Unveiling the Nature of the 321 Second Modulation in RX J0806.3+1527: Near-Simultaneous Chandra and Very Large Telescope Observations

We report on the first near-simultaneous X-ray and optical observations of RX J0806.3+1527. The source is believed to be a 321 s orbital period ultracompact binary system hosting an X-ray-emitting white dwarf. Data were obtained with Chandra and the ESO Very Large Telescope (VLT) in 2001 November. We found an optical/X-ray phase shift in the periodic modulation of about 0.5, strongly favoring the existence of two distinct emission regions in the two bands (for the pulsed fluxes). The Chandra data allow us to study, for the first time, the spectral continuum of RX J0806.3+1527 in soft X-rays. This was well fitted by a blackbody spectrum with kT~65 eV and hydrogen column density of NH~5×1020cm-2. The average (unabsorbed) source 0.1-2.5 keV luminosity during the modulation-on phase is LX~5×1032ergss-1 (assuming a distance of 500 pc). Such a value is lower than the luminosity expected if stable mass transfer between two white dwarfs were driven by gravitational radiation. Evidence for absorption-like features are present in the phase-averaged spectrum at about 0.53, 0.64, and 1.26 keV, which may be attributed to heavy elements (C and N). We compare and discuss these findings with other binary systems hosting an accreting white dwarf.