Early Time Spectropolarimetry of the Aspherical Type II Supernova SN 2023ixf

Abstract

We present six epochs of optical spectropolarimetry of the Type II supernova (SN) 2023ixf ranging from ∼2 to 15 days after the explosion. Polarimetry was obtained with the Kast double spectrograph on the Shane 3 m telescope at Lick Observatory, representing the earliest such observations ever captured for an SN. We observe a high continuum polarization p cont ≈ 1% on days +1.4 and +2.5 before dropping to 0.5% on day +3.5, persisting at that level up to day +14.5. Remarkably, this change coincides temporally with the disappearance of highly ionized “flash” features. The decrease of the continuum polarization is accompanied by a ∼70° rotation of the polarization position angle (PA) as seen across the continuum. The early evolution of the polarization may indicate different geometric configurations of the electron-scattering atmosphere as seen before and after the disappearance of the emission lines associated with highly ionized species (e.g., He ii, C iv, and N iii), which are likely produced by elevated mass loss shortly prior to the SN explosion. We interpret the rapid change of polarization and PA from days +2.5 to +4.5 as the time when the SN ejecta emerge from the dense asymmetric circumstellar material (CSM). The temporal evolution of the continuum polarization and the PA is consistent with an aspherical SN explosion that exhibits a distinct geometry compared to the CSM. The rapid follow-up spectropolarimetry of SN 2023ixf during the shock ionization phase reveals an exceptionally asymmetric mass-loss process leading up to the explosion.

Chandra’s Insights into SN 2023ixf

“…Assuming the CSM is ionized by the X-rays, we can estimate the ionization parameter ζ = L/nr 2 using the X-ray luminosity and will show a shift in the centroid energy, which has not been seen in our measurements. This could possibly be due to asymmetric distribution of the CSM, which has also been indicated by the spectropolarimetric data (Vasylyev et al 2023) as well as the lack of narrow blueshifted absorption profiles in optical spectra (Smith et al 2023).…”

“…The data have revealed prominent flash features of hydrogen, helium, carbon, and nitrogen in the spectra up to 5 days (Bostroem et al 2023;Hiramatsu et al 2023;Jacobson-Galan et al 2023;Singh Teja et al 2023;Smith et al 2023;Yamanaka et al 2023). The UV data along with the flash features indicate a temperature rise, which is not expected in a pure-shock cooling event, indicating a delayed shock breakout in a dense CSM (Hiramatsu et al 2023;Zimmerman et al 2023). Vasylyev et al (2023 measured high polarization and rapid evolution during 2-15 days, and explained it as asymmetric ejecta coming out of dense asymmetric CSM, suggesting a highly asymmetric mass-loss process.…”

New constraints on the gamma-ray and high energy neutrino fluxes from the circumstellar interaction of SN 2023ixf

“…The supernova, SN 2023ixf (distance, 6.9 Mpc) in NGC 5457, classified as a type II SN is found to be more than 2 magnitudes brighter than the typical type II SNe [1]. This bright emission is produced by photo-ionization of a dense circumstellar material (CSM) around the progenitor star as suggested by the flash spectroscopic analysis of early emission from SN 2023ixf [1][2][3][4]. Interaction of SN ejecta with this dense CSM may accelerate particles (e.g., protons) to PeV energies [5][6][7][8][9][10].…”

“…Beyond this radius, a sharp fall in the CSM density has been observed [1]. In addition, some studies of SN 2023ixf suggest a possible JCAP04(2024)083 asymmetry in the CSM [4,30]. The progenitor of SN 2023ixf is inferred to be a red supergiant (RSG) from observations [1,31,32].…”

SN 2023ixf in Messier 101: The Twilight Years of the Progenitor as Seen by Pan-STARRS

“…The discovery of SN 2023ixf led to a sustained spectroscopic and photometric follow-up effort (e.g., Berger et al 2023;Bostroem et al 2023;Jacobson-Galán et al 2023;Sgro et al 2023;Smith et al 2023). To date, these multiwavelength follow-up observations and archival data examination have revealed detections of a dusty RSG progenitor and signatures of interaction with circumstellar material (e.g., Berger et al 2023;Bostroem et al 2023;Grefenstette et al 2023;Guetta et al 2023;Hiramatsu et al 2023;Jacobson-Galán et al 2023;Jencson et al 2023;Kilpatrick et al 2023;Koenig 2023;Kong 2023;Li et al 2023;Neustadt et al 2024;Niu et al 2023;Panjkov et al 2023;Pledger & Shara 2023;Qin et al 2023;Sarmah 2023;Singh Teja et al 2023;Smith et al 2023;Soraisam et al 2023;Van Dyk et al 2023;Vasylyev et al 2023;Xiang et al 2024;Yamanaka et al 2023;Zhang et al 2023).…”