arXiv subjects

Magaretha L. Pretorius

Publications and source records attributed to Magaretha L. Pretorius.

At least 19 recordsLinked to original sources

arXiv2023

The Orbital Period vs. Absolute Magnitude Relationship of Intermediate Polars: Implications for Low States and Outbursts

Recent advances in time-domain astronomy have led to fresh observational insights into intermediate polars, a subtype of magnetic cataclysmic variables generally accreting via a partial accretion disc. These new discoveries include detections of superhumps, low states, and outbursts. However, these studies have largely relied on relative photometry. Here we tabulate the absolute G magnitudes of confirmed intermediate polars, plot them against their orbital periods, and compare the results to similar studies of dwarf novae during quiescence and in outburst. This exercise suggests the presence of two distinct luminosity classes of intermediate polars, with practical and physical implications for the studies of low states and outbursts. In particular, we point out that two of the optically luminous systems showing short outbursts are also seen to exhibit superhumps, suggesting that they may be caused by the same underlying mechanism.

astro-ph.HE↗

arXiv2017

A comparison between SALT/SAAO observations and kilonova models for AT 2017gfo: the first electromagnetic counterpart of a gravitational wave transient - GW170817

We report on SALT low resolution optical spectroscopy and optical/IR photometry undertaken with other SAAO telescopes (MASTER-SAAO and IRSF) of the kilonova AT 2017gfo (aka SSS17a) in the galaxy NGC4993 during the first 10 days of discovery. This event has been identified as the first ever electromagnetic counterpart of a gravitational wave event, namely GW170817, which was detected by the LIGO and Virgo gravitational wave observatories. The event is likely due to a merger of two neutron stars, resulting in a kilonova explosion. SALT was the third telescope to obtain spectroscopy of AT 2017gfo and the first spectrum, 1.2 d after the merger, is quite blue and shows some broad features, but no identifiable spectral lines and becomes redder over time. We compare the spectral and photometric evolution with recent kilonova simulations and conclude that they are in qualitative agreement for post-merger wind models with proton: nucleon ratios of $Y_e$ = 0.25$-$0.30. The blue colour of the first spectrum is consistent with the lower opacity of the Lathanide-free r-process elements in the ejecta. Differences between the models and observations are likely due to the choice of system parameters combined with the absence of atomic data for more elements in the ejecta models.

astro-ph.HE↗

arXiv2016

An irradiated brown-dwarf companion to an accreting white dwarf

Brown dwarfs and giant planets orbiting close to a host star are subjected to significant irradiation that can modify the properties of their atmospheres. In order to test the atmospheric models that are used to describe these systems, it is necessary to obtain accurate observational estimates of their physical properties (masses, radii, temperatures, albedos). Interacting compact binary systems provide a natural laboratory for studying strongly irradiated sub-stellar objects. As the mass-losing secondary in these systems makes a critical, but poorly understood transition from the stellar to the sub-stellar regime, it is also strongly irradiated by the compact accretor. In fact, the internal and external energy fluxes are both expected to be comparable in these objects, providing access to an unexplored irradiation regime. However, the atmospheric properties of such donors have so far remained largely unknown. Here, we report the direct spectroscopic detection and characterisation of an irradiated sub-stellar donor in an accreting white dwarf binary system. Our near-infrared observations allow us to determine a model-independent mass estimate for the donor of $M_2=0.055\pm0.008M_{\odot}$ and an average spectral type of ${\rm L1}\pm{\rm1}$, supporting both theoretical predictions and model-dependent observational constraints. Our time-resolved data also allow us to estimate the average irradiation-induced temperature difference between the day and night sides on the sub-stellar donor, $Δ{\rm T}\simeq57$~K, and the maximum difference between the hottest and coolest parts of its surface, of $Δ{\rm T}_{max}\simeq200$~K. The observations are well described by a simple geometric reprocessing model with a bolometric (Bond) albedo of $A_B<0.54$ at 2-$σ$ confidence level, consistent with high reprocessing efficiency, but poor lateral heat redistribution in the donor's atmosphere.

astro-ph.SR↗

arXiv2014

The space density of magnetic and non-magnetic cataclysmic variables, and implications for CV evolution

We present constraints on the space densities of both non-magnetic and magnetic cataclysmic variables, and discuss some implications for models of the evolution of CVs. The high predicted non-magnetic CV space density is only consistent with observations if the majority of these systems are extremely faint in X-rays. The data are consistent with the very simple model where long-period IPs evolve into polars and account for the whole short-period polar population. The fraction of WDs that are strongly magnetic is not significantly higher for CV primaries than for isolated WDs. Finally, the space density of IPs is sufficiently high to explain the bright, hard X-ray Galactic Centre source population.

astro-ph.SR↗

arXiv2014

Constraints on the space density of intermediate polars from the Swift-BAT survey

We construct a complete, hard X-ray flux-limited sample of intermediate polars (IPs) from the Swift-BAT 70-month survey, by imposing selection cuts in flux and Galactic latitude ($F_X > 2.5 \times 10^{-11}\,\mathrm{erg\,cm^{-2}s^{-1}}$ at 14--195~keV, and $|b|>5^\circ$). We then use it to estimate the space density ($ρ$) of IPs. Assuming that this sample of 15 long-period systems is representative of the intrinsic IP population, the space density of long-period IPs is $1^{+1}_{-0.5} \times 10^{-7}\,\mathrm{pc^{-3}}$. The Swift-BAT data also allow us to place upper limits on the size of a hypothetical population of faint IPs that is not included in the flux-limited sample. While most IPs detected by BAT have 14--195~keV luminosities of $\sim 10^{33} {\rm erg s^{-1}}$, there is evidence of a fainter population at $L_X \sim 10^{31} {\rm erg s^{-1}}$. We find that a population of IPs with this luminosity may have a space density as large as $5\times 10^{-6}\,\mathrm{pc^{-3}}$. Furthermore, these low-luminosity IPs, despite appearing rare in observed samples, are probably at least as intrinsically common as the brighter systems that are better represented in the known IP sample.

astro-ph.SR↗

arXiv2013

The space density of magnetic cataclysmic variables

We use the complete, X-ray flux-limited ROSAT Bright Survey (RBS) to measure the space density of magnetic cataclysmic variables (mCVs). The survey provides complete optical identification of all sources with count rate >0.2/s over half the sky ($|b|>30^\circ$), and detected 6 intermediate polars (IPs) and 24 polars. If we assume that the 30 mCVs included in the RBS are representative of the intrinsic population, the space density of mCVs is $8^{+4}_{-2} \times 10^{-7}\,{\rmpc^{-3}}$. Considering polars and IPs separately, we find $ρ_{polar}=5^{+3}_{-2} \times 10^{-7}\,{\rm pc^{-3}}$ and $ρ_{IP}=3^{+2}_{-1} \times 10^{-7}\,{\rm pc^{-3}}$. Allowing for a 50% high-state duty cycle for polars (and assuming that these systems are below the RBS detection limit during their low states) doubles our estimate of $ρ_{polar}$ and brings the total space density of mCVs to $1.3^{+0.6}_{-0.4} \times 10^{-6}\,{\rm pc^{-3}}$. We also place upper limits on the sizes of faint (but persistent) mCV populations that might have escaped detection in the RBS. Although the large uncertainties in the $ρ$ estimates prevent us from drawing strong conclusions, we discuss the implications of our results for the evolutionary relationship between IPs and polars, the fraction of CVs with strongly magnetic white dwarfs (WDs), and for the contribution of mCVs to Galactic populations of hard X-ray sources at $L_X \ga 10^{31} {\rm erg/s}$. Our space density estimates are consistent with the very simple model where long-period IPs evolve into polars and account for the whole short-period polar population. We find that the fraction of WDs that are strongly magnetic is not significantly higher for CV primaries than for isolated WDs. Finally, the space density of IPs is sufficiently high to explain the bright, hard X-ray source population in the Galactic Centre.

astro-ph.SR↗

arXiv2011

The space density and X-ray luminosity function of non-magnetic cataclysmic variables

We combine two complete, X-ray flux-limited surveys, the ROSAT Bright Survey (RBS) and the ROSAT North Ecliptic Pole (NEP) survey, to measure the space density (ρ) and X-ray luminosity function (Φ) of non-magnetic CVs. The combined survey has a flux limit of F_X \ga 1.1 \times 10^{-12} erg cm^{-2}s^{-1} over most of its solid angle of just over 2π, but is as deep as \simeq 10^{-14} erg cm^{-2}s^{-1} over a small area. The CV sample that we construct from these two surveys contains 20 non-magnetic systems. We carefully include all sources of statistical error in calculating ρand Φby using Monte Carlo simulations; the most important uncertainty proves to be the often large errors in distances estimates. If we assume that the 20 CVs in the combined RBS and NEP survey sample are representative of the intrinsic population, the space density of non-magnetic CVs is 4^{+6}_{-2} \times 10^{-6} pc^{-3}. We discuss the difficulty in measuring Φin some detail---in order to account for biases in the measurement, we have to adopt a functional form for Φ. Assuming that the X-ray luminosity function of non-magnetic CVs is a truncated power law, we constrain the power law index to -0.80 \pm 0.05. It seems likely that the two surveys have failed to detect a large, faint population of short-period CVs, and that the true space density may well be a factor of 2 or 3 larger than what we have measured; this is possible, even if we only allow for undetected CVs to have X-ray luminosities in the narrow range 28.7< log(L_X/erg\,s^{-1})<29.7. However, ρas high as 2 \times 10^{-4} pc^{-3} would require that the majority of CVs has X-ray luminosities below L_X = 4 \times 10^{28} erg s^{-1} in the 0.5--2.0 keV band.

astro-ph.SR↗

arXiv2011

Rapid variations of polarization in low-mass X-ray binaries

Time-resolved optical and infrared polarimetric observations of black hole and neutron star low-mass X-ray binaries are presented. Data were acquired with the VLT, UKIRT and HIPPO on the SAAO 1.9-m. We find that for some sources in outburst, a rapidly variable component of polarization is evident that is stronger in the redder wavebands. We attribute this to the polarimetric signature of synchrotron emission from jets in these systems, the emission of which is known to dominate these redder bands. Such synchrotron emission from jets launched close to black holes and neutron stars can be highly linearly polarized, depending on the configuration of the magnetic field. The variability of the polarization is suggestive of a tangled and turbulent magnetic field at the location of the compact jet. For some sources the position angle of polarization is consistent with a magnetic field that is parallel to the observed radio jet. These are some of the first observational constraints of the geometry and magnetic structure at the inner regions of the outflow. We also present the first ever simultaneous optical polarization and X-ray campaign of an X-ray binary, using data taken simultaneously with HIPPO and RXTE with sub-second time resolution.

astro-ph.HE↗

arXiv2009

Time-resolved optical observations of five cataclysmic variables detected by INTEGRAL

The ESA gamma-ray telescope, INTEGRAL, is detecting relatively more intrinsically rare cataclysmic variables (CVs) than were found by surveys at lower energies. Specifically, a large fraction of the CVs that are INTEGRAL sources consists of asynchronous polars and intermediate polars (IPs). IP classifications have been proposed for the majority of CVs discovered by INTEGRAL, but, in many cases, there is very little known about these systems. In order to address this, I present time-resolved optical data of five CVs discovered through INTEGRAL observations. The white dwarf spin modulation is detected in high-speed photometry of three of the new CVs (IGR J15094-6649, IGR J16500-3307, and IGR J17195-4100), but two others (XSS J12270-4859 and IGR J16167-4957) show no evidence of magnetism, and should be considered unclassified systems. Spectroscopic orbital period (P_orb) measurements are also given for IGR J15094-6649, IGR J16167-4957, IGR J16500-3307, and IGR J17195-4100.

astro-ph.SR↗

arXiv2008

An Halpha-selected sample of cataclysmic variables -- II. Implications for CV evolution

We use an independent new sample of cataclysmic variables (CVs), constructed by selecting objects for Halpha emission, to constrain the properties of the intrinsic CV population. This sample is restricted to systems that are likely to be non-magnetic and unevolved; it consists of 17 CVs, of which at least 10 have orbital periods above 3 h. We find that even very generous allowance for selection effects is not sufficient to reconcile the large ratio of short- to long-period CVs predicted by standard CV evolution theory with the observed sample, possibly implying that short-period systems evolve faster than predicted by the disrupted magnetic braking model. This would require that an angular momentum loss mechanism, besides gravitational radiation, acts on CVs with orbital periods below the period gap. To bring the model into agreement with observations, the rate of angular momentum loss below the period gap must be increased by a factor of at least 3, unless the model also over estimates the angular momentum loss rate of long-period CVs.

astro-ph↗

arXiv2008

An Halpha-selected sample of cataclysmic variables -- I. Observations of newly discovered systems

Strong selection effects are present in observational samples of cataclysmic variables (CVs), complicating comparisons to theoretical predictions. The selection criteria used to define most CV samples discriminate heavily against the discovery of short-period, intrinsically faint systems. The situation can be improved by selecting CVs for the presence of emission lines. For this reason, we have constructed a homogeneous sample of CVs selected on the basis of Halpha emission. We present discovery observations of the 14 CVs and 2 additional CV candidates found in this search. The orbital periods of 11 of the new CVs were measured; all are above 3 h. There are two eclipsing systems in the sample, and one in which we observed a quasi-periodic modulation on a \sim 1000 s time-scale. We also detect the secondary star in the spectrum of one system, and measure its spectral type. Several of the new CVs have the spectroscopic appearance of nova-like variables (NLs), and a few display what may be SW Sex star behaviour. In a companion paper, we discuss the implications of this new sample for CV evolution.

astro-ph↗

arXiv2007

Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables -- VI. Spin rates, propellering, and coherence

We examine published observations of dwarf nova oscillations (DNOs) on the rise and decline of outbursts and show that their rates of change are in reasonable agreement with those predicted from the magnetic accretion model. We find evidence for propellering in the late stages of outburst of several dwarf novae, as shown by reductions in EUVE fluxes and from rapid increases of the DNO periods. Reanalysis of DNOs observed in TY PsA, which had particularly large amplitudes, shows that the apparent loss of coherence during late decline is better described as a regular switching between two nearby periods. It is partly this and the rapid deceleration in some systems that make the DNOs harder to detect. We suggest that the 28.95 s periodicity in WZ Sge, which has long been a puzzle, is caused by heated regions in the disc, just beyond the corotation radius, which are a consequence of magnetic coupling between the primary and gas in the accretion disc. This leads to a possible new interpretation of the `longer period DNOs' (lpDNOs) commonly observed in dwarf novae and nova-like variables.

astro-ph↗

arXiv2007

Inflation after WMAP3

I discuss the current status of inflationary cosmology in light of the recent WMAP 3-year data release. The basic predictions of inflation are all supported by the data. Inflation also makes predictions which have not been well tested by current data but can be by future experiments, most notably a deviation from a scale-invariant power spectrum and the production of primordial gravitational waves. A scale-invariant spectrum is disfavored by current data, but not conclusively. Tensor modes are currently poorly constrained, and slow-roll inflation does not make an unambiguous prediction of the expected amplitude of primordial gravitational waves. A tensor/scalar ratio of $r \simeq 0.01$ is within reach of near-future measurements.

astro-ph↗

arXiv2007

Cool Companions to White Dwarf Stars from the Two Micron All Sky Survey All Sky Data Release

We present the culmination of our near-infrared survey of the optically spectroscopically identified white dwarf stars from the McCook & Sion catalog, conducted using photometric data from the Two Micron All Sky Survey final All Sky Data Release. The color-selection technique, which identifies candidate binaries containing a white dwarf and a low mass stellar (or sub-stellar) companion via their distinctive locus in the near-infrared color-color diagram, is demonstrated to be simple to apply and to yield candidates with a high rate of subsequent confirmation. We recover 105 confirmed binaries, and identify 28 firm candidates (20 of which are new to this work) and 21 tentative candidates (17 of which are new to this work) from the 2MASS data. Only a small number of candidates from our survey have likely companion spectral types later than M5, none of which is an obvious L type (i.e., potential brown dwarf) companion. Only one previously known WD + brown dwarf binary is detected. This result is discussed in the context of the 2MASS detection limits, as well as other recent observational surveys that suggest a very low rate of formation (or survival) for binary stars with extreme mass ratios.

astro-ph↗

arXiv2006

The influence of selection effects on the observed cataclysmic variable population: modelling and application to the Palomar-Green sample

Large differences between the properties of the known sample of cataclysmic variable stars (CVs) and the predictions of the theory of binary star evolution have long been recognised. However, because all existing CV samples suffer from strong selection effects, observational biases must be taken into account before it is possible to tell whether there is an inconsistency. In order to address this problem, we have modelled the impact of selection effects on observed CV samples using a Monte Carlo approach. By simulating the selection criteria of the Palomar-Green (PG) Survey, we show that selection effects cannot reconcile the predictions of standard CV evolution theory with the observed sample. More generally, we illustrate the effect of the biases that are introduced by magnitude limits, selection cuts in U-B, and restrictions in galactic latitude.

astro-ph↗

arXiv2006

Discovery of a new cataclysmic variable through optical variability and X-ray emission

Aims: We present discovery observations of the new cataclysmic variable star (CV) 1RXS J092737.4-191529, as well as spectra and photometry of SY Vol. The selection technique that turned up these two CVs is described; it should be efficient for finding dwarf novae with high outburst duty cycles. Methods: Two very common observational features of CVs, namely optical variability and X-ray emission, are combined to select targets for follow-up observations. Long-slit spectra were taken to identify CVs in the sample. Results: Two out of three objects selected in this way are CVs. One of these is the known dwarf nova SY Vol, while the second system, 1RXS J092737.4-191529, is a new discovery. We present medium resolution spectra, $UBVRI$ magnitudes, and high-speed photometry for both these CVs. Rapid flickering in the light curve of 1RXS J092737.4-191529 confirms the mass transferring binary nature of this object; it is probably a dwarf nova that was in quiescence during our observations.

astro-ph↗

arXiv2006

Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables -- V. Results from an extensive survey

We present observations of dwarf nova oscillations (DNOs), longer period dwarf nova oscillations (lpDNOs), and quasi-periodic oscillations (QPOs) in 13 cataclysmic variable stars (CVs). In the six systems WW Cet, BP CrA, BR Lup, HP Nor, AG Hya, and V1193 Ori, rapid, quasi-coherent oscillations are detected for the first time. For the remainder of the systems discussed we have observed more classes of oscillations, in addition to the rapid oscillations they were already known to display, or previously unknown aspects of the behaviour of oscillations. The period of a QPO in RU Peg is seen to change by 84% over ten nights of the decline from outburst--the largest evolution of a QPO period observed to date. A period-luminosity relation similar to the relation that has long been known to apply to DNOs is found for lpDNOs in X Leo; this is the first clear case of lpDNO frequency scaling with accretion luminosity. WX Hyi and V893 Sco are added to the small list of dwarf novae that have shown oscillations in quiescence.

astro-ph↗

arXiv2004

High speed photometry of faint Cataclysmic Variables: IV. V356 Aql, Aqr1, FIRST J1023+0038, Ha 0242-2802, GI Mon, AO Oct, V972 Oph, SDSS 0155+00, SDSS 0233+00, SDSS 1240-01, SDSS 1556-00, SDSS 2050-05, FH Ser

We present results from high speed photometry of a further thirteen faint cataclysmic variables. V356 Aql (Nova Aql 1936) shows flare-like outbursts with recurrence time scales ~ 3000 s, but no coherent periodicities. Aqr1 is an intermediate polar with a spin period of 6.7284 min and a probable orbital period P(orb) = 3.226 h derived from orbital sidebands. Its orbital sideband frequencies show very variable amplitudes. The published spectroscopic period of 2.0 h suggests that Aqr1 is similar to GW Lib and FS Aur in having an additional periodicity of unknown origin. FIRST J1023+0038 has P(orb) = 4.7548 h with an orbital modulation of range 0.45 mag, probably caused by reflection effect from a hot white dwarf primary; as such it may have been a nova sometime in the past few decades. Ha 0242 is a deeply eclipsing very low mass-transfer rate system, probably a dwarf nova of very long outburst interval, with P(orb) = 1.792 h. GI Mon, an old nova, has optical modulations at 4.325 h and possibly also at 48.6 min and is thus a candidate intermediate polar. AO Oct, an SU UMa type dwarf nova, shows orbital modulation at quiescence with P(orb) = 94.097 min. V972 Oph (Nova Ophiuchi 1957) showed no flickering activity during one set of observations, but did so at a later time, confirming the correctness of the identification of this object, but it shows no orbital modulation. SDSS 0155 is a deeply eclipsing polar with an orbital period of 87.13 min. SDSS 0233 shows flaring activity but no discernible periodicity. No periodicity was found in the new AM CVn candidate SDSS 1240. SDSS 1556 shows a periodic modulation at 1.778 h which is possibly due to orbital motion. SDSS 2050 is an eclipsing polar with P(orb) = 1.5702 h. FH Ser (Nova Serpentis 1970) has strong flickering activity but no detectable orbital modulation.

astro-ph↗