arXiv subjects

H. Falcke

Publications and source records attributed to H. Falcke.

At least 181 records · Page 10Linked to original sources

arXiv2007

LOFAR - Opening up a new window on the Universe

LOFAR, the Low Frequency Array, is a next-generation radio telescope that is being built in Northern Europe and expected to be fully operational at the end of this decade. It will operate at frequencies from 15 to 240 MHz (corresponding to wavelengths of 20 to 1.2 m). Its superb sensitivity, high angular resolution, large field of view and flexible spectroscopic capabilities will represent a dramatic improvement over previous facilities at these wavelengths. As such, LOFAR will carry out a broad range of fundamental astrophysical studies. The design of LOFAR has been driven by four fundamental astrophysical applications: (i) The Epoch of Reionisation, (ii) Extragalactic Surveys and their exploitation to study the formation and evolution of clusters, galaxies and black holes, (iii) Transient Sources and their association with high energy objects such as gamma ray bursts, and (iv) Cosmic Ray showers and their exploitation to study the origin of ultra-high energy cosmic rays. In this conference the foreseen LOFAR work on the epoch of reionisation has been covered by de Bruyn and on cosmic ray showers by Falcke. During this contribution we will first present the LOFAR project with an emphasis on the challenges faced when carrying out sensitive imaging at low radio frequencies. Subsequently, we will discuss LOFAR's capabilities to survey the low-frequency radio sky. Main aims for the planned surveys are studies of z>6 radio galaxies, diffuse emission associated with distant clusters and starbursting galaxies at z>2.

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arXiv2006

Towards a kinematic model of the Local Group

Measuring 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 limited only to the closest companions of the Milky Way. However, Very Long Baseline Interferometry (VLBI) provides the best angular resolution in astronomy and phase-referencing techniques yield astrometric accuracies of ~ 10 micro-arcseconds. This makes a measurement of proper motions and angular rotation rates of galaxies out to a distance of ~ 1 Mpc feasible. This article presents results of VLBI observations of regions of H2O maser activity in the Local Group galaxies M33 and IC10. These measurements promise a new handle on dynamical models for the Local Group and the mass and dark matter halo of Andromeda and the Milky Way. (Abridged)

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arXiv2006

The Proper Motion of the Local Group Galaxy IC 10

We 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 10, we obtain a motion of -39 +/- 9 microas/yr toward the East and 31 +/- 8 microas/yr toward the North. This corresponds to a total space velocity of 215 +/- 42 km/s relative to the Milky Way for an assumed distance of 660 +/- 66 kpc. Assuming that IC 10 and M33, for which also a proper motion measurement exists, are bound to M31, we calculate a lower limit for the mass of M31 of 7.5 x 10^11 Msol.

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arXiv2006

A very brief description of LOFAR - the Low Frequency Array

LOFAR (Low Frequency Array) is an innovative radio telescope optimized for the frequency range 30-240 MHz. The telescope is realized as a phased aperture array without any moving parts. Digital beam forming allows the telescope to point to any part of the sky within a second. Transient buffering makes retrospective imaging of explosive short-term events possible. The scientific focus of LOFAR will initially be on four key science projects (KSPs): 1) detection of the formation of the very first stars and galaxies in the universe during the so-called epoch of reionization by measuring the power spectrum of the neutral hydrogen 21-cm line (Shaver et al. 1999) on the ~5' scale; 2) low-frequency surveys of the sky with of order $10^8$ expected new sources; 3) all-sky monitoring and detection of transient radio sources such as gamma-ray bursts, x-ray binaries, and exo-planets (Farrell et al. 2004); and 4) radio detection of ultra-high energy cosmic rays and neutrinos (Falcke & Gorham 2003) allowing for the first time access to particles beyond 10^21 eV (Scholten et al. 2006). Apart from the KSPs open access for smaller projects is also planned. Here we give a brief description of the telescope.

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arXiv2006

Detecting UHE Cosmics & Neutrinos off the Moon; an Optimal Radio Window

We 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, for the first time, the realistic possibility of measuring UHE neutrino fluxes below the Waxman-Bahcall limit. It is shown that in only one month of observing with the upcoming LOFAR radio telescope, cosmic-ray events can be measured beyond the GZK-limit, at a sensitivity level of two orders of magnitude below the extrapolated values.

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arXiv2006

Optimal Radio Window for the Detection of Ultra-High-Energy Cosmic Rays and Neutrinos off the Moon

When high-energy cosmic rays impinge on a dense dielectric medium, radio waves are produced through the Askaryan effect. We 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, for the first time, the realistic possibility of measuring UHE neutrino fluxes below the Waxman-Bahcall limit. It is shown that in only one month of observing with the upcoming LOFAR radio telescope, cosmic-ray events can be measured beyond the GZK-limit, at a sensitivity level of two orders of magnitude below the extrapolated values.

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arXiv2006

Water masers in the Local Group of galaxies

We compare the number of detected 22 GHz H2O masers in the Local Group galaxies M31, M33, NGC6822, IC10, IC1613, DDO187, GR8, NGC185, and the Magellanic Clouds with the water maser population of the Milky Way. To accomplish this we searched for water maser emission in the two Local Group galaxies M33 and NGC6822 using the Very Large Array (VLA) and incorporated results from previous studies. We observed 62 HII regions in M33 and 36 regions with H-alpha emission in NGC6822. Detection limits are 0.0015 and 0.0008 Lsun for M33 and NGC6822, respectively (corresponding to 47 and 50 mJy in three channels with 0.7 km/s width). M33 hosts three water masers above our detection limit, while in NGC6822 no maser source was detected. We find that the water maser detection rates in the Local Group galaxies M31, M33, NGC6822, IC1613, DDO187, GR8, NGC185, and the Magellanic Clouds are consistent with expectations from the Galactic water masers if one considers the different star formation rates of the galaxies. However, the galaxy IC10 exhibits an overabundance of masers, which may result from a compact central starburst.

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arXiv2006

Why the fundamental plane of black hole activity is not simply a distance driven artifact

The 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, by Falcke, Koerding and Markoff (2004). Here we further examine a number of statistical issues related to this correlation. In particular, we discuss the issue of sample selection and quantify the bias introduced by the effect of distance in two of the correlated quantities. We demonstrate that the fundamental plane relation cannot be a distance artifact, and that its non-linearity must represent an intrinsic characteristic of accreting black holes. We also discuss possible future observational strategies to improve our understanding of this correlation.

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arXiv2005

Detection and imaging of atmospheric radio flashes from cosmic ray air showers

The 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 sufficient powerful accelerators within this distance from the Galaxy, explanations for UHECRs range from unusual astrophysical sources to exotic string physics. Also unclear is whether UHECRs consist of protons, heavy nuclei, neutrinos or gamma-rays. To resolve these questions, larger detectors with higher duty cycles and which combine multiple detection techniques are needed. Radio emission from UHECRs, on the other hand, is unaffected by attenuation, has a high duty cycle, gives calorimetric measurements and provides high directional accuracy. Here we report the detection of radio flashes from cosmic-ray air showers using low-cost digital radio receivers. We show that the radiation can be understood in terms of the geosynchrotron effect. Our results show that it should be possible to determine the nature and composition of UHECRs with combined radio and particle detectors, and to detect the ultrahigh-energy neutrinos expected from flavour mixing.

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arXiv2005

The Geometric Distance and Proper Motion of the Triangulum Galaxy (M33)

We 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, we obtained a distance of 730 +/- 168 kiloparsecs. This distance is consistent with the most recent Cepheid distance measurement. M33 is moving with a velocity of 190 +/- 59 km/s relative to the Milky Way. These measurements promise a new method to determine dynamical models for the Local Group and the mass and dark matter halos of M31, M33 and the Milky Way.

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arXiv2005

A Radio Monitoring Survey of Ultra-Luminous X-Ray Sources

We 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 sample of the 9 nearest ULXs has been monitored eight times during 5 months with the Very Large Array in A and B configuration. Our limiting sensitivity is approximately 0.15 mJy (4 sigma) for radio flares and around 60 uJy for continuous emission. In M82 two ULXs seem to have coincident compact radio sources, which are probably supernova remnants. No continuous or flaring radio emission has been detected from any other ULX. Thus, ULXs do not generally emit steady-state radio emission above radio powers of 1.5 10^17 W/Hz. The non-detections of the continuous emission are consistent with beamed or unbeamed radio emission from accreting black holes of <= 10^3 Msol based on the radio/X-ray correlation. Other published radio detections (M82, NGC 5408) are also discussed in this context. Both detections are significantly above our detection limit. If ULXs have flaring radio emission above 4 10^17 W/Hz we can give an upper limit on the duty circle of the flares of 6%. This upper limit is in agreement with the observed number of flares in Galactic radio transients. Additionally we present a yet unreported radio double structure in the nearby low-luminosity AGN NGC 4736.

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arXiv2005

The extreme flare in III Zw 2: Evolution of a radio jet in a Seyfert galaxy

A 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 source are closely linked and these observations allowed us to study in great detail a textbook example of a synchrotron self-absorbed jet. We observe a phase where the jet gets frustrated, without expansion and no spectral evolution. Then the jet breaks free and starts to expand with apparent superluminal motion. This expansion is accompanied by a strong spectral evolution. The results are a good confirmation of synchrotron theory and equipartition for jets.

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arXiv2004

VLBI observations of weak sources using fast frequency switching

We 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 here, one observes the target source continuously while switching rapidly between the target frequency and a lower reference frequency. We demonstrate that the technique allows phase calibration almost reaching the thermal noise limit and present the first detection of the AGN in the FR I radio galaxy NGC 4261 at 86 GHz. Although point-like, this is the weakest source ever detected with VLBI at this frequency.

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arXiv2004

Radio Luminosity Function, Importance of Jet Power, and Radio Properties of Nearby Low-Luminosity Active Galactic Nuclei

We 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 pc-scale radio nuclei, with implied brightness temperatures of the order of 10E7 K, and sub-parsec jets argue for accreting black holes in 50% of all LLAGNs; there is no evidence against all LLAGNs being mini-AGNs.The detected parsec-scale radio nuclei are preferentially found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad Halpha emission). The radio luminosity function (RLF) of Palomar Sample LLAGNs extends three orders of magnitude below, and is continuous with, that of "classical" AGNs.We find marginal evidence for a low-power turnover in the RLF; nevertheless LLAGNs are responsible for a significant fraction of accretion in the local universe. The accretion energy output in LLAGNs is dominated by the energy in the observed jets rather than the radiated bolometric luminosity. The Palomar LLAGNs follow the same scaling between jet power and narrow line region (NLR) luminosity as the parsec to kilo-parsec jets in powerful radio galaxies. Low accretion rates (of 10E-2 to 10E-6 of the Eddington rate) are impliedin both advection- and jet-type models, with evidence for increasing "radio-loudness" with decreasing Eddington fraction. The jets are energetically more significant than supernovae in the LLAGN host galaxies, and are potentially able to dump sufficient energy into the innermost parsecs to significantly slow the accretion inflow. Detailed results can be found in Nagar et al. (2002a) and Nagar et al. (2004, to appear in Astronomy & Astrophysics).

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arXiv2004

VLBI detections of a source weaker than 100 mJy at 86 GHz

We 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 the target frequency and a lower reference frequency. Self-calibration at the reference frequency yielded phase corrections which were multiplied with the frequency ratio and applied to the target frequency visibilities. The resulting detection of NGC 4261 is, to our knowledge, the first of NGC 4261 with 86 GHz VLBI, and it is also the weakest source so far detected with VLBI at that frequency.

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arXiv2004

Probing the Dark Ages with the Square Kilometer Array

The 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 wavelengths imply that we are finally probing into this key epoch of galaxy formation at $z \ge 6$. The Square Kilometer Array (SKA) will provide critical insight into the EoR in a number of ways. First, the ability of the SKA to image the neutral IGM in 21cm emission is a truly unique probe of the process of reionization, and is recognized as the next necessary and fundamental step in our study of the evolution of large scale structure and cosmic reionization. Second, study of HI 21cm absorption toward the first radio loud objects probes small to intermediate scale structure in the neutral 'cosmic web', as well as HI in the first collapsed structures (proto-disks and mini-halos). And third, the incomparable sensitivity of the SKA allows for the study of the molecular gas, dust, and star formation activity in the first galaxies, as well as the radio continuum emission from the first accreting massive black holes. Such objects will be obscured at optical wavelengths due to absorption by the neutral IGM.

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arXiv2004

Prospects for radio detection of ultra-high energy cosmic rays and neutrinos

The 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 accelerators and hence they are fundamental probes for particle physics as well as astrophysics. Because of the scarcity of these high-energy particles, larger and larger ground-based detectors have been built. The new generation of digital radio telescopes may play an important role in this, if properly designed. Radio detection of cosmic ray showers has a long history but was abandoned in the 1970's. Recent experimental developments together with sophisticated air shower simulations incorporating radio emission give a clearer understanding of the relationship between the air shower parameters and the radio signal, and have led to resurgence in its use. Observations of air showers by the SKA could, because of its large collecting area, contribute significantly to measuring the cosmic ray spectrum at the highest energies. Because of the large surface area of the moon, and the expected excellent angular resolution of the SKA, using the SKA to detect radio Cherenkov emission from neutrino-induced cascades in lunar regolith will be potentially the most important technique for investigating cosmic ray origin at energies above the photoproduction cut-off. (abridged)

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arXiv2004

Compact radio cores: from the first black holes to the last

One 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 detect the very first generation of supermassive black holes. We start by introducing some of the basic properties of compact radio cores and how they scale with accretion power. The relative contribution of jets and radio cores to the Spectral Energy Distribution (SED) is strongest in sub-Eddington black holes but also present in the most luminous objects. Radio and X-rays are correlated as a function of black hole mass such that the most massive black holes are most suited for radio detections. We present a radio core luminosity function for the present universe down to the least luminous AGN. The SKA will essentially detect all dormant black holes in the local universe, such as that in our Milky Way, out to several tens of Megaparsecs. It will also be able to see black holes in the making at redshifts z>10 for black hole masses larger than 10^7 M_sun. Finally, we suggest that the first generation of black holes may have jets that are frustrated in their dense environment and thus appear as Gigahertz-Peaked-Spectrum (GPS) sources. Since their intrinsic size and peak frequency are related and angular size and frequency scale differently with redshift, there is a unique region in parameter space that should be occupied by emerging black holes in the epoch of reionization. This can be well probed by radio-only methods with the SKA. (abridged)

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