arXiv subjects

P. Woudt

Publications and source records attributed to P. Woudt.

32 records · Page 2Linked to original sources

arXiv2019

The 2018 outburst of BHXB H1743-322 as seen with MeerKAT

In recent years, the black hole candidate X-ray binary system H1743-322 has undergone outbursts and it has been observed with X-ray and radio telescopes. We present 1.3 GHz MeerKAT radio data from the ThunderKAT Large Survey Project on radio transients for the 2018 outburst of H1743-322. We obtain seven detections from a weekly monitoring programme and use publicly available Swift X-ray Telescope and MAXI data to investigate the radio/X-ray correlation of H1743-322 for this outburst. We compare the 2018 outburst with those reported in the literature for this system and find that the X-ray outburst reported is similar to previously reported `hard-only' outbursts. As in previous outbursts, H1743-322 follows the `radio-quiet' correlation in the radio/X-ray plane for black hole X-ray binaries, and the radio spectral index throughout the outburst is consistent with the `radio-quiet' population.

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arXiv2018

The physical properties of AM CVn stars: new insights from Gaia DR2

AM CVn binaries are hydrogen deficient compact binaries with an orbital period in the 5-65 min range and are predicted to be strong sources of persistent gravitational wave radiation. Using Gaia Data Release 2, we present the parallaxes and proper motions of 41 out of the 56 known systems. Compared to the parallax determined using the HST Fine Guidance Sensor we find that the archetype star, AM CVn, is significantly closer than previously thought. This resolves the high luminosity and mass accretion rate which models had difficulty in explaining. Using Pan-STARRS1 data we determine the absolute magnitude of the AM CVn stars. There is some evidence that donor stars have a higher mass and radius than expected for white dwarfs or that the donors are not white dwarfs. Using the distances to the known AM CVn stars we find strong evidence that a large population of AM CVn stars have still to be discovered. As this value sets the background to the gravitational wave signal of LISA, this is of wide interest. We determine the mass transfer rate for 15 AM CVn stars and find that the majority have a rate significantly greater than expected from standard models. This is further evidence that the donor star has a greater size than expected.

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arXiv2018

Detection of a 23.6 min periodic modulation in the optical counterpart of 3XMMJ051034.6--670353

We present high speed optical photometric observations made using the NTT and ULTRACAM of the optical counterpart of 3XMMJ051034.6--670353, which was recently identified as an X-ray source showing a modulation on a period of 23.6 min. Although the optical counterpart is faint (g=21.4), we find that the u'g'r' light curves show a periodic modulation on a period which is consistent with the X-ray period. We also obtained three low resolution spectra of 3XMMJ051034.6--670353 using the Gemini South Telescope and GMOS. There is no evidence for strong emission lines in the optical spectrum of 3XMMJ051034.6--670353. We compare and contrast the optical and X-ray observations of 3XMMJ051034.6--670353 with the ultra compact binaries HM Cnc and V407 Vul. We find we can identify a distribution of binary masses in which stable direct impact accretion can occur.

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arXiv2017

Multiwavelength observations of nova SMCN 2016-10a --- one of the brightest novae ever observed

We report on multiwavelength observations of nova SMCN 2016-10a. The present observational set is one of the most comprehensive for any nova in the Small Magellanic Cloud, including: low, medium, and high resolution optical spectroscopy and spectropolarimetry from SALT, FLOYDS, and SOAR; long-term OGLE $V$- and $I$- bands photometry dating back to six years before eruption; SMARTS optical and near-IR photometry from $\sim$ 11 days until over 280 days post-eruption; $Swift$ satellite X-ray and ultraviolet observations from $\sim$ 6 days until 319 days post-eruption. The progenitor system contains a bright disk and a main sequence or a sub-giant secondary. The nova is very fast with $t_2 \simeq$ 4.0 $\pm$ 1.0 d and $t_3 \simeq$ 7.8 $\pm$ 2.0 d in the $V$-band. If the nova is in the SMC, at a distance of $\sim$ 61 $\pm$ 10 kpc, we derive $M_{V,\mathrm{max}} \simeq - 10.5$ $\pm$ 0.5, making it the brightest nova ever discovered in the SMC and one of the brightest on record. At day 5 post-eruption the spectral lines show a He/N spectroscopic class and a FWHM of $\sim$ 3500 kms$^{-1}$ indicating moderately high ejection velocities. The nova entered the nebular phase $\sim$ 20 days post-eruption, predicting the imminent super-soft source turn-on in the X-rays, which started $\sim$ 28 days post-eruption. The super-soft source properties indicate a white dwarf mass between 1.2 M$_{\odot}$ and 1.3 M$_{\odot}$ in good agreement with the optical conclusions.

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arXiv2017

Paving the way to simultaneous multi-wavelength astronomy

Whilst astronomy as a science is historically founded on observations at optical wavelengths, studying the Universe in other bands has yielded remarkable discoveries, from pulsars in the radio, signatures of the Big Bang at submm wavelengths, through to high energy emission from accreting, gravitationally-compact objects and the discovery of gamma-ray bursts. Unsurprisingly, the result of combining multiple wavebands leads to an enormous increase in diagnostic power, but powerful insights can be lost when the sources studied vary on timescales shorter than the temporal separation between observations in different bands. In July 2015, the workshop "Paving the way to simultaneous multi-wavelength astronomy" was held as a concerted effort to address this at the Lorentz Center, Leiden. It was attended by 50 astronomers from diverse fields as well as the directors and staff of observatories and spaced-based missions. This community white paper has been written with the goal of disseminating the findings of that workshop by providing a concise review of the field of multi-wavelength astronomy covering a wide range of important source classes, the problems associated with their study and the solutions we believe need to be implemented for the future of observational astronomy. We hope that this paper will both stimulate further discussion and raise overall awareness within the community of the issues faced in a developing, important field.

astro-ph.IM↗

arXiv2016

MANTIS: The Mid-Frequency Aperture Array Transient and Intensity-Mapping System

The objective of this paper is to present the main characteristics of a wide-field MFAA precursor that we envisage to be built at the SKA site in South Africa. Known as MANTIS (the Mid-Frequency Aperture Array Transient and Intensity-Mapping System), this ambitious instrument will represent the next logical step towards the MFAA based SKA telescope. The goal is to use innovative aperture array technology at cm wavelengths, in order to demonstrate the feasibility of deploying huge collecting areas at modest construction and operational cost. Such a transformative step is required in order to continue the exponential progress in radio telescope performance, and to make the ambitious scale of the SKA Phase 2 a realistic near-time proposition.

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arXiv2013

The evolving polarised jet of black hole candidate Swift J1745-26

Swift J1745-26 is an X-ray binary towards the Galactic Centre that was detected when it went into outburst in September 2012. This source is thought to be one of a growing number of sources that display "failed outbursts", in which the self-absorbed radio jets of the transient source are never fully quenched and the thermal emission from the geometrically-thin inner accretion disk never fully dominates the X-ray flux. We present multifrequency data from the Very Large Array, Australia Telescope Compact Array and Karoo Array Telescope (KAT- 7) radio arrays, spanning the entire period of the outburst. Our rich data set exposes radio emission that displays a high level of large scale variability compared to the X-ray emission and deviations from the standard radio--X-ray correlation that are indicative of an unstable jet and confirm the outburst's transition from the canonical hard state to an intermediate state. We also observe steepening of the spectral index and an increase of the linear polarization to a large fraction (~50%) of the total flux, as well as a rotation of the electric vector position angle. These are consistent with a transformation from a self-absorbed compact jet to optically-thin ejecta -- the first time such a discrete ejection has been observed in a failed outburst -- and may imply a complex magnetic field geometry.

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arXiv2013

A return to strong radio flaring by Circinus X-1 observed with the Karoo Array Telescope test array KAT-7

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.

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arXiv2009

The UV-Excess Survey of the Northern Galactic Plane (UVEX)

The UV-Excess Survey of the Northern Galactic Plane images a 10x185 degree wide band, centered on the Galactic Equator using the 2.5m Isaac Newton Telescope in four bands (U,g,r,HeI5875) down to ~21st-22nd magnitude (~20th in HeI5875). The setup and data reduction procedures are described. Simulations of the colours of main-sequence stars, giant, supergiants, DA and DB white dwarfs and AM CVn stars are made, including the effects of reddening. A first look at the data of the survey (currently 30% complete) is given.

astro-ph.GA↗

arXiv2009

SDSS J080449.49+161624.8: A peculiar AM CVn star from a colour-selected sample of candidates

We describe a spectroscopic survey designed to uncover an estimated ~40 AM CVn stars hiding in the photometric database of the Sloan Digital Sky Survey (SDSS). We have constructed a relatively small sample of about 1500 candidates based on a colour selection, which should contain the majority of all AM CVn binaries while remaining small enough that spectroscopic identification of the full sample is feasible. We present the first new AM CVn star discovered using this strategy, SDSS J080449.49+161624.8, the ultracompact binary nature of which is demonstrated using high-time-resolution spectroscopy obtained at the Magellan telescopes at Las Campanas Observatory, Chile. A kinematic 'S-wave' feature is observed on a period 44.5+/-0.1min, which we propose is the orbital period, although the present data cannot yet exclude its nearest daily aliases. The new AM CVn star shows a peculiar spectrum of broad, single-peaked helium emission lines with unusually strong series of ionised helium, reminiscent of the (intermediate) polars among the hydrogen-rich Cataclysmic Variables. We speculate that SDSS J0804+1616 may be the first magnetic AM CVn star. The accreted material appears to be enriched in nitrogen, to N/O>~10 and N/C>10 by number, indicating CNO-cycle hydrogen burning, but no helium burning, in the prior evolution of the donor star.

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arXiv2007

Outbursts of EX Hydrae Revisited

We present optical spectroscopy of EX Hya during its 1991 outburst. This outburst is characterised by strong irradiation of the front face of the secondary star by the white dwarf, an overflowing stream which is seen strongly in HeII and by a dip in the light curves, which extends from 0.1-0.6 in the binary and spin phases. Strong irradiation of the accretion curtain and that of the inner regions of the disc led to strong emission of HeII and to the suppression of the Hg and Hb emission. Disc overflow was observed in quiescence in earlier studies, where the overflow stream material was modulated at high velocities close to 1000 km/s. In outburst, the overflowing material is modulated at even higher velocities (~1500 km/s). These are streaming velocities down the field lines close to the white dwarf. Evidence for material collecting near the outer edge of the disc and corotating with the accretion curtain was observed. In decline, this material and the accretion curtain obscured almost all the emission near binary phase 0.4, causing a dip. The dip minimum nearly corresponds with spin pulse minimum. This has provided additional evidence for an extended accretion curtain, and for the corotation of material with the accretion curtain at the outer edge of the disc. From these observations we suggest that a mechanism similar to that of Spruit & Taam, where outbursts result due to the storage and release of matter outside the magnetosphere, triggers the outbursts of EX Hya. This is followed by the irradiation of the secondary star due to accretion induced radiation.

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arXiv2007

The Discovery of a Persistent Quasi-Periodic Oscillation in the Intermediate Polar TX Col

We report on the detection of an ~5900 s quasi-periodic variation in the extensive photometry of TX Col spanning 12 years. We discuss five different models to explain this period. We favour a mechanism where the quasi-periodic variation results from the beating of the Keplerian frequency of the `blobs' orbiting in the outer accretion disc with the spin frequency, and from modulated accretion of these `blobs' taking place in a shocked region near the disc/magnetosphere boundary.

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arXiv2005

Hubble Space Telescope and Ground-Based Observations of Type Ia Supernovae at Redshift 0.5: Cosmological Implications

We present observations of the Type Ia supernovae (SNe) 1999M, 1999N, 1999Q, 1999S, and 1999U, at redshift z~0.5. They were discovered in early 1999 with the 4.0~m Blanco telescope at Cerro Tololo Inter-American Observatory by the High-z Supernova Search Team (HZT) and subsequently followed with many ground-based telescopes. SNe 1999Q and 1999U were also observed with the Hubble Space Telescope. We computed luminosity distances to the new SNe using two methods, and added them to the high-z Hubble diagram that the HZT has been constructing since 1995. The new distance moduli confirm the results of previous work. At z~0.5, luminosity distances are larger than those expected for an empty universe, implying that a ``Cosmological Constant,'' or another form of ``dark energy,'' has been increasing the expansion rate of the Universe during the last few billion years.

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arXiv2004

The Nonradially-Pulsating Primary of the Cataclysmic Variable GW Librae

The dwarf nova GW Librae is the first cataclysmic variable discovered to have a primary in a white dwarf instability strip, making it the first multi-mode, nonradially-pulsating star known to be accreting. The primaries of CVs, embedded in hot, bright accretion discs, are difficult to study directly. Applying the techniques of asteroseismology to GW Librae could therefore give us an unprecedented look at a white dwarf that has undergone ~10^9 years of accretion. However, an accreting white dwarf may have characteristics sufficiently different from those of single pulsating white dwarfs to render the standard models of white dwarf pulsations invalid for its study. This paper presents amplitude spectra of GW Lib from a series of observing campaigns conducted during 1997, 1998 and 2001. We find that the dominant pulsation modes cluster at periods near 650, 370 and 230 s, which also appear in linear combinations with each other. GW Lib's pulsation spectrum is highly unstable on time-scales of months, and exhibits clusters of signals very closely spaced in frequency, with separations on the order of a few microHz.

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