arXiv subjects

P. J. Meintjes

Publications and source records attributed to P. J. Meintjes.

At least 19 recordsLinked to original sources

arXiv2026

Ellipsoidal modulation and multi-wavelength activity in the pre-cataclysmic binary RX J1553.0+4457

RX J1553.0+4457 (TMTS J15530469+4457458) is a detached post-common-envelope binary containing a cool white dwarf and an active late-type companion. We present a multi-wavelength study combining BOOTES multi-band photometry, six sectors of public TESS full-frame imaging, Einstein Probe/FXT X-ray observations, CAFOS optical spectroscopy, and archival ultraviolet-to-mid-infrared photometry. The BOOTES data reveal two short optical flares separated by about 3 h, with amplitudes of roughly 1-1.5 mag and faster decay at shorter wavelengths. The combined TESS light curve shows a stable signal at P = 0.083782 d, consistent with the first harmonic of the known spectroscopic orbital period, and its folded morphology indicates ellipsoidal modulation from a tidally distorted late-type companion rather than an irradiation-dominated waveform. TESS flare energetics lie in the active M-dwarf regime. The Einstein Probe/FXT spectra show a factor of about 4 decline in the 0.3-10 keV flux, mainly associated with decreasing emission measures. The broadband SED is reproduced by a cool white dwarf plus a late-type M dwarf, with no clear hot-continuum or mid-infrared excess. RX J1553.0+4457 is therefore best interpreted as a detached pre-cataclysmic binary whose rapid optical variability is dominated by magnetic activity and whose orbital modulation is ellipsoidal, although a weak wind-fed or intermittent accretion contribution remains possible.

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arXiv2026

Stochastic Optical Variability and an rms-flux Relation in the Intermediate Polar EP240309a

Magnetic cataclysmic variables provide a natural laboratory for studying how accretion interacts with compact-object magnetospheres and generates stochastic variability. We present an optical variability study of the intermediate-polar candidate EP240309a, an Einstein Probe X-ray transient, using BOOTES photometry, high-cadence TESS light curves, and a SOAR/Goodman optical spectrum. Previous studies found a white-dwarf spin period of 3.97 min (Pspin ~ 238 s) and an orbital period of Porb = 3.7614(4) h. Power spectral densities from the BOOTES data are consistent with single power laws with slopes alpha ~ 1.2-1.8, with no statistically significant evidence for a bend across the sampled frequency range. Using red-noise simulations and injection-recovery tests, we place one-sided constraints on any putative break frequency, which translate, under standard dynamical identifications, into an upper limit on the magnetospheric radius of Rm <= few x 10^10 cm for MWD = 0.8 Msun. In the TESS data, we detect a linear rms-flux relation on hour timescales in three high-cadence sectors, while two other sectors do not show a robust detection, indicating epoch-dependent rms-flux behavior. The SOAR spectrum shows Balmer and He II emission lines with FWHM about 1000-1600 km s^-1; under a Keplerian interpretation, these imply characteristic radii of r about (0.9-3.4) x 10^10 cm, broadly comparable to the timing-based constraints. Overall, the data provide conservative, order-of-magnitude radius constraints consistent with accretion onto a magnetic white dwarf, but they do not establish the detailed accretion geometry or exclude stream-fed or mixed accretion scenarios.

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arXiv2024

H.E.S.S. observations of the 2021 periastron passage of PSR B1259-63/LS 2883

PSR B1259-63 is a gamma-ray binary system that hosts a pulsar in an eccentric orbit, with a 3.4 year period, around an O9.5Ve star. At orbital phases close to periastron passages, the system radiates bright and variable non-thermal emission. We report on an extensive VHE observation campaign conducted with the High Energy Stereoscopic System, comprised of ~100 hours of data taken from $t_p-24$ days to $t_p+127$ days around the system's 2021 periastron passage. We also present the timing and spectral analyses of the source. The VHE light curve in 2021 is consistent with the stacked light curve of all previous observations. Within the light curve, we report a VHE maximum at times coincident with the third X-ray peak first detected in the 2021 X-ray light curve. In the light curve -- although sparsely sampled in this time period -- we see no VHE enhancement during the second disc crossing. In addition, we see no correspondence to the 2021 GeV flare in the VHE light curve. The VHE spectrum obtained from the analysis of the 2021 dataset is best described by a power law of spectral index $Γ= 2.65 \pm 0.04_{\text{stat}}$ $\pm 0.04_{\text{sys}}$, a value consistent with the previous H.E.S.S. observations of the source. We report spectral variability with a difference of $ΔΓ= 0.56 ~\pm~ 0.18_{\text{stat}}$ $~\pm~0.10_{\text{sys}}$ at 95% c.l., between sub-periods of the 2021 dataset. We also find a linear correlation between contemporaneous flux values of X-ray and TeV datasets, detected mainly after $t_p+25$ days, suggesting a change in the available energy for non-thermal radiation processes. We detect no significant correlation between GeV and TeV flux points, within the uncertainties of the measurements, from $\sim t_p-23$ days to $\sim t_p+126$ days. This suggests that the GeV and TeV emission originate from different electron populations.

astro-ph.HE↗

arXiv2024

Evidence for gamma-ray emission from the remnant of Kepler's supernova based on deep H.E.S.S. observations

Observations with imaging atmospheric Cherenkov telescopes (IACTs) have enhanced our knowledge of nearby supernova (SN) remnants with ages younger than 500 years by establishing Cassiopeia A and the remnant of Tycho's SN as very-high-energy (VHE) gamma-ray sources. The remnant of Kepler's SN, which is the product of the most recent naked-eye supernova in our Galaxy, is comparable in age to the other two, but is significantly more distant. If the gamma-ray luminosities of the remnants of Tycho's and Kepler's SNe are similar, then the latter is expected to be one of the faintest gamma-ray sources within reach of the current generation IACT arrays. Here we report evidence at a statistical level of 4.6 sigma for a VHE signal from the remnant of Kepler's SN based on deep observations by the High Energy Stereoscopic System (H.E.S.S.) with an exposure of 152 hours. The measured integral flux above an energy of 226 GeV is ~0.3% of the flux of the Crab Nebula. The spectral energy distribution (SED) reveals a gamma-ray emitting component connecting the VHE emission observed with H.E.S.S. to the emission observed at GeV energies with Fermi-LAT. The overall SED is similar to that of the remnant of Tycho's SN, possibly indicating the same non-thermal emission processes acting in both these young remnants of thermonuclear SNe.

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arXiv2023

Detection of extended gamma-ray emission around the Geminga pulsar with H.E.S.S

Geminga is an enigmatic radio-quiet gamma-ray pulsar located at a mere 250 pc distance from Earth. Extended very-high-energy gamma-ray emission around the pulsar was discovered by Milagro and later confirmed by HAWC, which are both water Cherenkov detector-based experiments. However, evidence for the Geminga pulsar wind nebula in gamma rays has long evaded detection by imaging atmospheric Cherenkov telescopes (IACTs) despite targeted observations. The detection of gamma-ray emission on angular scales > 2 deg poses a considerable challenge for the background estimation in IACT data analysis. With recent developments in understanding the complementary background estimation techniques of water Cherenkov and atmospheric Cherenkov instruments, the H.E.S.S. IACT array can now confirm the detection of highly extended gamma-ray emission around the Geminga pulsar with a radius of at least 3 deg in the energy range 0.5-40 TeV. We find no indications for statistically significant asymmetries or energy-dependent morphology. A flux normalisation of $(2.8\pm0.7)\times10^{-12}$ cm$^{-2}$s$^{-1}$TeV$^{-1}$ at 1 TeV is obtained within a 1 deg radius region around the pulsar. To investigate the particle transport within the halo of energetic leptons around the pulsar, we fitted an electron diffusion model to the data. The normalisation of the diffusion coefficient obtained of $D_0 = 7.6^{+1.5}_{-1.2} \times 10^{27}$ cm$^2$s$^{-1}$, at an electron energy of 100 TeV, is compatible with values previously reported for the pulsar halo around Geminga, which is considerably below the Galactic average.

astro-ph.HE↗

arXiv2021

Probing the Non-thermal Emission Geometry of AR Sco via Optical Phase-Resolved Polarimetry

AR Sco is a binary system that contains a white and red dwarf. The rotation rate of the white dwarf has been observed to slow down, analogous to rotation-powered radio pulsars; it has thus been dubbed a "white dwarf pulsar". We previously fit the traditional radio pulsar rotating vector model to the linearly polarised optical data from this source, constraining the system geometry as well as the white dwarf mass. Using a much more extensive dataset, we now explore the application of the same model to binary phase-resolved optical polarimetric data, thought to be the result of non-thermal synchrotron radiation, and derive the magnetic inclination angle $α$ and the observer angle $ζ$ at different orbital phases. We obtain a $\sim 10^{\circ}$ variation in $α$ and $\sim 30^{\circ}$ variation in $ζ$ over the orbital period. The variation patterns in these two parameters is robust, regardless of the binning and epoch of data used. We speculate that the observer is detecting radiation from an asymmetric emission region that is a stable structure over several orbital periods. The success of this simple model lastly implies that the pitch angles of the particles are small and the pulsed, non-thermal emission originates relatively close to the white dwarf surface.

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arXiv2021

H.E.S.S. follow-up observations of Binary Black Hole Coalescence events during the second and third Gravitational Waves observing runs of Advanced LIGO and Advanced Virgo

We report on the observations of four well-localized binary black hole (BBH) mergers by the High Energy Stereoscopic System (H.E.S.S.) during the second and third observing runs of Advanced LIGO and Advanced Virgo, O2 and O3. H.E.S.S. can observe $\mathrm{20\,deg^2}$ of the sky at a time and follows up gravitational-wave (GW) events by ``tiling'' localization regions to maximize the covered localization probability. During O2 and O3, H.E.S.S. observed large portions of the localization regions, between 35\% and 75\%, for four BBH mergers (GW170814, GW190512\_180714, GW190728\_064510, and S200224ca). For these four GW events, we find no significant signal from a pointlike source in any of the observations, and set upper limits on the very high energy ($>$100 GeV) $γ$-ray emission. The 1-10 TeV isotropic luminosity of these GW events is below $10^{45}$ erg s$^{-1}$ at the times of the H.E.S.S. observations, around the level of the low-luminosity GRB 190829A. Assuming no changes are made to how follow-up observations are conducted, H.E.S.S. can expect to observe over 60 GW events per year in the fourth GW observing run, O4, of which eight would be observable with minimal latency.

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arXiv2021

Probing the evolution of the EBL photon density out to $z~\sim 1$ via $γ$-ray propagation measurements with Fermi

The redshift ($z$) evolution of the Extragalactic Background Light (EBL) photon density is very important to understand the history of cosmological structure formation of galaxies and stars since the epoch of recombination. The EBL photons with the characteristic spectral energy distribution ranging from ultraviolet/optical to far-infrared provide a major source of opacity of the Universe to the GeV-TeV $γ$-rays travelling over cosmological distances. The effect of the EBL is very significant through $γγ\rightarrow e^- e^+$ absorption process on the propagation of the $γ$-ray photons with energy $E >$ 50 GeV emitted from the sources at $z \sim 1$. This effect is characterized by the optical depth ($τ$) which strongly depends on $E$, $z$ and density of the EBL photons. The proper density of the EBL photons increases with $z$ due to expansion of the Universe whereas evolution of radiation sources contributing to the EBL leads to a decrease in the density with increasing $z$. Therefore, the resultant volumetric evolution of the EBL photon density is approximated by a modified redshift dependence. In this work, we probe evolution of the EBL photon density predicted by two prominent models using cosmic gamma-ray horizon ($τ(E,z)=$ 1) determined by the measurements from the \emph{Fermi}-Large Area Telescope (LAT) observations. The modified redshift dependence of the EBL photon density is optimized for a given EBL model by estimating the same gamma-ray horizon as predicted by the \emph{Fermi}-LAT observations. We further compare the optical depth estimates in the energy range $E =$ 4 GeV-1 TeV and redshift range $z =0.01-1$ from the \emph{Fermi}-LAT observations with the values derived from the two EBL models to further constrain the evolution of the EBL photon density in the $z~\sim 1$ Universe.

astro-ph.CO↗

arXiv2021

Properties of white dwarf in the binary system AR Scorpii and its observed features

The binary system AR Scorpii hosts an M-type main sequence cool star orbiting around a magnetic white dwarf in the Milky Way Galaxy. The broadband non-thermal emission over radio, optical and X-ray wavebands observed from AR Scorpii indicates strong modulations on the spin frequency of the white dwarf as well as the spin-orbit beat frequency of the system. Therefore, AR Scorpii is also referred to as a white dwarf pulsar wherein a fast spinning white dwarf star plays very crucial role in the broadband non-thermal emission. Several interpretations for the observed features of AR Scorpii appear in the literature without firm conclusions. In this work, we investigate connection between some of the important physical properties like spin-down power, surface magnetic field, equation of state, temperature and gravity associated with the white dwarf in the binary system AR Scorpii and its observational characteristics. We explore the plausible effects of white dwarf surface magentic field on the absence of substantial accretion in this binary system and also discuss the gravitational wave emission due to magnetic deformation mechanism.

astro-ph.HE↗

arXiv2020

Modelling the broadband emission from the white dwarf binary system AR Scorpii

In this work, we have analyzed the $γ$-ray data in the energy range 100 MeV to 500 GeV from the \emph{Fermi}-Large Area Telescope (LAT) observations for the period August 4, 2008 to March 31, 2019. The $γ$-ray emission from AR Scorpii over the last decade is not statistically significant and therefore 2$σ$ upper limit on the integral flux above 100 MeV has been estimated. We reproduce the non-thermal broadband spectral energy distribution of AR Scorpii using an emission model having two synchrotron components due to the relativistic electrons in very high magnetic fields. The first component (Synchrotron-1) broadly describes the emissions at radio to high energy X-rays through the synchrotron radiation originating from a spherical region of radius $\sim$ 1.8$\times$10$^{10}$ cm and a magnetic field strength of $\sim$ 10$^3$ Gauss. The second component (Synchrotron-2) which reproduces the X-ray emission at lower energies and predicts the $γ$-ray emission, originates from another spherical region with radius $\sim$ 1.4$\times$10$^{10}$ cm and a magnetic field strength of $\sim$ 10$^6$ Gauss. The relativistic electron populations in both the emission regions are described by a smooth broken power law energy distribution. The $γ$-ray emission predicted by the Synchrotron-2 model is below the broadband sensitivity of the \emph{Fermi}-LAT and is also consistent with the 95$\%$ confidence level upper limit on the integral flux above 100 MeV derived from more than 10 years of observations. According to our model, the binary system AR Scorpii could be a $γ$-ray source, although its emission level must be below the current detection limit of the \emph{Fermi}-LAT.

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arXiv2020

Characterization of Variability in Blazar Light curves

Blazars represent dominant population of the extragalactic $γ$-ray sources in the Universe. These sources exhibit some characteristic properties like strong and non-thermal continuum emission over the entire electromagnetic spectrum from radio to TeV $γ$-rays with rapid variability on all timescales. The emission at radio and optical wavelengths is highly polarized with significant variation. The fastest variability in the blazar emission is observed during the flaring activity which is an important observational property of blazars. In this paper, we describe various methods to characterize the temporal variability in the multi-wavelength light curves of blazars. We also provide a detailed description of the set of statistical parameters which are used to quantify the level of variability present in the time-series. Implications of the informations derived from the variability study to probe the physics of blazars using multi-wavelength observations are also discussed.

astro-ph.HE↗

arXiv2020

Extragalactic background light models and GeV-TeV observation of blazars

In this work, we use two different methods to determine the opacity of the TeV gamma-rays caused by the extragalactic background light (EBL) via e-e+ production due to photon-photon interaction. The first method, Model-Dependent Approach, uses various EBL models for estimating the opacity as a function of the redshift and energy of the TeV photons. The second method, Model-Independent Approach, relies on using the simultaneous observations of blazars in the MeV-GeV energy range from the Fermi-LAT and in the TeV band from the ground-based gamma-ray telescopes. We make the underline assumption that the extrapolation of the LAT spectrum of blazars to TeV energies is either a good estimate or an upper limit for the intrinsic TeV spectrum of a source. We apply this method on the simultaneous observations of a few blazars at different redshifts to demonstrate a comparative study of six prominent EBL models. Opacities of the TeV photons predicted by the model-independent approach are systematically larger than the ones estimated from the model-dependent method. Therefore, the gamma-ray observations of blazars can be used to set a strict upper limit on the opacity of the Universe to the TeV photons at a given redshift.

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arXiv2020

Understanding the giant gamma-ray outburst on June 16, 2015 from the blazar 3C 279

A very bright and fast varying gamma-ray flare has been detected from the blazar 3C 279 on June 16, 2015. We have studied the broadband spectral energy distribution of the source during the flaring episode and in the low activity state using a simple one zone leptonic model. We find that an electron energy distribution described by a broken power law can be used to reproduce the broadband emissions during the high and low activity states. The flux measurements at radio, infrared and optical frequencies are reproduced by the synchrotron emission resulting from the relativistic electrons in a jet magnetic field strength of 0.37 G. The gamma-ray emission from the blazar 3C 279 is attributed to the Comptonization of the IR seed photons from the dusty torus with a temperature of 870 K. The outburst from the source observed on June 16, 2015 can be ascribed to an efficient acceleration process associated with a sudden enhancement in the electron energy density in the emitting region with respect to the low activity state. The fast gamma-ray variability at a minute timescale implies that the emission during the flare originates from a more compact region and the size of the emission zone in the low activity state is found to be four times larger than that during the flare. We have also used the model parameters derived from the broadband spectral energy distribution modelling to investigate a few physical properties of the jet during the outburst.

astro-ph.HE↗

arXiv2019

Long term multi-wavelength view of the blazar 1ES 1218+304

In this work, we present a multi-wavelength study of the blazar 1ES 1218+304 using near simultaneous observations over 10 years during the period September 1, 2008 to August 31, 2018 (MJD 54710-58361). We have analyzed data from \emph{Swift}-UVOT, \emph{Swift}-XRT and \emph{Fermi}-LAT to study the long term behaviour of 1ES 1218+304 in different energy bands over the last decade. We have also used the archival data from OVRO, MAXI and \emph{Swift}-BAT available during the above period. The near simultaneous data on 1ES 1218+304 suggest that the long term multi-wavelength emission from the source is steady and does not show any significant change in the source activity. The optical/UV fluxes are found to be dominated by the host galaxy emission and can be modelled using the $PEGASE$ code. However, the time averaged X-ray and $γ$-ray emisions from the source are reproduced using a single zone leptonic model with log-parabolic distribution for the radiating particles. The intrinsic very high energy $γ$-ray emission during a low activity state of the source is broadly consistent with the predictions of the leptonic model for blazars. We have investigated the physical properties of the jet and the mass of the super massive black hole at the center of the host galaxy using long term X-ray observations from the \emph{Swift}-XRT which is in agreement with the value derived using blackbody approximation of the host galaxy. We also discuss the extreme nature of the source on the basis of X-ray and $γ$-ray observations.

astro-ph.HE↗

arXiv2019

Results from the long-term Fermi-LAT observations of the white dwarf binary pulsar AR Scorpii

The discovery of the white dwarf binary system AR Scorpii (AR Sco) with its fascinating non-thermal dominated multi-frequency emission has sparked renewed interest in potential high energy gamma-ray emission from white dwarf pulsars. The Spectral Energy Distribution (SED) below and above optical shows evidence of non-thermal synchrotron emission, with pulsed emission in optical and X-ray bands at the white dwarf spin period ($P_{*} = 117 \, \mbox{s}$) as well as a beat period ($P_{\rm b} = 118.1 \, \mbox{s}$) with the binary period. From an energy perspective, the highly magnetic rotating white dwarf can accelerate particles to TeV energies. In this study, a search for high energy gamma-ray emission was conducted between 100 MeV - 500 GeV by analysing the newly available Fermi-LAT Pass 8 data with the new Fermi 1.0.1 Science Tools. Binned likelihood analysis was done using power law, broken power law and log parabola models. From the selected Region of Interest (ROI) centred on AR Sco's position, we calculated a significance of $\sqrt{TS}\leq$ 3.87$σ$ for the integrated gamma-ray activity between 100 MeV - 500 GeV at a photon flux level of 0.486 $\pm$ 0.261 x 10$^{-8}$ photons cm$^{-2}$ s$^{-1}$ using the broken power law model. This resulted in a 3$σ$ upper-limit detection from the position of AR Sco. The location of AR Sco inside the Rho Ophiuchi (Rho Oph) molecular cloud complex combined with the poor spatial resolution of Fermi-LAT, complicates any positive identification of low-level gamma-ray activity at the location that coincides with the position of AR Sco.

astro-ph.HE↗

arXiv2019

Intrinsic GeV-TeV gamma-ray emission from EHSP blazars

Extremely High Synchrotron Peak (EHSP) blazars are observed to form a small population of sources with high energy hump peaking at TeV energies in their broad-band spectral energy distributions. The observed gamma-ray emission from these sources at GeV-TeV energies is described by unusual hard spectral indices. The observed spectral and temporal characteristics of these sources challenge the standard leptonic models for the broad-band emissions from blazars. Therefore, such sources provide astrophysical sites to investigate directly the particle acceleration, cooling of relativistic particles and indirectly probe the cosmological quantities like extragalactic background light (EBL) and intergalactic magnetic field (IMF) in the Universe. In this study, we investigate the spectral properties of the gamma-ray emission from EHSP blazars using observations from the Fermi-LAT catalogues (3FGL and 3FHL) along with the TeV observations using ground based telescopes. The observed TeV gamma-ray spectra are corrected for the EBL absorption using the most recent and updated EBL models to determine the intrinsic spectrum at the source. The intrinsic TeV spectra are combined with the MeV-GeV observations from the Fermi-LAT to study the gamma-ray emission from EHSP blazars in the broad energy band. The intrinsic gamma-ray spectra are then used to estimate the position of high energy peak in the spectral energy distribution. We also present the qualitative description for the observed spectral properties of EHSP blazars using different physical scenarios and discuss their importance for the upcoming CTA observatory.

astro-ph.HE↗

arXiv2018

X-shooter and ALMA spectroscopy of GRB 161023A - A study of metals and molecules in the line of sight towards a luminous GRB

Long gamma-ray bursts are produced during the dramatic deaths of massive stars with very short lifetimes, meaning that they explode close to the birth place of their progenitors. During a short period they become the most luminous objects observable in the Universe, being perfect beacons to study high-redshift star-forming regions. To use the afterglow of GRB 161023A at a redshift $z=2.710$ as a background source to study the environment of the explosion and the intervening systems along its line-of-sight. r the first time, we complement UV/Optical/NIR spectroscopy with millimetre spectroscopy using ALMA, which allows us to probe the molecular content of the host galaxy. The X-shooter spectrum shows a plethora of absorption features including fine-structure and metastable transitions of Fe, Ni, Si, C and O. We present photometry ranging from 43 s to over 500 days after the burst. We infer a host-galaxy metallicity of [Zn/H] $=-1.11\pm0.07$, which corrected for dust depletion results in [X/H] $=-0.94\pm0.08$. We do not detect molecular features in the ALMA data, but we derive limits on the molecular content of $log(N_{CO}/cm^{-2})<15.7$ and $log(N_{HCO+}/cm^{-2})<13.2$, which are consistent with those that we obtain from the optical spectra, $log(N_{H_2}/cm^{-2})<15.2$ and $log(N_{CO}/cm^{-2})<14.5$. Within the host galaxy we detect three velocity systems through UV/Optical/NIR absorption spectroscopy, all with levels that were excited by the GRB afterglow. We determine the distance from these systems to the GRB to be in the range between 0.7 and 1.0 kpc. The sight-line to GRB 161023A shows 9 independent intervening systems, most of them with multiple components. (Abridged)

astro-ph.HE↗

arXiv2017

An eLIMA model for the 67 s X-ray periodicity in CAL 83

Supersoft X-ray sources (SSSs) are characterized by their low effective temperatures and high X-ray luminosities. The soft X-ray emission can be explained by hydrogen nuclear burning on the surface of a white dwarf (WD) accreting at an extremely high rate. A peculiar 67 s periodicity (P67) was previously discovered in the XMM-Newton light curves of the SSS CAL 83. P67 was detected in X-ray light curves spanning 9 years, but exhibits variability of several seconds on time-scales as short as a few hours, and its properties are remarkably similar to those of dwarf nova oscillations (DNOs). DNOs are short time-scale modulations often observed in dwarf novae during outburst. DNOs are explained by the well established low-inertia mag- netic accretor (LIMA) model. In this paper, we show that P67 and its associated period variability can be satisfactorily explained by an application of the LIMA model to the more extreme environment in a SSS (eLIMA), contrary to another recent study at- tempting to explain P67 and its associated variability in terms of non-radial g-mode oscillations in the extended envelope of the rapidly accreting white dwarf in CAL 83. In the eLIMA model, P67 originates in an equatorial belt in the WD envelope at the boundary with the inner accretion disc, with the belt weakly coupled to the WD core by a 100 000 G magnetic field. New optical light curves obtained with the Sutherland High-speed Optical Camera (SHOC) are also presented, exhibiting quasi-periodic modulations on time-scales of 1000 s, compatible with the eLIMA framework.

astro-ph.HE↗