arXiv subjects

Heino Falcke

Publications and source records attributed to Heino Falcke.

At least 163 records · Page 9Linked to original sources

arXiv2003

A Scheme to Unify Low-Power Accreting Black Holes - Jet-Dominated Accretion Flows and the Radio/X-Ray Correlation

We explore the evolution in power of black holes of all masses, and their associated jets, within the scheme of an accretion rate-dependent state transition. Below a critical value of the accretion rate all systems are assumed to undergo a transition to a state where the dominant accretion mode is optically thin and radiatively inefficient. In these significantly sub-Eddington systems, the spectral energy distribution is predicted to be dominated by non-thermal emission from a relativistic jet whereas near-Eddington black holes will be dominated instead by emission from the accretion disk. Reasonable candidates for such a sub-Eddington state include X-ray binaries in the hard and quiescent states, the Galactic Center (Sgr A*), LINERs, FR I radio galaxies, and a large fraction of BL Lac objects. Standard jet physics predicts non-linear scaling between the optically thick (radio) and optically thin (optical or X-ray) emission of these systems, which has been confirmed recently in X-ray binaries. We show that this scaling relation is also a function of black hole mass and only slightly of the relativistic Doppler factor. Taking the scaling into account we show that indeed hard and quiescent state X-ray binaries, LINERs, FR I radio galaxies, and BL Lacs can be unified and fall on a common radio/X-ray correlation. This suggests that jet domination is an important stage in the luminosity evolution of accreting black hole systems.

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arXiv2003

X-ray time lags from a pivoting power law in black holes

Most 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 a roughly logarithmic behavior. Up to now most theories fail to explain the observed magnitude and Fourier frequency dependence of the lags or fail other statistical tests. We show that the time lags can arise from a simple pivoting power law model, which creates the logarithmic dependence on the photon energy at once. A pivoting power law arises naturally from jet/synchrotron models for the X-ray emission, but may also be applicable to corona models. A hint to the coherence features of the light-curves can be obtained from the power spectral density, which can be decomposed into a few broad Lorentzians that could arise from a couple of strongly damped oscillators with low quality factors below one. Using small variations of the power law index for each Lorentzian separately the lags can be derived analytically. They show the correct Fourier frequency dependence of the time lags. If one assumes variations of the power law index by +- 0.2 the model can account for the observed magnitude of the time lags in Cyg X-1. As a further test we calculated the cross- and auto-correlation functions for our model, which also show qualitatively the observed behavior. The coherence function for the model is in agreement with the observed data in the Fourier regime, where the model is valid.

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arXiv2003

Radio Emission from Cosmic Ray Air Showers: Coherent Geosynchrotron Radiation

Cosmic ray air showers have been known for over 30 years to emit pulsed radio emission in the frequency range from a few to a few hundred MHz, an effect that offers great opportunities for the study of extensive air showers with upcoming fully digital "software radio telescopes" such as LOFAR and the enhancement of particle detector arrays such as KASCADE Grande or the Pierre Auger Observatory. However, there are still a lot of open questions regarding the strength of the emission as well as the underlying emission mechanism. Accompanying the development of a LOFAR prototype station dedicated to the observation of radio emission from extensive air showers, LOPES, we therefore take a new approach to modeling the emission process, interpreting it as "coherent geosynchrotron emission" from electron-positron pairs gyrating in the earth's magnetic field. We develop our model in a step-by-step procedure incorporating increasingly realistic shower geometries in order to disentangle the coherence effects arising from the different scales present in the air shower structure and assess their influence on the spectrum and radial dependence of the emitted radiation. We infer that the air shower "pancake" thickness directly limits the frequency range of the emitted radiation, while the radial dependence of the emission is mainly governed by the intrinsic beaming cone of the synchrotron radiation and the superposition of the emission over the air shower evolution as a whole. Our model succeeds in reproducing the qualitative trends in the emission spectrum and radial dependence that were observed in the past, and is consistent with the absolute level of the emission within the relatively large systematic errors in the experimental data.

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arXiv2003

The AGN Content of Ultraluminous IR Galaxies: High Resolution VLA Imaging of the IRAS 1Jy ULIRG Sample

This paper presents the results of a high resolution radio imaging survey of 83 of the 118 ultraluminous infrared galaxies (ULIRGs) in the IRAS 1 Jy ULIRG sample. We have observed these ULIRGs at 15GHz with the Very Large Array (VLA). We find that ~75% of Seyferts (both type 1 and 2) and LINERs have radio nuclei which are compact at our 150mas resolution. The detection rate of HII nuclei is significantly lower (32%); the detections among these are preferentially HII + LINER/Seyfert composite nuclei. Among ULIRGs with multiple optical or near-IR nuclei our observations detected only one (or no) nucleus; in these the radio detection is typically towards the brightest near-IR nucleus. The compactness of the radio sources, the higher detection rates in AGN-type nuclei than HII nuclei, the preferential detection of nuclei with unresolved point sources in the near-IR, the low soft X-ray to nuclear radio luminosity ratio (arguing against thermal emission powering the radio nuclei), and the lack of correlation between radio power and Halpha luminosity, all support an origin of the detected radio nuclei in AGN related activity. This result is especially interesting for LINER ULIRGs for which signatures of AGNs have often been ambiguous in other wavebands. Such a high incidence of AGN would provide, for the first time, a large sample in which to study the interplay between AGN, starbursts, and galaxy mergers.

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arXiv2003

Radio Emission From EAS - Coherent Geosynchrotron Radiation

Extensive air showers (EAS) have been known for over 30 years to emit pulses of radio emission at frequencies from a few to a few hundred MHz, an effect that offers great opportunities for the study of EAS with the next generation of "software radio interferometers" such as LOFAR and LOPES. The details of the emission mechanism, however, remain rather uncertain to date. Following past suggestions that the bulk of the emission is of geomagnetic origin, we model the radio pulses as "coherent geosynchrotron radiation" arising from the deflection of electrons and positrons in the earth's magnetic field. We analytically develop our model in a step-by-step procedure to disentangle the coherence effects arising from different scales present in the shower structure and infer which shower characteristics govern the frequency spectrum and radial dependence of the emission. The effect is unavoidable and our predictions are in good agreement with the available experimental data within their large margins of error.

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arXiv2003

Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz

We 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 demonstrate that the polarization does not arise from dust but from a synchrotron source associated with Sgr A*. We see no change in the polarization position angle and only a small change in the polarization fraction in four observations distributed over 60 days. We find a position angle 139 +/- 4 degrees that differs substantially from what was found in earlier JCMT observations at the same frequency. Polarized dust emission cannot account for this discrepancy leaving variability and observational error as the only explanations. The BIMA observations alone place an upper limit on the magnitude of the rotation measure of 2 x 10^6 rad m^-2. These new observations when combined with the JCMT observations at 150, 375 and 400 GHz suggest RM =-4.3 +/- 0.1 x 10^5 rad m^-2. This RM may be caused by an external Faraday screen. Barring a special geometry or a high number of field reversals, this RM rules out accretion rates greater than ~ 10^-7 M_sun y^-1. This measurement is inconsistent with high accretion rates necessary in standard advection dominated accretion flow and Bondi-Hoyle models for Sgr A*. It argues for low accretion rates as a major factor in the overall faintness of Sgr A*.

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arXiv2002

Single and binary Black Holes and their active environment

In this short review we describe some of the latest endeavours to understand the activity around Black Holes. First, it has been possible to demonstrate that a large part of the electromagnetic emission observed can be interpreted as arising from the jet; this explains at once all spectral features and their variability. Second we dwell on the concept that merging galaxies naturally lead to merging Black Holes. Here we emphasize two aspects: a) the torque exerted by the binary Black Holes carves a torus like distribution out of the stellar population near to the Black Hole binary; b) We consider the last stages of the Black Hole binary merger, taking into account the angle between the spin of the primary Black Hole, and the orbital spin of the second Black Hole. We show that the loss of orbital angular momentum is very strongly spin-dependent; for large angles between the two spins the angular momentum loss is strongly inhibited, allowing spin flip of the primary Black Hole which preserves a high angular momentum relative to the maximum allowed. This ensures that both before and after the merger the accretion disk may reach to very small distances from the central Black Hole, with very high local temperatures right near the base of the jet: This is especially interesting in the case that forming the jet requires the formation of an ADAF like ring near the inner edge of the disk, as suggested by some earlier work. It also may have consequences for the initial hadronic interactions right near the base of the jet. Finally, this may also have important implications for the discovery of gravitational radiation bursts from the merger of black holes; the spin dependence needs to be taken into account.

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arXiv2002

A Radio Survey for Linear and Circular Polarization in Low Luminosity Active Galactic Nuclei

We conducted a Very Large Array survey of eleven low luminosity active galactic nuclei for linear and circular polarization at 8.4 GHz. We detected circular polarization in one source (M81*) and linear polarization in 3 sources. Sensitivity limits were ~0.1% for both modes of polarization in 9 of 11 sources. The detections confirm the importance of nonthermal emission in LLAGN. However, detection rates for circular and linear polarization are lower for these sources than for more powerful AGN. Fractional linear polarization in detected sources is also lower than in more powerful AGN. The weak linear polarization in the survey sources indicates their overall similarity to Sgr A*. Confusion with thermal sources, depolarization and weaker, less extended jets may contribute to these differences. We detect a rotation measure >~ 7 x 10^4 rad m^-2 for NGC 4579. This may arise from magnetized plasma in the accretion, outflow or interstellar regions. Inverted spectra are present in both M81* and Sagittarius A* and absent from all sources in which circular polarization is not detected. This suggests that optical depth effects are important in the creation of circular polarization.

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arXiv2002

Radio-Emission from Cosmic Ray Air-Showers - A Theoretical Perspective for LOPES

High-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. Adopting a simplified shower geometry and electron-positron energy distribution, we calculate theoretical pulse spectra in the scheme of synchrotron emission from highly relativistic electron-positron pairs gyrating in the Earth's magnetic field. These calculations will play an important role for the calibration of observational data of radio-emission from cosmic ray air-showers acquired with LOPES and later LOFAR and SKA.

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arXiv2002

Detecting Radio Emission from Cosmic Ray Air Showers and Neutrinos with a Digital Radio Telescope

We discuss the possibilities of measuring ultra-high energy cosmic rays and neutrinos with radio techniques. We review a few of the properties of radio emission from cosmic ray air showers and show how these properties can be explained by coherent ``geosynchrotron'' emission from electron-positron pairs in the shower as they move through the geomagnetic field. This should allow one to use the radio emission as a useful diagnostic tool for cosmic ray research. A new generation of digital telescopes will make it possible to study this radio emission in greater detail. For example, the planned Low-Frequency Array (LOFAR), operating at 10-200 MHz, will be an instrument uniquely suited to study extensive air showers and even detect neutrino-induced showers on the moon. We discuss sensitivities, count rates and possible detection algorithms for LOFAR and a currently funded prototype station LOPES. This should also be applicable to other future digital radio telescopes such as the Square-Kilometer-Array (SKA). LOFAR will be capable of detecting air-shower radio emission from >2*10^14 eV to ~10^20 eV. The technique could be easily extended to include air shower arrays consisting of particle detectors (KASCADE, Auger), thus providing crucial additional information for obtaining energy and chemical composition of cosmic rays. It also has the potential to extend the cosmic ray search well beyond an energy of 10^21 eV if isotropic radio signatures can be found. Other issues that LOFAR can address are to determine the neutral component of the cosmic ray spectrum, possibly look for neutron bursts, and do actual cosmic ray astronomy.

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arXiv2002

Size and Structure of the Narrow-Line Region of Quasars

We have observed the narrow-line regions (NLRs) of the seven brightest radio-quiet PG (or BQS) quasars (z < 0.5) with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope (HST). Linear-ramp filters were used to image the [OIII] lambda 5007 line emission with 0.0455-0.1 arcsec pixel resolution. We find that the NLRs are very compact with typical extents of 2-4 arcsec. Two quasars show compact filamentary structures similar to Seyfert NLRs. They may be related to radio outflows. Most interestingly, when including a sample of Seyfert galaxies observed with HST, we tentatively find that the size of the NLR is proportional to the square root of the [OIII] luminosity. This is comparable to the scaling found for the size of the broad-line region with continuum luminosity, which has been interpreted in terms of a constant photoionization parameter. The relation determined here connects the NLR of radio-quiet quasars and Seyferts over three orders of magnitude in [OIII] luminosity.

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arXiv2002

NGC4258: a jet-dominated low-luminosity AGN?

Low-luminosity AGNs (LLAGNs) are a very important class of sources since they occupy a significant fraction of local galaxies. Their spectra differ significantly from the canonical luminous AGNs, most notably by the absence of the ``big blue bump''. In the present paper, taking a typical LLAGN--NGC4258--as an example, we investigate the origin of their spectral emission. The observational data of NGC4258 is extremely abundant, including water maser emission, putting very strict constraints to its theoretical models. The infrared (IR) spectrum is well described by a steep power-law form $f_ν\propto ν^{-1.4}$, and may extend to the optical/UV band. Up until now there is no model which can explain such a steep spectrum, and we here propose a coupled jet plus accretion disk model for NGC4258. The accretion disk is composed of an inner ADAF (or radiatively inefficient accretion flow) and an outer standard thin disk. A shock occurs when the accretion flow is ejected out of the ADAF to form the jet near the black hole, accelerating the electrons into a power-law energy distribution. The synchrotron and self-Comptonized emission from these electrons greatly dominates over the underlying accretion disk and can well explain the spectrum ranging from IR to X-ray bands. The further propagation of the shocked gas in the jet can explain the flat radio spectrum of NGC4258. Several predictions of our model are presented for testing against future observations, and we briefly discuss the application of the model to other LLAGNs.

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arXiv2002

The Spectrum and Variability of Circular Polarization in Sagittarius A* from 1.4 to 15 GHz

We report here multi-epoch, multi-frequency observations of the circular polarization in Sagittarius A*, the compact radio source in the Galactic Center. Data taken from the VLA archive indicate that the fractional circular polarization at 4.8 GHz was -0.31% with an rms scatter of 0.13% from 1981 to 1998, in spite of a factor of 2 change in the total intensity. The sign remained negative over the entire time range, indicating a stable magnetic field polarity. In the Summer of 1999 we obtained 13 epochs of VLA A-array observations at 1.4, 4.8, 8.4 and 15 GHz. In May, September and October of 1999 we obtained 11 epochs of Australia Telescope Compact Array observations at 4.8 and 8.5 GHz. In all three of the data sets, we find no evidence for linear polarization greater than 0.1% in spite of strong circular polarization detections. Both VLA and ATCA data sets support three conclusions regarding the fractional circular polarization: the average spectrum is inverted with a spectral index ~0.5 +/- 0.2; the degree of variability is roughly constant on timescales of days to years; and, the degree of variability increases with frequency. We also observed that the largest increase in fractional circular polarization was coincident with the brightest flare in total intensity. Significant variability in the total intensity and fractional circular polarization on a timescale of 1 hour was observed during this flare, indicating an upper limit to the size of 70 AU at 15 GHz. The fractional circular polarization at 15 GHz reached -1.1% and the spectral index is strongly inverted during this flare. We conclude that the spectrum has two components that match the high and low frequency total intensity components. (abridged)

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arXiv2001

A Jet-ADAF model for Sgr A*

The recent {\em Chandra} observation of the radio source at the center of our Galaxy, Sgr A$^*$, puts new constraints on its theoretical models. The spectrum is very soft, and the source is rapidly variable. We consider different models to explain the observations. We find that the features of the x-ray spectrum can be marginally explained with an advection-dominated accretion flow (ADAF) model while it does not well fit the radio spectrum. An ADAF with strong winds (ADIOS) model is not favored if we assume that the wind does not radiate. Alternatively, we propose a coupled jet plus accretion disk model to explain the observations for Sgr A$^*$. The accretion flow is described as an ADAF fed by Bondi-Hoyle accretion of hot plasma in the Galactic Center region. A small fraction of the accretion flow is ejected near the black hole, forming a jet after passing through a shock. As a result, the electron temperature increases to $\sim 2 \times 10^{11}{\rm K}$, which is about 10 times higher than the highest temperature attained in the ADAF. The model is self-consistent since the main jet parameters are determined by the underlying accretion disk at the inner edge. The emergent spectrum of Sgr A$^*$ is the sum of the emission from jet and underlying ADAF. The very strong Comptonization of synchrotron emission from the jet dominates the bremsstrahlung from the ADAF, therefore, a very short variability timescale is expected and the predicted X-ray slope and the radio spectrum is in very good agreement with the observations.

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arXiv2001

Population X: Are the super-Eddington X-ray sources beamed jets in microblazars or intermediate mass black holes?

Recent X-ray observations reveal an increasing number of X-ray sources in nearby galaxies exceeding luminosities of L_x > 2 10^39 erg/s. Assuming isotropic emission, the Eddington limit suggests a population of intermediate-mass black holes of M_bh >> 10 M_sun. However, Markoff, Falcke, & Fender (2001) proposed that jets may be contributing to the X-ray emission from X-ray binaries (XRBs), implying that some X-ray sources may be relativistically beamed. This could reduce the required black hole masses to standard values. To test this hypothesis, we investigate a simple X-ray population synthesis model for X-ray point sources in galaxies with relativistic beaming and compare it with an isotropic emission model. The model is used to explain a combined data set of X-ray point sources in nearby galaxies. We show that the current distributions are consistent with black hole masses M_bh <=10 M_sun and bulk Lorentz factors for jets in microquasars of gamma_j ~ 5. Alternatively, intermediate mass black holes up to 1000 M_sun are required which are distributed in a powerlaw with roughly dN/dM ~ M^-2.

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arXiv2001

Circular Polarization of Radio Emission from Relativistic Jets

In inhomogeneous optically thick synchrotron sources a substantial part of the electron population at low energies can be hidden by self-absorption and overpowered by high energy electrons in optically thin emission. These invisible electrons produce Faraday rotation and conversion, leaving their footprints in the linear and circular polarized radiation of the source. An important factor is also the magnetic field structure, which can be characterized in most cases by a global magnetic field and a turbulent component. We present the basic radiative transfer coefficients for polarized synchrotron radiation and apply them to the standard jet model for relativistic radio jets. The model can successfully explain the unusual circular and linear polarization of the Galactic Centre radio source Sgr A* and its sibling M81*. It also can account for the circular polarization found in jets of more luminous quasars and X-ray binaries. The high ratio of circular to linear polarization requires the presence of a significant fraction of hidden matter and low-energy electrons in these jets. The stable handedness of circular polarization requires stable global magnetic field components with non-vanishing magnetic flux along the jet, while the low degree of total polarization implies also a significant turbulent field. The most favoured magnetic field configuration is that of a helix, while a purely toroidal field is unable to produce significant circular polarization. If connected to the magnetosphere of the black hole, the circular polarization and the jet direction determine the magnetic poles of the system which is stable over long periods of time. This may also have implications for possible magnetic field configurations in accretion flows.

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arXiv2001

The Power of Jets: New Clues from Radio Circular Polarization and X-rays

Jets are ubiquitous in accreting black holes. Often ignored, they may be a major contributor to the emitted spectral energy distribution for sub-Eddington black holes. For example, recent observations of radio-to-X-ray correlations and broad band spectra of X-ray binaries in the low/hard state can be explained by a significant synchrotron contribution from jets also to their IR-to-X-ray spectrum as proposed by Markoff, Falcke, Fender 2001. This model can also explain state-transitions from low/hard to high/soft states. Relativistic beaming of the jet X-ray emission could lead to the appearance of seemingly Super-Eddington X-rays sources in other galaxies. We show that a simple population synthesis model of X-ray binaries with relativistic beaming can well explain the currently found distribution of off-nucleus X-ray point sources in nearby galaxies. Specifically we suggest that the so-called ultra-luminous X-ray sources (ULXs, also IXOs) could well be relativistically beamed microblazars. The same model that can be used to explain X-ray binaries also fits Low-Luminosity AGN (LLAGN) and especially Sgr A* in the Galactic Center. The recent detection of significant circular polarization in AGN radio cores, ranging from bright quasars down to low-luminosity AGN like M81*, Sgr A* and even X-ray binaries, now places additional new constraints on the matter contents of such jets. The emerging picture are powerful jets with a mix of hot and cold matter, a net magnetic flux, and a stable magnetic north pole.

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arXiv2001

The Silent Majority - Jets and Radio Cores from Low-Luminosity Black Holes

They 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 years, specifically focusing on the large population of low-luminosity AGN. Special attention is also given to Sgr A*, the supermassive black hole candidate at the Galactic Center, whose radio properties are reviewed in more detail. This source exhibits a submm-bump, possibly from an ultra-compact region around the black hole which should allow imaging of the event horizon of the black hole in the not too distant future. A jet model is proposed which explains the basic feature of Sgr A*: its slightly inverted radio spectrum, the submm-bump, the lack of extended emission, and the X-ray emission. This model also works for famous sources like M81*, NGC4258, or GRS1915+105 based on the argument that radio cores are jets whose emission can be scaled with the accretion power over many orders of magnitude. This scaling is corroborated by the detection of many Sgr A*-like radio cores in nearby Low-Luminosity AGN (LLAGN), some of which show jet structures on the VLBI (Very Long Baseline Interferometry) scale. These cores confirm an AGN origin of about half of the known low-luminosity AGN classified as LINERs and dwarf-Seyferts. It is argued that in fact most of the compact radio emission at centimeter waves in LLAGN is produced by a compact radio jet and not an Advection Dominated Accretion Flow (ADAF). In general one can say that compact radio cores are a genuine feature of AGN, allowing one to precisely pinpoint black holes in many galaxies - not only in luminous quasars.

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