arXiv subjects

H. Falcke

Publications and source records attributed to H. Falcke.

At least 109 records · Page 6Linked to original sources

arXiv2014

The LOFAR long baseline snapshot calibrator survey

Aims. An efficient means of locating calibrator sources for International LOFAR is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140 MHz. Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full International LOFAR array. Sources were pre-selected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators. Results. Over 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the density of calibrators on the sky that are sufficiently bright to calibrate dispersive and non-dispersive delays for the International LOFAR using existing methods is 1.0 per square degree. Conclusions. The observed density of satisfactory delay calibrator sources means that observations with International LOFAR should be possible at virtually any point in the sky, provided that a fast and efficient search using the methodology described here is conducted prior to the observation to identify the best calibrator.

astro-ph.IM↗

arXiv2014

On the radio properties of the intermediate-mass black hole candidate ESO 243-49 HLX-1

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 $M_{\rm{BH}}\leq2.8^{+7.5}_{-2.1} \times 10^{6}$ M$_{\odot}$ at 90% 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 suggests a variable radio source and we briefly discuss the properties of the previously detected flares and compare them with microquasars and active galactic nuclei.

astro-ph.HE↗

arXiv2014

LOFAR low-band antenna observations of the 3C295 and Bootes fields: source counts and ultra-steep spectrum sources

We present LOFAR Low Band observations of the Bootes and 3C295 fields. Our images made at 34, 46, and 62 MHz reach noise levels of 12, 8, and 5 mJy beam$^{-1}$, making them the deepest images ever obtained in this frequency range. In total, we detect between 300 and 400 sources in each of these images, covering an area of 17 to 52 deg$^{2}$. From the observations we derive Euclidean-normalized differential source counts. The 62 MHz source counts agree with previous GMRT 153 MHz and VLA 74 MHz differential source counts, scaling with a spectral index of $-0.7$. We find that a spectral index scaling of $-0.5$ is required to match up the LOFAR 34 MHz source counts. This result is also in agreement with source counts from the 38 MHz 8C survey, indicating that the average spectral index of radio sources flattens towards lower frequencies. We also find evidence for spectral flattening using the individual flux measurements of sources between 34 and 1400 MHz and by calculating the spectral index averaged over the source population. To select ultra-steep spectrum ($α< -1.1$) radio sources, that could be associated with massive high redshift radio galaxies, we compute spectral indices between 62 MHz, 153 MHz and 1.4 GHz for sources in the Boötes field. We cross-correlate these radio sources with optical and infrared catalogues and fit the spectral energy distribution to obtain photometric redshifts. We find that most of these ultra-steep spectrum sources are located in the $ 0.7 \lesssim z \lesssim 2.5$ range.

astro-ph.GA↗

arXiv2014

Polarized radio emission from extensive air showers measured with LOFAR

We present LOFAR measurements of radio emission from extensive air showers. We find that this emission is strongly polarized, with a median degree of polarization of nearly $99\%$, and that the angle between the polarization direction of the electric field and the Lorentz force acting on the particles, depends on the observer location in the shower plane. This can be understood as a superposition of the radially polarized charge-excess emission mechanism, first proposed by Askaryan and the geomagnetic emission mechanism proposed by Kahn and Lerche. We calculate the relative strengths of both contributions, as quantified by the charge-excess fraction, for $163$ individual air showers. We find that the measured charge-excess fraction is higher for air showers arriving from closer to the zenith. Furthermore, the measured charge-excess fraction also increases with increasing observer distance from the air shower symmetry axis. The measured values range from $(3.3\pm 1.0)\%$ for very inclined air showers at $25\, \mathrm{m}$ to $(20.3\pm 1.3)\%$ for almost vertical showers at $225\, \mathrm{m}$. Both dependencies are in qualitative agreement with theoretical predictions.

astro-ph.HE↗

arXiv2014

A method for high precision reconstruction of air shower Xmax using two-dimensional radio intensity profiles

The 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 highest energies. Composition can be studied by measuring the atmospheric depth of the shower maximum Xmax of air showers generated by high-energy cosmic rays hitting the Earth's atmosphere. We present a new method to reconstruct Xmax based on radio measurements. The radio emission mechanism of air showers is a complex process that creates an asymmetric intensity pattern on the ground. The shape of this pattern strongly depends on the longitudinal development of the shower. We reconstruct Xmax by fitting two-dimensional intensity profiles, simulated with CoREAS, to data from the LOFAR radio telescope. In the dense LOFAR core, air showers are detected by hundreds of antennas simultaneously. The simulations fit the data very well, indicating that the radiation mechanism is now well-understood. The typical uncertainty on the reconstruction of Xmax for LOFAR showers is 17 g/cm^2.

astro-ph.IM↗

arXiv2014

LOFAR observations of PSR B0943+10: profile evolution and discovery of a systematically changing profile delay in Bright mode

We present broadband, low-frequency (25-80 MHz and 110-190 MHz) LOFAR observations of PSR B0943+10, with the goal of better illuminating the nature of its enigmatic mode-switching behaviour. This pulsar shows two relatively stable states: a Bright (B) and Quiet (Q) mode, each with different characteristic brightness, profile morphology, and single-pulse properties. We model the average profile evolution both in frequency and time from the onset of each mode, and highlight the differences between the two modes. In both modes, the profile evolution can be well explained by radius-to-frequency mapping at altitudes within a few hundred kilometres of the stellar surface. If both B and Q-mode emission originate at the same magnetic latitude, then we find that the change of emission height between the modes is less than 6%. We also find that, during B-mode, the average profile is gradually shifting towards later spin phase and then resets its position at the next Q-to-B transition. The observed B-mode profile delay is frequency-independent (at least from 25-80 MHz) and asymptotically changes towards a stable value of about 0.004 in spin phase by the end of mode instance, much too large to be due to changing spin-down rate. Such a delay can be interpreted as a gradual movement of the emission cone against the pulsar's direction of rotation, with different field lines being illuminated over time. Another interesting explanation is a possible variation of accelerating potential inside the polar gap. This explanation connects the observed profile delay to the gradually evolving subpulse drift rate, which depends on the gradient of the potential across the field lines.

astro-ph.GA↗

arXiv2014

LORA: A scintillator array for LOFAR to measure extensive air showers

The measurement of the radio emission from extensive air showers, induced by high-energy cosmic rays is one of the key science projects of the LOFAR radio telescope. The LOfar Radboud air shower Array (LORA) has been installed in the core of LOFAR in the Netherlands. The main purpose of LORA is to measure the properties of air showers and to trigger the read-out of the LOFAR radio antennas to register extensive air showers. The experimental set-up of the array of scintillation detectors and its performance are described.

physics.ins-det↗

arXiv2014

Reconstruction of the energy and depth of maximum of cosmic-ray air-showers from LOPES radio measurements

LOPES is a digital radio interferometer located at Karlsruhe Institute of Technology (KIT), Germany, which measures radio emission from extensive air showers at MHz frequencies in coincidence with KASCADE-Grande. In this article, we explore a method (slope method) which leverages the slope of the measured radio lateral distribution to reconstruct crucial attributes of primary cosmic rays. First, we present an investigation of the method on the basis of pure simulations. Second, we directly apply the slope method to LOPES measurements. Applying the slope method to simulations, we obtain uncertainties on the reconstruction of energy and depth of shower maximum Xmax of 13% and 50 g/cm^2, respectively. Applying it to LOPES measurements, we are able to reconstruct energy and Xmax of individual events with upper limits on the precision of 20-25% for the primary energy and 95 g/cm^2 for Xmax, despite strong human-made noise at the LOPES site.

astro-ph.IM↗

arXiv2014

The wavefront of the radio signal emitted by cosmic ray air showers

Analyzing measurements of the LOPES antenna array together with corresponding CoREAS simulations for more than 300 measured events with energy above $10^{17}\,$eV and zenith angles smaller than $45^\circ$, we find that the radio wavefront of cosmic-ray air showers is of approximately hyperbolic shape. The simulations predict a slightly steeper wavefront towards East than towards West, but this asymmetry is negligible against the measurement uncertainties of LOPES. At axis distances $\gtrsim 50\,$m, the wavefront can be approximated by a simple cone. According to the simulations, the cone angle is clearly correlated with the shower maximum. Thus, we confirm earlier predictions that arrival time measurements can be used to study the longitudinal shower development, but now using a realistic wavefront. Moreover, we show that the hyperbolic wavefront is compatible with our measurement, and we present several experimental indications that the cone angle is indeed sensitive to the shower development. Consequently, the wavefront can be used to statistically study the primary composition of ultra-high energy cosmic rays. At LOPES, the experimentally achieved precision for the shower maximum is limited by measurement uncertainties to approximately $140\,$g/cm$^2$. But the simulations indicate that under better conditions this method might yield an accuracy for the atmospheric depth of the shower maximum, $X_\mathrm{max}$, better than $30\,$g/cm$^2$. This would be competitive with the established air-fluorescence and air-Cherenkov techniques, where the radio technique offers the advantage of a significantly higher duty-cycle. Finally, the hyperbolic wavefront can be used to reconstruct the shower geometry more accurately, which potentially allows a better reconstruction of all other shower parameters, too.

hep-ex↗

arXiv2014

The LOFAR Pilot Surveys for Pulsars and Fast Radio Transients

We have conducted two pilot surveys for radio pulsars and fast transients with the Low-Frequency Array (LOFAR) around 140 MHz and here report on the first low-frequency fast-radio burst limit and the discovery of two new pulsars. The first survey, the LOFAR Pilot Pulsar Survey (LPPS), observed a large fraction of the northern sky, ~1.4 x 10^4 sq. deg, with 1-hr dwell times. Each observation covered ~75 sq. deg using 7 independent fields formed by incoherently summing the high-band antenna fields. The second pilot survey, the LOFAR Tied-Array Survey (LOTAS), spanned ~600 sq. deg, with roughly a 5-fold increase in sensitivity compared with LPPS. Using a coherent sum of the 6 LOFAR "Superterp" stations, we formed 19 tied-array beams, together covering 4 sq. deg per pointing. From LPPS we derive a limit on the occurrence, at 142 MHz, of dispersed radio bursts of < 150 /day/sky, for bursts brighter than S > 107 Jy for the narrowest searched burst duration of 0.66 ms. In LPPS, we re-detected 65 previously known pulsars. LOTAS discovered two pulsars, the first with LOFAR or any digital aperture array. LOTAS also re-detected 27 previously known pulsars. These pilot studies show that LOFAR can efficiently carry out all-sky surveys for pulsars and fast transients, and they set the stage for further surveying efforts using LOFAR and the planned low-frequency component of the Square Kilometer Array.

astro-ph.SR↗

arXiv2014

LOFAR tied-array imaging of Type III solar radio bursts

The Sun is an active source of radio emission which is often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), the Sun has not been imaged extensively because of the instrumental limitations of previous radio telescopes. Here, the combined high spatial, spectral and temporal resolution of the Low Frequency Array (LOFAR) was used to study solar Type III radio bursts at 30-90 MHz and their association with CMEs. The Sun was imaged with 126 simultaneous tied-array beams within 5 solar radii of the solar centre. This method offers benefits over standard interferometric imaging since each beam produces high temporal (83 ms) and spectral resolution (12.5 kHz) dynamic spectra at an array of spatial locations centred on the Sun. LOFAR's standard interferometric output is currently limited to one image per second. Over a period of 30 minutes, multiple Type III radio bursts were observed, a number of which were found to be located at high altitudes (4 solar radii from the solar center at 30 MHz) and to have non-radial trajectories. These bursts occurred at altitudes in excess of values predicted by 1D radial electron density models. The non-radial high altitude Type III bursts were found to be associated with the expanding flank of a CME. The CME may have compressed neighbouring streamer plasma producing larger electron densities at high altitudes, while the non-radial burst trajectories can be explained by the deflection of radial magnetic fields as the CME expanded in the low corona.

astro-ph.SR↗

arXiv2014

Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz

We present radio observations of the Moon between $35$ and $80$ MHz to demonstrate a novel technique of interferometrically measuring large-scale diffuse emission extending far beyond the primary beam (global signal) for the first time. In particular, we show that (i) the Moon appears as a negative-flux source at frequencies $35<ν<80$ MHz since it is `colder' than the diffuse Galactic background it occults, (ii) using the (negative) flux of the lunar disc, we can reconstruct the spectrum of the diffuse Galactic emission with the lunar thermal emission as a reference, and (iii) that reflected RFI (radio-frequency interference) is concentrated at the center of the lunar disc due to specular nature of reflection, and can be independently measured. Our RFI measurements show that (i) Moon-based Cosmic Dawn experiments must design for an Earth-isolation of better than $80$ dB to achieve an RFI temperature $<1$ mK, (ii) Moon-reflected RFI contributes to a dipole temperature less than $20$ mK for Earth-based Cosmic Dawn experiments, (iii) man-made satellite-reflected RFI temperature exceeds $20$ mK if the aggregate cross section of visible satellites exceeds $80$ m$^2$ at $800$ km height, or $5$ m$^2$ at $400$ km height. Currently, our diffuse background spectrum is limited by sidelobe confusion on short baselines (10-15% level). Further refinement of our technique may yield constraints on the redshifted global $21$-cm signal from Cosmic Dawn ($40>z>12$) and the Epoch of Reionization ($12>z>5$).

astro-ph.IM↗

arXiv2014

Initial LOFAR observations of Epoch of Reionization windows: II. Diffuse polarized emission in the ELAIS-N1 field

This study aims to characterise the polarized foreground emission in the ELAIS-N1 field and to address its possible implications for the extraction of the cosmological 21-cm signal from the Low-Frequency Array - Epoch of Reionization (LOFAR-EoR) data. We use the high band antennas of LOFAR to image this region and RM-synthesis to unravel structures of polarized emission at high Galactic latitudes. The brightness temperature of the detected Galactic emission is on average 4 K in polarized intensity and covers the range from -10 to +13rad m^-2 in Faraday depth. The total polarized intensity and polarization angle show a wide range of morphological features. We have also used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology, at comparable brightness levels, but their spatial correlation is very low. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to 1.5%. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies. The wide frequency range, good angular resolution and good sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of 1-2 rad m^-2 in Faraday depth. The different polarised patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The presence of polarised foregrounds is a serious complication for Epoch of Reionization experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%.

astro-ph.GA↗

arXiv2014

Future mmVLBI Research with ALMA: A European vision

Very long baseline interferometry at millimetre/submillimetre wavelengths (mmVLBI) offers the highest achievable spatial resolution at any wavelength in astronomy. The anticipated inclusion of ALMA as a phased array into a global VLBI network will bring unprecedented sensitivity and a transformational leap in capabilities for mmVLBI. Building on years of pioneering efforts in the US and Europe the ongoing ALMA Phasing Project (APP), a US-led international collaboration with MPIfR-led European contributions, is expected to deliver a beamformer and VLBI capability to ALMA by the end of 2014 (APP: Fish et al. 2013, arXiv:1309.3519). This report focuses on the future use of mmVLBI by the international users community from a European viewpoint. Firstly, it highlights the intense science interest in Europe in future mmVLBI observations as compiled from the responses to a general call to the European community for future research projects. A wide range of research is presented that includes, amongst others: - Imaging the event horizon of the black hole at the centre of the Galaxy - Testing the theory of General Relativity an/or searching for alternative theories - Studying the origin of AGN jets and jet formation - Cosmological evolution of galaxies and BHs, AGN feedback - Masers in the Milky Way (in stars and star-forming regions) - Extragalactic emission lines and astro-chemistry - Redshifted absorption lines in distant galaxies and study of the ISM and circumnuclear gas - Pulsars, neutron stars, X-ray binaries - Testing cosmology - Testing fundamental physical constants

astro-ph.IM↗

arXiv2014

The shape of the radio wavefront of extensive air showers as measured with LOFAR

Extensive air showers, induced by high energy cosmic rays impinging on the Earth's atmosphere, produce radio emission that is measured with the LOFAR radio telescope. As the emission comes from a finite distance of a few kilometers, the incident wavefront is non-planar. A spherical, conical or hyperbolic shape of the wavefront has been proposed, but measurements of individual air showers have been inconclusive so far. For a selected high-quality sample of 161 measured extensive air showers, we have reconstructed the wavefront by measuring pulse arrival times to sub-nanosecond precision in 200 to 350 individual antennas. For each measured air shower, we have fitted a conical, spherical, and hyperboloid shape to the arrival times. The fit quality and a likelihood analysis show that a hyperboloid is the best parametrization. Using a non-planar wavefront shape gives an improved angular resolution, when reconstructing the shower arrival direction. Furthermore, a dependence of the wavefront shape on the shower geometry can be seen. This suggests that it will be possible to use a wavefront shape analysis to get an additional handle on the atmospheric depth of the shower maximum, which is sensitive to the mass of the primary particle.

astro-ph.HE↗

arXiv2014

Discovery of Carbon Radio Recombination Lines in absorption towards Cygnus~A

We present the first detection of carbon radio recombination line absorption along the line of sight to Cygnus A. The observations were carried out with the LOw Frequency ARray in the 33 to 57 MHz range. These low frequency radio observations provide us with a new line of sight to study the diffuse, neutral gas in our Galaxy. To our knowledge this is the first time that foreground Milky Way recombination line absorption has been observed against a bright extragalactic background source. By stacking 48 carbon $α$ lines in the observed frequency range we detect carbon absorption with a signal-to-noise ratio of about 5. The average carbon absorption has a peak optical depth of 2$\times$10$^{-4}$, a line width of 10 km s$^{-1}$ and a velocity of +4 km s$^{-1}$ with respect to the local standard of rest. The associated gas is found to have an electron temperature $T_{e}\sim$ 110 K and density $n_{e}\sim$ 0.06 cm$^{-3}$. These properties imply that the observed carbon $α$ absorption likely arises in the cold neutral medium of the Orion arm of the Milky Way. Hydrogen and helium lines were not detected to a 3$σ$ peak optical depth limit of 1.5$\times$10$^{-4}$ for a 4 km s$^{-1}$ channel width. Radio recombination lines associated with Cygnus A itself were also searched for, but are not detected. We set a 3$σ$ upper limit of 1.5$\times$10$^{-4}$ for the peak optical depth of these lines for a 4 km s$^{-1}$ channel width.

astro-ph.GA↗

arXiv2013

Unveiling recurrent jets of the ULX Holmberg II X-1: evidence for a massive stellar-mass black hole?

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.

astro-ph.HE↗

arXiv2013

Detecting cosmic rays with the LOFAR radio telescope

The low frequency array (LOFAR), is the first radio telescope designed with the capability to measure radio emission from cosmic-ray induced air showers in parallel with interferometric observations. In the first $\sim 2\,\mathrm{years}$ of observing, 405 cosmic-ray events in the energy range of $10^{16} - 10^{18}\,\mathrm{eV}$ have been detected in the band from $30 - 80\,\mathrm{MHz}$. Each of these air showers is registered with up to $\sim1000$ independent antennas resulting in measurements of the radio emission with unprecedented detail. This article describes the dataset, as well as the analysis pipeline, and serves as a reference for future papers based on these data. All steps necessary to achieve a full reconstruction of the electric field at every antenna position are explained, including removal of radio frequency interference, correcting for the antenna response and identification of the pulsed signal.

astro-ph.IM↗