Time‐resolvedHubble Space TelescopeSpectroscopy of Four Eclipsing Magnetic Cataclysmic Variables

Abstract

Time-resolved, multiepoch, Hubble Space Telescope ultraviolet eclipse spectrophotometry is presented for the magnetic cataclysmic variables (MCVs) V1309 Ori, MN Hya, V2301 Oph, and V1432 Aql. Separation of the eclipse light curves into specific wavebands allows the multiple emission components to be distinguished. Accretion-heated photospheric spots are detected in V1309 Ori and V2301 Oph, indicating covering factors f 150,000 and ~90,000 K, respectively. The cyclotron-emitting shock on MN Hya is detected in the optical/near-UV and found to occupy no more than f = 0.004 of the stellar surface. Emission from the accretion stream is a prominent component in V1309 Ori and V2301 Oph, and the protracted eclipse ingress of UV emission lines in the latter object indicates that the stream penetrates the white dwarf magnetosphere to a height of ~9Rwd before it is funneled onto the magnetic poles. The UV emission-line spectrum of V1309 Ori (and to a lesser extent MN Hya) is unusual among MCVs, with the relative strength of N V ?1240 in V1309 Ori matched only by the asynchronous system BY Cam. The prominence of N IV ?1718 suggests that an overabundance of nitrogen may be the most likely explanation. Three epochs of observation of the asynchronous V1432 Aql cover about one-third of a 50 day lap cycle between the white dwarf spin and binary orbit. The eclipse light curves vary enormously from epoch to epoch and as a function of waveband. The dereddened UV spectrum is extremely bright compared with the optical, and the overall spectral energy distribution coupled with the duration of eclipse ingress indicate that the dominant source of energy is a hot (Twd ~ 35,000 K) white dwarf. This explanation also accounts for the modest brightness variation out of eclipse and the weakness of optical circular polarization due to cyclotron emission. Undiminished line emission through eclipse indicates that the eclipse itself is caused by a dense portion of the accretion stream, not the secondary star. The temperature of the white dwarf in V1432 Aql greatly exceeds that in any other AM Her system except the recent nova V1500 Cyg. This, combined with its current asynchronous nature and rapid timescale for relocking, suggests that V1432 Aql underwent a nova eruption in the past ~75-150 yr. The reversed sense of asynchronism, with the primary star currently spinning up toward synchronism, is not necessarily at odds with this scenario if the rotation of the magnetic white dwarf can couple effectively to the ejecta during the dense wind phase of the eruption.