Newly Discovered Evolved SNRs

One of the most significant contributions of the ROSAT All Sky Survey is the revelation of X-ray emission from many previously undetected or unknown SNRs, especially remnants with large angular diameters and low surface brightness. Two of the best known of these, RX J1713.3-3946 and RX J0852.0-4622, have already been discussed. Two others worth mention are G189.6+3.3 and G156.2+5.7.

G189.6+3.3 lies partially in the line of sight to IC 443. The ROSAT All Sky Survey revealed the remnant to have a complete shell that partially overlaps IC 443 [4]. It is located in front of IC 443 and the thin molecular cloud with which IC 443 is colliding. Spectral analysis yields an age of <105 yrs, more than an order of magnitude older than IC 443. Where the G189.6+3.3 shell overlaps the bright northeastern region of IC 443, an increased amount of absorption is measured in the IC 443 spectrum.

G156.2+5.7, also known as RX J04591+5147, was the first new SNR discovered in the RASS [117]. It is a circular SNR with a diameter of 108 arcmin. One of the brightest X-ray SNRs, it is by contrast one of the lowest surface brightness remnants in the radio. This suggests the remnant is expanding into a very low-density medium. Unlike other low radio surface brightness remnants, the soft X-ray spectrum of G156.2+5.7 is dominated by a thermal component [180], and its X-ray and radio morphologies do not correspond closely. The thermal emission is described using a NEI model with a temperature of about 0.4 keV, and an ionization timescale of 1-3 x 1011 cm~3 s. The ambient density is —0.2 cm~3, higher than that found in the synchrotron-dominated shell-like remnants. The age is estimated to be —1.5x 104 yrs. The remnant also has a hard tail. ASCA observations indicate that the hard component arises in the remnant interior as well as along the rim, and were unable to discriminate between a thermal and a nonthermal origin for the hard component [180].

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