SN 2019vxm: A Shocking Coincidence between Fermi and TESS

TL;DR

SN 2019vxm is a luminous Type IIn/SLSN-IIn observed with unprecedented multi-wavelength, high-cadence coverage that captures its early rise with a broken-power-law behavior ($n=1.41\pm0.04$). MOSFiT-based Bayesian modeling of the light curve indicates a massive, compact progenitor embedded in an asymmetric, clumpy CSM, with a total mass around $\sim 40\,M_\odot$ and $E_{\rm KE}\approx 1.1\times10^{52}$ erg. The study identifies a strong spatial-temporal association with the Fermi X-ray transient GRB191117A, favoring shock breakout in dense CSM over a classical GRB jet and implying a breakout within an extended envelope reaching roughly $\sim10$ AU. The derived CSM slope $s\approx1.40$ points to a complex mass-loss history, and the host is a low-mass, star-forming dwarf, aligning SN 2019vxm with other SLSN-IIn environments. Together, these results advance our understanding of the earliest SN phases in extreme CSM conditions and constrain progenitor-channel scenarios such as LBV-to-WR evolution and aspherical mass loss.

Abstract

Shock breakout and, in some cases, jet-driven high-energy emission are increasingly recognized as key signatures of the earliest phases of core-collapse supernovae, especially in Type IIn systems due to their dense, interaction-dominated circumstellar environments. We present a comprehensive photometric analysis of SN 2019vxm, a long-duration, luminous Type IIn supernova, $M_V^{}=-21.41\pm0.05\;{\rm mag}$, observed from X-ray to near-infrared. SN 2019vxm is the first superluminous supernovae Type IIn to be caught with well-sampled TESS photometric data on the rise and has a convincing coincident X-ray source at the time of first light. The high-cadence TESS light curve captures the early-time rise, which is well described by a broken power law with an index of $n=1.41\pm0.04$, significantly shallower than the canonical $n=2$ behavior. From this, we constrain the time of first light to within 7.2 hours. We identify a spatial and temporal coincidence between SN 2019vxm and the X-ray transient GRB191117A, corresponding to a $3.3σ$ association confidence. Both the short-duration X-ray event and the lightcurve modeling are consistent with shock breakout into a dense, asymmetric circumstellar medium, indicative of a massive, compact progenitor such as a luminous blue variable transitioning to Wolf-Rayet phase embedded in a clumpy, asymmetric environment.

Figures (9)

• Figure 1: The rise of SN 2019vxm in the native 30 min cadence and 6 hr bins after adjusting the baseline to follow zero-flux. Both plots show the complete rise, and the data binned in $6\;{\rm hr}$ sections. Top: the flux for the Sector 18 with a simple broken power-law fit to the rise. We show the value for $c$ despite the baseline correction already having been applied in the figure. Bottom: the residual, $\Delta {\rm Flux} = {\rm Flux}_{\rm l.c.}^{} - {\rm Flux}_{\rm p.law.}^{}$, between the lightcurve and the fit power-law, to test for significant deviations in the lightcurve.
• Figure 2: An overlay of SN 2019vxm and the control lightcurve positions used in the ATClean reduction on a tricolor (g, r, i) Pan-STARRS1 image, colored using the AstroColour package. The control lightcurve positions were chosen relative to the bright source near SN 2019vxm, as that will be the main source of contamination.
• Figure 3: The detector six gamma-ray Fermi GBM data within the energy range 8--900$\;{\rm keV}$ binned in increments of 1$\,{\rm s}$. Top: the background subtracted Fermi lightcurve. Bottom: the energy-calibrated and background subtracted spectrum to show the peak energies of the X-rays photons detected at the time of the event.
• Figure 4: The 'redder' filters are plotted in flux-density space to show the GBM trigger alongside the complete rise and decay. The ATLAS data has been binned daily, and the TESS data is binned in groups of $6\,{\rm hr}$. We do not take into account band specific extinction. We note that most of the errorbars are not large enough to be visible. The vertical shaded region in the inset panel refers to the $1\sigma$ error around the TESS time of first light.
• Figure 5: The complete lightcurve in all of the different bands plotted with different magnitude offsets for display purposes. The ATLAS data has been binned daily, and the TESS data is binned in groups of $6\,{\rm hr}$. The plotted lines and the respective shaded regions represent the MOSFiT lightcurve fitting median and the 5% and 95% confidence region for each of the respective bands fit within the first 200 days. For further discussion and details for the MOSFiT fitting refer to §\ref{['sec:models']}. For this data we have not corrected for extinction in the magnitudes. We note that most of the errorbars are not large enough to be visible.