P. A. Woudt

University of Cape Town, Kaapstad, Western Cape, South Africa

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Publications (169)364.51 Total impact

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    ABSTRACT: The thermal radio emission of novae during outburst enables us to derive fundamental quantities such as the ejected mass, kinetic energy, and density profile of the ejecta. Recent observations with newly-upgraded facilities such as the VLA and e-MERLIN are just beginning to reveal the incredibly complex processes of mass ejection in novae (ejections appear to often proceed in multiple phases and over prolonged timescales). Symbiotic stars can also exhibit outbursts, which are sometimes accompanied by the expulsion of material in jets. However, unlike novae, the long-term thermal radio emission of symbiotics originates in the wind of the giant secondary star, which is irradiated by the hot white dwarf. The effect of the white dwarf on the giant's wind is strongly time variable, and the physical mechanism driving these variations remains a mystery (possibilities include accretion instabilities and time-variable nuclear burning on the white dwarf's surface). The exquisite sensitivity of SKA1 will enable us to survey novae throughout the Galaxy, unveiling statistically complete populations. With SKA2 it will be possible to carry out similar studies in the Magellanic Clouds. This will enable high-quality tests of the theory behind accretion and mass loss from accreting white dwarfs, with significant implications for determining their possible role as Type Ia supernova progenitors. Observations with SKA1-MID in particular, over a broad range of frequencies, but with emphasis on the higher frequencies, will provide an unparalleled view of the physical processes driving mass ejection and resulting in the diversity of novae, whilst also determining the accretion processes and rates in symbiotic stars.
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    ABSTRACT: The universal link between the processes of accretion and ejection leads to the formation of jets and outflows around accreting compact objects. Incoherent synchrotron emission from these outflows can be observed from a wide range of accreting binaries, including black holes, neutron stars, and white dwarfs. Monitoring the evolution of the radio emission during their sporadic outbursts provides important insights into the launching of jets, and, when coupled with the behaviour of the source at shorter wavelengths, probes the underlying connection with the accretion process. Radio observations can also probe the impact of jets/outflows (including other explosive events such as magnetar giant flares) on the ambient medium, quantifying their kinetic feedback. The high sensitivity of the SKA will open up new parameter space, enabling the monitoring of accreting stellar-mass compact objects from their bright, Eddington-limited outburst states down to the lowest-luminosity quiescent levels, whose intrinsic faintness has to date precluded detailed studies. A census of quiescently accreting black holes will also constrain binary evolution processes. By enabling us to extend our existing investigations of black hole jets to the fainter jets from neutron star and white dwarf systems, the SKA will permit comparative studies to determine the role of the compact object in jet formation. The high sensitivity, wide field of view and multi-beaming capability of the SKA will enable the detection and monitoring of all bright flaring transients in the observable local Universe, including the ULXs, ... [Abridged] This chapter reviews the science goals outlined above, demonstrating the progress that will be made by the SKA. We also discuss the potential of the astrometric and imaging observations that would be possible should a significant VLBI component be included in the SKA.
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    ABSTRACT: Radio observations of novae in outburst are of particular interest due to the physical parameters that may be retrieved from fitting the radio light curves. Most models that have fitted previous data assumed spherical symmetry however, it is becoming more and more clear that this is not the case. We explore morpho-kinematical techniques to retrieve the free-free radio light curves of non-spherical models and explore the effects of a non-spherical outburst on the physical parameters. In particular, we find that we may have been over estimating the ejected masses in the outburst of non-spherical novae.
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    ABSTRACT: Observations of novae at radio frequencies provide us with a measure of the total ejected mass, density profile and kinetic energy of a nova eruption. The radio emission is typically well characterized by the free-free emission process. Most models to date have assumed spherical symmetry for the eruption, although it has been known for as long as there have been radio observations of these systems, that spherical eruptions are to simplistic a geometry. In this paper, we build bipolar models of the nova eruption, assuming the free-free process, and show the effects of varying different parameters on the radio light curves. The parameters considered include the ratio of the minor- to major-axis, the inclination angle and shell thickness (further parameters are provided in the appendix). We also show the uncertainty introduced when fitting spherical model synthetic light curves to bipolar model synthetic light curves. We find that the optically thick phase rises with the same power law ($S_{\nu} \propto t^2$) for both the spherical and bipolar models. In the bipolar case there is a "plateau" phase -- depending on the thickness of the shell as well as the ratio of the minor- to major-axis -- before the final decline, that follows the same power law ($S_{\nu} \propto t^{-3}$) as in the spherical case. Finally, fitting spherical models to the bipolar model synthetic light curves requires, in the worst case scenario, doubling the ejected mass, more than halving the electron temperature and reducing the shell thickness by nearly a factor of 10. This implies that in some systems we have been over predicting the ejected masses and under predicting the electron temperature of the ejecta.
    The Astrophysical Journal 07/2014; 792(1). DOI:10.1088/0004-637X/792/1/57 · 6.28 Impact Factor
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    ABSTRACT: Current studies of the peculiar velocity flow field in the Local Universe are limited by either the lack of detection or accurate photometry for galaxies at low Galactic latitudes. The contribution to the dynamics of the Local Group of the largely unknown mass distribution in this 'Zone of Avoidance' remains controversial. We present here the results of a pilot project to obtain deep near infrared (NIR) observations of galaxies detected in the systematic Parkes deep HI survey of the ZoA - 578 galaxies with recession velocities out to 6000 km/s were observed with the 1.4m InfraRed Survey Facility SIRIUS camera providing J, H and K_s imaging ~2 mag deeper than 2MASS. After star-subtraction, the resulting isophotal magnitudes and inclinations of ZoA galaxies are of sufficient accuracy (magnitude errors under 0.1 mag even at high extinction) to ultimately be used to determine cosmic flow fields "in" the ZoA via the NIR Tully-Fisher relation. We further used the observed NIR colours to assess the ratio of the true extinction to the DIRBE/IRAS extinction deep into the dust layers of the Milky Way. The derived ratio was found to be 0.87 across the HIZOA survey region with no significant variation with Galactic latitude or longitude. This value is in excellent agreement with the completely independently derived factor of 0.86 by Schlafly & Finkbeiner based on Sloan data far away from the Milky Way.
    Monthly Notices of the Royal Astronomical Society 06/2014; 443(1). DOI:10.1093/mnras/stu1155 · 5.23 Impact Factor
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    ABSTRACT: While Norma (ACO3627) is the richest cluster in the Great Attractor (GA) region, its role in the local dynamics is poorly understood. The Norma cluster has a mean redshift (z_CMB) of 0.0165 and has been proposed as the "core" of the GA. We have used the Ks-band Fundamental Plane (FP) to measure Norma cluster's distance with respect to the Coma cluster. We report FP photometry parameters (effective radii and surface brightnesses), derived from ESO NTT SOFI images, and velocity dispersions, from AAT 2dF spectroscopy, for 31 early-type galaxies in the cluster. For the Coma cluster we use 2MASS images and SDSS velocity dispersion measurements for 121 early-type galaxies to generate the calibrating FP dataset. For the combined Norma-Coma sample we measure FP coefficients of a=1.465+/-0.059 and b=0.326+/-0.020. We find an rms scatter, in log(sigma) of 0.08 dex which corresponds to a distance uncertainty of 28% per galaxy. The zero point offset between Norma's and Coma's FPs is 0.154+\-0.014 dex. Assuming that the Coma cluster is at rest with respect to the cosmic microwave background frame and z_CMB(Coma)=0.0240, we derive a distance to the Norma cluster of 5026+/-160 km/s, and the derived peculiar velocity is -72+\-170 km/s, i.e., consistent with zero. This is lower than previously reported positive peculiar velocities for clusters/groups/galaxies in the GA region and hence the Norma cluster may indeed represent the GA's "core".
    Monthly Notices of the Royal Astronomical Society 01/2014; 439(4). DOI:10.1093/mnras/stu217 · 5.23 Impact Factor
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    ABSTRACT: Time series photometry of 20 Cataclysmic Variables detected by the Catalina Real-time Transient Survey is presented. 14 of these systems have not been observed previously and only two have been examined in-depth. From the observations we determined 12 new orbital periods and independently found a further two. Eight of the CVs are eclipsing systems, five of which have eclipse depths of more than 0.9 mag. Included in the sample are six SU UMa systems (three of which show superhumps in our photometry), a polar (SSS1944-42) and one system (CSS1417-18) that displays an abnormally fast decline from outburst.
    Monthly Notices of the Royal Astronomical Society 10/2013; 437(1). DOI:10.1093/mnras/stt1900 · 5.23 Impact Factor
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    ABSTRACT: We report on recent optical spectroscopy of Nova Sco 2012, which was discovered in June 2012 as a bright transient in the galactic bulge by the Microlensing Observations in Astrophysics (MOA) Collaboration (ATel #4157) and subsequently detected in gamma-rays by the Fermi Gamma-ray Space Telescope (ATel #4284). Follow-up spectra revealed a standard Fe II nova (ATel #4287).
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    ABSTRACT: Two identical new instruments, the Sutherland High-speed Optical Cameras (SHOC), have been developed for use on the South African Astronomical Observatory’s (SAAO) 1.9, 1.0 and 0.75 m telescopes at Sutherland. The SHOC systems are fast-frame-rate, accurately-timed, high-quality, visible-wavelength imagers. Each system consists of a camera, global positioning system (GPS), control computer and peripherals. The primary component is an Andor iXon X3 888 UVB camera, utilizing a 1024 × 1024 pixel, frame-transfer, thermoelectrically-cooled, back-illuminated CCD. One of SHOC’s most important features is that it can achieve frame rates of between 1 and 20 frames/s during normal operation (dependent on binning and subframing) with microsecond timing accuracy on each frame (achieved using frame-by-frame GPS triggering). Frame rates can be increased further, and fainter targets observed, by making use of SHOC’s electron-multiplying (EM) modes. SHOC is therefore ideally suited to time domain astronomy where high frame rates and extremely accurate timing are critical. Here, we present details of the instrument components, characteristics measured during commissioning, science demonstrations, and development plans. Attention is specifically given to exploration of the signal-to-noise (S/N) parameter space as a function of EM and conventional modes. These results enable observers to optimize instrumental settings for their observations and clearly demonstrate the advantages and potential pitfalls of the EM modes.
    Publications of the Astronomical Society of the Pacific 08/2013; 125(930):976-988. DOI:10.1086/672156 · 3.23 Impact Factor
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    ABSTRACT: We present ground-based optical and near infrared photometric observations and Hubble Space Telescope COS spectroscopic observations of the old nova V842 Cen (Nova Cen 1986). Analysis of the optical light curves reveals a peak at 56.5 +/- 0.3s with an amplitude of 8.9 +/- 4.2 mma, which is consistent with the rotation of a magnetic white dwarf primary in V842 Cen that was detected earlier by Woudt et al., and led to its classification as an intermediate polar.However, our UV lightcurve created from the COS time-tag spectra does not show this periodicity. Our synthetic spectral analysis of an HST COS spectrum rules out a hot white dwarf photosphere as the source of the FUV flux. The best-fitting model to the COS spectrum is a full optically thick accretion disk with no magnetic truncation, a low disk inclination angle, low accretion rate and a distance less than half the published distance that was determined on the basis of interstellar sodium D line strengths.Truncated accretion disks with truncation radii of 3Rwd and 5Rwd yielded unsatisfactory agreement with the COS data. The accretion rate is unexpectedly low for a classical nova only 24 years after the explosion when the accretion rate is expected to be high and the white dwarf should still be very hot, especially if irradiation of the donor star took place. Our low accretion rate is consistent with low accretion rates derived from X-ray and ground-based optical data.
    The Astrophysical Journal 05/2013; 772(2). DOI:10.1088/0004-637X/772/2/116 · 6.28 Impact Factor
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    ABSTRACT: Circinus X-1 is a bright and highly variable X-ray binary which displays strong and rapid evolution in all wavebands. Radio flaring, associated with the production of a relativistic jet, occurs periodically on a ~17-day timescale. A longer-term envelope modulates the peak radio fluxes in flares, ranging from peaks in excess of a Jansky in the 1970s to an historic low of milliJanskys during the years 1994 to 2007. Here we report first observations of this source with the MeerKAT test array, KAT-7, part of the pathfinder development for the African dish component of the Square Kilometre Array (SKA), demonstrating successful scientific operation for variable and transient sources with the test array. The KAT-7 observations at 1.9 GHz during the period 13 December 2011 to 16 January 2012 reveal in temporal detail the return to the Jansky-level events observed in the 1970s. We compare these data to contemporaneous single-dish measurements at 4.8 and 8.5 GHz with the HartRAO 26-m telescope and X-ray monitoring from MAXI. We discuss whether the overall modulation and recent dramatic brightening is likely to be due to an increase in the power of the jet due to changes in accretion rate or changing Doppler boosting associated with a varying angle to the line of sight.
    Monthly Notices of the Royal Astronomical Society 05/2013; 433(3). DOI:10.1093/mnras/stt860 · 5.23 Impact Factor
  • B. Warner · P. Woudt
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    ABSTRACT: We summarise the results of observing faint cataclysmic variables from the Catalina Real-Time Transient Survey, resulting in confirming the theoretically predicted maximum in the orbital period histogram near a period of 80 minutes.
    Proceedings of the International Astronomical Union 02/2013; 8(S290):339-340. DOI:10.1017/S1743921312020303
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    ABSTRACT: The spectroscopy described in ATel #4704 was taken on Jan 5, 2013, not Jan 3, 2013, as erroneously stated there. The authors apologize for the typo.
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    ABSTRACT: We present high-speed photometric observations of 20 faint cataclysmic variables (CVs) selected from the Sloan Digital Sky Survey (SDSS) and Catalina catalogues. Measurements are given of 15 new directly measured orbital periods, including four eclipsing dwarf novae (SDSS 0904+03, CSS 0826-00, CSS 1404-10 and CSS 1626-12), two new polars (CSS 0810+00 and CSS 1503-22) and two dwarf novae with superhumps in quiescence (CSS 0322+02 and CSS 0826-00). Whilst most of the dwarf novae presented here have periods below 2h, SDSS 0805+07 and SSS 0617-36 have relatively long orbital periods of 5.489 and 3.440h, respectively. The double-humped orbital modulations observed in SSS 0221-26, CSS 0345-01, CSS 1300+11 and CSS 1443-17 are typical of low-mass transfer rate dwarf novae. The white dwarf primary of SDSS 0919+08 is confirmed to have non-radial oscillations, and quasi-periodic oscillations were observed in the short-period dwarf nova CSS 1028-08 during outburst. We further report the detection of a new nova-like variable (SDSS 1519+06). The frequency distribution of orbital periods of CVs in the Catalina Real-time Transient Survey (CRTS) has a high peak near ~80min orbital period, independently confirming that found by Gansicke et al. (2009MNRAS.397.2170G) from SDSS sources. We also observe a marked correlation between the median in the orbital period distribution and the outburst class, in the sense that dwarf novae with a single observed outburst (over the 5-year baseline of the CRTS coverage) occur predominantly at shortest orbital period. (2 data files).
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    ABSTRACT: The moderately fast old nova V842 Cen (Nova Cen 1986) was last observed in the FUV spectrum with IUE in 1994. Since then, the UV energy distribution has became increasingly flatter, indicating the gradual cooling of the heated white dwarf. We obtained a Hubble COS spectrum on 18 March 2010. Emission lines of Si III +O I (1300) and N V 1240 have newly appeared.Woudt et al. (2009) carried out high-speed photometry revealing that the light curve of V842 Cen exhibited a coherent light modulation at 56.825 s which they took as the rotation period of the white dwarf. The system has an orbital period of 3.94 days, a low inclination and a WD spin period of 56 seconds, making it the fastest spinning WD in an intermediate polar. Luna et al. (2012) obtained X-ray spectra which revealed that the WD has a mass 0.88 Msun. However, they did not detect the 56s spin in X-rays. We have de-reddened the HST spectrum with E(B-V) = 0.55 and modeled it with optically thick, truncated, viscous accretion disks with vertical structure. The results of our study are presented. This research was supported by HST grants GO-11639.01-A and GO-11639.02-A to the University of Washington and Villanova University, respectively.
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    ABSTRACT: We have obtained optical spectroscopy of the bright optical transient SSS130101:122222-311525 reported in ATel 4699 and 4700) with the 4.2m William Herschel Telescope on La Palma. The observations were taken on Jan 3, 2013 with the dual arm spectrograph ISIS, covering both blue and red optical wavelengths. The total exposure time was 29 mins, centered on 06:33 UT. The data were obtained at high airmass in poor seeing, and no flux calibration was attempted.
  • P. A. Woudt · B. Warner · M. Motsoaledi
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    ABSTRACT: Further to ATel#4678, we report the detection of a photometric modulation of 47.28 ± 0.01 min in the helium dwarf nova CSS121123:045020-093113 with a possible 2-day alias at 46.50 ± 0.01 min. At this long period for AM CVn dwarf novae, CSS121123:045020-093113 is located uniquely amongst known AM CVn systems that are thought to be permanently in a quiescent state.
  • South African Journal of Science 12/2012; 109(7-8):01-02. DOI:10.1590/sajs.2013/a0030 · 1.03 Impact Factor
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    ABSTRACT: We present high speed optical, spectroscopic and Swift X-ray observations made during the dwarf nova superoutburst of CC Scl in November 2011. An orbital period of 1.383 h and superhump period of 1.443 h were measured, but the principal new finding is that CC Scl is a previously unrecognised intermediate polar, with a white dwarf spin period of 389.49 s which is seen in both optical and Swift X-ray light curves only during the outburst. In this it closely resembles the old nova GK Per, but unlike the latter has one of the shortest orbital periods among intermediate polars.
    Monthly Notices of the Royal Astronomical Society 08/2012; 427(2). DOI:10.1111/j.1365-2966.2012.22010.x · 5.23 Impact Factor
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    Patrick A. Woudt · Brian Warner · Ewald Zietsman
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    ABSTRACT: High speed photometric observations of the spectroscopically-discovered PG 1159 star SDSS J034917.41-005917.9 in 2007 and 2009 reveal a suite of pulsation frequencies in the range of 1038 - 3323 microHz with amplitudes between 3.5 and 18.6 mmag. SDSS J034917.41-005917.9 is therefore a member of the GW Vir class of pulsating pre-white dwarfs. We have identified 10 independent pulsation frequencies that can be fitted by an asymptotic model with a constant period spacing of 23.61 +/- 0.21 s, presumably associated with a sequence of l = 1 modes. The highest amplitude peak in the suite of frequencies shows evidence for a triplet structure, with a frequency separation of 14.4 microHz. Five of the identified frequencies do not fit the l = 1 sequence, but are, however, well-modeled by an independent asymptotic sequence with a constant period spacing of 11.66 +/- 0.13 s. It is unclear to which l mode these frequencies belong.
    Monthly Notices of the Royal Astronomical Society 08/2012; 426(3). DOI:10.1111/j.1365-2966.2012.21899.x · 5.23 Impact Factor