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ABSTRACT We analyze the standardizability of Type Ia supernovae (SNe Ia) in the near-infrared (NIR) by investigating the correlation between observed peak NIR (YJH) absolute magnitude and postmaximum B-band decline rate [Δm15(B)]. A sample of 27 low-redshift SNe Ia with well-observed NIR light curves observed by the Carnegie Supernova Project (CSP) between 2004 and 2007 is used. All 27 objects have premaximum coverage in optical bands, with a subset of 13 having premaximum NIR observations as well; coverage of the other 14 begins shortly after NIR maximum brightness. We describe the methods used to derive light-curve parameters (absolute peak magnitudes and decline rates) from both spline- and template-fitting procedures, and we confirm prior findings that fitting templates to SNe Ia light curves in the NIR is problematic due to the diversity of postmaximum behavior of objects that are characterized by similar Δm15(B) values, especially at high decline rates. Nevertheless, we show that NIR light curves can be reasonably fit with a template, especially if the observations begin within 5 days after NIR maximum. SNe Ia appear to be better \"standardizable candles\" in the NIR bands than in the optical bands. For the subset of 13 objects in our data set that excludes the highly reddened and fast-declining SNe Ia and includes only those objects for which NIR observations began prior to 5 days after maximum light, we find modest (1.7σ) evidence for a peak-luminosity versus decline-rate relation in Y, and stronger evidence (2.8σ) in J and H. Using RV values differing from the canonical value (RV = 3.1) is shown to have little effect on the results. A Hubble diagram is presented for the NIR bands and the B band. The resulting scatter for the combined NIR bands is 0.13 mag, while the B band produces a scatter of 0.22 mag. Finally, we find evidence for a bimodal distribution in the NIR absolute magnitudes of fast-declining SNe Ia [Δm15(B) > 1.7]. These data suggest that applying a correction to SNe Ia peak luminosities for decline rate is likely to be beneficial in the J and H bands to make SNe Ia more precise distance indicators, but of only marginal importance in the Y band.

The Neil Gehrels Swift Observatory has proven to be an extraordinary supernova (SN) observatory. The clearest application of Swift’s unique strengths is obtaining very early UV and X-ray data of young SNe, which enables robust constraints on their progenitor systems. As part of a year-long Swift Guest Investigator Key Project, we initiated a follow-up program to rapidly observe all of the nearest (distance < 35 Mpc or roughly z < 0.008) extragalactic transients without waiting for them to be spectroscopically classified as supernovae. Among the possible results were to measure any UV-bright radiative cooling following the shock breakout from core-collapse SNe and shock emission from the interaction of thermonuclear Type Ia SNe with a non-degenerate companion. Just as importantly, uniformly following up and analyzing a significant sample can constrain the fraction of events for which the shock emission is not seen. Here we present the UV and X-ray measurements performed during our campaign. Our sample of 24 observed triggers included three SNe Ia, six SNe II, three stripped-envelope, core-collapse SNe, five galactic transients, three extragalactic SN imposters, and four unconfirmed transients. For our sample, the median delay time from the discovery image to the first Swift image was 1.45 days. We tabulate the X-ray upper limits and find they are sufficiently deep to have detected objects as X-ray luminous as GRB060218/SN2006aj. Other X-ray-detected SNe such as SNe 2006bp, 2008D, and 2011dh would have been detectable in some of the observations. We highlight the spectroscopically classified Type II SN 2018hna with UV-optical light curves indicating a luminosity and flux evolution very similar to SN 1987A.

The detection and simulation of a type Ia supernova with an early, red flash suggests that it formed through detonation of the helium shell of a white dwarf, rather than by collision of the ejecta with a companion star or by merging with another white dwarf. A different kind of supernova Type Ia supernovae have rather uniform and normalizable light curves, making them suitable for cosmology, yet there remains uncertainty over what paths lead to the explosion. Several years ago a claim was made that a flash seen soon after the explosion was evidence of the shock wave hitting a normal companion star, although most other evidence so far suggests that the explosions arise from the merger of two white dwarfs. Ji-an Jiang and collaborators report observations of a red flash half a day after a type Ia explosion. Their observations lead them to the conclusion that the flash came from the detonation of a thin helium shell surrounding the exploding star. The authors conclude that their finding supports the existence of the previously proposed helium-ignition pathway. Type Ia supernovae arise from the thermonuclear explosion of white-dwarf stars that have cores of carbon and oxygen 1 , 2 . The uniformity of their light curves makes these supernovae powerful cosmological distance indicators 3 , 4 , but there have long been debates about exactly how their explosion is triggered and what kind of companion stars are involved 2 , 5 , 6 . 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 7 , 8 . 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 8 , 9 , 10 . 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 11 , 12 , 13 , 14 .

The detection of the luminous, blue progenitor system of the type Iax supernova 2012Z suggests that this supernova was the explosion of a white dwarf accreting material from a helium-star companion. A type Iax supernova progenitor SN 2012Z, discovered in the Lick Observatory Supernova Search on 29 January 2012, is a type Iax supernova. Sometimes referred to as 'mini supernovae', these are initially spectroscopically similar to some type-Ia supernovae but diverge with time and are much less energetic and fainter. It is not clear what triggers a type Iax explosion. This paper reports the detection of a progenitor in deep observations of NGC 1309, the host galaxy of SN 2012Z, obtained with the Hubble Space Telescope and including the location of the supernova before its explosion. Its optical properties and similarity to the progenitor of the helium nova V445 Puppis suggest that SN 2012Z was probably an explosion of a white dwarf accreting from a helium-star companion. Type Iax supernovae are stellar explosions that are spectroscopically similar to some type Ia supernovae at the time of maximum light emission, except with lower ejecta velocities 1 , 2 . They are also distinguished by lower luminosities. At late times, their spectroscopic properties diverge from those of other supernovae 3 , 4 , 5 , 6 , but their composition (dominated by iron-group and intermediate-mass elements 1 , 7 ) suggests a physical connection to normal type Ia supernovae. Supernovae of type Iax are not rare; they occur at a rate between 5 and 30 per cent of the normal type Ia rate 1 . The leading models for type Iax supernovae are thermonuclear explosions of accreting carbon–oxygen white dwarfs that do not completely unbind the star 8 , 9 , 10 , implying that they are ‘less successful’ versions of normal type Ia supernovae, where complete stellar disruption is observed. Here we report the detection of the luminous, blue progenitor system of the type Iax SN 2012Z in deep pre-explosion imaging. The progenitor system's luminosity, colours, environment and similarity to the progenitor of the Galactic helium nova V445 Puppis 11 , 12 , 13 suggest that SN 2012Z was the explosion of a white dwarf accreting material from a helium-star companion. Observations over the next few years, after SN 2012Z has faded, will either confirm this hypothesis or perhaps show that this supernova was actually the explosive death of a massive star 14 , 15 .

It is now accepted that long-duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. The standard 'collapsar' model predicts that a broad-lined and luminous type Ic core-collapse supernova accompanies every long-duration GRB. This association has been confirmed in observations of several nearby GRBs. Here we report that GRB 060505 (ref. 10) and GRB 060614 (ref. 11) were not accompanied by supernova emission down to limits hundreds of times fainter than the archetypal supernova SN 1998bw that accompanied GRB 980425, and fainter than any type Ic supernova ever observed. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration and show that the bursts originated in actively star-forming regions. The absence of a supernova to such deep limits is qualitatively different from all previous nearby long-duration GRBs and suggests a new phenomenological type of massive stellar death.

In this paper we use near-infrared (NIR) spectral observations of Type Ia supernovae (SNe Ia) to study the uncertainties inherent in NIR K corrections. To do so, 75 previously published NIR spectra of 33 SNe Ia are employed to determine K-correction uncertainties in the YJHKs passbands as a function of temporal phase and redshift. The resultant K corrections are then fed into an interpolation algorithm that provides mean K corrections as a function of temporal phase and robust estimates of the associated errors. These uncertainties are both statistical and intrinsic-i.e., due to the diversity of spectral features from object to object-and must be included in the overall error budget of cosmological parameters constrained through the use of NIR observations of SNe Ia. Intrinsic variations are likely the dominant source of error for all four passbands at maximum light. Given the present data, the total Y-band K-correction uncertainties at maximum are smallest, amounting to ± 0.04 mag at a redshift of z = 0.08. The J-band K-term errors are also reasonably small (± 0.06 mag), but intrinsic variations of spectral features and noise introduced by telluric corrections in the H-band currently limit its total K-correction errors at maximum to ± 0.10 mag at z = 0.08. Finally, uncertainties in the Ks-band K terms at maximum amount to ± 0.07 mag at this same redshift. These results are largely constrained by the small number of published NIR spectra of SNe Ia, which do not yet allow spectral templates to be constructed as a function of the light curve decline rate.

In this paper we use near-infrared (NIR) spectral observations of Type Ia supernovae (SNe Ia) to study the uncertainties inherent in NIRKcorrections. To do so, 75 previously published NIR spectra of 33 SNe Ia are employed to determineK-correction uncertainties in theYJHK s passbands as a function of temporal phase and redshift. The resultantKcorrections are then fed into an interpolation algorithm that provides meanKcorrections as a function of temporal phase and robust estimates of the associated errors. These uncertainties are both statistical and intrinsic—i.e., due to the diversity of spectral features from object to object—and must be included in the overall error budget of cosmological parameters constrained through the use of NIR observations of SNe Ia. Intrinsic variations are likely the dominant source of error for all four passbands at maximum light. Given the present data, the totalY-bandK-correction uncertainties at maximum are smallest, amounting to ± 0.04 mag at a redshift ofz = 0.08. TheJ-bandK-term errors are also reasonably small (± 0.06 mag), but intrinsic variations of spectral features and noise introduced by telluric corrections in theH-band currently limit its totalK-correction errors at maximum to ± 0.10 mag atz = 0.08. Finally, uncertainties in theK s -bandKterms at maximum amount to ± 0.07 mag at this same redshift. These results are largely constrained by the small number of published NIR spectra of SNe Ia, which do not yet allow spectral templates to be constructed as a function of the light curve decline rate.

We present optical photometric and spectroscopic observations of the peculiar Type Ia supernova ASASSN-20jq/SN 2020qxp. It is a low-luminosity object with a peak absolute magnitude of \\(M_B=-17.1\\pm0.5\\) mag. Despite its low luminosity, its post-peak light-curve decline rate (\\(\\Delta m_{15}(B)=1.35\\pm0.09\\) mag) and color-stretch parameter (sBV>0.82) are similar to normal SNe Ia, making it an outlier in the luminosity-width and luminosity-color-stretch relations. Early light curves suggest a \"bump\" during the first 1.4 days of explosion. ASASSN-20jq synthesized a low radioactive \\(^{56}\\)Ni mass of \\(0.09\\pm0.01M_\\odot\\). Near-maximum light spectra reveal strong Si II absorption lines, indicating a cooler photosphere than normal SNe Ia, but lack Ti II absorption lines. Unusually strong O I \\(\\lambda\\)7773 and Ca II near-infrared triplet absorption features are present. Nebular spectra show a strong, narrow forbidden [Ca II] \\(\\lambda\\lambda\\)7291,7324 doublet emission, rarely seen in SNe Ia except in some Type Iax events. Marginal detection of [O I] \\(\\lambda\\lambda\\)6300,6364 doublet emission, which is extremely rare, is observed. Both [Ca II] and [O I] lines are redshifted by \\(\\sim2000\\) km/s. A strong [Fe II] \\(\\lambda\\)7155 emission line with a tilted-top profile, identical to the [Fe II] \\(\\lambda\\)16433 profile, is also observed. These asymmetric [Fe II] profiles and redshifted [Ca II] and [O I] emissions suggest a high central density white dwarf progenitor undergoing an off-center delayed-detonation explosion mechanism, producing roughly equal amounts of \\(^{56}\\)Ni in deflagration and detonation phases. This distinguishes ASASSN-20jq from normal and subluminous SNe Ia. ASASSN-20jq's light curve and spectra do not align with any single SNe Ia subclass but show similarities to 2002es-like objects. Thus, we add it as an extreme candidate within the heterogeneous parameter space of 2002es-like SNe Ia.

We present optical and near-infrared observations of SN~2022crv, a stripped envelope supernova in NGC~3054, discovered within 12 hrs of explosion by the Distance Less Than 40 Mpc Survey. We suggest SN~2022crv is a transitional object on the continuum between SNe Ib and SNe IIb. A high-velocity hydrogen feature ($\\sim$$-\\(20,000 -- \\)-\\(16,000 \\)\\rm km\\,s^{-1}\\() was conspicuous in SN~2022crv at early phases, and then quickly disappeared around maximum light. By comparing with hydrodynamic modeling, we find that a hydrogen envelope of \\)\\sim 10^{-3}\\( \\msun{} can reproduce the behaviour of the hydrogen feature observed in SN~2022crv. The early light curve of SN~2022crv did not show envelope cooling emission, implying that SN~2022crv had a compact progenitor with extremely low amount of hydrogen. The analysis of the nebular spectra shows that SN~2022crv is consistent with the explosion of a He star with a final mass of \\)\\sim\\(4.5 -- 5.6 \\msun{} that has evolved from a \\)\\sim\\(16 -- 22 \\msun{} zero-age main sequence star in a binary system with about 1.0 -- 1.7 \\msun{} of oxygen finally synthesized in the core. The high metallicity at the supernova site indicates that the progenitor experienced a strong stellar wind mass loss. In order to retain a small amount of residual hydrogen at such a high metallicity, the initial orbital separation of the binary system is likely larger than \\)\\sim\\(1000~\\)\\rm R_{\\odot}\\(. The near-infrared spectra of SN~2022crv show a unique absorption feature on the blue side of He I line at \\)\\sim\\(1.005~\\)\\mu$m. This is the first time that such a feature has been observed in a Type Ib/IIb, and could be due to \\ion{Sr}{2}. Further detailed modelling on SN~2022crv can shed light on the progenitor and the origin of the mysterious absorption feature in the near infrared.

We present optical photometry and spectroscopy of the superluminous SN 2002gh from maximum light to \\(+204\\) days, obtained as part of the Carnegie Type II Supernova (CATS) project. SN 2002gh is among the most luminous discovered supernovae ever, yet it remained unnoticed for nearly two decades. Using Dark Energy Camera archival images we identify the potential SN host galaxy as a faint dwarf galaxy, presumably having low metallicity, and in an apparent merging process with other nearby dwarf galaxies. We show that SN 2002gh is among the brightest hydrogen-poor SLSNe with \\(M_{V} = -22.40 \\pm 0.02\\), with an estimated peak bolometric luminosity of \\(2.6 \\pm 0.1 \\times 10^{44}\\) erg s\\(^{-1}\\). We discount the decay of radioactive nickel as the main SN power mechanism, and assuming that the SN is powered by the spin down of a magnetar we obtain two alternative solutions. The first case, is characterized by significant magnetar power leakage, and \\(M_{\\mathrm{ej}}\\) between 0.6 and 3.2 \\(M_{\\odot}\\), \\(P_{\\mathrm{spin}} = 3.2\\) ms, and \\(B = 5 \\times 10^{13}\\) G. The second case does not require power leakage, resulting in a huge ejecta mass of about 30 \\(M_{\\odot}\\), a fast spin period of \\(P_{\\mathrm{spin}} \\sim 1\\) ms, and \\(B\\sim 1.6 \\times 10^{14}\\) G. We estimate a zero-age main-sequence mass between 14 and 25 \\(M_{\\odot}\\) for the first case and of about 135 \\(M_{\\odot}\\) for the second case. The latter case would place the SN progenitor among the most massive stars observed to explode as a SN.