The auroral radio emission of the magnetic B-type star rho OphC

Abstract

The non-thermal radio emission of main-sequence early-type stars is a signature of stellar magnetism. We present multi-wavelength (1.6-16.7 GHz) ATCA measurements of the early-type magnetic star rho OphC, which is a flat-spectrum non-thermal radio source. The rho OphC radio emission is partially circularly polarized with a steep spectral dependence: the fraction of polarized emission is about 60% at the lowest frequency sub-band (1.6 GHz) while is undetected at 16.7 GHz. This is clear evidence of coherent Auroral Radio Emission (ARE) from the rho OphC magnetosphere. Interestingly, the detection of the rho OphC's ARE is not related to a peculiar rotational phase. This is a consequence of the stellar geometry, which makes the strongly anisotropic radiation beam of the amplified radiation always pointed towards Earth. The circular polarization sign evidences mainly amplification of the ordinary mode of the electromagnetic wave, consistent with a maser amplification occurring within dense regions. This is indirect evidence of the plasma evaporation from the polar caps, a phenomenon responsible for the thermal X-ray aurorae. rho OphC is not the first early-type magnetic star showing the O-mode dominated ARE but is the first star with the ARE always on view.

5 pages, 4 figures; accepted to MNRAS Letter

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• Testing a scaling relation between coherent radio emission and physical parameters of hot magnetic stars
• What leads to premature upper cut-off frequencies of auroral radio emission from hot magnetic stars?
• Discovery and origin of the radio emission from the multiple stellar system KQVel
• Direct observation of the radial magnetic field gradient in HD58260 from spectropolarimetry of NLTE lines in emission
• Ultra-Wideband, Multi-epoch Radio Study of the First Discovered `Main sequence Radio Pulse emitter' CU Vir
• A scaling relationship for non-thermal radio emission from ordered magnetospheres - II. Investigating the efficiency of relativistic electron production in magnetospheres of BA-type stars
• X-ray and radio data obtained by XMM-Newton and VLA constrain the stellar wind of the magnetic quasi-Wolf-Rayet star in HD45166
• Discovery of eight 'Main-sequence Radio Pulse emitters' using the GMRT: clues to the onset of coherent radio emission in hot magnetic stars