportunity to study the hard X-ray properties of galactic black hole candidates (BHCs). We summarize BATSE observations of the persistent BHCs Cygnus X-1 and GX 339–4 and the transient BHCs X-Nova Muscae 1991 (GRS/GS 1124–68), GRO J0422+32 (X-Nova Persei 1992), GRS 1009–45 (X-Nova Velorum 1993), and GRO J1719–24 (GRS 1716–249≡X-Nova Ophiuchi 1993).

A Study of the Effects of Background Subtraction on Occultation Imaging

One of the crucial steps in the occultation transform imaging technique involves the removal of the time-varying instrumental background. Previous versions of this imaging technique applied to BATSE data have used simple high-pass filtering to eliminate background variations on timescales longer than the typical duration of an occultation step (of order 10 s). We have investigated an alternative technique in which the imaging algorithm is applied to the residuals generated from fitting the raw data with a semi-empirical model of the background. Comparison of the resulting maps shows that the latter does not significantly improve imaging performance.

Earth occultation measurements of galactic hard X‐ray/gamma‐ray sources: A survey of BATSE results

The large area detectors on the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) are being used to monitor about 40 sources in the energy range of 20–1800 keV and to search for transients on a daily basis. We present an overview of the sources which have been detected with the BATSE instrument, and discuss some of the properties of these sources which can be studied. Time histories of the flux and energy spectra of recently detected transient hard x‐ray sources are presented.

BATSE observations of GX 354‐0

The transient hard x‐ray source and burster GX 354‐0 (MXB 1728‐34) has been detected several times since April 1991 using the newly‐developed occultation transform imaging to process data from the BATSE large area detectors (20‐1800 keV) on the COMPTON Observatory. One period corresponds to observations of an outburst seen by GRANAT/SIGMA in February 1992 (Goldwurm et al. 1992). This exploratory effort uses the occultation imaging technique to guide the identification of bright galactic center region sources within a few‐degree field. Then, using this information, temporal and spectral information is extracted with the standard occultation software.