We present UV/optical observations of PS16aqv (SN 2016ard), a fast-evolving Type I superluminous supernova (SLSN-I) that reached a peak absolute magnitude of Mr −22.1. The lightcurves exhibit a ...significant undulation at 30 rest-frame days after peak, with a behavior similar to undulations seen in the slowly fading SLSN-I SN 2015bn. This similarity strengthens the case that fast and slow SLSNe-I form a continuum with a common origin. At 80 days after peak, the lightcurves exhibit a transition to a slow decline, followed by significant steepening, indicative of a plateau phase or a second significant undulation. Deep limits at 280 days after peak imply a tight constraint on the nickel mass, MNi 0.35 M (lower than for previous SLSNe-I), and indicate that some SLSNe-I do not produce significantly more nickel than normal Type Ic SNe. Using MOSFiT, we model the lightcurve with a magnetar central engine model and find Pspin 0.9 ms, B 1.5 × 1014 G, and Mej 16 M . The implied rapid spin-down time and large reservoir of available energy coupled with the high ejecta mass may account for the fast lightcurve and slow spectroscopic evolution. We also study PS16aqv's location within its host galaxy and find that it occurred at an offset of 2.46 0.21 kpc from the central star-forming region. Aside from high extinction, the host properties are similar to most other SLSN-I host galaxies. The complexity in the lightcurves of PS16aqv and other events highlights the importance of obtaining well-sampled lightcurves for exploring deviations from a uniform decline.
Abstract The existence of a secondary (in addition to compact object mergers) source of heavy element ( r -process) nucleosynthesis, the core-collapse of rapidly rotating and highly magnetized ...massive stars, has been suggested by both simulations and indirect observational evidence. Here, we probe a predicted signature of r -process enrichment, a late-time (≳40 days post-burst) distinct red color, in observations of gamma-ray burst supernovae (GRB-SNe), which are linked to these massive star progenitors. We present optical to near-IR color measurements of four GRB-SNe at z ≲ 0.4, extending out to >500 days post-burst, obtained with the Hubble Space Telescope and large-aperture ground-based telescopes. Comparison of our observations to models indicates that GRBs 030329, 100316D, and 130427A are consistent with both no enrichment and producing 0.01–0.15 M ⊙ of r -process material if there is a low amount of mixing between the inner r -process ejecta and outer supernova (SN) layers. GRB 190829A is not consistent with any models with r -process enrichment ≥0.01 M ⊙ . Taken together the sample of GRB-SNe indicates color diversity at late times. Our derived yields from GRB-SNe may be underestimated due to r -process material hidden in the SN ejecta (potentially due to low mixing fractions) or the limits of current models in measuring r -process mass. We conclude with recommendations for future search strategies to observe and probe the full distribution of r -process produced by GRB-SNe.
The existence of a secondary (in addition to compact object mergers) source of heavy element (\(r\)-process) nucleosynthesis, the core-collapse of rapidly-rotating and highly-magnetized massive ...stars, has been suggested by both simulations and indirect observational evidence. Here, we probe a predicted signature of \(r\)-process enrichment, a late-time (\(\gtrsim 40\) days post-burst) distinct red color, in observations of GRB-supernovae (GRB-SNe) which are linked to these massive star progenitors. We present optical to near-IR color measurements of four GRB-SNe at \(z \lesssim 0.4\), extending out to \(> 500\) days post-burst, obtained with the Hubble Space Telescope and large-aperture ground-based telescopes. Comparison of our observations to models indicates that GRBs 030329, 100316D and 130427A are consistent with both no enrichment and producing \(0.01 - 0.15 M_{\odot}\) of \(r\)-process material if there is a low amount of mixing between the inner \(r\)-process ejecta and outer SN layers. GRB 190829A is not consistent with any models with \(r\)-process enrichment \(\geq 0.01 M_{\odot}\). Taken together the sample of GRB-SNe indicates color diversity at late times. Our derived yields from GRB-SNe may be underestimated due to \(r\)-process material hidden in the SN ejecta (potentially due to low mixing fractions) or the limits of current models in measuring \(r\)-process mass. We conclude with recommendations for future search strategies to observe and probe the full distribution of \(r\)-process produced by GRB-SNe.
The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs), sources ...of high-frequency gravitational waves and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process). These heavy elements include some of great geophysical, biological and cultural importance, such as thorium, iodine and gold. Here we present observations of the exceptionally bright gamma-ray burst GRB 230307A. We show that GRB 230307A belongs to the class of long-duration gamma-ray bursts associated with compact object mergers, and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW170817. We obtained James Webb Space Telescope mid-infrared (mid-IR) imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns which we interpret as tellurium (atomic mass A=130), and a very red source, emitting most of its light in the mid-IR due to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy element nucleosynthesis across the Universe.
We present the results from multi-wavelength observations of a transient discovered during the follow-up of S191213g, a gravitational wave (GW) event reported by the LIGO-Virgo Collaboration as a ...possible binary neutron star merger in a low latency search. This search yielded SN2019wxt, a young transient in a galaxy whose sky position (in the 80\% GW contour) and distance (\(\sim\)150\,Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transient's tightly constrained age, its relatively faint peak magnitude (\(M_i \sim -16.7\)\,mag) and the \(r-\)band decline rate of \(\sim 1\)\,mag per 5\,days appeared suggestive of a compact binary merger. However, SN2019wxt spectroscopically resembled a type Ib supernova, and analysis of the optical-near-infrared evolution rapidly led to the conclusion that while it could not be associated with S191213g, it nevertheless represented an extreme outcome of stellar evolution. By modelling the light curve, we estimated an ejecta mass of \(\sim 0.1\,M_\odot\), with \(^{56}\)Ni comprising \(\sim 20\%\) of this. We were broadly able to reproduce its spectral evolution with a composition dominated by helium and oxygen, with trace amounts of calcium. We considered various progenitors that could give rise to the observed properties of SN2019wxt, and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling electromagnetic counterparts to GW events from transients such as SN2019wxt is challenging: in a bid to characterise the level of contamination, we estimated the rate of events with properties comparable to those of SN2019wxt and found that \(\sim 1\) such event per week can occur within the typical GW localisation area of O4 alerts out to a luminosity distance of 500\,Mpc, beyond which it would become fainter than the typical depth of current electromagnetic follow-up campaigns.
We present JWST and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A, the brightest gamma-ray burst (GRB) ever observed. This includes the first mid-IR spectra of any GRB, ...obtained with JWST/NIRSPEC (0.6-5.5 micron) and MIRI (5-12 micron), 12 days after the burst. Assuming that the intrinsic spectral slope is a single power-law, with \(F_{\nu} \propto \nu^{-\beta}\), we obtain \(\beta \approx 0.35\), modified by substantial dust extinction with \(A_V = 4.9\). This suggests extinction above the notional Galactic value, possibly due to patchy extinction within the Milky Way or dust in the GRB host galaxy. It further implies that the X-ray and optical/IR regimes are not on the same segment of the synchrotron spectrum of the afterglow. If the cooling break lies between the X-ray and optical/IR, then the temporal decay rates would only match a post jet-break model, with electron index \(p<2\), and with the jet expanding into a uniform ISM medium. The shape of the JWST spectrum is near-identical in the optical/nIR to X-shooter spectroscopy obtained at 0.5 days and to later time observations with HST. The lack of spectral evolution suggests that any accompanying supernova (SN) is either substantially fainter or bluer than SN 1998bw, the proto-type GRB-SN. Our HST observations also reveal a disc-like host galaxy, viewed close to edge-on, that further complicates the isolation of any supernova component. The host galaxy appears rather typical amongst long-GRB hosts and suggests that the extreme properties of GRB 221009A are not directly tied to its galaxy-scale environment.
Abridged We present UV/optical observations of PS16aqv (SN 2016ard), a Type I superluminous supernova (SLSN-I) classified as part of our search for low-\(z\) SLSNe. PS16aqv is a fast evolving SLSNe-I ...that reached a peak absolute magnitude of \(M_{r} \approx -22.1\). The lightcurves exhibit a significant undulation at 30 rest-frame days after peak, with a behavior similar to undulations seen in the slowly fading SLSN-I SN 2015bn. This similarity strengthens the case that fast and slow SLSNe-I form a continuum with a common origin. At \(\approx\!80\) days after peak, the lightcurves exhibit a transition to a slow decline, followed by significant subsequent steepening, indicative of a plateau phase or a second significant undulation. Deep limits at \(\approx280\) days after peak imply a tight constraint on the nickel mass, \(M_{\rm Ni} \lesssim 0.35\) M\(_{\odot}\) (lower than for previous SLSNe-I), and indicate that some SLSNe-I do not produce significantly more nickel than normal Type Ic SNe. Using MOSFiT, we model the lightcurve with a magnetar central engine model and find \(P_{\rm spin} \approx 0.9\) ms, \(B \approx 1.5 \times 10^{14}\) G, and \(M_{\rm ej} \approx 16\) M\(_{\odot}\). The implied rapid spin-down time and large reservoir of available energy coupled with the high ejecta mass may account for the fast evolving lightcurve and slow spectroscopic evolution. We also study the location of PS16aqv in its host galaxy and find that it occurred at an offset of \(2.46 \pm 0.21\) kpc from the central star-forming region. We find the host galaxy exhibits low metallicity and spatially varying extinction and star formation rate, with the explosion site exhibiting lower values than the central region. The complexity seen in the lightcurves of PS16aqv and other events highlights the importance of obtaining well-sampled lightcurves for exploring deviations from a uniform decline.
We present the first observations of a Type I superluminous supernova (SLSN) at 1000 days after maximum light. We observed SN 2015bn using the Hubble Space Telescope (HST) Advanced Camera for Surveys ...in the F475W, F625W and F775W filters at 721 days and 1068 days. SN 2015bn is clearly detected and resolved from its compact host, allowing reliable photometry. A galaxy template constructed from these data further enables us to isolate the SLSN flux in deep ground-based imaging. We measure a light curve decline rate at >700 days of 0.19 0.03 mag(100 d)−1, much shallower than the earlier evolution, and slower than previous SLSNe (at any phase) or the decay rate of 56Co. Neither additional radioactive isotopes nor a light echo can consistently account for the slow decline. A spectrum at 1083 days shows the same O i λ6300 and Ca ii λ7300 lines as seen at ∼300-400 days, with no new features to indicate strong circumstellar interaction. Radio limits with the Very Large Array rule out an extended wind for mass-loss rates yr−1 (where v10 is the wind velocity in units of 10 km s−1). The optical light curve is consistent with L ∝ t−4, which we show is expected for magnetar spin-down with inefficient trapping; furthermore, the evolution matches predictions from earlier magnetar model fits. The opacity to magnetar radiation is constrained at ∼0.01 cm2 g−1, consistent with photon-matter pair-production over a broad ∼GeV-TeV range. This suggests that the magnetar spectral energy distribution, and hence the "missing energy" leaking from the ejecta, may peak in this range.
The interaction of a supernova with a circumstellar medium (CSM) can dramatically increase the emitted luminosity by converting kinetic energy to thermal energy. In 'superluminous' supernovae (SLSNe) ...of Type IIn -- named for narrow hydrogen lines in their spectra -- the integrated emission can reach \(\sim 10^{51}\) erg, attainable by thermalising most of the kinetic energy of a conventional SN. A few transients in the centres of active galaxies have shown similar spectra and even larger energies, but are difficult to distinguish from accretion onto the supermassive black hole. Here we present a new event, SN2016aps, offset from the centre of a low-mass galaxy, that radiated \(\gtrsim 5 \times 10^{51}\) erg, necessitating a hyper-energetic supernova explosion. We find a total (SN ejecta \(+\) CSM) mass likely exceeding 50-100 M\(_\odot\), with energy \(\gtrsim 10^{52}\) erg, consistent with some models of pair-instability supernovae (PISNe) or pulsational PISNe -- theoretically-predicted thermonuclear explosions from helium cores \(>50\) M\(_\odot\). Independent of the explosion mechanism, this event demonstrates the existence of extremely energetic stellar explosions, detectable at very high redshifts, and provides insight into dense CSM formation in the most massive stars.
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