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We present a detailed investigation of photometric, spectroscopic, and polarimetric observations of the Type II SN 2023ixf. Earlier studies have provided compelling evidence for a delayed shock breakout from a confined dense circumstellar matter (CSM) enveloping the progenitor star. The temporal evolution of polarization in the SN 2023ixf phase revealed three distinct peaks in polarization evolution at 1.4 days, 6.4 days, and 79.2 days, indicating an asymmetric dense CSM, an aspherical shock front and clumpiness in the low-density extended CSM, and an aspherical inner ejecta/He-core. SN 2023ixf displayed two dominant axes, one along the CSM-outer ejecta and the other along the inner ejecta/He-core, showcasing the independent origin of asymmetry in the early and late evolution. The argument for an aspherical shock front is further strengthened by the presence of a high-velocity broad absorption feature in the blue wing of the Balmer features in addition to the P-Cygni absorption post-16 days. Hydrodynamical light-curve modeling indicated a progenitor mass of 10 M _⊙ with a radius of 470 R _⊙ and explosion energy of 2 × 10 ^51 erg, along with 0.06 M _⊙ of ^56 Ni, though these properties are not unique due to modeling degeneracies. The modeling also indicated a two-zone CSM: a confined dense CSM extending up to 5 × 10 ^14 cm with a mass-loss rate of 10 ^−2 M _⊙ yr ^−1 and an extended CSM spanning from 5 × 10 ^14 to at least 10 ^16 cm with a mass-loss rate of 10 ^−4 M _⊙ yr ^−1 , both assuming a wind-velocity of 10 km s ^−1 . The early-nebular phase observations display an axisymmetric line profile of [O i ], redward attenuation of the emission of H α post 125 days, and flattening in the Ks -band, marking the onset of dust formation.

We present He i /H β flux and He/H abundance ratios in three James Webb Space Telescope galaxies with significant constraints on N/O abundance ratios, GS-NDG-9422, RXCJ2248-ID, and GLASS150008 at z ∼ 6 mostly with the spectroscopic coverage from He i λ 4471 and He ii λ 4686 to He i λ 7065, and comparing with 68 local dwarf galaxies. We find that these high- z galaxies present strong He i emission with He i /H β flux ratios generally larger than those of local dwarf galaxies. We derive He/H with all of the detected He i , He ii , and 2−3 hydrogen Balmer lines in the same manner as the local He/H determination conducted for cosmology studies. These high- z galaxies show He overabundance He/H ≳0.10 or high electron density of n _e ∼ 10 ^3−4 cm ^−3 much larger than local values at low O/H, $12+\mathrm{log}({\rm{O}}/{\rm{H}})=7-8$ . In contrast, we obtain low He/H and n _e values for our local dwarf galaxies by the same technique with the same helium and hydrogen lines, and confirm that the difference between the high- z and local dwarf galaxies is not mimicked by systematics. While two scenarios of (1) He overabundance and (2) high electron density are not clearly concluded, we find that there is a positive correlation between the He/H–N/O or n _e –N/O plane by the comparison of the high- z and local dwarf galaxies. Scenario (1) suggests that the overabundant helium and nitrogen are not explained by the standard chemical enrichment of core-collapse supernovae, but by the CNO-cycle products and equilibrium ratios, respectively. Scenario (2) indicates that the strong helium lines originated from the central dense clouds of the high- z galaxies by excessive collisional excitation.

Core-collapse supernovae (CCSNe) are the terminal explosions of massive stars. While most massive stars explode as iron-core-collapse supernovae (FeCCSNe), slightly less massive stars explode as electron-capture supernovae (ECSNe), shaping the low-mass end of CCSNe. ECSNe were proposed ∼40 yr ago and first-principles simulations also predict their successful explosions. Observational identification and investigation of ECSNe are important for the completion of stellar evolution theory. To date, only one promising candidate has been proposed, SN 2018zd, other than the historical progenitor of the Crab Nebula, SN 1054. We present representative synthetic light curves of low-mass FeCCSNe and ECSNe, i.e., with theoretically or observationally plausible explosion energies and distributions of circumstellar media (CSM). The ECSNe have shorter, brighter, and bluer plateaus than the FeCCSNe. To investigate the robustness of their intrinsic differences, we also calculated light curves of ECSNe and low-mass FeCCSNe adopting various explosion energies and CSM. Although they may have similar bolometric light-curve plateaus, ECSNe are bluer than FeCCSNe in the absence of strong CSM interaction, illustrating that multicolor observations are essential to identify ECSNe. This provides a robust indicator of ECSNe because the bluer plateaus stem from the low-density envelopes of their super-asymptotic-giant-branch progenitors. We propose a distance-independent method to identify ECSNe: ${\left(g-r\right)}_{{t}_{{\rm{PT}}}/2}\lt 0.008\times {t}_{{\rm{PT}}}-0.4$ , i.e., blue g – r at the middle of the plateau ${(g-r)}_{{t}_{\mathrm{PT}}/2}$ , where t _PT is the transition epoch from plateau to tail. Using this method, we identified SN 2018zd as an ECSN, which we believe to be the first ECSN identified with modern observing techniques.

We studied the optical variability of 241 BL Lacertae (BL Lacs) and 83 flat-spectrum radio quasars (FSRQs) from the 4LAC catalog using data from the Tomo-e Gozen Northern Sky Transient Survey, with ∼50 epochs per blazar on average. We excluded blazars whose optical variability may be underestimated due to the influence of their host galaxy based on their optical luminosity ( L _O ). FSRQs with γ -ray photon index greater than 2.6 exhibit very low optical variability, and their distribution of standard deviation of repeated photometry is significantly different from that of the other FSRQs (Kolmogorov–Smirnov test p -value equal to 5 × 10 ^−6 ). Among a sample of blazars at any particular cosmological epoch, those with lower γ -ray luminosity ( L _γ ) tend to have lower optical variability, and those FSRQs with a γ -ray photon index greater than 2.6 tend to have low L _γ . We also measured the structure function of optical variability and found that the amplitude of the structure function for FSRQs is higher than previously measured and higher than that of BL Lacs at multiple time lags. Additionally, the amplitude of the structure function of FSRQs with high γ -ray photon index is significantly lower than that of FSRQs with low γ -ray photon index. The structure function of FSRQs of high γ -ray photon index shows a characteristic timescale of more than 10 days, which may be the variability timescale of the accretion disk. In summary, we infer that the optical component of FSRQs with high γ -ray photon index may be dominated by the accretion disk.

We present gas-phase elemental abundance ratios of thirteen local extremely metal-poor galaxies (EMPGs), including our new Keck/LRIS spectroscopy determinations together with 33 James Webb Space Telescope z ∼ 4–10 star-forming galaxies in the literature, and compare chemical evolution models. We develop chemical evolution models with the yields of core-collapse supernovae (CCSNe), Type Ia SNe, hypernovae (HNe), and pair-instability supernovae (PISNe), and compare the EMPGs and high- z galaxies in conjunction with dust depletion contributions. We find that high Fe/O values of EMPGs can (cannot) be explained by PISN metal enrichments (CCSN/HN enrichments even with the mixing-and-fallback mechanism enhancing iron abundance), while the observed Ar/O and S/O values are much smaller than the predictions of the PISN models. The abundance ratios of EMPGs can be explained by the combination of Type Ia SNe and CCSNe/HNe whose inner layers of argon and sulfur mostly fallback, which are comparable to the Sculptor stellar chemical abundance distribution, suggesting that early chemical enrichment has taken place in the EMPGs. Comparing our chemical evolution models with the star-forming galaxies at z ∼ 4–10, we find that the Ar/O and S/O ratios of the high- z galaxies are comparable to those of the CCSN/HN models, while the majority of high- z galaxies do not have constraints good enough to rule out contributions from PISNe. The high N/O ratio recently reported in GN-z11 cannot be explained even by rotating PISNe, but could be reproduced by the winds of rotating Wolf–Rayet stars that end up as a direct collapse.

We present the largest compilation to date of optical observations during and following fast radio bursts (FRBs). The data set includes our dedicated simultaneous and follow-up observations, as well as serendipitous archival survey observations, for a sample of 15 well-localized FRBs: eight repeating and seven one-off sources. Our simultaneous (and nearly simultaneous with a 0.4 s delay) optical observations of 13 (1) bursts from the repeating FRB 20220912A provide the deepest such limits to date for any extragalactic FRB, reaching a luminosity limit of ν L _ν ≲ 10 ^42 erg s ^−1 (≲2 × 10 ^41 erg s ^−1 ) with 15–400 s exposures; an optical-flux-to-radio-fluence ratio of f _opt / F _radio ≲ 10 ^−7 ms ^−1 (≲10 ^−8 ms ^−1 ); and a flux ratio of f _opt / f _radio ≲ 0.02–≲2 × 10 ^−5 (≲10 ^−6 ) on millisecond to second timescales. These simultaneous limits provide useful constraints in the context of FRB emission models, such as the pulsar magnetosphere and pulsar nebula models. Interpreting all available optical limits in the context of the synchrotron maser model, we find that they constrain the flare energies to ≲10 ^43 –10 ^49 erg (depending on the distances of the various repeating FRBs, with ≲10 ^39 erg for the Galactic SGR 1935+2154). These limits are generally at least an order of magnitude larger than those inferred from the FRBs themselves, although in the case of FRB 20220912A our simultaneous and rapid follow-up observations severely restrict the model parameter space. We conclude by exploring the potential of future simultaneous and rapid-response observations with large optical telescopes.

We present optical, near-infrared, and radio observations of supernova (SN) SN IIb 2022crv. We show that it retained a very thin H envelope and transitioned from an SN IIb to an SN Ib; prominent H α seen in the pre-maximum phase diminishes toward the post-maximum phase, while He i lines show increasing strength. SYNAPPS modeling of the early spectra of SN 2022crv suggests that the absorption feature at 6200 Å is explained by a substantial contribution of H α together with Si ii , as is also supported by the velocity evolution of H α . The light-curve evolution is consistent with the canonical stripped-envelope SN subclass but among the slowest. The light curve lacks the initial cooling phase and shows a bright main peak (peak M _V = −17.82 ± 0.17 mag), mostly driven by radioactive decay of ^56 Ni. The light-curve analysis suggests a thin outer H envelope ( M _env ∼ 0.05 M _⊙ ) and a compact progenitor ( R _env ∼ 3 R _⊙ ). An interaction-powered synchrotron self-absorption model can reproduce the radio light curves with a mean shock velocity of 0.1 c . The mass-loss rate is estimated to be in the range of (1.9−2.8) × 10 ^−5 M _⊙ yr ^−1 for an assumed wind velocity of 1000 km s ^−1 , which is on the high end in comparison with other compact SNe IIb/Ib. SN 2022crv fills a previously unoccupied parameter space of a very compact progenitor, representing a beautiful continuity between the compact and extended progenitor scenario of SNe IIb/Ib.

We present chemical abundance ratios of 70 star-forming galaxies at z ∼ 4–10 observed by the JWST/NIRSpec Early Release Observations, GLASS, and CEERS programs. Among the 70 galaxies, we have pinpointed two galaxies, CEERS_01019 at z = 8.68 and GLASS_150008 at z = 6.23, with extremely low C/N ([C/N] ≲ −1), evidenced with C iii ] λλ 1907,1909, N iii ] λ 1750, and N iv ] λλ 1483,1486, which show high N/O ratios ([N/O] ≳ 0.5) comparable with the one of GN-z11, regardless of whether stellar or active galactic nucleus radiation is assumed. Such low C/N and high N/O ratios found in CEERS_01019 and GLASS_150008 (additionally identified in GN-z11) are largely biased toward the equilibrium of the CNO cycle, suggesting that these three galaxies are enriched by metals processed by the CNO cycle. On the C/N versus O/H plane, these three galaxies do not coincide with Galactic H ii regions, normal star-forming galaxies, and nitrogen-loud quasars with asymptotic giant branch stars, but with globular-cluster (GC) stars, indicating a connection with GC formation. We compare the C/O and N/O of these three galaxies with those of theoretical models and find that these three galaxies are explained by scenarios with dominant CNO-cycle materials, i.e., Wolf–Rayet stars, supermassive (10 ^3 –10 ^5 M _⊙ ) stars, and tidal disruption events, interestingly with a requirement of frequent direct collapses. For all the 70 galaxies, we present measurements of Ne/O, S/O, and Ar/O, together with C/O and N/O. We identify four galaxies with very low Ne/O, log(Ne/O) < −1.0, indicating abundant massive (≳30 M _⊙ ) stars.

We report on the chemical composition of the very metal-poor (VMP; [Fe/H] = −2.9) star LAMOST J1645+4357, which was identified as a red giant having peculiar abundance ratios by Li et al. The standard abundance analysis is carried out for this object and the well-studied metal-poor star HD 122563, which has similar atmospheric parameters. LAMOST J1645+4357 has a remarkable abundance set, highlighted by these features: (1) nitrogen is significantly enhanced ([N/Fe] = +1.4), and the total abundance of C and N is also very high ([(C+N)/Fe] = +0.9); (2) α -elements are overabundant with respect to iron, as generally found in VMP stars; (3) Ti, Sc, Co, and Zn are significantly deficient; (4) Cr and Mn are enhanced compared to most of VMP stars; and (5) Sr and Ba are deficient, and the Sr/Ba ratio ([Sr/Ba] = −1.0) is significantly lower than the value expected for the r -process. The overall abundance pattern of this object from C to Zn is well reproduced by a faint supernova model assuming spherical explosion, except for the excess of Cr and Mn, which requires enhancement of incomplete Si burning or small contributions from a Type Ia or pair-instability supernova. There remains, however, a question as to why the abundance pattern of this star is so unique among VMP stars.

In unveiling the nature of the first stars, the main astronomical clue is the elemental compositions of the second generation of stars, observed as extremely metal-poor (EMP) stars, in the Milky Way. However, no observational constraint was available on their multiplicity, which is crucial for understanding early phases of galaxy formation. We develop a new data-driven method to classify observed EMP stars into mono- or multi-enriched stars with support vector machines. We also use our own nucleosynthesis yields of core-collapse supernovae with mixing fallback that can explain many of the observed EMP stars. Our method predicts, for the first time, that 31.8% ± 2.3% of 462 analyzed EMP stars are classified as mono-enriched. This means that the majority of EMP stars are likely multi-enriched, suggesting that the first stars were born in small clusters. Lower-metallicity stars are more likely to be enriched by a single supernova, most of which have high carbon enhancement. We also find that Fe, Mg. Ca, and C are the most informative elements for this classification. In addition, oxygen is very informative despite its low observability. Our data-driven method sheds a new light on solving the mystery of the first stars from the complex data set of Galactic archeology surveys.

The LIGO/Virgo detected a gravitational wave (GW) event, named GW200224_222234 (also known as S200224ca) and classified as a binary-black hole coalescence, on 2020 February 24. Given its relatively small localization skymap (71 deg ^2 for a 90% credible region; revised to 50 deg ^2 in GWTC-3), we performed target-of-opportunity observations using the Subaru/Hyper Suprime-Cam (HSC) in the r 2 and z bands. Observations were conducted on 2020 February 25 and 28 and March 23, with the first epoch beginning 12.3 hr after the GW detection. The survey covered the highest-probability sky area of 56.6 deg ^2 , corresponding to a 91% probability. This was the first deep follow-up ( m _r ≳ 24, m _z ≳ 23) for a binary-black hole merger covering >90% of the localization. By performing image subtraction and candidate screening including light-curve fitting with transient templates and examples, we found 22 off-nucleus transients that were not ruled out as the counterparts of GW200224_222234 with our Subaru/HSC data alone. We also performed GTC/OSIRIS spectroscopy of the probable host galaxies for five candidates; two are likely to be located within the 3D skymap, whereas the others are not. In conclusion, 19 transients remain as possible optical counterparts of GW200224_222234; but we could not identify a unique promising counterpart. If there are no counterparts in the remaining candidates, the upper limits of the optical luminosity are $\nu {L}_{\nu }\lt {5.2}_{-1.9}^{+2.4}\times {10}^{41}$ erg s ^−1 and $\nu {L}_{\nu }\lt {1.8}_{-0.6}^{+0.8}\times {10}^{42}$ erg s ^−1 in the r 2 and z bands, respectively, at ∼12 hr after GW detection. We also discuss improvements in the strategies of optical follow-ups for future GW events.

We perform a z-band survey for an optical counterpart of the binary neutron star coalescence GW170817 with Subaru/Hyper Suprime-Cam. Our untargeted transient search covers 23.6 deg² corresponding to the 56.6% credible region of GW170817 and reaches the 50% completeness magnitude of 20.6mag on average. As a result, we find 60 candidate extragalactic transients, including J-GEM17btc (also known as SSS17a/DLT17ck). While J-GEM17btc is associated with NGC 4993, which is firmly located inside the 3D skymap of GW170817, the other 59 candidates do not have distance information in the GLADE v2 catalog or NASA/IPAC Extragalactic Database. Among 59 of the candidates, 58 are located at the center of extended objects in the Pan-STARRS1 catalog, while one candidate has an offset. We present location, z-band apparent magnitude, and time variability of the candidates and evaluate the probabilities that they are located within the 3D skymap of GW170817. The probability for J-GEM17btc is 64%, which is much higher than for the other 59 candidates (9.3 × 10-3-2.1 × 10-1%). Furthermore, the possibility that at least one of the other 59 candidates is located within the 3D skymap is only 3.2%. Therefore, we conclude that J-GEM17btc is the most likely and distinguished candidate to be the optical counterpart of GW170817. [ABSTRACT FROM AUTHOR]

We present the results of detailed analysis of an optical imaging survey conducted using the Subaru/Hyper Suprime-Cam (HSC) that aimed to identify an optical counterpart to the gravitational wave event GW151226. In half a night, the i- and z-band imaging survey by HSC covered 63.5 deg² of the error region, which contains about 7% of the LIGO localization probability, and the same fieldwas observed in three different epochs. The detectable magnitude of the candidates in a differenced image is evaluated as i ~ 23.2mag for the requirement of at least two 5 σ detections, and 1744 candidates are discovered. Assuming a kilonova as an optical counterpart, we compare the optical properties of the candidates with model predictions. A red and rapidly declining light curve condition enables the discrimination of a kilonova from other transients, and a small number of candidates satisfy this condition. The presence of stellar-like counterparts in the reference frame suggests that the surviving candidates are likely to be flare stars. The fact that most of those candidates are in the galactic plane, |b| < 5°, supports this interpretation. We also check whether the candidates are associated with the nearby GLADE galaxies, which reduces the number of contaminants even with a looser color cut. When a better probability map (with localization accuracy of ~50 deg²) is available, kilonova searches of up to approximately 200Mpc will become feasible by conducting immediate follow-up observations with an interval of 3-6 d. [ABSTRACT FROM AUTHOR]

We present monitoring campaign observations at optical and near-infrared (NIR) wavelengths for a radio-loud active galactic nucleus (AGN) at z = 0.840, SDSSJ110006.07+442144.3 (hereafter, J1100+4421), which was identified during a flare phase in late 2014 February. The campaigns consist of three intensive observing runs from the discovery to 2015 March, mostly within the scheme of the OISTER collaboration. Optical-NIR light curves and simultaneous spectral energy distributions (SEDs) are obtained. Our measurements show the strongest brightening in 2015 March. We found that the optical-NIR SEDs of J1100+4421 show an almost steady shape despite the large and rapid intranight variability. This constant SED shape is confirmed to extend to ~5μm in the observed frame using the archival WISE data. Given the lack of absorption lines and the steep power-law spectrum of αν ~ -1.4, where fν - ναν , synchrotron radiation by a relativistic jet with no or small contributions from the host galaxy and the accretion disk seemsmost plausible as an optical-NIR emission mechanism. The steep optical-NIR spectral shape and the large amplitude of variability are consistent with this object being a low νpeak jet-dominated AGN. In addition, sub-arcsecond resolution optical imaging data taken with Subaru Hyper Suprime-Cam does not show a clear extended component and the spatial scales are significantly smaller than the large extensions detected at radio wavelengths. The optical spectrum of a possible faint companion galaxy does not show any emission lines at the same redshift, and hence a merging hypothesis for this AGN-related activity is not supported by our observations. [ABSTRACT FROM AUTHOR]

We report the results of optical-infrared follow-up observations of the gravitational wave (GW) event GW151226 detected by the Advanced LIGO in the framework of JGEM (Japanese collaboration for Gravitational wave ElectroMagnetic follow-up).We performed wide-field optical imaging surveys with the Kiso Wide Field Camera (KWFC), Hyper Suprime-Cam (HSC), and MOA-cam3. The KWFC survey started at 2.26 d after the GW event and covered 778 deg² centered at the high Galactic region of the skymap of GW151226. We started the HSC follow-up observations from ~12 d after the event and covered an area of 63.5 deg² of the highest probability region of the northern sky with limiting magnitudes of 24.6 and 23.8 for the i and z bands, respectively. MOA-cam3 covered 145 deg² of the skymap with the MOA-red filter ~2.5 mon after the GW alert. The total area covered by the wide-field surveys was 986.5 deg². The integrated detection probability for the observed area was ~29%. We also performed galaxy-targeted observations with six optical and near-infrared telescopes from 1.61 d after the event. A total of 238 nearby (≤100 Mpc) galaxies were observed with a typical I band limiting magnitude of ~19.5. We detected 13 supernova candidates with the KWFC survey, and 60 extragalactic transients with the HSC survey. Two thirds of the HSC transients were likely supernovae and the remaining one third were possible active galactic nuclei. With our observational campaign, we found no transients that are likely to be associated with GW151226. [ABSTRACT FROM AUTHOR]

We present an application of machine-learning (ML) techniques to source selection in the optical transient survey data with the Hyper Suprime-Cam (HSC) on the Subaru telescope. Our goal is to select real transient events accurately and in a timely manner out of a large number of false candidates, obtained by the standard difference-imaging method. We have developed the transient selector, which is based on majority voting of the three ML machines of AUC Boosting, Random Forest, and Deep Neural Networks. We applied it to our observing runs of Subaru-HSC in 2015 May and August, and proved it to be efficient in selecting optical transients. The false positive rate was 1.0% at the true positive rate of 90% in the magnitude range of 22.0-25.0mag for the May data. For the August run, we successfully detected and reported 10 supernovae candidates within the same day as the observation. From these runs, we learned the following lessons: (1) training using artificial objects is effective in filtering false candidates out, especially for faint objects, and (2) a combination of ML by majority voting is advantageous. [ABSTRACT FROM AUTHOR]

We present extensively dense observations of the super-Chandrasekhar supernova (SC SN) candidate SN2012dn from -11 to +140 d after the date of its B-band maximum in the optical and near-infrared (NIR) wavelengths conducted through the OISTER ToO (Optical and Infrared Synergetic Telescopes for Education and Research Target of Opportunity) program. The NIR light curves and color evolutions up to 35 days after the B-band maximum provided an excellent match with those of another SC SN 2009dc, providing further support to the nature of SN 2012dn as an SC SN. We found that SN 2012dn exhibited strong excesses in the NIR wavelengths from 30 d after the B-band maximum. The H- and Ks-band light curves exhibited much later maximum dates at 40 and 70 d after the B-band maximum, respectively, compared with those of normal SNe Ia. The H- and Ks-band light curves subtracted by those of SN2009dc displayed plateaued evolutions, indicating an NIR echo from the surrounding dust. The distance to the inner boundary of the dust shell is limited to 4.8-6.4 x 10-2 pc. No emission lines were found in its early phase spectra, suggesting that the ejecta-circumstellar material interaction could not occur. On the other hand, we found no signature that strongly supports the scenario of dust formation. The mass-loss rate of the pre-explosion system is estimated to be 10-6 - 10-5 M⊙ yr-1, assuming that the wind velocity of the system is 10-100 kms-1, which suggests that the progenitor of SN 2012dn could be a recurrent nova system. We conclude that the progenitor of this SC SN could be explained by the single-degenerate scenario. [ABSTRACT FROM AUTHOR]

We present a newmethod for effectively selecting objects which may be low-mass active black holes (BHs) at galaxy centers using high-cadence optical imaging data, and our first spectroscopic identification of an active 2.7 × 106 M☉ BH at z = 0.164. This active BH was originally selected due to its rapid optical variability, from a few hours to a day, based on Subaru Hyper Suprime-Cam g-band imaging data taken with a 1 hr cadence. Broad and narrow Hα lines and many other emission ones are detected in our optical spectra taken with Subaru FOCAS, and the BH mass is measured via the broad Hα emission line width (1880 kms-1) and luminosity (4.2 × 1040 erg s-1) after careful correction to the atmospheric absorption around 7580-7720 A° . We measure the Eddington ratio and find it to be as low as 0.05, considerably smaller than those in a previous SDSS sample with similar BH mass and redshift, which indicates one of the special potentials of our Subaru C survey. The g - r color and morphology of the extended component indicate that the host galaxy is a star-forming galaxy. We also show the effectiveness of our variability selection for low-mass active BHs. [ABSTRACT FROM AUTHOR]

A rapidly rotating neutron star with strong magnetic fields, called a magnetar, is a possible candidate for the central engine of long gamma-ray bursts and hypernovae (HNe). We solve the evolution of a shock wave driven by the wind from a magnetar and we evaluate the temperature evolution, by which we estimate the amount of 56Ni that produces the bright emission of HNe. We obtain a constraint on the magnetar parameters (i.e. the poloidal magnetic field strength Bp and initial angular velocity Ωi) for synthesizing enough 56Ni mass to explain HNe (... ≳ 0.2 M⊙), that is, (Bp/1016G)1/2 (Ωi/104rads-1) ≳ 0.7. [ABSTRACT FROM AUTHOR]

We investigate the fraction of metal nuclei in the relativistic jets of gamma-ray bursts associated with core-collapse supernovae. We simulate the fallback in jet-induced explosions with two-dimensional relativistic hydrodynamics calculations and the jet acceleration with steady, radial, relativistic magnetohydrodynamics calculations, and derive the detailed nuclear composition of the jet by post-processing calculation. We found that if the temperature at the jet launch site is above 4.7 x 109 K, quasi-statistical equilibrium is established and heavy nuclei are dissociated into light particles such as 4He during the acceleration of the jets. The criterion for the survival of metal nuclei is written in terms of the isotropic jet luminosity as Ljiso ≲ 3.9 x 1050(Ri/107 cm)²(1 + σi) erg s-1, where Ri and σi are the initial radius of the jets and the initial magnetization parameter, respectively. If the jet is initially dominated by radiation field (i.e., σi << 1) and the isotropic luminosity is relatively high (Ljiso ≳ 4 x 1052 erg s-1), the metal nuclei cannot survive in the jet. On the other hand, if the jet is mainly accelerated by magnetic field (i.e., σi >> 1), metal nuclei initially contained in the jet can survive without serious dissociation even in the case of a high-luminosity jet. If the jet contains metal nuclei, the dominant nuclei are 28Si, 16O, and 32S and the mean mass number can be ~ 25. [ABSTRACT FROM AUTHOR]

After the Big Bang, production of heavy elements in the early Universe takes place starting from the formation of the first stars, their evolution, and explosion. The first supernova explosions have strong dynamical, thermal, and chemical feedback on the formation of subsequent stars and evolution of galaxies. However, the nature of the Universe's first stars and supernova explosions has not been well clarified. The signature of the nucleosynthesis yields of the first stars can be seen in the elemental abundance patterns observed in extremely metal-poor stars. Interestingly, those patterns show some peculiarities relative to the solar abundance pattern, which should provide important clues to understanding the nature of early generations of stars. We thus review the recent results of the nucleosynthesis yields of mainly massive stars for a wide range of stellar masses, metallicities, and explosion energies. We also provide yields tables and examine how those yields are affected by some hydrodynamical effects during supernova explosions, namely, explosion energies from those of hypernovae to faint supernovae, mixing and fallback of processed materials, asphericity, etc. Those parameters in the su-pernova nucleosynthesis models are constrained from observational data of supernovae and metal-poor stars. Nucleosynthesis yields are then applied to the chemical evolution model of our Galaxy and other types of galaxies to discuss how the chemical enrichment process occurred during evolution. [ABSTRACT FROM AUTHOR]

We present detailed chemical abundances for the bright carbon-enhanced metal-poor (CEMP) star BD+44°493, previously reported on by Ito et al.Our measurements confirm that BD+44°493 is an extremelymetal-poor ([Fe/H]= -3.8) subgiant star with excesses of carbon and oxygen. No significant excesses are found for nitrogen and neutroncapture elements (the latter of which place it in the CEMP-no class of stars). Other elements that we measure exhibit abundance patterns that are typical for non-CEMP extremely metal-poor stars. No evidence for variations of radial velocity has been found for this star. These results strongly suggest that the carbon enhancement in BD+44°493 is unlikely to have been produced by a companion asymptotic giant-branch star and transferred to the presently observed star, nor by pollution of its natal molecular cloud by rapidly-rotating, massive, mega metal-poor ([Fe/H]<-6.0) stars. A more likely possibility is that this star formed from gas polluted by the elements produced in a "faint" supernova, which underwent mixing and fallback, and only ejected small amounts of elements of metals beyond the lighter elements. The Li abundance of BD+44°493 (A(Li) = log(Li/H)+12 = 1.0) is lower than the Spite plateau value, as found in other metal-poor subgiants. The upper limit on Be abundance (A(Be) = log(Be/H)+12<-1.8) is as low as those found for stars with similarly extremely-low metallicity, indicating that the progenitors of carbon- (and oxygen-) enhanced stars are not significant sources of Be, or that Be is depleted in metal-poor subgiants with effective temperatures of ~5400 K. [ABSTRACT FROM AUTHOR]

\We propose an alternative explanation for the post-breakout emission of SN 2008D associated with the X-ray transient 080109. Observations of this object show a very small contrast of 0.35 dex between the light-curve minimum occurring soon after the breakout, and the main luminosity peak which is due to radioactive heating of the ejecta. Hydrodynamical models show that the cooling of a shocked Wolf-Rayet star leads to a much greater difference (≳0.9 dex). Our proposed scenario is that of a jet produced during the explosion which deposits 56Ni-rich material in the outer layers of the ejecta. The presence of high-velocity radioactive material allows us to reproduce the complete luminosity evolution of the object. Without outer 56Ni it could be possible to reproduce the early emission purely from cooling of the shocked envelope by assuming a larger progenitor than a Wolf-Rayet star, but that would require an initial density structure significantly different from what is predicted by stellar evolution models. Analytic models of the cooling phase have been proposed reproduce the early emission of SN 2008D with an extended progenitor. However, we found that the models are valid only until 1.5 days after the explosion where only two data of SN 2008D are available. We also discuss the possibility of the interaction of the ejecta with a binary companion, based on published analytic expressions. However, the binary separation required to fit the early emission should be ≲3R☉, which is too small for a system containing two massive stars. [ABSTRACT FROM AUTHOR]

We show that the diversity in the density slope of the dense wind due to non-steady mass loss can be one way to explain the spectral diversity of Type II luminous supernovae (LSNe). The interaction of SN ejecta and wind surrounding it is considered to be a power source to illuminate LSNe because many LSNe show the wind signature in their spectra (Type IIn LSNe). However, there also exist LSNe without the spectral features caused by the wind (Type IIL LSNe).We show that, even if LSNe are illuminated by the interaction, it is possible that they do not show the narrow spectra from the wind if we take into account the non-steady mass loss of their progenitors. When the shock breakout takes place in a dense wind with the density structure ρ ∝ r-w, the ratio of the diffusion timescale in the optically thick region of the wind (td) and the shock propagation timescale of the entire wind after the shock breakout (ts) strongly depends on w. For the case w ≾ 1, both timescales are comparable (td/ts ⋍ 1) and td/ts gets smaller as w gets larger. For the case td/ts ⋍ 1, the shock goes through the entire wind just after the light-curve (LC) peak, and narrow spectral lines from the wind cannot be observed after the LC peak (Type IIL LSNe). If td/ts is much smaller, the shock wave continues to propagate in the wind after the LC peak, and unshocked wind remains (Type IIn LSNe). This difference can be obtained only through careful treatment of the shock breakout condition in a dense wind. The lack of narrow Lorentzian line profiles in Type IIL LSNe before the LC peak can also be explained by the difference in the density slope. Furthermore, we apply our model to Type IIn LSN 2006gy and Type IIL LSN 2008es and find that our model is consistent with the observations. [ABSTRACT FROM AUTHOR]

With a booming number of Type Ia supernovae (SNe Ia) discovered within a few days of their explosions, a fraction of SNe Ia that show luminosity excess in the early phase (early-excess SNe Ia) have been confirmed. In this article, we report early-phase observations of seven photometrically normal SNe Ia (six early detections and one deep non detection limit) at the COSMOS field through a half-year transient survey as a part of the Hyper Suprime-Cam Subaru Strategic Program (HSC SSP). In particular, a blue light-curve excess was discovered for HSC17bmhk, a normal SN Ia with rise time longer than 18.8 days, during the first four days after the discovery. The blue early excess in optical wavelength can be explained not only by interactions with a nondegenerate companion or surrounding dense circumstellar matter but also radiation powered by radioactive decays of 56Ni at the surface of the SN ejecta. Given the growing evidence of the early-excess discoveries in normal SNe Ia that have longer rise times than the average, and a similarity in the nature of the blue excess to a luminous SN Ia subclass, we infer that early excess discovered in HSC17bmhk and other normal SNe Ia are most likely attributed to radioactive 56Ni decay at the surface of the SN ejecta. In order to successfully identify normal SNe Ia with early excess similar to that of HSC17bmhk, early UV photometries or high-cadence blue-band surveys are necessary. [ABSTRACT FROM AUTHOR]

We perform a high-cadence transient survey with the Subaru Hyper Suprime-Cam (HSC), which we call the Subaru HSC survey Optimized for Optical Transients (SHOOT). We conduct HSC imaging observations with time intervals of about one hour on two successive nights, and spectroscopic and photometric follow-up observations. A rapidly declining blue transient SHOOT14di at z = 0.4229 is found in observations on two successive nights using an image-subtraction technique. The rate of brightness change is () in the observer (rest) frame and the rest-frame color between 3400 and 4400 Å is . The nature of the object is investigated by comparing its peak luminosity, decline rate, and color with those of transients and variables previously observed, and with those of theoretical models. None of the transients or variables share the same properties as SHOOT14di. Comparisons with theoretical models demonstrate that, while the emission from the cooling envelope of a SN IIb shows a slower decline rate than SHOOT14di, and the explosion of a red supergiant star with a dense circumstellar wind shows a redder color than SHOOT14di, the shock breakout at the stellar surface of the explosion of a red supergiant star with a small explosion energy of erg reproduces the multicolor light curve of SHOOT14di. This discovery shows that a high-cadence, multicolor optical transient survey at intervals of about one hour, and continuous and immediate follow-up observations, is important for studies of normal core-collapse supernovae at high redshifts. [ABSTRACT FROM AUTHOR]

We report our observations of HSC16aayt (SN 2016jiu), which was discovered by the Subaru/Hyper Suprime-Cam (HSC) transient survey conducted as part of the Subaru Strategic Program. It shows very slow photometric evolution and its rise time is more than 100 days. The optical magnitude change in 400 days remains within 0.6 mag. Spectra of HSC16aayt show a strong narrow emission line and we classify it as a Type IIn supernova. The redshift of HSC16aayt is 0.6814 ± 0.0002 from the spectra. Its host galaxy center is at 5 kpc from the supernova location and HSC16aayt might be another example of isolated Type IIn supernovae, although the possible existence of underlying star-forming activity of the host galaxy at the supernova location is not excluded. [ABSTRACT FROM AUTHOR]

Recent supernova searches revealed a number of fast-evolving luminous transients. We perform radiation-hydrodynamic simulations of light curves of several models of supernova from super-asymptotic giant branch (super-AGB, SAGB) stars with low-mass envelopes (Menv ∼ 0.05–1.25 M⊙). The differences in the light curves found among the models are used to link the observed events to the most appropriate models. In particular, we propose that KSN 2015K is an electron-capture supernova. We assume an “optically” thick circumstellar medium (CSM) around SAGB and the circumstellar interaction powers the peak luminosity of the light curve with a short rise time. The faint tail might be influenced by the spin-down luminosity of a newborn Crab-like pulsar. Our fits indicate an ejecta mass of 0.02–0.05 M⊙, a CSM mass of 0.10–0.12 M⊙, a radius of the CSM photosphere ∼1014 cm, a kinetic energy of ∼3 × 1050 erg, a photospheric velocity vph ≳ 10,000 km s−1, and a pulsar total spin energy (2.5–4) × 1049 erg. [ABSTRACT FROM AUTHOR]

We present observational evidence that an aspherical supernova explosion could have occurred in the first stars in the early universe. Our results are based on the first determination of a Zn abundance in a Hubble Space Telescope/Cosmic Origins Spectrograph high-resolution UV spectrum of a hyper-metal-poor (HMP) star, HE 1327−2326, with . We determine [Zn/Fe] = 0.80 ± 0.25 from a UV Zn i line at 2138 Å, detected at 3.4σ. Yields of a 25 M⊙ aspherical supernova model with artificially modified densities exploding with E = 5 × 1051 erg best match the entire abundance pattern of HE 1327−2326. Such high-entropy hypernova explosions are expected to produce bipolar outflows, which could facilitate the external enrichment of small neighboring galaxies. This has already been predicted by theoretical studies of the earliest star-forming minihalos. Such a scenario would have significant implications for the chemical enrichment across the early universe, as HMP carbon-enhanced metal-poor (CEMP) stars such as HE 1327−2326 might have formed in such externally enriched environments. [ABSTRACT FROM AUTHOR]

We present optical and near-infrared observations of the rapidly evolving supernova (SN) 2017czd that shows hydrogen features. The optical light curves exhibit a short plateau phase (∼13 days in the R-band) followed by a rapid decline by 4.5 mag ∼20 days after the plateau. The decline rate is larger than those of any standard SNe, and close to those of rapidly evolving transients. The peak absolute magnitude is −16.8 mag in the V band, which is within the observed range for SNe IIP and rapidly evolving transients. The spectra of SN 2017czd clearly show the hydrogen features and resemble those of SNe IIP at first. The Hα line, however, does not evolve much with time, and it becomes similar to those in SNe IIb at the decline phase. We calculate the synthetic light curves using a SN IIb progenitor that has 16 at the zero-age main sequence and evolves into a binary system. The model with a low explosion energy (5 × 1050 erg) and a low 56Ni mass () can reproduce the short plateau phase, as well as the sudden drop of the light curve, as observed in SN 2017czd. We conclude that SN 2017czd might be the first weak explosion identified from a SN IIb progenitor. We suggest that some rapidly evolving transients can be explained by such a weak progenitor explosion with a barely hydrogen-rich envelope. [ABSTRACT FROM AUTHOR]

We report the discovery of an SN 1988Z–like type IIn supernova KISS15s found in a low-mass star-forming galaxy at redshift z = 0.038 during the course of the Kiso Supernova Survey (KISS). KISS15s shows long-duration optical continuum and emission line light curves, indicating that KISS15s is powered by a continuous interaction between the expanding ejecta and dense circumstellar medium (CSM). The Hα emission line profile can be decomposed into four Gaussians of narrow, intermediate, blueshifted intermediate, and broad velocity width components, with a full width at half maximum of ≲100, ∼2000, and ∼14,000 km s−1 for the narrow, intermediate, and broad components, respectively. The presence of the blueshifted intermediate component, of which the line-of-sight velocity relative to the systemic velocity is about −5000 km s−1, suggests that the ejecta-CSM interaction region has an inhomogeneous morphology and anisotropic expansion velocity. We found that KISS15s shows increasing infrared continuum emission, which can be interpreted as hot dust thermal emission of T ∼ 1200 K from newly formed dust in a cool, dense shell in the ejecta-CSM interaction region. The progenitor mass-loss rate, inferred from bolometric luminosity, is , where vw is the progenitor’s stellar wind velocity. This implies that the progenitor of KISS15s was a red supergiant star or a luminous blue variable that had experienced a large mass loss in the centuries before the explosion. [ABSTRACT FROM AUTHOR]

We investigate a progenitor mass distribution of core-collapse supernova remnants (CCSNRs) in our Galaxy and Large and Small Magellanic Clouds for the first time. We count the number of CCSNRs in three mass ranges divided by the zero-age main-sequence (ZAMS) mass, MZAMS; A: MZAMS < 15 M⊙, B: 15 M⊙ < MZAMS < 22.5 M⊙, C: MZAMS > 22.5 M⊙. A simple compilation of progenitor masses in the literature yields a progenitor mass distribution of fA : fB : fC = 0.27:0.27:0.46, where f is the number fraction of the progenitors. The distribution is inconsistent with any standard initial mass functions (IMFs). We notice, however, that previous mass estimates are subject to large systematic uncertainties because most of the relative abundances (X/Si) are not good probes for the progenitor masses. Instead, we rely only on the Fe/Si ratio, which is sensitive to the CO core mass (MCOcore) and MZAMS. Comparing Fe/Si ratios in SN remnants in the literature with the newest theoretical model, we estimate 33 MCOcore and MZAMS, leading to a revised progenitor mass distribution of fA : fB : fC = 0.47:0.32:0.21. This is consistent with the standard Salpeter IMF. However, the relation between MCOcore and MZAMS could be affected by binary evolution, which is not taken into account in this study and should be considered in future work to derive a better progenitor mass distribution estimate. [ABSTRACT FROM AUTHOR]

We present optical and near-infrared observations of a low-luminosity (LL) Type IIP supernova (SN) 2016bkv from the initial rising phase to the plateau phase. Our observations show that the end of the plateau is extended to ≳140 days since the explosion, indicating that this SN takes one of the longest times to finish the plateau phase among Type IIP SNe (SNe IIP), including LL SNe IIP. The line velocities of various ions at the middle of the plateau phase are as low as 1000–1500 km s−1, which is the lowest even among LL SNe IIP. These measurements imply that the ejecta mass in SN 2016bkv is larger than that of the well-studied LL IIP SN 2003Z. In the early phase, SN 2016bkv shows a strong bump in the light curve. In addition, the optical spectra in this bump phase exhibit a blue continuum accompanied by a narrow Hα emission line. These features indicate an interaction between the SN ejecta and the circumstellar matter (CSM) as in SNe IIn. Assuming the ejecta–CSM interaction scenario, the mass loss rate is estimated to be yr−1 within a few years before the SN explosion. This is comparable to or even larger than the largest mass loss rate observed for the Galactic red supergiants ( yr−1 for VY CMa). We suggest that the progenitor star of SN 2016bkv experienced a violent mass loss just before the SN explosion. [ABSTRACT FROM AUTHOR]

We compare the elemental abundance patterns of ∼200 extremely metal-poor (EMP; [Fe/H] < −3) stars to the supernova yields of metal-free stars, in order to obtain insights into the characteristic masses of the first (Population III or Pop III) stars in the universe. The supernova yields are prepared with nucleosynthesis calculations of metal-free stars with various initial masses (M = 13, 15, 25, 40 and 100 M⊙) and explosion energies (E51 = E/1051[erg] = 0.5–60), to include low-energy, normal-energy, and high-energy explosions. We adopt the mixing-fallback model, to take into account possible asymmetry in the supernova explosions, and the yields that best fit the observed abundance patterns of the EMP stars are searched by varying the model parameters. We find that the abundance patterns of the EMP stars are predominantly best-fitted by the supernova yields with initial masses M < 40 M⊙, and that more than than half of the stars are best-fitted by the M = 25 M⊙ hypernova (E51 = 10) models. The results also indicate that the majority of the primordial supernovae have ejected 10−2–10−1M⊙ of 56Ni, leaving behind a compact remnant (either a neutron star or a black hole), with a mass in the range of ∼1.5–5 M⊙. These results suggest that the masses of the first stars responsible for the first metal enrichment are predominantly <40 M⊙. This implies that the higher-mass first stars were either less abundant, directly collapsed into a black hole without ejecting heavy elements, or a supernova explosion of a higher-mass first star inhibits the formation of the next generation of low-mass stars at [Fe/H] < −3. [ABSTRACT FROM AUTHOR]

The birth properties of neutron stars (NSs) yield important information about the still-debated physical processes that trigger the explosion as well as on intrinsic neutron-star physics. These properties include the high space velocities of young neutron stars with average values of several 100 km s−1, with an underlying “kick” mechanism that is not fully clarified. There are two competing possibilities that could accelerate NSs during their birth: anisotropic ejection of either stellar debris or neutrinos. Here we present new evidence from X-ray measurements that chemical elements between silicon and calcium in six young gaseous supernova remnants are preferentially expelled opposite to the direction of neutron star motion. There is no correlation between the kick velocities and magnetic field strengths of these neutron stars. Our results support a hydrodynamic origin of neutron-star kicks connected to asymmetric explosive mass ejection, and they conflict with neutron-star acceleration scenarios that invoke anisotropic neutrino emission caused by particle and nuclear physics in combination with very strong neutron-star magnetic fields. [ABSTRACT FROM AUTHOR]

Low-mass population III (PopIII) stars of could survive up until the present. The nondetection of low-mass PopIII stars in our Galaxy has already put a stringent constraint on the initial mass function (IMF) of PopIII stars, suggesting that PopIII stars have a top-heavy IMF. On the other hand, some claim that the lack of such stars stems from metal enrichment of their surfaces by the accretion of heavy elements from the interstellar medium (ISM). We investigate the effects of the stellar wind on metal accretion onto low-mass PopIII stars because accretion of the local ISM onto the Sun is prevented by the solar wind, even for neutrals. The stellar wind and radiation of low-mass PopIII stars are modeled based on knowledge of nearby low-mass stellar systems, including our Sun. We find that low-mass PopIII stars traveling across the Galaxy form a stellar magnetosphere in most of their life. Once the magnetosphere is formed, most of the neutral interstellar particles are photoionized before reaching the stellar surface and are blown away by the wind. Especially, the accretion abundance of iron will be reduced by a factor of compared with Bondi–Hoyle–Lyttleton accretion. The metal accretion can enhance iron abundance [Fe/H] only up to ∼−14. This demonstrates that low-mass PopIII stars remain pristine and will be found as metal-free stars and that further searches for them are valuable in constraining the IMF of PopIII stars. [ABSTRACT FROM AUTHOR]

The properties of the first generation of stars and their supernova (SN) explosions remain unknown due to the lack of their actual observations. Pop III stars may have been very massive and predicted to be exploded as pair-instability SNe, but the observed metal-poor stars show the abundance patterns which are more consistent with yields of core-collapse SNe. We study the multicolor light curves for a metal-free core-collapse SN models (massive stars of 25- 100 solar mass range) to determine the indicators for the detection and identification of first generation SNe. We use mixing-fallback supernova explosion models which explain the observed abundance patterns of metal poor stars. Numerical calculations of the multicolor light curves are performed using the multigroup radiation hydrodynamic code STELLA. The calculated light curves of metal-free SNe are compared with our calculations of non-zero metallicity models and observed SNe. [ABSTRACT FROM AUTHOR]

We study the number and the distribution of low-mass Population III (Pop III) stars in the Milky Way. In our numerical model, hierarchical formation of dark matter minihalos and Milky-Way-sized halos are followed by a high-resolution cosmological simulation. We model the Pop III formation in H2 cooling minihalos without metal under UV radiation of the Lyman–Werner bands. Assuming a Kroupa initial mass function (IMF) from 0.15 to 1.0 M⊙ for low-mass Pop III stars, as a working hypothesis, we try to constrain the theoretical models in reverse by current and future observations. We find that the survivors tend to concentrate on the center of halo and subhalos. We also evaluate the observability of Pop III survivors in the Milky Way and dwarf galaxies, and constraints on the number of Pop III survivors per minihalo. The higher latitude fields require lower sample sizes because of the high number density of stars in the galactic disk, the required sample sizes are comparable in the high- and middle-latitude fields by photometrically selecting low-metallicity stars with optimized narrow-band filters, and the required number of dwarf galaxies to find one Pop III survivor is less than 10 at <100 kpc for the tip of red giant stars. Provided that available observations have not detected any survivors, the formation models of low-mass Pop III stars with more than 10 stars per minihalo are already excluded. Furthermore, we discuss the way to constrain the IMF of Pop III stars at a high mass range of ≳10 M⊙. [ABSTRACT FROM AUTHOR]

The properties of the first generation of stars and their supernova (SN) explosions remain unknown due to the lack of actual observations. Recently, many transient surveys have been conducted and the feasibility of detecting supernovae (SNe) of Pop III stars is growing. In this paper, we study the multicolor light curves for a number of metal-free core-collapse SN models (25–100 ) to determine the indicators for the detection and identification of first generation SNe. We use mixing-fallback supernova explosion models that explain the observed abundance patterns of metal-poor stars. Numerical calculations of the multicolor light curves are performed using the multigroup radiation hydrodynamic code stella. The calculated light curves of metal-free SNe are compared with non-zero-metallicity models and several observed SNe. We have found that the shock breakout characteristics, the evolution of the photosphere’s velocity, the luminosity, and the duration and color evolution of the plateau, that is, all of the SN phases from shock breakout to 56Co decay, are helpful for estimating the parameters of the SN progenitor: the mass, the radius, the explosion energy, and the metallicity. We conclude that the multicolor light curves could potentially be used to identify first-generation SNe in current (Subaru/HSC) and future transient surveys (LSST, James Webb Space Telescope). They are also suitable for identifying low-metallicity SNe in the nearby universe (PTF, Pan-STARRS, Gaia). [ABSTRACT FROM AUTHOR]

We present rapidly rising transients discovered by a high-cadence transient survey with the Subaru telescope and Hyper Suprime-Cam. We discovered five transients at z = 0.384–0.821, showing a rate of rise faster than 1 mag per day in the restframe near-ultraviolet wavelengths. The fast rate of rise and brightness are most similar to SN 2010aq and PS1-13arp, for which ultraviolet emission was detected within a few days after the shock breakout. The lower limit of the event rate of rapidly rising transients is ∼9% of core-collapse supernova rates, assuming the duration of rapid rise to be 1 day. We show that the light curves of the three faint objects agree with the cooling envelope emission from the explosion of red supergiants. The other two luminous objects, however, are brighter and faster than the cooling envelope emission. We interpret these two objects to be the shock breakout from a dense wind with a mass loss rate of ∼10−3 yr−1, as also proposed for PS1-13arp. This mass loss rate is higher than that typically observed for red supergiants. The event rate of these luminous objects is ≳1% of the core-collapse supernova rate, and thus our study implies that more than ∼1% of massive stars can experience intense mass loss a few years before the explosion. [ABSTRACT FROM AUTHOR]

We report the first detection of thermal X-ray line emission from the supernova remnant (SNR) RX J1713.7-3946, the prototype of the small class of synchrotron-dominated SNRs. A softness-ratio map generated using XMM-Newton data shows that faint interior regions are softer than bright shell regions. Using Suzaku and deep XMM-Newton observations, we have extracted X-ray spectra from the softest area, finding clear line features at 1 and ∼1.35 keV. These lines can be best explained as Ne Lyα and Mg Heα from a thermal emission component. Since the abundance ratios of metals to Fe are much higher than solar values in the thermal component, we attribute the thermal emission to reverse-shocked SN ejecta. The measured Mg/Ne, Si/Ne, and Fe/Ne ratios of 2.0–2.6, 1.5–2.0, and <0.05 solar suggest that the progenitor star of RX J1713.7-3946 was a relatively low-mass star (≲20 M⊙), consistent with a previous inference based on the effect of stellar winds of the progenitor star on the surrounding medium. Since the mean blastwave speed of ∼6000 km s−1 (the radius of 9.6 pc divided by the age of 1600 years) is relatively fast compared with other core-collapse SNRs, we propose that RX J1713.7-3946 is a result of an SN Ib/c whose progenitor was a member of an interacting binary. While our analysis provides strong evidence for X-ray line emission, our interpretation of its nature as thermal emission from SN ejecta requires further confirmation especially through future precision spectroscopic measurements using ASTRO-H. [ABSTRACT FROM AUTHOR]

We report observations of the Type Iax supernova (SN Iax) 2012Z at optical and near-infrared (NIR) wavelengths from immediately after the explosion until ∼260 days after the maximum luminosity using the Optical and Infrared Synergetic Telescopes for Education and Research Target-of-Opportunity program and the Subaru Telescope. We found that the NIR light curve evolutions and color evolutions are similar to those of SNe Iax 2005hk and 2008ha. The NIR absolute magnitudes ( mag and mag) and the rate of decline of the light curve (m15(B) mag) are very similar to those of SN 2005hk ( mag, mag, and m15(B) 1.6 mag), yet differ significantly from SNe 2008ha and 2010ae (MJ ∼ −14 to −15 mag and m15(B) 2.4–2.7 mag). The estimated rise time is 12.0 ± 3.0 days, which is significantly shorter than that of SN 2005hk or any other SNe Ia. The rapid rise indicates that the 56Ni distribution may extend into the outer layer or that the effective opacity may be lower than that in normal SNe Ia. The late-phase spectrum exhibits broader emission lines than those of SN 2005hk by a factor of six to eight. Such high velocities of the emission lines indicate that the density profile of the inner ejecta extends more than that of SN 2005hk. We argue that the most favored explosion scenario is a “failed deflagration” model, although the pulsational delayed detonations is not excluded. [ABSTRACT FROM AUTHOR]