Searching for precursor activity of Type IIn supernovae

Abstract

We conducted a search for luminous outbursts prior to the explosion of Type IIn supernovae (SNe IIn). We built a sample of 27 objects spectroscopically classified as SNe IIn and all located at $z<0.015$. Using deep archival SN fields images ---taken up to nearly 20 years prior to the SN explosions themselves--- from transient surveys (PTF, ZTF, DES and CHASE) and major astronomical observatories (European Southern Observatory, ESO and National Optical Astronomy Observatory, NOAO), we found at least one outburst years to months before the explosion of seven SNe IIn, with the earliest precursor being 10 years prior to the explosion of SN 2019bxq. The maximum absolute magnitudes of the outbursts range between $-$11.5 mag and $-$15 mag, and the eruptive phases last for a few weeks to a few years. The $g-r$ colour measured for three objects during their outburst is relatively red, with $g-r$ ranging between 0.5 and 1.0 mag. This is similar to the colour expected during the eruptions of luminous blue variables. We note that the light curves of SNe with pre-SN outbursts have faster decline rates than those of the SNe that do not show pre-SN outbursts. SN 2011fh is remarkable, as it is still visible 12 years after the luminous SN-like event, indicating that the progenitor possibly survived, or that the interaction is still ongoing. We detect precursor activity in 29<!PCT!> of bona fide SNe IIn in our sample. However, a quantitative assessment of the observational biases affecting the sample suggests that this fraction is an underestimation of the intrinsic precursor occurrence rate.