Search

Using the ULTRASPEC instrument mounted on the 2.4-m Thai National Telescope, we observed two large flares, each with a total energy close to 1034 erg with sub-second cadence. A combination of a wavelet analysis, a Fourier transform plus an empirical mode decomposition, reveals quasi-periodic pulsations (QPPs) that exhibit an apparent doubling of the oscillation period. Both events showed oscillations of a few minutes over a interval of several minutes, and despite the availability of sub-second cadence, there was no evidence of sub-minute oscillations. The doubling of the QPP periods and shorter lifetime of shorter-period QPP modes strongly favour resonant dynamics of magnetohydrodynamic waves in a coronal loop. We estimate loop lengths to be 0.2–0.7 R⋆, in agreement with a typical length of solar coronal loops. These observations presents rare and compelling evidence for the presence of compact plasma loops in a stellar corona.

We present results from a circular polarimetric survey of candidate detached magnetic white dwarf - M dwarf binaries obtained using the Nordic Optical Telescope, La Palma. We obtained phase resolved spectropolarimetry and imaging polarimetry of seven systems, five of which show clearly variable circular polarisation. The data indicate that these targets have white dwarfs with magnetic field strengths > 80 MG. Our study reveals that cyclotron emission can dominate the optical luminosity at wavelengths corresponding to the cyclotron emission harmonics, even in systems where the white dwarfs are only wind-accreting. This implies that a very significant fraction of the the stellar wind of the companion star is captured by the magnetic white dwarf reducing the magnetic braking in pre-CVs. Furthermore, the polarimetric confirmation of several detached, wind-accreting magnetic systems provides observational constraints on the models of magnetic CV evolution and white dwarf magnetic field generation. We also find that the white dwarf magnetic field configuration in at least two of these systems appears to be very complex., 13 pages, 10 figures, accepted for publication in MNRAS, Figure 10 was replaced by a corrected one

Numerous studies have analysed inferred power-law distributions between frequency and energy of impulsive events in the outer solar atmosphere in an attempt to understand the predominant energy supply mechanism in the corona. Here, we apply a burst detection algorithm to high-resolution imaging data obtained by the Interface Region Imaging Spectrograph to further investigate the derived power-law index, γ, of bright impulsive events in the transition region. Applying the algorithm with a constant minimum event lifetime (of either 60 s or 110 s) indicated that the target under investigation, such as Plage and Sunspot, has an influence on the observed power-law index. For regions dominated by sunspots, we always find γ < 2; however, for data sets where the target is a plage region, we often find that γ > 2 in the energy range (∼1023, ∼1026) erg. Applying the algorithm with a minimum event lifetime of three time-steps indicated that cadence was another important factor, with the highest cadence data sets returning γ > 2 values. The estimated total radiative power obtained for the observed energy distributions is typically 10–25 per cent of what would be required to sustain the corona indicating that impulsive events in this energy range are not sufficient to solve coronal heating. If we were to extend the power-law distribution down to an energy of 1021 erg, and assume parity between radiative energy release and the deposition of thermal energy, then such bursts could provide 25–50 per cent of the required energy to account for the coronal heating problem.

Solar radio emission at low frequencies (, 10 pages, 6 figures, 3 tables

Blue Large Amplitude Pulsators (BLAPs) are a relatively new class of blue variable stars showing periodic variations in their light curves with periods shorter than a few tens of mins and amplitudes of more than ten percent. We report nine blue variable stars identified in the OmegaWhite survey conducted using ESO's VST, which show a periodic modulation in the range 7-37 min and an amplitude in the range 0.11-0.28 mag. We have obtained a series of followup photometric and spectroscopic observations made primarily using SALT and telescopes at SAAO. We find four stars which we identify as BLAPs, one of which was previously known. One star, OW J0820--3301, appears to be a member of the V361 Hya class of pulsating stars and is spatially close to an extended nebula. One further star, OW J1819--2729, has characteristics similar to the sdAV pulsators. In contrast, OW J0815--3421 is a binary star containing an sdB and a white dwarf with an orbital period of 73.7 min, making it only one of six white dwarf-sdB binaries with an orbital period shorter than 80 min. Finally, high cadence photometry of four of the candidate BLAPs show features which we compare with notch-like features seen in the much longer period Cepheid pulsators., Comment: Accepted by MNRAS

We have performed a search for flares and quasi-periodic pulsations (QPPs) from low-mass M-dwarf stars using Transient Exoplanet Survey Satellite (TESS) two-minute cadence data. We find seven stars that show evidence of QPPs. Using Fourier and empirical mode decomposition techniques, we confirm the presence of 11 QPPs in these seven stars with a period between 10.2 and 71.9 minutes, including an oscillation with strong drift in the period and a double-mode oscillation. The fraction of flares that showed QPPs (7%) is higher than other studies of stellar flares, but it is very similar to the fraction of solar C-class flares. Based on the stellar parameters taken from the TESS Input Catalog, we determine the lengths and magnetic-field strengths of the flare coronal loops using the period of the QPPs and various assumptions about the origin of the QPPs. We also use a scaling relationship based on flares from the Sun and solar-type stars and the observed energy, plus the duration of the flares, finding that the different approaches predict loop lengths that are consistent to within a factor of about two. We also discuss the flare frequency of the seven stars determining whether this could result in ozone depletion or abiogenesis in any orbiting exoplanet. Three of our stars have a sufficiently high rate of energetic flares, which are likely to cause abiogenesis. However, two of these stars are also in the range where ozone depletion is likely to occur. We speculate on the implications of the flare rates, loop lengths, and QPPs for life on potential exoplanets orbiting in their host star’s habitable zone.

We present the results of near continuous TESS optical observations of the asynchronous polar CD Ind (RX J2115-5840). The 27.9 d long light curve, with 2 min resolution, reveals remarkable changes in the magnetic accretion geometry of the system over the 7.3 d beat period. We have modelled the changes in the optical spin period pulse shape using a cyclotron emission mapping technique. The resulting cyclotron emission maps of the magnetic white dwarf reveal how the accretion geometry changes from single to two pole accretion and back over the beat cycle. Finally, we present the results from particle based numerical magnetic accretion simulations, that agree with our interpretation of the changing accretion scenario., Comment: Accepted for publication in MNRAS on 4th Apr 2019, originally submitted 28th Feb 2019. 8 pages, 6 figures. See also a preprint by Littlefield et al. (1903.00490) that uses the same dataset

MQ Dra is a strongly magnetic Cataclysmic Variable whose white dwarf accretes material from its secondary star through a stellar wind at a low rate. TESS observations were made of MQ Dra in four sectors in Cycle 2 and show a short duration, high energy flare (~10^35 erg) which has a profile characteristic of a flare from the M5V secondary star. This is one of the few occasions where an energetic flare has been seen from a Polar. We find no evidence that the flare caused a change in the light curve following the event and consider whether a coronal mass ejection was associated with the flare. We compare the frequency of energetic flares from the secondary star in MQ Dra with M dwarf stars and discuss the overall flare rate of stars with rotation periods shorter than 0.2 d and how such fast rotators can generate magnetic fields with low differential rotation rates., Accepted by MNRAS

DW Cnc is an intermediate polar which has previously been observed in both high and low states. Observations of the high state of DW Cnc have previously revealed a spin period at ~ 38.6 min, however observations from the 2018/19 low state showed no evidence of the spin period. We present results from our analysis of 12 s cadence photometric data collected by NGTS of DW Cnc during the high state which began in 2019. Following the previously reported suppression of the spin period signal we identify the return of this signal during the high state, consistent with previous observations of it. We identify this as the restarting of accretion during the high state. We further identified three short outbursts lasting ~ 1 d in DW Cnc with a mean recurrence time of ~ 60 d and an amplitude of ~ 1 mag. These are the first outbursts identified in DW Cnc since 2008. Due to the short nature of these events we identify them not as a result of accretion instabilities but instead either from instabilities originating from the interaction of the magnetorotational instability in the accretion disc and the magnetic field generated by the white dwarf or the result of magnetic gating., Comment: Accepted to MNRAS; 8 pages, 4 figues

In our previous study of low mass stars using TESS, we found a handful which show a periodic modulation on a period, Accepted for publication in MNRAS

We report K2 observations of the eclipsing cataclysmic variable V729 Sgr which covered nearly 80 days in duration. We find five short outbursts and two long outbursts, one of which shows a clear plateau phase in the rise to maximum brightness. The mean time between successive short outbursts is ~10 d while the time between the two long outbursts is ~38 d. The frequency of these outbursts are unprecedented for a CV above the orbital period gap. We find evidence that the mid-point of the eclipse occurs systematically earlier in outburst than in quiescence. During five of the six quiescent epochs we find evidence for a second photometric period which is roughly 5 percent shorter than the 4.16 h orbital period which we attribute to negative superhumps. V729 Sgr is therefore one of the longest period CVs to show negative superhumps during quiescence., Comment: Accepted for publication in MNRAS

HD144941 is an evolved early-type metal-poor low-mass star with a hydrogen-poor surface. It is frequently associated with other intermediate helium-rich subdwarfs and extreme helium stars. Previous photometric studies have failed to detect any variability. New observations with the K2 mission show complex but periodic variations with a full amplitude of 4 parts per thousand. It is proposed that these are due to an inhomogeneous surface brightness distribution (spots) superimposed on a rotation period of 13.9+/-0.2 d. The cause of the surface inhomogeneity is not identified, although an oblique dipolar magnetic field origin is plausible., Comment: Accepted by MNRAS

Detailed studies of the Sun have shown that sunspots and solar flares are closely correlated. Photometric data from Kepler/K2 has allowed similar studies to be carried out on other stars. Here, we utilise TESS photometric 2-min cadence of 167 low mass stars from Sectors 1 - 3 to investigate the relationship between starspots and stellar flares. From our sample, 90 percent show clear rotational modulation likely due to the presence of a large, dominant starspot and we use this to determine a rotational period for each star. Additionally, each low mass star shows one or more flares in its lightcurve and using Gaia DR2 parallaxes and SkyMapper magnitudes we can estimate the energy of the flares in the TESS band-pass. Overall, we have 1834 flares from the 167 low mass stars with energies from $6.0\times 10^{29}$ - $2.4\times 10^{35}$~erg. We find none of the stars in our sample show any preference for rotational phase suggesting the lack of a correlation between the large, dominant star spot and flare number. We discuss this finding in greater detail and present further scenarios to account for the origin of flares on these low mass stars., 10 pages, 9 figures, 2 tables, accepted for publication in MNRAS

We present an analysis of K2 short cadence data of 34 M dwarfs which have spectral types in the range M0 - L1. Of these stars, 31 showed flares with a duration between $\sim$10-90 min. Using distances obtained from Gaia DR2 parallaxes, we determined the energy of the flares to be in the range $\sim1.2\times10^{29}-6\times10^{34}$ erg. In agreement with previous studies we find rapidly rotating stars tend to show more flares, with evidence for a decline in activity in stars with rotation periods longer than $\sim$10 days. The rotational modulation seen in M dwarf stars is widely considered to result from a starspot which rotates in and out of view. Flux minimum is therefore the rotation phase where we view the main starspot close to the stellar disk center. Surprisingly, having determined the rotational phase of each flare in our study we find none show any preference for rotational phase. We outline three scenarios which could account for this unexpected finding. The relationship between rotation phase and flare rate will be explored further using data from wide surveys such as NGTS and TESS., Accepted main Journal MNRAS

We report the discovery of the ultracompact hot subdwarf (sdOB) binary OW J074106.0-294811.0 with an orbital period of P$_{\rm orb}=44.66279\pm1.16\times10^{-4}$ min, making it the most compact hot subdwarf binary known. Spectroscopic observations using the VLT, Gemini and Keck telescopes revealed a He-sdOB primary with an intermediate helium abundance, T$_{\rm eff}=39 400\pm500$ K and log(g)=$5.74\pm0.09$. High signal-to-noise ratio lightcurves show strong ellipsoidal modulation resulting in a derived sdOB mass $M_{\rm sdOB}=0.23\pm0.12$ M$_\odot$ with a WD companion ($M_{\rm WD}=0.72\pm0.17$ M$_\odot$). The mass ratio was found to be $q = M_{\rm sdOB}/M_{\rm WD}=0.32\pm0.10$. The derived mass for the He-sdOB is inconsistent with the canonical mass for hot sbudwarfs of $\approx0.47$ M$_\odot$. To put constraints on the structure and evolutionary history of the sdOB star we compared the derived T$_{\rm eff}$, log(g) and sdOB mass to evolutionary tracks of helium stars and helium white dwarfs calculated with Modules for Experiments in Stellar Astrophysics (MESA). We find that the best fitting model is a helium white dwarf with a mass of $0.320$ M$_\odot$, which left the common envelope ${\approx}1.1$ Myr ago, is consistent with the observations. As a helium white dwarf with a massive white dwarf companion the object will reach contact in 17.6 Myr at an orbital period of 5 min. Depending on the spin-orbit synchronization timescale the object will either merge to form an R CrB star or end up as a stably accreting AM CVn-type system with a helium white dwarf donor., Accepted for publication in ApJ, 13 pages, 7 figures

We present Kepler and Swift observations of StHa 169 which is currently classified as a symbiotic binary. The Kepler light curve shows quasi periodic behaviour with a mean period of 34 d and an amplitude of a few percent. Using Swift data we find a relatively strong UV source at the position of StHa 169 but no X-ray counterpart. Using a simple two component blackbody fit to model the combined Swift and 2MASS spectral energy distribution and an assessment of the previously published optical spectrum, we find that the source has a hot (~10,000K) component and a cooler (~3700K) component. The Kepler light is dominated by the cool component and we attribute the variability to pulsations in a red giant star. If we remove this approximate month long modulation from the light curve, we find no evidence for additional variability in the light curve. The hotter source is assigned to a late B or early A main sequence star. We briefly discuss the implications of these findings and conclude that StHA 169 is a red giant plus main sequence binary., Accepted for publication in MNRAS

Symbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. However, in most symbiotics, the origin of this burning is unclear. In symbiotic slow novae, however, it is linked to a past thermonuclear runaway. In June 2015, the symbiotic slow nova AG Peg was seen in only its second optical outburst since 1850. This recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks -- like outbursts in classical symbiotic stars such as Z And. We report Swift X-ray and UV observations of AG Peg made between June 2015 and January 2016. The X-ray flux was markedly variable on a time scale of days, particularly during four days near optical maximum, when the X-rays became bright and soft. This strong X-ray variability continued for another month, after which the X-rays hardened as the optical flux declined. The UV flux was high throughout the outburst, consistent with quasi-steady shell burning on the white dwarf. Given that accretion disks around white dwarfs with shell burning do not generally produce detectable X-rays (due to Compton-cooling of the boundary layer), the X-rays probably originated via shocks in the ejecta. As the X-ray photo-electric absorption did not vary significantly, the X-ray variability may directly link to the properties of the shocked material. AG Peg's transition from a slow symbiotic nova (which drove the 1850 outburst) to a classical symbiotic star suggests that shell burning in at least some symbiotic stars is residual burning from prior novae., Accepted by MNRAS 23 June 2016. Manuscript submitted in original form 5 April 2016

We present the results of an analysis of data covering 1.5 years of the dwarf nova V447 Lyr. We detect eclipses of the accretion disk by the mass donating secondary star every 3.74 hrs which is the binary orbital period. V447 Lyr is therefore the first dwarf nova in the Kepler field to show eclipses. We also detect five long outbursts and six short outbursts showing V447 Lyr is a U Gem type dwarf nova. We show that the orbital phase of the mid-eclipse occurs earlier during outbursts compared to quiescence and that the width of the eclipse is greater during outburst. This suggests that the bright spot is more prominent during quiescence and that the disk is larger during outburst than quiescence. This is consistent with an expansion of the outer disk radius due to the presence of high viscosity material associated with the outburst, followed by a contraction in quiescence due to the accretion of low angular momentum material. We note that the long outbursts appear to be triggered by a short outburst, which is also observed in the super-outbursts of SU UMa dwarf novae as observed using Kepler.

KL Dra is a helium-accreting AM CVn binary system with an orbital period close to 25 min. Approximately every 60 d there is a 4-mag optical outburst lasting ∼10 d. We present the most sensitive X-ray observations made of an AM CVn system during an outburst cycle. A series of eight observations were made using XMM–Newton which started shortly after the onset of an optical outburst. We find that X-rays are suppressed during the optical outburst. There is some evidence for a spectral evolution of the X-ray spectrum during the course of the outburst. A periodic modulation is seen in the ultraviolet data at three epochs – this is a signature of the binary orbital or the superhump period. The temperature of the X-ray-emitting plasma is cooler compared to dwarf novae, which may suggest that a wind is the origin of a significant fraction of the X-ray flux.

We present the results of a two and a half year optical photometric monitoring programme covering 16 AM CVn binaries using the Liverpool Telescope on La Palma. We detected outbursts in seven systems, one of which (SDSS J0129) was seen in outburst for the first time. Our study coupled with existing data shows that ~1/3 of these helium-rich accreting compact binaries show outbursts. The orbital period of the outbursting systems lie in the range 24-44 mins and is remarkably consistent with disk-instability predictions. The characteristics of the outbursts seem to be broadly correlated with their orbital period (and hence mass transfer rate). Systems which have short periods (

The bright X-ray source, 2XMMi J180438.7-145647 is fortunate to have long baseline observations in INTEGRAL that compliment observations taken by other missions. Optical spectroscopy of this object has suggested a distance of ~7 kpc and an identification with a low mass X-ray binary. We instead use the X-ray data from 0.3-40 keV to identify the source as a bright intermediate polar (IP) with an estimate for the white dwarf mass of~0.60 M_solar. This identification is supported by the presence of an iron triplet, the component lines of which are some of the strongest seen in IPs; and the signature of the spin period of the white dwarf at ~24 mins. We note that the lack of broad-band variability may suggest that this object is a stream-fed IP, similar in many respects to the well studied IP, V2400 Oph. Phase-binning has allowed us to create spectra corresponding to the peaks and troughs of the lightcurve from which we determine that the spectra appear harder in the troughs, consistent with the behaviour of other IPs binned on their spin periods. This work strongly suggests a mis-identification in the optical due to the presence of large columns of enshrouding material. We instead propose a distance to the source of

We report the discovery of 31 blue, short period, pulsators made using data taken as part of the Rapid Temporal Survey (RATS). We find they have periods between 51-83 mins and full-amplitudes between 0.05-0.65 mag. Using the period-luminosity relationship for short period pulsating stars we determine their distance. Assuming they are pulsating in either the fundamental or first over-tone radial mode the majority are located at a distance greater than 3kpc, with several being more than 20 kpc distant. Most stars are at least 1 kpc from the Galactic plane, with three being more than 10 kpc. One is located in the direction of the Galactic anti-center and has Galactocentric distance of ~30 kpc and is ~20 kpc below the plane: they are therefore potential tracers of Galactic structure. We have obtained low-resolution spectra for a small number our targets and find they have temperatures between 7200--7900K and a metal content less than Solar. The colours of the pulsators and the spectral fits to those stars for which we have spectra indicate that they are either SX Phe or delta Scuti stars. We estimate the number of SX Phe stars in our Galaxy and find significantly fewer per unit mass than reported in massive globular clusters or dwarf spheroidal galaxies.

We present multi-wavelength observations of the helium-dominated accreting binary KL Dra which has an orbital period of 25 mins. Our ground-based optical monitoring programme using the Liverpool Telescope has revealed KL Dra to show frequent outbursts. Although our coverage is not uniform, our observations are consistent with the outbursts recurring on a timescale of ~60 days. Observations made using Swift show that the outbursts occur with a similar amplitude at both UV and optical energies and a duration of 2 weeks. Although KL Dra is a weak X-ray source we find no significant evidence that the X-ray flux varies over the course of an outburst cycle. We can reproduce the main features of the 60 day outburst cycle using the Disc Instability Model and a helium-dominated accretion flow. Although the outbursts of KL Dra are very similar to those of the hydrogen accreting dwarf novae, we cannot exclude that they are the AM CVn equivalent of WZ Sge type outbursts. With outbursts occurring every ~2 months, KL Dra is an excellent target to study helium-dominated accretion flows in general.

We present phase resolved optical spectroscopy and photometry of V4580 Sagittarii, the optical counterpart to the accretion powered millisecond pulsar SAX J1808.4-3658, obtained during the 2008 September/October outburst. Doppler tomography of the N III 4640.64 Bowen blend emission line reveals a focused spot of emission at a location consistent with the secondary star. The velocity of this emission occurs at 324 +/- 15 km/s; applying a "K-correction", we find the velocity of the secondary star projected onto the line of sight to be 370 +/- 40 km/s. Based on existing pulse timing measurements, this constrains the mass ratio of the system to be 0.044^{+0.005}_{-0.004}, and the mass function for the pulsar to be 0.44^{+0.16}_{-0.13} Msun. Combining this mass function with various inclination estimates from other authors, we find no evidence to suggest that the neutron star in SAX J1808.4-3658 is more massive than the canonical value of 1.4 Msun. Our optical light curves exhibit a possible superhump modulation, expected for a system with such a low mass ratio. The equivalent width of the Ca II H and K interstellar absorption lines suggest that the distance to the source is ~2.5 kpc. This is consistent with previous distance estimates based on type-I X-ray bursts which assume cosmic abundances of hydrogen, but lower than more recent estimates which assume helium-rich bursts., 12 pages, 10 figures; Accepted for publication in MNRAS

We use K2 short cadence data obtained over a duration of 50 days during Campaign 0 to observe two M1V dwarf stars, TYC 1330-879-1 and RXJ 0626+2349. We provide an overview of our data analysis, in particular, making a comparison between using a fixed set of pixels and an aperture which follows the position of the source. We find that this moving aperture approach can give fewer non-astrophysical features compared to a fixed aperture. Both sources shows flares as energetic as observed from several M4V stars using both Kepler and ground based telescopes. We find that the flare energy distribution of the sources shown here are very similar to the less active M3-M5 stars but are ~8 times less likely to produce a flare of a comparable energy to the more active M0--M5 stars. We discuss the biases and sources of systematic errors when comparing the activity of stars derived from different instruments. We conclude that K2 observations will provide an excellent opportunity to perform a census of flare activity across the full range of M dwarf spectral class and hence the physical mechanisms which power them., Accepted for publication in MNRAS

Contains fulltext : 34840.pdf ( ) (Open Access) Contains fulltext : 34840.pdf (Publisher’s version ) (Open Access) We present the results of XMM-Newton observations of two AM CVn systems - V396 Hya and SDSS J1240-01. Both systems are detected in X-rays and in the UV: neither shows coherent variability in their light curves. We compare the rms variability of the X-ray and UV power spectra of these sources with other AM CVn systems. Apart from ES Cet, AM CVn sources are not strongly variable in X-rays, while in the UV the degree of variability is related to the systems apparent brightness. The X-ray spectra of V396 Hya and SDSS J1240-01 show highly non-solar abundances, requiring enhanced nitrogen to obtain good fits. We compare the UV and X-ray luminosities for 7 AM CVn systems using recent distances. We find that the X-ray luminosity is not strongly dependent upon orbital period. However, the UV luminosity is highly correlated with orbital period with the UV luminosity decreasing with increasing orbital period. We expect that this is due to the accretion disk making an increasingly strong contribution to the UV emission at shorter periods. The implied luminosities are in remarkably good agreement with predictions.

We present high time-resolution optical energy-resolved photometry of the eclipsing cataclysmic variable V2301 Oph made using the ESA S-Cam detector, an array of photon counting super-conducting tunnel junction (STJ) devices with intrinsic energy resolution. Three eclipses were observed, revealing considerable variation in the eclipse shape, particularly during ingress. The eclipse shape is shown to be understood in terms of AM Her accretion via a bright stream, with very little contribution from the white dwarf photosphere and/or hotspot. About two thirds of the eclipsed light arises in the threading region. Variation in the extent of the threading region can account for most of the variations observed between cycles. Spectral fits to the data reveal a 10,000K blackbody continuum with strong, time-varying emission lines of hydrogen and helium. This is the first time that stellar emission lines have been detected in the optical band using a non-dispersive photon-counting system., Accepted A&A

We present the results of a search for X-ray variability in the galactic open cluster NGC 2516. This cluster has been observed on 8 occasions using XMM-Newton making it an excellent target for such a study. We find 4 sources which show significant variability, implying the detection of one significantly variable source every 25ksec. All sources exhibit spectra which can be fitted using an absorbed one or two temperature thermal plasma model. The brightest of these flares also show a prominent Iron line near ~7.0keV. All 4 sources lie near the observed main sequence of NGC 2516 suggesting they are cluster members. We propose that at least 3 of the 4 objects are RS CVn systems. We compare the properties of the brightest flare with those of solar flares., Comment: Accepted for publication in MNRAS, 6 pages

We present XMM-Newton observations of the luminous star Eta Carinae, including a high resolution soft X-ray spectrum of the surrounding nebula obtained with the Reflection Grating Spectrometer. The EPIC image of the field around Eta Car shows many early-type stars and diffuse emission from hot, shocked gas. The EPIC spectrum of the star is similar to that observed in previous X-ray observations, and requires two temperature components. The RGS spectrum of the surrounding nebula shows K-shell emission lines from hydrogen- and helium-like nitrogen and neon and L-shell lines from iron, but little or no emission from oxygen. The observed emission lines are not consistent with a single temperature, but the range of temperatures observed is not large, spanning ~0.15-0.6 keV. We obtain upper limits for oxygen line emission and derive a lower limit of N/O > 9. This is consistent with previous abundance determinations for the ejecta of Eta Car, and with theoretical models for the evolution of massive, rotating stars.

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., Comment: Accepted for publication in MNRAS (11 pages)

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.

Accretion in white-dwarf binary systems can occur through discs, accretion columns or a combination of these, depending on the magnetic field of the white dwarf. Recent high-quality X-ray observations with the XMM-Newton and Chandra observatories have significantly advanced our understanding of the physics of the accretion process, and place severe tests on our existing models. There have been some surprises, such as the strong dependence of atmospheric heating on accretion rate. However, we believe that we are now confident that we understand in general the physical processes in the accretion region, although some complicating factors, such as absorption, remain. We also discuss new developments in ultra-short-period white-dwarf binary systems.

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., Comment: 8 pages, 7 figures, accepted for publication in MNRAS

We present high time-resolution optical photometry of the eclipsing binary UZ For using a superconducting tunnel junction (STJ) device, a photon-counting array detector with intrinsic energy resolution. Three eclipses of the $\sim$18 mag 126.5 min orbital binary were observed using a $6\times6$ array of Tantalum STJs at the 4.2-m William Herschel Telescope on La Palma. The detector presently provides individual photon arrival time accuracy to about 5 $\mu$s, and a wavelength resolution of about 60 nm at 500 nm, with each array element capable of counting up to $\sim$5000 photons s$^{-1}$. The data allow us to place accurate constraints on the accretion geometry from our time- and spectrally-resolved monitoring, especially of the eclipse ingress and egress. We find that there are two small accretion regions, located close to the poles of the white dwarf. The positions of these are accurately constrained, and show little movement from eclipse to eclipse, even over a number of years. The colour of the emission from the two regions appears similar, although their X-ray properties are known to be significantly different: we argue that the usual accretion shock may be absent at the non-X-ray emitting region, and instead the flow here interacts directly with the white dwarf surface; alternatively, a special grazing occultation of this region is required. There is no evidence for any quasi-periodic oscillations on time-scales of the order of seconds, consistent with relatively stable cyclotron cooling in each accretion region., Comment: Accepted by MNRAS: 11 pages, 8 figures

An analysis of X-ray and optical light curves of the magnetic cataclysmic variable (MCV) BY Cam is presented. This system is one of three MCVs in which the spin period of the white dwarf and the binary orbital period differ by ∼1 per cent. As such these ‘BY Cam’ stars are important objects with which to probe the field structure of the magnetic white dwarf and ultimately the nature of synchronization of AM Her binaries. We confirm asynchronous rotation of the magnetic white dwarf with respect to the binary. We find evidence that the accretion stream accretes directly on to the white dwarf as in AM Her systems, but further, the stream impacts on to different magnetic poles over the course of the beat period. We present evidence that the optical and hard X-ray light curves modulate in phase, but together they are out of phase with the soft X-ray light curve. We confirm the spin down of the white dwarf which is expected to lead to the synchronization of the spin and orbital periods of BY Cam.

One method of obtaining the mass of the white dwarf in magnetic cataclysmic variables (mCVs) is through their hard X-ray spectra. However, previous mass estimates using this method give lower limits because the temperature of the plasma in the post-shock region (where the hard X-rays are emitted) is lower than the temperature of the shock itself. In AM Her systems, the additional cooling of the post-shock plasma by cyclotron emission will further lower the derived mass. Here we present estimates of the masses of the white dwarf in 13 mCVs derived using Ginga data and a model in which X-rays are emitted from a multi-temperature emission region with the appropriate temperature and density profile. We include in the model reflection from the surface of the white dwarf and a partially ionized absorber. We are able to achieve good fits to the data. We compare the derived masses with previous estimates and the masses for larger samples of isolated white dwarfs and those in CVs.

RX J1914.4 + 2456 exhibits a light curve with a strong modulation at 569 s: this period is characteristic of the white dwarf spin period in intermediate polar (IF) systems. However, the X-ray light curve is difficult to reconcile with current models for IP emission. We argue that a simpler explanation is that the system is a polar with a degenerate secondary star, which would make it the first known system of its kind. As such it should contain the first example of an He-dominated radial accretion flow on to the white dwarf surface, and RX J1914.4 + 2456 would have the shortest known orbital period of any binary system.

We present the results of simultaneous optical polarimetry and EUV spectroscopy and photometry of PQ Gem, a magnetic cataclysmic variable which shows observational properties of the strongly magnetic AM Her class, as well as the weaker field DQ Her stars. The EUV spectrum of PQ Gem is weak, showing continuum blueward of 80 A and a few possible weak emission lines due to Mg, Si, and Ne. The EUV light curve has a similar appearance to previous X-ray data obtained for PQ Gem, including a narrow "dip" feature that is modulated with the white dwarf spin period. Observed polarization variations on the spin period, modeled by a slightly modified version of that used in Potter et al., matched the optical light curve and linear polarization curve reasonably well, but not the position angle variation. The EUV properties of PQ Gem can also be understood in the context of this model.

A method to determine the masses of white dwarfs in magnetic cataclysmic variables from X-ray observations is presented. The method is appropriate for both eclipsing and non-eclipsing systems, for it is insensitive to the orbital inclination of the systems.

We report optical and X-ray observations of the AM Her system RE J1844 - 74 using the Anglo-Australian Telescope (AAT) and ROSAT. From previously published data and these new data we derive a new accurate ephemeris for the spin period of the white dwarf. We find we cannot simultaneously fit the green and red polarization data. However, we can fit them separately using different values of the plasma parameter, Lambda. We investigate the possible reasons for this result. Furthermore, we find that the polarization data require long narrow accretion arcs. This is in contrast to results on eclipsing AM Her systems which tend to favour accretion regions that are relatively small, We briefly explore the possible effects that may influence the determination of the size of the accretion region from these different methods. We find evidence that the main accretion region of RE J1844 - 74 has a magnetic field strength similar to 10 MG less than its secondary pole, and that the system inclination is moderately high, Our X-ray data are out of phase with the optical data, consistent with our current view of the accretion flow in AM Her systems.

We show that the degree of excess soft X-ray emission measured in AM Her stars during their high intensity state is correlated with the magnetic field strength of the accretion region and the radius at which material couples on to the magnetic field lines of the white dwarf. AM Her in an intermediate intensity state, however, does not follow the same correlation and shows an apparent soft X-ray deficiency. ST LMi in a low intensity state exhibits a bremsstrahlung temperature that is over an order of magnitude lower than in the high state. One interpretation of these results is that only a small fraction of the accretion region accretes at a rate sufficiently high for a shock to form in the intermediate state so that the observed blackbody flux severely underestimates the true bolometric flux. In the low state, no shock at all forms over the accretion region.

We present the results of ROSAT pointed observations of the AM Her stars AN UMa and MR Ser. AN UMa was in a high-brightness state. We examine its soft X-ray light curve, and fit its spectrum with a two-component model consisting of an absorbed soft blackbody and a harder thermal bremsstrahlung distribution. MR Ser was found to be in a low-brightness state, and had a spectrum which was well fitted with a single-component, thermal bremsstrahlung model.The question of the energy balance in AM Her stars is reviewed with a view to quantifying the incidence of the 'soft X-ray excess'. The excess has been explained as due to the deposition of energy directly into the white dwarf photosphere by the accretion stream, supplementing the energy of photons from the hot accretion column which irradiate the photosphere. We have calculated the energy balance in AN UMa and 16 other AM Her stars observed with ROSAT. We find that, although the majority of AM Her stars exhibit a soft X-ray excess, a number of objects are consistent with a model which includes radiative heating only. We examine possible factors which could determine the value of the soft X-ray excess. We find a correlation of soft X-ray excess with magnetic field strength or, alternatively, the radius at which material couples on to the magnetic field lines.

We report the discovery of an eclipsing polar, 2XMMi J225036.9+573154, using XMM-Newton. It was discovered by searching the light curves in the 2XMMi catalogue for objects showing X-ray variability. Its X-ray light curve shows a total eclipse of the white dwarf by the secondary star every 174 mins. An extended pre-eclipse absorption dip is observed in soft X-rays at phi=0.8-0.9, with evidence for a further dip in the soft X-ray light curve at phi~0.4. Further, X-rays are seen from all orbital phases (apart from the eclipse) which makes it unusual amongst eclipsing polars. We have identified the optical counterpart, which is faint (r=21), and shows a deep eclipse (>3.5 mag in white light). Its X-ray spectrum does not show a distinct soft X-ray component which is seen in many, but not all, polars. Its optical spectrum shows Halpha in emission for a fraction of the orbital period., Accepted for publication in MNRAS

The compactness of short‐period white‐dwarf binaries allows strong electro‐magnetic interaction between their component stars. For systems with a magnetic white dwarf a large e.m.f can be generated via unipolar induction, and it drives the flows of electric currents between the two white dwarfs. The electro‐magnetic interaction between the white dwarfs and the tidal interaction between the orbit leads to spin‐orbit coupling. This alters the orbital dynamics and evolution of the system. It also modifies the gravitational waves emitted from the system. The converging of current flows at the field foot points of the magnetic white dwarf may lead to the development of particle instabilities and the generation of electron‐cyclotron masers. The unipolar induction model of ultra‐compact white‐dwarf binaries is also applicable to white‐dwarf planetary systems. The model predicts electron‐cyclotron masers from white‐dwarf planetary systems, and the masers provide a useful mean for the study of ‘dead’ solar systems.

We investigate the hydrodynamics of accretion channelled by a dipolar magnetic field (funnel flows). We consider situations in which the electrons and ions in the flow cannot maintain thermal equilibrium (two-temperature effects) due to strong radiative loss, and determine the effects on the keV X-ray properties of the systems. We apply this model to investigate the accretion shocks of white dwarfs in magnetic cataclysmic variables. We have found that the incorporation of two-temperature effects could harden the keV X-rays. Also, the dipolar model yields harder X-ray spectra than the standard planar model if white dwarf is sufficiently massive (>~1M_sun). When fitting observed keV X-ray spectra of magnetic cataclysmic variables, the inclusion of two-temperature hydrodynamics and a dipolar accretion geometry lowers estimates for white-dwarf masses when compared with masses inferred from models excluding these effects. We find mass reductions, Comment: MNRAS accepted; 13 pages, 10 figures, 2 tables

We present XMM-Newton observations of the eclipsing polar V2301 Oph which cover nearly 2.5 binary orbital cycles and 2 eclipses. This polar is believed to have the lowest magnetic field strength (7 MG) of any known polar. We find evidence for structure in the X-ray eclipse profile which shows a `standstill' feature lasting 26+/-4 sec. This allows us to place an upper limit on the mass of the white dwarf of ~1.2 Msun. We find no evidence for QPOs in the frequency range 0.02-10 Hz. This coupled with the absence of QPOs in RXTE data suggest that, if present, any oscillations in the shock front have a minimal effect on the resultant X-ray flux. We find no evidence for a distinct soft X-ray component in its spectrum - it therefore joins another 7 systems which do not show this component. We suggest that those systems which are asynchronous, have low mass transfer rates, or have accretion occurring over a relatively large fraction of the white dwarf are more likely to show this effect. We find that the specific mass transfer rate has to be close to 0.1 g cm^-2 s^-1 to predict masses which are consistent with that derived from our eclipse analysis. This maybe due to the fact that the low magnetic field strength allows accretion to take place along a wide range of azimuth., Comment: Accepted MNRAS

The intermediate polar PQ Geminorum shows a complex pulsation, caused by a spinning white dwarf, which varies markedly with wavelength. We report XMM-Newton observations, including the soft and hard X-ray bands and the first UV lightcurves of this star. We update the ephemeris for PQ Gem allowing us to align these data with a compilation of lightcurves from the optical to the X-ray. Building on work by previous authors, we show how a model in which accretion flows along skewed field lines, viewed at the correct inclination, can explain the major features of the lightcurves in all bands. We discuss how the skew of the field lines relates to the spinning down of the white-dwarf rotation., 7 pages, 7 figures, accepted for publication in MNRAS

We present the results of a spectral study of the soft X-ray emission (0.2-2.5 keV) from low-latitude (`disk') regions of Jupiter. The data were obtained during two observing campaigns with XMM-Newton in April and November 2003. While the level of the emission remained approximately the same between April and the first half of the November observation, the second part of the latter shows an enhancement by about 40% in the 0.2-2.5 keV flux. A very similar, and apparently correlated increase, in time and scale, was observed in the solar X-ray and EUV flux. The months of October and November 2003 saw a period of particularly intense solar activity, which appears reflected in the behaviour of the soft X-rays from Jupiter's disk. The X-ray spectra, from the XMM-Newton EPIC CCD cameras, are all well fitted by a coronal model with temperatures in the range 0.4-0.5 keV, with additional line emission from Mg XI (1.35 keV) and Si XIII (1.86 keV): these are characteristic lines of solar X-ray spectra at maximum activity and during flares. The XMM-Newton observations lend further support to the theory that Jupiter's disk X-ray emission is controlled by the Sun, and may be produced in large part by scattering, elastic and fluorescent, of solar X-rays in the upper atmosphere of the planet., Comment: 17 pages, 7 figures, accepted for publication in a special issue of Planetary and Space Science

We have carried out a Chandra observation of the globular cluster M54. We detected 7 sources located within the half-mass radius of M54, at a flux limit of 1.5x10^-15 erg/s/cm in the 0.3-8 keV energy band. The spatial distribution and the colour/spectral properties of the 7 sources suggest that they are likely to be cataclysmic variables or LMXBs in the globular cluster. M54 shows the largest number of X-ray sources with luminosities greater than 10^32 ergs/s compared to other globular clusters observed using Chandra and XMM-Newton. We searched for a correlation between the number of sources above this luminosity level with globular cluster parameters. We found evidence that the number of sources peaks at a King concentration parameter c~1.7-1.9, with globular clusters which are core-collapsed or have low-c values having a smaller number of sources. We speculate on possible reasons for this., Accepted A&A, 5 pages

V407 Vul and RX J0806.3+1527 are X-ray emitting stars with X-ray light curves that are 100% modulated on periods of 569 and 321 seconds respectively. These periods and no others are also seen at optical and infra-red wavelengths. These properties have led to the suggestion that the periods are the orbital period of ultra-compact pairs of white dwarfs. There are two main double white dwarf models: the unipolar inductor model analogous to the Jupiter-Io system and the direct impact model analogous to Algol. In this paper we consider geometrical constraints on the unipolar inductor model, in particular what parameter values (component masses, orbital inclination and magnetic co-latitude) can describe the X-ray and optical light curves. We find that for a dipole field on the primary star, the unipolar inductor model fails to match the data on V407 Vul for any combination of parameters, and can only match RX 0806.3+1527 if the sparser set of observations of this star have been unluckily timed., Comment: 7 pages, 7 figures (figure 6 has been compressed), accepted for publication in MNRAS