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Register a new userNeglected X-ray discovered polars: III. RX J0154.0-5947, RX J0600.5-2709, RX J0859.1+0537, RX J0953.1+1458, and RX J1002.2-1925
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K. Beuermann
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We report results on the ROSAT-discovered noneclipsing short-period polars RX J0154.0-5947, RX J0600.5-2709, RX J0859.1+0537, RX J0953.1+1458, and RX J1002.2-1925 collected over 30 years. We present accurate linear orbital ephemerides that allow a correct phasing of data taken decades apart. Three of the systems show cyclotron and Zeeman lines that yield magnetic field strengths of 36 MG, 19 MG, and 33 MG for the last three targets, respectively. RX J0154.0-5947, RX J0859.1+0537, and RX J1002.2-1925 show evidence for part-time accretion at both magnetic poles, while RX J0953.1+1458 is a polar with a stable one-pole geometry. RX J1002.2-1925 shows large variations in the shapes of its light curves that we associate with an unstable accretion geometry. Nevertheless, it appears to be synchronized. We determined the bolometric soft and hard X-ray fluxes and the luminosities at the Gaia distances of the five stars. Combined with estimates of the cyclotron luminosities, we derived high-state accretion rates that range from $dot M = 2.9 times 10^{-11}$ $M_{odot}$yr$^{-1}$ to $9.7 times 10^{-11}$ $M_{odot}$yr$^{-1}$ for white dwarf masses between 0.61 and 0.82 $M_odot$, in agreement with predictions based on the observed effective temperatures of white dwarfs in polars and the theory of compressional heating. Our analysis lends support to the hypothesis that different mean accretion rates appply for the subgroups of short-period polars and nonmagnetic cataclysmic variables.
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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.
In the present paper an alternative theoretical interpretation to the generally assumed thermal emission models of the observed X-ray spectrum of isolated pulsar RX J0420.0-5022 is presented. It is well known that the distribution function of relativistic particles is one-dimensional at the pulsar surface. However, cyclotron instability causes an appearance of transverse momenta of relativistic electrons, which as a result, start to radiate in the synchrotron regime. On the basis of the Vlasovs kinetic equation we study the process of the quasi-linear diffusion (QLD) developed by means of the cyclotron instability. This mechanism provides generation of optical and X-ray emission on the light cylinder lengthscales. The analysis of the three archival XMM-Newton observations of RX J0420.0-5022 is performed. Considering a different approach of the synchrotron emission theory, the spectral energy distribution is obtained that is in a good agreement with the observational data. A fit to the X-ray spectrum is performed using both the present synchrotron emission model spectrum absorbed by cold interstellar matter and generally assumed absorbed black-body model.
We report on submillimetre bolometer observations of the isolated neutron star RX J1856.5--3754 using the LABOCA bolometer array on the Atacama Pathfinder Experiment (APEX) Telescope. No cold dust continuum emission peak at the position of RX J1856.5--3754 was detected. The 3 sigma flux density upper limit of 5 mJy translates into a cold dust mass limit of a few earth masses. We use the new submillimetre limit, together with a previously obtained H-band limit, to constrain the presence of a gaseous, circumpulsar disc. Adopting a simple irradiated-disc model, we obtain a mass accretion limit of dM/dt less than 10^{14} g/s, and a maximum outer disc radius of around 10^{14} cm. By examining the projected proper motion of RX J1856.5--3754, we speculate about a possible encounter of the neutron star with a dense fragment of the CrA molecular cloud a few thousand years ago.
RX J1856.5$-$3754 is the brightest and nearest ($sim 120$ pc) source among thermally emitting isolated neutron stars. Its spectra observed with {sl XMM-Newton} and {sl Chandra} satellites are well-fitted with the two-temperature ($kT^infty sim$ 32 and 63 eV) blackbody model. Fitting ten sets of the data from {sl Suzaku} XIS0, XIS1, XIS3 and {sl XMM-Newton} EPIC-pn with the two-temperature blackbody model, we discover an excess emission, 16--26% in 0.8--1.2,keV. We examine possible causes of this keV-X-ray excess; uncertainty in the background, pile up of the low energy photons and confusion of other sources. None of them succeeds in explaining the keV-X-ray excess observed with different instruments. We thus consider this keV-X-ray excess is most likely originated in RX J1856.5$-$3754. However, it is difficult to constrain the spectral shape of the keV-X-ray excess. The third blackbody component with $kT^infty = 137^{+18}_{-14}$ eV, additional power-law component with a photon index $Gamma = 3.4^{+0.5}_{-0.6}$, or Comptonization of blackbody seed photons into power-law with a photon index $Gamma_c = 4.3^{+0.8}_{-0.8}$ can reproduce the keV-X-ray excess. We also search for the periodicity of 0.8--1.2,keV data, since 7.055 s pulsation is discovered from 0.15--1.2,keV band in the XMM Newton EPIC-pn data ($sim$1.5%). We only obtain the upper limit of pulsed fraction $<$ 3% in the keV-X-ray excess. We shortly discuss the possible origin of the keV-X-ray excess, such as synchrotron radiation and Comptonization of blackbody photons.
We report optical and X-ray observations of the high-field polar RXJ1007.5-2017 performed between 1990 and 2012. It has an orbital period of 208.60 min determined from the ellipsoidal modulation of the secondary star in an extended low state. The spectral flux of the dM3- secondary star yields a distance of 790+-105 pc. At low accretion levels, RX{} exhibits pronounced cyclotron emission lines. The second and third harmonic fall in the optical regime and yield a field strength in the accretion spot of 94 MG. The source is highly variable on a year-to-year basis and was encountered at visual magnitudes between V sim 20 and V sim 16. In the intermediate state of 1992 and 2000, the soft X-ray luminosity exceeds the sum of the luminosities of the cyclotron source, the hard X-ray source, and the accretion stream by an order of magnitude. An X-ray high state, corresponding to the brightest optical level, has apparently not been observed so far.
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