V2301 Ophiuchi: An X‐Ray–bright Eclipsing AM Herculis Object

Abstract

We present hard X-ray and optical observations of the eclipsing AM Her system V2301 Oph. The X-ray data were obtained using the PCA detector of the Rossi X-Ray Timing Explorer satellite during 1997 May, and the optical data were obtained using the 1 and 1.5 m telescopes of the Cerro Tololo Inter-American Observatory during 1996 May and 1997 June. V2301 Oph was bright in both the optical and hard X-rays during our observations. This, when coupled with its eclipsing nature, makes V2301 Oph an ideal testbed for theories of the large-scale topology of AM Her flows and the radiative shocks in AM Her systems. The X-ray emission from V2301 Oph was modulated strongly on the orbital period. During the bright orbital phases, the X-ray flux was Fx≈3.6 × 10−11 ergs cm-2 s-1 over the energy range E=2-10 keV. The X-ray emission did not go to zero during the faint orbital phases; it was ~10% of the bright phase level. The X-ray spectrum could be fitted by (1) optically thin thermal bremsstrahlung (temperature kTx≈9-19 keV) models with an absorption line at 5.1-5.2 keV or an emission line at ~7 keV, and (2) power-law continuum (index ≈2) models with an absorption line at 5.1-5.2 keV or an emission line at ~7 keV. The absorption columns were large for all fits, nH~(3-10) × 1022 cm-2. The nH are model dependent, but their large sizes are secure because they are set by the rollover in the X-ray spectrum at 3-4 keV. The hardness of the X-ray spectrum was roughly constant during the bright orbital phases. During the faint orbital phases, the X-ray spectral properties were not well determined, but it did appear that the spectrum hardened. There were total eclipses in both the X-ray and optical light curves. The X-ray light curves and eclipses were consistent with a dominant hot spot and a secondary hot spot. The dominant hot spot was not a point source; it had to cover about 50° in longitude on the surface of the white dwarf. We argue that the X-ray light curve and eclipse shape also suggest that the accretion occurs in a sheetlike geometry rather than in a columnar geometry. The optical light curves and eclipses were consistent with emission from the white dwarf photosphere, an extended emission region that sat above the surface of the white dwarf, and the X-ray-heated face of the companion star.