GRB 180728A and SN 2018fip: the nearest high-energy cosmological gamma-ray burst with an associated supernova

TL;DR

GRB 180728A is a nearby, high-energy long GRB at $z=0.1171$ with SN 2018fip. Through dense multi-wavelength observations, the study separates afterglow and SN light, showing a jet break near $t_{\rm jet}\approx0.2$ d and a SN that is energetically typical but somewhat fainter than SN 1998bw. Spectral modelling with TARDIS supports a two-component, aspherical SN ejecta with a high-velocity outer region (>20{,}000 km s$^{-1}$) and a slower inner component, and the HV component may be confined to a small solid angle. The GRB energy appears decoupled from the SN energy, with a modest GRB efficiency of a few percent, underscoring the diversity of GRB–SN energy budgets and the central role of ejecta geometry in these explosions.

Abstract

The long GRB 180728A, at a redshift of $z = 0.1171$, stands out due to its high isotropic energy of $E_{γ,iso} \sim 2.5 \times 10^{51}$ erg, in contrast with most events at redshift $z<0.2$. We analyze the properties of GRB 180728A's prompt emission, afterglow, and associated supernova SN 2018fip, comparing them with other GRB-SN events. This study employs a dense photometric and spectroscopic follow-up of the afterglow and the SN up to 80 days after the burst, supported by image subtraction to remove the presence of a nearby bright star, and modelling of both the afterglow and the supernova. GRB 180728A lies on the $E_{p,i}-E_{γ,iso}$ plane occupied by classical collapsar events, and the prompt emission is one of the most energetic at $z < 0.2$ after GRB 030329 and GRB 221009A. The afterglow of GRB 180728A is less luminous than that of most long GRBs, showing a shallow early phase that steepens around 5 hours (0.2 days). The GRB exploded in an irregular, low-mass, blue, star-forming galaxy, typical of low-z collapsar events. Because of the relatively faint afterglow, the light curve bump of SN 2018fip dominates the optical emission already after $\sim$3 days and is one of the best sampled to date. The strong suppression below $\sim$ 4000 angstrom and a largely featureless continuum in the early 6--9 days spectra favor aspherical two-component ejecta with a high-velocity collimated component ($> 20,000 km s^{-1}$), dominant early-on, and a more massive, low-velocity component, which dominates at much later epochs. Our findings indicate that asymmetries need to be considered in order to better understand GRB-SNe. In any case, SN 2018fip shares many characteristics with typical GRB-SNe. Its kinetic energy is below the common range of $10^{52}-10^{53}$ erg and does not correlate with the high energy of the GRB, highlighting the diversity of the GRB-SN energy budget partition.

Figures (21)

• Figure 1: $r$-band image of the optical afterglow of GRB 180728A obtained with the X-shooter acquisition camera on 2018-07-28, 0.23 days after the burst trigger. We highlight the position of the afterglow (AG) and of the nearby star, together with their projected distance. The rectangle shows the X-shooter slit with a position angle of 118.1 deg, which we selected to cover both the AG and the nearby star starting with the 4th observation at 6.26 days. The green contour lines indicate equal count levels.
• Figure 2: GRB 180728A (red) in the $E_\mathrm{p,i}-E_\mathrm{\gamma,iso}$ plane. GRBs with an associated SN are highlighted in green, outliers in blue. Dot-dashed lines indicate the $\pm2\sigma$ region. GRB data are from Amati2019a.
• Figure 3: Fit to the light curve and supernova of GRB 180728A. Magnitudes are in the AB system and corrected for Galactic foreground extinction, and time is in observer-frame. For reasons of clarity, light curves in each filter are offset by the magnitude values noted in the legend. Starting at $\approx300$ s, the light curve in all bands except for $H$ and $K$ is fitted with a broken power-law plus individual SN component. The dashed line for the $white$ filter is an extrapolation because the points at $>4$ days were not fitted. See text for details.
• Figure 4: X-ray to optical SED at 0.03, 0.1, 0.24 and 1.5 days. The last 2 epochs are well modelled by a joint-fit with a single power-law with spectral slope $\beta\sim0.7$. In the first 2 epochs the optical data has a slightly shallower spectral slope $\beta_{opt}\sim0.6$, but not the X-ray data, and needs a break that moves from $2.1$ to $2.6$ keV (with $\beta_{X}=\beta_{opt}+0.5$). The solid line is the unextinguished/unabsorbed model. The dashed line represents the extinguished/absorbed model, dominated by the Galactic foreground extinction in the optical/NIR bands and MW absorption in the X-rays.
• Figure 5: The Swift/XRT and optical light curves (MASTER CR band, and X-shooter, GROND $r^\prime$). Note that the optical data of the transient was obtained via image subtraction after 0.2 days, and therefore the contribution of the host is also subtracted. We have highlighted in cyan the epochs used in the optical-to-XRT SED fitting. Vertical lines highlight epochs with spectroscopic coverage. The dotted lines are the best fit of the optical light curve obtained in section \ref{['sect:lcana']}, which we have also normalized to the X-rays for data after 5 ks, i.e., excluding the first Swift orbit, which clearly shows that these early data are offset from the fit (see Sect.\ref{['sect:SED']}).