Abstract
RX J1007.5-2017 is a polar: a compact binary system in which matter flows from a low-mass main-sequence star to a magnetized white dwarf without the formation of an accretion disk. RX J1007.5-2017 has some observational peculiarities (conspicuous optical cyclotron harmonics, a very soft X-ray spectrum, and no polarization in R and I bands), which may be related to extreme conditions at the accretion flow: a very strong white-dwarf magnetic field (around 100 MG on surface) and a low accretion rate. To study the accretion, from the mass-donor star to the white dwarf, we obtained time-resolved spectroscopy using the Goodman spectrograph at the SOAR telescope in observing runs distributed around the first semester of 2012. We found the object in different brightness states. In the low state, we gathered data with two spectral resolutions (219 km/s and 170 km/s). In a brighter state, the spectral resolution was ≈ 170 km/s. The low (high) spectral resolution data cover the spectral region from 360 to 760 nm (435 to 700 nm). The continuum varies in both states and the cyclotron humps are visible at some orbital phases. The low-state spectra show Balmer emission lines superimposed on absorption features from the mass-donor star. The bright-state spectra show strong Balmer, HeI, and HeII emission lines. The Balmer and HeII lines are not single Gaussians: in bright state the lines are broader and have three components; in low state, the lines are narrower and two components are distinguished in some phases. Doppler tomography of the low state reveals that line emission arises mainly from a region near the white dwarf. The orbital dependence of the cyclotron emission was modeled using the Cyclops code, which adopts a 3D representation of the accretion column.
