Swift X-ray and ultraviolet observations of the shortest orbital period double-degenerate system RX J0806.3+1527 (HM Cnc) [HEAP]


RX J0806.3+1527 (HM Cnc) is a pulsating X-ray source with 100 per cent modulation on a period of 321.5 s (5.4 min). This period reflects the orbital motion of a close binary system consisting of two interacting white dwarfs. Here we present a series of simultaneous X-ray (0.2-10 keV) and near-ultraviolet (2600 angstrom and 1928 angstrom) observations carried out with the Swift satellite. In the near-ultraviolet the counterpart of RX J0806.3+1527 was detected at flux densities consistent with a blackbody with temperature 27E+3 K. We found that the emission at 2600 angstrom is modulated at the 321.5-s period, with the peak ahead of the X-ray one by 0.28 cycles and coincident, within 0.05 cycles, with the optical. This phase-shift measurement confirms that the X-ray hot spot (located on the primary white dwarf) is at about 80-100 degrees from the direction connecting the two white dwarfs. Albeit at lower significance, the 321.5-s signature is present also in the 1928-angstrom data; at this wavelength, however, the pulse peak is better aligned with that observed at X-rays. We use the constraints on the source luminosity and on the geometry of the emitting regions to discuss the merits and limits of the main models for RX J0806.3+1527.

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Thu, 28 Nov 13