Recent time-domain surveys in the optical have revealed rapid transients that evolve on timescales of <= 10 d, expanding the population of transients toward the short-duration regime. The transient search on even shorter timescales, particularly those lasting only seconds, or less, remains a largely unexplored frontier, offering the significant potential for discovering objects from unexpected populations. Very short-duration optical transients could be potential counterparts to millisecond-duration fast radio bursts (FRBs), providing clues about their origins. However, the optical search for transients on such short timescales has been limited primarily due to instrumental constraints. Here, we report the discovery of an optical transient candidate (TMG20200322) with a duration of less than or similar to 2 s by wide-field video observations in the direction of the Earth's shadow. TMG20200322 was detected in just two consecutive images of 1-s exposure time, with its shape becoming elongated in the second frame. PSF shape variability analysis of the field stars reveals such an elongated PSF cannot be explained by atmospheric fluctuations. We investigate the potential origins of TMG20200322 in two scenarios: meteoroid impact flashes on near-Earth asteroids (NEAs) and headon meteors in the Earth's atmosphere. None of the scenarios provides a satisfactory explanation for this transient. We derive a sky-projected rate of the TMG20200322 event to be R-trans = (3.4 & times; 10(-2))(+0.13)(-0.028) deg(-2) d(-1) and an upper limit of second-timescale transients with durations of 1 s <= tau less than or similar to 15 s to be R-trans less than or similar to 0.10 deg(-2) d(-1) for the non-detection case. We highlight that continuous monitoring observations in the direction of the Earth's shadow could be a key strategy to unveiling a new population of optical transients on timescales of seconds or less.

Attempts to reveal the spectroscopic diversity of Type Ia supernovae (SNe Ia) have led to subclassification schemes such as the Branch system, which classifies SNe Ia into four categories: core normal (CN), broad line (BL), cool (CL), and shallow silicon (SS). The physical origin of these spectroscopic differences, including progenitor channels, explosion mechanisms, or other parameters, however, remains unclear. Moreover, previous work has concentrated primarily on properties near peak luminosity, yielding limited insight into their behavior at later epochs. In this study, we compile $UBVRI$ photometry for 109 SNe Ia and construct the first set of average light curves for each Branch subgroup, spanning from pre-maximum through the late tail. We find pronounced diversity in the I band, especially in the timing of the secondary maximum across subgroups and in the late-time decline of CL events. After correcting for light-curve stretch, which reflects the combined influence of ${}<^>{56}$Ni and ejecta masses, we show that the secondary maximum is powered by Fe ii recombination, and its timing is particularly sensitive to the amount of stable iron-group elements (IGEs) synthesized in the explosion. This implies an anti-correlation between the mass of stable IGEs and ${}<^>{56}$Ni: CL (and possibly BL) events have a larger mass ratio of IGEs/$<^>{56}$Ni, resulting in earlier secondary maxima, while SS events have a smaller ratio and thus later secondary maxima. This trend is naturally explained within the near-$M_{\rm Ch}$ delayed-detonation scenario, whereas it is inconsistent with the positive correlation predicted by the sub-$M_{\rm Ch}$ double-detonation scenario. Finally, we show that stretch-corrected late-time slopes provide a practical diagnostic for CL events, likely linked to an emission feature around 7200 & Aring;.

In recent years, with the increasing number of type Ia supernovae (SNe Ia) discovered soon after their explosions, a nonnegligible fraction of SNe Ia with early-excess emissions (EExSNe Ia) have been confirmed. In this paper, we present a total of 67 early-phase normal SNe Ia from published papers and ongoing transient survey projects to systematically investigate their photometric behaviors from very early time. We found that EExSNe Ia in our sample have longer rise and brighter peak luminosities compared to non-EExSNe Ia. Moreover, EExSNe Ia commonly have “tiny red bump” or “blue plateau” features in the early B − V color while non-EExSNe Ia show blueward evolution from the very beginning. Here, we propose that the thin He-shell double detonation scenario can phenomenologically explain the photometric diversities of normal SNe Ia considering different white dwarf–He-shell mass combinations and the viewing angle effect, implying a unified explosion mechanism of normal-type SNe Ia. To further verify the possible common origin of normal SNe Ia, systematical studies of multiband photometric and spectral properties of early-phase SNe Ia through the new-generation wide-field time-domain survey facilities and global real-time follow-up networks are highly needed.

Blazars are active galactic nuclei known for their extreme variability, offering unique opportunities to study jet physics and high-energy emission mechanisms. In 2024, the Flat Spectrum Radio Quasar (FSRQ) OP313 underwent a remarkable flare event, during which the gamma-ray flux observed by the Fermi Large Area Telescope increased by a factor of 60 over its average value. The flare peak lasted less than two days. Using optical telescopes, we conducted 100 day timescale observations. Multiwavelength data revealed that OP313 entered an active state 50 days prior to the flare and remained active for at least 50 days afterward. We propose that this prolonged activity resulted from variations in electron density within the shock front due to changes in the accretion rate. Concurrently, OP313’s spectrum transitioned from an FSRQ-like state to a BL Lac-like state, characterized by a significant increase in the synchrotron peak frequency and the disappearance of broad-line region emission lines. In the post-flare phase, we observed a decoupling between synchrotron radiation and inverse Compton scattering, along with a possible decrease in the magnetic field strength within the shock front.

Measurement of the alignment error of the telescope mirrors is an essential and demanding task in the telescope assembly phase. One method is to examine the aberration over the whole telescope field of view from sky images with stars, but there are complicated issues in the case of large telescopes. The focal plane of the University of Tokyo Atacama Observatory (TAO) 6.5-m telescope has a large diameter of 546 mm and a field curvature. Therefore, many imaging sensors must be arrayed on the curved focal plane. We propose a concept of a screen camera for the TAO 6.5-m telescope. To lower the cost, we accept the degradation of the spatial resolution up to similar to 2 arcseconds and the decrease in optical throughput. This system consists of a transparent screen, a camera lens, and a CMOS sensor, and it obtains sky images through the telescope on the screen. The transparent spherical screen with one side sanded is placed at the telescope focal plane. A CMOS sensor with a commercially available camera lens and filters is placed at about 1.5 meters from the screen and captures the starry sky on the screen. The pixel scale on the CMOS sensor is calculated to be 0.31 arcseconds, and the estimated limiting magnitude is about 13 in a 10-second exposure at a 10s level. After the telescope mirror alignment, the screen camera will provide focused sky images in the whole field of view, 25 arcminutes diameter.

In time-domain astronomy, achieving high-speed optical spectroscopy within 10 seconds presents a significant challenge. We are developing an efficient three-channel, image slicer-type integral field unit (IFU) for the Seimei Telescope's TriCCS instrument. To maximize throughput within the constraints of limited space, the IFU optics consist exclusively of a slicer unit and spherical mirrors that form an Offner optical system, omitting the magnification optics commonly used in conventional IFUs. The slicer unit employs a medium transmission slit in the transmission channel instead of an air slit, resulting in a 4.6 mm backward shift of the pseudo-focal plane, which effectively reduces dust impact on the mirror at the image plane. Our design achieves both sufficient imaging performance and over 90% throughput in the 400-1000 nm wavelength range, despite the limited available space, making it adaptable to other slit spectroscopy instruments. This presentation provides an overview of the TriCCS IFU design and its current development status.

The University of Tokyo Atacama Observatory (TAO) is a project to build and operate a 6.5m telescope at the summit of Co. Chajnantor (5640 m.a.s.l). This is promoted by the Institute of Astronomy, School of Science, the University of Tokyo in collaboration with many institutes and universities in Japan and Chile. The site construction started in 2018 and was successfully completed by April 2024. An operation support building and an enclosure have already been constructed and are operational at the summit. Electricity is supplied by two generators installed in the operations building. The telescope mount and mirrors have already completed their tests in Japan and the U.S., respectively. They were transported to Chile and wait for the assembly. The first light instruments, NICE and MIMIZUKU, are undergoing final adjustment in Japan and will be transported to Chile as the telescope assembly progresses. The near-infrared instrument SWIMS has completed its open use on the Subaru telescope and returned back to Japan in Aug. 2023 for upgrading for TAO. The near-infrared spectrograph TARdYS, which is being developed in collaboration with Pontificia Universidad Catolica de Chile, is also making progress in the development of its optics and detectors. In addition to these, the development of a new optical instrument has been started this year. Allocation of the observing time was also determined. TAO will use approximately 45% of its scientific observation time as project time, 35% as Japan open time for the Japanese community, and 10% as Chilean time for the Chilean community. 5-15% will be provided as paid observing time.

We present the luminosity functions and stellar mass functions of supernova (SN) host galaxies and test if they differ from the functions of normal field galaxies. We utilize homogeneous samples consisting of 273 SNe Ia ( z ≤ 0.3) and 44 core-collapse (CC) SNe ( z ≤ 0.1) from the Sloan Digital Sky Survey II Supernova Survey and the high-signal-to-noise-ratio photometry of galaxies from the Hyper Suprime-Cam Subaru Strategic Program. SN hosts are classified into star-forming and passive galaxy groups based on the spectral energy distribution fitting. We find that the SN host luminosity functions and stellar mass functions deviate from those of normal field galaxies. Star-forming galaxies dominate the low-mass end of the SN Ia host mass function, while passive galaxies dominate the high-mass end. CC SNe are predominantly hosted by star-forming galaxies. In addition, intermediate-mass hosts produce CC SNe with the highest efficiency, while the efficiency of producing SNe Ia monotonically increases as the hosts become more massive. Furthermore, we derive the pseudo mass normalized SN rates (pSNuM) based on the mass functions. We find that the star-forming component of pSNuM _Ia is less sensitive to the changes in stellar mass, in comparison with the total rate. The behavior of pSNuM _CC suggests that the CC rate is proportional to the star-forming rate.

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.

Over-luminous type Ia supernovae (SNe Ia) show peculiar observational features, for which an explosion of a super-massive white dwarf (WD) beyond the classical Chandrasekhar-limiting mass has been suggested, largely based on their high luminosities and slow light-curve evolution. However, their observational features are diverse, with a few extremely peculiar features whose origins have not been clarified; strong and persisting C II lines, late-time accelerated luminosity decline and red spectra, and a sub-day time-scale initial flash clearly identified so far at least for three over-luminous SNe Ia. In the present work, we suggest a scenario that provides a unified solution to these peculiarities, through hydrodynamic and radiation transfer simulations together with analytical considerations; a C+O-rich envelope ( 0.01 - 0.1 Msun) attached to an exploding WD. Strong C II lines are created within the shocked envelope. Dust formation is possible in the late phase, providing a sufficient optical depth thereafter. The range of the envelope mass considered here predicts an initial flash with time-scale of 0.5 - 3 days. The scenario thus can explain some of the key diverse observational properties by a different amount of the envelope, but additional factors are also required; we argue that the envelope is distributed in a disc-like structure, and also the ejecta properties, e.g., the mass of the WD, plays a key role. Within the context of the hypothesized super-Chandrasekhar-mass WD scenario, we speculatively suggest a progenitor WD evolution including a spin-up accretion phase followed by a spin-down mass-ejection phase.

In this Letter, we report the discovery of an ultraluminous fast-evolving transient in rest-frame UV wavelengths, MUSSES2020J, soon after its occurrence by using the Hyper Suprime-Cam (HSC) mounted on the 8.2 m Subaru telescope. The rise time of about 5 days with an extremely high UV peak luminosity shares similarities to a handful of fast blue optical transients whose peak luminosities are comparable with the most luminous supernovae while their timescales are significantly shorter (hereafter “fast blue ultraluminous transient,” FBUT). In addition, MUSSES2020J is located near the center of a normal low-mass galaxy at a redshift of 1.063, suggesting a possible connection between the energy source of MUSSES2020J and the central part of the host galaxy. Possible physical mechanisms powering this extreme transient such as a wind-driven tidal disruption event and an interaction between supernova and circumstellar material are qualitatively discussed based on the first multiband early-phase light curve of FBUTs, although whether the scenarios can quantitatively explain the early photometric behavior of MUSSES2020J requires systematical theoretical investigations. Thanks to the ultrahigh luminosity in UV and blue optical wavelengths of these extreme transients, a promising number of FBUTs from the local to the high-z universe can be discovered through deep wide-field optical surveys in the near future.

We report on a one-second-cadence wide-field survey for M-dwarf flares using the Tomoe Gozen camera mounted on the Kiso Schmidt telescope. We detect 22 flares from M3-M5 dwarfs with a rise time of 5 s less than or similar to t(rise) less than or similar to 100 s and an amplitude of 0.5 less than or similar to Delta F/F-* less than or similar to 20. The flare light-curves mostly show steeper rises and shallower decays than those obtained from the Kepler one-minute cadence data and tend to have flat peak structures. Assuming a blackbody spectrum with a temperature of 9000-15000 K, the peak luminosities and energies are estimated to be 10(29) erg s(-1) less than or similar to L-peak less than or similar to 10(31) erg s(-1) and 10(31) erg less than or similar to E-flare less than or similar to 10(34 )erg, which constitutes the bright end of fast optical flares for M dwarfs. We confirm that more than 90% of the host stars of the detected flares are magnetically active based on their H alpha-emission-line intensities obtained by LAMOST. An estimated occurrence rate of detected flares is similar to 0.7 per day per active star, indicating they are common in magnetically active M dwarfs. We argue that the flare light-curves can be explained by the chromospheric compression model: the rise time is broadly consistent with the Alfven transit time of a magnetic loop with a length scale of l(loop) similar to 10(4) km and a field strength of 1000 gauss, while the decay time is likely determined by the radiative cooling of the compressed chromosphere down near to the photosphere with a temperature of greater than or similar to 10000 K. These flares from M dwarfs could be a major contamination source for a future search of fast optical transients of unknown types.

We report the results of video observations of tiny (diameter less than 100 m) near-Earth objects (NEOs) with Tomo-e Gozen on the Kiso 105 cm Schmidt telescope. The rotational period of a tiny asteroid reflects its dynamical history and physical properties since smaller objects are sensitive to the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. We carried out video observations of 60 tiny NEOs at 2 fps from 2018 to 2021 and successfully derived the rotational periods and axial ratios of 32 NEOs including 13 fast rotators with rotational periods less than 60 s. The fastest rotator found during our survey is 2020 HS7 with a rotational period of 2.99 s. We statistically confirmed that there is a certain number of tiny fast rotators in the NEO population, which have been missed with all previous surveys. We have discovered that the distribution of the tiny NEOs in a diameter and rotational period (D-P) diagram is truncated around a period of 10 s. The truncation with a flat-top shape is not explained well by either a realistic tensile strength of NEOs or the suppression of YORP by meteoroid impacts. We propose that the dependence of the tangential YORP effect on the rotational period potentially explains the observed pattern in the D-P diagram.

We conduct 24.4 fps optical observations of repeating fast radio burst (FRB) 20190520B using Tomo-e Gozen, a high-speed CMOS camera mounted on the Kiso 105 cm Schmidt telescope, simultaneously with radio observations carried out using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We succeeded in the simultaneous optical observations of 11 radio bursts that FAST detected. However, no corresponding optical emission was found. The optical fluence limits as deep as 0.068 Jy ms are obtained for the individual bursts (0.029 Jy ms on the stacked data) corrected for the dust extinction in the Milky Way. The fluence limit is deeper than those obtained in the previous simultaneous observations for an optical emission with a duration ≳0.1 ms. Although the current limits on radio-optical spectral energy distribution (SED) of FRBs are not constraining, we show that SED models based on observed SEDs of radio variable objects such as optically detected pulsars, and a part of parameter spaces of theoretical models in which FRB optical emission is produced by inverse Compton scattering in a pulsar magnetosphere or a strike of a magnetar blastwave into a hot wind bubble, can be ruled out once a similar fluence limit as in our observation is obtained for a bright FRB with a radio fluence ≳5 Jy ms.

Metal-deficient stars are important tracers for understanding the early formation of the Galaxy. Recent large-scale surveys with both photometric and spectroscopic data have reported an increasing number of metal-deficient stars whose kinematic features are consistent with those of the disk stellar populations. We report the discovery of an RR Lyrae variable (hereafter RRL) that is located within the thick disk and has an orbit consistent with the thick-disk kinematics. Our target RRL (HD 331986) is located at around 1 kpc from the Sun and, with V ≃ 11.3, is among the ∼130 brightest RRLs known so far. However, this object has scarcely been studied because it is in the midplane of the Galaxy, at a Galactic latitude around –1°. Its near-infrared spectrum (0.91–1.32 μm) shows no absorption line except hydrogen lines of the Paschen series, suggesting [Fe/H] ≲ –2.5. It is the most metal-deficient RRL, at least among RRLs whose orbits are consistent with the disk kinematics, although we cannot determine to which of the disk and the halo it belongs. This unique RRL would provide us with essential clues for studying the early formation of stars in the inner Galaxy with further investigations, including high-resolution optical spectroscopy.

There has recently been an increasing interest in a possible population of type Ia supernovae (SNe Ia) triggered by helium detonation on the surface of a massive white dwarf. In this paper, we first summarize possible observational signatures of the He detonation-triggered SNe Ia, emphasizing the new diagnostics of the He detonation mode potentially seen in the SN light within the first few days since the explosion. We then argue that observational properties of a peculiar SN Ia, MUSSES1604D as discovered by the Hyper Suprime-Cam (HSC) attached with the Subaru telescope, are best explained by the He-detonation scenario. We then discuss possible origins, including the He detonation scenario, of the diversity seen in the photometric properties of SNe Ia in the first few days. While the He detonation could reproduce observational properties of a fraction of SNe Ia showing the excessive emission in the first few days, it is likely that a bulk of them are linked to a different explosion mechanism where the early excess would arise due to an extensive mixing of Ni-56 during the explosion. A combined analysis of the very early phase observations and the maximum-phase observations will be key in mapping the diverse SN Ia zoo into different populations reflecting different progenitors and/or different explosion modes.

MIMIZUKU is the first-generation mid-infrared instrument for the TAO 6.5-m telescope. It has three internal optical channels to cover a wide wavelength range from 2 to 38 µm. Of the three channels, the NIR channel is responsible for observations in the shortest wavelength range, shorter than 5.3 µm. The performance of the NIR channel is evaluated in the laboratory. Through the tests, we confirm the followings: 1) the detector (HAWAII 1RG with 5.3-µm cutoff) likely achieves ∼80% quantum efficiency; 2) imaging performance is sufficient to achieve seeing-limit spatial resolution; 3) system efficiencies in imaging mode are 2.4–31%; and 4) the system efficiencies in spectroscopic modes is 5–18%. These results suggest that the optical performance of the NIR channel is achieved as expected from characteristics of the optical components. However, calculations of the background levels and on-sky sensitivity based on these results suggest that neutral density (ND) filters are needed to avoid saturation in L ′ - and M′ -band observations and that the ND filters and the entrance window, made of chemical-vapor-deposition (CVD) diamond, significantly degrade the sensitivity in these bands. This means that the use of different window materials and improvements of the detector readout speed are required to achieve both near-infrared and long-wavelength mid-infrared (>30 µm) observations.

The University of Tokyo Atacama Observatory (TAO) is a project to build and operate an infrared-optimized 6.5m telescope at the summit of Cerro Chajnantor (5640 m.a.s.l). This is promoted by Institute of Astronomy, Graduate School of Science, the University of Tokyo in collaboration with many universities and institutes. The project is now approaching the final phase of the construction. Production of major components are almost completed. The primary mirror fabricated by Steward Observatory Richard F. Caris Mirror Lab in the University of Arizona was temporarily assembled in its support system and confirmed its performance by the optical test in the laboratory. The telescope mount, the enclosure system, and the mirror coating system were fabricated in Japan and already shipped to Chile. They are now stored in an open yard located in the foot area of Cerro Chajanator. The expansion of the summit access road, the summit leveling, the foundation work was completed. Now the construction work of the summit facilities is on-going. TAO will equip three instruments in early science phase. A near-infrared instrument SWIMS is completed, and now used as a PI-type instrument of Subaru telescope. A near-infrared spectrograph NICE which was used on the 1.6m Pirka telescope in Japan is being refurbished for TAO. A mid-infrared instrument MIMIZUKU successfully saw the first light on Subaru telescope and is being prepared for TAO in Japan. We expect to start science operation in FY2023.

ABSTRACT Permafrost occurs in the high Atacama Desert, and its thermal state was characterized at a study site 5,075 m a.s.l., at the lower regional altitude boundary for permafrost. The permafrost body is about 5 m thick and located in the hydrothermal alteration zone. The freeze–thaw layer and upper part of the permafrost layer temperatures were measured at 0 to 39 cm depth at 1-cm resolution throughout the year. The upper 3 cm of the ground experienced more than 100 freeze–thaw cycles in 2019. The maximum thaw depth was 14 cm. No significant thermal offset is observed between the annual mean of the surface temperature and the top permafrost boundary. The 14-m borehole reveals that the geothermal gradient was quite high at 200°C/km. In 2019 the seventy days of snow cover impacted the surface energy budget. Winter and summer snow conditions contribute to cooling the surface temperature regime in different ways.

In this Letter we report a discovery of a prominent flash of a peculiar overluminous Type Ia supernova, SN 2020hvf, in about 5 hr of the supernova explosion by the first wide-field mosaic CMOS sensor imager, the Tomo-e Gozen Camera. The fast evolution of the early flash was captured by intensive intranight observations via the Tomo-e Gozen high-cadence survey. Numerical simulations show that such a prominent and fast early emission is most likely generated from an interaction between 0.01 M (circle dot) circumstellar material (CSM) extending to a distance of similar to 10(13) cm and supernova ejecta soon after the explosion, indicating a confined dense CSM formation at the final evolution stage of the progenitor of SN 2020hvf. Based on the CSM-ejecta interaction-induced early flash, the overluminous light curve, and the high ejecta velocity of SN 2020hvf, we suggest that the SN 2020hvf may originate from a thermonuclear explosion of a super-Chandrasekhar-mass white dwarf ("super-M (Ch) WD"). Systematical investigations on explosion mechanisms and hydrodynamic simulations of the super-M (Ch) WD explosion are required to further test the suggested scenario and understand the progenitor of this peculiar supernova.