We investigate the light-curve properties of a sample of 26 spectroscopically confirmed hydrogen-poor superluminous supernovae (SLSNe-I) in the Palomar Transient Factory survey. These events are ...brighter than SNe Ib/c and SNe Ic-BL, on average, by about 4 and 2 mag, respectively. The peak absolute magnitudes of SLSNe-I in rest-frame g band span −22 Mg −20 mag, and these peaks are not powered by radioactive 56Ni, unless strong asymmetries are at play. The rise timescales are longer for SLSNe than for normal SNe Ib/c, by roughly 10 days, for events with similar decay times. Thus, SLSNe-I can be considered as a separate population based on photometric properties. After peak, SLSNe-I decay with a wide range of slopes, with no obvious gap between rapidly declining and slowly declining events. The latter events show more irregularities (bumps) in the light curves at all times. At late times, the SLSN-I light curves slow down and cluster around the 56Co radioactive decay rate. Powering the late-time light curves with radioactive decay would require between 1 and 10 M of Ni masses. Alternatively, a simple magnetar model can reasonably fit the majority of SLSNe-I light curves, with four exceptions, and can mimic the radioactive decay of 56Co, up to ∼400 days from explosion. The resulting spin values do not correlate with the host-galaxy metallicities. Finally, the analysis of our sample cannot strengthen the case for using SLSNe-I for cosmology.
Abstract
A non-local-thermodynamic-equilibrium (NLTE) level population model of the first and second ionisation stages of iron, nickel and cobalt is used to fit a sample of XShooter optical + ...near-infrared (NIR) spectra of Type Ia supernovae (SNe Ia). From the ratio of the NIR lines to the optical lines limits can be placed on the temperature and density of the emission region. We find a similar evolution of these parameters across our sample. Using the evolution of the Fe ii 12 570 Å to 7 155 Å line as a prior in fits of spectra covering only the optical wavelengths we show that the 7200 Å feature is fully explained by Fe ii and Ni ii alone. This approach allows us to determine the abundance of Ni ii/Fe ii for a large sample of 130 optical spectra of 58 SNe Ia with uncertainties small enough to distinguish between Chandrasekhar mass (MCh) and sub-Chandrasekhar mass (sub-MCh) explosion models. We conclude that the majority (85%) of normal SNe Ia have a Ni/Fe abundance that is in agreement with predictions of sub-MCh explosion simulations of ∼Z⊙ progenitors. Only a small fraction (11%) of objects in the sample have a Ni/Fe abundance in agreement with MCh explosion models.
We present near-infrared (NIR) spectroscopy of the nearby supernova 2014J obtained ∼450 d after explosion. We detect the Ni II 1.939 μm line in the spectra indicating the presence of stable 58Ni in ...the ejecta. The stable nickel is not centrally concentrated but rather distributed as the iron. The spectra are dominated by forbidden Fe II and Co II lines. We used lines, in the NIR spectra, arising from the same upper energy levels to place constraints on the extinction from host galaxy dust. We find that that our data are in agreement with the high AV and low RV found in earlier studies from data near maximum light. Using a 56Ni mass prior from near maximum light γ-ray observations, we find 0.053 ± 0.018 M⊙ of stable nickel to be present in the ejecta. We find that the iron group features are redshifted from the host galaxy rest frame by ∼600 km s−1.
We obtained optical and near infrared spectra of Type Ia supernovae (SNe Ia) at epochs ranging from 224 to 496 days after the explosion. The spectra show emission lines from forbidden transitions of ...singly ionised iron and cobalt atoms. We used non-local thermodynamic equilibrium (NLTE) modelling of the first and second ionisation stages of iron, nickel, and cobalt to fit the spectra using a sampling algorithm allowing us to probe a broad parameter space. We derive velocity shifts, line widths, and abundance ratios for iron and cobalt. The measured line widths and velocity shifts of the singly ionised ions suggest a shared emitting region. Our data are fully compatible with radioactive 56Ni decay as the origin for cobalt and iron. We compare the measured abundance ratios of iron and cobalt to theoretical predictions of various SN Ia explosion models. These models include, in addition to 56Ni, different amounts of 57Ni and stable 54,56Fe. We can exclude models that produced only 54,56Fe or only 57Ni in addition to 56Ni. If we consider a model that has 56Ni, 57Ni, and 54,56Fe then our data imply that these ratios are 54,56Fe / 56Ni = 0.272 ± 0.086 and 57Ni / 56Ni = 0.032 ± 0.011.
ABSTRACT
Even though the electromagnetic counterpart AT2017gfo to the binary neutron star merger GW170817 is powered by the radioactive decay of r-process nuclei, only few tentative identifications ...of light r-process elements have been made so far. One of the major limitations for the identification of heavy nuclei is incomplete or missing atomic data. While substantial progress has been made on lanthanide atomic data over the last few years, for actinides there has been less emphasis, with the first complete set of opacity data only recently published. We perform atomic structure calculations of neodymium (Z = 60) as well as the corresponding actinide uranium (Z = 92). Using two different codes flexible atomic code (fac) and hartree–fock-relativistic (hfr) for the calculation of the atomic data, we investigate the accuracy of the calculated data (energy levels and electric dipole transitions) and their effect on kilonova opacities. For the fac calculations, we optimize the local central potential and the number of included configurations and use a dedicated calibration technique to improve the agreement between theoretical and available experimental atomic energy levels (AELs). For ions with vast amounts of experimental data available, the presented opacities agree quite well with previous estimations. On the other hand, the optimization and calibration method cannot be used for ions with only few available AELs. For these cases, where no experimental nor benchmarked calculations are available, a large spread in the opacities estimated from the atomic data obtained with the various atomic structure codes is observed. We find that the opacity of uranium is almost double the neodymium opacity.
This work presents the observations and analysis of ATLAS19dqr/SN 2019bkc, an extraordinary rapidly evolving transient event located in an isolated environment, tens of kiloparsecs from any likely ...host. Its light curves rise to maximum light in 5−6 d and then display a decline of Δm15 ∼ 5 mag. With such a pronounced decay, it has one of the most rapidly evolving light curves known for a stellar explosion. The early spectra show similarities to normal and “ultra-stripped” type Ic SNe, but the early nebular phase spectra, which were reached just over two weeks after explosion, display prominent calcium lines, marking SN 2019bkc as a Ca-rich transient. The Ca emission lines at this phase show an unprecedented and unexplained blueshift of 10 000–12 000 km s−1. Modelling of the light curve and the early spectra suggests that the transient had a low ejecta mass of 0.2−0.4 M⊙ and a low kinetic energy of (2−4) × 1050 erg, giving a specific kinetic energy Ek/Mej ∼ 1 1051 erg/M⊙. The origin of this event cannot be unambiguously defined. While the abundance distribution used to model the spectra marginally favours a progenitor of white dwarf origin through the tentative identification of Ar II, the specific kinetic energy, which is defined by the explosion mechanism, is found to be more similar to an ultra-stripped core-collapse events. SN 2019bkc adds to the diverse range of physical properties shown by Ca-rich events.
Context.
Type II supernovae offer a direct way of estimating distances via the expanding photosphere method, which is independent of the cosmic distance ladder. A Gaussian process-based method was ...recently introduced, allowing for a fast and precise modelling of spectral time series and placing accurate and computationally cheap Type II-based absolute distance determinations within reach.
Aims.
The goal of this work is to assess the internal consistency of this new modelling technique coupled with the distance estimation in an empirical way, using the spectral time series of supernova siblings, that is, supernovae that exploded in the same host galaxy.
Methods.
We used a recently developed spectral emulator code, trained on T
ARDIS
radiative transfer models that is capable of a fast maximum-likelihood parameter estimation and spectral fitting. After calculating the relevant physical parameters of supernovae, we applied the expanding photosphere method to estimate their distances. Finally, we tested the consistency of the obtained values by applying the formalism of Bayes factors.
Results.
The distances to four different host galaxies were estimated based on two supernovae in each. The distance estimates are not only consistent within the errors for each of the supernova sibling pairs, but in the case of two hosts, they are precise to better than 5%. The analysis also showed that the main limiting factor of this estimation is the number and quality of spectra available for the individual objects, rather than the physical differences of the siblings.
Conclusions.
Even though the literature data we used was not tailored to the requirements of our analysis, the agreement of the final estimates shows that the method is robust and is capable of inferring both precise and consistent distances. By using high-quality spectral time series, this method can provide precise distance estimates independent of the distance ladder, which are of high value for cosmology.
This work presents the observations and analysis of ATLAS19dqr/SN 2019bkc, an extraordinary rapidly evolving transient event located in an isolated environment, tens of kiloparsecs from any likely ...host. Its light curves rise to maximum light in 5−6 d and then display a decline of Δ
m
15
∼ 5 mag. With such a pronounced decay, it has one of the most rapidly evolving light curves known for a stellar explosion. The early spectra show similarities to normal and “ultra-stripped” type Ic SNe, but the early nebular phase spectra, which were reached just over two weeks after explosion, display prominent calcium lines, marking SN 2019bkc as a Ca-rich transient. The Ca emission lines at this phase show an unprecedented and unexplained blueshift of 10 000–12 000 km s
−1
. Modelling of the light curve and the early spectra suggests that the transient had a low ejecta mass of 0.2−0.4
M
⊙
and a low kinetic energy of (2−4) × 10
50
erg, giving a specific kinetic energy
E
k
/
M
ej
∼ 1 10
51
erg/
M
⊙
. The origin of this event cannot be unambiguously defined. While the abundance distribution used to model the spectra marginally favours a progenitor of white dwarf origin through the tentative identification of Ar
II
, the specific kinetic energy, which is defined by the explosion mechanism, is found to be more similar to an ultra-stripped core-collapse events. SN 2019bkc adds to the diverse range of physical properties shown by Ca-rich events.
We present the results of our photometric and spectroscopic follow-up of the intermediate-luminosity optical transient AT 2017jfs. At peak, the object reaches an absolute magnitude of Mg = −15.46 ± ...0.15 mag and a bolometric luminosity of 5.5 × 1041 erg s−1. Its light curve has the double-peak shape typical of luminous red novae (LRNe), with a narrow first peak bright in the blue bands, while the second peak is longer-lasting and more luminous in the red and near-infrared (NIR) bands. During the first peak, the spectrum shows a blue continuum with narrow emission lines of H and Fe II. During the second peak, the spectrum becomes cooler, resembling that of a K-type star, and the emission lines are replaced by a forest of narrow lines in absorption. About 5 months later, while the optical light curves are characterized by a fast linear decline, the NIR ones show a moderate rebrightening, observed until the transient disappears in solar conjunction. At these late epochs, the spectrum becomes reminiscent of that of M-type stars, with prominent molecular absorption bands. The late-time properties suggest the formation of some dust in the expanding common envelope or an IR echo from foreground pre-existing dust. We propose that the object is a common-envelope transient, possibly the outcome of a merging event in a massive binary, similar to NGC 4490−2011OT1.
Even though the electromagnetic counterpart AT2017gfo to the binary neutron star merger GW170817 is powered by the radioactive decay of r-process nuclei, only few tentative identifications of light ...r-process elements have been made so far. One of the major limitations for the identification of heavy nuclei is incomplete or missing atomic data. While substantial progress has been made on lanthanide atomic data over the last few years, for actinides there has been less emphasis, with the first complete set of opacity data only recently published. We perform atomic structure calculations of neodymium \((Z=60)\) as well as the corresponding actinide uranium \((Z=92)\). Using two different codes (FAC and HFR) for the calculation of the atomic data, we investigate the accuracy of the calculated data (energy levels and electric dipole transitions) and their effect on kilonova opacities. For the FAC calculations, we optimise the local central potential and the number of included configurations and use a dedicated calibration technique to improve the agreement between theoretical and available experimental atomic energy levels (AELs). For ions with vast amounts of experimental data available, the presented opacities agree quite well with previous estimations. On the other hand, the optimisation and calibration method cannot be used for ions with only few available AELs. For these cases, where no experimental nor benchmarked calculations are available, a large spread in the opacities estimated from the atomic data obtained with the various atomic structure codes is observed.We find that the opacity of uranium is almost double the neodymium opacity.
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