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, #

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.

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.

Over five years of daily hard X-ray (>20 keV) monitoring of the 2-min accretion-powered pulsar GX 1+4 with the Compton Gamma Ray Observatory/BATSE large-area detectors has found nearly continuous rapid spin-down, interrupted by a bright 200-d spin-up episode. During spin-down, the torque becomes more negative as the luminosity increases (assuming that the 20-60 keV pulsed flux traces bolometric luminosity), the opposite of what is predicted by standard accretion torque theory. No changes in the shape of the 20-100 keV pulsed energy spectrum were detected, so that a very drastic change in the spectrum below 20 keV or the pulsed fraction would be required to make the 20-60 keV pulsed flux a poor luminosity tracer. These are the first observations which flatly contradict standard magnetic disk accretion theory, and they may have important implications for understanding the spin evolution of X-ray binaries, cataclysmic variables, and protostars. We briefly discuss the possibility that GX 1+4 may be accreting from a retrograde disk during spin-down, as previously suggested.Comment: 10 pages including 3 PS figures. To appear in ApJ Letter

No abstract

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 CGRO. 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 $\dot M_{\pm}$ producing accretion torques of similar magnitude, but always of opposite sign. Moreover, in at least one pulsar (GX 1+4) undergoing secular spin down the neutron star spins down faster during brief ($\sim 20$ day) hard X-ray flares -- this is opposite the correlation expected from standard theory, assuming 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 time scale 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 surprisingly consistent with an earlier suggestion by Makishima \etal (1988) 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.Comment: 8 pages plus 2 figures, Latex, uses aaspp4.sty; to be published in ApJ Let

Most Galactic point sources of gamma rays remain unidentiÐed. The few (extrasolar) sources that have been identiÐed are all young, rotation-powered pulsars, all but one of which were identiÐed using radio ephemerides. The radio-quiet Geminga pulsar was identiÐed only after pulsations were discovered in a coincident X-ray source. Observational evidence indicates that many of the unidentiÐed Galactic sources are likely to be pulsars, and some theoretical models predict a potentially large population of radio-quiet gamma-ray pulsars. We present a new method for performing sensitive gamma-ray pulsar searches. We used this method to search several of the strongest EGRET sources for pulsations. This was a blind search for new pulsars, covering a frequency and a frequency-derivative phase space large enough to detect Crab-like pulsars as well as lower frequency, high magnetic Ðeld "" magnetars.ÏÏ No new pulsars were discovered, and we report upper limits constraining the characteristics of any signals contained in the data sets searched.

We report on the discovery and hard X-ray (20È70 keV) observations of the 4.45 s period transient X-ray pulsar GRO J1750[27 with the BATSE all-sky monitor on board CGRO. A relatively faint outburst (\30 mcrab peak) lasting at least 60 days was observed during which the spin-up rate peaked at 38 pHz s~1 and was correlated with the pulsed intensity. An orbit with a period of 29.8 days was found. The large spin-up rate, spin period, and orbital period together suggest that accretion is occurring from a disk and that the outburst is a "" giant ÏÏ outburst typical of a Be/X-ray transient system. No optical counterpart has yet been reported.