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We present the Spectroscopic Classification of Astronomical Transients (SCAT) survey, which is dedicated to spectrophotometric observations of transient objects such as supernovae and tidal disruption events. SCAT uses the SuperNova Integral-Field Spectrograph (SNIFS) on the University of Hawai’i 2.2 m (UH2.2m) telescope. SNIFS was designed specifically for accurate transient spectrophotometry, including absolute flux calibration and host-galaxy removal. We describe the data reduction and calibration pipeline including spectral extraction, telluric correction, atmospheric characterization, nightly photometricity, and spectrophotometric precision. We achieve ≲5% spectrophotometry across the full optical wavelength range (3500–9000 Å) under photometric conditions. The inclusion of photometry from the SNIFS multi-filter mosaic imager allows for decent spectrophotometric calibration (10%–20%) even under unfavorable weather/atmospheric conditions. SCAT obtained ≈640 spectra of transients over the first 3 yr of operations, including supernovae of all types, active galactic nuclei, cataclysmic variables, and rare transients such as superluminous supernovae and tidal disruption events. These observations will provide the community with benchmark spectrophotometry to constrain the next generation of hydrodynamic and radiative transfer models.

The discovery of a newly born type IIb supernova reveals a rapid brightening at optical wavelengths that corresponds to the shock-breakout phase of the explosion. A newborn supernova Supernovae do not provide warning signs that they are about to explode—at least not that we have been able to recognize. With rare exceptions (such as the supernova SN 1987A), supernovae are caught some undetermined time after their explosion. Melina Bersten et al . report observations of the supernova SN 2016gkg in the galaxy NGC 613. The explosion of this star was caught serendipitously by an amateur astronomer who was testing a new camera with 20-second exposures, so the timing of the beginning of the event is tightly constrained. The brightness of the supernova increased very rapidly, at a rate of about 40 magnitudes per day. It is difficult to establish the properties of massive stars that explode as supernovae 1 , 2 . The electromagnetic emission during the first minutes to hours after the emergence of the shock from the stellar surface conveys important information about the final evolution and structure of the exploding star 3 , 4 , 5 , 6 . However, the unpredictable nature of supernova events hinders the detection of this brief initial phase 7 , 8 , 9 . Here we report the serendipitous discovery of a newly born, normal type IIb supernova (SN 2016gkg) 10 , which reveals a rapid brightening at optical wavelengths of about 40 magnitudes per day. The very frequent sampling of the observations allowed us to study in detail the outermost structure of the progenitor of the supernova and the physics of the emergence of the shock. We develop hydrodynamical models of the explosion that naturally account for the complete evolution of the supernova over distinct phases regulated by different physical processes. This result suggests that it is appropriate to decouple the treatment of the shock propagation from the unknown mechanism that triggers the explosion.

Type II supernovae (SNe II) originate from the explosion of hydrogen-rich supergiant massive stars. Their first electromagnetic signature is the shock breakout (SBO), a short-lived phenomenon that can last for hours to days depending on the density at shock emergence. We present 26 rising optical light curves of SN II candidates discovered shortly after explosion by the High Cadence Transient Survey and derive physical parameters based on hydrodynamical models using a Bayesian approach. We observe a steep rise of a few days in 24 out of 26 SN II candidates, indicating the systematic detection of SBOs in a dense circumstellar matter consistent with a mass loss rate of M ̇  > 10 −4 M ⊙  yr −1 or a dense atmosphere. This implies that the characteristic hour-timescale signature of stellar envelope SBOs may be rare in nature and could be delayed into longer-lived circumstellar material SBOs in most SNe II. The shock breakout (SBO) is the first electromagnetic signature of a supernova (SN) explosion. Förster et al. find that in nearly all type II SNe they survey that the SBO occurs on a timescale of days, indicating that the progenitors were surrounded by thick circumstellar matter when they exploded.

In the version of this Article originally published, the authors Pablo Huijse and Pablo Huentelemu were mistakenly affiliated with the University of California, Berkeley, and their affiliation to the University of Chile was omitted. Pablo Huijse’s affiliation to the Universidad Austral de Chile was also omitted.

We present optical photometry and spectrosopy of the transient SN 2011A. Our data spans 140 days after discovery including BVRIu'g'r'i'z' photometry and a sequence of 11 spectra. First classified as a type IIn supernova due to the presence of narrow Hα emission, this object shows exceptional characteristics. Firstly, the light curve shows a double plateau; a property only before observed in the impostor object SN 1997bs. Secondly SN 2011A has a very low luminosity for a type IIn supernova placing it between the type IIn supernovae and impostor classes in terms of luminosity. Thirdly, SN 2011A shows low velocity and high equivalent width sodium doublet absorption which increases with time and is most likely of circumstellar origin. This evolution is also accompanied by a change of line profile. When the absorption becomes stronger, a P-Cygni profile appears.

We present the Spectroscopic Classification of Astronomical Transients (SCAT) survey, which is dedicated to spectrophotometric observations of transient objects such as supernovae and tidal disruption events. SCAT uses the SuperNova Integral-Field Spectrograph (SNIFS) on the University of Hawai’i 2.2 m (UH2.2m) telescope. SNIFS was designed specifically for accurate transient spectrophotometry, including absolute flux calibration and host-galaxy removal. We describe the data reduction and calibration pipeline including spectral extraction, telluric correction, atmospheric characterization, nightly photometricity, and spectrophotometric precision. We achieve ≲5% spectrophotometry across the full optical wavelength range (3500–9000 Å) under photometric conditions. The inclusion of photometry from the SNIFS multi-filter mosaic imager allows for decent spectrophotometric calibration (10%–20%) even under unfavorable weather/atmospheric conditions. SCAT obtained ≈640 spectra of transients over the first 3 yr of operations, including supernovae of all types, active galactic nuclei, cataclysmic variables, and rare transients such as superluminous supernovae and tidal disruption events. These observations will provide the community with benchmark spectrophotometry to constrain the next generation of hydrodynamic and radiative transfer models.

The use of multiple independent methods with their own systematic uncertainties is crucial for resolving the ongoing tension between local and distant measurements of the Hubble constant (\\(H_{0}\\)). While type Ia supernovae (SNe Ia) have historically been the most widely used distance indicators, recent studies have shown that type II supernovae (SNe II) can provide independent measurements of extragalactic distances with different systematic uncertainties. Unlike SNe Ia, the progenitors of SNe II are well understood, arising from the explosion of red supergiants in late-type galaxies via core-collapse. While SNe II do not exhibit the same level of uniformity in peak luminosity as SNe Ia, their differences can be calibrated using theoretical or empirical methods. Overall, this chapter presents a comprehensive overview of the use of SNe II as extragalactic distance indicators, with a particular focus on their application to measuring \\(H_0\\) and addressing the Hubble tension. We describe the underlying theory of each method, discuss the challenges associated with them, including uncertainties in the calibration of the supernova absolute magnitude, and present a comprehensive list of the most updated Hubble constant measurements.

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=-17.10.5\\) mag. Despite its low luminosity, its post-peak light-curve decline rate (\\( m_15(B)=1.350.09\\) 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 \\(0.090.01M_\\). 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 \\(\\)7773 and Ca II near-infrared triplet absorption features are present. Nebular spectra show a strong, narrow forbidden [Ca II] \\(\\)7291,7324 doublet emission, rarely seen in SNe Ia except in some Type Iax events. Marginal detection of [O I] \\(\\)6300,6364 doublet emission, which is extremely rare, is observed. Both [Ca II] and [O I] lines are redshifted by \\(2000\\) km/s. A strong [Fe II] \\(\\)7155 emission line with a tilted-top profile, identical to the [Fe II] \\(\\)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.

The success of Type Ia supernova (SN Ia) distance standardisation for cosmology relies on a single global linear relationship between their peak luminosity and colour, the \\(\\beta\\) parameter. However, there are several pieces of evidence and physical reasons to believe that this relation is not universal and may change within different subgroups, or even among individual objects. In this work, we allow \\(\\beta\\) to vary among subpopulations with different observed properties in the cosmological fits. Although the inferred cosmological parameters are consistent with previous studies that assume a single colour-luminosity relation, we find that the SN data favour nonuniversal distributions of \\(\\beta\\) when split according to SN colour and/or host-galaxy mass. For galaxy mass, we obtain a \\(\\beta\\)-step relation in which low \\(\\beta\\) values occur in more massive galaxies, a trend that can be explained by differing dust reddening laws for two types of environments. For colour, we find that bluer/redder SNe Ia are consistent with a lower/larger \\(\\beta\\). This trend is explained with \\(\\beta\\) being a combination of a low intrinsic colour-luminosity relation dominant in bluer SNe and a higher extrinsic reddening relation dominant at redder colours. The host galaxy mass-step correction always provides better distance calibration, regardless of the multiple \\(\\beta\\) approaches, and we suggest that it may come from a difference in intrinsic colour-luminosity properties of SNe Ia in two types of environments. Additionally, we find that blue SNe in low-mass environments are better standard candles than the others.

Understanding Type Ia supernovae (SNe~Ia) and the empirical standardisation relations that make them excellent distance indicators is vital to improving cosmological constraints. SN~Ia ``siblings\", i.e. two or more SNe~Ia in the same host or parent galaxy offer a unique way to infer the standardisation relations and their diversity across the population. We analyse a sample of 25 SN~Ia pairs, observed homogeneously by the Zwicky Transient Factory (ZTF) to infer the SNe~Ia light curve width-luminosity and colour-luminosity parameters \\(\\) and \\(\\). Using the pairwise constraints from siblings, allowing for a diversity in the standardisation relations, we find \\( = 0.218 0.055 \\) and \\( = 3.084 0.312\\), respectively, with a dispersion in \\(\\) and \\(\\) of \\( 0.195\\) and \\( 0.923\\), respectively, at 95\\(\\%\\) C.L. While the median dispersion is large, the values within \\( 1 \\) are consistent with no dispersion. Hence, fitting for a single global standardisation relation, we find \\( = 0.228 0.029 \\) and \\( = 3.160 0.191\\). We find a very small intrinsic scatter of the siblings sample \\(_ int 0.10\\) at 95\\% C.L. compared to \\(_ int = 0.22 0.04\\) when computing the scatter using the Hubble residuals without comparing them as siblings. Splitting the sample based on host galaxy stellar mass, we find that SNe~Ia in both subsamples have consistent \\(\\) and \\(\\). The \\(\\) value is consistent with the value for the cosmological sample. However, we find a higher \\(\\) by \\( 2.5 - 3.5 \\). The high \\(\\) is driven by low \\(x_1\\) pairs, potentially suggesting that the slow and fast declining SN~Ia have different slopes of the width-luminosity relation. We can confirm or refute this with increased statistics from near future time-domain surveys. (abridged)