Updated with real 2009-2010 stuff

Massive star forming regions

Torstensson and van Langevelde continued their work on measuring the large-scale thermal methanol distribution in methanol maser bearing high mass star-forming regions. In the nearby star forming region Cep A, which is studied as the archetypical source in the sample, intersting results were obtained which relate to the origin of the maser in collaboration with van der Tak (SRON), Vlemmings (Bonn) and Kristensen (Leiden). Analysis of HARP data taken with the JCMT allows the derivation of the rotation temperature and column density of the thermal methanol gas. The methanol is clearly associated with the central source in this famous HII region and the derived temperature peaks at the location where the maser is found. Excitation models indicate that radiative excitation is limited to the very central region.

Thermal methanol in Cep A, from left to right the integrated flux, the velocity field and the line width of the methanol (7 1 7 – 6 1 6) E-type transition is shown. This is the strongest unblended line in the JCMT HARP spectra

The direction of the outflow is also roughly consistent with the orientation of the methanol masers on a much smaller scale as observed with the European VLBI Network. The methanol masers that straddle the waist of Cep A are interpreted to outline a large scale ring structure perpendicular to the outflow axis of the central source. Remarkably, the velocity field does not show a rotation signature, but seems to be dominated by a small radial motion. It could be hypothesized that the ring outlines an accretion shock, where in-falling gas hits the accretion disk.

In a project led by Vlemmings the same Cep A methanol masers are observed with MERLIN to obtain measurements of the magnetic field direction. Although the interpretation of maser polarization can be quite complex, the measurements reveal a structured large scale magnetic field, which is perpendicular to the accretion disk in this high mass young stellar object. Combined with measurements of the field strength, this observation indicated that magnetic fields could be the dominant force in regulating the formation of high mass stars. This is an important clue that the formation mechanism of high mass stars may resemble that of less massive stars.

The magnetic field configuration around the young massive star Cepheus A HW2 and the location of the methanol regions from which the 3D structure was inferred. The figure shows that the magnetic field is almost perfectly perpendicular to the disk through which matter is transported onto the star.

Torstensson and van Langevelde are extending their studies of the relation between methanol masers and thermal methanol to a small sample. Using JCMT HARP data and the same methods previously applied to Cep A it is possible to determine temperatures and column densities in a number of maser sources. Although some of the well studied sources appear to be very complex, a significant sub-sample of the sources can be characterised by very well confined (young) outflows, that seem to originate from the same location as the maser emission. Like in the relatively nearby source Cep A, this argues for a single location for methanol production in this source.

Velocity fields of the methanol 7_1 – 6_1 E-type transition in four methanol maser sources. The crosses define the position of the maser, which is argued to be associated with the origin of methanol in these high-mass star-forming regions.

Together with Bourke, 2010 summer student Cseh worked on the VLBI mapping of these sources. This will allow one to relate the orientation of the thermal methanol outflow to the VLBI structure, possibly yielding clues on the physical component that is associated with the maser structures.

The methanol masers feature in several other projects by van Langevelde and Torstensson. Together with Vlemmings and Surcis (Bonn) progress has been made in interpreting the methanol and water maser Zeeman effect and observed linear polarization, for example in the source NGC7538-IRS1. Together with Bartkiewcz a sample defines by a blind survey with the Torun telescope has been followed up. For a number of these sources, parallax based distances have been obtained and water masers and infrared speckle observations have been processed in order to understand the geometry of methanol masers in this sample.

Evolved stars

Amiri continued her studies of the shaping of evolved stars circumstellar material using masers, together with van Langevelde and Vlemmings (Bonn). Software was developed to interpret MERLIN observations of OH in so-called water fountain sources. While it was found that in the archetypical source W43A the OH most likely resides in the equatorial region, in one other case the OH may be entrained in the bipolar flow. In all cases the OH polarization measurements point to a large scale magnetic field that could be important in shaping the outflow.

The same team also started a research project to use the SiO masers in evolved stars to measure the effect of magnetic shaping. While previous SiO maser imaging has focused on Mira variable, this resulted in a beautfiful ring image of the classic OH/IR star OH44.8-2.3. Unlike the predominant tangential linear polarization morphology previously detected for Mira variables, the observations likely show a dipole magnetic field morphology for the SiO maser region of this star. In a collaboration with Kemball (Univ of Illinois) the hard problems of 43 GHz polarization calibration are addressed.

The SiO masers around OH44.8 are observed to lie in a 4.75mas ring, corresponding to 5.6 AU for a distance of 1.13 kpc.

In order to address the questions of the shaping of Planetray Nebulae a continued observing program is aiming to study candidate water fountain sources with the Effelsberg telescope. Two candidates were followed up with the European VLBI Network.

S. Mühle's contribution (draft version)

Nearby galaxies

The JCMT Nearby Galaxy Legacy Survey (NGLS) investigates the molecular gas and dust content in an HI flux limited sample of 155 galaxies within a distance of 25 Mpc by mapping the CO(3-2) line emission and the submillimeter continuum emission at 850μm and at 450 μm using the latest instruments at the JCMT, HARP-B and SCUBA-2. Stefanie Mühle and several other collaborators completed the observations of the sample galaxies in the CO(3-2) line with the 4 x 4 pixel receiver HARP-B receiver and the ACSIS spectrometer at the JCMT. The spectral line data are now being reduced and analyzed. So far, fully sampled maps of the CO(3-2) intensity (moment 0) of 9 galaxies - 4 galaxies in the Virgo cluster and 5 galaxies in the field - have been published, as well as 9 maps of the velocity dispersion (moment 2) and other plots of the kinematics. These maps were compared to ancillary data from the literature like CO(1-0) line emission, HI cubes and Spitzer data in order to derive correlations of the warm/dense gas traced by CO(3-2) with other components of the ISM. Some highlights include: Extended CO(3-2) emission with small-scale structure has been found in almost all galaxies published so far. While the gas depletion times appear to be similar in cluster galaxies and field galaxies, the CO(3-2) intensity itself may depend strongly on the environment, i.e. on local excitation conditions, leading to a range of CO(3-2)/CO(1-0) intensity ratios. The gas-to-dust ratio traced in NGC2403 increases with distance to the nucleus and may be a function of metalicity. The cloud-cloud velocity dispersion of the molecular gas in the sample galaxies is significantly smaller than that of the atomic gas in these objects, suggesting that the molecular gas as the dynamically coldest component determines the stability of a galactic disk. In NGC 4631, most of the molcular gas is concentrated within a radius of ~ 5 kpc, but weaker disk emission has been traced out to 12.4 kpc from the nucleus and up to heights of 1.4 kpc, including evidence for molecular outflows. some final edits?

Images of NGC 2403 in multiple wave bands. The 3.6 μm image traces starlight. The PAH 8 μm image traces primarily PAH emission. The 70 and 160 μm images trace cold dust emission. The CO(3–2) image shows CO emission associated with molecular gas. The HI image shows atomic gas emission. All images are 12 × 9 arcmin² with north up and east to the left. Logarithmic colour scales are used for all images except the 160 μm image, where the logarithmic scale to the second power was used to enhance the structure in the image, and the CO(3–2) image, where a linear colour scale was used. The colour scales were selected so as to best enhance the structures visible in the optical disc of the galaxy. The colours in the two CO(1–0) images [from the BIMA SONG survey and from the NRAO 12 m observations by Thornley & Wilson (1995)] are scaled linearly. Green circles in the upper left corners of the images show the beam size when the FWHM is greater than 10 arcsec; smaller beam sizes are difficult to illustrate in this image. The blue circles in the PAH 8 μm image show the positions of foreground stars that were brighter than or as bright as the continuum emission from the galaxy at 8 μm. These stars were masked out in the analysis. The magenta boxes in the CO images show the regions observed in the observations. Pixels with non-detections in the CO data are set to black. The cyan square indicates the location of VS 44, which is region 44 in the catalogue by Véron & Sauvayre (1965) and region 128 in the catalogue by Hodge & Kennicutt (1983). [Figure 2 in Bendo et al. (2010)] {unabridged caption; please edit as necessary}

In addition, Stefanie Mühle continued to lead her own investigations on the properties of the molecular gas in starburst galaxies and near AGN. In particular, the kinetic temperature of the dense molecular gas may prove to be crucial for the evolution of a galaxy. Hydrodynamical simulations suggest that the initial mass function (IMF) may be top-heavy if stars form out of dense molecular gas with a kinetic temperature of ~100 K or more. Unfortunately, the kinetic temperature of the molecular gas is rarely well constrained in external galaxies by the usual molecular tracers. Stefanie Mühle and her collaborators C. Henkel (Max-Planck-Institut für Radioastronomie), S. Aalto (Onsala Space Observatory), M. Rodríguez (Instituto de Astrofísica de Andalucía) and R. Herrero Illana (Instituto de Astrofísica de Andalucía) exploit the diagnostic power of a selected set of formaldehyde (H2CO) lines in combination with their Large Velocity Gradient (LVG) radiative transfer model to derive fundamental properties of the (dense) molecular gas in external galaxies like the kinetic temperature and gas density, completely independent of other molecular tracers (see e.g. Mühle et al. 2007). With the IRAM 30-m telescope and the new EMIR receivers, they detected the full set of diagnostic formaldehyde lines towards the nearby starburst galaxy Maffei 2. In addition, the group obtained very sensitive spectra of the H2CO lines towards the AGN of NGC 1068, which often serves as a template for nearby AGN, using the same telescope and receivers. Stefanie Mühle and her summer student Tahlia de Maio (University of Colorado) reduced and and started to analyze the H_2CO emission of the nearby starburst galaxy NGC 253. The spectra show significant variations in the physical and chemical properties of the molecular gas along the mapped major axis of the galaxy. A first comparison of the derived line intensity ratios with the non-LTE radiative transfer models yield kinetic temperatures that are significantly higher than the temperature of the dense molecular gas in the Galactic disk. The results imply the presence of a warm molecular gas phase even in a galaxy whose starburst is considered to be young.

Hubble Heritage image of the prototypical starburst galaxy M82 and formaldehyde lines observed towards the southwestern lobe of its circumnuclear molecular ring. A non-LTE radiative transfer analysis based on the intensity ratio of these diagnostic lines suggests a kinetic temperature of ~200 K in the traced molecular gas phase (NASA, ESA, and The Hubble Heritage Team (STScI/AURA), Mühle et al. 2007).

Amanda Kepley (University of Virginia) and Stefanie Mühle, with collaborators at the University of Wisconsin-Madison and the Argelander Institut für Astronomie in Bonn, investigated the polarized radio continuum emission of the nearby starburst dwarf galaxy NGC 1569 using data at four wavelengths (3 cm, 6 cm, 13 cm and 20 cm) observed with the MPIfR 100-m telescope in Effelsberg, the VLA and the WSRT. They found an extended radio continuum halo at 20 cm. The radio continuum feature associated with the western Hα arm is also present at wavelengths shorter than 20 cm. While the spectral indices derived for this galaxy generally steepen towards the halo of the galaxy, there are also filamentary regions of flat spectral indices extending to the edge of the galaxy. In general, the spatial distribution of the spectral indices supports the theory that a convective wind is at work in this galaxy. There is strong polarized emission at 3 cm and at 6 cm and weak polarized emission at 13 cm and at 20 cm. The thermal fraction of the continuum emission is 40-50% in the center of the galaxy and falls off rapidly with height above the disk. The total magnetic field strength is estimated to be 38 μG in the central regions and 10-15 μG in the halo. The magnetic field is largely random in the center of the galaxy; the uniform field is ~3-9 μG and is strongest in the halo. The magnetic pressure seems to be of the same order of magnitude but, in general, a factor of a few less than the other components of the interstellar medium in this galaxy. The uniform magnetic field in NGC 1569 is closely associated with the Hα bubbles and filaments. Based on pressure estimates and the morphology of the magnetic field, the outflow of hot gas from NGC 1569 is clearly shaping the magnetic field, but the magnetic field in turn may be aiding the outflow by channeling gas out of the disk of the galaxy. [A comprehensive paper with all the details has been published in the Astrophysical Journal.]

kepley_fig6.pdf
E-vectors showing the direction and distribution of polarized intensity at 3 cm and 6 cm. The contours outline the total intensity radio continuum emission at each wavelength, superposed on a greyscale image of the distribution of the Hα emission. The contours start at 3σ and increase by a factor of 2 for each subsequent contour. A polarization vector with a length of 1“ has a polarized intensity of 12.8 μJy/beam. The beams are indicated in the lower left corner of each panel [Fig. 6 in Keply et al. (2010)].

Y. Pidopryhora's contribution (draft version)

Neutral Hydrogen in the Milky Way's Halo

Yurii Pidopryhora continued his work on the large project to study the extra-planar hydrogen in the Inner Milky Way based on the observations of 21 cm HI line with the Green Bank Telescope (GBT), the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT) performed in 2003-2007. The project is done in collaboration with Felix Lockman (NRAO) and, in parts, with K.S. Dwarakanath (RRI), Harvey Liszt (NRAO), Joseph Shields (Ohio University) and Michael Rupen (NRAO).

The effort to make high-resolution maps of a chosen number of bright clouds in the disk-halo interface of the Galaxy continued. VLA observations in D, and in some cases also in C and B configurations provide high angular resolution view of the clouds and GBT data supply short spacing information, to detect extended, low surface brightness emission which might not have been detected by the VLA observations. Using previously developed successful interferometric-single dish data reduction recipes (involving procedures of both AIPS and MIRIAD packages) the maps for a number of clouds were made, including maps of two clouds (G19.4+6.3 and G27.0+6.3) observed in both D and C VLA configurations. A preliminary analysis of these latter clouds was published (Pidopryhora, Lockman, and Rupen 2009, in press). Previous hypotheses about formation of halo clouds due to superbubble envelope disintegration and of their unstable nature were given a strong experimental support. The findings of this analysis were also reported at several conferences.

Total HI column density map of halo cloud G19.4+6.3, one of the maps produced by combining observations of the VLA in D and C configurations and the GBT. The map shown is integrated over 48 spectral channels of 0.64 km/s width, rms noise is 8.7×10¹⁸ cm^-2, synthesized beam FWHM 32.5”x25.6“, which at the estimated distance of 8.0 kpc is equivalent to the linear resolution of 1.12 pc (the beam is shown in the bottom-left corner). This map was produced using the resulting AIPS VTESS model of the signal.

J. Yang's contribution (draft version)

Extragalactic Jets: GHz-Peaked-Spectrum Radio Sources

Collaborating with a group of young Chinese Astronomer: Tao An (Shanghai Astronomical Observatory), Lang Cur(Urumqi Observatory), Xinying Zhu (National Astronomical Observatories), and Longfei Hao (Yunnan Astronomical Observatory), Jun Yang organised the first EVN observations with the two New Chinese telescopes: the Kunming 40m and Miyun 50m radio telescope. With four Chinese telescopes and two European telescopes (On & Mc) on 5 August 2009, the team observed five GHz-Peaked-spectrum radio sources: J0132+5620, J1135+4258, J1324+4048, J1756+5748, and J2312+3847. The first EVN fringes to the Miyun 50m radio telescope were successfully detected during the observations with JIVE SFXC software correlator.

Compared with kiloparsec-scale radio sources, the five GPS sources show much more simple morphology. Usually, they only have a few compact jet features. Thus, they are top-priority objects to study jet kinematics of GPS sources and investigate the explanation of youth interpretation and against the radio-loud AGNs “frustrated” by the ambient medium. Fig1. shows the imaging results of the observations. All the images used naturally weighting. Since these sources have the high declination and u-v tracks look nearly circular, these images have nice synthesized beam. The image sensitivity reaches 0.2-0.5 mJy/beam, close to their theoretical values. By model fitting, we have obtained their position information. Compared with previous VLBA observations (BY020, PI: Jun YANG) in 2005, these jet components have already displayed a hint of increasing separation distance (e.g. ~0.1 mas for J0132+5620), i.e. proper motion. To extend the time baseline up to more than a decade, the team has recently reanalysized the existing VLBA archive data. A new EVN observations with Kunming and Miyun in 2011 is in preparation.

The EVN imaging results of five GPS radio sources. The contours start from 3-sigma noise level in each image and increase by a factor of 2. The beam is shown in the bottom-left corner.

Galactic Pulsars

Since 2009, Jun Yang (JIVE) has started to provide technical supports for Ding Chen (National Time Service Centre, Chinese Academy of Sciences, P.R. China) to perform VLBI pulsar astrometry. The first interesting pulsar for the group is the millisecond pulsar PSR J0218+4232. The pulsar is one of most shinning pulsars in X-rays and has been identified as a key beacon in building a GPS-like navigation system for deep space satellites carrying small X-ray detectors.

The first VLBI observation of PSR J0218+4232 was done by the group with the EVN on 4 Nov 2010. The data was correlated on the new SFXC software correlator at JIVE via binning correlation to align pulse signal and to remove off-pulse data.

The EVN detection of the millisecond pulsar PSR J0218+4232. The contours start from 3-sigma noise level in each image and increase by a factor of 2. The beam is shown in the bottom-left corner.

The above image shows the preliminary total intensity image from the EVN pipeline calibration. The millisecond pulsar has a peak brightness ~0.7 mJy/beam (SNR ~20). The image origin reflects the position inferred from timing observations, including timing-derived proper motion, and the offset shows the residual error (~50 mas). A proposal for follow-up multi-epoch EVN observations was submitted on 1 February 2011 to measure its proper motion and distance. VLBI astrometry will help to reduce the input covariances of the pulsar timing solutions. A possible application could be a network of X-ray pulsars with good astrometric and timing solutions serving as beacons for navigation of deep-space satellites.

Variable Radio Sources in the Galactic Plane

This is a new collaboration research between Chinese Academy of Sciences and JIVE. The involved people are Yongjun Chen, Zhiqiang Shen, and Weihua Wang from Shanghai Astronomical Observatory; Jinjin Li from Purple Mountain Observatory; Leonid Gurvits, Zsolt Paragi, and Jun Yang from JIVE.

Recently, Becker et al. (AJ, 2010) has presented a sample of intriguing variable radio source in the Galactic plane. These sources show compact morphology in the three epochs of VLA surveys with a flux density in the range of 2 – 50 mJy at 5 GHz and much higher variability (>50% on a time scale of a year) than most extragalactic objects. These variable radio sources have high population density toward the Galactic centre, and have no counterparts at other wavelengths. Thus, the sample is likely dominated by Galactic objects. However, none of the well-known Galactic sources can be well associated with them. To further unveil their nature, the team proposed a pilot e-VLBI experiment at 5 GHz to directly image a subsample of these sources. The exploratory observations aim to answer whether they are non-thermal radio emitters and whether follow-up observations can be initiated to measure their proper motion and to identify their Galactic origin.

The proposal has been approved and scheduled in the e-VLBI session of 15 Dec 2010. Through the EVN pipeline calibrations and manually Difmap imaging, the subsample has a detection rate of 8/17 sources under a sensitivity level of ~0.1 mJy/beam. Among them, the source G23.66-0.03 shows clearly fringes on the baseline of Ef-Wb. In the upcoming months, the group are going to manually calibrate the EVN data to improve the image quality and to reduce the WSRT data to answer whether these non-detections were caused by high resolution.

Galactic black hole candidates

Triggered by a call for VLBI observations of the new X-ray transient XTE J1752-223, posted by Catherine Brocksopp (Mullard Space Science Laboratory, University College London) in the Astronomers’ Telegram 2400, JIVE support scientists Zsolt Paragi and Jun Yang, in collaboration with the group, proposed and carried out four ToO (Target of Opportunity) VLBI experiments to image the Galactic black hole candidate during its radio-loud state in 2010.

The decelerating jet in the Glactic black hole candidate XTE J1752-223. The cross shows the location of the first detection of component A. The contours start from 3-sigma noise level in each image and increase by a factor of 1.4. The right-hand planel plots the fitting results of the proper motion data of component A using the modes with (solid curve) and without deceleration rate (dotted line).

The group observed the black hole transient at four epochs with the EVN and VLBA in February 2010. The above image shows the VLBI imaging results. A moving jet component (marked as A) was successfully detected. The proper motion analysis shows that it was significantly (0.34 mas per square day) decelerated by last epoch. The overall picture is consistent with a mildly relativistic jet interacting with the interstellar medium or with pre-existing material along the jet. Beside the decelerating ejecta, a newly-birth ejecta (marked as B) was also detected at the fourth epoch. The first VLBI detection of the direct jet deceleration has been published in MNRAS Letters (Yang et al. 2010, vol. 409. p. 64).

In view of later new outburst, the group performed an EVN experiment during the EVN session on 22 March 2010. Thanks to the high sensitivity of the EVN observations, the team successfully detected another two ejectas along the jet direction. One is on the approaching side; the other is on the receding side. Together with the ATCA (Australia Telescope Compact Array) radio light curves, the results will be discussed in a new paper.

After the transition from X-ray soft to hard states in XTE J1752-223, another two VLBA observations were performed to hunt for the re-appeared radio core. A compact component was detected in the first observations. The component has been identified as the radio core as it is within the error circle of the accurate optical position. The latest results have been submitted to MNRAS by James Miller-Jones (International Centre fro Radio Astronomy Research, Curtin University); P.G. Jonker, E. Ratti, and M.A.P. Torres (Netherlands Institute for Space Research); Catherine Brocksopp; Jun Yang; and N. Morrel (Las Campanas Observatory, Chile).

M. Mahmud's contribution (draft version)

Ultra-Steep Spectrum Radio Sources

Mehreen Mahmud was part of a new collaboration with Huub Rottgering (Leiden Observatory, Universiteit Leiden), Zsolt Paragi (JIVE), Hans-Rainer Kloeckner (University of Oxford, UK and MPIfR, Bonn) and George Miley (Leiden Observatory, Universiteit Leiden). The EVN project EP070A-B (PI: Huub Rottgering), was observed at L-Band in June 2010 in e-VLBI mode with the aim to elucidate the nature of a group of ultra-steep spectrum radio sources that were unresolved at the arcsecond scale.

From a sample of five target sources, the group found two detections (on milliarcsecond scales) of two of these sources, 0722+291 and 1512+470, that were a first step in understanding their radio morphologies and classification, with total flux densities of 2 mJy and 42 mJy, respectively. 0722+291 did not show clear evidence of structure on VLBI scales and was consistent with a point source model, albeit at significantly lower flux density than detected by the VLA. 1512+470 on the other hand was clearly resolved with a peak brightness to total flux density ratio of 1:4.

L-Band EVN total intensity maps of 0722+291 (left) and 1512+470 (right). The color bar on top shows the Flux distribution in millijansky. For 1512+470, the peak I contour is 9.9 mJy/Beam, the rms is 0.36 mJy and the restoring beam is 23.88 x 22.31 mas at a position angle of 70.07 degrees. For 0722+291, the peak I contour is 1.88 mJy/Beam, the rms is 0.085 mJy and the restoring beam is 32.15 x 26.43 mas at a position angle of -9.95 degrees.

Because the redshifts were not known, it was difficult to classify the sources, but the group was still able to make some tentative conclusions. Furthermore, their results showed that a moderate fraction of ultra-steep spectrum sources may have compact structures that can be studied only at the highest resolution with the VLBI technique. Also, the factor of 5 difference in the flux density on arcsecond and milliarcsecond scales of both sources indicated that there was extended emission associated with these sources. The group is thus planning e-MERLIN observations that will be crucial for these sources; to image these sources on intermediate scales, with better sensitivity than VLBI to the diffuse emission and provide more information on the classification of these sources.

Jet kinematics of the blazar S5 0716+714

Mahmud was also a member of the collaboration led by Elizaveta Rastorgueva (University of Turku, Tuorla Observatory, Finland) and included K.J.Wiik (University of Turku, Tuorla Observatory) , A.T.Bajkova (Central (Pulkovo) Astronomical Observatory, Russian Academy of Sciences, Russia), E.Valtaoja (University of Turku, Tuorla Observatory), L.O. Takalo (University of Turku, Tuorla Observatory) and Y. N. Vetukhnovskaya (Pushchino Radio Astronomy Observatory, Lebedev Physical Institute, Russia) which investigated large scale jet kinematics and the comparison of VLBI data imaging as applied to kinematic studies of AGN, based on observations of the blazar S5 0716+714 during the active state in 2004 (Rastorgueva et al. 2011, in press). The kinematic analysis of the large-scale (0-12 mas) jet of 0716+714 was based on six epochs of VLBA monitoring at 5 GHz.

The large-scale diffuse jet of 0716+714 was found to be mostly stationary with a large scatter of the component positions, that were systematically slower and fainter than those of the inner (< 1mas) jet components. Differences between the inner (0-1 mas) and outer (1-12 mas) regions of the jet in brightness and velocity of the components could be explained by the bending of the jet, which causes the angle between the jet direction and the line of sight to change from ~5 degrees to ~ 11 degrees.

Additionally, to obtain accurate kinematics of this source, the group applied two imaging techniques to the raw data: the conventional method, based on difference mapping, which uses the CLEAN deconvolution, and the generalized maximum entropy method (GMEM). The group found that while both methods yielded similar results, determination of the jet component positions by the conventional method was less precise and recommend GMEM in combination with the difference mapping technique for restoring the structure of compact AGN and other objects that have a diffuse structure with bright point-like features.

G. Cimo's contribution (draft version)

Interstellar Scintillation

Giuseppe Cimo' has continued his work on Intraday Variability of flat spectrum radio sources focusing on the scintillator J1819+3845. This quasar has shown extreme variability with flux density variations in the radio regime up to 600% in less than one hour. In case of intrinsic variability, the brightness temperature calculated via standard causality argument would exceed 10^21 K and therefore would violate the Inverse Compton limit (10^12 K). The variability of this source has been proven to be due to scattering in the interstellar medium by a number of different experiments. The extreme variability of J1819+3548 has been observed since its first observation in 1999 and it has continued for more than 8 years. In 2008, Giuseppe Cimo' and collaborators conducted EVN observations of J1819+3548 and they observed no signs of any variability (Cimo' et al. 2008, 2009).

The source-extrinsic explanation of the extreme variability of J1819+3845 required a close-by scattering screen (few parsecs) and it results in a brightness temperature of about 10^14 K, still exceeding the inverse Compton limit and implying a very compact structure. A possible explanation for the cessation of the fast variations in the source is that the scattering screen has moved away from our line of sight. On the other hand, the disappearance of the extreme scintillation of J1819+3845 could be due to variations in the source structure at μ-arcsecond scale.

Due to its high brightness temperature and the compact nature, J1819+3845 was expected to expand quenching the scintillation or at least changing its variability characteristics following its internal structural variations. However, the variations have been observed continuously along the years showing also a clear annual modulation due to relative motion of the Earth and the scattering screen. On one hand, the scintillation has prevented to image the milli-arcsecond structure of this puzzling object. On the other hand, it has been a powerful tool to study indirectly the source structure at μ-arcsecond scale. Macquart & de Bruyn (2007) have compared observations taken at identical epochs each year in order to disentangle the effects of source structural evolution and asymmetry in the scintillation pattern. Analysing the light curves and their power spectra, it was possible to indicate the evolution of the internal structure of J1829+3845. Comparing the scintillation characteristics in 2003, 2004 and 2006, Macquart & de Bruyn (2007) found an expanding double structure at μ-arcsecond scale.

Following the work of Macquart & de Bruyn (2007) and since the discovery of the lack of variability, Giuseppe Cimo' has been working with his collaborators on a model of the J1819+3845 in its current quiescent phase. New VLBI observations have also been requested to map the source without the limitation of the interstellar scattering that has effected all the previous attempts to map the innermost jet structures of J1819+3845 Furthermore, a retroactive interpretation of the past data has been investigated to use new scintillation-free data to describe the structure of the innermost jet structures of the source and their evolution in the course of the previous 10 years.