SN 2008iy: an unusual Type IIn Supernova with an enduring 400-d rise time

Abstract

We present spectroscopic and photometric observations of the Type IIn supernova (SN) 2008iy. SN 2008iy showed an unprecedentedly long rise time of ∼400 d, making it the first known SN to take significantly longer than 100 d to reach peak optical luminosity. The peak absolute magnitude of SN 2008iy was Mr≈−19.1 mag, and the total radiated energy over the first ∼700 d was ∼2 × 1050 erg. Spectroscopically, SN 2008iy is very similar to the Type IIn SN 1988Z at late times and, like SN 1988Z, it is a luminous X‐ray source (both SNe had an X‐ray luminosity LX > 1041 erg s−1). SN 2008iy has a growing near‐infrared excess at late times similar to several other SNe IIn. The Hα emission‐line profile of SN 2008iy shows a narrow P Cygni absorption component, implying a pre‐SN wind speed of ∼100 km s−1. We argue that the luminosity of SN 2008iy is powered via the interaction of the SN ejecta with a dense, clumpy circumstellar medium. The ∼400‐d rise time can be understood if the number density of clumps increases with distance over a radius ∼1.7 × 1016 cm from the progenitor. This scenario is possible if the progenitor experienced an episodic phase of enhanced mass loss <1 century prior to explosion or if the progenitor wind speed increased during the decades before core collapse. We favour the former scenario, which is reminiscent of the eruptive mass‐loss episodes observed for luminous blue variable (LBV) stars. The progenitor wind speed and increased mass‐loss rates serve as further evidence that at least some, and perhaps all, Type IIn SNe experience LBV‐like eruptions shortly before core collapse. We also discuss the host galaxy of SN 2008iy, a subluminous dwarf galaxy, and offer a few reasons why the recent suggestion that unusual, luminous SNe preferentially occur in dwarf galaxies may be the result of observational biases.

THE DETECTION OF A SN IIn IN OPTICAL FOLLOW-UP OBSERVATIONS OF ICECUBE NEUTRINO EVENTS

“…Comparing with other SNe IIn, e.g., SN 2008iy (Miller et al 2010b) which had a measured X-ray luminosity of L X = (2.4 ± 0.8) × 10 41 erg s −1 or SN 2010jl (Ofek et al 2014b) with L X ≈ 1.5 × 10 41 erg s −1 , we cannot exclude X-ray emission from PTF12csy with our measured upper limit. However, Svirski et al (2012) suggest that L X be about 10 4…”

“…The corrected absolute magnitude light curves are fitted to obtain the linear decline rates in different photometric filters, during different epochs (see Figure 7 and Table 4). For some epochs and filters, especially g and r, the decline rates are close to 0.98 mag (100 days) −1 , the decline rate expected for radioactive 56 Co decay (Miller et al 2010b), while in general decline rates are slower, indicating that at least part of the radiated energy is powered by interaction of the SN ejecta with a dense CSM (Miller et al 2010b).…”

“…The upper limit count rate is converted to unabsorbed flux UL F using the HEASARC Tool WebPIMMS, 67 assuming a blackbody model with T = 0.6 keV as in Miller et al (2010b), a Galactic hydrogen column density of 1.31 × 10 21 cm −2 (Willingale et al 2013) 68 and a redshift of z 0.0684 = . The result is a 0.2-10 keV X-ray flux < 4.6 × 10 −14 erg cm −2 s −1 for the most constraining upper limits, corresponding to a 0.2-10 keV X-ray luminosity of L X < 5.2 × 10 41 erg s −1 with a luminosity distance of about 308 Mpc.…”

A Long-duration Luminous Type IIn Supernova KISS15s: Strong Recombination Lines from the Inhomogeneous Ejecta–CSM Interaction Region and Hot Dust Emission from Newly Formed Dust*

“…It should be noted that the He I emission lines and the Hγ emission line also have similar line profiles(Figure 8), although the He I emission line luminosities are much weaker than the Hβ and Hα emission lines. As shown inFigure 18(see alsoFigure 8), the optical spectral properties of KISS15s are similar to type IIn SN 1988Z and other SN 1988Z-like subclasses of SN IIn (see e.g.,Stathakis & Sadler 1991;Pastorello et al 2002;Miller et al 2010;Stritzinger et al 2012).…”

MULTI-WAVELENGTH OBSERVATIONS OF THE ENDURING TYPE IIn SUPERNOVAE 2005ip AND 2006jd

“…At the top of the luminosity scale are the exceedingly bright (M V ∼ −22 mag) objects (see , for a review), including among others SN 2006gy (Ofek et al 2007;Woosley et al 2007;Smith & McCray 2007;Agnoletto et al 2009), SN 2006tf (Smith et al 2008a), SN 2008am (Chatzopoulos et al 2011, and SN 2008iy (Miller et al 2010). The exact nature of these objects is a matter of debate, but the general consensus seems to require one or more dense shells of CSM ejected from the progenitor during sequential luminous blue variable (LBV)-like eruptions in the decades prior to explosion.…”