Change in Accretion Torque in the Binary Accreting Pulsar 4U 1626-67

We describe two years of BATSE observations of the 7.66 s accreting pulsar 4U 1626-6i. Power spectral analysis and period folding techniques with the 1.024 s resolution DISCLA data froru the Large Area Detectors (LADs) have provided nearly continuous monitoring of 4U 1626-6i in the 20-60 ke V range. The long term frequency history of the source shows nearly constant spin-down at a rate of f ~ -7.4 x 10^(-13) s^(-2) , in contrast to the continuous spin-up {with f ~ 8.5 x^(-13) t 3 s- 2) observed previously. We discuss the implications of these results for accretion torque theories and compare this behavior to that of GX 1+4

Long-Term Monitoring of the Accreting Pulsar GX 1+4

We present preliminary results from two years of GRO/BATSE hard X‐ray (20–100 keV) monitoring of the ∼120 s accretion‐powered pulsar GX 1+4. Daily pulse frequency measurements from 1991 April to 1993 September show an average spin‐down of ḟ≊−5×10^(−12) s^(−2), with increases in the spin‐down rate during high‐luminosity intervals. The 20–100 keV pulsed flux spectrum for the interval TJD 8393–8406 is fit by a power‐law index of 2.56±0.04. Optical spectroscopy of the suggested red giant companion V2116 Oph taken during the 1993 September X‐ray outburst from GX 1+4 show considerably strengthened emission line features, supporting the association

Discovery of the 18.7-Second Accreting Pulsar GRO J1948+32

We have detected an 18.7 s accreting X-ray pulsar in the Cygnus region, using the BATSE large-area detec­tors on the Compton Gamma Ray Observatory. GRO 11948 + 32 has been localized to within 10 deg^2 using a method we developed for positioning weak pulsed sources with BATSE. During the 33 day outburst, the phase-averaged 20-75 keV pulsed flux rose from 25 mCrab to 50 mCrab over 10 days and then decayed below our detection .threshold over nearly 25 days. A photon spectral index of γ = 2.65 ± 0.15 (assuming photon flux density d N/ dE α E^(-γ) was measured during a bright interval. The observed modulation of the neutron star's pulse frequency is suggestive of orbital variation over less than one orbit cycle. Assuming a constant spin frequency derivative over the outburst, we can place the following individual 95% confidence limits on each of the pulsar parameters: orbital period 35d < P_(orb) < 70d; orbital radius 75 It-sec < α_x sin i < 300 It-sec, eccentricity e < 0.25, spin frequency derivative 5 x 10^(-13) Hz s^(-1) < v < 2.5 x 10^(-11) Hz s^(-1), X-ray mass function 0.5 M_⊙ <f_x( M ) < 5 M_⊙ . As the stellar type of the mass-providing companion is still not known for this source, we briefly speculate on the nature of mass transfer in this system

Continuous Long Term Observations of Accreting Pulsars

The all-sky monitoring ability of the BATSE instrument on the GRO has enabled the first uniform, continuous and long term observations of the torque and fluxes of many known accreting pulsars. In the first part of this thesis, I describe the capabilities of BATSE for detecting hard X-rays, the techniques developed to perform timing and flux measurements, and the steps involved in reducing the raw data sets to a standardized database of data products for each accreting pulsar monitored by BATSE. In the second part of this thesis, I describe several studies performed with this database. The first is a broad overview of all the accreting pulsars monitored by BATSE. For each source, I display the frequency and pulsed flux histories from ≈ five years of BATSE observations, and provide a brief summary of the BATSE findings. I then focus on the wind-fed accreting pulsar GX 301-2. The most striking features in the pulsar frequency history are two steady and rapid spin-up episodes, with v [over-dot] ≈ (3-5) x 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 time scales. We suggest that the long-term spin-up trend observed since 1984 (v [over-dot] ≈ 2 x 10⁻¹³ Hz s⁻¹) may be due entirely to brief (≈ 20 d) spin-up episodes similar to those we have discovered. Next, I highlight the most significant trends revealed by the BATSE observations and discuss their implications for our current understanding of the spin-evolution and torque-luminosity relations in accreting pulsars. Alternating episodes of steady spin-up and spin-down were found to be a surprisingly common characteristic of many persistent sources. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars, but uncorrelated, or even anticorrelated, in persistent sources. I describe the various competing models that currently exist and critically assess each model within the context of the BATSE observations. Finally, I describe how BATSE observations of the recurrence rate of transient systems can be used to infer the galactic population of high-mass Be-transient systems.</p