OGLE-IV Real-Time Transient Search
  We present the design and first results of a real-time search for transients
within the 650 sq. deg. area around the Magellanic Clouds, conducted as part of
the OGLE-IV project and aimed at detecting supernovae, novae and other events.
The average sampling of about 4 days from September to May, yielded a detection
of 238 transients in 2012/2013 and 2013/2014 seasons. The superb photometric
and astrometric quality of the OGLE data allows for numerous applications of
the discovered transients.
  We use this sample to prepare and train a Machine Learning-based automated
classifier for early light curves, which distinguishes major classes of
transients with more than 80% of correct answers. Spectroscopically classified
49 supernovae Type Ia are used to construct a Hubble Diagram with statistical
scatter of about 0.3 mag and fill the least populated region of the redshifts
range in the Union sample. We investigate the influence of host galaxy
environments on supernovae statistics and find the mean host extinction of
A_I=0.19+-0.10 mag and A_V=0.39+-0.21 mag based on a subsample of supernovae
Type Ia. We show that the positional accuracy of the survey is of the order of
0.5 pixels (0.13 arcsec) and that the OGLE-IV Transient Detection System is
capable of detecting transients within the nuclei of galaxies. We present a few
interesting cases of nuclear transients of unknown type.
  All data on the OGLE transients are made publicly available to the
astronomical community via the OGLE website.
