F. Mantovani

Deceleration in the Expansion of SN 1993J

A rarity among supernova, SN 1993J in M81 can be studied with high spatial resolution. Its radio power and distance permit VLBI observations to monitor the expansion of its angular structure. This radio structure was previously revealed to be shell-like and to be undergoing a self-similar expansion at a constant rate. From VLBI observations at wavelengths of 3.6 and 6 cm in the period 6-42 months after explosion, we have discovered that the expansion is decelerating. Our measurement of this deceleration yields estimates of the density profiles of the supernova ejecta and circumstellar material in standard supernova explosion models.

A Decade of SN 1993J: Discovery of Radio Wavelength Effects in the Expansion Rate

We studied the growth of the shell-like radio structure of supernova SN 1993J in M 81 from September 1993 to October 2003 with very-long-baseline interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18 cm. We developed a method to accurately determine the outer radius (R) of any circularly symmetric compact radio structure such as SN 1993J. The source structure of SN 1993J remains circularly symmetric (with deviations from circularity under 2%) over almost 4000 days. We characterize the decelerated expansion of SN 1993J until approximately day 1500 after explosion with an expansion parameter m = 0.845 ± 0.005 (R ∝ t m ). However, from that day onwards the expansion differs when observed at 6 and 18 cm. Indeed, at 18 cm, the expansion can be well characterized by the same m as before day 1500, while at 6 cm the expansion appears more decelerated, and is characterized by another expansion parameter, m6 = 0.788 ± 0.015. Therefore, since about day 1500 onwards, the radio source size has been progressively smaller at 6 cm than at 18 cm. These findings differ significantly from those of other authors in the details of the expansion. In our interpretation, the supernova expands with a single expansion parameter, m = 0.845 ± 0.005, and the 6 cm results beyond day 1500 are caused by physical effects, perhaps also coupled to instrumental limitations. Two physical effects may be involved: (a) a changing opacity of the ejecta to the 6 cm radiation; and (b) a radial decrease of the magnetic field in the emitting region. We also found that at 6 cm about 80% of the radio emission from the backside of the shell behind the ejecta is absorbed (our average estimate, since we cannot determine any possible evolution of the opacity), and the width of the radio shell is (31 ± 2)% of the outer radius. The shell width at 18 cm depends on the degree of assumed absorption. For 80% absorption, the width is (33.5 ± 1.7)%, and for 100% absorption, it is (37.8 ± 1.3)%. A comparison of our VLBI results with optical spectral line velocities shows that the deceleration is more pronounced in the radio than in the optical. This difference might be due to a progressive penetration of ejecta instabilities into the shocked circumstellar medium, as also suggested by other authors.

Expansion of SN 1993J

A sequence of images from very long baseline interferometry shows that the young radio supernova SN 1993J is expanding with circular symmetry. However, the circularly symmetric images show emission asymmetries. A scenario in which freely expanding supernova ejecta shock mostly isotropic circumstellar material is strongly favored. The sequence of images constitutes the first “movie” of a radio supernova.

Effelsberg 100-m polarimetric observations of a sample of compact steep-spectrum sources

Aims. We completed observations with the Effelsberg 100-m radio telescope to measure the polarised emission from a complete sample of Compact Steep-Spectrum sources and improve our understanding of the physical conditions inside and around regions of radio emission embedded in dense interstellar environments. Methods. We observed the sources at four different frequencies, namely 2.64 GHz, 4.85GHz, 8.35 GHz, and 10.45 GHz, making use of the polarimeters available at the Effelsberg telescope. We complemented these measurements with polarisation parameters at 1.4 GHz derived from the NRAO VLA Sky Survey. Previous single dish measurements were taken from the catalogue of Tabara and Inoue. Results. The depolarisation index DP was computed for four pairs of frequencies. A drop in the fractional polarisation appeared in the radio emission when observing at frequencies below �2GHz. Rotation measures were derived for about 25% of the sources in the sample. The values, in the source rest frame, range from about –20 rad m −2 found for 3C138 to 3900 rad m −2 in 3C119. In all cases, the � 2 law is closely followed. Conclusions. The presence of a foreground screen as predicted by the Tribble model or with “partial coverage” as defined by ourselves can explain the polarimetric behaviour of the CSS sources detected in polarisation by the present observations. Indication of repolarisation at lower frequencies was found for some sources. A case of possible variability in the fractional polarisation is also suggested. The most unexpected result was found for the distribution of the fractional polarisations versus the linear sizes of the sources. Our results appear to disagree with the findings of Cotton and collaborators and Fanti and collaborators for the B3-VLA sample of CSS sources, the so-called “Cotton effect”, i.e., a strong drop in polarised intensity for the most compact sources below a given frequency. This apparent contradiction may, however, be caused by the large contamination of the sample by quasars with respect to the B3-VLA.

Radio monitoring of a sample of X-and

In this paper we present the results of a 4-year (1996{1999) radio flux density monitoring program for as ample of X{ and{ray loud blazars. Our program started in January 1996 and was carried out on a monthly basis at the frequencies of 5 GHz and 8.4 GHz with the 32-m antennas located at Medicina (Bologna, Italy) and Noto (Siracusa, Italy). 22 GHz data collected at Medicina from January 1996 to June 1997 will also be presented. The sample of selected sources comprises most radio-loud blazars with 10 characterised by emission in the X{ and {ray regimes, and target sources for the BeppoSAX X{ray mission. All sources in the sample, except J1653+397 (MKN 501), are variable during the four years of our monitoring program. We classied the type of variability in each source by means of a structure-function analysis. We also computed the spectral index 8:4 5 for all epochs with nearly simultaneous observations at these two frequencies, and found that 8:4 5 starts flattening at the very beginning of a radio flare, or flux density increase.

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From a series of simultaneous 8.4 and 2.3 GHz VLBI observations of the quasar 4C 39.25 phase referenced to the radio source 0920+390, carried out in 1990-1992, we have measured the proper motion of component b in 4C 39.25: mu(sub alpha) = 90 +/- 43 (mu)as/yr, mu(sub beta) = 7 +/- 68 (mu)as/yr, where the quoted uncertainties account for the contribution of the statistical standard deviation and the errors assumed for the parameters related to the geometry of the interferometric array, the atmosphere, and the source structure. This proper motion is consistent with earlier interpretations of VLBI hybrid mapping results, which showed an internal motion of this component with respect to other structural components. Our differential astrometry analyses show component b to be the one in motion. Our results thus further constrain models of this quasar.

-ray loud blazars

We present the status of the Sardinia Radio Telescope (SRT) project, a new general purpose, fully steerable 64 m diameter parabolic radiotelescope capable to operate with high efficiency in the 0.3-116 GHz frequency range. The instrument is the result of a scientific and technical collaboration among three Structures of the Italian National Institute for Astrophysics (INAF): the Institute of Radio Astronomy of Bologna, the Cagliari Astronomy Observatory (in Sardinia,) and the Arcetri Astrophysical Observatory in Florence. Funding agencies are the Italian Ministry of Education and Scientific Research, the Sardinia Regional Government, and the Italian Space Agency (ASI,) that has recently rejoined the project. The telescope site is about 35 km North of Cagliari. The radio telescope has a shaped Gregorian optical configuration with a 7.9 m diameter secondary mirror and supplementary Beam-WaveGuide (BWG) mirrors. With four possible focal positions (primary, Gregorian, and two BWGs), SRT will be able to allocate up to 20 remotely controllable receivers. One of the most advanced technical features of the SRT is the active surface: the primary mirror will be composed by 1008 panels supported by electromechanical actuators digitally controlled to compensate for gravitational deformations. With the completion of the foundation on spring 2006 the SRT project entered its final construction phase. This paper reports on the latest advances on the SRT project.

Proper Motion of Components in 4C 39.25

We report here on 5 GHz global very-long-baseline interferometry (VLBI) observations of SN1986J, 16 yr after its explosion. We obtained a high-resolution image of the supernova, which shows a distorted shell of radio emission, indicative of a deformation of the shock front. The angular size of the shell is � 4.7mas, corresponding to a linear size of � 6.8 × 10 17 cm for a distance of 9.6 Mpc to NGC 891. The average speed of the shell has decreased from �7400 km s −1 in 1988.74 down to about 6300kms −1 in 1999.14, indicative of a mild deceleration in the expansion of SN1986J. Assuming a standard density profile for the progenitor wind (�cs / r −s ,s = 2), the swept-up mass by the shock front is �2.2M⊙. This large swept-up mass, coupled with the mild deceleration suffered by the supernova, suggests that the mass of the hydrogen-rich envelope ejected at explosion was > 12 M⊙. Thus, the supernova progenitor must have kept intact most of its hydrogen-rich envelope by the time of explosion, which favours a single, massive star progenitor scenario. We find a flux density for SN1986J of �7.2 mJy at the observing frequency of 5 GHz, which results in a radio luminosity of � 1.4 × 10 37 ergs −1 for the frequency range 10 7 –10 10 Hz (� = 0.69;S� / � � ). We detect four bright knots that delineate the shell structure, and an absolute minimum of emission, which we tentatively identify with the centre of the supernova explosion. If this is the case, SN1986J has then suffered an asymmetric expansion. We suggest that this asymmetry is due to the collision of the supernova ejecta with an anisotropic, clumpy (or filamentary) medium.

Status of the Sardinia Radio Telescope project

We report multiepoch VLBI observations of the source PKS 1741-038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the sources diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the sources diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741-038. We also see no evidence for ESE-induced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in flux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741-038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order 1 AU, the level of scattering within an ESE lens may be a factor of 107 larger than that in the ambient medium. A filamentary structure could reduce the difference between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a comparable lens passing in front of a pulsar. The pulse broadening would be no more than 1.1 μs, consistent with the lack of pulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of sight toward PKS 1741-038 is consistent with a turbulent origin for the structures responsible for ESEs. The source PKS 1741-038 lies near the radio Loop I and is seen through a local minimum in 100 μm emission.

A distorted radio shell in the young supernova SN 1986J

N are systematically high with respect to this prior expression. (3) Our estimates for the speed at which the irregularities move with respect to the antennas, and in the radial direction from the Sun, are in the range of 190{340 km s 1 for heliocentric distances of 16{26 R. These values are in good agreement with observational estimates of the solar wind flow speed in this part of space based on coronagraph observations. The speeds are lower than the sum of the estimated solar wind flow speed and the local Alfv en speed. The potential signicance of this result is discussed.