SN 2018hti: a nearby superluminous supernova discovered in a metal-poor galaxy

Abstract

<jats:title>Abstract</jats:title> <jats:p>SN 2018hti is a Type I superluminous supernova (SLSN I) with an absolute g-band magnitude of −22.2 at maximum brightness, discovered in a metal-poor galaxy at a redshift of 0.0612. We present extensive photometric and spectroscopic observations of this supernova, covering the phases from ∼−35 days to more than +340 days from the r-band maximum. Combining our BVgri-band photometry with Swift UVOT optical/ultraviolet photometry, we calculated the peak luminosity as ∼3.5 × 1044 erg s−1. Modeling the observed light curve reveals that the luminosity evolution of SN 2018hti can be produced by an ejecta mass of 5.8 M⊙ and a magnetar with a magnetic field of B = 1.8 × 1013 G having an initial spin period of P0 = 1.8 ms. Based on such a magnetar-powered scenario and a larger sample, a correlation between the spin of the magnetar and the kinetic energy of the ejecta can be inferred for most SLSNe I, suggesting a self-consistent scenario. Like for other SLSNe I, the host galaxy of SN 2018hti is found to be relatively faint (Mg = −17.75 mag) and of low metallicity (Z = 0.3 Z⊙), with a star-formation rate of 0.3 M⊙ yr−1. According to simulation results of single-star evolution, SN 2018hti could originate from a massive, metal-poor star with a zero-age main sequence (ZAMS) mass of 25–40 M⊙, or from a less massive rotating star with MZAMS ≈ 16–25 M⊙. For the case of a binary system, its progenitor could also be a star with MZAMS ≳ 25 M⊙.</jats:p>

Close, bright and boxy: the superluminous SN 2018hti

“…As expected, the results point towards a metal-poor site with 12 + log 10 (O/H) ≈ 8.17, which corresponds to a metallicity 𝑍 ≈ 0.3 𝑍 (assuming 12 + log 10 (O/H) = 8.69 for the solar metallicity, Asplund et al 2009). This estimate nicely agrees with the results obtained by Lin et al (2020a). Moreover, we estimated the star formation rate (SFR) of the host galaxy of SN 2018hti based on the measurements of the flux emitted by the reddening corrected narrow H𝛼 using equation 2 of Kennicutt (1998).…”

“…17), opacity 𝜅 ≈ 0.1 cm 2 g −1 , gamma-ray opacity 𝜅 𝛾 ≈ 0.02 cm 2 g −1 , an average ejecta velocity 𝑣 ej ≈ 8500 km s −1 and a temperature floor 𝑇 min ≈ 9300 ± 250 K. This corresponds to a kinetic energy 𝐸 kin = 2.2 × 10 51 erg s −1 . These results are absolutely reasonable for what is expected by the magnetar scenario for SLSNe I (e. g. Nicholl et al 2017b) and are in perfect agreement with the estimates of Lin et al (2020a).…”

“…2 and are plotted against the phase corrected for time dilation. To estimate the maximum luminosity epoch, we fit a fourth order polynomial to the 𝑟-band LC and infer a magnitude at maximum 𝑟 max 16.5 ± 0.2 mag at MJD = 58 464.5 ± 4.0 (in agreement with the maximum found by Lin et al 2020a). Given a distance modulus 𝜇 = 37.17 ± 0.1 mag and a Galactic extinction 5 𝐴 𝑉 = 1.280 mag (Schlafly & Finkbeiner 2011), assuming the extinction law 𝐴 V = 3.1 × 𝐸 (𝐵 − 𝑉) (Cardelli et al 1988(Cardelli et al , 1989, the absolute peak 𝑟-filter magnitude is 𝑀 𝑟 = −22.2 ± 0.2 mag.…”

“…It was then classified on 2018 November 6 by Burke et al (2018) as a SLSN I. Independent spectro-photometric data of SN 2018hti are already presented by Lin et al (2020a) and imaging polarimetry data of SN 2018hti are presented by Lee (2019). Here we present the photometric and spectroscopic data of SN 2018hti in Sec.…”

Post maximum light and late time optical imaging polarimetry of type I superluminous supernova 2020znr

“…The results are dominated by the ISP and do not show > 3𝜎 signal-to-noise polarization detection variations. Its light curve is similar to the one of 2015bn (Lin et al 2020) suggesting it is part of the Slow rising event group. Polarimetry obtained by (Leloudas…”