New $\mathsf{\lambda}$6 cm observations of the Cygnus Loop

Abstract

Radio continuum and polarization observations of the entire Cygnus Loop at λ6 cm wavelength were made with the Urumqi 25 m telescope. The λ6 cm map is analysed together with recently published maps from the Effelsberg 100 m telescope at λ21 cm and λ11 cm. The integrated flux density of the Cygnus Loop at λ6 cm is 90 ± 9 Jy, which implies a spectral index of α = −0.40 ± 0.06. This rules out any global spectral steepening up to λ6 cm. However, small spectral index variations in some regions of the source are possible, but there are no indications for any spectral curvature. The linear polarization data at λ6 cm show percentage polarizations up to 35% in some areas of the Cygnus Loop, exceeding those observed at λ11 cm. The Rotation Measure is around −21 rad m −2 in the southern area, which agrees with previous observations. However, the distribution of Rotation Measures is rather complex in the northern part of the Cygnus Loop, where the λ21 cm emission is totally depolarized. Rotation Measures based on λ11 cm and λ6 cm data are significantly larger than in the southern part. The difference in the polarization characteristics between the northern and southern part supports previous ideas that the Cygnus Loop consists of two supernova remnants.Key words. ISM: magnetic fields -ISM: supernova remnants -radio continuum: ISM -techniques: polarimetric IntroductionThe Cygnus Loop (G74.0−8.5) is a large and intense nonthermal Galactic radio source. It is a rather well studied object at all observing bands. It has an apparent size of ∼4• × 3• and its distance was recently revised to 540 pc by Blair et al. (2005) using Hubble Space Telescope data. The small distance and its location well out of the Galactic plane lead to little obscuration or confusion with Galactic emission. Its non-thermal radio spectrum makes its identification as a supernova remnant (SNR) most likely, which is supported by observations in other wavelength ranges.In the radio band a recent spectral index study of the Cygnus Loop was made by Uyanıker et al. (2004) using high quality data from the Effelsberg 100 m telescope and the DRAO synthesis telescope in the frequency range from 408 MHz to 2675 MHz at arcmin angular resolution. For the integrated radio emission they obtained a spectral index of α = −0.42 ± 0.06 (with α defined as S ν ∝ ν α , with S ν being the flux density and ν the frequency). This straight spectrum rules out any spectral break around 1 GHz as reported earlier (DeNoyer 1974). However, the flux density measured by Kundu & Becker (1972) at 5 GHz, the highest frequency where the Cygnus Loop was observed so far, is inconsistent with the spectrum of Uyanıker et al. (2004) and indicates some spectral steepening above 2.7 GHz. A steepening in the spectrum of a SNR is an important and characteristic feature, which is closely connected to the age and the evolution of the source in the interstellar medium. This is for instance observed for the SNR S147, which is similar to the Cygnus Loop in many aspects (Fürst & Reich 1986). It is the...

A Sino-Germanλ6 cm polarization survey of the Galactic plane

“…Thus, intrinsically unpolarized H ii regions show polarized intensity, which exhibits a ring-like pattern for compact sources. The radius of the ring is about that of the beam width, and the maximum intensity is measured to be less than 3% of the peak value of total intensity (Sun et al 2006). Instrumental polarization at this level is not important towards the anti-centre region, where only a few strong sources are located.…”

Newλ6 cm andλ11 cm observations of the supernova remnant CTA 1

“…This project is part of the Sino-German λ6 cm polarisation survey of the Galactic plane Gao et al 2010;Sun et al 2011;Xiao et al 2011;Gao et al 2011). Results on some large SNRs have been already reported, such as G65.2+5.7 (Xiao et al 2009), the Cygnus Loop (Sun et al 2006), HB 3 (Shi et al 2008), G156.2+5.7 (Xu et al 2007), and S 147 (Xiao et al 2008).…”

A Sino-Germanλ6 cm polarization survey of the Galactic plane

“…Stokes parameters I, U, and Q were stored simultaneously for each map. The system set-up and the data reduction were described in detail by Sun et al (2006Sun et al ( , 2007 and Gao et al (2010).…”

“…The surface brightness limit for the survey is Σ 1 GHz = 3.9 × 10 −23 W m −2 Hz −1 sr −1 , below the faintest SNRs known today. Using this system, we have studied the magnetic fields and the spectral index distribution of SNR G74.0−8.5 (Cygnus Loop, Sun et al 2006), G126.2+1.6 and G127.1+0.5 ), G156.2+5.7 (Xu et al 2007), G180.0−1.7 (S147, Xiao et al 2008), G130.7+3.1 (HB3, Shi et al 2008), and G65.3+5.7 (Xiao et al 2009). In addition, Foster et al (2006) disapproved G166.2+2.5 (OA 184) as being a SNR.…”