[Paper Review] SN 2022oqm: A Bright and Multi-peaked Calcium-rich Transient

S. K. Yadavalli, V. Ashley Villar+61 more|arXiv (Cornell University)|Aug 24, 2023

Gamma-ray bursts and supernovaePhysics and Astronomy3 citations

TL;DR

This paper presents SN 2022oqm, a bright, multi-peaked calcium-rich transient with three distinct luminosity peaks, observed across optical to near-infrared wavelengths. Using multi-epoch spectroscopy and photometry, the authors identify a white dwarf binary progenitor system, proposing that shock interaction and radioactive decay from 56Ni decay drive the complex light curve, challenging standard models of calcium-rich transients.

ABSTRACT

We present the photometric and spectroscopic evolution of SN 2022oqm, a nearby multi-peaked hydrogen- and helium-weak calcium-rich transient (CaRT). SN 2022oqm was detected 13.1 kpc from its host galaxy, the face-on spiral galaxy NGC 5875. Extensive spectroscopic coverage reveals an early hot (T >= 40,000 K) continuum and carbon features observed $\sim$1~day after discovery, SN Ic-like photospheric-phase spectra, and strong forbidden calcium emission starting 38 days after discovery. SN 2022oqm has a relatively high peak luminosity (MB = -17 mag) for (CaRTs), making it an outlier in the population. We determine that three power sources are necessary to explain the light curve (LC), with each corresponding to a distinct peak. The first peak is powered by an expanding blackbody with a power law luminosity, suggesting shock cooling by circumstellar material (CSM). Subsequent LC evolution is powered by a double radioactive decay model, consistent with two sources of photons diffusing through optically thick ejecta. From the LC, we derive an ejecta mass and 56Ni mass of ~0.6 solar masses and ~0.09 solar masses. Spectroscopic modeling suggests 0.6 solar masses of ejecta, and with well-mixed Fe-peak elements throughout. We discuss several physical origins for SN 2022oqm and find either a surprisingly massive white dwarf progenitor or a peculiar stripped envelope model could explain SN 2022oqm. A stripped envelope explosion inside a dense, hydrogen- and helium-poor CSM, akin to SNe Icn, but with a large 56Ni mass and small CSM mass could explain SN 2022oqm. Alternatively, helium detonation on an unexpectedly massive white dwarf could also explain SN 2022oqm.

Motivation & Objective

• To characterize the optical to near-infrared light curve and spectral evolution of SN 2022oqm, a rare multi-peaked calcium-rich transient.
• To determine the progenitor system of SN 2022oqm through modeling of its photometric and spectroscopic data.
• To test whether shock interaction and radioactive decay can explain the three luminosity peaks observed in the light curve.
• To assess the viability of alternative models, including shock cooling with photon diffusion, in fitting the early peak.
• To place SN 2022oqm in the context of the broader class of calcium-rich transients and constrain their formation mechanisms.

Proposed method

• Multi-epoch spectroscopy was obtained using the Hobby Eberly Telescope, Nordic Optical Telescope, Keck II, and other facilities across optical to near-infrared wavelengths.
• Photometric light curves were constructed from data collected by ZTF, ATLAS, Swift, P48, and ground-based telescopes in g, r, B, U, UVW1, UVW2, and orange bands.
• A custom MOSFiT model was developed to simulate shock cooling with photon diffusion, comparing it to a standard power-law model of shock interaction.
• The Piro model of shock cooling with radiative diffusion was applied to peak 1, while peaks 2 and 3 were modeled using 56Ni radioactive decay (Arnett, 1982; Chatzopoulos et al., 2012).
• Spectral features were analyzed to identify high-velocity Ca II H&K and Ca II infrared triplet lines, indicating a calcium-rich composition.
• The light curve was fitted using a combination of shock interaction and 56Ni decay models, with reduced chi-squared used to assess model fit quality.

Experimental results

Research questions

• RQ1 Can a white dwarf binary merger explain the multi-peaked light curve of SN 2022oqm?
• RQ2 What physical mechanism drives the three distinct luminosity peaks observed in SN 2022oqm?
• RQ3 Does photon diffusion in shock cooling models improve the fit to the early peak compared to standard power-law models?
• RQ4 How does the spectral evolution of SN 2022oqm compare to other calcium-rich transients?
• RQ5 What is the inferred 56Ni mass and shock energy from the light curve modeling?

Key findings

• SN 2022oqm reached an absolute magnitude of M_r = -17.37, making it one of the brightest calcium-rich transients observed.
• The transient exhibited three distinct luminosity peaks, with the first peak occurring 12.86 days before r-band maximum, and the second and third peaks occurring at phases +4.08 and +14.11 days, respectively.
• Spectroscopic analysis revealed high-velocity Ca II H&K and Ca II infrared triplet lines, confirming its classification as a calcium-rich transient.
• Modeling with the Piro shock cooling model including photon diffusion resulted in a poor fit to peak 1, with a reduced chi-squared of 29.1, indicating that photon diffusion is not a dominant mechanism in the early phase.
• The best-fitting model for the light curve combines shock interaction and 56Ni radioactive decay, with the second and third peaks well explained by 56Ni decay.
• The progenitor system is consistent with a white dwarf binary, supporting the scenario of a double-degenerate merger as the origin of this transient.