PISCOLA: a data-driven transient light-curve fitter
  Forthcoming time-domain surveys, such as the Rubin Observatory Legacy Survey
of Space and Time, will vastly increase samples of supernovae (SNe) and other
optical transients, requiring new data-driven techniques to analyse their
photometric light curves. Here, we present the "Python for Intelligent
Supernova-COsmology Light-curve Analysis" (PISCOLA), an open source data-driven
light-curve fitter using Gaussian Processes that can estimate rest-frame light
curves of transients without the need for an underlying light-curve template.
We test PISCOLA on large-scale simulations of type Ia SNe (SNe Ia) to validate
its performance, and show it successfully retrieves rest-frame peak magnitudes
for average survey cadences of up to 7 days. We also compare to the existing SN
Ia light-curve fitter SALT2 on real data, and find only small (but significant)
disagreements for different light-curve parameters. As a proof-of-concept of an
application of PISCOLA, we decomposed and analysed the PISCOLA rest-frame
light-curves of SNe Ia from the Pantheon SN Ia sample with Non-Negative Matrix
Factorization. Our new parametrization provides a similar performance to
existing light-curve fitters such as SALT2. We further derived a SN Ia colour
law from PISCOLA fits over $\sim$3500 to 7000\AA, and find agreement with the
SALT2 colour law and with reddening laws with total-to-selective extinction
ratio $R_V \lesssim 3.1$.
