Publications (14)19.6 Total impact
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Article: The Chandra Multi-Wavelength Project: Optical Spectroscopy and the Broadband Spectral Energy Distributions of X-ray Selected AGN
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ABSTRACT: From optical spectroscopy of X-ray sources observed as part of ChaMP, we present redshifts and classifications for a total of 1569 Chandra sources from our targeted spectroscopic follow up using the FLWO, SAAO, WIYN, CTIO, KPNO, Magellan, MMT and Gemini telescopes, and from archival SDSS spectroscopy. We classify the optical counterparts as 50% BLAGN, 16% NELG, 14% ALG, and 20% stars. We detect QSOs out to z~5.5 and galaxies out to z~3. We have compiled extensive photometry from X-ray to radio bands. Together with our spectroscopic information, this enables us to derive detailed SEDs for our extragalactic sources. We fit a variety of templates to determine bolometric luminosities, and to constrain AGN and starburst components where both are present. While ~58% of X-ray Seyferts require a starburst event to fit observed photometry only 26% of the X-ray QSO population appear to have some kind of star formation contribution. This is significantly lower than for the Seyferts, especially if we take into account torus contamination at z>1 where the majority of our X-ray QSOs lie. In addition, we observe a rapid drop of the percentage of starburst contribution as X-ray luminosity increases. This is consistent with the quenching of star formation by powerful QSOs, as predicted by the merger model, or with a time lag between the peak of star formation and QSO activity. We have tested the hypothesis that there should be a strong connection between X-ray obscuration and star-formation but we do not find any association between X-ray column density and star formation rate both in the general population or the star-forming X-ray Seyferts. Our large compilation also allows us to report here the identification of 81 XBONG, 78 z>3 X-ray sources and 8 Type-2 QSO candidates. Also we have identified the highest redshift (z=5.4135) X-ray selected QSO with optical spectroscopy.04/2012; -
Article: On the spin modulated circular polarization from the intermediate polars NY Lup and IGRJ1509-6649
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ABSTRACT: We report on high time resolution, high signal/noise, photo-polarimetry of the intermediate polars NY Lup and IGRJ1509-6649. Our observations confirm the detection and colour dependence of circular polarization from NY Lup and additionally show a clear white dwarf, spin modulated signal. From our new high signal/noise photometry we have unambiguously detected wavelength dependent spin and beat periods and harmonics thereof. IGRJ1509-6649 is discovered to also have a particularly strong spin modulated circularly polarized signal. It appears double peaked through the I filter and single peaked through the B filter, consistent with cyclotron emission from a white dwarf with a relatively strong magnetic field. We discuss the implied accretion geometries in these two systems and any bearing this may have on the possible relationship with the connection between polars and soft X-ray-emitting IPs. The relatively strong magnetic fields is also suggestive of them being polar progenitors.11/2011; -
Article: Possible detection of two giant extrasolar planets orbiting the eclipsing polar UZ Fornacis
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ABSTRACT: We present new high-speed, multi-observatory, multi-instrument photometry of the eclipsing polar UZ For in order to measure precise mid-eclipse times with the aim of detecting any orbital period variations. When combined with published eclipse times and archival data spanning ~27 years, we detect departures from a linear and quadratic trend of ~60 s. The departures are strongly suggestive of two cyclic variations of 16(3) and 5.25(25) years. The two favoured mechanisms to drive the periodicities are either two giant extrasolar planets as companions to the binary (with minimum masses of 6.3(1.5)M(Jupiter) and 7.7(1.2)M(Jupiter)) or a magnetic cycle mechanism (e.g. Applegate's mechanism) of the secondary star. Applegate's mechanism would require the entire radiant energy output of the secondary and would therefore seem to be the least likely of the two, barring any further refinements in the effect of magnetic fieilds (e.g. those of Lanza et al.). The two planet model can provide realistic solutions but it does not quite capture all of the eclipse times measurements. A highly eccentric orbit for the outer planet would fit the data nicely, but we find that such a solution would be unstable. It is also possible that the periodicities are driven by some combination of both mechanisms. Further observations of this system are encouraged.06/2011; -
Article: Determining the extragalactic extinction law with SALT – II. Additional sample★
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ABSTRACT: We present new results from an ongoing programme to study the dust extragalactic extinction law in E/S0 galaxies with dust lanes with the Southern African Large Telescope (SALT) during its performance verification phase. The wavelength dependence of the dust extinction for seven galaxies is derived in six spectral bands ranging from the near-ultraviolet atmospheric cut-off to the near-infrared. The derivation of an extinction law is performed by fitting model galaxies to the unextinguished parts of the image in each spectral band, and subtracting from these the actual images. We compare our results with the derived extinction law in the Galaxy and find them to run parallel to the Galactic extinction curve with a mean total-to-selective extinction value of RV= 2.71 ± 0.43. We use total optical extinction values to estimate the dust mass for each galaxy, compare these with dust masses derived from IRAS measurements, and find them to range from 104 to 107 M⊙. We study the case of the well-known dust-lane galaxy NGC 2685 for which Hubble Space Telescope/Wide Field Planetary Camera 2 (HST/WFPC2) data are available to test the dust distribution on different scales. Our results imply a scale-free dust distribution across the dust lanes, at least within ∼1 arcsec (∼60 pc) regions.Monthly Notices of the Royal Astronomical Society 11/2010; 409(2):727 - 736. · 4.90 Impact Factor -
Article: The Herschel-SPIRE Legacy Survey (HSLS): the scientific goals of a shallow and wide submillimeter imaging survey with SPIRE
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ABSTRACT: A large sub-mm survey with Herschel will enable many exciting science opportunities, especially in an era of wide-field optical and radio surveys and high resolution cosmic microwave background experiments. The Herschel-SPIRE Legacy Survey (HSLS), will lead to imaging data over 4000 sq. degrees at 250, 350, and 500 micron. Major Goals of HSLS are: (a) produce a catalog of 2.5 to 3 million galaxies down to 26, 27 and 33 mJy (50% completeness; 5 sigma confusion noise) at 250, 350 and 500 micron, respectively, in the southern hemisphere (3000 sq. degrees) and in an equatorial strip (1000 sq. degrees), areas which have extensive multi-wavelength coverage and are easily accessible from ALMA. Two thirds of the of the sources are expected to be at z > 1, one third at z > 2 and about a 1000 at z > 5. (b) Remove point source confusion in secondary anisotropy studies with Planck and ground-based CMB data. (c) Find at least 1200 strongly lensed bright sub-mm sources leading to a 2% test of general relativity. (d) Identify 200 proto-cluster regions at z of 2 and perform an unbiased study of the environmental dependence of star formation. (e) Perform an unbiased survey for star formation and dust at high Galactic latitude and make a census of debris disks and dust around AGB stars and white dwarfs. Comment: White paper supplement to the proposal submitted by the HSLS science team to ESA for Herschel open-time programs07/2010; -
Article: The Herschel-SPIRE Legacy Survey (HSLS): the scientific goals of a shallow and wide submillimeter imaging survey with SPIRE
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ABSTRACT: A large sub-mm survey with Herschel will enable many exciting science opportunities, especially in an era of wide-field optical and radio surveys and high resolution cosmic microwave background experiments. The Herschel-SPIRE Legacy Survey (HSLS), will lead to imaging data over 4000 sq. degrees at 250, 350, and 500 micron. Major Goals of HSLS are: (a) produce a catalog of 2.5 to 3 million galaxies down to 26, 27 and 33 mJy (50% completeness; 5 sigma confusion noise) at 250, 350 and 500 micron, respectively, in the southern hemisphere (3000 sq. degrees) and in an equatorial strip (1000 sq. degrees), areas which have extensive multi-wavelength coverage and are easily accessible from ALMA. Two thirds of the of the sources are expected to be at z > 1, one third at z > 2 and about a 1000 at z > 5. (b) Remove point source confusion in secondary anisotropy studies with Planck and ground-based CMB data. (c) Find at least 1200 strongly lensed bright sub-mm sources leading to a 2% test of general relativity. (d) Identify 200 proto-cluster regions at z of 2 and perform an unbiased study of the environmental dependence of star formation. (e) Perform an unbiased survey for star formation and dust at high Galactic latitude and make a census of debris disks and dust around AGB stars and white dwarfs.06/2010; -
Article: Polarized QPOs from the INTEGRAL polar IGRJ14536-5522 (=Swift J1453.4-5524)
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ABSTRACT: We report optical spectroscopy and high speed photometry and polarimetry of the INTEGRAL source IGRJ14536-5522 (=Swift J1453.4-5524). The photometry, polarimetry and spectroscopy are modulated on an orbital period of 3.1564(1) hours. Orbital circularly polarized modulations are seen from 0 to -18 per cent, unambiguously identifying IGRJ14536-5522 as a polar. Some of the high speed photometric data show modulations that are consistent with quasi-periodic oscillations (QPOs) on the order of 5-6 minutes. Furthermore, for the first time, we detect the (5-6) minute QPOs in the circular polarimetry. We discuss the possible origins of these QPOs. We also include details of HIPPO, a new high-speed photo-polarimeter used for some of our observations. Comment: Accepted for publication by MNRAS. The paper contains 7 figures and 1 table11/2009; -
Article: Discovery of the first symbiotic star in NGC 6822★
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ABSTRACT: We report the discovery of the first symbiotic star (V= 21.6, KS= 15.8 mag) in the Local Group dwarf irregular galaxy NGC 6822. This star was identified during a spectral survey of Hα emission-line objects using the Southern African Large Telescope during its performance-verification phase. The observed strong emission lines of H i and He ii suggest a high electron density and T* < 130 000 K for the hot companion. The infrared colours allow us to classify this object as an S-type symbiotic star, comprising a red giant losing mass to a compact companion. The red giant is an asymptotic giant branch (AGB) carbon star, and a semiregular variable, pulsating in the first overtone with a period of 142 d. Its bolometric magnitude is Mbol=−4.4 mag.We review what is known about the luminosities of extragalactic symbiotic stars, showing that most, possibly all, contain AGB stars. We suggest that a much larger fraction of Galactic symbiotic stars may contain AGB stars than was previously realized.Monthly Notices of the Royal Astronomical Society 05/2009; 395(2):1121 - 1126. · 4.90 Impact Factor -
Article: Determining the extragalactic extinction law with SALT★
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ABSTRACT: We present CCD imaging observations of early-type galaxies with dark lanes obtained with the Southern African Large Telescope (SALT) during its performance-verification phase. The observations were performed in six spectral bands that span the spectral range from the near-ultraviolet atmospheric cut-off to the near-infrared. We derive the extinction law by the extragalactic dust in the dark lanes in the spectral range 1.11 < λ−1 < 2.94 μm−1 by fitting model galaxies to the unextinguished parts of the image, and subtracting from these the actual images. This procedure allows the derivation, with reasonably high signal-to-noise ratio, of the extinction in each spectral band we used for each resolution element of the image. We also introduce an alternative method to derive the extinction values by comparing various colour-index maps under the assumption of negligible intrinsic colour gradients in these galaxies. We than compare the results obtained using these two methods.We compare the total-to-selective extinction derived for these galaxies with previously obtained results and with similar extinction values of Milky Way dust to derive conclusions about the properties of extragalactic dust in different objects and conditions.We find that the extinction curves run parallel to the Galactic extinction curve, which implies that the properties of dust in the extragalactic environment are similar to those of the Milky Way, despite our original expectations. The ratio of the total V-band extinction to the selective extinction between the V and B bands is derived for each galaxy with an average of 2.82 ± 0.38, compared to a canonical value of 3.1 for the Milky Way. The similar values imply that galaxies with well-defined dark lanes have characteristic dust grain sizes similar to those of Galactic dust. We use total optical extinction values to estimate the dust mass for each galaxy, compare these with dust masses derived from IRAS measurements, and find them in the range 104–107 M⊙.Monthly Notices of the Royal Astronomical Society 10/2008; 390(3):969 - 984. · 4.90 Impact Factor -
Article: Astronomical Telescopes and Instrumentation: Synergies Between Ground and Space
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ABSTRACT: Construction of the Southern African Large Telescope (SALT) was largely completed by the end of 2005 and since then it has been in intensive commissioning. This has now almost been completed except for the telescope's image quality which shows optical aberrations, chiefly a focus gradient across the focal plane, along with astigmatism and other less significant aberrations. This paper describes the optical systems engineering investigation that has been conducted since early 2006 to diagnose the problem. A rigorous approach has been followed which has entailed breaking down the system into the major sub-systems and subjecting them to testing on an individual basis. Significant progress has been achieved with many components of the optical system shown to be operating correctly. The fault has been isolated to a major optical sub-system. We present the results obtained so far, and discuss what remains to be done.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.07/2008; -
Article: Photometric and polarimetric observations of the eclipsing polar SDSS J205017.84‐053626.8
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ABSTRACT: Photometric and polarimetric observations of the eclipsing polar (AM Herculis system) Sloan Digital Sky Survey 2050-05 are reported, in which both sets of data are found to be modulated on a period of ∼94.2 min. This is the first detection of polarization in this system, unambiguously confirming its identification. Circular polarization variations are seen from ∼−10 to +4 per cent, consistent with a ‘two-pole’ accretor. Both accreting poles are self-occulted by the white dwarf during parts of the orbit. The photopolarimetry from different epochs shows the system to be in different states. During the epochs showing a higher state the polarimetry suggests accretion occurs preferentially near the pole displaying positive circular polarization, and vice versa during the epoch showing a lower state. From the eclipses we estimate some of the system's parameters.Monthly Notices of the Royal Astronomical Society 07/2006; 371(2):727 - 731. · 4.90 Impact Factor -
Article: PySALT: the SALT science pipeline
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ABSTRACT: PySALT is the python/PyRAF-based data reduction and analysis pipeline for the Southern African Large Telescope (SALT), a modern 10m class telescope with a large user community consisting of 13 partner institutions. The two first generation instruments on SALT are SALTICAM, a wide-field imager, and the Robert Stobie Spectrograph (RSS). Along with traditional imaging and spectroscopy modes, these instruments provide a wide range of observing modes, including Fabry-Perot imaging, polarimetric observations, and high-speed observations. Due to the large user community, resources available, and unique observational modes of SALT, the development of reduction and analysis software is key to maximizing the scientific return of the telescope. PySALT is developed in the Python/PyRAF environment and takes advantage of a large library of open-source astronomical software. The goals in the development of PySALT are: (1) Provide science quality reductions for the major operational modes of SALT, (2) Create analysis tools for the unique modes of SALT, and (3) Create a framework for the archiving and distribution of SALT data. The data reduction software currently provides support for the reduction and analysis of regular imaging, high-speed imaging, and long slit spectroscopy with planned support for multi-object spectroscopy, high-speed spectroscopy, Fabry-Perot imaging, and polarimetric data sets. We will describe the development and current status of PySALT and highlight its benefits through early scientific results from SALT. -
Article: Handling observation proposals for SALT
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ABSTRACT: SALT uses the Principal Investigator Proposal Tool (PIPT) for generating, checking, submitting and editing proposals. The PIPT maps XML into Java classes with immediate error and consistency checking, and thus prevents non-feasible observation requests. Various tools allow the user to simulate SALT observations. These include standard source spectra (e.g. black body, power law, Kurucz model atmospheres), and allow users to add their own library spectra. The PIPT is complemented by the Web Manager for administering submitted proposals. It is discussed how the code of these tools can easily be extended for future instruments and used for other projects. -
Article: The Herschel-SPIRE Legacy Survey (HSLS): the scientific goals of a shallow and wide submillimeter imaging survey with SPIRE
[show abstract] [hide abstract]
ABSTRACT: A large sub-mm survey with Herschel will enable many exciting science opportunities, especially in an era of wide-field optical and radio surveys and high resolution cosmic microwave background experiments. The Herschel-SPIRE Legacy Survey (HSLS), will lead to imaging data over 4000 sq. degrees at 250, 350, and 500 micron. Major Goals of HSLS are: (a) produce a catalog of 2.5 to 3 million galaxies down to 26, 27 and 33 mJy (50% completeness; 5 sigma confusion noise) at 250, 350 and 500 micron, respectively, in the southern hemisphere (3000 sq. degrees) and in an equatorial strip (1000 sq. degrees), areas which have extensive multi-wavelength coverage and are easily accessible from ALMA. Two thirds of the of the sources are expected to be at z ensuremath> 1, one third at z ensuremath> 2 and about a 1000 at z ensuremath> 5. (b) Remove point source confusion in secondary anisotropy studies with Planck and ground-based CMB data. (c) Find at least 1200 strongly lensed bright sub-mm sources leading to a 2% test of general relativity. (d) Identify 200 proto-cluster regions at z of 2 and perform an unbiased study of the environmental dependence of star formation. (e) Perform an unbiased survey for star formation and dust at high Galactic latitude and make a census of debris disks and dust around AGB stars and white dwarfs.
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Institutions
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2006–2010
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South African Astronomical Observatory
Cape Town, Province of the Western Cape, South Africa
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