Coordinated Optical and X‐Ray Observations of the AM Herculis Objects VV Puppis, V834 Centauri, and EF Eridani

Abstract

ABSTRACTWe obtained coordinated broadband optical photometry and hard X‐ray data on the AM Her objects VV Pup, V834 Cen, and EF Eri. The X‐ray data were obtained using the Proportional Counter Array detectors of the Rossi X‐Ray Timing Explorer, and the optical data were obtained using the 1 m telescope of the Cerro Tololo Inter‐American Observatory. During our observations, VV Pup and V834 Cen were in their high optical luminosity states with average mV∼17 and 15.1, respectively. EF Eri was in a very low state at the time of our observation: mV>17.7. VV Pup and V834 Cen had average 2–10 keV X‐ray fluxes of 1.7 × 10-12 and 2.13 × 10-11 ergs cm−2 s−1, respectively. EF Eri was not detected in X‐rays. The X‐ray spectrum of V834 Cen was adequately modeled using an absorbed bremsstrahlung continuum with a Gaussian emission line. The best‐fit model had continuum temperature kTX = 11.8 ± 1.0 keV and absorption column nH = (1.03 ± 0.39) × 1022 cm−2. The Gaussian emission line had line center energy E = 6.7 ± 0.1 keV and equivalent width EW = 0.64 keV. VV Pup was too faint to produce reliable spectral fits. The 2–10 keV flux for VV Pup quoted above was based on the best‐fit spectral model. Holding kTX fixed in the fitting process yielded roughly the same flux for kTX = 0.5–30 keV.We detected ∼1 Hz optical quasi‐periodic oscillations (QPOs) with amplitude ∼2.2% from VV Pup and ∼1% from V834 Cen. We did not detect optical QPOs over the range 0.2–1 Hz from EF Eri at a 90% confidence upper limit of 4.5%. The QPO properties are consistent with those found in the literature. None of the targets showed X‐ray QPO activity. However, only V834 Cen yielded a meaningful upper limit: ∼14% at the 90% confidence level over 0.2–1.2 Hz.