The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment

TL;DR

SN 2021hem is a Type Ia supernova with pronounced 2003fg-like characteristics, occurring in an apparently hostless environment. The authors combine dense optical/NIR photometry, spectroscopy, and deep, late-time imaging to derive a high $^{56}$Ni mass of about $1.0\pm0.1\,M_\odot$ and a pre-fireball early emission best explained by shallow $^{56}$Ni mixing or an asymmetric Ni distribution, with $t_{first}\approx-16.4$ days relative to $B$-band maximum. Deep GTC imaging places stringent limits on a detectable host, suggesting the SN either originated from a hyper-velocity progenitor ejected from a nearby AGN host at $\sim$104 kpc or from an ultra-faint or ultra-diffuse dwarf galaxy below current detection limits. Together, these findings reinforce SN 2021hem as a strong hostless SN Ia candidate and underscore the diversity of luminous, slowly evolving 2003fg-like explosions and the challenges in identifying extremely faint hosts.

Abstract

We report observations of Type Ia SN 2021hem, located in an apparently hostless environment. With a peak absolute B-band magnitude of -19.96 mag, and a lack of secondary maximum in near-infrared and i-band light curves make it resemble 2003fg-like events. The slowly evolving light curves, and the earliest spectrum showing CII absorption lines, further support this classification. Fireball model fit to early light curves yields a time of first light of -16.43 days relative to B-band maximum. The first detection occurs 1.51 days before the onset of the fireball-like flux rise. This early emission, and $(g - r)_0$ color, is inconsistent with circumstellar or companion interaction. Instead, shallow $^{56}$Ni mixing or an asymmetric $^{56}$Ni distribution offers a plausible explanation. SN2021hem is the fifth known 2003fg-like SN with early-time excess flux emission. The estimated mass of radioactive $^{56}$Ni in SN2021hem is $1.00\pm0.09 M_\odot$. Deep GTC imaging obtained 2.5 yr after the explosion (with $m_{lim,r}=24.4$ mag and $μ_{lim,r} = 26.3\rm~mag~arsec^{-2}$), reveals no coincident host, thereby ruling out most faint dwarf and UDGs. Alternatively, assuming the nearest plausible AGN host galaxy, at a distance of 104 kpc, implies a hyper-velocity progenitor ejected at $\sim$2200 km/s by AGN interaction. A faint diffuse feature ~6 kpc from the SN site has also been detected in the image, with its surface brightness of a UDGs. However, it is unclear whether it is a galaxy and is associated with SN2021hem. Considering its large normalized directional light distance ($d_{DLR}\sim3-4$) from SN, and its unusual elongation, it is a candidate of low probability to be the host galaxy of SN2021hem. These results identify SN2021hem as one of the strongest candidates for a hostless SN Ia, underscoring the diversity of luminous, slowly evolving, 2003fg-like explosions.

Figures (15)

• Figure 1: $r$-band image of SN 2021hem at peak brightness observed with the Nordic Optical Telescope. The location of SN 2021hem is positioned at the center in both panels. Two of the closest cataloged galaxies are marked in blue, both at the projected distance of roughly $\sim100\hbox{kpc}$ (see Sect. \ref{['sec:arunawayburnoutnooneloved']})
• Figure 2: Large-scale structure around the position of SN 2021hem (marked with a cyan cross) shown in a field of a size of 2$\times$2 square degrees DSS image. The colored circles indicate the galaxies with measured redshifts from SIMBAD catalog within redshift range of 0.02 to 0.05. The color-bar indicates the redshifts for each color. There appears to be an overdensity of galaxies with redshifts close to $z \sim 0.027$, $\sim0.035$ within a radius of 1 Mpc from the position of the SN. The open star symbol corresponds to the AGN galaxy 2MASX J16212572+1431537, which has the projected distance of 104$\hbox{kpc}$ from the SN.
• Figure 3: Optical and NIR light curves of SN 2021hem, shown alongside the best-fit SNooPy template light curves using the 'max_model'. Our own observations are shown with filled circles, while photometry from ZTF, YSE, and ATLAS surveys are shown in different symbols. The SNooPy template fits are overplotted as solid lines; those in the $gri$ bands are derived specifically from fits to the ZTF photometry. The indicated offsets have been added to the light curves for clarity. The vertical dashed line corresponds to the estimated "time of first light" by fitting a "fireball" model (see Sect. \ref{['sec:tfirst']}).
• Figure 4: MCMC power-law model fit to ZTF $g$ and $r$-band fluxes to estimate the time of first-light $t_{first}$. The fitting is limited to the first 12 days after $t_{first}$, during which the light curve clearly follows a power-law rise. Power-law model fits are performed separately on the ZTF $g$- and $r$-band light curves to determine $t_{first}$ and $\alpha$ independently for each band. In the top panel, the $g$- and $r$-band fluxes are shown with arbitrary and distinct scales for visual clarity. The inferred (weighted mean) $t_{first}$ is shown by a vertical dashed line, with the shaded region indicating the uncertainty limits. In the bottom panels, the vertical and horizontal lines indicate the $t_{first}$ and $\alpha$ values, as inferred from each MCMC fit.
• Figure 5: UVOIR bolometric light curve of SN 2021hem fit with a radioactive $^{56}$Ni decay model. The time of explosion is taken as the epoch of first detection, which is $\approx17.9$ days before the $B$-band maximum. Only data beyond 60 days are used for fitting the model. The UVOIR light curve of SN 2021hem (top panel), its time-weighted integrated luminosity (middle panel), and the ratio $t^2L/\left(\int L t~dt\right)$ (bottom panel), which is defined independent of the $^{56}$Ni mass are shown.