[Paper Review] High resolution spectroscopy of SN~2023ixf's first week: Engulfing the Asymmetric Circumstellar Material

Nathan Smith, Jeniveve Pearson+5 more|arXiv (Cornell University)|Jun 13, 2023

Gamma-ray bursts and supernovae8 citations

TL;DR

The paper presents nightly high-resolution echelle spectra of SN 2023ixf in the first week after discovery, revealing rapidly evolving, asymmetric circumstellar material signatures and constraining the CSM extent to ~20–30 AU. Narrow lines fade as the SN photosphere engulfs the asymmetric CSM.

ABSTRACT

We present a series of high-resolution echelle spectra of SN~2023ixf in M101, obtained nightly during the first week or so after discovery using PEPSI on the LBT. NaID absorption in these spectra indicates a host reddening of $E(B-V)$=0.031~mag and a systemic velocity of $+$7~km~s$^{-1}$ relative to the average redshift of M101. Dramatic changes are seen in in the strength and shape of strong emission lines emitted by CSM, including HeII4686, CIV5801,5811, H$α$, and NIV7109,7123. In general, these narrow lines broaden to become intermediate-width lines before disappearing from the spectrum within a few days, indicating a limited extent to the dense CSM of around 20-30 AU (or $\la$10$^{14.7}$ cm). H$α$ persists in the spectrum for about a week as an intermediate-width emission line with P~Cyg absorption at 700-1300 km s$^{-1}$ arising in the post-shock shell of swept-up CSM. Early narrow emission lines are blueshifted and indicate an expansion speed in the pre-shock CSM of about 115 km s$^{-1}$, but with even broader emission in higher ionization lines. This is faster than the normal winds of red supergiants, suggesting some mode of eruptive mass loss from the progenitor or radiative acceleration of the CSM. A lack of narrow blueshifted absorption suggests that most of the CSM is not along our line of sight. This and several other clues indicate that the CSM of SN~2023ixf is significantly aspherical. We find that CSM lines disappear after a few days because the asymmetric CSM is engulfed by the SN photosphere.

Motivation & Objective

• Characterize the immediate circumstellar environment of SN 2023ixf through time-series high-resolution spectroscopy.
• Quantify the reddening and systemic velocity from interstellar Na I D absorption to anchor the host environment.
• Track the evolution of high-ionization and H lines to infer CSM geometry and mass-loss history.

Proposed method

• Obtain nightly high-resolution echelle spectra with PEPSI on the LBT across days 2–9 after explosion (R~50,000, 6 km s^-1).
• Measure Na I D equivalent widths to determine host reddening using Poznanski et al. (2012) relations.
• Decompose and track evolution of narrow, intermediate-width emission lines (He II 4686, C IV 5801/5811, N IV 7109/7123, Hα, He I 5876).
• Subtract line profiles with Lorentzian/Gaussian fits to compare electron-scattering wings and post-shock cooling signatures.
• Use line centroid shifts and widths to infer pre-shock CSM speeds (~115 km s^-1) and post-shock CDS dynamics (~500–1000 km s^-1).

Experimental results

Research questions

• RQ1 What are the properties and timescales of the CSM interaction signatures in SN 2023ixf during the first week after explosion?
• RQ2 Is the CSM geometry predominantly asymmetric, and how does this manifest in the line profiles and their evolution?
• RQ3 What are the constraints on reddening, systemic velocity, and progenitor mass-loss history derived from early high-resolution spectroscopy?
• RQ4 How does the evolution of high-ionization lines (He II, C IV, N IV) relate to the ionization source and the progression of the shock through the CSM?

Key findings

• Na I D interstellar absorption yields host reddening E(B-V) = 0.031 mag and a systemic velocity of +7 ± 1 km s^-1 relative to M101.
• High-ionization narrow lines (He II 4686, C IV 5801/5811, N IV 7109/7123) are blueshifted by ~50–150 km s^-1 and fade within 3–4 days, indicating a compact dense CSM of ~20–30 AU.
• Hα evolves from a narrow component with broad Lorentzian wings to a clear intermediate-width P Cygni profile within about a week, with maximum red-wing velocities declining from ~2000 to ~1000 km s^-1.
• He II decays and He I 5876 grows in strength over 4–5 days, suggesting cooling and recombination of post-shock gas at ~500–1000 km s^-1.
• The lack of persistent narrow blueshifted absorption and disappearance of CSM lines by day ~7–8 implies most CSM is engulfed by the SN photosphere and is highly aspherical.
• CSM velocities and rapid evolution point to eruptive or episodic pre-SN mass loss rather than steady winds.