Cyclotron spectroscopy of HU Aquarii^*

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
² Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching, Germany
³ Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany

Corresponding author: A. D. Schwope, aschwope@aip.de

Received: 30 October 2002
Accepted: 13 February 2003

Abstract

We present the results of a combined analysis of low-resolution spectroscopy and high-speed optical photometry of the bright eclipsing polar HU Aqr, obtained in its high accretion state. Cyclotron harmonic emission in a field of  MG became evident and could be traced through the whole optical bright phase. The cyclotron harmonics display an asymmetric motion as a function of phase which requires a certain forward tilt of the magnetic field lines in the emission region. The optical bright phase is more extended than the bright phase in soft X-rays suggesting a more extended emission region. Also, ingress into and egress from the optical eclipse last much longer in the optical than at X-ray wavelengths. These observations require the existence of an accretion arc with total extent of about 20°. The soft X-ray accretion spot is located at the far end of the arc, where the bulk of matter is accreted. The arc is likely to be more extended than the foot-line of field lines connecting to the ballistic accretion stream in a dipolar geometry. This together with the required tilt of the accreting field lines suggest a more complex magnetic geometry than a simple dipole.