Results for "H. Falcke"

total 85913took 0.14s
Simulation of radio emission from air showers in atmospheric electric fieldsFeb 25 2010We study the effect of atmospheric electric fields on the radio pulse emitted by cosmic ray air showers. Under fair weather conditions the dominant part of the radio emission is driven by the geomagnetic field. When the shower charges are accelerated ... More
Coupled jet-disk model for Sgr A*: explaining the flat-spectrum radio core with GRMHD simulations of jetsOct 18 2013The supermassive black hole in the center of the Milky Way, Sgr A*, displays a nearly flat radio spectrum that is typical for jets in active galactic nuclei. Indeed, time-dependent magnetized models of radiatively inefficient accretion flows (RIAFs), ... More
Starving Accretion Disks in the Center of Galaxies: the Examples of Sgr A* and M31Oct 20 1994The low accretion rate inferred for the GC source Sgr A* requires a distinct type of accretion disk -- a starving disk. We investigate the structure of this type of disk within the framework of thin Keplerian disk models taking into account relativistic ... More
Constraints on the low frequency spectrum of FRB 121102Feb 05 2019While repeating fast radio bursts (FRBs) remain scarce in number, they provide a unique opportunity for follow-up observations that enhance our knowledge of their sources and potentially of the FRB population as a whole. Attaining more burst spectra could ... More
Cosmic-ray energy spectrum and composition up to the ankle - the case for a second Galactic componentMay 10 2016Nov 11 2016We have carried out a detailed study to understand the observed energy spectrum and composition of cosmic rays with energies up to ~10^18 eV. Our study shows that a single Galactic component with subsequent energy cut-offs in the individual spectra of ... More
The NuMoon experiment: first resultsAug 13 2008The NuMoon project uses the Westerbork Synthesis Radio Telescope to search for short radio pulses from the Moon. These pulses are created when an ultra high energy cosmic ray or neutrino initiates a particle cascade inside the Moon's regolith. The cascade ... More
Prospects for radio detection of ultra-high energy cosmic rays and neutrinosSep 09 2004The origin and nature of the highest energy cosmic ray events is currently the subject of intense investigation by giant air shower arrays and fluorescent detectors. These particles reach energies well beyond what can be achieved in ground-based particle ... More
Atmosphere-Corrected Phase-ReferencingSep 22 2003One major problem of phase-referencing VLBI observations are phase errors due to the unknown tropospheric zenith delay at each antenna. These errors degrade the quality of the phase-referenced image and limit the achievable astrometric accuracy. We will ... More
Air Shower Measurements with LOFARMar 13 2009Air showers from cosmic rays emit short, intense radio pulses. LOFAR is a new radio telescope, that is being built in the Netherlands and Europe. Designed primarily as a radio interferometer, the core of LOFAR will have a high density of radio antennas, ... More
Compact radio cores: from the first black holes to the lastSep 06 2004One of the clearest signs of black hole activity is the presence of a compact radio core in the nuclei of galaxies. With the Square Kilometer Array (SKA) these cores can be used to study the evolution of black holes throughout the universe and even to ... More
Status report of the NuMoon experimentOct 19 2008We show that at wavelengths comparable to the length of the shower produced by an Ultra-High Energy cosmic ray or neutrino, radio signals are an extremely efficient way to detect these particles. First results are presented of an analysis of 20 hours ... More
Constraints on the flux of Ultra-High Energy neutrinos from WSRT observationsApr 02 2010Ultra-high energy (UHE) neutrinos and cosmic rays initiate particle cascades underneath the Moon's surface. These cascades have a negative charge excess and radiate Cherenkov radio emission in a process known as the Askaryan effect. The optimal frequency ... More
A Radio Monitoring Survey of Ultra-Luminous X-Ray SourcesFeb 14 2005We present the results of a radio monitoring campaign to search for radio emission from nearby ultra-luminous X-ray sources (ULXs). These sources are bright off-nuclear X-ray point sources with luminosities exceeding L_X > 10^39 erg/sec. A well-defined ... More
Precision measurements of cosmic ray air showers with the SKAAug 22 2014Jan 08 2015Supplemented with suitable buffering techniques, the low-frequency part of the SKA can be used as an ultra-precise detector for cosmic-ray air showers at very high energies. This would enable a wealth of scientific applications: the physics of the transition ... More
VLBI observations of weak sources using fast frequency switchingDec 21 2004Dec 21 2004We have developed a new phase referencing technique for high frequency VLBI observations. In conventional phase referencing, one interleaves short scans on a nearby phase calibrator between the target source observations. In fast frequency switching described ... More
Modeling the X-ray Contribution of X-ray Binary JetsJan 23 2003Astrophysical jets exist in both XRBs and AGN, and seem to share common features, particularly in the radio. While AGN jets are known to emit X-rays, the situation for XRB jets is not so clear. Radio jets have been resolved in several XRBs in the low/hard ... More
The lunar Askaryan technique: a technical roadmapSep 17 2015The lunar Askaryan technique, which involves searching for Askaryan radio pulses from particle cascades in the outer layers of the Moon, is a method for using the lunar surface as an extremely large detector of ultra-high-energy particles. The high time ... More
The effect of the atmospheric refractive index on the radio signal of extensive air showersJan 25 2017For the interpretation of measurements of radio emission from extensive air showers, an important systematic uncertainty arises from natural variations of the atmospheric refractive index $n$. At a given altitude, the refractivity $N=10^6\, (n-1)$ can ... More
Measurement of the cosmic-ray energy spectrum above $10^{16}$ eV with the LOFAR Radboud Air Shower ArrayJun 30 2015Jul 01 2015The energy reconstruction of extensive air showers measured with the LOFAR Radboud Air Shower Array (LORA) is presented in detail. LORA is a particle detector array located in the center of the LOFAR radio telescope in the Netherlands. The aim of this ... More
Realtime processing of LOFAR data for the detection of nano-second pulses from the MoonDec 20 2016The low flux of the ultra-high energy cosmic rays (UHECR) at the highest energies provides a challenge to answer the long standing question about their origin and nature. Even lower fluxes of neutrinos with energies above $10^{22}$ eV are predicted in ... More
Assessing the X-ray Contribution from Jets in X-ray BinariesAug 05 2002Astrophysical jets exist from the stellar scale up to AGN, and seem to share common features particularly in the radio. But while AGN jets are known to emit X-rays, the situation for XRB jets is not so clear. Radio jets have been resolved in several XRBs ... More
High-precision measurements of extensive air showers with the SKAAug 14 2015As of 2023, the Square Kilometre Array will constitute the world's largest radio telescope, offering unprecedented capabilities for a diverse science programme in radio astronomy. At the same time, the SKA will be ideally suited to detect extensive air ... More
Search for Cosmic Particles with the Moon and LOFARSep 21 2016The low flux of the ultra-high energy cosmic rays (UHECR) at the highest energies provides a challenge to answer the long standing question about their origin and nature. A significant increase in the number of detected UHECR is expected to be achieved ... More
Cosmic Ray Physics with the LOFAR Radio TelescopeMar 20 2019The LOFAR radio telescope is able to measure the radio emission from cosmic ray induced air showers with hundreds of individual antennas. This allows for precision testing of the emission mechanisms for the radio signal as well as determination of the ... More
VLBA Observations and Effelsberg Monitoring of a Flaring Water Maser in Mrk 348Jul 09 2001We report on single-dish monitoring as well as extremely high angular resolution observations of the flaring H2O maser in the Seyfert 2 galaxy Mrk 348. The H2O line is redshifted by ~130 km/s with respect to the systemic velocity, is very broad, with ... More
Detection of Circular Polarization in M81*Sep 11 2001We report the detection of circular polarization in the compact radio jet of the nearby spiral galaxy M81 (M81*). The observations were made with the Very Large Array at 4.8 and 8.4 GHz and circular polarization was detected at both frequencies. We estimate ... More
VLBI detections of a source weaker than 100 mJy at 86 GHzSep 27 2004We use a new phase-calibration strategy to calibrate the phase of 86 GHz VLBI observations of the FR I radio galaxy NGC 4261. Instead of switching between a calibrator source and the target source, the target was observed while rapidly switching between ... More
An air shower array for LOFAR: LORAFeb 04 2011LOFAR is a new form of radio telescope which can detect radio emission from air showers induced by very high-energy cosmic rays. It can also look for radio emission from particle cascades on the Moon induced by ultra high-energy cosmic rays or neutrinos. ... More
Monte Carlo simulations of air showers in atmospheric electric fieldsOct 28 2009The development of cosmic ray air showers can be influenced by atmospheric electric fields. Under fair weather conditions these fields are small, but the strong fields inside thunderstorms can have a significant effect on the electromagnetic component ... More
Probing spacetime around Sagittarius A* using modeled VLBI closure phasesFeb 04 2016The emission region and black hole shadow of Sagittarius A*, the supermassive black hole at the Galactic Center, can be probed with millimeter Very Long Baseline Interferometry. Our goal is to probe the geometry of the emitting plasma around Sgr A* by ... More
Radio Luminosity Function, Importance of Jet Power, and Radio Properties of Nearby Low-Luminosity Active Galactic NucleiDec 13 2004We present the completed results of a high resolution radio imaging survey of all (around 200) low-luminosity active galactic nuclei (LLAGNs) in the Palomar spectroscopic Sample of all (around 470) nearby bright northern galaxies. The high incidence of ... More
Prospects for direct cosmic ray mass measurements through the Gerasimova-Zatsepin effectApr 16 2008The Solar radiation field may break apart ultra high energy cosmic nuclei, after which both remnants will be deflected in the interplanetary magnetic field in different ways. This process is known as the Gerasimova-Zatsepin effect after its discoverers. ... More
Lunar detection of ultra-high-energy cosmic rays and neutrinos with the Square Kilometre ArrayAug 26 2014Dec 19 2014The origin of the most energetic particles in nature, the ultra-high-energy (UHE) cosmic rays, is still a mystery. Only the most energetic of these have sufficiently small angular deflections to be used for directional studies, and their flux is so low ... More
A method for high precision reconstruction of air shower Xmax using two-dimensional radio intensity profilesAug 29 2014Sep 01 2014The mass composition of cosmic rays contains important clues about their origin. Accurate measurements are needed to resolve long-standing issues such as the transition from Galactic to extragalactic origin, and the nature of the cutoff observed at the ... More
Status of the Lunar Detection Mode for Cosmic Particles of LOFARMar 20 2019Cosmic particles hitting Earth's moon produce radio emission via the Askaryan effect. If the resulting radio ns-pulse can be detected by radio telescopes, this technique potentially increases the available collective area for ZeV scale particles by several ... More
The jet-disk symbiosis model for Gamma Ray Bursts: cosmic ray and neutrino background contributionMar 02 2000Feb 03 2001The relation between the cosmological evolution of the jet-disk symbiosis model for GRBs and the cosmic rays energy distribution is presented. We used two different Star Formation Rates (SFR) as a function of redshift and a Luminosity Function (LF) distribution ... More
Detection and imaging of atmospheric radio flashes from cosmic ray air showersMay 18 2005The nature of ultrahigh-energy cosmic rays (UHECRs) at energies >10^20 eV remains a mystery. They are likely to be of extragalactic origin, but should be absorbed within ~50 Mpc through interactions with the cosmic microwave background. As there are no ... More
Prospects for determining air shower characteristics through geosynchrotron emission arrival timesApr 05 2010Using simulations of geosynchrotron radiation from extensive air showers, we present a relation between the shape of the geosynchrotron radiation front and the distance of the observer to the maximum of the air shower. By analyzing the relative arrival ... More
Exploring the Role of Jets in the Radio/X-ray Correlations of GX339-4Oct 18 2002The Galactic black hole candidate X-ray binary GX339-4 spends most of its time in the low/hard state, making it an ideal candidate for modeling the assumedly low accretion phase. The radio emission correlates very tightly with the X-rays over more than ... More
III Zw 2: superluminal expansion and compact lobe expansion in a Seyfert galaxySep 28 2000So far, all relativistically boosted jets with superluminal motion have only been detected in typical radio galaxies with early type host galaxies. We have now discovered superluminal motion in the Seyfert I galaxy III Zw 2, classified as a spiral. The ... More
The extreme flare in III Zw 2: Evolution of a radio jet in a Seyfert galaxyJan 27 2005A very detailed monitoring of a radio flare in the Seyfert I galaxy III Zw 2 with the VLA and the VLBA is presented. The relative astrometry in the VLBA observations was precise on a level of a few microarcseconds. Spectral and spatial evolution of the ... More
Radio Variability of Sgr A* at Centimeter WavelengthsSep 27 1999Results of two years of continuous monitoring of flux density variations at 8.3 and 2.3 GHz of the Galactic Center super-massive black hole candidate Sgr A* are reported. The average RMS modulation indices are 6% and 2.5% at 8.3 & 2.3 GHz respectively. ... More
Black Hole, Jet, and Disk: The Universal EngineDec 14 1995In this paper I review the results of our ongoing project to investigate the coupling between accretion disk and radio jet in galactic nuclei and stellar mass black holes. We find a good correlation between the UV bump luminosity and the radio luminosities ... More
The Silent Majority - Jets and Radio Cores from Low-Luminosity Black HolesNov 06 2001They are weak, they are small, and they are often overlooked, but they are numerous and an ubiquitous sign of accreting black holes: compact radio cores and jets in low-power AGN. Here I summarize our work concerning these radio cores and jets in recent ... More
The Nuclear Jet in M81Apr 04 1996In this paper we apply the jet-disk symbiosis model developed for Sgr A* to M81* -- the nucleus of the nearby galaxy M81. The model accurately predicts radio flux and size of M81* for the observed bolometric luminosity of the nuclear source with no major ... More
Dust lanes causing structure in the extended narrow line region of early-type Seyfert galaxiesFeb 16 1999Jun 02 1999We construct near-infrared to visible broad band NICMOS/WFPC color maps for 4 early-type Seyfert galaxies with S-shaped or one-sided ionization cones. We find that dust lanes are near or connected to many of the features seen in the [OIII] and H$\alpha$+[NII] ... More
Calibration of the LOFAR low-band antennas using the Galaxy and a model of the signal chainMar 14 2019The LOw-Frequency ARray (LOFAR) is used to make precise measurements of radio emission from extensive air showers, yielding information about the primary cosmic ray. Interpreting the measured data requires an absolute and frequency-dependent calibration ... More
Compact Radio Cores in the Galactic Center and ElsewhereOct 14 1996Compact radio cores are not only common in radio galaxies and quasars but also in many nearby galaxies with low-active, supermassive black holes. One famous example is the Galactic Center source Sgr A*. Recent studies of proper motions and radial velocities ... More
Jets in AGN -- New Results from HST and VLAFeb 18 1998This paper summarizes some of our recent projects which try to illuminate the nature and importance of jets associated with active nuclei and compact objects. After a short introduction on jets in radio galaxies and radio loud quasars the paper focuses ... More
WHAT IS SGR A*? The Starved Black Hole in the Center of the Milky WayNov 16 1994Sgr A* is the unique 1 Jy flat spectrum radio point source located at the dynamical center of the Galaxy and in the very center of the central star cluster. Due to its unusual appearance it has long been speculated that this source is powered by a supermassive ... More
The Jet Model for Sgr A*Sep 27 1999In this paper the jet model for the supermassive black hole candidate Sgr A* in the Center of the Galaxy is reviewed. The most recent model, with a reduced set of parameters, is able to account for all major radio properties of the source: size, structure, ... More
Sgr A* and its siblings in nearby galaxiesMay 01 1996We have proposed previously that Sgr A* is simply a scaled down AGN with a black hole, an accretion disk and a radio jet operating at a very low power. It appears as if M81* - the nuclear source in the nearby galaxy M81 - is an ideal laboratory to study ... More
Gigahertz Peaked Spectrum sources from the Jodrell Bank-VLA Astrometric SurveyNov 09 1998Jan 15 1999Observations with MERLIN at 408 MHz have been used to establish the low-frequency part of the spectra of more than a hundred compact radio sources taken from the part of the Jodrell Bank--VLA Astrometric Survey limited by 35 deg. < \delta < 75 deg. These ... More
Proper Motions in the Andromeda SubgroupAug 13 2007This article presents results of VLBI observations of regions of H2O maser activity in the Local Group galaxies M33 and IC10. Since all position measurements were made relative to extragalactic background sources, the proper motions of the two galaxies ... More
Radio Sources in Low-Luminosity Active Galactic Nuclei. III. "AGNs" in a Distance-Limited Sample of "LLAGNs"Jul 08 2002(abbreviated): This paper presents the results of a high resolution radio imaging survey of all known (96) low-luminosity active galactic nuclei (LLAGNs) at D<19Mpc. We find that almost half of all LINERs and low-luminosity Seyferts have flat-spectrum ... More
Evidence of a pure starburst nature of the nuclear region of NGC 253Feb 06 2009We present high-resolution spectral line and continuum VLBI and VLA observations of the nuclear region of NGC 253 at 22 GHz. While the water vapor masers in this region were detected on arcsecond and milliarcsecond scales, we could not detect any compact ... More
VLBI observations of jupiter with the initial test station of LOFAR and the nancay decametric arraySep 16 2008AIMS: To demonstrate and test the capability of the next generation of low-frequency radio telescopes to perform high resolution observations across intra-continental baselines. Jupiter's strong burst emission is used to perform broadband full signal ... More
The Geometric Distance and Proper Motion of the Triangulum Galaxy (M33)Mar 02 2005We measured the angular rotation and proper motion of the Triangulum Galaxy (M33) with the Very Long Baseline Array by observing two H2O masers on opposite sides of the galaxy. By comparing the angular rotation rate with the inclination and rotation speed, ... More
Influence of Atmospheric Electric Fields on the Radio Emission from Extensive Air ShowersNov 10 2015The atmospheric electric fields in thunderclouds have been shown to significantly modify the intensity and polarization patterns of the radio footprint of cosmic-ray-induced extensive air showers. Simulations indicated a very non-linear dependence of ... More
Towards a dynamical mass of the ultraluminous X-ray source NGC 5408 X-1Aug 06 2013We obtained multi-epoch Very Large Telescope (VLT) optical spectroscopic data in 2011 and 2012 on the ultraluminous X-ray source (ULX) NGC 5408 X-1. We confirm that the HeII\lambda4686 line has a broad component with an average FWHM of v=780\pm64 km/s ... More
Why the fundamental plane of black hole activity is not simply a distance driven artifactJan 13 2006The fundamental plane of black hole activity is a non-linear correlation among radio core luminosity, X-ray luminosity and mass of all accreting black holes, both of stellar mass and supermassive, found by Merloni, Heinz and Di Matteo (2003) and, independently, ... More
The Linear Polarization of Sagittarius A* II. VLA and BIMA Polarimetry at 22, 43 and 86 GHzJul 21 1999We present a search for linear polarization at 22 GHz, 43 GHz and 86 GHz from the nearest super massive black hole candidate, Sagittarius A*. We find upper limits to the linear polarization of 0.2%, 0.4% and 1%, respectively. These results strongly support ... More
Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHzFeb 11 2003We measured the linear polarization of Sagittarius A* to be 7.2 +/- 0.6 % at 230 GHzusing the BIMA array with a resolution of 3.6 x 0.9 arcsec. This confirms the previously reported detection with the JCMT 14-m antenna. Our high resolution observations ... More
Molecular Hydrogen and Paschen-alpha Emission in Cooling Flow GalaxiesDec 18 1997We present near-infrared spectra obtained to search for Pa-alpha and molecular hydrogen lines in edge-darkened (FR I-type) radio galaxies with bright Halpha emission in the redshift range 0.0535<z<0.15. We find that all three galaxies in our sample (PKS ... More
The Twin--Jet System in NGC 1052: VLBI-Scrutiny of the Obscuring TorusJul 14 2004NGC 1052 offers the possibility to study the obscuring torus around a supermassive black hole, predicted by the standard model of active galactic nuclei, over a wide range of wavelengths from the radio to the X-ray regime. We present a detailed VLBI study ... More
Towards Proper Motions in the Local GroupJun 12 2002Key and still largely missing parameters for measuring the mass content and distribution of the Local Group are the proper motion vectors of its member galaxies. The problem when trying to derive the gravitational potential of the Local Group is that ... More
The Proper Motion of the Local Group Galaxy IC 10Oct 26 2006We have measured the proper motion of the Local Group galaxy IC 10 with the Very Long Baseline Array by measuring the position of an H2O maser relative to two background quasars over 4.3 years. After correction for the rotation of the Milky Way and IC ... More
Discovery of a bright radio transient in M82: a new radio supernova?Apr 15 2009May 28 2009In this Letter, we report the discovery of a new bright radio transient in M82. Using the Very Large Array, we observed the nuclear region of M82 at several epochs at 22 GHz and detected a new bright radio source in this galaxy's central region. We find ... More
Probing the Dark Ages with the Square Kilometer ArraySep 13 2004Sep 20 2004The epoch of reionization (EoR) sets a fundamental benchmark in cosmic structure formation, corresponding to the formation of the first luminous objects that act to ionize the neutral intergalactic medium (IGM). Recent observations at near-IR and radio ... More
Gigahertz-Peaked Spectrum Radio Sources in Nearby GalaxiesJan 10 2002There is now strong evidence that many low-luminosity AGNs (LLAGNs) contain accreting massive black holes and that the nuclear radio emission is dominated by parsec-scale jets launched by these black holes. Here, we present preliminary results on the ... More
Universality of electron-positron distributions in extensive air showersFeb 03 2009Using a large set of simulated extensive air showers, we investigate universality features of electron and positron distributions in very-high-energy cosmic-ray air showers. Most particle distributions depend only on the depth of the shower maximum and ... More
Jet emission in NGC1052 at radio, optical, and X-ray frequenciesMar 08 2004We present a combined radio, optical, and X-ray study of the nearby LINER galaxy NGC 1052. Data from a short (2.3 ksec) {\it CHANDRA} observation of NGC 1052 reveal the presence of various jet-related X-ray emitting regions, a bright compact core and ... More
The Rotation Measure and 3.5mm Polarization of Sgr A*Jun 15 2006We report the detection of variable linear polarization from Sgr A* at a wavelength of 3.5mm, the longest wavelength yet at which a detection has been made. The mean polarization is 2.1 +/- 0.1% at a position angle of 16 +/- 2 deg with rms scatters of ... More
Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion FlowNov 18 2004We report the discovery of variability in the linear polarization from the Galactic Center black hole source, Sagittarius A*. New polarimetry obtained with the Berkeley-Illinois-Maryland Association array at a wavelength of 1.3 mm shows a position angle ... More
Measurement of the circular polarization in radio emission from extensive air showers confirms emission mechanismsNov 02 2016Nov 19 2016We report here on a novel analysis of the complete set of four Stokes parameters that uniquely determine the linear and/or circular polarization of the radio signal for an extensive air shower. The observed dependency of the circular polarization on azimuth ... More
Towards a kinematic model of the Local GroupNov 07 2006Measuring the proper motions and geometric distances of galaxies within the Local Group is very important for our understanding of the history, present state and future of the Local Group. Currently, proper motion measurements using optical methods are ... More
Dense Gas in Nearby Galaxies: XIV. Detection of hot ammonia in Maffei 2Oct 25 2000The (J,K) = (1,1), (2,2), (3,3), and (4,4) inversion lines of ammonia (NH3) have been detected toward the nuclear 40'' sized bar of the nearby spiral galaxy Maffei 2. The relative intensities of the ammonia lines are characterized by a rotational temperature ... More
H_2O megamaser emission from FR I radio galaxiesApr 17 1998A systematic search for 22 GHz H2O megamaser emission is reported for 50 nearby (z < 0.15) FR I galaxies. No detection was obtained, implying that ultraluminous H2O masers (L_H2O>10^3 L_sun) must be rare in early-type galaxies with FR I radio morphology. ... More
NGC1052 - A study of the pc-scale twin jetApr 03 2002We present results of a VLBA multi-frequency study of the pc-scale twin jet in NGC1052. We observed this object at epoch 1998.99 with the VLBA at 5, 8.4, 22 and 43 GHz both in total and linearly polarized intensity. The spectral analysis confirms the ... More
BIMA Observations of Linear Polarization in Sagittarius A* at 112 GHzJun 08 2001We report here BIMA array observations of linear polarization in Sagittarius A*, the compact radio source in the Galactic Center. These observations had a resolution of 20 arcsec x 5 arcsec oriented North-South. We do not detect linear or circular polarization ... More
Higher Resolution VLBI Imaging with Fast Frequency SwitchingJul 03 2002Millimetre-VLBI is an important tool in AGN astrophysics, but it is limited by short atmospheric coherence times and poor receiver and antenna performance. We demonstrate a new kind of phase referencing for the VLBA, enabling us to increase the sensitivity ... More
Asymmetric structure in Sgr A* at 3mm from closure phase measurements with VLBA, GBT and LMTAug 23 2016We present the results of a closure phase analysis of 3 mm very long baseline interferometry (VLBI) measurements performed on Sagittarius A* (Sgr A*). We have analyzed observations made in May 2015 using the Very Long Baseline Array, the Robert C. Byrd ... More
Sgr A* and Company - Multiwavelength observations of Sgr A* and VLA search of ``Sgr A*'s'' in LINERsJun 24 1997We report first results from a multiwavelength campaign to measure the simultaneous spectrum of Sgr A* from cm to mm wavelengths. The observations confirm that the previously detected submm-excess is not due to variability; the presence of an ultracompact ... More
The MHz-peaked radio spectrum of the unusual gamma-ray source PMN J1603-4904Sep 07 2016Context. The majority of bright extragalactic gamma-ray sources are blazars. Only a few radio galaxies have been detected by Fermi/LAT. Recently, the GHz-peaked spectrum source PKS 1718-649 was confirmed to be gamma-ray bright, providing further evidence ... More
The Flaring H2O Megamaser and Compact Radio Source in Mrk 348Mar 18 2003We report on single-dish monitoring and extremely high angular resolution observations of the flaring H2O megamaser in the Seyfert 2 galaxy Mrk 348. The H2O line is redshifted by ~130 km/s with respect to the systemic velocity, is very broad, with a FWHM ... More
A Flaring Megamaser in Mrk 348May 17 2001We report new observations of the H2O megamaser in the Seyfert 2 galaxy Mrk 348. Following our initial detection in 2000 March using the Effelsberg 100 m telescope, re-analysis of previous data on this source indicates that the maser was present but only ... More
Detecting UHE Cosmics & Neutrinos off the Moon; an Optimal Radio WindowSep 07 2006Sep 25 2006We show that at wavelengths comparable to the length of the shower produced by an Ultra-High Energy cosmic ray or neutrino, radio signals are an extremely efficient way to detect these particles. Through an example it is shown that this new approach offers, ... More
Back to the Moon: The Scientific Rationale for Resuming Lunar Surface ExplorationJun 04 2012The lunar geological record has much to tell us about the earliest history of the Solar System, the origin and evolution of the Earth-Moon system, the geological evolution of rocky planets, and the near-Earth cosmic environment throughout Solar System ... More
The Twin-Jet of NGC1052 at Radio, Optical, and X-ray FrequenciesMar 14 2003We present results from a combined radio, optical, and X-ray study of the jet-associated emission features in NGC1052. We analyse the radio-optical morphology and find a good positional correlation between the radio jet and the optical emission cone. ... More
Sub-Relativistic Radio Jets and Parsec-Scale Absorption in Two Seyfert GalaxiesMar 24 1999The Very Long Baseline Array has been used at 15 GHz to image the milliarcsecond structure of the Seyfert galaxies Mrk 231 and Mrk 348 at two epochs separated by about 1.7 yr. Both galaxies contain parsec-scale double radio sources whose components have ... More
Radio-Emission from Cosmic Ray Air-Showers - A Theoretical Perspective for LOPESJul 30 2002High-energy cosmic ray air-showers have been known for over 30 years to emit strong radio pulses in the regime between a few and several 100 MHz (Allan 1971). To date, however, a thorough analysis of the emission mechanisms has not yet been conducted. ... More
X-ray time lags from a pivoting power law in black holesOct 22 2003Most black hole candidate X-ray binaries show Fourier time lags between softer and harder X-rays. The hard photons seem to arrive up to a few ms after the soft for a given Fourier frequency of the perturbation. The energy dependence of the time lags has ... More
Radio galaxies of the local universe: all-sky catalog, luminosity functions, and clusteringMay 31 2012Jul 20 2012To understand the feedback of black holes on their environment or the acceleration of ultra-high energy cosmic rays in the present cosmic epoch, a systematic, all-sky inventory of radio galaxies in the local universe is needed. Here we present the first ... More
Cosmic-ray energy spectrum and composition up to the ankle - the case for a second Galactic componentMay 10 2016We have carried out a detailed study to understand the observed energy spectrum and composition of cosmic rays with energies up to ~10^18 eV. Our study shows that a single Galactic component with subsequent energy cut-offs in the individual spectra of ... More
Monte Carlo simulations of radio emission from cosmic ray air showersOct 17 2005As a basis for the interpretation of data gathered by LOPES and other experiments, we have carried out Monte Carlo simulations of geosynchrotron radio emission from cosmic ray air showers. The simulations, having been verified carefully with analytical ... More
Radio emission from cosmic ray air showers: simulation results and parametrizationJan 26 2005Oct 17 2005We have developed a sophisticated model of the radio emission from extensive air showers in the scheme of coherent geosynchrotron radiation, providing a theoretical foundation for the interpretation of experimental data from current and future experiments. ... More
Simulations of Radio Emission from Cosmic Ray Air ShowersOct 28 2004Radio emission from cosmic ray air showers has the potential to become an additional, cost-effective observing technique for cosmic ray research, being largely complementary to the well-established particle detector and air fluorescence techniques. We ... More
Fast radio bursts: the last sign of supramassive neutron starsJul 04 2013Jan 21 2014Several fast radio bursts have been discovered recently, showing a bright, highly dispersed millisecond radio pulse. The pulses do not repeat and are not associated with a known pulsar or gamma-ray burst. The high dispersion suggests sources at cosmological ... More
A parameterization for the radio emission of air showers as predicted by CoREAS simulations and applied to LOFAR measurementsFeb 12 2014Jun 19 2014Measuring radio emission from air showers provides excellent opportunities to directly measure all air shower properties, including the shower development. To exploit this in large-scale experiments, a simple and analytic parameterization of the distribution ... More