We examine the early phase intrinsic (B − V)0 color evolution of a dozen SNe Ia discovered within three days of the inferred time of first light (tfirst) and have (B − V)0 color information beginning ...within five days of tfirst. The sample indicates there are two distinct early populations. The first is a population exhibiting blue colors that slowly evolve, and the second population exhibits red colors and evolves more rapidly. We find that the early blue events are all 1991T/1999aa-like with more luminous, slower declining light curves than those exhibiting early red colors. Placing the first sample on the Branch diagram (i.e., ratio of Si ii λλ5972, 6355 pseudo-Equivalent widths) indicates that all blue objects are of the Branch shallow silicon (SS) spectral type, while all early red events except for the 2000cx-like SN 2012fr are of the Branch Core Normal (CN) or CooL (CL) type. A number of potential processes contributing to the early emission are explored, and we find that, in general, the viewing-angle dependance inherent in the companion collision model is inconsistent with all of the SS objects with early-time observations being blue and exhibiting an excess. We caution that great care must be taken when interpreting early phase light curves as there may be a variety of physical processes that are possibly at play and significant theoretical work remains to be done.
Hydrogen-poor superluminous supernovae (SLSNe-I) have been predominantly found in low-metallicity, star-forming dwarf galaxies. Here we identify Gaia17biu/SN 2017egm as an SLSN-I occurring in a ..."normal" spiral galaxy (NGC 3191) in terms of stellar mass (several times 1010 M ) and metallicity (roughly solar). At redshift z = 0.031, Gaia17biu is also the lowest-redshift SLSN-I to date, and the absence of a larger population of SLSNe-I in dwarf galaxies of similar redshift suggests that metallicity is likely less important to the production of SLSNe-I than previously believed. With the smallest distance and highest apparent brightness for an SLSN-I, we are able to study Gaia17biu in unprecedented detail. Its pre-peak near-ultraviolet to optical color is similar to that of Gaia16apd and among the bluest observed for an SLSN-I, while its peak luminosity (Mg = −21 mag) is substantially lower than that of Gaia16apd. Thanks to the high signal-to-noise ratios of our spectra, we identify several new spectroscopic features that may help to probe the properties of these enigmatic explosions. We detect polarization at the ∼0.5% level that is not strongly dependent on wavelength, suggesting a modest, global departure from spherical symmetry. In addition, we put the tightest upper limit yet on the radio luminosity of an SLSN-I with <5.4 × 1026 erg s−1 Hz−1 at 10 GHz, which is almost a factor of 40 better than previous upper limits and one of the few measured at an early stage in the evolution of an SLSN-I. This limit largely rules out an association of this SLSN-I with known populations of gamma-ray-burst-like central engines.
Abstract
We present five far- and near-ultraviolet spectra of the Type II plateau supernova, SN 2022acko, obtained 5, 6, 7, 19, and 21 days after explosion, all observed with the Hubble Space ...Telescope/Space Telescope Imaging Spectrograph. The first three epochs are earlier than any Type II plateau supernova has been observed in the far-ultraviolet revealing unprecedented characteristics. These three spectra are dominated by strong lines, primarily from metals, which contrasts with the featureless early optical spectra. The flux decreases over the initial time series as the ejecta cool and line blanketing takes effect. We model this unique data set with the non–local thermodynamic equilibrium radiation transport code
CMFGEN
, finding a good match to the explosion of a low-mass red supergiant with energy
E
kin
= 6 × 10
50
erg. With these models we identify, for the first time, the ions that dominate the early ultraviolet spectra. We present optical photometry and spectroscopy, showing that SN 2022acko has a peak absolute magnitude of
V
= − 15.4 mag and plateau length of ∼115 days. The spectra closely resemble those of SN 2005cs and SN 2012A. Using the combined optical and ultraviolet spectra, we report the fraction of flux as a function of bluest wavelength on days 5, 7, and 19. We create a spectral time-series of Type II supernovae in the ultraviolet, demonstrating the rapid decline of flux over the first few weeks of evolution. Future observations of Type II supernovae are required to map out the landscape of exploding red supergiants, with and without circumstellar material, which is best revealed in high-quality ultraviolet spectra.
Abstract
When discovered, SN 2017egm was the closest (redshift
z
= 0.03) hydrogen-poor superluminous supernova (SLSN-I) and a rare case that exploded in a massive and metal-rich galaxy. Thus, it has ...since been extensively observed and studied. We report spectroscopic data showing strong emission at around He
i
λ
10830 and four He
i
absorption lines in the optical. Consequently, we classify SN 2017egm as a member of an emerging population of helium-rich SLSNe-I (i.e., SLSNe-Ib). We also present our late-time photometric observations. By combining them with archival data, we analyze high-cadence ultraviolet, optical, and near-infrared light curves spanning from early pre-peak (∼−20 days) to late phases (∼+300 days). We obtain its most complete bolometric light curve, in which multiple bumps are identified. None of the previously proposed models can satisfactorily explain all main light-curve features, while multiple interactions between the ejecta and circumstellar material (CSM) may explain the undulating features. The prominent infrared excess with a blackbody luminosity of 10
7
–10
8
L
⊙
detected in SN 2017egm could originate from the emission of either an echo of a pre-existing dust shell or newly formed dust, offering an additional piece of evidence supporting the ejecta–CSM interaction model. Moreover, our analysis of deep Chandra observations yields the tightest-ever constraint on the X-ray emission of an SLSN-I, amounting to an X-ray-to-optical luminosity ratio ≲10
−3
at late phases (∼100–200 days), which could help explore its close environment and central engine.
Abstract
SN 2017dio shows both spectral characteristics of a type-Ic supernova (SN) and signs of a hydrogen-rich circumstellar medium (CSM). Prominent, narrow emission lines of H and He are ...superposed on the continuum. Subsequent evolution revealed that the SN ejecta are interacting with the CSM. The initial SN Ic identification was confirmed by removing the CSM interaction component from the spectrum and comparing with known SNe Ic and, reversely, adding a CSM interaction component to the spectra of known SNe Ic and comparing them to SN 2017dio. Excellent agreement was obtained with both procedures, reinforcing the SN Ic classification. The light curve constrains the pre-interaction SN Ic peak absolute magnitude to be around
M
g
=
−
17.6
mag. No evidence of significant extinction is found, ruling out a brighter luminosity required by an SN Ia classification. These pieces of evidence support the view that SN 2017dio is an SN Ic, and therefore the first firm case of an SN Ic with signatures of hydrogen-rich CSM in the early spectrum. The CSM is unlikely to have been shaped by steady-state stellar winds. The mass loss of the progenitor star must have been intense,
M
˙
∼
0.02
(
ϵ
H
α
/
0.01
)
−
1
(
v
wind
/
500
km s
−1
)
(
v
shock
/
10,000 km s
−1
)
−3
M
⊙
yr
−1
, peaking at a few decades before the SN. Such a high mass-loss rate might have been experienced by the progenitor through eruptions or binary stripping.
Type Ia supernovae arise from the thermonuclear explosion of white-dwarf stars that have cores of carbon and oxygen. The uniformity of their light curves makes these supernovae powerful cosmological ...distance indicators, but there have long been debates about exactly how their explosion is triggered and what kind of companion stars are involved. For example, the recent detection of the early ultraviolet pulse of a peculiar, subluminous type Ia supernova has been claimed as evidence for an interaction between a red-giant or a main-sequence companion and ejecta from a white-dwarf explosion. Here we report observations of a prominent but red optical flash that appears about half a day after the explosion of a type Ia supernova. This supernova shows hybrid features of different supernova subclasses, namely a light curve that is typical of normal-brightness supernovae, but with strong titanium absorption, which is commonly seen in the spectra of subluminous ones. We argue that this early flash does not occur through previously suggested mechanisms such as the companion-ejecta interaction. Instead, our simulations show that it could occur through detonation of a thin helium shell either on a near-Chandrasekhar-mass white dwarf, or on a sub-Chandrasekhar-mass white dwarf merging with a less-massive white dwarf. Our finding provides evidence that one branch of previously proposed explosion models-the helium-ignition branch-does exist in nature, and that such a model may account for the explosions of white dwarfs in a mass range wider than previously supposed.
Abstract
We present observations of ASASSN-20hx, a nearby ambiguous nuclear transient (ANT) discovered in NGC 6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We observed ASASSN-20hx ...from −30 to 275 days relative to the peak UV/optical emission using high-cadence, multiwavelength spectroscopy and photometry. From Transiting Exoplanet Survey Satellite data, we determine that the ANT began to brighten on 2020 June 22.8 with a linear rise in flux for at least the first week. ASASSN-20hx peaked in the UV/optical 30 days later on 2020 July 22.8 (MJD = 59052.8) at a bolometric luminosity of
L
= (3.15 ± 0.04) × 10
43
erg s
−1
. The subsequent decline is slower than any TDE observed to date and consistent with many other ANTs. Compared to an archival X-ray detection, the X-ray luminosity of ASASSN-20hx increased by an order of magnitude to
L
x
∼ 1.5 × 10
42
erg s
−1
and then slowly declined over time. The X-ray emission is well fit by a power law with a photon index of Γ ∼ 2.3–2.6. Both the optical and near-infrared spectra of ASASSN-20hx lack emission lines, unusual for any known class of nuclear transient. While ASASSN-20hx has some characteristics seen in both tidal disruption events and active galactic nuclei, it cannot be definitively classified with current data.
Abstract
In this paper, we present photometric and spectroscopic observations of the subluminous Type Ia supernova (SN Ia) 2012ij, which has an absolute
B
-band peak magnitude
M
B
,
max
=
−
17.95
±
...0.15
mag. The
B
-band light curve exhibits a fast postpeak decline with Δ
m
15
(
B
) = 1.86 ± 0.05 mag. All the
R
- and
I
/
i
-band light curves show a weak secondary peak/shoulder feature at about 3 weeks after the peak, like some transitional subclass of SNe Ia, which could result from an incomplete merger of near-infrared (NIR) double peaks. The spectra are characterized by Ti
ii
and strong Si
ii
λ
5972 absorption features that are usually seen in low-luminosity objects like SN 1999by. The NIR spectrum before maximum light reveals weak carbon absorption features, implying the existence of unburned materials. We compare the observed properties of SN 2012ij with those predicted by the sub-Chandrasekhar-mass and the Chandrasekhar-mass delayed-detonation models and find that both optical and NIR spectral properties can be explained to some extent by these two models. By comparing the secondary maximum features in the
I
and
i
bands, we suggest that SN 2012ij is a transitional object linking normal SNe Ia to typical 91bg-like ones. From the published sample of SNe Ia from the Carnegie Supernova Project II, we estimate that the fraction of SN 2012ij–like SNe Ia is not lower than ∼2%.
We present photometric and spectroscopic observations of the interacting transient SN 2009ip taken during the 2013 and 2014 observing seasons. We characterize the photometric evolution as a steady ...and smooth decline in all bands, with a decline rate that is slower than expected for a solely 56Co-powered supernova at late phases. No further outbursts or eruptions were seen over a two year period from 2012 December until 2014 December. SN 2009ip remains brighter than its historic minimum from pre-discovery images. Spectroscopically, SN 2009ip continues to be dominated by strong, narrow (≲2000 km s− 1) emission lines of H, He, Ca, and Fe. While we make tenuous detections of Fe ii λ7155 and O i λλ6300, 6364 lines at the end of 2013 June and the start of 2013 October, respectively, we see no strong broad nebular emission lines that could point to a core-collapse origin. In general, the lines appear relatively symmetric, with the exception of our final spectrum in 2014 May, when we observe the appearance of a redshifted shoulder of emission at +550 km s− 1. The lines are not blueshifted, and we see no significant near- or mid-infrared excess. From the spectroscopic and photometric evolution of SN 2009ip until 820 d after the start of the 2012a event, we still see no conclusive evidence for core-collapse, although whether any such signs could be masked by ongoing interaction is unclear.
We present observations of ASASSN-20hx, a nearby ambiguous nuclear transient (ANT) discovered in NGC 6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We observed ASASSN-20hx from -30 to ...275 days relative to the peak UV/optical emission using high-cadence, multiwavelength spectroscopy and photometry. From Transiting Exoplanet Survey Satellite data, we determine that the ANT began to brighten on 2020 June 22.8 with a linear rise in flux for at least the first week. ASASSN-20hx peaked in the UV/optical 30 days later on 2020 July 22.8 (MJD = 59052.8) at a bolometric luminosity ofL= (3.15 ± 0.04) × 1043erg s-1. The subsequent decline is slower than any TDE observed to date and consistent with many other ANTs. Compared to an archival X-ray detection, the X-ray luminosity of ASASSN-20hx increased by an order of magnitude to Lx ~ 1.5 × 1042erg s-1and then slowly declined over time. The X-ray emission is well fit by a power law with a photon index of Γ ~ 2.3–2.6. Both the optical and near-infrared spectra of ASASSN-20hx lack emission lines, unusual for any known class of nuclear transient. While ASASSN-20hx has some characteristics seen in both tidal disruption events and active galactic nuclei, it cannot be definitively classified with current data.
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