Could SNAD160 be a Pair-instability Supernova?

During the search for supernova candidates in the Zwicky Transient Facility (Graham et al. 2019) data release 3 (ZTF DR3), the SNAD ¹¹ team found a transient at the position α = 13^h43^m53357, δ = +61°33'1724. It has been sent to the Transient Name Server ¹² (TNS) as a possible supernova and received an official TNS identifier, AT2018lzi, as well as an internal name SNAD160. In the official ZTF alert stream it is denoted as ZTF18aautopz and automatically classified as a superluminous supernova by the ALeRCE ZTF Explorer. ¹³

On the archival images provided by the Legacy Surveys Sky Viewer, ¹⁴ a possible host with less than 1'' separation from SNAD160 is detected with photometric redshift estimation z_ph = 0.676 ± 0.246 (Zhou et al. 2021). The host photometry gives 2291 ± 045 in r-band (DATE-OBS = "2017-05-31T07:13:31.066000"), calibrated with SDSS-DR12.

SNAD160 has photometry in three ZTF-bands (zg, zr, zi) and reaches ∼19^m in zr-band. Its multicolor light curve is available via the SNAD viewer ¹⁵ . Using a polynomial fit, we estimate a decline in zr-band magnitude from ${m}_{\max }$ to ${m}_{\max }+1$ over a period of ∼140 days in the observer frame.

Given its long and slow evolution resulting in a broad light-curve shape, as well as the poor comparison with standard SN models, we consider this object a good pair-instability supernova (PISN) candidate—a theoretical class of thermonuclear explosions of very massive stars (Barkat et al. 1967; Rakavy & Shaviv 1967; Gal-Yam 2019), for which no observational confirmation has been reported. PISN explosions are supposed to produce large quantities of radioactive ⁵⁶Ni which powers its superluminous light curve (e.g., Kasen et al. 2011; Dessart et al. 2013; Kozyreva et al. 2014).

We explore the possibility of SNAD160 being explained by a few different classes of well known supernovae. Using the Python library sncosmo (Barbary et al. 2022) we fit its multicolor light curve with several supernova models: Peter Nugent's spectral templates ¹⁶ which cover the main supernova types (Ia, Ib/c, IIP, IIL, IIn) and the normal IIP 1999em supernova template from Vincenzi et al. (2019). Each model represents simple spectral time series that can be scaled up and down. The model parameters are redshift z, observer-frame time corresponding to the zero source's phase, t₀, and the amplitude. The zero phase is defined relative to the moment of explosion and the observed time t is related to phase via t = t₀ + phase × (1 + z).

We subtract the reference magnitude from the ZTF light curves to roughly account for the host-galaxy contamination. Since we do not have the spectroscopic redshift for SNAD160, we adopt [−15^m; −22^m] as an acceptable region for the supernovae absolute magnitude (Richardson et al. 2014) and then, using the apparent maximum magnitude, roughly transform it to the possible redshift range. None of the explored models fits SNAD160 satisfactorily.

In Figure 1 we show the zr-light curve of SNAD160 in comparison with Nugent's SN Ia template and the SN IIP model from Vincenzi et al. (2019). For the plot we fixed the zero-phase t₀ to MJD = 58,200. We note that the light curves of normal SNe Ia and SNe II are too narrow to provide a good match to SNAD160.

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We also compare the observations with the synthetic R-band light curves of luminous PISN models of Kasen et al. (2011)—the red supergiant PISN explosion models for 250 M_⊙ (R250) star at the zero-age main sequence and the WolfRayet PISN model with a helium core mass of 130 M_⊙ (He130). Both models are shifted to the observer frame assuming z = 0.3 (dashed line) and z = 0.4 (solid line). The observed light curve shows a good agreement with PISN models, however we recognize that agreement with other SLSN models are also possible. Further detailed analysis of SNAD160 properties, including light-curve modeling with a radiation hydrodynamical code (e.g., STELLA; Blinnikov et al. 1998, 2006) is beyond the scope of this work, but can certainly clarify this question. Such investigations will be included in a subsequent analysis.

We present SNAD160—the first PISN candidate found by the SNAD team in ZTF data releases. This represents still another reported evidence that the tools and expertise developed in recent years by the SNAD team (Pruzhinskaya et al. 2019; Aleo et al. 2021; Ishida et al. 2021; Malanchev et al. 2021) are suited to find interesting astrophysical anomalies within large astronomical data sets. In light of the upcoming large surveys like the Vera Rubin Observatory Legacy Survey of Space and Time (LSST, LSST Science Collaboration et al. 2009), we plan to further develop tailored machine learning algorithms which can scale to LSST-like requirements. This will enable early discovery of sources similar to SNAD160 and increase the chances of their spectroscopic classification as PISN.

We thank Anais Möller for helpful discussions and Alexandre Moskvitin for assistance with acquiring host follow-up resources. The reported study was funded by RFBR and CNRS according to the research project No21-52-15024. The authors acknowledge the support by the Interdisciplinary Scientific and Educational School of Moscow University "Fundamental and Applied Space Research." P.D.A. is supported by the Center for Astrophysical Surveys at the National Center for Supercomputing Applications (NCSA) as an Illinois Survey Science Graduate Fellow. S.B. acknowledges support from the 'Programme National de Physique Stellaire (PNPS) of CNRS/INSU co-funded by CEA and CNES and the ESO Scientific Visitor Programme in Garching.