The bumpy light curve of supernova iPTF13z
  A Type IIn supernova (SN) is dominated by the interaction of SN ejecta with
the circumstellar medium (CSM). Some SNe IIn (e.g., SN 2006jd) have episodes of
re-brightening ("bumps") in their light curves. We present iPTF13z, a SN IIn
discovered by the intermediate Palomar Transient Factory (iPTF) and
characterised by several bumps in its light curve. We analyse this peculiar
behaviour trying to infer the properties of the CSM and of the SN explosion, as
well as the nature of its progenitor star. We obtained multi-band optical
photometry for over 1000 days after discovery with the P48 and P60 telescopes
at Palomar Observatory. We obtained low-resolution optical spectra in the same
period. We did an archival search for progenitor outbursts. We analyse our
photometry and spectra, and compare iPTF13z to other SNe IIn. A simple
analytical model is used to estimate properties of the CSM. iPTF13z was a SN
IIn showing a light curve with five bumps during its decline phase. The bumps
had amplitudes between 0.4 and 0.9 mag and durations between 20 and 120 days.
The most prominent bumps appeared in all our different optical bands. The
spectra showed typical SN IIn characteristics, with emission lines of H$\alpha$
(with broad component FWHM ~$10^{3}-10^{4} ~{\rm ~km ~s^{-1}}$ and narrow
component FWHM ~$10^2 \rm ~km ~s^{-1}$) and He I, but also with Fe II, Ca II,
Na I D and H$\beta$ P-Cygni profiles (with velocities of ~$10^{3}$ ${\rm ~km
~s^{-1}}$). A pre-explosion outburst was identified lasting $\gtrsim 50$ days,
with $M_r \approx -15$ mag around 210 days before discovery. Large, variable
progenitor mass-loss rates (~> 0.01 $M_{\odot} \rm ~yr^{-1}$) and CSM densities
(~> 10$^{-16}$ g cm$^{-3}$) are derived. We suggest that the light curve bumps
of iPTF13z arose from SN ejecta interacting with denser regions in the CSM,
possibly produced by the eruptions of a luminous blue variable star.
