GSH 90-28-17: a Possible Old Supernova Remnant
  GSH 90-28-17 is a high-latitude galactic HI supershell, identified in the HI
supershell catalogs with a velocity of $v_{lsr}\sim-17$ \kms. We used the new
Arecibo GALFA-HI survey data which have much higher resolution and sensitivity
than what were previously available to re-examine the properties of the
supershell. We derived a new distance of 400 pc for GSH 90-28-17 and suggested
that it is related to the Lac OB1 association. The radius of GSH 90-28-17 is
66.0$\pm$3.5 pc. The HI mass of the shell is (3.1$\pm0.1)\times10^{4}$
M$_{\odot}$. It has an age of $\sim4.5$ Myr and a total kinetic energy of
(8.2$\pm0.3)\times10^{48}$ ergs. We extracted radio continuum data for the GSH
90-28-17 region from the 408 MHz all-sky Survey and Bonn 1420 MHz survey, and
filtered the diffuse background Galactic emission. A radio loop-like ridge is
found to be associated with the HI shell at both frequencies, and shows a
nonthermal origin with a TT-plot index of $\alpha$=-1.35$\pm$0.69. In addition,
the pulsar J2307+2225 with a similar distance is found in the shell region. We
conclude that GSH 90-28-17 is probably an old, type II supernova remnant in the
solar neighborhood.
