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

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica – Milano, INAF, via E. Bassini 15, 20133 Milano, Italy
e-mail: paoloesp@iasf-milano.inaf.it
² Osservatorio Astronomico di Roma, INAF, via di Frascati 33, 00040 Monteporzio Catone (Roma), Italy
³ Theoretische Astrophysik, IAAT, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁴ Osservatorio Astronomico di Brera, INAF, via E. Bianchi 46, 23807 Merate (LC), Italy

Received: 20 September 2013
Accepted: 27 November 2013

Abstract

The system 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 consisting of two interacting white dwarfs. Here we present a series of simultaneous X-ray (0.2–10 keV) and near-ultraviolet (2600 Å and 1928 Å) observations that were 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 a temperature of (27 ± 8) × 10³ K. We found that the emission at 2600 Å is modulated at the 321.5-s period with the peak ahead of the X-ray one by 0.28 ± 0.02 cycles and is 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° from the direction that connects the two white dwarfs. Albeit at lower significance, the 321.5-s signature is also present in the 1928-Å 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 the geometry of the emitting regions to discuss the merits and limits of the main models for RX J0806.3+1527.