X‐Ray Spectral and Timing Observations of AO Piscium

Abstract

ABSTRACTHard X‐ray observations of AO Piscium acquired using the Proportional Counter Array detector on board the Rossi X‐Ray Timing Explorer (RXTE) satellite on 1997 July 11–15 are presented. The data are modeled using theoretical spectra from two‐temperature radiative shocks. Based on fits to the X‐ray continuum, we find best‐fit shock models for M* = 0.60 ± 0.05 M⊙, consistent with earlier works. Using our models, we next consider the result of Mukai et al., who found that the soft X‐ray emission‐line spectrum for AO Psc acquired by Chandra was better fitted by a photoionization model powered by a power‐law spectrum rather than by a cooling‐flow model. The power‐law spectral index found by Mukai et al. is not consistent with the RXTE data but could be produced by a shock generated by accretion onto a massive white dwarf, M*≳1.2 M⊙. Furthermore, a high‐mass white dwarf shock model, M* ≈ 1 M⊙, is consistent with the Fe lines at 6.7 and 6.97 keV found by Mukai et al. These lines are generally thought to be produced by thermal plasma. The Chandra result that the white dwarf in AO Psc is more massive than implied by earlier X‐ray observations is intriguing and warrants further study. Optical studies have found that the white dwarf in AO Psc is spinning up. We show that based on X‐ray data acquired from 1983 to 1997, the spin period had small P˙ consistent with zero.