MHz Continuum Research Articles

The LOFAR Two-metre Sky Survey

In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey we present 120–168 MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44°30′ and 1h00m +28°00′ and spanning 4178 and 1457 square degrees respectively. The images were derived from 3451 h (7.6 PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4 396 228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6″ resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144 MHz have: a median rms sensitivity of 83 μJy beam−1; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2″; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8 mJy beam−1. By creating three 16 MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of > ± 0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20″ resolution 120–168 MHz continuum images have a median rms sensitivity of 95 μJy beam−1, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480 × 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8 mJy beam−1 at 4′ and 2.2 mJy beam−1 at 20″; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data to facilitate the thorough scientific exploitation of this unique dataset.

Probing the Physical Conditions and Star Formation Processes in the Galactic H II Region S305

Abstract We present multiscale and multiwavelength observations of the Galactic H ii region S305, which is excited by massive O8.5V and O9.5V stars. Infrared images reveal an extended sphere-like shell (extension ∼7.5 pc; at T d = 17.5–27 K) enclosing the S305 H ii region (size ∼5.5 pc; age ∼1.7 Myr). The extended structure observed in the Herschel temperature map indicates that the molecular environment of S305 is heated by the massive O-type stars. Regularly spaced molecular condensations and dust clumps are investigated toward the edges of the infrared shell, where the polycyclic aromatic hydrocarbon (PAH) and H2 emission is also observed. The molecular line data show a signature of an expanding shell of molecular gas in S305. Giant Metrewave Radio Telescope 610 and 1280 MHz continuum maps reveal overdensities of the ionized emission distributed around two O-type stars, which are surrounded by the horseshoe envelope (extension ∼2.3 pc). A molecular gas deficient region/cavity is identified toward the center of the horseshoe envelope, which is well traced with PAH, H2, molecular, and dust emission. The edges of the infrared shell are found to be located in the front of the horseshoe envelope. All these outcomes provide the observational evidence of the feedback of O-type stars in S305. Moreover, nonthermal radio emission is detected in S305 with an average spectral index α ∼ −0.45. The variations in α, ranging from −1.1 to 1.3, are explained due to soft synchrotron emission and either optically thicker thermal emission at high frequencies or a suppression of the low-frequency emission by the Razin–Tsytovich effect.

Probing Star Formation in Galaxies at z ≈ 1 via a Giant Metrewave Radio Telescope Stacking Analysis

Abstract We report deep Giant Metrewave Radio Telescope (GMRT) 610 MHz continuum imaging of four subfields of the DEEP2 Galaxy Redshift Survey. We stacked the radio emission in the GMRT images from a near-complete (absolute blue magnitude M B ≤ −21) sample of 3698 blue star-forming galaxies with redshifts 0.7 ≲ z ≲ 1.45 to detect (at ≈17σ significance) the median rest-frame 1.4 GHz radio continuum emission of the sample galaxies. The stacked emission is unresolved, with a rest-frame 1.4 GHz luminosity of L 1.4GHz = (4.13 ± 0.24) × 1022 W Hz−1. We used the local relation between total star formation rate (SFR) and 1.4 GHz luminosity to infer a median total SFR of (24.4 ± 1.4) M ⊙ yr−1 for blue star-forming galaxies with M B ≤ −21 at 0.7 ≲ z ≲ 1.45. We detect the main-sequence relation between SFR and stellar mass, M ⋆, obtaining SFR = (13.4 ± 1.8) × [(M ⋆/1010 M ⊙)]0.73±0.09 M ⊙ yr−1; the power-law index shows no change over z ≈ 0.7–1.45. We find that the nebular line emission suffers less extinction than the stellar continuum, contrary to the situation in the local universe; the ratio of nebular extinction to stellar extinction increases with decreasing redshift. We obtain an upper limit of 0.87 Gyr to the atomic gas depletion time of a subsample of DEEP2 galaxies at z ≈ 1.3; neutral atomic gas thus appears to be a transient phase in high-z star-forming galaxies.

DISTANCE AND EVOLUTIONARY STATE OF THE SUPERNOVA REMNANT 3C 397 (G41.1-0.3)

ABSTRACT In this paper, we analyze 1420 MHz continuum and H i observations of the supernova remnant (SNR) 3C 397 (G41.1-0.3). The H i absorption spectra show clear absorption up to the tangent point velocity and also the absence of absorption at 50–60 km s−1. This yields lower and upper limits to the distances of 6.3 ± 0.1 and 9.7 ± 0.3 kpc, which are better and more robust than previous estimates. We apply generalized SNR models to 3C 397, including the ejecta-dominated phase and the transition-to-Sedov phase. Using emission measures from the X-ray and mean gas density from the infrared, we show that the hard X-ray component has the dominant filling factor and the soft X-ray component has a very small filling factor. The models are required to be consistent with 3C 397's measured properties, including the observed shock temperatures and shock radii. Consistent models are found if 3C 397 has a distance in the range of ≃8–9.7 kpc. For an 8 kpc distance, the estimated age is ≃1350 years and the explosion energy is 1.0 × 1051 erg, while for 9.7 kpc, the the most probable age is ≃1750 years and the energy 1.5 × 1051 erg.

The search for faint radio supernova remnants in the outer Galaxy: five new discoveries

High resolution and sensitivity large-scale radio surveys of the Milky Way are critical in the discovery of very low surface brightness supernova remnants (SNRs), which may constitute a significant portion of the Galactic SNRs still unaccounted for (ostensibly the Missing SNR problem). The overall purpose here is to present the results of a systematic, deep data-mining of the Canadian Galactic Plane Survey (CGPS) for faint, extended non-thermal and polarized emission structures that are likely the shells of uncatalogued supernova remnants. We examine 5$\times$5 degree mosaics from the entire 1420 MHz continuum and polarization dataset of the CGPS after removing unresolved point sources and subsequently smoothing them. Newly revealed extended emission objects are compared to similarly-prepared CGPS 408 MHz continuum mosaics, as well as to source-removed mosaics from various existing radio surveys at 4.8 GHz, 2.7 GHz, and 327 MHz, to identify candidates with non-thermal emission characteristics. We integrate flux densities at each frequency to characterise the radio spectra behaviour of these candidates. We further look for mid- and high-frequency (1420 MHz, 4.8 GHz) ordered polarized emission from the limb brightened shell-like continuum features that the candidates sport. Finally, we use IR and optical maps to provide additional backing evidence. Here we present evidence that five new objects, identified as filling all or some of the criteria above, are strong candidates for new SNRs. These five are designated by their Galactic coordinate names G108.5+11.0, G128.5+2.6, G149.5+3.2, G150.8+3.8, and G160.1$-$1.1. These discoveries represent a significant increase in the number of SNRs known in the outer Galaxy second quadrant of longitude (90$^{\circ} \leq \ell \leq $180$^{\circ}$).

Investigation of Radio Frequency Interference (RFI) Profile and Determination of Potential Astronomical Radio Sources

In this article, we have recognized a Radio Frequency Interference (RFI) sources that can potentially affect for radio astronomical observation. The main objective of this surveying is to test and qualify the potential of radio astronomical sources that can be observed in Malaysia generally. Analysis process focuses on the high sources that contribute the pollution and the significant region that can be considered for astronomical purpose beginning 1-2000 MHz. It was found that 13 individual sources contribute as a noise and mostly are telecommunication and radio navigation applications. We then compared of the RFI profiles based on three different periods in order to observe the variety of the signals. The main regions that still excellent to do an observation are: 13.36 -13.41 MHz (solar), (25.55-25.67) MHz (Jupiter) and (37.50-38.25) MHz (Continuum) respectively. This work is also an initiative of the International Space Weather Initiative (ISWI) project where Malaysia is one of the countries that involve in e- CALLISTO (Compound Astronomical Low Cost Low Frequency Transportable Observatory) network project. Some suggestions are recommended in order to improve the quality of the radio frequency profile.

Investigation of Radio Frequency Interference (RFI) Profile and Determination of Potential Astronomical Radio Sources

In this article, we have recognized a Radio Frequency Interference (RFI) sources that can potentially affect for radio astronomical observation. The main objective of this surveying is to test and qualify the potential of radio astronomical sources that can be observed in Malaysia generally. Analysis process focuses on the high sources that contribute the pollution and the significant region that can be considered for astronomical purpose beginning 1-2000 MHz. It was found that 13 individual sources contribute as a noise and mostly are telecommunication and radio navigation applications. We then compared of the RFI profiles based on three different periods in order to observe the variety of the signals. The main regions that still excellent to do an observation are: 13.36 -13.41 MHz (solar), (25.55-25.67) MHz (Jupiter) and (37.50-38.25) MHz (Continuum) respectively. This work is also an initiative of the International Space Weather Initiative (ISWI) project where Malaysia is one of the countries that involve in e- CALLISTO (Compound Astronomical Low Cost Low Frequency Transportable Observatory) network project. Some suggestions are recommended in order to improve the quality of the radio frequency profile.

74 MHz Nonthermal Emission from Molecular Clouds: Evidence for a Cosmic Ray Dominated Region at the Galactic Center

We present 74 MHz radio continuum observations of the Galactic center region. These measurements show nonthermal radio emission arising from molecular clouds that is unaffected by free–free absorption along the line of sight. We focus on one cloud, G0.13-0.13, representative of the population of molecular clouds that are spatially correlated with steep spectrum (α(327MHz)(74MHz) = 1.3 ± 0.3) nonthermal emission from the Galactic center region. This cloud lies adjacent to the nonthermal radio filaments of the Arc near l 0.2° and is a strong source of 74 MHz continuum, SiO (2-1), and Fe I Kα 6.4 keV line emission. This three-way correlation provides the most compelling evidence yet that relativistic electrons, here traced by 74 MHz emission, are physically associated with the G0.13-0.13 molecular cloud and that low-energy cosmic ray electrons are responsible for the Fe I Kα line emission. The high cosmic ray ionization rate 10(–1)3 s(–1) H(–1) is responsible for heating the molecular gas to high temperatures and allows the disturbed gas to maintain a high-velocity dispersion. Large velocity gradient (LVG) modeling of multitransition SiO observations of this cloud implies H2 densities 10(4–5) cm(–3) and high temperatures. The lower limit to the temperature of G0.13-0.13 is 100 K, whereas the upper limit is as high as 1000 K. Lastly, we used a time-dependent chemical model in which cosmic rays drive the chemistry of the gas to investigate for molecular line diagnostics of cosmic ray heating. When the cloud reaches chemical equilibrium, the abundance ratios of HCN/HNC and N2H+/HCO+ are consistent with measured values. In addition, significant abundance of SiO is predicted in the cosmic ray dominated region of the Galactic center. We discuss different possibilities to account for the origin of widespread SiO emission detected from Galactic center molecular clouds.

A search for steep spectrum radio relics and halos with the GMRT

Context: Diffuse radio emission, in the form of radio halos and relics, traces regions in clusters with shocks or turbulence, probably produced by cluster mergers. Some models of diffuse radio emission in clusters indicate that virtually all clusters should contain diffuse radio sources with a steep spectrum. External accretion shocks associated with filamentary structures of galaxies could also accelerate electrons to relativistic energies and hence produce diffuse synchrotron emitting regions. Here we report on Giant Metrewave Radio Telescope (GMRT) observations of a sample of steep spectrum sources from the 74 MHz VLSS survey. These sources are diffuse and not associated with nearby galaxies. Aims: The main aim of the observations is to search for diffuse radio emission associated with galaxy clusters or the cosmic web. Methods: We carried out GMRT 610 MHz continuum observations of unidentified diffuse steep spectrum sources. Results: We have constructed a sample of diffuse steep spectrum sources, selected from the 74 MHz VLSS survey. We identified eight diffuse radio sources probably all located in clusters. We found five radio relics, one cluster with a giant radio halo and a radio relic, and one radio mini-halo. By complementing our observations with measurements from the literature we find correlations between the physical size of relics and the spectral index, in the sense that smaller relics have steeper spectra. Furthermore, larger relics are mostly located in the outskirts of clusters while smaller relics are located closer to the cluster center.

Radio observations of ZwCl 2341.1+0000: a double radio relic cluster

Context: Hierarchal models of large scale structure (LSS) formation predict that galaxy clusters grow via gravitational infall and mergers of (smaller) mass concentrations, such as clusters and galaxy groups. Diffuse radio emission, in the form of radio halos and relics, is found in clusters undergoing a merger, indicating that shocks or turbulence associated with the merger are capable of accelerating electrons to highly relativistic energies. Here we report on radio observations of ZwCl 2341.1+0000, a complex merging structure of galaxies located at z=0.27, using Giant Metrewave Radio Telescope (GMRT) observations. Aims: The main aim of the observations is to study the nature of the diffuse radio emission in the galaxy cluster ZwCl 2341.1+0000. Methods: We have carried out GMRT 610, 241, and 157 MHz continuum observations of ZwCl 2341.1+0000. The radio observations are combined with X-ray and optical data of the cluster. Results: The GMRT observations show the presence of a double peripheral radio relic in the cluster ZwCl 2341.1+0000. The spectral index is -0.49 \pm 0.18 for the northern relic and -0.76 \pm 0.17 for the southern relic respectively. We have derived values of 0.48-0.93 microGauss for the equipartition magnetic field strength. The relics are probably associated with an outwards traveling merger shock waves.

Diffuse radio emission in the merging cluster MACS J0717.5+3745: the discovery of the most powerful radio halo

<i>Context. <i/>Hierarchical models of structure formation predict that galaxy clusters grow via mergers of smaller clusters and galaxy groups, as well as through continuous accretion of gas. MACS J0717.5+3745 is an X-ray luminous and complex merging cluster, located at a redshift of 0.5548. The cluster is suspected to host a bright radio relic, but up until now no detailed radio observations have been reported. Here we present Giant Metrewave Radio Telescope (GMRT) radio observations at 610 MHz of this cluster. <i>Aims. <i/>The main aim of the observations is to study the diffuse radio emission within the galaxy cluster MACS J0717.5+3745 related to the ongoing merger. <i>Methods. <i/>We have carried out GMRT 610 MHz continuum observations of MACS J0717.5+3745. These are complemented by Very Large Array (VLA) archival observations at 1.4, 4.9 and 8.5 GHz. <i>Results. <i/>We have discovered a radio halo in the cluster MACS J0717.5+3745 with a size of about 1.2 Mpc. The monochromatic radio power at 1400 MHz (<i>P<i/><sub>1.4<sub/>) is 5 10<sup>25<sup/> W Hz<sup>-1<sup/>, which makes it the most powerful radio halo known to date. A 700 kpc radio structure, which we classify as a radio relic, is located in between the merging substructures of the system. The global spectral index of radio emission within the cluster is found to be -1.24 <i>±<i/> 0.05 between 4.9 GHz and 610 MHz. We derive a value of 5.8 <i>μ<i/>G for the equipartition magnetic field strength at the location of the radio halo. The location of the relic roughly coincides with regions of the intra-cluster medium (ICM) that have a significant enhancement in temperature as shown by Chandra. The major axis of the relic is also roughly perpendicular to the merger axis. This shows that the relic might be the result of a merger-related shock wave, where particles are accelerated via the diffuse shock acceleration (DSA) mechanism. Alternatively, the relic might trace an accretion shock of a large-scale galaxy filament to the south-east.

DISTANCE DETERMINATION TO THE CRAB-LIKE PULSAR WIND NEBULA G54.1+0.3 AND THE SEARCH FOR ITS SUPERNOVA REMNANT SHELL

We discovered a large-scale shell G53.9+0.2 around the Crab-like pulsar wind nebula (PWN) G54.1+0.3 with 1420 MHz continuum Very Large Array observations. This is confirmed by a new infrared (IR) image at 8 μm from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire project, which reveals an intriguing IR shell just surrounding the large radio shell. We analyze the 21 cm H I absorption spectra and 13CO emission spectra toward PWN G54.1+0.3 and bright sources on both radio and IR shells. Continuous H I absorption up to the tangent point and absence of negative H I absorption features imply that PWN G54.1+0.3 has a distance beyond the tangent point but within the solar circle, i.e., 4.5-9 kpc. G54.1+0.3 is likely at a distance of 6.2 kpc due to the morphological association of the PWN with a CO molecular cloud at a velocity of 53 km s–1, as revealed by high-resolution 13CO images. Based on the H I absorption spectrum and recombination line velocity (40 km s–1) of the bright H II region G54.09–0.06, which is on the IR shell, the IR shell is likely located at a distance of 7.3 kpc, which is also the distance of the associated large-scale radio shell. At this distance, the radio shell has a radius of ~30 pc. The radio shell may be thermal and lack IR emission due to dust destruction, or it may be nonthermal and part of a newly found old supernova remnant. In either case, it is located at a distance different from PWN G54.1+0.3.

Radio spectrum and distance of the SNR HB9

New images are presented of the supernova remnant (SNR) HB9 based on 408 MHz and 1420 MHz continuum emission and HI-line emission data from the Canadian Galactic Plane Survey (CGPS). Two different T-T plot methods and new integrated flux densities give spectral index ($S_{\nu}$ $\propto$ ν $^{-\alpha}$) for the whole of HB9 of 0.48 ± 0.03; 0.49; and 0.47 ± 0.06, respectively. These values are lower than the previous spectral index estimated for HB9 ( α = 0.61). The change is mainly due to improved 1420 MHz data. No difference in spectral index is detected between strong and weak filaments. A new result is that the spectral index for interior regions is steeper than for the rim. This can be explained by a standard curved interstellar electron energy spectrum combined with lower interior magnetic field compared to that near the outer shock. This results in a larger proportion of steep spectrum emission for lines-of-sight through the central body of the SNR. HI observations show structures probably associated with the SNR in the radial velocity range -3 to -9 km s -1 , suggesting a kinematic distance of 0.8 ± 0.4 kpc for the SNR. This is consistent with the distance to the radio pulsar PSR B0458+46, offset from the center of HB9 by 23′. However the pulsar spindown and kinematic ages are significantly greater than estimates of the SNR age: the Sedov age for HB9 is 6600 yr and the evaporative cloud model yields ages of 4000–7000 yr.

The radio SNR G65.1+0.6 and its associated pulsar J1957+2831

New images of the radio Supernova Remnant (SNR) G65.1+0.6 are presented, based on the 408 MHz and 1420 MHz continuum emission and the HI-line emission data of the Canadian Galactic Plane Survey (CGPS). A large shell-like structure seen in the 2695 MHz Effelsberg map appears to have nonthermal spectral index. HI observations show structures associated with the SNR G65.1+0.6 in the radial velocity range of -20 to -26 km s-1 and suggest a distance of 9.2 kpc for the SNR. The estimated Sedov age for G65.1+0.6 is yr. The pulsar (PSR) J1957+2831 is possibly associated with G65.1+0.6, with consistent distance and kinematic age estimate, but different characteristic age than the SNR. The EGRET source 3EG J1958+2909 and γ-ray source 2CG 065+00 are also near the eastern edge of the SNR but do not agree in position with the pulsar and are likely not associated with the SNR. The SNR's flux densities at 408 MHz ( Jy), 1420 MHz ( Jy) and 2695 MHz ( Jy) have been corrected for flux densities from compact sources within the SNR. The integrated flux density based spectral index () between 1420 MHz and 408 MHz is and agrees with the T-T plot spectral index of . The nearby SNR DA495 has a T-T plot spectral index of .

Radio observations and spectrum of the SNR G127.1+0.5 and its central source 0125+628

We present new images of the Supernova Remnant (SNR) G127.1+0.5 (R5), based on the 408 MHz and 1420 MHz continuum emission and the HI-line emission data of the Canadian Galactic Plane Survey (CGPS). The radio spectrum of the central compact source (0125+628) is analyzed in the range 178 MHz - 8.7 GHz, indicating a flat spectrum with synchrotron self-absorption below 800 MHz. The SNR's flux density at 408 MHz is 17.1$\pm$1.7 Jy and at 1420 MHz is 10.0$\pm$0.8 Jy, corrected for flux densities from compact sources within the SNR. The SNR's integrated flux density based spectral index (S$_{\nu}$$\propto$$\nu$$^{-\alpha}$) is 0.43$\pm$0.10. The respective T-T plot spectral index (derived from the relative size of brightness temperature variations between two frequencies, see text for details) is 0.46$\pm$0.01. There is no evidence at 1$\sigma$ for spatial variations in spectral index within G127.1+0.5. In particular, we compared the northern shell, southern shell and central diffuse region. HI observations show structures associated with the SNR in the radial velocity range of -12 to -16 km$/$s, suggesting G127.1+0.5's distance is 1.15 kpc. The estimated Sedov age is 2 - 3 $\times$$10^{4}$ yr.

Radio observations and spectral index study of the SNR G126.2+1.6

We present new images of the low radio surface brightness Supernova Remnant (SNR) G126.2+1.6, based on the 408 MHz and 1420 MHz continuum emission and the HI-line emission data of the Canadian Galactic Plane Survey (CGPS). We find the SNR's flux densities at 408 MHz ( Jy) and 1420 MHz ( Jy) which have been corrected for flux densities from compact sources within the SNR. We list properties of the 19 brightest compact sources within G126.2+1.6 at both 408 MHz and 1420 MHz. The integrated flux density based spectral index () is . The respective T-T plot spectral index is . We also find spatial variations of spectral index within the SNR: 0.2–0.6. HI observations show structures probably associated with the SNR, i.e., features associated with the SNR's southeastern filaments in the radial velocity range of –33 to –42 km s-1, and with its northwestern filaments in –47 to –52 km s-1. This association suggests a distance of 5.6 kpc for SNR G126.2+1.6. The estimated Sedov age for G126.2+1.6 is less than 5 yr.

The Overlooked HiiRegion DA 568

During the inspection of Canadian Galactic Plane Survey 1420 MHz continuum images of the outer Galaxy we have rediscovered the large, bright, H II region DA 568. Intriguingly, this region does not appear in any of the commonly used catalogs of H II regions and radio sources in the outer Galaxy. After demonstrating how DA 568 was essentially overlooked, we present new radio, infrared, and molecular line observations of the region. We show that, far from being a minor overlooked H II region, DA 568 is actually the most prominent part of a much larger star-forming complex in the Perseus arm of our Galaxy. The complex spans ~45 × 20 pc, contains over 104 M⊙ of ionized and molecular gas, and includes the BFS 10 H II region and numerous associated IRAS sources. The star-forming complex is apparently quite evolved with most of the original giant molecular cloud having been severely disrupted by the action of DA 568.

A Westerbork Synthesis Radio Telescope 1400 and 350 MHz Continuum Survey of the Cygnus OB2 Association, in Search of Hot Massive Stars

We present a radio continuum survey at 1400 and 350 MHz of a region of 2degrees x 2degrees centered on the Cygnus OB2 association (d = 1.7 kpc), using the Westerbork Synthesis Radio Telescope (WSRT) with angular resolutions of, respectively, 13 and 55. The resulting 5 sigma flux-density limits of, respectively, similar to2 mJy and similar to10-15 mJy are a significant improvement over previous surveys. We detected 210 discrete sources with sizes less than 1.9theta(beam) (beam size), 98 of which at both frequencies. We also detected 28 resolved sources (sizes > 1.9theta(beam)) still having well-defined peak intensities. The observed spectral index alpha(350)(1400) distribution and source count strongly suggest an excess of sources of Galactic origin in the direction of Cyg OB2. We have searched for positional coincidences of the detected sources in our list with other radio, infrared, and optical objects from various surveys by using the likelihood ratio (LR) method. Furthermore, we looked for objects that show characteristics of either optically thick stellar winds (alpha greater than or similar to +0.6), or nonthermal emission (-1 less than or similar to alpha less than or similar to +0.6) and/or variable spectral flux density. The LR method resulted in 108 identifications. Eighty unidentified sources, i.e., similar to2/3, show characteristics of sources of Galactic origin, 10 of which may be stars. The remaining unidentified sources are probably of extragalactic origin. We identified one source with the O7 star Cyg OB2-335 and consider it to be a candidate colliding-wind binary. We also identify 19 point sources with known infrared and optical objects: these have nearly. at or inverted spectral indices, and some of them show flux-density variability. Follow-up multifrequency monitoring of these sources will be important in establishing the reality of the flux variabilities and to assess the nature of these sources.

G107.5-1.5, a new SNR discovered through its highly polarized radio emission

A new highly polarized shell-type supernova remnant (SNR) has been discovered in the Canadian Galactic Plane Survey (CGPS). The only part of the remnant visible in radio continuum is a thin shell segment sitting on top of diffuse emission. The curvature of this segment indicates a much larger object which is not detectable in our observations most likely due to an imhomogenous ambient medium. A comparison of 408 MHz and 1420 MHz continuum emission reveals a spectral index of α = −0.6 ± 0. 1( S ∼ ν α ), typical for a shell-type SNR. The polarized intensity averaged over the object is 50% of the total intensity, and the peak fractional polarization is close to the theoretical maximum, making this remnant the most highly polarized SNR known. At the projected centre of the radio shell is the unidentified X-ray point source 1RXS J225203.8+574249 which could be the neutron star left behind by the supernova explosion or its pulsar wind nebula. From the low rotation measure and possibly related H  features a distance of about 1.1 kpc is proposed. At this distance the radius of G107.5−1.5 is about 6 pc. The morphology and the structure of the ambient neutral hydrogen around the SNR suggests that this supernova remnant is in a late stage of evolution.

The Supernova Remnant CTB 104A: Magnetic Field Structure and Interaction with the Environment

We present new, high-resolution 1420 and 408 MHz continuum images and H I and 12CO (J = 1-0) spectral line maps of the diffuse supernova remnant (SNR) CTB 104A (G93.7-0.3). Analysis of the complex continuum emission reveals no significant spectral index variations across the remnant. Three prominences around CTB 104A are found to be related to the SNR, while one extension to the east is identified as an H II region associated with a background molecular shell. Small-scale polarization and rotation measure (RM) structures are turbulent in nature, but we find a well-ordered RM gradient across the remnant, extending from southeast to northwest. This gradient does not agree with the direction of the global Galactic magnetic field but does agree with a large-scale RM anomaly inferred from RM data by Clegg et al. We show that the observed morphology of CTB 104A is consistent with expansion in a uniform magnetic field, and this is supported by the observed RM distribution. By modeling the RM gradient with a simple compression model we have determined the magnetic field strength within the remnant as B0 ≈ 2.3 μG. We have identified signatures of the interaction of CTB 104A with the surrounding neutral material and determined its distance, from the kinematics of the H I structure encompassing the radio emission, as 1.5 kpc. We also observed clear breaks in the H I shell that correspond well to the positions of two of the prominences, indicating regions where hot gas is escaping from the interior of the SNR.