  Microquasars are neutron star or black hole X-ray binaries with jets. These
jets can create shock-ionized bubbles of hot plasma that can masquerade as
peculiar supernova remnants (SNRs) in extragalactic surveys. To see if this is
the case in the well-studied spiral galaxy M83, where one microquasar candidate
(M83-MQ1) has already been identified, we studied the properties of nine SNR
candidates, selected because of their elongated or irregular morphology, from
the set of previously identified SNRs in that galaxy. Using multiwavelength
data from Chandra, the Hubble Space Telescope, Gemini, and the Australia
Telescope Compact Array, we found that at least six of our nine sources are
best interpreted as SNRs. For one source, we do not have enough observational
data to explain its linear morphology. Another source shows a nebular optical
spectrum dominated by photo-ionization by O stars, but its excess [Fe II] and
radio luminosity suggest a possible hidden SNR. Finally, one source (S2) shows
an elongated structure of ionized gas, two radio sources along that line, and
an accretion-powered X-ray source in between them (the Chandra source L14-139).
While S2 could be a chance alignment of multiple SNRs and one X-ray binary, it
seems more likely that it is a single physical structure powered by the jet
from the accreting compact object. In the latter case, the size and luminosity
of S2 suggest a kinetic power of around 10^{40} erg/s, in the same class as the
most powerful microquasars in the local universe (e.g., S26 in NGC7793 and
SS433 in our own Galaxy).
