Direct Observation of Cosmic Strings via their Strong Gravitational
  Lensing Effect: II. Results from the HST/ACS Image Archive
  We have searched 4.5 square degrees of archival HST/ACS images for cosmic
strings, identifying close pairs of similar, faint galaxies and selecting
groups whose alignment is consistent with gravitational lensing by a long,
straight string. We find no evidence for cosmic strings in five large-area HST
treasury surveys (covering a total of 2.22 square degrees), or in any of 346
multi-filter guest observer images (1.18 square degrees). Assuming that
simulations ccurately predict the number of cosmic strings in the universe,
this non-detection allows us to place upper limits on the unitless Universal
cosmic string tension of G mu/c^2 < 2.3 x 10^-6, and cosmic string density of
Omega_s < 2.1 x 10^-5 at the 95% confidence level (marginalising over the other
parameter in each case). We find four dubious cosmic string candidates in 318
single filter guest observer images (1.08 square degrees), which we are unable
to conclusively eliminate with existing data. The confirmation of any one of
these candidates as cosmic strings would imply G mu/c^2 ~ 10^-6 and Omega_s ~
10^-5. However, we estimate that there is at least a 92% chance that these
string candidates are random alignments of galaxies. If we assume that these
candidates are indeed false detections, our final limits on G mu/c^2 and
Omega_s fall to 6.5 x 10^-7 and 7.3 x 10^-6. Due to the extensive sky coverage
of the HST/ACS image archive, the above limits are universal. They are quite
sensitive to the number of fields being searched, and could be further reduced
by more than a factor of two using forthcoming HST data.
