Elmar Koerding

Radboud University Nijmegen, Nymegen, Gelderland, Netherlands

Are you Elmar Koerding?

Claim your profile

Publications (92)406.42 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The outburst catalogue contains a wide variety of observational properties for 722 dwarf nova (DN)-type cataclysmic variables (CVs) and 309 CVs of other types from the Catalina Real-time Transient Survey. In particular, it includes the apparent outburst and quiescent V-band magnitudes, duty cycles, limits on the recurrence time, upper and lower limits on the distance and absolute quiescent magnitudes, colour information, orbital parameters and X-ray counterparts. These properties were determined by means of a classification script presented in this paper. The DN in the catalogue show a correlation between the outburst duty cycle and the orbital period (and outburst recurrence time), as well as between the quiescent absolute magnitude and the orbital period (and duty cycle). This is the largest sample of DN properties collected to date. Besides serving as a useful reference for individual systems and a means of selecting objects for targetted studies, it will prove valuable for statistical studies that aim to shed light on the formation and evolution of cataclysmic variables.
    No preview · Article · Dec 2015 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies.
    Preview · Article · Oct 2015 · Science Advances
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present quasi-simultaneous, multi-epoch radio and X-ray measurements of Holmberg II X-1 using the European VLBI Network (EVN), the Karl G. Jansky Very Large Array (VLA), and the Chandra and Swift X-ray telescopes. The X-ray data show apparently hard spectra with steady X-ray luminosities four months apart from each other. In the high-resolution EVN radio observations, we have detected an extended milliarcsecond scale source with unboosted radio emission. The source emits non-thermal, likely optically thin synchrotron emission, and its morphology is consistent with a jet ejection. The 9-GHz VLA data show an arcsecond-scale triple structure of Holmberg II X-1 similar to that seen at lower frequencies. However, we find that the central ejection has faded by at least a factor of 7.3 over 1.5 yr. We estimate the dynamical age of the ejection to be higher than 2.1 yr. We show that such a rapid cooling can be explained with simple adiabatic expansion losses. These properties of Holmberg II X-1 imply that ULX radio bubbles may be inflated by ejecta instead of self-absorbed compact jets.
    No preview · Article · Jun 2015 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Radio emission from non-magnetic cataclysmic variables (CVs, accreting white dwarfs) could allow detailed studies of outflows and possibly accretion flows in these nearby, numerous and non-relativistic compact accretors. Up to now, however, very few CVs have been detected in the radio. We have conducted a Very Large Array pilot survey of four close and optically bright novalike CVs at 6 GHz, detecting three, and thereby doubling the number of radio detections of these systems. TT Ari, RW Sex and the old nova V603 Aql were detected in both of the epochs, while V1084 Her was not detected (to a 3σ upper limit of 7.8 $\mu \rm {Jy} \rm {beam}^{-1}$). These observations clearly show that the sensitivity of previous surveys was typically too low to detect these objects and that non-magnetic CVs can indeed be significant radio emitters. The three detected sources show a range of properties, including flaring and variability on both short (∼200 s) and longer term (days) time-scales, as well as circular polarization levels of up to 100 per cent. The spectral indices range from steep to inverted; TT Ari shows a spectral turnover at ∼6.5 GHz, while the spectral index of V603 Aql flattened from α = 0.54 ± 0.05 to 0.16 ± 0.08 (Fν ∝ να) in the week between observations. This range of properties suggests that more than one emission process can be responsible for the radio emission in non-magnetic CVs. In this sample we find that individual systems are consistent with optically thick synchrotron emission, gyrosynchrotron emission or cyclotron maser emission.
    Preview · Article · May 2015 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Sco X-1 has been the subject of many multi-wavelength studies in the past, being the brightest persistent extra-solar X-ray source ever observed. Here we revisit Sco X-1 with simultaneous short cadence Kepler optical photometry and MAXI X-ray photometry over a 78 day period, as well as optical spectroscopy obtained with HERMES. We find Sco X-1 to be highly variable in all our datasets. The optical fluxes are clearly bimodal, implying the system can be found in two distinct optical states. These states are generally associated with the known flaring/normal branch X-ray states, although the flux distributions associated with these states overlap. Furthermore, we find that the optical power spectrum of Sco X-1 differs substantially between optical luminosity states. Additionally we find rms-flux relations in both optical states, but only find a linear relation during periods of low optical luminosity. The full optical/X-ray discrete correlation function displays a broad ~12.5 hour optical lag. However during the normal branch phase the X-ray and optical fluxes are anti-correlated, whilst being correlated during the flaring branch. We also performed a Cepstrum analysis on the full Kepler light curve to determine the presence of any echoes within the optical light curve alone. We find significant echo signals, consistent with the optical lags found using the discrete cross-correlation. We speculate that whilst some of the driving X-ray emission is reflected by the disk, some is absorbed and re-processed on the thermal timescale, giving rise to both the observed optical lags and optical echoes.
    Full-text · Article · May 2015 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of the physics of accretion and ejection around compact objects. For a summary, we refer to the paper.
    Full-text · Article · Jan 2015
  • Source
    Sjoert van Velzen · Heino Falcke · Elmar Koerding
    [Show abstract] [Hide abstract]
    ABSTRACT: Current wide-area radio surveys are dominated by active galactic nuclei, yet many of these sources have no identified optical counterparts. Here we investigate whether one can constrain the nature and properties of these sources, using Fanaroff–Riley type II (FR II) radio galaxies as probes. These sources are easy to identify since the angular separation of their lobes remains almost constant at some tens of arcseconds for z > 1. Using a simple algorithm applied to the Faint Images of the Radio Sky at Twenty-cm survey, we obtain the largest FR II sample to date, containing over 104 double-lobed sources. A subset of 459 sources is matched to Sloan Digital Sky Survey quasars. This sample yields a statistically meaningful description of the fraction of quasars with lobes as a function of redshift and luminosity. This relation is combined with the bolometric quasar luminosity function and a disc–lobe correlation to obtain a robust prediction for the density of FR IIs on the radio sky. We find that the observed density can be explained by the population of known quasars, implying that the majority of powerful jets originate from a radiatively efficient accretion flow with a linear jet–disc coupling. Finally, we show that high-redshift jets are more often quenched within 100 kpc, suggesting a higher efficiency of jet-induced feedback into their host galaxies.
    Preview · Article · Nov 2014 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present follow-up radio observations of ESO 243-49 HLX-1 from 2012 using the Australia Telescope Compact Array (ATCA) and the Karl G. Jansky Very Large Array (VLA). We report the detection of radio emission at the location of HLX-1 during its hard X-ray state using the ATCA. Assuming that the ‘Fundamental Plane’ of accreting black holes is applicable, we provide an independent estimate of the black hole mass of $M_{\rm {BH}}\le 2.8^{+7.5}_{-2.1} \times 10^{6}$ M⊙ at 90 per cent confidence. However, we argue that the detected radio emission is likely to be Doppler-boosted and our mass estimate is an upper limit. We discuss other possible origins of the radio emission such as being due to a radio nebula, star formation, or later interaction of the flares with the large-scale environment. None of these were found adequate. The VLA observations were carried out during the X-ray outburst. However, no new radio flare was detected, possibly due to a sparse time sampling. The deepest, combined VLA data suggest a variable radio source and we briefly discuss the properties of the previously detected flares and compare them with microquasars and active galactic nuclei.
    Full-text · Article · Nov 2014 · Monthly Notices of the Royal Astronomical Society
  • Elmar Körding
    [Show abstract] [Hide abstract]
    ABSTRACT: Accretion is a ubiquitous phenomenon—it is seen in sources ranging from young stars to accreting supermassive black holes in the centres of galaxies. Here, we present the known empirical connections between stellar mass X-ray binaries and active galactic nuclei. We argue that this implies that both the accretion disc and the jet are scale invariant with respect to the black hole mass. Finally, we show that also accretion discs and jets in sources with a different accretor can be connected empirically to accreting black holes, hinting towards a common mechanism of accretion in all sources.
    No preview · Article · Sep 2014 · Space Science Reviews
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on the discovery of an apparent triple radio structure hidden inside the radio bubble of the ultraluminous X-ray source Holmberg II X-1. The morphology is consistent with a collimated jet structure, which is observed to emit optically thin synchrotron radiation. The central component has a steep radio spectrum and is brighter than the outer components indicating a renewed radio activity. We estimate a minimum time-averaged jet power of 2 x 10^{39} erg/s that is associated with a time-averaged isotropic X-ray luminosity of at least 4 x 10^{39} erg/s. Our results suggest that Holmberg II X-1 is powered by a black hole of M_BH \geq 25 M_sun, that is inferred to be accreting at a high Eddington rate with intermittent radio activity.
    Full-text · Article · Nov 2013 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    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.
    Full-text · Article · Oct 2013 · Monthly Notices of the Royal Astronomical Society
  • Source
    P. -O. Petrucci · C. Cabanac · S. Corbel · E. Koerding · R. Fender
    [Show abstract] [Hide abstract]
    ABSTRACT: The microquasar GX 339-4 was observed by Suzaku five times, spaced by a few days, during its transition back to the hard state at the end of its 2010-2011 outburst. The 2-10 keV source flux decreases by a factor ~10 between the beginning and the end of the monitoring. Simultaneous radio and OIR observations highlighted the re-ignition of the radio emission just before the beginning of the campaign, the maximum radio emission being reached between the two first Suzaku pointings, while the IR peaked a few weeks latter. A fluorescent iron line is always significantly detected. Fits with a gaussian or Laor profiles give statistically equivalent results. In the case of a Laor profile, fits of the five data sets simultaneously agree with a disk inclination angle of ~20 degrees. The disk inner radius is <10-30 R_g in the first two observations but almost unconstrained in the last three. A soft X-ray excess is also present in these two first observations. Fits with a multicolor disk component give disk inner radii in agreement with those obtained with the iron line fits. The use of a physically more realistic model, including a blurred reflection component and a comptonization continuum, give some hints of the increase of the disk inner radius but the significances are always weak. Interestingly, the addition of warm absorption significantly improves the fit of OBS1 while it is not needed in the other observations. The radio-jet re-ignition occurring between OBS1 and OBS2, these absorption features may indicate the natural evolution from a disk wind and a jet. The comparison with a long 2008 Suzaku observation of GX 339-4 in a persistent faint hard state where a narrow iron line clearly indicates a disk recession, is discussed.
    Preview · Article · Oct 2013 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An international consortium is presently constructing a beamformer for the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile that will be available as a facility instrument. The beamformer will aggregate the entire collecting area of the array into a single, very large aperture. The extraordinary sensitivity of phased ALMA, combined with the extremely fine angular resolution available on baselines to the Northern Hemisphere, will enable transformational new very long baseline interferometry (VLBI) observations in Bands 6 and 7 (1.3 and 0.8 mm) and provide substantial improvements to existing VLBI arrays in Bands 1 and 3 (7 and 3 mm). The ALMA beamformer will have impact on a variety of scientific topics, including accretion and outflow processes around black holes in active galactic nuclei (AGN), tests of general relativity near black holes, jet launch and collimation from AGN and microquasars, pulsar and magnetar emission processes, the chemical history of the universe and the evolution of fundamental constants across cosmic time, maser science, and astrometry.
    Full-text · Article · Sep 2013
  • Source
    D. Cseh · F. Grise · P. Kaaret · S. Corbel · S. Scaringi · P. Groot · H. Falcke · E. Koerding
    [Show abstract] [Hide abstract]
    ABSTRACT: We obtained multi-epoch Very Large Telescope optical spectroscopic data in 2011 and 2012 on the ultraluminous X-ray source NGC 5408 X-1. We confirm that the He ii λ4686 line has a broad component with an average full width at half-maximum of v = 780 ± 64 km s−1 with a variation of ∼13 per cent during observations spanning over four years, and is consistent with the origin in the accretion disc. The deepest optical spectrum does not reveal any absorption line from a donor star. Our aim was to measure the radial velocity curve and estimate the parameters of the binary system. We find an upper limit on the semi-amplitude of the radial velocity of K = 132 ± 42 km s−1. A search for a periodic signal in the data resulted in no statistically significant period. The mass function and constraints on the binary system imply a black hole mass of less than ∼510 M⊙. Whilst, a disc irradiation model may imply a black hole mass smaller than ∼431–1985 M⊙, depending on inclination. Our data can also be consistent with an unexplored orbital period range from a couple of hours to a few days, thus with a stellar-mass black hole and a subgiant companion.
    Full-text · Article · Aug 2013 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Dwarf novae are white dwarfs accreting matter from a nearby red dwarf companion. Their regular outbursts are explained by a thermal-viscous instability in the accretion disc, described by the disc instability model that has since been successfully extended to other accreting systems. However, the prototypical dwarf nova, SS Cygni, presents a major challenge to our understanding of accretion disc theory. At the distance of 159 ± 12 parsecs measured by the Hubble Space Telescope, it is too luminous to be undergoing the observed regular outbursts. Using very long baseline interferometric radio observations, we report an accurate, model-independent distance to SS Cygni that places the source substantially closer at 114 ± 2 parsecs. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects.
    Full-text · Article · May 2013 · Science
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report striking changes in the broadband spectrum of the compact jet of the black hole transient MAXI J1836–194 over state transitions during its discovery outburst in 2011. A fading of the optical-infrared (IR) flux occurred as the source entered the hard-intermediate state, followed by a brightening as it returned to the hard state. The optical-IR spectrum was consistent with a power law from optically thin synchrotron emission, except when the X-ray spectrum was softest. By fitting the radio to optical spectra with a broken power law, we constrain the frequency and flux of the optically thick/thin break in the jet synchrotron spectrum. The break gradually shifted to higher frequencies as the source hardened at X-ray energies, from ~1011 to ~4 × 1013 Hz. The radiative jet luminosity integrated over the spectrum appeared to be greatest when the source entered the hard state during the outburst decay (although this is dependent on the high-energy cooling break, which is not seen directly), even though the radio flux was fading at the time. The physical process responsible for suppressing and reactivating the jet (neither of which are instantaneous but occur on timescales of weeks) is uncertain, but could arise from the varying inner accretion disk radius regulating the fraction of accreting matter that is channeled into the jet. This provides an unprecedented insight into the connection between inflow and outflow, and has implications for the conditions required for jets to be produced, and hence their launching process.
    Preview · Article · Apr 2013 · The Astrophysical Journal Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the first study of Fourier-frequency-dependent coherence and phase/time lags at optical wavelengths of cataclysmic variables (MV Lyr and LU Cam) displaying typical flickering variability in white light. Observations were performed on the William Herschel Telescope using ULTRACAM. Light curves for both systems have been obtained with the SDSS filters u′, g′ and r′ simultaneously with cadences in the range ≈0.5–2 s, and allow us to probe temporal frequencies between ≈10−3 and ≈1 Hz. We find high levels of coherence between the u′, g′ and r′ light curves up to at least ≈10−2 Hz. Furthermore, we detect red/negative lags where the redder bands lag the bluer ones at the lowest observed frequencies. For MV Lyr time lags up to ≈3 s are observed, whilst LU Cam displays larger time lags of ≈10 s. Mechanisms which seek to explain red/negative lags observed in X-ray binaries and active galactic nuclei involve reflection of photons generated close to the compact object on to the surface layers of the accretion disc, where the lag delay is simply the light travel time from the emitting source to the reflecting accretion disc area. Although this could be a viable explanation for the lags observed in MV Lyr, the lags observed in LU Cam are too large to be explained by reflection from the disc and/or the donor star. We suggest reprocessing on the thermal time-scale of boundary layer photons on to the accretion disc as a possible mechanism to explain the lags observed in accreting white dwarfs, or reverse (inside-out) shocks within the disc travelling through cooler disc regions as they move outwards.
    Full-text · Article · Feb 2013 · Monthly Notices of the Royal Astronomical Society
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present the first results of an ongoing spectroscopic follow-up programme of blue-Hα-excess sources within the Kepler field of view in order to identify new cataclysmic variables. Kepler observations of the identified targets in this work will then provide detailed, time-resolved, studies of accretion. Candidates selected from the Kepler-Isaac Newton Telescope Survey were observed with the 4.2-m William Herschel Telescope. Out of 38 observed candidates, we found 11 new cataclysmic variables reported here for the first time, as well as 13 new quasars. Our target selection has a success rate of 29 per cent when searching for cataclysmic variables, and we show how this can be improved by including photometry obtained with the Wide-field Infrared Survey Explorer.
    No preview · Article · Jan 2013 · Monthly Notices of the Royal Astronomical Society
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Jan 2013
  • [Show abstract] [Hide abstract]
    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.
    No preview · Article · Jan 2013

Publication Stats

2k Citations
406.42 Total Impact Points

Institutions

  • 2011-2015
    • Radboud University Nijmegen
      • Department of Astrophysics
      Nymegen, Gelderland, Netherlands
  • 2009-2011
    • Paris Diderot University
      • AstroParticule et Cosmologie (APC) UMR 7164
      Lutetia Parisorum, Île-de-France, France
  • 2006-2009
    • University of Southampton
      • • Department of Physics and Astronomy
      • • Institute of Developmental Sciences
      Southampton, England, United Kingdom
  • 2006-2007
    • University of New Hampshire
      Durham, New Hampshire, United States
  • 2005-2006
    • Max Planck Institute for Radio Astronomy
      Bonn, North Rhine-Westphalia, Germany
    • Universität Heidelberg
      • Institute of Theoretical Physics
      Heidelburg, Baden-Württemberg, Germany
  • 2004
    • Johns Hopkins University
      Baltimore, Maryland, United States