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Register a new userSwift X-ray and ultraviolet observations of the shortest orbital period double-degenerate system RX J0806.3+1527 (HM Cnc)
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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 angstrom and 1928 angstrom) 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 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 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 degrees from the direction that connects 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 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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We carried out optical observations of the field of the X-ray pulsator RXJ0806.3+1527. A blue V=21.1 star was found to be the only object consistent with the X-ray position. VLT FORS spectra revealed a blue continuum with no intrinsic absorption lines. Broad (v~1500 km/s), low equivalent width (about -1/-6A) emission lines from the HeII Pickering series were clearly detected. B, V and R time-resolved photometry revealed the presence of about 15% pulsations at the 321s X-ray period, confirming the identification. These findings, together with the period stability and absence of any additional modulation in the 1min-5hr period range, argue in favour of the orbital interpretation of the 321s pulsations. The most likely scenario is thus that RXJ0806.3+1527 is a double degenerate system of the AM CVn class. This would make RXJ0806.3+1527 the shortest orbital period binary currently known and one of the best candidates for gravitational wave detection.
We present imaging circular polarimetry and near-infrared photometry of the suspected ultra-short period white-dwarf binary RX J0806.3+1527 obtained with the ESO VLT and discuss the implications for a possible magnetic nature of the white dwarf accretor and the constraints derived for the nature of the donor star. Our V-filter data show marginally significant circular polarization with a modulation amplitude of ~0.5% typical for cyclotron emission from an accretion column in a magnetic field of order 10 MG and not compatible with a direct-impact accretor model. The optical to near-infrared flux distribution is well described by a single blackbody with temperature kT_bb = 35000 K and excludes a main-sequence stellar donor unless the binary is located several scale heights above the galactic disk population.
We report on the temporal and spectral properties of the HMXB IGR J16283-4838 in the hard X-ray band. We searched the first 88 months of Swift BAT survey data for long-term periodic modulations. We also investigated the broad band (0.2--150 keV) spectral properties of IGR J16283--4838 complementing the BAT dataset with the soft X-ray data from the available Swift-XRT pointed observations. The BAT light curve of IGR J16283-4838 revealed a periodic modulation at P_o=287.6+7-1.7 days (with a significance higher than 4 standard deviations). The profile of the light curve folded at P_o shows a sharp peak lasting ~ 12 d, over a flat plateau. The long-term light curve shows also a ~300 d interval of prolonged enhanced emission. The observed phenomenology is suggestive of a Be nature of IGR J16283-4838, where the narrow periodic peaks and the ~300 d outburst can be interpreted as Type I and Type II outbursts, respectively. The broad band 0.2-150 keV spectrum can be described with an absorbed power-law and a steepening in the BAT energy range.
RX J1914.4+2456 and RX J0806.3+1527 have been proposed as double degenerate binaries with orbital periods of 569s and 321s respectively. An alternative model, in which the periods are related to the spin of a magnetic white dwarf in an intermediate polar system, has been rejected by other authors. We show that a face-on, stream-fed intermediate polar model for the two systems is viable and preferable to the other models. In each case, the X-ray modulation periods then represent the rotation of the white dwarf in the binary reference frame. The model explains the fully modulated X-ray pulse profiles, the X-ray spectra, the antiphase between X-ray and optical/infrared modulation, the lack of longer period modulation, and the low level of polarization. The optical spectrum of RX J0806.3+1527 suggests that Balmer series lines are present, blended with HeII lines. This is unlike the spectra of any of the known AM CVn stars and suggests that the system is not a double degenerate binary. The optical spectrum of RX J1914.4+2456 has spectral features that are consistent with those of a K star, ruling out the double degenerate models in this case. The lack of optical/infrared emission lines in RX J1914.4+2456 may be attributed to a high mass accretion rate and its face-on orientation. Its reported period decrease may be a short term spin-up episode driven by the current high M-dot. Finally we suggest that there is an observational selection effect such that the face-on intermediate polars that are detected will all have a stream-fed component, and the purely stream-fed intermediate polars that are detected will all be face-on systems.
We present the discovery of the orbital period of Swift J1626.6-5156. Since its discovery in 2005, the source has been monitored with Rossi X-ray Timing Explorer, especially during the early stage of the outburst and into the X-ray modulating episode. Using a data span of $sim$700 days, we obtain the orbital period of the system as 132.9 days. We find that the orbit is close to a circular shape with an eccentricity 0.08, that is one of the smallest among Be/X-ray binary systems. Moreover, we find that the timescale of the X-ray modulations varied, which led to earlier suggestions of orbital periods at about a third and half of the orbital period of Swift J1626.6-5156.
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