Sub-millimeter photometry of the white dwarf pulsar AR Sco

The white dwarf pulsar AR Sco is a magnetic cataclysmic variable with a white dwarf spin period of 117.23 seconds and an orbital period of 3.56 hours. Marsh et al. discovered highly pulsed (~90% pulse fraction) non-thermal emission over a wavelength range extending from the radio to ultraviolet. Those pulsations are seen predominantly at the 118.31 s beat period. They assert that the bulk of the luminosity is a result of the spin-down power of the white dwarf with a rate of (-4.82±0.18)x10^-17 s s^-1 and the overall spectral energy distribution appears to be dominated by two non-thermal power-law components. From the optical to X-rays, the power law slope is ≈-0.2, while from radio to infrared, it is believed to be ≈1.3. We present the first sub-millimeter photometric observations of AR Sco at 220 and 345 GHz (1.36 and 0.87 mm) using the Sub-Millimeter Array (SMA). The 220 GHz observations show pulsed non-thermal emission with a pulse fraction of ≈27% at half the spin period (58.616 s), but no signals at the spin and beat periods. The observations show that the white dwarf has a dipolar magnetic field and the non-thermal radio emission is from near the white dwarf. The sensitivity of the 345 GHz data is insufficient for a time series analysis. However, the flux density at this frequency is lower than is expected of a single power law. It suggests that the non-thermal emission may turnover at sub-millimeter wavelengths and the sub-millimeter range contains two emission components and not the assumed one component.