Oliver Butters

The accretion flows and evolution of magnetic cataclysmic variables

We have used a model of magnetic accretion to investigate the accretion flows of magnetic cataclysmic variables (mCVs). Numerical simulations demonstrate that four types of flow are possible: disks, streams, rings, and propellers. The fundamental observable determining the accretion flow, for a given mass ratio, is the spin-to-orbital-period ratio of the system. If intermediate polars (IPs) are accreting at their equilibrium spin rates, then for a mass ratio of 0.5, those with Pspin/Porb 0.1 will be disklike, those with 0.1 Pspin/Porb 0.6 will be streamlike, and those with Pspin/Porb ~ 0.6 will be ringlike. The spin-to-orbital-period ratio at which the systems transition between these flow types increases as the mass ratio of the stellar components decreases. For the first time we present evolutionary tracks of mCVs, which make it possible to investigate how their accretion flow changes with time. As systems evolve to shorter orbital periods and smaller mass ratios, in order to maintain spin equilibrium their spin-to-orbital-period ratio will generally increase. As a result, the relative occurrence of ringlike flows will increase, and the occurrence of disklike flows will decrease, at short orbital periods. The growing number of systems observed at high spin-to-orbital-period ratios with orbital periods below 2 hr and the observational evidence for ringlike accretion in EX Hya are fully consistent with this picture.

Polarized QPOs from the INTEGRAL polar IGRJ14536-5522 (=Swift J1453.4-5524)

We report optical spectroscopy and high-speed photometry and polarimetry of the INTEGRAL source IGRJ14536-5522 (=Swift J1453.4-5524). The photometry, polarimetry and spectroscopy are modulated on an orbital period of 3.1564(1) h. Orbital circularly polarized modulations are seen from ∼0 to ∼−18 per cent, unambiguously identifying IGRJ14536-5522 as a polar. The negative circular polarization is seen over ∼95 per cent of the orbit, which is consistent (as viewed from the Earth) with a single-pole accretor. We estimate some of the system parameters by modelling the polarimetric observations. n nSome of the high-speed photometric data show modulations that are consistent with quasi-periodic oscillations (QPOs) on the order of 5–6 min. Furthermore, for the first time, we detect the (5–6) min QPOs in the circular polarimetry. We discuss the possible origins of these QPOs. In addition, we note that the source undergoes frequent changes between different accretion states. n nWe also include details of HIgh-speed Photo-POlarimeter (HIPPO), a new high-speed photo-polarimeter, used for some of our observations. This instrument is capable of high-speed, multi-filtered, simultaneous all-Stokes observations. It is therefore ideal for investigating rapidly varying astronomical sources such as magnetic cataclysmic variables.

RXTE determination of the intermediate polar status of XSS J00564+4548, IGR J17195-4100, and XSS J12270-4859

Aims. We determine the nature of the intermediate polar candidates XSS J00564+4548, IGR J17195–4100, and XSS J12270–4859. Methods. Pointed RXTE observations searched for intermediate polar characteristics in these candidate systems. Results. XSS J00564+4548 exhibits a period of 465.68 ± 0.07 s, which we interpret as the spin period, an energy dependent modulation depth, and a spectrum that is fit by a 22 keV photoelectrically absorbed bremsstrahlung with an iron line profile. IGR J17195–4100 shows several candidate periodicities and a spectrum that is fit by a power law with an iron line. XSS J12270–4859 exhibits a candidate spin period of 859.57 ± 0.64 s and a spectrum that is fit by a power law with no evidence of an iron line. Conclusions. XSS J00564+4548 is confirmed to be an intermediate polar. IGR J17195–4100 and XSS J12270–4859 both show some properties of intermediate polars, but cannot be confirmed as definite members of the class here.

Circular polarization survey of intermediate polars I. Northern targets in the range 17 h < RA < 23 h

Context. The origin, evolution, and ultimate fate of magnetic cataclysmic variables are poorly understood. It is largely the nature of the magnetic fields in these systems that leads to this poor understanding. Fundamental properties, such as the field strength and the axis alignment, are unknown in a majority of these systems. Aims. We undertake to put all the previous circular polarization measurements into context and systematically survey intermediate polars for signs of circular polarization, hence to get an indication of their true magnetic field strengths and try to understand the evolution of magnetic cataclysmic variables. Methods. We used the TurPol instrument at the Nordic Optical Telescope to obtain simultaneous UBVRIphoto-polarimetric observations of a set of intermediate polars, during the epoch 2006 July 31–August 2. Results. Of this set of eight systems two (1RXS J213344.1+510725 and 1RXS J173021.5–055933) were found to show significant levels of circular polarization, varying with spin phase. Five others (V2306 Cyg, AO Psc, DQ Her, FO Aqr, and V1223 Sgr) show some evidence for circular polarization and variation of this with spin phase, whilst AE Aqr shows little evidence for polarized emission. We also report the first simultaneous UBVRI photometry of the newly identified intermediate polar 1RXS J173021.5–055933. Conclusions. Circular polarization may be ubiquitous in intermediate polars, albeit at a low level of one or two percent or less. It is stronger at longer wavelengths in the visible spectrum. Our results lend further support to the possible link between the presence of soft X-ray components and the detectability of circular polarization in intermediate polars.

Discovery of polarized emission from the long period intermediate polar RX J2133.7+5107

Aims. We intended to investigate the magnetic field properties of the recently identified intermediate polar RX J2133.7+5107. Methods. We carried out UBVRI photopolarimetric observations of the target using TURPOL on the Nordic Optical Telescope over 2 nights in July/August 2006. Results. We found that RX J2133.7+5107 emits circularly polarized light in all UBVRI bands (up to 3%). This is the first detection of circular polarization in this object. The circular polarization modulations and flux variations give hints of cyclotron beaming effects and suggest that the field strength in RX J2133.7+5107 is possibly one of the highest found amongst the IPs. Conclusions. The highly asynchronous rotation of RX J2133.7+5107 (the spin to orbital period ratio is ∼0.022), suggests that it has only recently come into contact and although it is likely to evolve into a polar, it is currently a long way from doing so. We suggest a possible link between the detection of a soft X-ray blackbody component and polarized optical emission in intermediate polars.

RXTE confirmation of the intermediate polar status of Swift J0732.5-1331

Aims. We intend to establish the X-ray properties of Swift J0732.5–1331 and therefore confirm its status as an intermediate polar. Methods. We analysed 36 240 s of X-ray data from RXTE. Frequency analysis was used to constrain temporal variations and spectral analysis used to characterise the emission and absorption properties. Results. The X-ray spin period is confirmed to be 512.4(3) s with a strong first harmonic. No modulation is detected at the candidate orbital period of 5.6 h, but a coherent modulation is present at the candidate 11.3 h period. The spectrum is consistent with a 37 keV bremsstrahlung continuum with an iron line at 6.4 keV absorbed by an equivalent hydrogen column density of around 10 22 atoms cm −2 . Conclusions. Swift J0732–1331 is confirmed to be an intermediate polar.

DISCOVERY OF POLARIZED EMISSION FROM TWO SOFT X-RAY-EMITTING INTERMEDIATE POLARS: UU Col AND NY Lup

We aim to investigate the magnetic field strengths and cyclotron emission of the two soft X-ray-emitting intermediate polars (IPs) UU Col and NY Lup. We study the connection between polars and soft X-ray-emitting IPs by searching for evidence of circularly polarized light in these two systems, which may be examples of progenitors of polars. We carried out photopolarimetric observations of our targets using the Very Large Telescope (UT2) and FORS1 at Paranal. Imaging polarimetry with good signal-to-noise and relatively high time resolution is possible for these targets using such a large telescope. Detection of circular polarization, modulated according to a white dwarf (WD) spin period, is clear evidence of cyclotron emission processes near the WD surface. The color dependence of the polarization allows us to make estimates of the magnetic field strength. We found that both UU Col and NY Lup emit circularly polarized light in the B and I bands, modulated at the spin period of the WD in each case. We add further confirmation to the idea that soft X-ray-emitting IPs emit circularly polarized light and that cyclotron emission plays an important role in these systems. This also suggests that some soft X-ray-emitting IPs might be progenitors of polars.

RXTE confirmation of the intermediate polar status of IGR J15094–6649

Aims. We aim to establish the X-ray properties of the intermediate polar candidate IGR J15094–6649 and therefore confirm its inclusion into the class. Methods. 42 856 s of X-ray data from RXTE was analysed. Frequency analysis was used to constrain temporal variations and spectral analysis used to characterise the emission and absorption properties. Results. A spin period of 809.7 ± 0.6 s is present, revealed as a complex pulse profile whose modulation depth decreases with increasing X-ray energy. The spectrum is well fitted by either a 17 ± 4 keV Bremsstrahlung or Γ= 1.8 ± 0.1 power law, with an iron emission line feature and significant absorption in each case. Conclusions. IGR J15094–6649 is confirmed to be an intermediate polar.

On the spin modulated circular polarization from the intermediate polars NY Lup and IGR J15094–6649

We report on high-time-resolution, high-signal-to-noise ratio (S/N), photopolarimetry of the intermediate polars NY Lup and IGR J15094–6649. Our observations confirm the detection and colour dependence of circular polarization from NY Lup and additionally show a clear white dwarf, spin modulated signal. From our new high-S/N photometry, we have unambiguously detected wavelength-dependent spin and beat periods and harmonics thereof. IGR J15094–6649 is also discovered to have a particularly strong spin modulated circularly polarized signal. It appears double peaked through the I filter and single peaked through the B filter, consistent with cyclotron emission from a white dwarf with a relatively strong magnetic field. n n n nWe discuss the implied accretion geometries in these two systems and any bearing this may have on the possible relationship with the connection between polars and soft X-ray-emitting intermediate polars. The relatively strong magnetic fields are also suggestive of them being polar progenitors.

A new two channel high-speed photo-polarimeter (HIPPO) for the SAAO

We report on the completion of a new 2 channel, HIgh speed Photo-POlarimeter (HIPPO) for the 1.9m optical telescope of the South African Astronomical Observatory. The instrument makes use of rapidly counter-rotating (10Hz), super-achromatic half- and quarter-waveplates, a fixed Glan-Thompson beamsplitter and two photo-multiplier tubes that record the modulated O and E beams. Each modulated beam permits an independent measurement of the polarisation and therefore simultaneous 2 filter observations. All Stokes parameters are recorded every 0.1sec and photometry every 1 millisecond. Post-binning of data is possible in order to improve the signal. This is ideal for measuring e.g. the rapid variability of the optical polarisation from magnetic Cataclysmic Variable stars. First light was obtained in February 2008.