2. Data analysis and results
We have studied the surroundings of 3EG J1828+0142 using radio data from the surveys by Haslam et al. (1981) and Reich & Reich (1986). The background filtering method developed by Sofue & Reich (1979) was applied to remove the diffuse galactic emission that hides weak and extended radio structures at low galactic latitudes. The procedure and its application to find low surface brightness SNRs is described in detail by Combi et al. (1998, 1999). In the present case we have applied a gaussian filtering beam of to 0.408- and 1.42-GHz maps of a field around 3EG J1828+0142, identifying a previously unnoticed shell-type structure centered at . It is a large () source with a total flux density at 1.42 GHz of Jy and a nonthermal spectral index , very similar to other shell-type SNRs (see Green 1998). In Fig. 1, upper panel, we show the filtered 1.42-GHz image of the source. The large continuum sources at the South of the new remnant candidate are probably related to the North Polar Spur (see Jonas's 1999 large-scale map of the region at 2.326 GHz). Table 1 lists the main characteristics of the new SNR candidate. The distance of pc has been estimated using the relationship derived by Allakhverdiyev et al. (1986) for low surface brightness SNRs, and it should be considered a very rough estimate. The large apparent size of the remnant suggests that it is located nearby.
Table 1. Measured properties of the SNR
If we assume a standard energy release of erg for the supernova explosion and a typical intercloud density of 0.1 cm-3 for the interstellar medium (Spitzer 1998), we get from the Sedov solutions that the age of the remnant is yr. A transverse velocity of km s-1 is then required for the compact object left by the explosion to be currently located near the outer boundary of the SNR. A denser ambient medium would imply, of course, smaller velocities. For instance, with a density cm-3 we get a more reasonable velocity of km s-1. It should be also taken into account that the determination of the distance has an uncertainty of %, so even lower velocities are possible. Lyne & Lorimer (1994) have recently estimated that the space velocity of pulsars at birth has a mean value of km s-1, which is a factor three higher than earlier estimates (e.g. Lyne et al. 1982). Stellar black holes should present a velocity distribution similar to that of pulsars.
In order to explore the small-scale radio emission within the inner probability contours of the gamma-ray source we have used the NVSS Sky Survey with an angular resolution of 45 arcseconds (Condon et al. 1998). There are just four point sources with fluxes above 10 mJy within the 65% probability contour of 3EG J1828+0142. They have been labeled from S1 to S4 in Fig. 1, lower panel. All these sources are nonthermal, with flux densities of between and 100 mJy at 1.4 GHz. Sources S1, S2 and S4 do not present a resolved structure, whereas the source S3 has two weak extensions towards the Northeast and Southwest. It is hard to say, however, whether these features are artifacts due to confusion with weak, neighboring sources, or real components of S3. The characteristics of the four sources are summarized in Table 2. The sources S2, S3 and S4 have no entry in any point source catalog at present. We have estimated lower limits to their radio spectral index using the 5 GHz survey by Condon et al. (1994), which is sensitive down to 5 mJy.
Table 2. Characteristics of the point radio sources inside the -ray contour.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001