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Register a new userCharacterization of unresolved and unclassified sources detected in radio continuum surveys of the Galactic plane
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The continuum emission from 1 to 2 GHz of The HI/OH/Recombination line survey of the inner Milky Way (THOR) at $lesssim$18 resolution covers $sim 132$ square degrees of the Galactic plane and detects 10387 sources. Similarly, the first data release of the Global View of Star Formation in the Milky Way (GLOSTAR) surveys covers $sim 16$ square degrees of the Galactic plane from 4-8 GHz at 18 resolution and detects 1575 sources. However, a large fraction of the unresolved discrete sources detected in these radio continuum surveys of the Galactic plane remain unclassified. Here, we study the Euclidean-normalized differential source counts of unclassified and unresolved sources detected in these surveys and compare them with simulated extragalactic radio source populations as well as previously established source counts. We find that the differential source counts for THOR and GLOSTAR surveys are in excellent agreement with both simulation and previous observations. We also estimate the angular two-point correlation function of unclassified and unresolved sources detected in THOR survey. We find a higher clustering amplitude in comparison with the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey up to the angular separation of $5^{circ}$. The decrease in angular correlation with increasing flux cut and the excellent agreement of clustering pattern of sources above 1 mJy with high $z$ samples ($z >0.5$) of the FIRST survey indicates that these sources might be high $z$ extragalactic compact objects. The similar pattern of one-point and two-point statistics of unclassified and compact sources with extragalactic surveys and simulations confirms the extragalactic origin of these sources.
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We investigate how the imprint of Faraday rotation on radio spectra can be used to determine the geometry of radio sources and the strength and structure of the surrounding magnetic fields. We model spectra of Stokes Q and U for frequencies between 200 MHz and 10 GHz for Faraday screens with large-scale or small-scale magnetic fields external to the source. These sources can be uniform or 2D Gaussians on the sky with transverse linear gradients in rotation measure (RM), or cylinders or spheroids with an azimuthal magnetic field. At high frequencies the spectra of all these models can be approximated by the spectrum of a Gaussian source; this is independent of whether the magnetic field is large-scale or small-scale. A sinc spectrum in polarized flux density is not a unique signature of a volume where synchrotron emission and Faraday rotation are mixed. A turbulent Faraday screen with a large field coherence length produces a spectrum which is similar to the spectrum of a partial coverage model. At low and intermediate frequencies, such a Faraday screen produces a significantly higher polarized signal than the depolarization model by Burn, as shown by a random walk model of the polarization vectors. We calculate RM spectra for four frequency windows. Sources are strongly depolarized at low frequencies, but RMs can be determined accurately if the sensitivity of the observations is sufficient. Finally, we show that RM spectra can be used to differentiate between turbulent foreground models and partial coverage models.
We present a comprehensive multi-frequency catalogue of radio sources behind the Large Magellanic Cloud between 0.2 and 20 GHz, gathered from a combination of new and legacy radio continuum surveys. This catalogue covers an area of $sim$144~deg$^2$ at angular resolutions from 45 arcsec to $sim$3 arcmin. We find 6434 discrete radio sources in total, of which 3789 are detected at two or more radio frequencies. We estimate the median spectral index ($alpha$; where $S_{v}sim u^alpha$) of $alpha = -0.89 $ and mean of $-0.88 pm 0.48$ for 3636 sources detected exclusively at two frequencies (0.843 and 1.384 GHz) with similar resolution (FWHM $sim$40-45 arcsec). The large frequency range of the surveys makes it an effective tool to investigate Gigahertz Peak Spectrum (GPS), Compact Steep Spectrum (CSS) and Infrared Faint Radio sources populations within our sample. We find 10 GPS candidates with peak frequencies near 5 GHz, from which we estimate their linear size. 1866 sources from our catalogue are (CSS) candidates with $alpha <-0.8$. We found six candidates for High Frequency Peaker (HFP) sources, whose radio fluxes peak above 5 GHz and no sources with unconstrained peaks and $alpha~>0.5$. We found optical counterparts for 343 of the radio continuum sources, of which 128have a redshift measurement. Finally, we investigate the population of 123 Infrared Faint Radio Sources (IFRSs) found in this study.
We present high sensitivity ($sigma_P simeq 0.6,$mJy) polarimetric observations in seven bands, from $2.1$ to $38,$GHz, of a complete sample of $104$ compact extragalactic radio sources brighter than $200,$mJy at $20,$GHz. Polarization measurements in six bands, in the range $5.5-38,$GHz, for $53$ of these objects were reported by citet{Galluzzi2017}. We have added new measurements in the same six bands for another 51 sources and measurements at $2.1,$GHz for the full sample of $104$ sources. Also, the previous measurements at $18$, $24$, $33$ and $38,$GHz were re-calibrated using the updated model for the flux density absolute calibrator, PKS1934-638, not available for the earlier analysis. The observations, carried out with the Australia Telescope Compact Array (ATCA), achieved a $90%$ detection rate (at $5sigma$) in polarization. $89$ of our sources have a counterpart in the $72$ to $231,$MHz GLEAM survey citep{HurleyWalker2017}, providing an unparalleled spectral coverage of $2.7$ decades of frequency for these sources. While the total intensity data from $5.5$ to $38,$GHz could be interpreted in terms of single component emission, a joint analysis of more extended total intensity spectra presented here, and of the polarization spectra, reveals that over $90%$ of our sources show clear indications of at least two emission components. We interpret this as an evidence of recurrent activity. Our high sensitivity polarimetry has allowed a $5,sigma$ detection of the weak circular polarization for $sim 38%$ of the dataset, and a deeper estimate of $20,$GHz polarization source counts than has been possible so far.
Spectral energy distributions (SEDs) and radio continuum spectra are presented for a northern sample of 104 extragalactic radio sources, based on the Planck Early Release Compact Source Catalogue (ERCSC) and simultaneous multifrequency data. The nine Planck frequencies, from 30 to 857 GHz, are complemented by a set of simultaneous observations ranging from radio to gamma-rays. This is the first extensive frequency coverage in the radio and millimetre domains for an essentially complete sample of extragalactic radio sources, and it shows how the individual shocks, each in their own phase of development, shape the radio spectra as they move in the relativistic jet. The SEDs presented in this paper were fitted with second and third degree polynomials to estimate the frequencies of the synchrotron and inverse Compton (IC) peaks, and the spectral indices of low and high frequency radio data, including the Planck ERCSC data, were calculated. SED modelling methods are discussed, with an emphasis on proper, physical modelling of the synchrotron bump using multiplecomponents. Planck ERCSC data also suggest that the original accelerated electron energy spectrum could be much harder than commonly thought, with power-law index around 1.5 instead of the canonical 2.5. The implications of this are discussed for the acceleration mechanisms effective in blazar shock. Furthermore in many cases the Planck data indicate that gamma-ray emission must originate in the same shocks that produce the radio emission.
We present observations of a region of the Galactic plane taken during the Early Science Program of the Australian Square Kilometre Array Pathfinder (ASKAP). In this context, we observed the SCORPIO field at 912 MHz with an uncompleted array consisting of 15 commissioned antennas. The resulting map covers a square region of ~40 deg^2, centred on (l, b)=(343.5{deg}, 0.75{deg}), with a synthesized beam of 24x21 and a background rms noise of 150-200 {mu}Jy/beam, increasing to 500-600 {mu}Jy/beam close to the Galactic plane. A total of 3963 radio sources were detected and characterized in the field using the CAESAR source finder. We obtained differential source counts in agreement with previously published data after correction for source extraction and characterization uncertainties, estimated from simulated data. The ASKAP positional and flux density scale accuracy were also investigated through comparison with previous surveys (MGPS, NVSS) and additional observations of the SCORPIO field, carried out with ATCA at 2.1 GHz and 10 spatial resolution. These allowed us to obtain a measurement of the spectral index for a subset of the catalogued sources and an estimated fraction of (at least) 8% of resolved sources in the reported catalogue. We cross-matched our catalogued sources with different astronomical databases to search for possible counterparts, finding ~150 associations to known Galactic objects. Finally, we explored a multiparametric approach for classifying previously unreported Galactic sources based on their radio-infrared colors.
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