arXiv subjects

C. Henkel

Publications and source records attributed to C. Henkel.

At least 235 records · Page 13Linked to original sources

arXiv2010

Optical BCS conductivity at imaginary frequencies and dispersion energies of superconductors

We present an efficient expression for the analytic continuation to arbitrary complex frequencies of the complex optical and AC conductivity of a homogeneous superconductor with arbitrary mean free path. Knowledge of this quantity is fundamental in the calculation of thermodynamic potentials and dispersion energies involving type-I superconducting bodies. When considered for imaginary frequencies, our formula evaluates faster than previous schemes involving Kramers--Kronig transforms. A number of applications illustrates its efficiency: a simplified low-frequency expansion of the conductivity, the electromagnetic bulk self-energy due to longitudinal plasma oscillations, and the Casimir free energy of a superconducting cavity.

cond-mat.supr-con↗

arXiv2010

Searching for chameleon-like scalar fields with the ammonia method

(Abridged) The ammonia method, which has been proposed to explore the electron-to-proton mass ratio, mu = m_e/m_p, is applied to nearby dark clouds in the Milky Way. This ratio, which is measured in different physical environments of high (terrestrial) and low (interstellar) densities of baryonic matter is supposed to vary in chameleon-like scalar field models, which predict strong dependence of both masses and coupling constant on the local matter density. High resolution spectral observations of molecular cores in lines of NH3 (J,K) = (1,1), HC3N J = 2-1, and N2H+ J = 1-0 were performed at three radio telescopes to measure the radial velocity offsets, DeltaV = V_rot - V_inv, between the inversion transition of NH3 (1,1) and the rotational transitions of other molecules with different sensitivities to the parameter dmm = (mu_obs - mu_lab)/mu_lab. The measured values of DeltaV exhibit a statistically significant velocity offset of 23 +/- 4_stat +/- 3_sys m/s. When interpreted in terms of the electron-to-proton mass ratio variation, this infers that dmm = (2.2 +/- 0.4_stat +/- 0.3_sys)x10^{-8}. If only a conservative upper bound is considered, then the maximum offset between ammonia and the other molecules is |DeltaV| <= 30 m/s. This gives the most accurate reference point at z = 0 for dmm: |dmm| <= 3x10^{-8}.

astro-ph.CO↗

arXiv2010

Temperature dependence of the magnetic Casimir-Polder interaction

We analyze the magnetic dipole contribution to atom-surface dispersion forces. Unlike its electrical counterpart, it involves small transition frequencies that are comparable to thermal energy scales. A significant temperature dependence is found near surfaces with a nonzero DC conductivity, leading to a strong suppression of the dispersion force at T > 0. We use thermal response theory for the surface material and discuss both normal metals and superconductors. The asymptotes of the free energy of interaction and of the entropy are calculated analytically over a large range of distances. Near a superconductor, the onset of dissipation at the phase transition strongly changes the interaction, including a discontinuous entropy. We discuss the similarities with the Casimir interaction beween two surfaces and suggest that precision measurements of the atom-surface interaction may shed new light upon open questions around the temperature dependence of dispersion forces between lossy media.

quant-ph↗

arXiv2009

X-ray vs. water maser emission in AGN

Correlations between X-ray and water maser emission in AGN have been recently reported. However, the lack of systematic studies affects the confidence level of these results. In the following, we introduce a project aimed at studying all the water maser sources believed to be associated with AGN activity through X-ray data obtained with the XRT and BAT instruments on-board the Swift satellite. Preliminary results of this work indicate a promising rate of XRT detections allowing us to refine follow-up observing strategies focused on investigating the nuclei of individual galaxies and deriving, on statistical basis, the main characteristics of water maser hosts. In addition, a cross-correlation between our sample and the BAT 22-months all-sky survey provides an exceptionally high detection rate at hard X-ray energies when compared to other AGN-related catalogs.

astro-ph.CO↗

arXiv2009

On the nuclear obscuration of H2O maser galaxies

To shed light onto the circumnuclear environment of 22 GHz H2O maser galaxies, we have analyzed some of their multi-wavelength properties, including the far infrared luminosity (FIR), the luminosity of the [O III]\lambda5007 emission line, the nuclear X-ray luminosity, and the equivalent width of the neutral iron Ka emission line (EW (Ka)). Our statistical analysis includes a total of 85 sources, most of them harboring an active galactic nucleus (AGN). There are strong anti-correlations between EW (Ka) and two "optical thickness parameters", i.e. the ratios of the X-ray luminosity versus the presumably more isotropically radiated [O III] and far infrared (FIR) luminosities. Based on these anti-correlations, a set of quantitative criteria, EW (Ka) > 300eV, L_{2-10keV} < 2L_[O III] and L_{FIR} > 600L_{2-10keV} can be established for Compton-thick nuclear regions. 18 H2O maser galaxies belong to this category. There are no obvious correlations between the EW (Ka), the [O III] luminosity and the isotropic H2O maser luminosity. When comparing samples of Seyfert 2s with and without detected H2O maser lines, there seem to exist differences in EW (Ka) and the fraction of Compton-thick nuclei. This should be studied further. For AGN masers alone, there is no obvious correlation between FIR and H2O maser luminosities. However, including masers associated with star forming regions, a linear correlation is revealed. Overall, the extragalactic FIR-H2O data agree with the corresponding relation for Galactic maser sources, extrapolated by several orders of magnitude to higher luminosities.

astro-ph.CO↗

arXiv2009

Thermal effects in the magnetic Casimir-Polder interaction

We investigate the magnetic dipole coupling between a metallic surface and an atom in a thermal state, ground state and excited hyperine state. This interaction results in a repulsive correction and - unlike the electrical dipole contribution - depends sensitively on the Ohmic dissipation in the material.

quant-ph↗

arXiv2009

Discovery of a bright radio transient in M82: a new radio supernova?

In this Letter, we report the discovery of a new bright radio transient in M82. Using the Very Large Array, we observed the nuclear region of M82 at several epochs at 22 GHz and detected a new bright radio source in this galaxy's central region. We find a flux density for this flaring source that is ~300 times larger than upper limits determined in previous observations. The flare must have started between 2007 October 29 and 2008 March 24. Over the last year, the flux density of this new source has decreased from ~100 mJy to ~11 mJy. The lightcurve (based on only three data points) can be fitted better with an exponential decay than with a power law. Based on the current data we cannot identify the nature of this transient source. However, a new radio supernova seems to be the most natural explanation. With it's flux density of more than 100 mJy, it is at least 1.5 times brighter than SN1993J in M81 at the peak of its lightcurve at 22 GHz.

astro-ph.CO↗

arXiv2009

New H2O masers in Seyfert and FIR bright galaxies. III. The Southern Sample

Recently, a relationship between the water maser detection rate and far infrared (FIR) flux densities has been established as a result of two 22 GHz maser surveys in a complete sample of galaxies (Dec>-30 degree) with 100 micron flux densities of > 50 Jy and > 30 Jy. This survey has been extended to the southern galaxies in order to discover new maser sources and to investigate the galaxies hosting the maser spots with particular emphasis on their nuclear regions. A sample of 12 galaxies with Dec<-30 degree and S(100 micron)>50 Jy was observed with the 70-m telescope of the Canberra Deep Space Communication Complex (CDSCC) at Tidbinbilla (Australia) in a search for water maser emission. The average 3 sigma noise level of the survey is 15 mJy for a 0.42 km/s channel, corresponding to a detection threshold of ~0.1 solar luminosities for the isotropic maser luminosity at a distance of 25 Mpc. Two new detections are reported: a kilomaser with an isotropic luminosity L_H2O ~5 solar luminosities in NGC3620 and a maser with about twice this luminosity in the merger system NGC3256. The detections have been followed-up through continuum and spectral line interferometric observations with the Australia Telescope Compact Array (ATCA). In NGC3256, a fraction (about a third) of the maser emission arises from two hot spots associated with star formation activity, which are offset from the galactic nuclei of the system. The remaining emission may arise from weaker centers of maser activity distributed over the central 50 arcsec. [abridged]

astro-ph.CO↗

arXiv2009

The density, the cosmic microwave background, and the proton-to-electron mass ratio in a cloud at redshift 0.9

Based on measurements with the Effelsberg 100-m telescope, a multi-line study of molecular species is presented toward the gravitational lens system PKS1830-211. Obtaining average radial velocities and performing Large Velocity Gradient radiative transfer calculations, the aims of this study are (1) to determine the density of the gas, (2) to constrain the temperature of the cosmic microwave background, and (3) to evaluate the proton-to-electron mass ratio at redshift 0.9. Analyzing data from six rotational HC3N transitions (this includes the J=7-6 line, which is likely detected for the first time in the interstellar medium) we obtain about 2000 cm-3 for the gas density of the south-western absorption component. Again toward the south-western source, excitation temperatures of molecular species with optically thin lines and higher rotational constants are, on average, consistent with the expected temperature of the cosmic microwave background, T_CMB = 5.14 K. However, individually, there is a surprisingly large scatter which far surpasses expected uncertainties. A comparison of CS J=1-0 and 4-3 optical depths toward the weaker north-western absorption component results in an excitation temperature of 11 K and a 1-sigma error of 3 K. For the south-eastern main component, a comparison of velocities determined from ten optically thin NH3 inversion lines with those from five optically thin rotational transitions of HC3N, observed at similar frequencies, constrains potential variations of the proton-to-electron mass ratio, with respect to its present value, to <1.4 x 10^-6 with 3-sigma confidence. Also including optically thin rotational lines from other molecular species, it is emphasized that systematic errors are smaller than 1 km/s, corresponding to an uncertainty of 10-6.

astro-ph.CO↗

arXiv2009

Molecular line mapping of the giant molecular cloud associated with RCW 106 - III. Multi-molecular line mapping

We present multi-molecular line maps obtained with the Mopra Telescope towards the southern giant molecular cloud (GMC) complex G333, associated with the HII region RCW 106. We have characterised the GMC by decomposing the 3D data cubes with GAUSSCLUMPS, and investigated spatial correlations among different molecules with principal component analysis (PCA). We find no correlation between clump size and line width, but a strong correlation between emission luminosity and line width. PCA classifies molecules into high and low density tracers, and reveals that HCO+ and N2H+ are anti-correlated.

astro-ph.SR↗

arXiv2009

Evidence of a pure starburst nature of the nuclear region of NGC 253

We present high-resolution spectral line and continuum VLBI and VLA observations of the nuclear region of NGC 253 at 22 GHz. While the water vapor masers in this region were detected on arcsecond and milliarcsecond scales, we could not detect any compact continuum emission with a 5 sigma upper limit of ~ 1 mJy. The observations reveal that the water maser emission is not related to a possible low-luminosity active galactic nucleus but is almost certainly associated with star-formation activity. Not detecting any compact continuum source on milliarcsecond scales also questions the presence of a - previously assumed - active nucleus in NGC 253.

astro-ph.CO↗

arXiv2009

Properties of the Molecular Gas in Starburst Galaxies and AGN

There is growing evidence that the properties of the molecular gas in the nuclei of starburst galaxies and in AGN may be very different from those seen in Galactic star forming regions and that a high kinetic temperature in the molecular gas may lead to a non-standard initial mass function in the next generation of stars. Unfortunately, among the fundamental parameters derived from molecular line observations, the kinetic temperature of the molecular gas in external galaxies is often not well determined due to a lack of suitable tracer molecules. We discuss the diagnostic power of selected transition lines of formaldehyde (H_2CO), which can be used as a molecular thermometer as well as an excellent tracer of the molecular gas density. As a proof of concept, we present the results of our multi-transition line study of the H_2CO emission from the prototypical starburst galaxy M82. Using our large velocity gradient model, we tightly constrain the physical properties of the dense gas in the prominent molecular lobes, completely independent of the standard "cloud thermometer" ammonia (NH_3) or other molecular tracers. Our results agree well with the properties of the high-excitation molecular gas component found in the most comprehensive CO studies. Our observations also indicate that there may be an asymmetry between the two molecular lobes.

astro-ph.GA↗

arXiv2009

Disentangling the circumnuclear environs of Centaurus A: I. High resolution molecular gas imaging

We present high resolution images of the 12CO(2-1) emission in the central 1' (1 kpc) of NGC 5128 (Centaurus A), observed using the SMA. We elucidate for the first time the distribution and kinematics of the molecular gas in this region with a resolution of 6'.0 x 2'.4 (100 pc x 40 pc). We spatially resolve the circumnuclear molecular gas in the inner 24'' x 12'' (400 pc x 200 pc), which is elongated along a position angle P.A. = 155 deg and perpendicular to the radio/X-ray jet. The SE and NW components of the circumnuclear gas are connected to molecular gas found at larger radii. This gas appears as two parallel filaments at P.A. = 120 deg, which are coextensive with the long sides of the 3 kiloparsec parallelogram shape of the previously observed dust continuum, as well as ionized and pure rotational H2 lines. Spatial and kinematical asymmetries are apparent in both the circumnuclear and outer gas, suggesting non-coplanar and/or non-circular motions. We extend to inner radii (r < 200 pc) previously studied warped disk models built to reproduce the central parallelogram-shaped structure. Adopting the warped disk model we would confirm a gap in emission between the radii r = 200 - 800 pc (12'' - 50''), as has been suggested previously. Although this model explains this prominent feature, however, our 12CO(2-1) observations show relevant deviations from this model. Namely, the physical connection between the circumnuclear gas and that at larger radii, brighter SE and NW sides on the parallelogram-shaped feature, and an outer curvature of its long sides. Overall it resembles more closely an S-shaped morphology, a trend that is also found in other molecular species. Hence, we explore qualitatively the possible contribution of a weak bi-symmetric potential which would naturally explain these peculiarities.

astro-ph.GA↗

arXiv2009

A gravitationally lensed water maser in the early Universe

Water masers are found in dense molecular clouds closely associated with supermassive black holes in the centres of active galaxies. Based upon the understanding of the local water maser luminosity function, it was expected that masers at intermediate and high redshifts would be extremely rare, but galaxies at redshifts z > 2 might be quite different from those found locally, not least because of more frequent mergers and interaction events. Using gravitational lensing as a tool to enable us to search higher redshifts than would otherwise be possible, we have embarked on a survey of lensed galaxies, looking for masers. Here we report the discovery of a water maser at redshift 2.64 in the dust- and gas-rich gravitationally lensed type 1 quasar MG J0414+0534, which, with an isotropic luminosity of 10,000 L_solar, is twice as luminous as the most powerful local water maser, and half that of the most distant maser previously known. Using the locally-determined luminosity function, the probability of finding a maser this luminous associated with any single active galaxy is 10^{-6}. The fact that we saw such a maser in the first galaxy we observed must mean that the volume densities and luminosities of masers are higher at redshift 2.64.

astro-ph.CO↗

arXiv2008

The Megamaser Cosmology Project: I. VLBI observations of UGC 3789

The Megamaser Cosmology Project (MCP) seeks to measure the Hubble Constant (Ho) in order to improve the extragalactic distance scale and constrain the nature of dark energy. We are searching for sources of water maser emission from AGN with sub-pc accretion disks, as in NGC 4258, and following up these discoveries with Very Long Baseline Interferometric (VLBI) imaging and spectral monitoring. Here we present a VLBI map of the water masers toward UGC 3789, a galaxy well into the Hubble Flow. We have observed masers moving at rotational speeds up to 800 km/s at radii as small as 0.08 pc. Our map reveals masers in a nearly edge-on disk in Keplerian rotation about a 10^7 Msun supermassive black hole. When combined with centripetal accelerations, obtained by observing spectral drifts of maser features (to be presented in Paper II), the UGC 3789 masers may provide an accurate determination of Ho, independent of luminosities and metallicity and extinction corrections.

astro-ph↗

arXiv2008

Experimental and theoretical challenges for the trapped electron quantum computer

We discuss quantum information processing with trapped electrons. After recalling the operation principle of planar Penning traps we sketch the experimental conditions to load, cool and detect single electrons. Here we present a detailed investigation of a scalable scheme including feasibility studies and the analysis of all important elements, relevant for the experimental stage. On the theoretical side, we discuss different methods to couple electron qubits. We estimate the relevant qubit coherence times and draw implications for the experimental setting. A critical assessment of quantum information processing with trapped electrons is concluding the article.

quant-ph↗

arXiv2008

Submillimeter water and ammonia absorption by the peculiar z~0.89 interstellar medium in the gravitational lens of the PKS 1830-211 system

Using the Atacama Pathfinder Experiment (APEX) telescope we have detected the rotational ground-state transitions of ortho-ammonia and ortho-water toward the redshift ~0.89 absorbing galaxy in the PKS 1830-211 gravitational lens system. We discuss our observations in the context of recent space-borne data obtained for these lines with the SWAS and Odin satellites toward Galactic sources. We find commonalities, but also significant differences between the interstellar media in a galaxy at intermediate redshift and in the Milky Way. Future high-quality observations of the ground-state ammonia transition in PKS 1830-211, together with inversion line data, will lead to strong constraints on the variation of the proton to electron mass ratio over the past 7.2 Gyr.

astro-ph↗

arXiv2008

The CO line SED and atomic carbon in IRAS F10214+4724

Using the IRAM 30m telescope and the Plateau de Bure interferometer we have detected the \ctwo and the CO 3$-$2, 4$-$3, 6$-$5, 7$-$6 transitions as well as the dust continuum at 3 and 1.2 mm towards the distant luminous infrared galaxy IRAS F10214+4724 at $z=2.286$. The \ctwo line is detected for the first time towards this source and IRAS F10214+4724 now belongs to a sample of only 3 extragalactic sources at any redshift where both of the carbon fine structure lines have been detected. The source is spatially resolved by our \ctwo\ observation and we detect a velocity gradient along the east-west direction. The CI line ratio allows us to derive a carbon excitation temperature of 42$^{+12}_{-9}$ K. The carbon excitation in conjunction with the CO ladder and the dust continuum constrain the gas density to $n(\hh)$ = $10^{3.6-4.0}$ cm$^{-3}$ and the kinetic temperature to $T\rm_{kin}$ = 45--80 K, similar to the excitation conditions found in nearby starburst galaxies. The rest-frame 360 $μ$m dust continuum morphology is more compact than the line emitting region, which supports previous findings that the far infrared luminosity arises from regions closer to the active galactic nucleus at the center of this system.

astro-ph↗