SN 2002bu—Another SN 2008S-like Transient

Abstract

We observed supernova (SN) 2002bu in the near-IR with the Hubble Space Telescope (HST), the mid-IR with the Spitzer Space Telescope, and in X-rays with Swift 10 years after the explosion. If the faint L _H ~ 10² L _⊙ HST near-IR source at the transient position is the near-IR counterpart of SN 2002bu, then the source has dramatically faded between 2004 and 2012, from L ~= 10^6.0 L _⊙ to L ~= 10^4.5 L _⊙. It is still heavily obscured, τ _V ~= 5 in graphitic dust models, with almost all the energy radiated in the mid-IR. The radius of the dust emission is increasing as Rvpropt ^{0.7 ± 0.4} and the optical depth is dropping as τ _V vpropt ^{-1.3 ± 0.4}. The evolution expected for an expanding shell of material, τ _V vpropt ^-2, is ruled out at approximately 2σ, while the τ _V vpropt ^-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. SN 2002bu is clearly a member of the SN 2008S class of transients, but continued monitoring of the evolution of the spectral energy distribution is needed to conclusively determine the nature of the transient.