Abstract
Abstract We report the detailed history of spin-period changes in five intermediate polars (DQ Herculis, AO Piscium, FO Aquarii, V1223 Sagittarii, and BG Canis Minoris) during the 30–60 yr since their original discovery. Most are slowly spinning up, although there are sometimes years-long episodes of spin-down. This is supportive of the idea that the underlying magnetic white dwarfs are near spin equilibrium. In addition to the ∼40 stars sharing many properties and defined by their strong, pulsed X-ray emission, there are a few rotating much faster (P < 80 s), whose membership in the class is still in doubt—and who are overdue for closer study.
Highlights
31 Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., (AURA) under cooperative agreement with the National Science Foundation
These periods come from rotation of the radially accreting, magnetic white dwarf (WD), some show “sideband” signals which arise from interaction between the spin and orbital clocks
X-ray telescopes since 1980 have revealed many more, because most IPs radiate most of their energy in X-rays, as accreting matter plunges radially to the WD surface
Summary
Intermediate polars (IPs, called DQ Her stars after the prototype) are magnetic cataclysmic variables with stable periodic signals in optical and X-ray light, and periods typically. Much of the material in Mukai’s (2017) review of X-ray emission in cataclysmic variables is very pertinent to IPs. By tracking period changes from year to year, one can in principle measure torques on the rotating WD. Several authors have attempted this, based on time-series photometry obtained over a baseline of several years This has been sufficient to yield a rough estimate for the several stars studied: they change their pulse periods on timescales [=P/(dP/dt)] around 106 yr. We here present a summary of the period history of five IPs with the longest baseline of observation These typify the patterns and timescales found in other class members, but are more clearly defined, because the baseline is longer (at least 35 yr)
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