Radio Emission (radio + emission)

Distribution by Scientific Domains

Selected Abstracts

Evolution of the Chandra CCD spectra of SNR 1987A: probing the reflected-shock picture

Svetozar A. Zhekov
ABSTRACT We continue to explore the validity of the reflected-shock structure (RSS) picture in SNR 1987A that was proposed in our previous analyses of the X-ray emission from this object. We used an improved version of our RSS model in a global analysis of 14 CCD spectra from the monitoring program with Chandra. In the framework of the RSS picture, we are able to match both the expansion velocity curve deduced from the analysis of the X-ray images and light curve. Using a simplified analysis, we also show that the X-rays and the non-thermal radio emission may originate from the same shock structure (the blast wave). We believe that using the RSS model in the analysis of grating data from the Chandra monitoring program of SNR 1987A that cover a long enough time interval will allow us to build a more realistic physical picture and model of SNR 1987A. [source]

Annual cycles in the interstellar scintillation time-scales of PKS B1519,273 and PKS B1622,253

Steven J. B. Carter
ABSTRACT We have used the University of Tasmania's 30-m radio telescope at Ceduna in South Australia to regularly monitor the flux density of a number of southern blazars. We report the detection of an annual cycle in the variability time-scale of the centimetre radio emission of PKS B1622,253. Observations of PKS B1519,273 over a period of nearly 2 yr confirm the presence of an annual cycle in the variability time-scale in that source. These observations prove that interstellar scintillation is the principal cause of inter-day variability at radio wavelengths in these sources. The best-fitting annual cycle model for both sources implies a high degree of anisotropy in the scattering screen and that it has a large velocity offset with respect to the local standard of rest. This is consistent with a greater screen distance for these ,slow' intra-day variability (IDV) sources than for rapid scintillators such as PKS B0405,385 or J1819+3845. [source]

Jets from black hole X-ray binaries: testing, refining and extending empirical models for the coupling to X-rays

R. P. Fender
ABSTRACT In this paper we study the relation of radio emission to X-ray spectral and variability properties for a large sample of black hole X-ray binary systems. This is done to test, refine and extend , notably into the timing properties , the previously published ,unified model' for the coupling of accretion and ejection in such sources. In 14 outbursts from 11 different sources we find that in every case the peak radio flux, on occasion directly resolved into discrete relativistic ejections, is associated with the bright hard to soft state transition near the peak of the outburst. We also note the association of the radio flaring with periods of X-ray flaring during this transition in most, but not all, of the systems. In the soft state, radio emission is in nearly all cases either undetectable or optically thin, consistent with the suppression of the core jet in these states and ,relic' radio emission from interactions of previously ejected material and the ambient medium. However, these data cannot rule out an intermittent, optically thin, jet in the soft state. In attempting to associate X-ray timing properties with the ejection events we find a close, but not exact, correspondence between phases of very low integrated X-ray variability and such ejections. In fact the data suggest that there is not a perfect one-to-one correspondence between the radio, X-ray spectral or X-ray timing properties, suggesting that they may be linked simply as symptoms of the underlying state change and not causally to one another. We further study the sparse data on the reactivation of the jet during the transition back to the hard state in decay phase of outbursts, and find marginal evidence for this in one case only. In summary we find no strong evidence against the originally proposed model, confirming and extending some aspects of it with a much larger sample, but note that several aspects remain poorly tested. [source]

Radio constraints on the volume filling factors of AGN winds

A. J. Blustin
ABSTRACT The calculation of mass outflow rates of active galactic nuclei (AGN) winds is of great importance in understanding the role that such winds play in AGN-galaxy feedback processes. The mass outflow rates are, however, difficult to estimate since the volume filling factors of the winds are unknown. In this paper, we use constraints imposed by the observed radio emission to obtain upper limits to the volume filling factors of wind components in certain nearby AGN. We do this by predicting the 1.4 GHz radio flux densities emitted by those components, assuming a uniform wind, and then comparing these with the observed flux densities for each AGN at this frequency. We find that the upper limits to the volume filling factors are in the range of 10,4,0.5. [source]

A census of young stellar populations in the warm ULIRG PKS 1345+12

J. Rodríguez Zaurín
ABSTRACT We present a detailed investigation of the young stellar populations (YSP) in the radio-loud ultraluminous infrared galaxy (ULIRG), PKS 1345+12 (z= 0.12), based on high-resolution Hubble Space Telescope (HST) imaging and long-slit spectra taken with the William Herschel Telescope (WHT) at La Palma. While the images clearly show bright knots suggestive of super star clusters (SSCs), the spectra reveal the presence of YSP in the diffuse light across the full extent of the halo of the merging double nucleus system. Spectral synthesis modelling has been used to estimate the ages of the YSP for both the SSC and the diffuse light sampled by the spectra. For the SSC, we find ages tSSC < 6 Myr with reddenings 0.2 < E(B,V) < 0.5 and masses 106 < MYSPSSC < 107 M,. In the region to the south of the western nucleus that contains the SSC our modelling of the spectrum of the diffuse light is also consistent with a relatively young age for the YSP (,5 Myr), although older YSP ages cannot be ruled out. However, in other regions of the galaxy we find that the spectra of the diffuse light component can only be modelled with a relatively old post-starburst YSP (0.04,1.0 Gyr) or with a disc galaxy template spectrum. The results demonstrate the importance of accounting for reddening in photometric studies of SSC and highlight the dangers of focusing on the highest surface brightness regions when trying to obtain a general impression of the star formation activity in the host galaxies of ULIRGs. The case of PKS 1345+12 provides clear evidence that the star formation histories of the YSP in ULIRGs are complex. While the SSC represent the vigorous phase of star formation associated with the final stages of the merger, the YSP in the diffuse light are likely to represent star formation in one or more of the merging galaxies at an earlier stage or prior to the start of the merger. Intriguingly, our long-slit spectra show line splitting at the locations of the SSC, indicating that they are moving at up to 450 km s,1 with respect to the local ambient gas. Given their kinematics, it is plausible that the SSCs have been formed either in fast moving gas streams/tidal tails that are falling back into the nuclear regions as part of the merger process or as a consequence of jet-induced star formation linked to the extended, diffuse radio emission detected in the halo of the galaxy. [source]

G315.1+2.7: a new Galactic supernova remnant from the AAO/UKST H, survey

M. Stupar
ABSTRACT New narrow-band H, imaging and subsequent optical spectra confirm G315.1+2.7, a previously identified candidate supernova remnant (SNR), as a bona fide Galactic SNR. Present observations are based on independent discovery of filamentary optical emission nebulosity on images of the Anglo-Australian Observatory/United Kingdom Schmidt Telescope H, survey of the southern Galactic plane which were found to coincide with existing multifrequency radio detections. Separate medium- and high-dispersion spectra were taken across two locations of this 11-arcmin north,south (NS) aligned optical filament. The resulting spectral signatures were found to strongly confirm the SNR identification based on standard emission-line ratio discriminators which characterize emission from shock-heated gas. The average observed ratios of S ii/H,= 1.13, N ii/H,= 1.43 and S ii 6717/6731 = 1.46, together with the simultaneous detection of [O ii] 3727, [O iii] 5007 and [O i] 6300 Å, all point to a SNR origin of the observed optical emission. There is also an excellent positional coincidence between the new H, filament and the north-east radio arc of G315.1+2.7 seen at several frequencies. Careful scrutiny of the low-resolution but high-sensitivity Southern H, Sky Survey Atlas also revealed a low-level but distinct optical emission arc. This arc precisely correlates with the large, 2.5°, NS angular extent of the proposed new SNR also seen as a fractured structure in the extant radio data. G315.1+2.7 was detected previously at 2400 and 4800 MHz and at 408 and 1420 MHz. We also identified associated radio emission at 843 MHz from the now publicly available Sydney University Molonglo Sky Survey. On the basis of optical imaging and spectra and radio observations at five frequencies, we identify G315.1+2.7 as a new Galactic SNR. The large projected angular extent of the new remnant, together with the distance estimate of ,1.7 kpc and diameter of ,80 pc, make G315.1+2.7 one of the largest remnants known. [source]

Coherent synchrotron emission from cosmic ray air showers

Qinghuan Luo
ABSTRACT Coherent synchrotron emission by particles moving along semi-infinite tracks is discussed, with a specific application to radio emission from air showers induced by high-energy cosmic rays. It is shown that in general, radiation from a particle moving along a semi-infinite orbit consists of usual synchrotron emission and modified impulsive bremsstrahlung. The latter component is due to the instantaneous onset of the curved trajectory of the emitting particle at its creation. Inclusion of the bremsstrahlung leads to broadening of the radiation pattern and a slower decay of the spectrum at the cut-off frequency than the conventional synchrotron emission. Possible implications of these features for air shower radio emission are discussed. [source]

An ATCA radio-continuum study of the Small Magellanic Cloud , IV.

A multifrequency analysis of the N 66 region
ABSTRACT Traditional identification of supernova remnants (SNRs) include the use of radio spectral index, optical spectral studies (including strong [S ii], [N ii], [O i], [O ii] and [O iii] lines) and X-ray co-identifications. Each of these can have significant limitations within the context of a particular SNR candidate and new identification methods are continually sought. In this paper, we explore subtraction techniques by Ye, Turtle and Kennicutt to remove thermal emission estimated from H, flux from radio-continuum images. The remaining non-thermal emission allows the identification of SNRs embedded within these H ii regions. Subtraction images of the N 66 region in the Small Magellanic Cloud (SMC) using H, wide-field optical CCD images from the Curtis Schmidt Telescope and the recent Australia Telescope Compact Array (ATCA)/Parkes radio-continuum (1420, 2370, 4800 and 8640 MHz) data are presented as an example. These show three SNRs (B0057 , 724, B0056 , 724 and B0056 , 725) separated from their surrounding H ii radio emission. 2.3-m dual-beam spectrograph long-slit spectra from selected regions within N 66 suggest the presence of an additional SNR with no radio or X-ray emission. Radio spectral index, [S ii]/H, ratio and archived Chandra images of N 66 combine to give a more coherent picture of this region, confirming B0057 , 724 as an SNR. The N 66 nebula complex is divided into 10 components, composed separately of these SNRs and H ii regions. [source]

An empirical model for the polarization of pulsar radio emission

Don Melrose
ABSTRACT We present an empirical model for single pulses of radio emission from pulsars based on Gaussian probability distributions for relevant variables. The radiation at a specific pulse phase is represented as the superposition of radiation in two (approximately) orthogonally polarized modes (OPMs) from one or more subsources in the emission region of the pulsar. For each subsource, the polarization states are drawn randomly from statistical distributions, with the mean and the variance on the Poincaré sphere as free parameters. The intensity of one OPM is chosen from a lognormal distribution, and the intensity of the other OPM is assumed to be partially correlated, with the degree of correlation also chosen from a Gaussian distribution. The model is used to construct simulated data described in the same format as real data: distributions of the polarization of pulses on the Poincaré sphere and histograms of the intensity and other parameters. We concentrate on the interpretation of data for specific phases of PSR B0329+54 for which the OPMs are not orthogonal, with one well defined and the other spread out around an annulus on the Poincaré sphere at some phases. The results support the assumption that the radiation emerges in two OPMs with closely correlated intensities, and that in a statistical fraction of pulses one OPM is invisible. [source]

Turbulent gas motions in galaxy cluster simulations: the role of smoothed particle hydrodynamics viscosity

K. Dolag
ABSTRACT Smoothed particle hydrodynamics (SPH) employs an artificial viscosity to properly capture hydrodynamic shock waves. In its original formulation, the resulting numerical viscosity is large enough to suppress structure in the velocity field on scales well above the nominal resolution limit, and to damp the generation of turbulence by fluid instabilities. This could artificially suppress random gas motions in the intracluster medium (ICM), which are driven by infalling structures during the hierarchical structure formation process. We show that this is indeed the case by analysing results obtained with an SPH formulation where an individual, time-variable viscosity is used for each particle, following a suggestion by Morris & Monaghan. Using test calculations involving strong shocks, we demonstrate that this scheme captures shocks as well as the original formulation of SPH, but, in regions away from shocks, the numerical viscosity is much smaller. In a set of nine high-resolution simulations of cosmological galaxy cluster formation, we find that this low-viscosity formulation of SPH produces substantially higher levels of turbulent gas motions in the ICM, reaching a kinetic energy content in random gas motions (measured within a 1-Mpc cube) of up to 5,30 per cent of the thermal energy content, depending on cluster mass. This also has significant effects on radial gas profiles and bulk cluster properties. We find a central flattening of the entropy profile and a reduction of the central gas density in the low-viscosity scheme. As a consequence, the bolometric X-ray luminosity is decreased by about a factor of 2. However, the cluster temperature profile remains essentially unchanged. Interestingly, this tends to reduce the differences seen in SPH and adaptive mesh refinement simulations of cluster formation. Finally, invoking a model for particle acceleration by magnetohydrodynamics waves driven by turbulence, we find that efficient electron acceleration and thus diffuse radio emission can be powered in the clusters simulated with the low-viscosity scheme provided that more than 5,10 per cent of the turbulent energy density is associated with fast magneto-sonic modes. [source]

Spectral index of the H2O-maser-emitting planetary nebula IRAS 17347 , 3139

J. F. Gómez
ABSTRACT We present radio-continuum observations of the planetary nebula (PN) IRAS 17347 , 3139 (one of the only two known to harbour water maser emission), made to derive its spectral index and the turnover frequency of the emission. The spectrum of the source rises in the whole frequency range sampled, from 2.4 to 24.9 GHz, although the spectral index seems to decrease at the highest frequencies (0.79 ± 0.04 between 4.3 and 8.9 GHz, and 0.64 ± 0.06 between 16.1 and 24.9 GHz). This suggests a turnover frequency of around 20 GHz (which is unusual among PNe, whose radio emission usually becomes optically thin at frequencies <10 GHz), and a relatively high emission measure (1.5 × 109 cm,6 pc). The radio-continuum emission has increased by a factor of ,1.26 at 8.4 GHz in 13 yr, which can be explained as expansion of the ionized region by a factor of ,1.12 in radius with a dynamical age of ,120 yr and at an expansion velocity of ,5,40 km s ,1. These radio-continuum characteristics, together with the presence of water maser emission and a strong optical extinction, suggest that IRAS 17347 , 3139 is one of the youngest PNe known, with a relatively massive progenitor star. [source]

On the origin of the drifting subpulses in radio pulsars

G. Gogoberidze
ABSTRACT We present a model for the main observational characteristics of the radio emission of pulsars with well-organized drifting subpulses. We propose that drifting subpulses result from the modulation of the radio emission mechanism due to long-wavelength drift waves in the magnetosphere. The drift waves are generated at shorter wavelengths, and their non-linear evolution favours accumulation in a specific azimuthal eigenmode with an integral number, m, of nodes encircling the magnetic pole. The electric field of the drift waves is along the magnetic field lines, and this modulates the distribution for particles and hence the radio emission mechanism. The ratio of the frequency of the eigenmode to the rotation frequency of the star is insensitive to the magnetic field strength and the period of rotation, and is of order unity. The period, P3, of the drifting subpulses is attributed to the mismatch between this frequency and the nearest harmonic of the rotation frequency of the star. [source]

Radio detections of the neutron star X-ray binaries 4U 1820 , 30 and Ser X-1 in soft X-ray states

S. Migliari
ABSTRACT We present the analysis of simultaneous X-ray (RXTE) and radio (VLA) observations of two atoll-type neutron star X-ray binaries: 4U 1820 , 30 and Ser X-1. Both sources were steadily in the soft (,banana') X-ray state during the observations. We have detected the radio counterpart of 4U 1820 , 30 at 4.86 and 8.46 GHz at a flux density of ,0.1 mJy. This radio source is positionally coincident with the radio pulsar PSR 1820 , 30A. However, the radio emission of the pulsar falls rapidly with frequency (,,,3), and we argue that the radio emission of the X-ray binary is dominant above ,2 GHz. Supporting this interpretation, comparison with previous observations reveals variability at the higher radio frequencies that is likely to be due to the X-ray binary. We have detected for the first time the radio counterpart of Ser X-1 at 8.46 GHz, also at a flux density of ,0.1 mJy. The position of the radio counterpart has allowed us to identify its optical counterpart unambiguously. We briefly discuss similarities and differences between the disc,jet coupling in neutron star and black hole X-ray binaries. In particular, we draw attention to the fact that, contrary to other states, neutron star X-ray binaries seem to be more radio-loud than persistent black hole candidates when the emission is ,quenched' in the soft state. [source]

Extended X-ray emission in the high-redshift quasar GB 1508+5714 at z= 4.3

W. Yuan
ABSTRACT We report the discovery of extended X-ray emission around the powerful high-redshift quasar GB 1508+5714 at z= 4.3, revealed in a long Chandra ACIS observation. The emission feature is 3,4 arcsec away from the quasar core, which corresponds to a projected distance of about 25 kpc. The X-ray spectrum is best fitted with a power law of photon index 1.92 ± 0.35 (90 per cent confidence limit). The X-ray flux and luminosity reach 9.2 × 10,15 erg cm,2 s,1 (0.5,8 keV) and 1.6 × 1045 erg s,1 (2.7,42.4 keV rest frame, ,,= 0.73, ,m= 0.27, H0= 71 km s,1 Mpc,1), which is about 2 per cent of the total X-ray emission of the quasar. We interpret the X-ray emission as inverse Compton scattering of cosmic microwave background photons. The scattering relativistic electron population could either be a quasi-static diffuse cloud fed by the jet, or an outer extension of the jet with a high bulk Lorentz factor. We argue that the lack of an obvious detection of radio emission from the extended component could be a consequence of Compton losses on the electron population, or of a low magnetic field. Extended X-ray emission produced by inverse Compton scattering may be common around high-redshift radio galaxies and quasars, demonstrating that significant power is injected into their surroundings by powerful jets. [source]

Sunyaev,Zel'dovich effect from quasar-driven blast waves

P. Platania
Abstract Quasar-driven winds are currently the best candidates for accounting for the pre-heating of the intergalactic medium in clusters. Such winds, occurring during early phases of the evolution of spheroidal galaxies, shock-heat the interstellar gas, thus inducing a detectable Sunyaev,Zel'dovich effect. We estimate the amplitude and the angular scale of such an effect as well as its counts as a function of the Comptonization parameter y. The contamination arising from radio emission by the quasar itself is also discussed. The corresponding mean Compton distortion of the cosmic microwave background spectrum is found to be well below the COBE/FIRAS upper limit. [source]

Scaling and correlation analysis of galactic images

P. Frick
Different scaling and autocorrelation characteristics and their application to astronomical images are discussed: the structure function, the autocorrelation function, Fourier spectra and wavelet spectra. The choice of the mathematical tool is of great importance for the scaling analysis of images. The structure function, for example, cannot resolve scales that are close to the dominating large-scale structures, and can lead to the wrong interpretation that a continuous range of scales with a power law exists. The traditional Fourier technique, applied to real data, gives very spiky spectra, in which the separation of real maxima and high harmonics can be difficult. We recommend as the optimal tool the wavelet spectrum with a suitable choice of the analysing wavelet. We introduce the wavelet cross-correlation function, which enables us to study the correlation between images as a function of scale. The cross-correlation coefficient strongly depends on the scale. The classical cross-correlation coefficient can be misleading if a bright, extended central region or an extended disc exists in the galactic images. An analysis of the scaling and cross-correlation characteristics of nine optical and radio maps of the nearby spiral galaxy NGC 6946 is presented. The wavelet spectra allow us to separate structures on different scales like spiral arms and diffuse extended emission. Only the images of thermal radio emission and H, emission give indications of three-dimensional Kolmogorov-type turbulence on the smallest resolved scales . The cross-correlations between the images of NGC 6946 show strong similarities between the images of total radio emission, red light and mid-infrared dust emission on all scales. The best correlation is found between total radio emission and dust emission. Thermal radio continuum and H, emission are best correlated on a scale of about , the typical width of a spiral arm. On a similar scale, the images of polarized radio and H, emission are anticorrelated, a fact that remains undetected with classical cross-correlation analysis. [source]

Emission-line outflows in PKS1549,79: the effects of the early stages of radio-source evolution?

C. Tadhunter
We present new spectroscopic observations of the southern radio galaxy . Despite the flat-spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad-line radio galaxies. However, the high-ionization forbidden lines, including [O iii],,5007, 4959, are unusually broad for a narrow-line radio galaxy , and are blueshifted by 600 km s,1 relative to the low-ionization lines such as [O ii],,3726,3729. The [O ii] lines are also considerably narrower than the [O iii] lines, and have a redshift consistent with that of the recently detected H i 21-cm absorption-line system. Whereas the kinematics of the [O iii] emission lines are consistent with outflow in an inner narrow-line region, the properties of the [O ii] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the H i absorption-line system. The unusually broad [O iii] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that is a radio source in an early stage of evolution. [source]

A high-resolution radio survey of Class I protostars

P. W. Lucas
We report the results of a survey of low-mass Class I protostars in the cm continuum. In the initial survey, seven sources in the Taurus star formation region were observed with the VLA at 0.25-arcsec resolution. All seven sources drive CO outflows and display Herbig,Haro flows in the optical or near-infrared (NIR) wavebands. Four out of seven sources were detected, two of which are new discoveries in systems of very low luminosity, one being the lowest luminosity system detected to date in the cm continuum. Notably, three sources were not detected to a 3, limit of 0.10 mJy/beam, which indicates that significant cm continuum emission is not a universal feature of Class I systems with outflow activity. Subsequent observations of HH30, a more evolved Class II system, found no emission to a 3, limit of 0.03 mJy/beam. After comparison with near infrared data, we suggest that the discriminating feature of the detected systems is a relatively high ionization fraction in the stellar wind. Temporal variability of the outflow may also play a role: only recently ejected knots may have sufficiently dense plasma to be optically thick to free,free emission, and hence produce detectable flux. The one relatively bright source, IRAS 04016+2610 (L1489 IRS), is clearly resolved on a 0.4-arcsec scale at 2 and 3.5 cm. Additional imaging with MERLIN did not detect this source with a 0.04-arcsec beam, indicating that the radio emission is generated in a region with a radius of ,25 au, which is broadly similar to the radius of the bipolar cavities inferred from models of NIR data. Interpretation of this system is complicated by the existence of a quadrupolar outflow, i.e. two bipolar outflows along roughly perpendicular axes, which we originally detected through polarimetric imaging. We present an NIR H2 image in which a bow shock in the secondary outflow is clearly seen. This complicated structure may have been caused by a gravitational interaction between two protostars. [source]

Limits on radio emission from pulsar wind nebulae

B. M. Gaensler
We report on a sensitive survey for radio pulsar wind nebulae (PWN) towards 27 energetic and/or high-velocity pulsars. Observations were carried out at 1.4 GHz using the Very Large Array and the Australia Telescope Compact Array and utilized pulsar-gating to search for off-pulse emission. These observing parameters resulted in a considerably more sensitive search than previous surveys and could detect PWN over a much wider range of spatial scales (and hence ambient densities and pulsar velocities). However, no emission clearly corresponding to a PWN was discovered. Based on these non-detections we argue that the young and energetic pulsars in our sample have winds which are typical of young pulsars, but produce unobservable PWN because they reside in low-density (n,0.003 cm,3) regions of the interstellar medium. However, non-detection of PWN around older and less energetic pulsars can only be explained if the radio luminosity of their winds is less than 10,5 of their spin-down luminosity, implying an efficiency at least an order of magnitude smaller than that seen for young pulsars. [source]

Discovery of an unusual new radio source in the star-forming galaxy M82: faint supernova, supermassive black hole or an extragalactic microquasar?

T. W. B. Muxlow
ABSTRACT A faint new radio source has been detected in the nuclear region of the starburst galaxy M82 using Multi-Element Radio-Linked Interferometer Network radio observations designed to monitor the flux density evolution of the recent bright supernova SN 2008iz. This new source was initially identified in observations made between 2009 May 1 and 5 but had not been present in observations made 1 week earlier, or in any previous observations of M82. In this Letter, we report the discovery of this new source and monitoring of its evolution over its first 9 months of existence. The true nature of this new source remains unclear, and we discuss whether this source is an unusual and faint supernova, a supermassive black hole associated with the nucleus of M82 or intriguingly the first detection of radio emission from an extragalactic microquasar. [source]

The pulsar synchrotron: coherent radio emission

I. Contopoulos
ABSTRACT We propose a simple physical picture for the generation of coherent radio emission in the axisymmetric pulsar magnetosphere that is quite different from the canonical paradigm of radio emission coming from the magnetic polar caps. In this first paper, we consider only the axisymmetric case of an aligned rotator. Our picture capitalizes on an important element of the magnetohydrodynamic (MHD) representation of the magnetosphere, namely the separatrix between the corotating closed-line region (the ,dead zone') and the open-field lines that originate in the polar caps. Along the separatrix flows the return current that corresponds to the main magnetospheric electric current emanating from the polar caps. Across the separatrix, both the toroidal and poloidal components of the magnetic field change discontinuously. The poloidal component discontinuity requires the presence of a significant annular electric current which has up to now been unaccounted for. We estimate the position and thickness of this annular current at the tip of the closed line region, and show that it consists of electrons (positrons) corotating with Lorentz factors on the order of 105, emitting incoherent synchrotron radiation that peaks in the hard X-rays. These particles stay in the region of highest annular current close to the equator for a path-length of the order of 1 m. We propose that, at wavelengths comparable to that path-length, the particles emit coherent radiation, with radiated power proportional to N2, where N is the population of particles in the above path-length. We calculate the total radio power in this wavelength regime and its scaling with pulsar period and stellar magnetic field and show that it is consistent with estimates of radio luminosity based on observations. [source]

The radio expansion and brightening of the very young supernova remnant G1.9 + 0.3

D. A. Green
ABSTRACT Recent radio observations of the small Galactic supernova remnant G1.9 + 0.3 made at 4.86 GHz with the Very Large Array are presented, and compared with earlier observations at 1.49 GHz which have a comparable resolution (10 × 4 arcsec2). These show that the radio emission from this remnant has expanded significantly, by about 15 per cent over 23 yr, with a current outer diameter of ,92 arcsec. This expansion confirms that G1.9 + 0.3 is the youngest Galactic remnant yet identified, only about 150 yr old at most. Recent, lower resolution, 1.43-GHz observations are also discussed, and the integrated flux densities from these and the 4.86-GHz observations are compared with earlier results. This shows that the integrated flux density of G1.9 + 0.3 has been increasing recently. [source]

Detection of compact radio emission from Circinus X-1 with the first Southern hemisphere e-VLBI experiment

C. J. Phillips
ABSTRACT Circinus X-1 has recently returned to a state of strong radio flaring. Here we report on the first VLBI observations, and detection, undertaken in the 25 years since the 1975,1985 period of strong recurrent flaring activity. We detected Circinus X-1 with the first observations conducted by a recently developed Southern hemisphere e-VLBI array, at both 1.6 and 8.4 GHz, over a three-day period. At 1.6 GHz, the compact source has a total flux density of 11 mJy and a size of 60 ± 15 mas (Gaussian model full width at half maximum). At 8.4 GHz, the compact source is less than 60 mas. The size variation with frequency is consistent with a broadened image due to scattering in the turbulent, ionized interstellar medium of our Galaxy. However, these size measurements appear inconsistent with the ,2.2 variation expected for strong interstellar scattering and previous VLBI observations made at 2.3 GHz in the early 1980s. To explain this apparent inconsistency, we suggest that Circinus X-1 supports a weak, non-varying component of 35 mas extent (175 au at 5 kpc distance), corresponding to compact structure in the extended radio nebula. No significant variation in the flux density at 1.6 GHz is evident between two observations 24 h apart. No jet-like structures are evident on scales of tens of mas, simply a scatter broadened source, presumably coincident with the suggested neutron star in the binary system. [source]

Antenna performance analysis for decameter solar radio observations

A.A. Stanislavsky
Abstract Decameter wavelength radio emission is finely structured in solar bursts. For their research it is very important to use a sufficient sensitivity of antenna systems. In this paper we study an influence of the radiotelescope-antenna effective area on the results of decameter solar radio observations. For this purpose we compared the solar bursts received by the array of 720 ground-based dipoles and the single dipole of the radiotelescope UTR-2. It is shown that a larger effective area of the ground-based antenna allows us to measure a weaker solar emission and to distinguish a fine structure of strong solar events. This feature has been also verified by simultaneous ground- and space-based observations in the overlapping frequency range (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

From Gauß to Biermann: Highlights from the first 117 years of publications in Astronomische Nachrichten/Astronomical Notes

R. von Berlepsch
Abstract We present facsimiles of some of the scientifically and historically most relevant papers published in Astronomische Nachrichten/Astronomical Notes (AN) between 1821 and 1938. Almost all of these papers were written and printed in German and it is sometimes not completely straightforward to find these original works and then to cite the historically correct version, e.g. in case of a series of articles or editorial letters. It was common during the early years that many contributions were made in form of letters to the editor. We present a summary for these original works with an English translation of their titles. Among the highlights are the originals of the discovery of stellar parallaxes by FriedrichWilhelm Bessel, the discovery of the solar cycle by Heinrich Schwabe, the discovery of the planet Neptune by Johann Gottfried Galle, the first ever measured stellar radial velocity by Hermann Vogel, the discovery of radio emission from the Sun by Wilsing and Scheiner, the first ever conducted photoelectric photometry of stars by Paul Guthnick and up to the pioneering work by Karl Schwarzschild, Ejnar Hertzsprung, Erwin Finlay Freundlich and others. As a particular gimmick we present the still world record holding shortest paper ever published; by Johannes Hartmann in AN 226, 63 (1926) on Nova Pictoris. Our focus is on contributions in the early years and published until 1938 near the verge of the second world war (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Polarimetric observations of a sample of Compact Steep-Spectrum sources

F. Mantovani
Abstract Measurements of the polarised emission from Compact Steep-Spectrum (CSS) sources can give important information about the physical conditions inside and around the region of radio emission, essentially on the Narrow Line Region. We have observed the sample of CSS sources constructed by Fanti et al. (1990) with the Effelsberg 100-m radio telescope at four frequencies between 2.7 GHz and 10.4 GHz in order to measure the polarisation parameters. Results from the NRAO VLA Sky Survey (Condon et al. 1998) at 1.4 GHz were combined with our measurements for the full sample. The sample is considered statistically complete and allows comprehensive statistical analysis. Some of our conclusions are in disagreement with previous results obtained by polarimetric observations of other samples, for example the B3-VLA sample (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Probing the nature of multiple lobe-like emissions in 3C 84

H. Nagai
Abstract The bright radio source 3C 84 has a CSO-like feature in the central 10-pc scale with extended emissions on scale up to kpc. Our kinematic and synchrotron age measurements support that the innermost feature is a CSO formed by recent restarted activity before the radio emission that originated in the earlier activities has completely faded. The average rate of the jet power of the innermost feature over the source age is at least ,1044 erg s,1. Synchrotron age constraint for an outer lobe-like feature beyond the central CSO indicates that this feature originates in the outburst from ,1.3×105 years ago. Inferred duty cycle of the jet ejection is much shorter than the time scale expected from microquasars (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Relationship between the [O III] , 5007 line and 5 GHz radio emission

3Article first published online: 13 FEB 200, A. Labiano
Abstract I have compiled observations of [O III] , 5007 line and 5 GHz radio emission for a large sample of GPS, CSS and FR sources. Several properties were studied and compared. The most relevant findings are that the FWHM and the luminosity of the [O III] , 5007 line are correlated with the size of the radio source. I present the data and discuss the correlations, with special focus on jet-host interaction, triggering and enhancing of [O III] , 5007 emission (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Environment and luminosity of supernova remnants

L. K. Hunt
Abstract The explosion of supernovae and the evolution of their remnants (SNRs) accelerate cosmic rays over a vast range of timescales. Magnetic fields can be investigated indirectly through one of the observational signatures of this acceleration, namely radio synchrotron emission. With the aim of better understanding the role of the magnetic field in supernova evolution, we explore the variation of SNR radio luminosities with physical conditions in the surrounding interstellar medium. With a data set that comprises more than 90 individual SNRs in 10 galaxies, and a range of 3000 in ISM density and 104 in radio synchrotron luminosity, we find a significant correlation between the two quantities. The observed trends support the hypothesis that adiabatic compression of magnetic fields by itself is insuf.cient to explain the radio emission of the brighter and more luminous in SNRs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The Granada workshop on High Redshift Radio Galaxies: An overview

H. J. A. Röttgering
Abstract The Granada workshop on High Redshift Radio Galaxies (HzRGs) gave an excellent overview of the progress that has been made in this field during the last 3 years. Here we briefly review some of the results, with an emphasis on what studies of HzRGs can teach us about the formation and evolution of massive galaxies, clusters and active galactic nuclei (AGN). Of great relevance for this workshop are scenarios that describe certain aspects of the evolution of radio galaxies, including (i) the sequence of events after merging of galaxies that ultimately lead to extended powerful radio sources and (ii) the mass assembly and virialization of the hosting massive galaxies and their associated (proto-)clusters. Furthermore, I briefly discuss two projects that are important for a further understanding of AGN and high redshift radio galaxies. First, using the MIDI instrument mounted on the VLT Interferometer, the dusty tori of nearby AGN can be studied in the range of 8,13 micron at high angular resolution. The first result on the nearby AGN NGC 1068 as presented by Jaffe et al. (2004) indicated the presence of a hot (T > 800 K), compact (,1 pc) component, possible identified with the base of the jet and a warm (270 K), well-resolved (3 × 4 pc) component associated with the alleged torus. Second, LOFAR is a new low frequency radio telescope that is currently being build in the Netherlands and is expected to be operational in 2008. With 50 stations spread over an area of 100 km in diameter, its resolution and sensitivity will be unprecedented in the frequency range 10,240 MHz. LOFAR will be a unique instrument that will impact a broad range of astrophysical topics varying from the epoch of reionisation, to gamma ray bursts and cosmic rays. Surveys with LOFAR will be of paramount importance for studies of HzRGs: It will enable (i) defining samples of radio galaxies with redshifts higher than 6, (ii) observations of starbursting galaxies in proto-clusters, and (iii) mapping out the low-frequency radio emission of virtually all northern radio-loud AGN in revolutionary detail. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]