We summarize five years of continuous monitoring of accretion-powered pulsars with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Our 20-70 keV observations have determined or refined the orbital parameters of 13 binaries, discovered 5 new transient accreting pulsars, measured the pulsed flux history during outbursts of 12 transients (GRO J1744-28, 4U 0115+634, GRO J1750-27, GS 0834-430, 2S 1417-624, GRO J1948+32, EXO 2030+375, GRO J1008-57, A 0535+26, GRO J2058+42, 4U 1145-619 and A 1118-616), and also measured the accretion torque history of during outbursts of 6 of those transients whose orbital parameters were also known. We have also continuously measured the pulsed flux and spin frequency for eight persistently accreting pulsars (Her X-1, Cen X-3, Vela X-1, OAO 1657-415, GX 301-2, 4U 1626-67, 4U 1538-52, and GX 1+4). Because of their continuity and uniformity over a long baseline, BATSE observations have provided new insights into the long-term behavior of accreting magnetic stars. We have found that all accreting pulsars show stochastic variations in their spin frequencies and luminosities, including those displaying secular spin-up or spin-down on long time scales, blurring the conventional distinction between disk-fed and wind-fed binaries. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars, but uncorrelated, or even anticorrelated, in persistent sources.Comment: LaTeX, psfig, 90 pages, 42 figures. To appear in Dec. 1997 ApJS, Vol 113, #

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We describe how the coherence function-a Fourier frequency-dependent measure of the linear correlation between time series measured simultaneously in two energy channels-can be used in conjunction with energy spectra, power spectra, and time delays between energy channels to constrain models of the spectrum and variability of X-ray binaries. Here we present a procedure for estimating the coherence function in the presence of counting noise. We apply this method to the black hole candidates Cyg X-1 and GX 339-4, and find that the near perfect coherence between low-and high-energy X-ray photons rules out a wide range of models that postulate spatially extended fluctuating emission, thermal flares, and overlapping shot-noise.

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 Hz s~1, l5 B (3È5) ] 10~12 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 Hz s~1) may be due entirely to brief (B20 days) spin-up episodes similar to those we (l5 B 2 ] 10~13 have discovered. We assess di †erent accretion models and their ability to explain the orbital phase dependence of the observed Ñux. In addition to the previously observed preperiastron peak at orbital phase 0.956^0.022, we also Ðnd a smaller peak close to apastron at orbital phase 0.498^0.057. We show that if the companion starÏs e †ective temperature is less than 22,000 K, then it must have a mass and a M c \ 70 M _ radius so as not to overÐll the tidal lobe at periastron. In order not to overÑow the Roche R c \ 85 R _ lobe at periastron, the corresponding values are and These constraints are M c \ 55 M _ R c \ 68 R _. nearly at odds with the reclassiÐcation by Kaper et al. of the companion as a B1 Ia] hypergiant.

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We report the discovery, with NASA's Rossi X-ray Timing Explorer (RXTE), of the first sub-millisecond oscillation found in a celestial X-ray source. The quasi-periodic oscillations (QPO) come from Sco X-1 and have a frequency of approximately 1100 Hz, amplitudes of 0.6-1.2% (rms) and are relatively coherent, with Q up to ∼10 2 . The frequency of the QPO increases with accretion rate, rising from 1050 to 1130 Hz when the source moves from top to bottom along the normal branch in the X-ray color-color diagram, and shows a strong, approximately linear correlation with the frequency of the well-known 6-20 Hz normal/flaring branch QPO. We also report the discovery of QPO with a frequency near 800 Hz that occurs, simultaneously with the 1100 Hz QPO, in the upper normal branch. We discuss several possible interpretations, one involving a millisecond X-ray pulsar whose pulses we see reflected off accretion-flow inhomogeneities. Finally, we report the discovery of ∼45 Hz QPO, most prominent in the middle of the normal branch, which might be magnetospheric beat-frequency QPO.

Over 5 yr of hard X-ray (20È60 keV) monitoring of the 7.66 s accretion-powered pulsar 4U 1626[67 with the Compton Gamma Ray Observatory/BATSE large-area detectors has revealed that the neutron star is now steadily spinning down, in marked contrast to the steady spin-up observed during 1977È1989. This is the second accreting pulsar (the other is GX 1]4) that has shown extended, steady intervals of both spin-up and spin-down. Remarkably, the magnitudes of the spin-up and spin-down torques di †er by only 15%, with the neutron star spin changing on a timescale yr in both states. The o l/l5 o B 5000 current spin-down rate is itself decreasing on a timescale yr. The long-term timing history o l5 /l o B 26 shows small-amplitude variations on a 4000 day timescale, which are probably due to variations in the mass transfer rate. The pulsed 20È60 keV emission from 4U 1626[67 is well-Ðtted by a power-law spectrum with photon index c \ 4.9 and a typical pulsed intensity of 1.5 ] 10~10 ergs cm~2 s~1. The low count rates with BATSE prohibited us from constraining the reported 42 minute binary orbit, but we can rule out long-period orbits in the range 2 days. days [ P orb [ 900 We compare the long-term torque behavior of 4U 1626[67 to other disk-fed accreting pulsars and discuss the implications of our results for the various theories of magnetic accretion torques. The abrupt change in the sign of the torque is difficult to reconcile with the extremely smooth spin-down now observed. The strength of the torque noise in 4U 1626[67, D10~22 Hz2 s~2 Hz~1, is the smallest ever measured for an accreting X-ray pulsar, and it is comparable to the timing noise seen in young radio pulsars. We close by pointing out that the core temperature and external torque (the two parameters potentially relevant to internal sources of timing noise) of an accreting neutron star are also comparable to those of young radio pulsars.

We observed the low-mass X-ray binary and Z source GX 17+2 with the Rossi Xray Timing Explorer during 6-. The X-ray color-color diagram shows a clear Z track. Two simultaneous kHz quasi-periodic oscillations (QPOs) are present in each observation, whose frequencies are well correlated with the position of the source on the Z-track. At the left end of the horizontal branch (HB) only the higher frequency peak is observed, at 645±9 Hz, with an rms amplitude of 5.7±0.5%, and a FWHM of 183±35 Hz. When the source moves down the Z track to the upper normal branch the frequency of the kHz QPO increases to 1087±12 Hz, and the rms amplitude and FWHM decrease by a factor 2. Further down the Z the QPO becomes undetectable, with rms upper limits of typically 2.0%. Halfway down the HB a second QPO appears in the power spectra with a frequency of 480±23 Hz. The frequency of this QPO also increases when the source moves along the Z track, up to 781±11 Hz halfway down the normal branch, while the rms amplitude and FWHM stay approximately constant at 2.5% and 70 Hz. The QPO frequency difference is constant at 293.5±7.5 Hz. Simultaneously with the kHz QPOs we detect HB QPOs (HBOs). The simultaneous presence of HBOs and kHz QPOs excludes the magnetospheric beatfrequency model as the explanation for at least one of these two phenomena.