Why did it take so long for SN1987A to reach peak brightness?

Question

The light from SN1987A first reached us on February 23, 1987, and its brightness peaked in mid May, almost 3 months later. However, every source that I can find states that it normally only takes a few weeks for Supernovae to reach their peak brightness. Here, it states that "Supernovae of both types reach maximum optical brightness 2 to 3 weeks after their explosions." And here it states that "A typical supernova reaches its maximum brightness about 20 days after explosion."

Finally here is a graph from Wikipedia of what typical Supernova light curves look like. What's the reason for this discrepancy?

Answer 1

Here's the abstract of a relevant article from Astrophysical Journal (December 1993), by Li, Hongwei; McCray, Richard; Sunyaev, Rashid A. DOI: 10.1086/173534

Iron, Cobalt, and Nickel in SN 1987A

During the first several weeks after the explosion of SN 1987A, Fe/Co/Ni clumps, containing ∼1% of the mass of the supernova envelope, absorbed most of the energy released by $\rm ^{56}Ni$ and $\rm ^{56}Co$ decay. As a result, the clumps expanded relative to the substrate, forming a "nickel bubble" of low-density Fe/Co/Ni. Later the clumps captured ∼10% of the radioactive luminosity of gamma rays and positrons. An analysis of the light curves of several infrared emission lines of Fe I, Fe II, Co I, Co II, Ni I, and Ni II in the spectrum of SN 1987A confirms these predictions.

Assuming that these elements are not mixed microscopically with other elements, we find that the clumps must occupy > ≲30% of the volume of the emitting region (radial velocity ≲ 2500 km s-1). We can account for the light curves (for 150 days ≲ t ≲ 2 yr) of all the emission lines of Fe, Co, and Ni with a model consisting of ∼60 to 100 identical clumps. The temperature decreases from T ≍ 4200 K at 200 days to T ≍ 600 K at 800 days. For t ≳ 2 yr, the predicted fluxes are considerably lower than the data.

This result indicates that the emission at late times is dominated by an extra source of heating and ionization, most likely photoionization by two-photon continuum from metastable helium in the gas surrounding the clumps. The resulting "frothy" structure, consisting of bubbles of low-density Fe surrounded by higher-density filaments of H, He, and other elements, will persist and may be seen in the spectra and structure of supernova remnants.

(Link to PDF of the full article)

So it appears that the delay is due to those Iron / Cobalt / Nickel clumps initially absorbing energy and later being energised "by an extra source of heating and ionization".

However, that doesn't exactly tell us why these processes caused the delay in the light curve of SN 1987A compared to other core-collapse supernovae.