arXiv subjects

Mark Cropper

Publications and source records attributed to Mark Cropper.

At least 91 records · Page 5Linked to original sources

arXiv2002

STJ Observations of the Eclipsing Polar HU Aqr

We apply an eclipse mapping technique to observations of the eclipsing magnetic cataclysmic variable HU Aqr. The observations were made with the S-Cam2 Superconducting Tunnel Junction detector at the WHT in October 2000, providing high signal-to-noise observations with simultaneous spectral and temporal resolution. HU Aqr was in a bright (high accretion) state (V=14.7) and the stream contributes as much to the overall system brightness as the accretion region on the white dwarf. The stream is modelled assuming accretion is occuring onto only one pole of the white dwarf. We find enhanced brightness towards the accretion region from irradiation and interpret enhanced brightness in the threading region, where the ballistic stream is redirected to follow the magnetic field lines of the white dwarf, as magnetic heating from the stream-field interaction, which is consistent with recent theoretical results. Changes in the stream eclipse profile over one orbital period indicate that the magnetic heating process is unstable.

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arXiv2002

X-ray Spectroscopy of the IP PQ Gem

Using RXTE and ASCA data, we investigate the roles played by occultation and absorption in the X-ray spin pulse profile of the Intermediate Polar PQ Gem. From the X-ray light curves and phase-resolved spectroscopy, we find that the intensity variations are due to a combination of varying degrees of absorption and the accretion regions rotating behind the visible face of the white dwarf. These occultation and absorption effects are consistent with those expected from the accretion structures calculated from optical polarisation data. We can reproduce the changes in absorber covering fraction either from geometrical effects, or by considering that the material in the leading edge of the accretion curtain is more finely fragmented than in other parts of the curtain. We determine a white dwarf mass of ~1.2 using the RXTE data.

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arXiv2002

First X-ray observations of the polar CE Gru

We report the detection of the polar CE Gru in X-rays for the first time. We find evidence for a dip seen in the hard X-ray light curve which we attribute to the accretion stream obscuring the accretion region in the lower hemisphere of the white dwarf. The X-ray spectrum can be fitted using only a shock model: there is no distinct soft X-ray component. We suggest that this is because the reprocessed component is cool enough so that it is shifted into the UV. We determine a mass for the white dwarf of ~1.0Msun.

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arXiv2002

XMM-Newton observations of the asynchronous polar BY Cam

We report observations of the asynchronous polar BY Cam made using XMM-Newton. We find evidence for two accretion regions which have significantly different spectra. In both regions we find evidence for hard X-ray emission from the post-shock flow while we observe a distinct soft X-ray blackbody component in only one. We detect two emission lines in the RGS detectors which we attribute to Nitrogen NVI and NVII: the first time that Nitrogen lines have been detected in an X-ray spectrum of a cataclysmic variable. In the time interval where we observe short timescale variability we find the hard X-ray light curve is correlated with the softest energies in the sense that the hard X-rays trail the soft X-rays and are anti-correlated. We suggest that there are a small number of dense blobs of material which impact the white dwarf without forming a shock and release an amount of optically thick material which obscures hard X-rays for a short duration.

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arXiv2002

Timing study of the isolated neutron star RX J0720.4-3125

We present a combined analysis of XMM-Newton, Chandra and Rosat observations of the isolated neutron star RX J0720.4-3125, spanning a total period of \sim 7 years. We develop a maximum likelihood periodogramme based on ΔC statistic and maximum likelihood method, which are appropriate for sparse event lists. As an "a posteriori" check, we have folded a further BeppoSAX dataset with the period predicted at the time of that observation, finding that the phase is consistent. The value of the spin down rate, here measured for the first time, is \approx 10^{-14} s/s and can not be explained in terms of propeller or torque from a fossil disk. When interpreted in terms of dipolar losses, it gives a magnetic field of B \approx 10^{13} G, making also implausible that the source is accreting from the underdense surroundings. We discuss the implications of this measure for the different mechanisms that have been suggested to explain the X-ray emission. We conclude that the observed properties are more compatible with a scenario in which the source is \approx 10^6 yrs old, and its magnetic field has not changed substantially over the lifetime.

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arXiv2002

Timing analysis of the isolated neutron star RX J0720.4-3125

We present a combined analysis of XMM-Newton, Chandra and Rosat observations of the isolated neutron star RXJ0720.4-3125, spanning a total period of \sim 7 years. We develop a maximum likelihood periodogramme for our analysis based on the ΔC-statistic and the maximum likelihood method, which are appropriate for the treatment of sparse event lists. Our results have been checked "a posteriori" by folding a further BeppoSAX dataset with the period predicted at the time of that observation: the phase is found to be consistent. The study of the spin history and the measure of the spin-down rate is of extreme importance in discriminating between the possible mechanisms suggested for the nature of the X-ray emission. The value of \dot P, here measured for the first time, is \approx 10^{-14} s/s. This value can not be explained in terms of torque from a fossil disk. When interpreted in terms of dipolar losses, it gives a magnetic field of B \approx 10^{13} G, making also implausible that the source is accreting from the underdense surroundings. On the other hand, we also find unlikely that the field decayed from a much larger value (B\approx 10^{15} G, as expected for a magnetar powered by dissipation of a superstrong field) since this scenario predicts a source age of \approx 10^4 yrs, too young to match the observed X-ray luminosity. The observed properties are more compatible with a scenario in which the source is \approx 10^6 yrs old, and its magnetic field has not changed substantially over the lifetime.

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arXiv2001

First XMM-Newton observations of strongly magnetic cataclysmic variables - II. Timing studies of DP Leo and WW Hor

XMM-Newton was used to observe two eclipsing, magnetic cataclysmic variables, DP Leo and WW Hor, continuously for three orbital cycles each. Both systems were in an intermediate state of accretion. For WW Hor we also obtained optical light curves with the XMM-Newton Optical Monitor and from ground-based observations. Our analysis of the X-ray and optical light curves allows us to constrain physical and geometrical parameters of the accretion regions and derive orbital parameters and eclipse ephemerides of the systems. For WW Hor we directly measure horizontal and vertical temperature variations in the accretion column. From comparisons with previous observations we find that changes in the accretion spot longitude are correlated with the accretion rate. For DP Leo the shape of the hard X-ray light curve is not as expected for optically thin emission, showing the importance of optical depth effects in the post-shock region. We find that the spin period of the white dwarf is slightly shorter than the orbital period and that the orbital period is decreasing faster than expected for energy loss by gravitational radiation alone.

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arXiv2001

Indirect imaging of the accretion stream in eclipsing polars IV: V895 Cen

We present spectroscopic and high speed photometric data of the eclipsing polar V895 Cen. We find that the eclipsed component is consistent with it being the accretion regions on the white dwarf. This is in contrast to Stobie et al who concluded that the eclipsed component was not the white dwarf. Further, we find no evidence for an accretion disc in our data. From our Doppler tomography results, we find that the white dwarf has M geq 0.7 Msun. Our indirect imaging of the accretion stream suggests that the stream is brightest close to the white dwarf. When we observed V895 Cen in its highest accretion state, emission is concentrated along field lines leading to the upper pole. There is no evidence for enhanced emission at the magnetic coupling region.

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arXiv2001

An electrically powered binary star?

We propose a model for stellar binary systems consisting of a magnetic and a non-magnetic white-dwarf pair which is powered principally by electrical energy. In our model the luminosity is caused by resistive heating of the stellar atmospheres due to induced currents driven within the binary. This process is reminiscent of the Jupiter-Io system, but greatly increased in power because of the larger companion and stronger magnetic field of the primary. Electrical power is an alternative stellar luminosity source, following on from nuclear fusion and accretion. We find that this source of heating is sufficient to account for the observed X-ray luminosity of the 9.5-min binary RX J1914+24, and provides an explanation for its puzzling characteristics.

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arXiv2001

Ionisation Structure in Accretion Shocks with a Composite Cooling Function

We have investigated the ionisation structure of the post-shock regions of magnetic cataclysmic variables using an analytic density and temperature structure model in which effects due to bremsstrahlung and cyclotron cooling are considered. We find that in the majority of the shock-heated region where H- and He-like lines of the heavy elements are emitted, the collisional-ionisation and corona-condition approximations are justified. We have calculated the line emissivity and ionisation profiles for Iron as a function of height within the post-shock flow. For low-mass white dwarfs, line emission takes place near the shock. For high-mass white dwarfs, most of the line emission takes place in regions well below the shock and hence it is less sensitive to the shock temperature. Thus, the line ratios are useful to determine the white-dwarf masses for the low-mass white dwarfs, but the method is less reliable when the white dwarfs are massive. Line spectra can, however, be used to map the hydrodynamic structure of the post-shock accretion flow.

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arXiv2001

The lower boundary of the accretion column in magnetic cataclysmic variables

Using a parameterised function for the mass loss at the base of the post-shock region, we have constructed a formulation for magnetically confined accretion flows which avoids singularities, such as the infinity in density, at the base associated with all previous formulations. With the further inclusion of a term allowing for the heat input into the base from the accreting white dwarf we are able also to obtain the hydrodynamic variables to match the conditions in the stellar atmosphere. (We do not, however, carry out a mutually consistent analysis for the match). Changes to the emitted X-ray spectra are negligible unless the thickness of mass leakage region at the base approaches or exceeds one percent of the height of the post-shock region. In this case the predicted spectra from higher-mass white dwarfs will be harder, and fits to X-ray data will predict lower white-dwarf masses than previous formulations.

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arXiv2000

First XMM-Newton observations of an isolated neutron star: RXJ0720.4-3125

We present the high resolution spectrum of the isolated neutron star RXJ0720.4-3125, obtained with the Reflection Grating Spectrometer on XMM-Newton, complemented with the broad band spectrum observed with the EPIC PN camera. The spectrum appears smooth, with no evidence for strong photospheric absorption or emission features. We briefly discuss the implications of our failure to detect structure in the spectrum.

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arXiv2000

Modelling the spin pulse profile of the isolated neutron star RX J0720.4--3125 observed with XMM-Newton

We model the spin pulse intensity and hardness ratio profiles of the isolated neutron star RXJ0720.4--3125 using XMM-Newton data. The observed variation is approximately sinusoidal with a peak-to-peak amplitude of 15%, and the hardness ratio is softest slightly before flux maximum. By using polar cap models we are able to derive maximum polar cap sizes and acceptable viewing geometries. The inferred sizes of the caps turn out to be more compatible with a scenario in which the neutron star is heated by accretion, and place limits on the magnetic field strength. The hardness ratio modulation can then be explained in terms of energy-dependent beaming effects, and this constrains the acceptable models of the emerging radiation to cases in which softer photons are more strongly beamed than harder photons. An alternative explanation in terms of spatially variable absorption co-rotating in the magnetosphere is also discussed.

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arXiv2000

Simultaneous optical polarimetry and X-ray data of the near synchronous polar RX J2115-5840

We present simultaneous optical polarimetry and X-ray data of the near synchronous polar RX J2115-5840. We model the polarisation data using the Stokes imaging technique of Potter et al. We find that the data are best modelled using a relatively high binary inclination and a small angle between the magnetic and spin axes. We find that for all spin-orbit beat phases, a significant proportion of the accretion flow is directed onto the lower hemisphere of the white dwarf, producing negative circular polarisation. Only for a small fraction of the beat cycle is a proportion of the flow directed onto the upper hemisphere. However, the accretion flow never occurs near the upper magnetic pole, whatever the orientation of the magnetic poles. This indicates the presence of a non-dipole field with the field strength at the upper pole significantly higher. We find that the brightest parts of the hard X-ray emitting region and the cyclotron region are closely coincident.

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arXiv1999

Detection of the optical counterpart of the proposed double degenerate polar RX J1914+24

We have detected the optical counterpart of the proposed double degenerate polar RX J1914+24. The I band light curve is modulated on the 9.5 min period seen in X-rays. There is no evidence for any other periods. No significant modulation is seen in J. The infrared colours of RX J1914+24 are not consistent with a main sequence dwarf secondary star. Our ASCA spectrum of RX J1914+24 is typical of a heavily absorbed polar and our ASCA light curve also shows only the 9.5 min period. We find that the folded I band and X-ray light curves are out of phase. We attribute the I band flux to the irradiated face of the donor star. The long term X-ray light curve shows a variation in the observed flux of up to an order of magnitude. These observations strengthen the view that RX J1914+24 is indeed the first double degenerate polar to be detected. In this light, we discuss the synchronising mechanisms in such a close binary and other system parameters.

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arXiv1999

The Emission from Post-Shock Flows in mCVs

We re-examine the vertical structure of the post-shock flow in the accretion region of mCVs, and the X-ray emission as a function of height. We then predict X-ray light curves and phase-resolved spectra, taking into account the vertical structure, examine the implications and check whether the predicted heights are compatible with observation.

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arXiv1999

Effects of Gravity on the Structure of Postshock Accretion Flows in Magnetic Cataclysmic Variables

We have calculated the temperature and density structure of the hot postshock plasma in magnetically confined accretion flows, including the gravitational potential. This avoids the inconsistency of previous calculations which assume that the height of the shock is negligible. We assume a stratified accretion column with 1-d flow along the symmetry axis. We find that the calculations predict a lower shock temperature than previous calculations, with a flatter temperature profile with height. We have revised previous determinations of the masses of the white dwarf primary stars and find that for higher mass white dwarfs there is a general reduction in derived masses when the gravitational potential is included. This is because the spectrum from such flows is harder than that of previous prescriptions at intermediate energies.

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arXiv1998

RX J2115--5840: confirmation of a new near-synchronous polar

Following the suggestion of Schwope et al that the magnetic cataclysmic variable RX J2115-5840 maybe a near-synchronous polar, we obtained optical polarimetry of this system over a 2 week period. From a power spectrum of the circular polarimetry data we determine that the spin period of the white dwarf and the binary orbital period which differ by 1.2%. RX J2115-5840 is thus the fourth near synchronous polar and has the shortest spin-orbit beat period: 6.3 days. By folding the data on spin, beat and orbital periods we find evidence that the accretion stream is directed towards opposite magnetic poles as the stream precesses around the white dwarf on the spin-orbit beat period. The phasing requires that the accretion flow must be directed onto the same magnetic field line at all spin-orbit beat phases implying that at some phases the flow must follow a path around the white dwarf before accreting. This is difficult to reconcile with simple views of how the accretion stream attaches onto the magnetic field of the white dwarf.

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