SN2002bu -- Another SN2008S-like Transient

TL;DR

This study tracks the decade-long evolution of SN2002bu, a transient similar to SN2008S, revealing its fading, dust obscuration, and shock-driven emission, with implications for understanding such stellar explosions.

Contribution

It provides detailed multi-wavelength observations and models of SN2002bu's evolution, clarifying its nature and relation to the SN2008S class of transients.

Findings

Significant fading of near-IR source from 2004 to 2012.

Dust obscuration remains high, with optical depth decreasing over time.

X-ray emission not detected, consistent with shock-powered emission.

Abstract

We observed SN2002bu in the near-IR with the Hubble Space Telescope, the mid-IR with the Spitzer Space Telescope and in X-rays with Swift 10 years after the explosion. If the faint L_H\sim100 Lsun HST near-IR source at the transient position is the near-IR counterpart of SN2002bu, then the source has dramatically faded between 2004 and 2012, from L\sim10^6.0 Lsun to L\sim10^4.5 Lsun. It is still heavily obscured, tau_V\sim5 in graphitic dust models, with almost all the energy radiated in the mid-IR. The radius of the dust emission is increasing as R\simt^(0.7+/-0.4) and the optical depth is dropping as tau_V\simt^(-1.3+/-0.4). The evolution expected for an expanding shell of material, tau_V\sim1/t^2, is ruled out at approximately 2 sigma while the tau_V\simt^(-0.8) to t^(-1) optical depth scaling for a shock passing through a pre-existing wind is consistent with the data. If the near-IR source is a chance superposition, the present day source can be moderately more luminous, significantly more obscured and evolving more slowly. While we failed to detect X-ray emission, the X-ray flux limits are consistent with the present day emissions being powered by an expanding shock wave. SN2002bu is clearly a member of the SN2008S class of transients, but continued monitoring of the evolution of the spectral energy distribution is needed to conclusively determine the nature of the transient.