Radio and X-ray Observations of the Transitional Supernova 2019yvr: Insights into the Progenitor Mass-Loss History

Technical Summary: Radio and X-ray Observations of the Transitional Supernova 2019yvr

Problem Statement
The final evolutionary stages of massive star progenitors leading to stripped-envelope supernovae (SESNe) remain an open question, particularly regarding the timing and magnitude of envelope stripping. While SESNe constitute roughly 30% of core-collapse supernovae (CCSNe), the mechanisms driving mass loss—whether through strong Wolf-Rayet winds or binary interaction—are debated. A specific subset of SESNe, such as SN 2019yvr, exhibits a "transitional" behavior, evolving from a Type Ib (helium-rich, hydrogen-poor) spectrum to a Type IIn (hydrogen-rich, interaction-driven) spectrum roughly 100 days post-explosion. This transition implies the ejecta encountered dense, hydrogen-rich circumstellar matter (CSM) late in the explosion's evolution. The central problem addressed is whether this transition is driven by a dramatic, sudden increase in mass-loss rate (as seen in SN 2014C) or by a more gradual, complex mass-loss history that remains undetected in early optical spectra.

Methodology
The authors present a multi-wavelength analysis of SN 2019yvr using five years of radio and X-ray data to constrain the CSM density profile and mass-loss history.

• Radio Observations: The study utilized 11 epochs of data from the Giant Meterwave Radio Telescope (GMRT) and 5 epochs from the Karl G. Jansky Very Large Array (VLA), covering frequencies from 0.4 GHz to 26.5 GHz over 18 to 1784 days post-explosion. Data reduction involved self-calibration and flux extraction using pipelines such as CAPTURE and VLA standard tools.
• X-ray Observations: The team analyzed data from the Swift-XRT (30+ epochs up to 4 years post-explosion) and a single Chandra observation at 42 days. Due to the host galaxy (NGC 4666) being an X-ray bright AGN, the authors employed pre-explosion Swift data to model and subtract the host contribution, isolating the supernova flux.
• Modeling: Radio data were fitted using synchrotron self-absorption (SSA) and free-free absorption (FFA) models based on Chevalier (1998) formalism. The authors performed Markov Chain Monte Carlo (MCMC) fits to the full dataset and single-epoch broken power-law fits to derive shock radius ($R$), magnetic field ($B$), and shock velocity ($V_{sh}$). X-ray data were analyzed to distinguish between thermal and non-thermal emission mechanisms, utilizing constraints from the radio-derived shock parameters.

Key Results

1. Radio Evolution and CSM Profile: The radio light curves are best-fit by a synchrotron self-absorbed (SSA) model. The derived CSM density profile follows $\rho \propto r^{-1.65 \pm 0.25}$, which is shallower than the standard $r^{-2}$ wind profile. This suggests a mass-loss rate that was decreasing in the years leading up to the explosion, ranging from $\sim 3 \times 10^{-5}$ to $1 \times 10^{-5} M_\odot \text{yr}^{-1}$ (assuming a CSM speed of 100 km/s).
2. Shock Dynamics: The radio-derived shock speed was exceptionally high at early times, exceeding 30,000 km/s at 52 days post-explosion. This high velocity supports the hypothesis of a compact progenitor star (radius $\sim 20 R_\odot$), consistent with later optical re-analyses of the progenitor candidate. The shock decelerated to $\sim 17,000$ km/s by day 419.
3. Mass-Loss History: The combined radio and X-ray data probe CSM extending from $< 10^{16}$ cm to $\sim 20 \times 10^{16}$ cm. Crucially, the data rule out a dramatic jump in CSM density associated with the emergence of optical hydrogen emission at $\sim 100$ days. Unlike SN 2014C, which showed a density increase of orders of magnitude, SN 2019yvr shows only a slight increase (factor of 2–3) in the inferred mass-loss rate as the ejecta encountered material lost further pre-explosion.
4. X-ray Constraints: The Chandra detection at 42 days yielded a luminosity of $L_X \approx 1.03 \times 10^{39}$ erg s$^{-1}$. The emission is determined to be thermal (likely from an adiabatic reverse shock) rather than synchrotron or inverse Compton, providing a mass-loss estimate consistent with radio results ($\sim 1.4 \times 10^{-4} M_\odot \text{yr}^{-1}$, subject to model uncertainties).

Significance and Claims
The paper claims that SN 2019yvr presents a distinct evolutionary path compared to other transitional SNe like SN 2014C. The primary significance lies in demonstrating that a transition from Type Ib to Type IIn optical spectra does not necessarily require a catastrophic, sudden increase in mass loss (such as a common-envelope ejection) immediately prior to the explosion. Instead, the hydrogen-rich CSM in SN 2019yvr likely existed at a relatively constant, moderate density ($\sim 10^{-5} M_\odot \text{yr}^{-1}$) for hundreds of years before the explosion.

The authors propose two potential scenarios to explain the late appearance of hydrogen features without a density jump:

1. Geometric/Optical Depth Effects: The hydrogen may have been hidden behind the photosphere or distributed asymmetrically, only becoming visible as the shock expanded.
2. Composition Stratification: The inner CSM may have been hydrogen-poor (helium-rich) while the outer CSM was hydrogen-rich, implying the progenitor lost its hydrogen envelope thousands of years prior, followed by a phase of hydrogen-poor mass loss, and finally a return to hydrogen-rich loss or the exposure of a companion's envelope.

The study concludes that high-cadence radio and X-ray monitoring is essential for distinguishing between these complex mass-loss histories, as optical spectra alone can be misleading regarding the true density and composition of the circumstellar environment. The results suggest that the "stripping" mechanism in SESNe may be more diverse, with binary interactions potentially playing a significant role in creating the specific CSM environments observed in transitional objects.