Spectral Mapping of the Intermediate Polar DQ Herculis

TL;DR

This study uses time-resolved spectroscopy and eclipse mapping to analyze the accretion disk and magnetic accretion processes in the intermediate polar DQ Her, revealing detailed disk structure, temperature distribution, and evidence of magnetically-controlled inflow.

Contribution

It provides the first detailed eclipse maps of DQ Her's accretion disk and uncovers the role of magnetic accretion curtains through spectral and spatial analysis.

Findings

Bright spot at disk rim reprocesses X-rays from magnetic poles.

Inner disk shows no Balmer or Helium lines, with a redder continuum outward.

Radial inflow velocities are 200-500 km/s, indicating magnetically-controlled accretion.

Abstract

We report an eclipse mapping study of the intermediate polar DQ Her based on time-resolved optical spectroscopy (~3800-5000A) covering 4 eclipses. Eclipse maps of the HeII 4686 line indicate that an azimuthally-and vertically-extended bright spot at disk rim is important source of reprocessing of x-rays from the magnetic poles. The disk spectrum is flat with no Balmer or Helium lines in the inner regions, and shows double-peaked emission lines in the intermediate and outer disk regions while the slope of the continuum becomes progressively redder with increasing radius. The inferred disk temperatures are in the range T~13500-5000K and can be reasonably well described by a steady-state disk with mass accretion rate of dM/dt=(2.7+/-1.0)x10^-9 Msun/yr. A comparison of the radial intensity distribution for the Balmer lines reveals a linear correlation between the slope of the distribution and the transition energy. The spectrum of the uneclipsed light is dominated by Balmer and HeI lines in emission with narrow absorption cores. The observed narrow and redshifted CaII 3934 absorption line in the total light spectra plus the inverse P-Cygni profiles of the Balmer and HeII 4686 emission lines in spectra of the asymmetric component indicate radial inflow of gas in the innermost disk regions and are best explained in terms of magnetically-controlled accretion inside the white dwarf magnetosphere. We infer projected radial inflow velocities of ~200-500km/s, significantly lower than both the rotational and the free-fall velocities for the corresponding range of radii. A combined net emission HeII plus Hbeta low-velocity eclipse map reveals a twisted dipole emitting pattern near disk center. This is interpreted as being the projection of accretion curtains onto the orbital plane at two specific spin phases, as a consequence of the selection in velocity provided by the spectral eclipse mapping.