Radio Polarization from the Galactic Plane in Cygnus

We present 1420 MHz (lambda=21cm) observations of polarized emission from an area of 117 degree square in the Galactic plane in Cygnus, covering 82 < l < 95, -3.5 < b < +5.5, a complex region where the line of sight is directed nearly along the Local spiral arm. The angular resolution is ~ 1', and structures as large as 45' are fully represented in the images. Polarization features bear little resemblance to features detected in total power: while the polarized signal arises in diffuse Galactic synchrotron emission regions, the appearance of the polarized sky is dominated by Faraday rotation occurring in small-scale structure in the intervening Warm Ionized Medium. There is no concentration of polarization structure towards the Galactic plane, indicating that both the emission and Faraday rotation occur nearby. We develop a conceptual framework for interpretation of the observations. We can detect only that polarized emission which has its origin closer than the polarization horizon, at a distance d_ph; more distant polarized emission is undetectable because of depth depolarization (differential Faraday rotation) and/or beam depolarization (due to internal and external Faraday dispersion). d_ph depends on the instrument used (frequency and beamwidth) as well as the direction being studied. In our data we find that d_ph ~ 2 kpc, consistent with the polarization features originating in the Local arm

A 1.4 GHz radio continuum and polarization survey at medium Galactic latitudes: I. Observation and reduction technique

A radio continuum survey at medium Galactic latitudes with the Effelsberg 100-m telescope is being carried out at a centre frequency of 1.4 GHz in total power and linear polarization. Areas up to +/- 20 degree of Galactic latitude are now being observed at a sensitivity of 15 mK TB total intensity and 8 mK TB in linear polarization with an angular resolution of 9'35. This paper describes the observing and reduction technique applied which results in absolutely calibrated maps. The methods are illustrated by examples of images from the survey.Comment: 12 pages, 9 figures, Accepted for publication in Astronomy and Astrophysics Supp. Se

The distance to the SNR CTB109 deduced from its environment

We conducted a study of the environment around the supernova remnant CTB109. We found that the SNR is part of a large complex of HII regions extending over an area of 400 pc along the Galactic plane at a distance of about 3 kpc at the closer edge of the Perseus spiral arm. At this distance CTB109 has a diameter of about 24 pc. We demonstrated that including spiral shocks in the distance estimation is an ultimate requirement to determine reliable distances to objects located in the Perseus arm. The most likely explanation for the high concentration of HII regions and SNRs is that the star formation in this part of the Perseus arm is triggered by the spiral shock.Comment: 6 pages, 6 figures. accepted for publication in the Astrophysical Journa

Detection of a radio halo in the Virgo cluster

New Effelsberg 1.4 GHz observations of the central 10\degr x 10\degr of the Virgo cluster are presented. NVSS data are used to subtract point sources from our map. During the data reduction process special care is taken (i) to disentangle emission from the North Polar Spur from emission from the Virgo cluster, (ii) to disentangle emission from the strong M87 sidelobes from emission from the Virgo cluster, and (iii) to correct for non-linear ground emission due to the long scans. We detect a low surface brightness radio halo with a flux density of 5 +/- 1.5 Jy centered close to the elliptical galaxy M86. This halo is much weaker than that observed in the Coma cluster. It is reminiscent of a past interaction between the intracluster medium of M86 and a low density gas, belonging most probably to the Virgo cluster.Comment: 9 pages, 8 figures. Accepted for publication in A&

A Highly Ordered Faraday-Rotation Structure in the Interstellar Medium

We describe a Faraday-rotation structure in the Interstellar Medium detected through polarimetric imaging at 1420 MHz from the Canadian Galactic Plane Survey (CGPS). The structure, at l=91.8, b=-2.5, has an extent of ~2 degree, within which polarization angle varies smoothly over a range of ~100 degree. Polarized intensity also varies smoothly, showing a central peak within an outer shell. This region is in sharp contrast to its surroundings, where low-level chaotic polarization structure occurs on arcminute scales. The Faraday-rotation structure has no counterpart in radio total intensity, and is unrelated to known objects along the line of sight, which include a Lynds Bright Nebula, LBN 416, and the star cluster M39 (NGC7092). It is interpreted as a smooth enhancement of electron density. The absence of a counterpart, either in optical emission or in total intensity, establishes a lower limit to its distance. An upper limit is determined by the strong beam depolarization in this direction. At a probable distance of 350 +/- 50 pc, the size of the object is 10 pc, the enhancement of electron density is 1.7 cm-3, and the mass of ionized gas is 23 M_sun. It has a very smooth internal magnetic field of strength 3 microG, slightly enhanced above the ambient field. G91.8-2.5 is the second such object to be discovered in the CGPS, and it seems likely that such structures are common in the Magneto-Ionic Medium.Comment: 16 pages, 5 figures, ApJ accepte

Radio emission from the Cygnus Loop and its spectral characteristics

We present a new sensitive 2675 MHz radio continuum map of the Cygnus Loop, which is used in conjunction with 408 MHz, 863 MHz and 1420 MHz maps from both the Effelsberg 100-m telescope and the DRAO Synthesis Telescope for a spectral analysis. Between 408 MHz and 2675 MHz we find an overall integrated spectral index of $\rm \alpha = -0.42 \pm 0.06$ ($S \sim \nu^{\alpha}$), close to previous results. There is no indication of a spectral break in the integrated spectrum. Spatially highly varying and rather strong spectral curvature was previously reported, but is not confirmed on the basis of new, higher sensitivity observations. We found spectral variations across the Cygnus Loop reaching up to $\Delta\alpha = 0.2$ from a TT-plot analysis. The flattest spectra are seen towards enhanced emission areas. Spectral index maps produced between different frequency pairs, as well as all four maps, revealed that there are at least three flat spectrum regions. In regions interior to the high emission filaments, we have detected at least two spectral components across the whole object with $\alpha=-0.4$ and $\alpha=-0.6$ towards northern and southern parts of the object, respectively.Comment: 22 pages, 13 figures in jpeg format, accepted for publication in Astrophysics & Astronom

Distance of three Supernova Remnants from HI line observations in a complex region: G114.3+0.3, G116.5+1.1, and CTB 1 (G116.9+0.2)

We present new radio continuum and HI images towards the supernova remnants (SNRs) G114.3+0.3, G116.5+1.1, and G116.9+0.2 (CTB 1) taken from the Canadian Galactic Plane Survey (CGPS). We discuss the dynamics of their HI environment and a possible relationship of these SNRs with each other. We discovered patches of HI emission surrounding G114.3+0.3 indicating a location in the Local arm at a distance of about 700 pc in contrast to previous publications which proposed a Perseus arm location. The other two SNRs have radial velocities of -17 km/s (G116.5+1.1) and -27 km/s (CTB 1) according to related HI. However, the structure of the HI and its dynamics in velocity space suggest a possible relation between them, placing both remnants at a distance of about 1.6 kpc. CTB 1 appears to be embedded in an HI feature which is moving as a whole towards us with a velocity of about 10 km/s. Furthermore, the off-centered location of CTB 1 in a large HI bubble indicates that the so-called breakout region of the remnant is in fact due to its expansion towards the low density interior of this bubble. We believe that the progenitor star of CTB 1 was an early B or O-type star shaping its environment with a strong stellar wind in which case it exploded in a Ib or Ic event.Comment: 16 pages, 9 figures, accepted for publication in The Astrophysical Journa

The GMRT EoR Experiment: Limits on Polarized Sky Brightness at 150 MHz

The GMRT reionization effort aims to map out the large scale structure of the Universe during the epoch of reionization (EoR). Removal of polarized Galactic emission is a difficult part of any 21 cm EoR program, and we present new upper limits to diffuse polarized foregrounds at 150 MHz. We find no high significance evidence of polarized emission in our observed field at mid galactic latitude (J2000 08h26m+26). We find an upper limit on the 2-dimensional angular power spectrum of diffuse polarized foregrounds of [l^2 C_l/(2 PI)]^{1/2}< 3K in frequency bins of width 1 MHz at 300<l<1000. The 3-dimensional power spectrum of polarized emission, which is most directly relevant to EoR observations, is [k^3 P_p(k)/(2 PI^2)]^{1/2} 0.03 h/Mpc, k < 0.1 h/Mpc. This can be compared to the expected EoR signal in total intensity of [k^3 P(k)/ (2 PI^2) ]^{1/2} ~ 10 mK. We find polarized structure is substantially weaker than suggested by extrapolation from higher frequency observations, so the new low upper limits reported here reduce the anticipated impact of these foregrounds on EoR experiments. We discuss Faraday beam and depth depolarization models and compare predictions of these models to our data. We report on a new technique for polarization calibration using pulsars, as well as a new technique to remove broadband radio frequency interference. Our data indicate that, on the edges of the main beam at GMRT, polarization squint creates ~ 3% leakage of unpolarized power into polarized maps at zero rotation measure. Ionospheric rotation was largely stable during these solar minimum night time observations.Comment: 17 pages, 6 figures, 2 tables; changed figures, added appendices. To appear in MNRA

A large atomic hydrogen shell in the outer Galaxy: SNR or stellar wind bubble?

We report the detection of a ring like HI structure toward l=90.0, b=2.8 with a velocity of v_LSR=-99 km/s. This velocity implies a distance of d=13 kpc, corresponding to a Galactocentric radius of R_gal=15 kpc. The l-v_LSR diagram implies an expansion velocity of v_exp ~ 15 km/s for the shell. The structure has an oblate, irregular shell-like appearance which surrounds weak infrared emission as seen in the 60 micrometer IRAS data. At a distance of 13 kpc the size of the object is about 110 x 220 pc and placed 500 pc above the Galactic plane with a mass of 1e5 solar mass. An expanding shell with such a high mass and diameter cannot be explained by a single supernova explosion or by a single stellar wind bubble. We interpret the structure as a relic of a distant stellar activity region powered by the joint action of strong stellar winds from early type stars and supernova explosions.Comment: Accepted for publication by The Astrophysical Journal, 5 Pages, 4 Figure