5 Observations at other wavelengths

SNRs often produce emission in other bands of the electromagnetic spectrum, the characteristics of which may provide clues to the nature of the object. In particular, nonthermal radio emission is an almost certain indicator of shock acceleration in the blast wave of a SNR.

5.1 Radio

We have looked for radio observations of suitable resolution and sensitivity which might have detected emission from the new candidate remnant, and find: a 6 cm survey made with the Greenbank 91 m telescope (Condon et al. 1994), the 11 cm Galactic plane survey made by Reich et al. (1990), and the 92 cm WENSS survey (Rengelink et al. 1997). All three data sets are publicly available from various servers on the internet. In addition, we have reanalyzed a Westerbork 92 cm synthesis map of the adjacent object CTB 80, since its field of view does include the field observed in the optical, albeit with somewhat attenuated intensities. We will consider these data sets in order of increasing wavelength.

The 6 cm (4.85 GHz) northern sky survey was made with the 91 m Greenbank meridian transit telescope (Condon et al. 1994) at an angular resolution near $3\hbox{$.\mkern-4mu^\prime$ }5$. The total estimated noise (thermal plus confusion) for the galactic longitude of $l = 70\hbox{$.\!\!^\circ$ }1$ is under 4 mJy beam^-1, although examination of this low latitude field itself suggests that there may be weak, diffuse emission from the Galaxy, increasing the confusion level. Condon et al. (1994) note that the reduction technique used will tend to suppress emission extended by more than 20$^\prime$ in declination, and this could affect the proposed candidate remnant with its overall size of about 1$^\circ$.

The radio emission is dominated by the extended sources associated with the bright HII regions Sh 2-99/100 (Sharpless 1959) to the southeast, and the SNR CTB 80, to the southwest. Nonetheless, radio contours overlaid on the optical nebulosity (Fig. 4) do show faint emission with fairly good morphological correspondence to filaments I and IV. There may also be faint emission from II, but its proximity to the strong emission from Sh 2-99/100 means that its possible detection should be treated with caution. The faint feature near Ia bears little resemblence to the narrow optical filament. No emission is detected from the position of filament III. The rather strong emission from the filaments of LBN 156 comes from an extension of the northern arc of CTB 80, and may not be related to this candidate remnant. All of the 6 cm radio emission which might be associated with the SNR is very weak, with typical surface brightnesses of 5-10 mJy beam^-1. These can be considered marginal detections at best; only the morphological similarity to filaments I, IV, and perhaps II, indicates that the emission is likely to be genuine.

We next considered the 11 cm (2.695 GHz) galactic plane survey made with the Effelsberg 100 m telescope (Reich et al. 1990). The angular resolution was $4\hbox{$.\mkern-4mu^\prime$ }3$, with an expected noise level near 5 mJy beam^-1. In our analysis of a map obtained from the MPI website we were able to locate emission similar to that at 6 cm near filaments I, Ia and IV. The strength is typically 10-15 mJy beam^-1, although the emission from IV (which falls on a gradient increasing to the east) may be stronger than this. The emission associated with filament I (Fig. 5) is the most isolated, with a typical brightness of between 9 and 16 mJy beam^-1 (to be compared with 3-14 mJy beam^-1 at 6 cm). We conclude that the radio emission is probably nonthermal, supporting our interpretation that the detected filaments are part of a SNR.

The WENSS maps (Rengelink et al. 1997) for this region could not be used to image extended emission because of the relative proximity of the strong source Cyg A, and have not been considered further. However, we did analyze a 92 cm map centered on the well-known SNR CTB 80, with a resolution of about 0.9$^\prime$ $\times$ 1.6$^\prime$, and a sensitivity near 5 mJy beam^-1. Although of good quality, there are faint remnants of grating lobes from many extended galactic sources, and the distant source Cyg A. The extended radio emission near filament Ia is clearly present, and there is a hint of the emission arc from I, although it is crossed by a distant grating lobe from Cyg A. There are also large-scale gradients crossing the map, which make the determination of the intensity of faint features uncertain. A brightness estimate of some of the extended emission near Ia, when compared with the 6 cm emission, gives a spectral index of, $\alpha\simeq-1$, once again consistent with nonthermal emission.

5.2 Other wavebands

Emission might also be expected in the infrared and X-ray bands. We have inspected maps from the IRAS survey in the four bands (12, 25, 60 and 100 $\mu$m), but there are no obvious features which might suggest an interaction of the candidate supernova remnant shell with the source of IR emission like e.g. in CTB 80 (Fesen et al. 1988). There is a depression seen at all four bands which closely matches the western edge of the shell, extending from the southern end of filament I. Whether it is physically associated, or just a chance coincidence, is difficult to say; it is the region of lowest IR emission in the region.

A perusal of X-ray data available reveals no obvious associated emission features. The 3$\sigma$ upper limit, calculated from photons collected in $\sim$430 s from the ROSAT All-sky survey, close to this area, is $\sim$ $6 \times 10^{-4}$ cts s^-1 arcmin^-2 in the 0.1-2.0 keV band.

5.3 A hypothesis

The optical observations revealed the presence of filamentary structures which display the characteristic signature of emission from shock heated gas. In addition, faint radio emission seems spatially correlated with almost all of these new structures. We propose that the emission from areas I, II, and IV belongs to a single expanding supernova remnant shell. The observed variations in the absolute line fluxes could be due to density variations of the interstellar "clouds'' leading to shock velocity variations, or due to intrinsic absorption or due to possible abundance variations. The relation of the network of filaments seen in area III to the candidate remnant is not clear but cannot be ruled out at the moment. The estimated center of this candidate remnant is located at a galactic longitude of $70\hbox{$.\!\!^\circ$ }1$ and a galactic latitude of $2\hbox{$.\!\!^\circ$ }3$. The angular radius is $\sim$42$^\prime$ corresponding to $\sim$24 pc at a distance of 2 kpc (see Sect. 6.1). Further radio spectral observations would be required to establish the non-thermal nature of the emission from these structures and determine the extent to which they are related.