ABSTRACT Nebular-phase observations and spectral models of Type Ic superluminous supernovae (SLSNe) are presented. LSQ14an and SN 2015bn both display late-time spectra similar to galaxy-subtracted ...spectra of SN 2007bi, and the class shows strong similarity with broad-lined SNe Ic such as SN 1998bw. Near-infrared observations of SN 2015bn show a strong Ca ii triplet, O i 9263, O i 1.13 m, and Mg i 1.50 m, but no distinct He, Si, or S emission. The high Ca ii NIR/Ca ii 7291, 7323 ratio of ∼2 indicates a high electron density of cm−3. Spectral models of oxygen-zone emission are investigated to put constraints on the emitting region. Models require M to produce enough O i 6300, 6364 luminosity, irrespective of the powering situation and the density. The high oxygen-zone mass, supported by high estimated magnesium masses, points to explosions of massive CO cores, requiring . Collisions of pair-instability pulsations do not provide enough mass to account for the emission. O ii and O iii lines emerge naturally in many models, which strengthens the identification of broad O ii 7320, 7330, O iii 4363, and O iii 4959, 5007 in some spectra. A small filling factor for the O/Mg zone is needed to produce enough luminosity in Mg i 4571, Mg i 1.504 m, and O i recombination lines, which shows that the ejecta is clumped. We review the constraints from the nebular spectral modeling in the context of the various scenarios proposed for SLSNe.
Abstract
The late-time spectra of Type Ia supernovae (SNe Ia) are powerful probes of the underlying physics of their explosions. We investigate the late-time optical and near-infrared spectra of ...seven SNe Ia obtained at the VLT with XShooter at >200 d after explosion. At these epochs, the inner Fe-rich ejecta can be studied. We use a line-fitting analysis to determine the relative line fluxes, velocity shifts, and line widths of prominent features contributing to the spectra (Fe ii, Ni ii, and Co iii). By focusing on Fe ii and Ni ii emission lines in the ∼7000–7500 Å region of the spectrum, we find that the ratio of stable Ni ii to mainly radioactively produced Fe ii for most SNe Ia in the sample is consistent with Chandrasekhar-mass delayed-detonation explosion models, as well as sub-Chandrasekhar mass explosions that have metallicity values above solar. The mean measured Ni/Fe abundance of our sample is consistent with the solar value. The more highly ionized Co iii emission lines are found to be more centrally located in the ejecta and have broader lines than the Fe ii and Ni ii features. Our analysis also strengthens previous results that SNe Ia with higher
Si ii velocities at maximum light preferentially display blueshifted Fe ii 7155 Å lines at late times. Our combined results lead us to speculate that the majority of normal SN Ia explosions produce ejecta distributions that deviate significantly from spherical symmetry.
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.
ABSTRACT
Using the stokes Monte Carlo radiative transfer code, we revisit the predictions of the spectropolarimetric signal from a disc-like broad emission line region (BLR) in type I active galactic ...nuclei due to equatorial scattering. We reproduce the findings of previous works, but only for a scatterer that is much more optically and geometrically thick than previously proposed. We also find that when taking into account the polarized emission from all regions of the scatterer, the swing of the polarization position angle (PA) is in the opposite direction to that originally proposed. Furthermore, we find that the presence of outflows in the scattering media can significantly change the observed line profiles, with the PA of the scattering signal being enhanced in the presence of radially outflowing winds. Finally, a characteristically different PA profile, shaped like an ‘M’, is seen when the scatterer is co-spatial with the BLR and radially outflowing.
Abstract
We present near-IR (NIR) and optical observations of the Type Ic supernova (SN Ic) SN 2020oi in the galaxy M100 and the broad-lined SN Ic SN 2020bvc in UGC 9379, using Gemini, Las Cumbres ...Observatory, Southern Astrophysical Telescope, and other ground-based telescopes. The NIR spectrum of SN 2020oi at day 63 since the explosion shows strong CO emissions and a rising
K
-band continuum, which is the first unambiguous dust detection from an SN Ic. Non-LTE CO modeling shows that CO is still optically thick and that the lower limit to the CO mass is 10
−3
M
⊙
. The dust temperature is 810 K, and the dust mass is ∼10
−5
M
⊙
. We explore the possibilities that the dust is freshly formed in the ejecta, heated dust in the preexisting circumstellar medium, and an infrared echo. The light curves of SN 2020oi are consistent with a STELLA model with canonical explosion energy, 0.07
M
⊙
Ni mass, and 0.7
M
⊙
ejecta mass. A model of high explosion energy of 10
52
erg, 0.4
M
⊙
Ni mass, and 6.5
M
⊙
ejecta mass with the circumstellar matter reproduces the double-peaked light curves of SN 2020bvc. We observe temporal changes of absorption features of the IR Ca
ii
triplet, S
i
at 1.043
μ
m, and Fe
ii
at 5169 Å. The blueshifted lines indicate high velocities, up to 60,000 km s
−1
for SN 2020bvc and 20,000 km s
−1
for SN 2020oi, and the expansion velocity rapidly declines before the optical maximum. We present modeled spectral signatures and diagnostics of CO and SiO molecular bands between 1.4 and 10
μ
m.
We present nebular phase optical and near-infrared spectroscopy of the Type Ia supernova (SN) 2017cbv. The early light curves of SN 2017cbv showed a prominent blue bump in the U, B, and g bands ...lasting for ∼5 days. One interpretation of the early light curve is that the excess blue light is due to shocking of the SN ejecta against a nondegenerate companion star-a signature of the single degenerate scenario. If this is the correct interpretation, the interaction between the SN ejecta and the companion star could result in significant H (or helium) emission at late times, possibly along with other species, depending on the companion star and its orbital separation. A search for H emission in our +302 d spectrum yields a nondetection, with a LH < 8.0 × 1035 erg s−1 (given an assumed distance of D = 12.3 Mpc), which we verified by implanting simulated H emission into our data. We make a quantitative comparison to models of swept-up material stripped from a nondegenerate companion star and limit the mass of hydrogen that might remain undetected to MH < 1 × 10−4 M . A similar analysis of helium star related lines yields a MHe < 5 × 10−4 M . Taken at face value, these results argue against a nondegenerate H- or He-rich companion in Roche lobe overflow as the progenitor of SN 2017cbv. Alternatively, there could be weaknesses in the envelope-stripping and radiative transfer models necessary to interpret the strong H and He flux limits.
Abstract
We use the Gaia EDR3 to explore the Galactic supernova remnant (SNR) G272.2-3.2, produced by the explosion of a Type Ia supernova (SN Ia) about 7500 yr ago, to search for a surviving ...companion. From the abundances in the SNR ejecta, G272.2-3.2 is a normal SN Ia. The Gaia parallaxes allow us to select the stars located within the estimated distance range of the SNR, and the Gaia proper motions allow us to study their kinematics. From the Gaia EDR3 photometry, we construct the H-R diagram of the selected sample, which we compare with the theoretical predictions for the evolution of possible star companions of SNe Ia. We can discard several proposed types of companions by combining kinematics and photometry. We can also discard hypervelocity stars. We focus our study on the kinematically most peculiar star, Gaia EDR3 5323900215411075328 (hereafter MV-G272), an 8.9
σ
outlier in proper motion. It is of M1–M2 stellar type. Its trajectory on the sky locates it at the center of the SNR, 6000–8000 yr ago, a unique characteristic among the sample. Spectra allow a stellar parameter determination and a chemical abundance analysis. In conclusion, we have a candidate to be the surviving companion of the SN Ia that resulted in SNR G272.2-3.2. It is supported by its kinematical characteristics and its trajectory within the SNR. This opens the possibility of a single-degenerate scenario for an SN Ia with an M-type dwarf companion.
We present our statistical study of near-infrared (NIR) variability of X-ray-selected active galactic nuclei (AGNs) in the COSMOS field, using UltraVISTA data. This is the largest sample of AGN light ...curves in YJHKs bands, making it possible to have a global description of the nature of AGNs for a large range of redshifts and for different levels of obscuration. To characterize the variability properties of the sources, we computed the structure function. Our results show that there is an anticorrelation between the structure function A parameter (variability amplitude) and the wavelength of emission and a weak anticorrelation between A and the bolometric luminosity. We find that broad-line (BL) AGNs have a considerably larger fraction of variable sources than narrow-line (NL) AGNs and that they have different distributions of the A parameter. We find evidence that suggests that most of the low-luminosity variable NL sources correspond to BL AGNs, where the host galaxy could be damping the variability signal. For high-luminosity variable NL sources, we propose that they can be examples of "true type II" AGNs or BL AGNs with limited spectral coverage, which results in missing the BL emission. We also find that the fraction of variable sources classified as unobscured in the X-ray is smaller than the fraction of variable sources unobscured in the optical range. We present evidence that this is related to the differences in the origin of the obscuration in the optical and X-ray regimes.
ABSTRACT We present observations of SN 2015bn (=PS15ae = CSS141223-113342+004332 = MLS150211-113342+004333), a Type I superluminous supernova (SLSN) at redshift z = 0.1136. As well as being one of ...the closest SLSNe I yet discovered, it is intrinsically brighter ( ) and in a fainter galaxy ( ) than other SLSNe at . We used this opportunity to collect the most extensive data set for any SLSN I to date, including densely sampled spectroscopy and photometry, from the UV to the NIR, spanning −50 to +250 days from optical maximum. SN 2015bn fades slowly, but exhibits surprising undulations in the light curve on a timescale of 30-50 days, especially in the UV. The spectrum shows extraordinarily slow evolution except for a rapid transformation between +7 and +20-30 days. No narrow emission lines from slow-moving material are observed at any phase. We derive physical properties including the bolometric luminosity, and find slow velocity evolution and non-monotonic temperature and radial evolution. A deep radio limit rules out a healthy off-axis gamma-ray burst, and places constraints on the pre-explosion mass loss. The data can be consistently explained by a M stripped progenitor exploding with erg kinetic energy, forming a magnetar with a spin-down timescale of ∼20 days (thus avoiding a gamma-ray burst) that reheats the ejecta and drives ionization fronts. The most likely alternative scenario-interaction with ∼20 M of dense, inhomogeneous circumstellar material-can be tested with continuing radio follow-up.
We present photometry and time series spectroscopy of the nearby Type Ia supernova (SN Ia) SN 2015F over -16 d to +80 d relative to maximum light, obtained as part of the Public ESO Spectroscopic ...Survey of Transient Objects. SN 2015F is a slightly sub-luminous SN Ia with a decline rate of ...m sub( 15)(B) = 1.35 plus or minus 0.03 mag, placing it in the region between normal and SN 1991bg-like events. Our densely sampled photometric data place tight constraints on the epoch of first light and form of the early-time light curve. The spectra exhibit photospheric C ii ...6580 absorption until -4 days, and high-velocity Ca ii is particularly strong at <-10 d at expansion velocities of ...23 000 km s super( -1). At early times, our spectral modelling with syn++ shows strong evidence for iron-peak elements (Fe ii, Cr ii, Ti ii, and V ii) expanding at velocities >14 000 km s super( -1), suggesting mixing in the outermost layers of the SN ejecta. Although unusual in SN Ia spectra, including V ii in the modelling significantly improves the spectral fits. Intriguingly, we detect an absorption feature at ~6800 A that persists until maximum light. Our favoured explanation for this line is photospheric Al ii, which has never been claimed before in SNe Ia, although detached high-velocity C ii material could also be responsible. In both cases, the absorbing material seems to be confined to a relatively narrow region in velocity space. The nucleosynthesis of detectable amounts of Al ii would argue against a low-metallicity white dwarf progenitor. We also show that this 6800 A feature is weakly present in other normal SN Ia events and common in the SN 1991bg-like sub-class. (ProQuest: ... denotes formulae/symbols omitted.)
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