There are at least three astrophysical settings where stellar accretion is mediated by a magnetosphere: (1) highly magnetized ( ) neutron stars in binaries (X-ray pulsars), (2) magnetic white dwarfs in compact binaries (the DQ Her stars) and, (3) protostellar accretion onto a young magnetized low mass star. One would like to know, for all of these systems, how the accretion changes the stellar angular momentum. The small moment of inertia of a neutron star makes it the ideal system to study, as the torque leads to rapid and easily measureable spin period changes. In most cases, one can accurately measure the accretion torque in much less than a week, whereas for accreting white dwarfs a single torque measurement can take over a decade (see Patterson 1994, Patterson, J. , PASP, 106, 209).
The torque measurements inform us about the interaction between the accreting matter and the magnetosphere, and the resulting long term spin evolution. In particular, one of the paradigms for creating a millisecond radio pulsar is the long-term spin-up of a low magnetic field neutron star. The accretion torque theory of Ghosh & Lamb (1979) has been used to explain several aspects of accretion torque behavior, such as the torque-luminosity relationship in outbursts of EXO 2030+375. However, serious gaps remain in our understanding, in particular on the details of how and why torque reversals occur in accreting systems. An intriguing variety of spin-up and spin-down behavior has been found in accreting X-ray pulsars with the BATSE all-sky monitor on the Compton Gamma Ray Observatory (GRO) (Prince et al. 1994). Two pulsars, GX 1+4 and 4U 1626-67, monotonically spin-down or spin-up at a nearly constant rate, with a reversal every 10 years. In contrast, other disk-fed systems, such as Cen X-3, exhibit more frequent (days to weeks) changes in the sign of the torque. Wind-fed systems show even more eratic torque histories. While spin-up by disk accretion seems to be commonly accepted, alternative mechanisms for spin-down have been proposed () and may be significant in the spin evolution of accreting systems and the formation of magnetic winds.
A key diagnostic is the relationship between the torque and accretion rate. BATSE continuously measures spin frequency and accretion torque for about ten X-ray pulsars, but is capable of measuring only the pulsed component of the flux, and only above 20 keV. To study the correlation between accretion rate and flux with BATSE alone, we are forced to presume the pulsed flux measured with BATSE is proportional to the accretion rate. The ASM on XTE will provide daily, independent estimates of both the continuum and the pulsed flux in 2-10keV. Even for relatively hard-spectrum X-ray pulsars, most of the bolometric luminosity is emitted in the ASM band, and hence daily ASM monitoring, coupled with BATSE monitoring, yields a substantial improvement on the current situation. We will use the ASM combined with BATSE to measure daily 2-60 keV spectra and to study the degree of correlation between the BATSE pulsed flux and the low-energy pulsed and DC flux as determined by the ASM. To further improve the estimates of total accretion rate, we are also requesting in this proposal a single pointed observation of each of seven of the pulsars monitored by BATSE. The pointed observations will be used to measure the energy spectrum in the range 2-60 keV and to determine the pulsed fraction as a function of energy. For ``old'' BATSE data, taken prior to the launch of XTE, the energy spectra and pulsed fractions measured with PCA/HEXTE enable us to make a ``bolometric correction'' and estimate the accretion luminosity from the 20-60 keV pulsed flux which BATSE measures, assuming the spectral shape is constant. For ``new'' data, we utilize daily BATSE and ASM measurements to estimate the spectrum and the luminosity.