Highlights of the expansion of SN1993J

SN 1993J in M 81, a powerful radio supernova high in the northern sky, has been carefully monitored with VLBI throughout its lifetime. Its extremely circular shell-like radio structure has expanded over 15 years in a rather self-similar way in reasonable agreement with Chevalier’s model. An extension of Chevalier’s model simultaneously accounts for all the light-curve and VLBI results. Beyond the first year a single deceleration parameter, mtrue = 0.87± 0.02, characterizes the shock expansion. However, at short wavelengths and beyond 5 years, an enhanced deceleration, mshortwavelengths = 0.79±0.01, is measured. This enhancement is interpreted as due to a combination of effects ranging from varying free-free opacity in the supernova ejecta at the short wavelengths to the radial shape of the intensity of the magnetic fields in the emitting region. The final radio-loud stage is characterized by an abrupt decrease of radio emission. This large flux-density decay rate can be explained as due to the supernova shock surpassing the outer boundary of the circumstellar medium generated by the supernova progenitor. Presently, the supernova expansion beyond the circumstellar/interstellar boundary is rather radio silent, although there are indications that the supernova remnant could be studied by LOFAR and eMERLIN

UVMULTIFIT: A versatile tool for fitting astronomical radio interferometric data

Context. The analysis of astronomical interferometric data is often performed on the images obtained after deconvolving the interferometer's point spread function. This strategy can be understood (especially for cases of sparse arrays) as fitting models to models, since the deconvolved images are already non-unique model representations of the actual data (i.e., the visibilities). Indeed, the interferometric images may be affected by visibility gridding, weighting schemes (e. g., natural vs. uniform), and the particulars of the (non-linear) deconvolution algorithms. Fitting models to the direct interferometric observables (i.e., the visibilities) is preferable in the cases of simple (analytical) sky intensity distributions. Aims. We present UVMULTIFIT, a versatile library for fitting visibility data, implemented in a Python-based framework. Our software is currently based on the CASA package, but can be easily adapted to other analysis packages, provided they have a Python API. Methods. The user can simultaneously fit an indefinite number of source components to the data, each of which depend on any algebraic combination of fitting parameters. Fits to individual spectral-line channels or simultaneous fits to all frequency channels are allowed. Results. We have tested the software with synthetic data and with real observations. In some cases (e. g., sources with sizes smaller than the diffraction limit of the interferometer), the results from the fit to the visibilities (e. g., spectra of close by sources) are far superior to the output obtained from the mere analysis of the deconvolved images. Conclusions. UVMULTIFIT is a powerful improvement of existing tasks to extract the maximum amount of information from visibility data, especially in cases close to the sensitivity/resolution limits of interferometric observations

A first search of transients in the Galactic Center from 230 GHz ALMA observations

The Galactic Center (GC) presents one of the highest stellar densities in our Galaxy, making its surroundings an environment potentially rich in radio transients, such as pulsars and different kinds of flaring activity. In this paper, we present the first study of transient activity in the region of the GC based on Atacama Large Millimeter/submillimeter (mm/submm) Array (ALMA) continuum observations at 230 GHz. This search is based on a new self-calibration algorithm, especially designed for variability detection in the GC field. Using this method, we have performed a search of radio transients in the effective field of view of~$\sim 30\,$arcseconds of the GC central supermassive black hole Sagittarius A* (SgrA*) using ALMA 230 GHz observations taken during the 2017 Event Horizon Telescope (EHT) campaign, which span several observing hours (5-10) on 2017 April 6, 7, and 11. This calibration method allows one to disentangle the variability of unresolved SgrA* from any potential transient emission in the wider field of view and residual effects of the imperfect data calibration. Hence, a robust statistical criterion to identify real transients can be established: the event should survive at least three times the correlation time and it must have a peak excursion of at least seven times the instantaneous root-mean-square between consecutive images. Our algorithms are successfully tested against realistic synthetic simulations of transient sources in the GC field. Having checked the validity of the statistical criterion, we provide upper limits for transient activity in the effective field of view of the GC at 230 GHz.Comment: Accepted for publication in Astronomy and Astrophysic

VLBI imaging of M81* at 3.4 mm with source-frequency phase-referencing

We report on the first VLBI image of the M81 nucleus (M81*) at a wavelength of 3.4 mm, obtained with the source-frequency phase-referencing (SFPR) technique. Thanks to the SFPR calibration, the coherent integration time could be eventually increased by more than an order of magnitude, which enabled the detection of fringes at the level of 45 mJy beam-1 with a dynamic range higher than 130:1. This paves the way toward future mm/sub-mm VLBI observations of weaker sources. From the analysis of the M81* visibilities, a core size of ~50 uas at 3.4 mm was estimated. This follows the power-law relationship with wavelength lambda^0.88, reported previously at lower frequencies. These results constrain the core size (at 3.4 mm) to a minimum of ~80 Schwarzschild radii of M81*.Comment: 10 pages, 5 figures,accepted by ApJ Lette

Rapid angular expansion of the ionized core of CRL 618

Context. During the transition from the asymptotic giant branch (AGB) to the planetary nebulae phase the circumstellar envelopes of most low- and intermediate-mass stars experience a dramatic change in morphology. CRL 618 exhibits characteristics of both an AGB and post-AGB star. It also displays a spectacular array of bipolar lobes with a dense equatorial region, which makes it an excellent object for studying the development of asymmetries in evolved stars. In recent decades, an elliptical compact HII region located in the center of the nebula has been seen to be increasing in size and flux. This seems to be due to the ionization of the circumstellar envelope by the central star, and it probably indicates the beginning of the planetary nebula phase for CRL 618. Aims. We aim to determine the physical conditions under which the ionization of the circumstellar envelope of CRL 618 began to take place as well as the subsequent propagation of the ionization front. Methods. We analyzed interferometric radio continuum data at ~5 and 22 GHz from observations carried out at seven epochs with the VLA. We traced the flux increase of the ionized region over a period of ~26 years. We measured the dimensions of the HII region directly from the brightness distribution images to determine the increase of its size over time. For one of the epochs we analyzed observations at six frequencies from which we estimated the electron density distribution. We carried out model calculations of the spectral energy distribution at two different epochs to corroborate our observational results. Results. We found that the radio continuum flux and the size of the ionized region have been increasing monotonically in the past three decades. The size of the major axis of the HII region shows a dependance on frequency, which has been interpreted as a result of the gradient of the electron density in this direction. The growth of the HII region is due to the expansion of an ionized wind whose mass-loss rate increased continuously for a period of ~100 years until a few decades ago, when the mass-loss rate experienced a sudden decline. Our results indicate that the circumstellar envelope began to be ionized around 1971, which marks the start of the planetary nebula phase of CRL 618. © ESO, 2013

Core-shifts and proper-motion constraints in the S5 polar cap sample at the 15 and 43 GHz bands

We have studied a complete radio sample of active galactic nuclei with the very-long-baseline-interferometry (VLBI) technique and for the first time successfully obtained high-precision phase-delay astrometry at Q band (43 GHz) from observations acquired in 2010. We have compared our astrometric results with those obtained with the same technique at U band (15 GHz) from data collected in 2000. The differences in source separations among all the source pairs observed in common at the two epochs are compatible at the 1σ level between U and Q bands. With the benefit of quasi-simultaneous U and Q band observations in 2010, we have studied chromatic effects (core-shift) at the radio source cores with three different methods. The magnitudes of the core-shifts are of the same order (about 0.1 mas) for all methods. However, some discrepancies arise in the orientation of the core-shifts determined through the different methods. In some cases these discrepancies are due to insufficient signal for the method used. In others, the discrepancies reflect assumptions of the methods and could be explained by curvatures in the jets and departures from conical jets

Evidence of internal rotation and a helical magnetic field in the jet of the quasar NRAO 150

The source NRAO 150 is a very prominent millimeter to radio emitting quasar at redshift z = 1.52 for which previous millimeter VLBI observations revealed a fast counterclockwise rotation of the innermost regions of the jet. Here we present new polarimetric multi-epoch VLBI-imaging observations of NRAO 150 performed at 8, 15, 22, 43, and 86 GHz with the Very Long Baseline Array (VLBA), and the Global Millimeter VLBI Array (GMVA) between 2006 and 2010. All new and previous observational evidence - i.e., spectral index maps, multi-epoch image cross-correlation, and low level of linear polarization degree in optically thin regions - are consistent with an interpretation of the source behavior where the jet is seen at an extremely small angle to the line of sight, and the high frequency emitting regions in NRAO 150 rotate at high speeds on the plane of the sky with respect to a reference point that does not need to be related to any particularly prominent jet feature. The observed polarization angle distribution at 22, 43, and 86 GHz during observing epochs with high polarization degree suggests that we have detected the toroidal component of the magnetic field threading the innermost jet plasma regions. This is also consistent with the lower degree of polarization detected at progressively poorer angular resolutions, where the integrated polarization intensity produced by the toroidal field is explained by polarization cancellation inside the observing beam. All this evidence is fully consistent with a kinematic scenario where the main kinematic and polarization properties of the 43 GHz emitting structure of NRAO 150 are explained by the internal rotation of such emission regions around the jet axis when the jet is seen almost face on. A simplified model developed to fit helical trajectories to the observed kinematics of the 43GHz features fully supports this hypothesis. This explains the kinematics of the innermost regions of the jet in NRAO 150 in terms of internal jet rotation

Molecular line study of the S-type AGB star W Aquilae. ALMA observations of CS, SiS, SiO and HCN

Context. With the outstanding spatial resolution and sensitivity of the Atacama Large Millimeter/sub-millimeter Array (ALMA), molecular gas other than the abundant CO can be observed and resolved in circumstellar envelopes (CSEs) around evolved stars, such as the binary S-type Asymptotic Giant Branch (AGB) star W Aquilae. Aims. We aim to constrain the chemical composition of the CSE and determine the radial abundance distribution, the photospheric peak abundance, and isotopic ratios of a selection of chemically important molecular species in the innermost CSE of W Aql. The derived parameters are put into the context of the chemical evolution of AGB stars and are compared with theoretical models. Methods. We employ one-dimensional radiative transfer modeling - with the accelerated lambda iteration (ALI) radiative transfer code - of the radial abundance distribution of a total of five molecular species (CS, SiS, 30SiS, 29SiO and H13CN) and determine the best fitting model parameters based on high-resolution ALMA observations as well as archival single-dish observations. The additional advantage of the spatially resolved ALMA observations is that we can directly constrain the radial profile of the observed line transitions from the observations. Results. We derive abundances and e-folding radii for CS, SiS, 30SiS, 29SiO and H13CN and compare them to previous studies, which are based only on unresolved single-dish spectra. Our results are in line with previous results and are more accurate due to resolution of the emission regions