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Register a new userRevealing compact structures of interstellar plasma in the Galaxy with RadioAstron
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The aim of our work was to study the spatial structure of inhomogeneities of interstellar plasma in the directions of five pulsars: B0823+26, B0834+06, B1237+25, B1929+10, and B2016+28. Observations of these pulsars were made with RadioAstron space-ground radio interferometer at 324 MHz. We measured the angular size of the scattering disks to be in range between 0.63 and 3.2 mas. We determined the position of scattering screens on the line of sight. Independent estimates of the distances to the screens were made from the curvature of parabolic arcs revealed in the secondary spectra of four pulsars. The model of uniform distribution of inhomogeneities on the line of sight is not suitable. According to the results, we came to the conclusion that scattering is mainly produced by compact plasma layers and the uniform model of inhomogeneties distribution on the line of sight in not applicable.
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RadioAstron space-ground VLBI observations of the pulsar B0950+08, conducted with the 10-m space radio telescope in conjunction with the Arecibo 300-m telescope and Westerbork Synthesis Radio Telescope at a frequency of 324 MHz, were analyzed in order to investigate plasma inhomogeneities in the direction of this nearby pulsar. The observations were conducted at a spacecraft distance of 330,000 km, resulting in a projected baseline of 220,000 km, providing the greatest angular resolution ever achieved at meter wavelengths. Our analysis is based on fundamental behavior of structure and coherence functions. We find that the pulsar shows scintillation on two frequency scales, both much less than the observing frequency; but modulation is less than 100%. We infer that the scattering is weak, but a refracting wedge disperses the scintillation pattern. The refraction angle of this cosmic prism is measured as theta_0=1.1 - 4.4 mas, with the refraction direction being approximately perpendicular to the observer velocity. We show that the observed parameters of scintillation effects indicate that two plasma layers lie along the line of sight to the pulsar, at distances of 4.4 - 16.4 pc and 26 - 170 pc, and traveling in different directions relative to the line of sight. Spectra of turbulence for the two layers are found to follow a power law with the indices gamma_1 = gamma_2 = 3.00 +/- 0.08, significantly different from the index expected for a Kolmogorov spectrum of turbulence, gamma=11/3.
The RadioAstron space radio telescope provides a unique opportunity to study the extreme brightness temperatures ($mathrm{T_B }$) in AGNs with unprecedented long baselines of up to 28 Earth diameters. Since interstellar scintillation (ISS) may affect the visibilities observed with space VLBI (sVLBI), a complementary ground based flux density monitoring of the RadioAstron targets, which is performed near in time to the VLBI observation, could be beneficial. The combination/comparison with the sVLBI data can help to unravel the relative influence of source intrinsic and ISS induced effects, which in the end may alter the conclusions on the $mathrm{T_B }$ measurements from sVLBI. Since 2013, a dedicated monitoring program has been ongoing to observe the ISS of RadioAstron AGN targets with a number of radio telescopes. Here we briefly introduce the program and present results from the statistical analysis of the Effelsberg monitoring data. We discuss the possible effects of ISS on $mathrm{T_B }$ measurements for the RadioAstron target B0529+483 as a case study.
The origin of the Galactic center diffuse X-ray emission (GCDX) is still under intense investigation. In particular, the interpretation of the hot (kT ~ 7 keV) component of the GCDX, characterised by the strong Fe 6.7 keV line emission, has been contentious. If the hot component originates from a truly diffuse interstellar plasma, not a collection of unresolved point sources, such plasma cannot be gravitationally bound, and its regeneration would require a huge amount of energy. Here we show that the spatial distribution of the GCDX does NOT correlate with the number density distribution of an old stellar population traced by near-infrared light, strongly suggesting a significant contribution of the diffuse interstellar plasma. Contributions of the old stellar population to the GCDX are implied to be about 50 % and 20 % in the Nuclear stellar disk and Nuclear star cluster, respectively. For the Nuclear stellar disk, a scale height of 0.32 +- 0.02 deg is obtained for the first time from the stellar number density profiles. We also show the results of the extended near-infrared polarimetric observations in the central 3 deg * 2 deg region of our Galaxy, and confirm that the GCDX region is permeated by a large scale, toroidal magnetic field as previously claimed. Together with observed magnetic field strengths close to energy equipartition, the hot plasma could be magnetically confined, reducing the amount of energy required to sustain it.
We present deep polarimetric observations at 154 MHz with the Murchison Widefield Array (MWA), covering 625 deg^2 centered on RA=0 h, Dec=-27 deg. The sensitivity available in our deep observations allows an in-band, frequency-dependent analysis of polarized structure for the first time at long wavelengths. Our analysis suggests that the polarized structures are dominated by intrinsic emission but may also have a foreground Faraday screen component. At these wavelengths, the compactness of the MWA baseline distribution provides excellent snapshot sensitivity to large-scale structure. The observations are sensitive to diffuse polarized emission at ~54 resolution with a sensitivity of 5.9 mJy beam^-1 and compact polarized sources at ~2.4 resolution with a sensitivity of 2.3 mJy beam^-1 for a subset (400 deg^2) of this field. The sensitivity allows the effect of ionospheric Faraday rotation to be spatially and temporally measured directly from the diffuse polarized background. Our observations reveal large-scale structures (~1 deg - 8 deg in extent) in linear polarization clearly detectable in ~2 minute snapshots, which would remain undetectable by interferometers with minimum baseline lengths >110 m at 154 MHz. The brightness temperature of these structures is on average 4 K in polarized intensity, peaking at 11 K. Rotation measure synthesis reveals that the structures have Faraday depths ranging from -2 rad m^-2 to 10 rad m^-2 with a large fraction peaking at ~+1 rad m^-2. We estimate a distance of 51+/-20 pc to the polarized emission based on measurements of the in-field pulsar J2330-2005. We detect four extragalactic linearly polarized point sources within the field in our compact source survey. Based on the known polarized source population at 1.4 GHz and non-detections at 154 MHz, we estimate an upper limit on the depolarization ratio of 0.08 from 1.4 GHz to 154 MHz.
To understand the relation between the small obscuring torus and dusty structures at larger scales (5-10 pc) in NGC 1068, we use ESOs Mid-Infrared Interferometer (MIDI) with the 1.8 m Auxiliary Telescopes to achieve the necessary spatial resolution (~ 20-100 millarcsec). We use the chromatic phases in the data to improve the spatial fidelity of the analysis. We present interferometric data for NGC 1068 obtained in 2007 and 2012. We find no evidence of source variability. Many (u,v) points show non-zero chromatic phases indicating significant asymmetries. Gaussian model fitting of the correlated fluxes and chromatic phases provides a 3-component best fit with estimates of sizes, temperatures and positions of the components. A large, warm, off-center component is required at a distance approximately 90 mas to the north-west at a PA ~ -18 deg. The dust at 5-10 pc in the polar region contributes 4 times more to the mid-infrared flux at 12 um than the dust located at the center. This dust may represent the inner wall of a dusty cone. If similar regions are heated by the direct radiation from the nucleus, then they will contribute substantially to the classification of many Seyfert galaxies as Type 2. Such a region is also consistent in other Seyfert galaxies (the Circinus galaxy, NGC 3783 and NGC 424).
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