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Session 92 - Pulsars.
Oral session, Wednesday, January 15
Harbour C,
We report on phase-averaged and phase-resolved spectroscopy of the ``Bursting Pulsar'', GRO J1744-28, using the Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE) on the Rossi X-ray Timing Explorer. The PCA and HEXTE together cover an energy band from 2 to 250 keV. We discuss the average continuum shape and its variations as a function of pulse phase as well as a search for cyclotron absorption features and excess high energy emission.
Preliminary work presented at the 1996 High Energy Astrophysics Division meeting in San Diego on observations made 1996 March 16, showed that the average spectrum is typically well fit by a power law at low energies which is exponentially cut off above \approx18 keV with a folding energy of \approx12 keV. This work suggested that a cyclotron absorption line is present near 20 keV and that a hard power law tail may also exist. We report improved analyses of these data as well as two additional observations from 1996 March. Improvements made in the analysis include the use of more accurate detector response matrices for both the PCA and HEXTE and a better estimate of the PCA background. The cyclotron line search is improved through the use of a Fermi-Dirac term for the exponential cutoff, 1/(1 + e^(E - E_c)/E_f) . The exponential cutoff model used previously was discontinuous in its first derivative, which caused residuals in spectral fits near the discontinuity to appear as cyclotron features.
