arXiv subjects

Heino Falcke

Publications and source records attributed to Heino Falcke.

At least 145 records · Page 8Linked to original sources

arXiv2007

How to hide large scale outflows: size constraints on the jets of Sgr A*

Despite significant strides made towards understanding accretion, outflow, and emission processes in the Galactic Center supermassive black hole Sagittarius A*, the presence of jets has neither been rejected nor proven. We investigate here whether the combined spectral and morphological properties of the source at radio through near infrared wavelengths are consistent with the predictions for inhomogeneous jets. In particular, we construct images of jets at a wavelength of 7mm based on models that are consistent with the spectrum of Sgr A*. We then compare these models through closure quantities with data obtained from the Very Long Baseline Array at 7mm. We find that the best-fit jet models give comparable or better fits than best-fit Gaussian models for the intrinsic source found in previous analyses. The best fitting jet models are bipolar, are highly inclined to the line of sight ($θ\ge$ 75 degrees), may favor a position angle on the sky of 105 degrees, and have compact bases with sizes of a few gravitational radii.

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arXiv2006

The LOFAR Transients Key Project

LOFAR, the Low Frequency Array, is a new radio telescope under construction in the Netherlands, designed to operate between 30 and 240 MHz. The Transients Key Project is one of the four Key Science Projects which comprise the core LOFAR science case. The remit of the Transients Key Project is to study variable and transient radio sources detected by LOFAR, on timescales from milliseconds to years. This will be achieved via both regular snapshot monitoring of historical and newly-discovered radio variables and, most radically, the development of a `Radio Sky Monitor' which will survey a large fraction of the northern sky on a daily basis.

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arXiv2006

The Intrinsic Size of Sagittarius A* from 0.35 cm to 6 cm

We present new high-resolution observations of Sagittarius A* at wavelengths of 17.4 to 23.8 cm with the Very Large Array in A configuration with the Pie Town Very Long Baseline Array antenna. We use the measured sizes to calibrate the interstellar scattering law and find that the major axis size of the scattering law is smaller by ~6% than previous estimates. Using the new scattering law, we are able to determine the intrinsic size of Sgr A* at wavelengths from 0.35 cm to 6 cm using existing results from the VLBA. The new law increases the intrinsic size at 0.7 cm by ~20% and <5% at 0.35 cm. The intrinsic size is 13^{+7}_{-3} Schwarzschild radii at 0.35 cm and is proportional to lambda^gamma, where gamma is in the range 1.3 to 1.7.

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arXiv2006

The radio structure of radio-quiet quasars

We investigate the radio emitting structures of radio-quiet active galactic nuclei with an emphasis on radio-quiet quasars to study their connection to Seyfert galaxies. We present and analyse high-sensitivity VLA radio continuum images of 14 radio-quiet quasars and six Seyfert galaxies. Many of the low redshift radio-quiet quasars show radio structures that can be interpreted as jet-like outflows. However, the detection rate of extended radio structures on arcsecond scales among our sample decreases with increasing redshift and luminosity, most likely due to a lack of resolution. The morphologies of the detected radio emission indicate strong interactions of the jets with the surrounding medium. We also compare the radio data of seven quasars with corresponding HST images of the [OIII] emitting narrow-line region (NLR). We find that the scenario of interaction between the radio jet and the NLR gas is confirmed in two sources by structures in the NLR gas distribution as previously known for Seyfert galaxies. The extended radio structures of radio-quiet quasars at sub-arcsecond resolution are by no means different from that of Seyferts. Among the luminosities studied here, the morphological features found are similar in both types of objects while the overall size of the radio structures increases with luminosity. This supports the picture where radio-quiet quasars are the scaled-up versions of Seyfert galaxies. In addition to known luminosity relations we find a correlation of the NLR size and the radio size shared by quasars and Seyferts.

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arXiv2006

The Rotation Measure and 3.5mm Polarization of Sgr A*

We 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 0.4% and 9 deg over the five epochs. We also detect polarization variability on a timescale of days. Combined with previous detections over the range 150-400GHz (750-2000 microns), the average polarization position angles are all found to be consistent with a rotation measure of -4.4 +/- 0.3 x 10^5 rad/m^2. This implies that the Faraday rotation occurs external to the polarized source at all wavelengths. This implies an accretion rate ~0.2 - 4 x 10^-8 Msun/yr for the accretion density profiles expected of ADAF, jet and CDAF models and assuming that the region at which electrons in the accretion flow become relativistic is within 10 R_S. The inferred accretion rate is inconsistent with ADAF/Bondi accretion. The stability of the mean polarization position angle between disparate polarization observations over the frequency range limits fluctuations in the accretion rate to less than 5%. The flat frequency dependence of the inter-day polarization position angle variations also makes them difficult to attribute to rotation measure fluctuations, and suggests that both the magnitude and position angle variations are intrinsic to the emission.

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arXiv2006

Radio Linear and Circular Polarization from M81*

We present results from archival Very Large Array (VLA) data and new VLA observations to investigate the long term behavior of the circular polarization of M81*, the nuclear radio source in the nearby galaxy M81. We also used the Berkeley-Illinois-Maryland Association (BIMA) array to observe M81* at 86 and 230 GHz. M81* is unpolarized in the linear sense at a frequency as high as 86 GHz and shows variable circular polarization at a frequency as high as 15 GHz. The spectrum of the fractional circular polarization is inverted in most of our observations. The sign of circular polarization is constant over frequency and time. The absence of linear polarization sets a lower limit to the accretion rate of $10^{-7} M_\odot y^{-1}$. The polarization properties are strikingly similar to the properties of Sgr A*, the central radio source in the Milky Way. This supports the hypothesis that M81* is a scaled up version of Sgr A*. On the other hand, the broad band total intensity spectrum declines towards milimeter wavelengths which differs from previous observations of M81* and also from Sgr A*.

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arXiv2006

Refining the fundamental plane of accreting black holes

The idea of a unified description of supermassive and stellar black holes has been supported by the extension of the empirical radio/X-ray correlation from X-ray binaries to active galactic nuclei through the inclusion of a mass term. This has lead to the so-called fundamental plane of black hole activity in the black hole mass, radio and X-ray luminosity space. Two incarnations of this fundamental plane have so far been suggested using different underlying models and using two different samples of accreting black holes. We present revised samples for both studies together with a refined statistical analysis using measured errors of the observables. This method is used to compare the two samples, discuss selection effects, and infer parameters for the fundamental plane in a homogeneous way. We show that strongly sub-Eddington objects in a state equivalent to the low/hard state of X-ray binaries follow the fundamental plane very tightly; the scatter is comparable to the measurement errors. However, we find that the estimated parameters depend strongly on the assumptions made on the sources of scatter and the relative weight of the different AGN classes in the sample. Using only hard state objects, the fundamental plane is in agreement with the prediction of a simple uncooled synchrotron/jet model for the emitted radiation. Inclusion of high-state objects increases the scatter and moves the correlation closer to a disk/jet model. This is qualitatively consistent with a picture where low-state objects are largely dominated by jet emission while high-state objects have a strong contribution from an accretion disk.

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arXiv2005

Radio emission from cosmic ray air showers: simulation results and parametrization

We 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. Having verified the model through comparison of analytic calculations, Monte Carlo simulations and historical experimental data, we now present the results of extensive simulations performed with our Monte Carlo code. Important results are the absence of significant asymmetries in the total field strength emission pattern, the spectral dependence of the radiation, the polarization characteristics of the emission (allowing an unambiguous test of the geomagnetic emission mechanism), and the dependence of the radio emission on important air shower and observer parameters such as the shower zenith angle, the primary particle energy, the depth of the shower maximum and the observer position. An analytic parametrization incorporating the aforementioned dependences summarizes our results in a particularly useful way.

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arXiv2005

Monte Carlo simulations of radio emission from cosmic ray air showers

As 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 calculations, reveal a wealth of information on the characteristics of the radio signal and their dependence on specific air shower parameters. In this article, we review the spatial characteristics of the radio emission, its predicted frequency spectrum and its dependence on important air shower parameters such as the shower zenith angle, the primary particle energy and the depth of the shower maximum, which can in turn be related to the nature of the primary particle.

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arXiv2005

Constraints on the Proper Motion of the Andromeda Galaxy Based on the Survival of Its Satellite M33

A major uncertainty in the dynamical history of the local group of galaxies originates from the unknown transverse speed of the Andromeda galaxy (M31) relative to the Milky Way. We show that the recent VLBA measurement of the proper motion of Andromeda's satellite, M33, severely constrains the possible values of M31's proper motion. The condition that M33's stellar disk will not be tidally disrupted by either M31 or the Milky Way over the past 10 billion years, favors a proper motion amplitude of 100+-20km/s for M31 with the quadrant of a negative velocity component along Right Ascension and a positive component along Declination strongly ruled-out. This inference can be tested by future astrometric measurements with SIM, GAIA, or the SKA. Our results imply that the dark halos of Andromeda and the Milky Way will pass through each other within the next 5-10 billion years.

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arXiv2005

Simulations and parametrisation of radio emission from cosmic ray air showers

Cosmic ray air showers are known to emit pulsed radio emission at frequencies around a few tens of MHz. Accompanying the experimental efforts of the LOPES project, situated at the KASCADE-Grande site of the Forschungszentrum Karlsruhe, we have modeled the underlying emission mechanism in the scheme of coherent geosynchrotron radiation from electron-positron pairs deflected in the earth's magnetic field. As a follow-up to our earlier analytical calculations, we have developed a Monte Carlo simulation based on analytic parametrisations of air shower properties, including longitudinal and lateral particle distributions, particle energy and track-length distributions, and the longitudinal shower development as a whole. Here we present detailed simulation results. Important findings are the absence of significant asymmetries in the total field strength emission pattern in spite of the asymmetry introduced by the geomagnetic field, the polarisation characteristics of the geosynchrotron emission, allowing an unambiguous test of the geomagnetic emission mechanism, and the dependence of the radio emission on important shower parameters such as the shower zenith angle, the primary particle energy and the depth of the shower maximum. As a particularly useful result, these dependences have been summarized in a simple parametrisation formula, providing a solid basis for the interpretation of experimental data gathered with current and future experiments.

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arXiv2005

Radio Sources in Low-Luminosity Active Galactic Nuclei.IV. Radio Luminosity Function, Importance of Jet Power, and Radio Properties of the Complete Palomar Sample

We present the completed results of a high resolution radio imaging survey of all (~200) low-luminosity active galactic nuclei (LLAGNs) and AGNs in the Palomar Spectroscopic Sample. The high incidences of pc-scale radio nuclei, with T(brightness) >=10^7 K, and sub-parsec jets argue for accreting black holes in >=50% of all LINERs and low-luminosity Seyferts; there is no evidence against all LLAGNs being mini-AGNs. The radio luminosity function (RLF) of Palomar Sample LLAGNs and AGNs 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 present day mass accretion. Adopting the jet model of Falcke & Biermann, we show that the accretion energy output in LLAGNs is dominated by the energy in the observed jets rather than the radiated bolometric luminosity. The Palomar LLAGNs and AGNs follow the same scaling between jet power and narrow line region (NLR) luminosity as the pc to kpc jets in powerful radio galaxies. Eddington ratios of <= 10^{-1} - 10^{-5} are implied in jet models of the radio emission. We find evidence that, in analogy to Galactic black hole candidates, LINERs are in a `low/hard' state (gas poor nuclei, low Eddington ratio, ability to launch collimated jets) while low-luminosity Seyferts are in a `high' state (gas rich nuclei, higher Eddington ratio, less likely to launch collimated jets). The jets are energetically more significant than supernovae in the LLAGN host galaxies, and are potentially able to deposit sufficient energy into the innermost parsecs to significantly slow the gas supply to the accretion disk.

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arXiv2005

The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies

LINER nuclei, which are present in many nearby galactic bulges, may be the manifestation of low-rate or low-radiative-efficiency accretion onto supermassive central black holes. However, it has been unclear whether the compact UV nuclear sources present in many LINERs are clusters of massive stars, rather than being directly related to the accretion process. We have used HST to monitor the UV variability of a sample of 17 galaxies with LINER nuclei and compact nuclear UV sources. Fifteen of the 17 galaxies were observed more than once, with two to five epochs per galaxy, spanning up to a year. We detect significant variability in most of the sample, with peak-to-peak amplitudes from a few percent to 50%. In most cases, correlated variations are seen in two independent bands (F250W and F330W). Comparison to previous UV measurements indicates, for many objects, long-term variations by factors of a few over decade timescales. Variability is detected in LINERs with and without detected compact radio cores, in LINERs that have broad H-alpha wings detected in their optical spectra (``LINER 1's''), and in those that do not (``LINER 2s''). This variability demonstrates the existence of a non-stellar component in the UV continuum of all types, and sets a lower limit to the luminosity of this component. We note a trend in the UV color (F250W/F330W) with spectral type - LINER 1s tend to be bluer than LINER 2s. This trend may indicate a link between the shape of the nonstellar continuum and the presence or the visibility of a broad-line region. In one target, the post-starburst galaxy NGC 4736, we detect variability in a previously noted UV source that is offset by 2.5" (60 pc in projection) from the nucleus. This may be the nearest example of a binary active nucleus, and of the process leading to black hole merging.

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arXiv2004

Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion Flow

We 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 that differs by 28 +/- 5 degrees from observations 6 months prior and then remains stable for 15 months. This difference may be due to a change in the source emission region on a scale of 10 Schwarzschild radii or due to a change of 3 x 10^5 rad m^-2 in the rotation measure. We consider a change in the source physics unlikely, however, since we see no corresponding change in the total intensity or polarized intensity fraction. On the other hand, turbulence in the accretion region at a radius ~ 10 to 1000 R_s could readily account for the magnitude and time scale of the position angle change.

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arXiv2004

Simulations of Radio Emission from Cosmic Ray Air Showers

Radio 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 present Monte Carlo simulatios of radio emission from extensive air showers in the scheme of coherent geosynchrotron radiation from electron-positron pairs gyrating in the earth's magnetic field. Preliminary results of our simulations are the predicted frequency, primary particle energy, shower zenith angle, shower azimuth angle and polarization dependence of the radio emission. These properties can be directly related to data measured by LOPES and other experiments.

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arXiv2004

Radio emission from cosmic ray air showers: Monte Carlo simulations

We present time-domain Monte Carlo simulations of radio emission from cosmic ray air showers in the scheme of coherent geosynchrotron radiation. Our model takes into account the important air shower characteristics such as the lateral and longitudinal particle distributions, the particle track length and energy distributions, a realistic magnetic field geometry and the shower evolution as a whole. The Monte Carlo approach allows us to retain the full polarisation information and to carry out the calculations without the need for any far-field approximations. We demonstrate the strategies developed to tackle the computational effort associated with the simulation of a huge number of particles for a great number of observer bins and illustrate the robustness and accuracy of these techniques. We predict the emission pattern, the radial and the spectral dependence of the radiation from a prototypical 10^17 eV vertical air shower and find good agreement with our analytical results (Huege & Falcke 2003) and the available historical data. Track-length effects in combination with magnetic field effects surprisingly wash out any significant asymmetry in the total field strength emission pattern in spite of the magnetic field geometry. While statistics of total field strengths alone can therefore not prove the geomagnetic origin, the predicted high degree of polarisation in the direction perpendicular to the shower and magnetic field axes allows a direct test of the geomagnetic emission mechanism with polarisation-sensitive experiments such as LOPES. Our code provides a robust, yet flexible basis for detailed studies of the dependence of the radio emission on specific shower parameters and for the inclusion of additional radiation mechanism in the future.

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arXiv2004

Comparing AGN broad- and narrow-line regions

We compare recent HST observations of Seyfert and quasar NLRs and find that type-2 AGNs follow a relation consistent with that expected for a distribution of gas ionized by a central source R(NLR,2) \propto L^{0.32 +/- 0.05}, while type-1 objects are fit with a steeper slope of 0.55 +/- 0.05. The latter is comparable to the scaling found for the BLR size with continuum luminosity (slope: 0.5-0.7). Therefore, we investigate what we can learn about the BLR size if the NLR size is only determined by the AGN luminosity. We find that NLR and BLR size are related linearly following R(BLR) \propto R(NLR,1)^{0.88 +/- 0.1}. This relation can be used to estimate BH masses.

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arXiv2004

Detection of the Intrinsic Size of Sagittarius A* through Closure Amplitude Imaging (includes Online Supporting Material)

We have detected the intrinsic size of Sagittarius A*, the Galactic Center radio source associated with a supermassive black hole, showing that the short-wavelength radio emission arises from very near the event horizon of the black hole. Radio observations with the Very Long Baseline Array show that the source has a size of $24 \pm 2$ Schwarzschild radii at 7 mm wavelength. In one of eight 7-mm epochs we also detect an increase in the intrinsic size of $60^{+25}_{-17}$%. These observations place a lower limit to the mass density of Sgr A* of $1.4\times10^{4}$ solar masses per cubic astronomical unit.

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