arXiv subjects

Nozomu Tominaga

Publications and source records attributed to Nozomu Tominaga.

At least 55 records · Page 3Linked to original sources

arXiv2022

Deep simultaneous limits on optical emission from FRB 20190520B by 24.4 fps observations with Tomo-e Gozen

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 $\gtrsim 0.1$ 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 $\gtrsim 5$ Jy ms.

astro-ph.HE↗

arXiv2022

Video Observations of Tiny Near-Earth Objects with Tomo-e Gozen

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. A rotational period of a tiny asteroid reflects its dynamical history and physical properties since smaller objects are sensitive to the 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 any 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 either by a realistic tensile strength of NEOs or 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.

astro-ph.EP↗

arXiv2022

Deep-learning Real/Bogus classification for the Tomo-e Gozen transient survey

We present a deep neural network Real/Bogus classifier that improves classification performance in the Tomo-e Gozen transient survey by handling label errors in the training data. In the wide-field, high-frequency transient survey with Tomo-e Gozen, the performance of conventional convolutional neural network classifier is not sufficient as about $10^6$ bogus detections appear every night. In need of a better classifier, we have developed a new two-stage training method. In this training method, label errors in the training data are first detected by normal supervised learning classification, and then they are unlabeled and used for training of semi-supervised learning. For actual observed data, the classifier with this method achieves an area under the curve (AUC) of 0.9998 and a false positive rate (FPR) of 0.0002 at true positive rate (TPR) of 0.9. This training method saves relabeling effort by humans and works better on training data with a high fraction of label errors. By implementing the developed classifier in the Tomo-e Gozen pipeline, the number of transient candidates was reduced to $\sim$40 objects per night, which is $\sim$1/130 of the previous version, while maintaining the recovery rate of real transients. This enables more efficient selection of targets for follow-up observations.

astro-ph.IM↗

arXiv2022

MUSSES2020J: The Earliest Discovery of a Fast Blue Ultraluminous Transient at Redshift 1.063

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.

astro-ph.HE↗

arXiv2022

Four-hundred Very Metal-Poor Stars Studied with LAMOST and Subaru. II. Elemental abundances

We present homogeneous abundance analysis of over 20 elements for 385 very metal-poor (VMP) stars based on the LAMOST survey and follow-up observations with the Subaru Telescope. It is the largest high-resolution VMP sample (including 363 new objects) studied by a single program, and the first attempt to accurately determine evolutionary stages for such a large sample based on Gaia parallaxes. The sample covers a wide metallicity range from [Fe/H]=-1.7 down to [Fe/H]=-4.3, including over 110 objects with [Fe/H]<-3.0. The expanded coverage in evolutionary status makes it possible to define abundance trends respectively for giants and turn-off stars. The newly obtained abundance data confirm most abundance trends found by previous studies, but also provide useful update and new sample of outliers. The Li plateau is seen in -2.5 < [Fe/H] <-1.7 in our sample, whereas the average Li abundance is clearly lower at lower metallicity. Mg, Si, and Ca are overabundant with respect to Fe, showing decreasing trend with increasing metallicity. Comparisons with chemical evolution models indicate that the over-abundance of Ti, Sc, and Co are not well reproduced by current theoretical predictions. Correlations are seen between Sc and alpha-elements, while Zn shows a detectable correlation only with Ti but not with other alpha-elements. The fraction of carbon-enhanced stars ([C/Fe]> 0.7) is in the range of 20-30% for turn-off stars depending on the treatment of objects for which C abundance is not determined, which is much higher than that in giants (~8%). Twelve Mg-poor stars ([Mg/Fe] < 0.0) have been identified in a wide metallicity range from [Fe/H] =-3.8 through -1.7. Twelve Eu-rich stars ([Eu/Fe]> 1.0) have been discovered in -3.4 <[Fe/H]< -2.0, enlarging the sample of r-process-enhanced stars with relatively high metallicity.

astro-ph.SR↗

arXiv2022

Four-hundred Very Metal-poor Stars studied with LAMOST and Subaru. I. Survey Design, Follow-up Program, and Binary Frequency

The chemical abundances of very metal-poor stars provide important constraints on the nucleosynthesis of the first generation of stars and early chemical evolution of the Galaxy. We have obtained high-resolution spectra with the Subaru Telescope for candidates of very metal-poor stars selected with a large survey of Galactic stars carried out with LAMOST. In this series of papers, we report on the elemental abundances of about 400 very metal-poor stars and discuss the kinematics of the sample obtained by combining the radial velocities measured in this study and recent astrometry obtained with Gaia. This paper provides an overview of our survey and follow-up program, and reports radial velocities for the whole sample. We identify seven double-lined spectroscopic binaries from our high-resolution spectra, for which radial velocities of the components are reported. We discuss the frequency of such relatively short-period binaries at very low metallicity.

astro-ph.SR↗

arXiv2022

Can Population III stars be major origins of both merging binary black holes and extremely metal poor stars?

Population (Pop) III stars, first stars, or metal-free stars are made of primordial gas. We have examined if they can be dominant origins of merging binary black holes (BHs) and extremely metal-poor stars. The abundance pattern of EMP stars is helpful to trace back the properties of Pop III stars. We have confirmed previous arguments that the observed BH merger rate needs Pop III star formation efficiency 10 times larger than theoretically predicted values, while the cosmic reionization history still permits such a high Pop III star formation efficiency. On the other hand, we have newly found that the elemental abundance pattern of EMP stars only allows the Pop III initial mass function with the minimum mass of $\sim 15 - 27$ $M_\odot$. In other words, the minimum mass must not deviate largely from the critical mass below and above which Pop III stars leave behind neutron stars and BHs, respectively. Pop III stars may be still a dominant origin of merging binary BHs but our study has reduced the allowed parameter space under a hypothesis that EMP stars are formed from primordial gas mixed with Pop III supernova ejecta.

astro-ph.HE↗

arXiv2021

Top-heavy stellar mass distribution in galactic nuclei inferred from the universally high abundance ratio of [Fe/Mg]

Recent observations of active galactic nuclei (AGNs) have shown a high Fe~II/Mg~II line-flux ratio in their broad-line regions, nearly independent of redshift up to $z \gtrsim 6$. The high flux ratio requires rapid production of iron in galactic nuclei to reach an abundance ratio of ${\rm [Fe/Mg]} \gtrsim 0.2$ as high as those observed in matured galaxies in the local universe. We propose a possible explanation of rapid iron enrichment in AGNs by massive star formation that follows a top-heavy initial mass function (IMF) with a power-law index of $Γ$ larger than the canonical value of $Γ=-2.35$ for a Salpeter IMF. Taking into account metal production channels from different types of SNe, we find that the high value of ${\rm [Fe/Mg]} \gtrsim 0.2$ requires the IMF to be characterized with $Γ\gtrsim -1$ ($Γ\gtrsim 0$) and a high-mass cutoff at $M_{\rm max} \simeq 100$--$150~{\rm M_\odot}$ $(M_{\rm max} \gtrsim 250~{\rm M_\odot})$. Given the conditions, core-collapse SNe with $M_\ast \gtrsim 70~{\rm M_\odot}$ and pair-instability SNe give a major contribution for iron enrichment. Such top-heavy stellar IMFs would be a natural consequence from mass growth of stars formed in dense AGN disks under Bondi-like gas accretion that is regulated by feedback at $M_\ast \gtrsim 10~{\rm M_\odot}$. The massive stellar population formed in AGN disks also leave stellar-mass black hole remnants, whose mergers associated with gravitational-wave emission account for at most 10 \% of the merger rate inferred from LIGO/Virgo observations to simultaneously explain the high ${\rm [Fe/Mg]}$ ratio with metal ejection.

astro-ph.GA↗

arXiv2021

Discovery of the Fastest Early Optical Emission from Overluminous SN Ia 2020hvf: A Thermonuclear Explosion within a Dense Circumstellar Environment

In this Letter we report a discovery of a prominent flash of a peculiar overluminous Type Ia supernova, SN 2020hvf, in about 5 hours 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 $0.01~M_{\odot}$ circumstellar material (CSM) extending to a distance of $\sim$$10^{13}~\text{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-$M\rm_{Ch}$ WD"). Systematical investigations on explosion mechanisms and hydrodynamic simulations of the super-$M\rm_{Ch}$ WD explosion are required to further test the suggested scenario and understand the progenitor of this peculiar supernova.

astro-ph.HE↗

arXiv2021

A VLA Survey of Late-time Radio Emission from Superluminous Supernovae and the Host Galaxies

We present the results of 3 GHz radio continuum observations of 23 superluminous supernovae (SLSNe) and their host galaxies by using the Karl G. Jansky Very Large Array conducted 5-21 years after the explosions. The sample consists of 15 Type I and 8 Type II SLSNe at z < 0.3, providing one of the largest sample of SLSNe with late-time radio data. We detected radio emission from one SLSN (PTF10hgi) and 5 hosts with a significance of >5$σ$. No time variability is found in late-time radio light curves of the radio-detected sources in a timescale of years except for PTF10hgi, whose variability is reported in a separate study. Comparison of star-formation rates (SFRs) derived from the 3 GHz flux densities with those derived from SED modeling based on UV-NIR data shows that four hosts have an excess of radio SFRs, suggesting obscured star formation. Upper limits for undetected hosts and stacked results show that the majority of the SLSN hosts do not have a significant obscured star formation. By using the 3 GHz upper limits, we constrain the parameters for afterglows arising from interaction between initially off-axis jets and circumstellar medium (CSM). We found that the models with higher energies ($E_{\rm iso} \gtrsim$ several $\times 10^{53}$ erg) and CSM densities ($n \gtrsim 0.01$ cm$^{-3}$) are excluded, but lower energies or CSM densities are not excluded with the current data. We also constrained the models of pulsar wind nebulae powered by a newly born magnetar for a subsample of SLSNe with model predictions in the literature.

astro-ph.HE↗

arXiv2021

Origin of metals in old Milky Way halo stars based on GALAH and Gaia

Stellar and supernova nucleosynthesis in the first few billion years of the cosmic history have set the scene for early structure formation in the Universe, while little is known about their nature. Making use of stellar physical parameters measured by GALAH Data Release 3 with accurate astrometry from the Gaia EDR3, we have selected $\sim 100$ old main-sequence turn-off stars (ages $\gtrsim 12$ Gyrs) with kinematics compatible with the Milky Way stellar halo population in the Solar neighborhood. Detailed homogeneous elemental abundance estimates by GALAH DR3 are compared with supernova yield models of Pop~III (zero-metal) core-collapse supernovae (CCSNe), normal (non-zero-metal) CCSNe, and Type Ia supernovae (SN Ia) to examine which of the individual yields or their combinations best reproduce the observed elemental abundance patterns for each of the old halo stars ("OHS"). We find that the observed abundances in the OHS with [Fe/H]$>-1.5$ are best explained by contributions from both CCSNe and SN~Ia, where the fraction of SN~Ia among all the metal-enriching SNe is up to 10-20 % for stars with high [Mg/Fe] ratios and up to 20-27 % for stars with low [Mg/Fe] ratios, depending on the assumption about the relative fraction of near-Chandrasekhar-mass SNe Ia progenitors. The results suggest that, in the progenitor systems of the OHS with [Fe/H]$>-1.5$, $\sim$ 50-60% of Fe mass originated from normal CCSNe at the earliest phases of the Milky Way formation. These results provide an insight into the birth environments of the oldest stars in the Galactic halo.

astro-ph.GA↗

arXiv2021

The electron-capture origin of supernova 2018zd

In the transitional mass range ($\sim$ 8-10 solar masses) between white dwarf formation and iron core-collapse supernovae, stars are expected to produce an electron-capture supernova. Theoretically, these progenitors are thought to be super-asymptotic giant branch stars with a degenerate O+Ne+Mg core, and electron capture onto Ne and Mg nuclei should initiate core collapse. However, no supernovae have unequivocally been identified from an electron-capture origin, partly because of uncertainty in theoretical predictions. Here we present six indicators of electron-capture supernovae and show that supernova 2018zd is the only known supernova having strong evidence for or consistent with all six: progenitor identification, circumstellar material, chemical composition, explosion energy, light curve, and nucleosynthesis. For supernova 2018zd, we infer a super-asymptotic giant branch progenitor based on the faint candidate in the pre-explosion images and the chemically-enriched circumstellar material revealed by the early ultraviolet colours and flash spectroscopy. The light-curve morphology and nebular emission lines can be explained with the low explosion energy and neutron-rich nucleosynthesis produced in an electron-capture supernova. This identification provides insights into the complex stellar evolution, supernova physics, cosmic nucleosynthesis, and remnant populations in the transitional mass range.

astro-ph.HE↗

arXiv2021

J-GEM optical and near-infrared follow-up of gravitational wave events during LIGO's and Virgo's third observing run

The Laser Interferometer Gravitational-wave Observatory Scientific Collaboration and Virgo Collaboration (LVC) sent out 56 gravitational-wave (GW) notices during the third observing run (O3). Japanese collaboration for Gravitational wave ElectroMagnetic follow-up (J-GEM) performed optical and near-infrared observations to identify and observe an electromagnetic (EM) counterpart. We constructed web-based system which enabled us to obtain and share information of candidate host galaxies for the counterpart, and status of our observations. Candidate host galaxies were selected from the GLADE catalog with a weight based on the three-dimensional GW localization map provided by LVC. We conducted galaxy-targeted and wide-field blind surveys, real-time data analysis, and visual inspection of observed galaxies. We performed galaxy-targeted follow-ups to 23 GW events during O3, and the maximum probability covered by our observations reached to 9.8%. Among them, we successfully started observations for 10 GW events within 0.5 days after the detection. This result demonstrates that our follow-up observation has a potential to constrain EM radiation models for a merger of binary neutron stars at a distance of up to $\sim$100~Mpc with a probability area of $\leq$ 500~deg$^2$.

astro-ph.HE↗

arXiv2021

Gravitational Wave Physics and Astronomy in the nascent era

The detections of gravitational waves (GW) by LIGO/Virgo collaborations provide various possibilities to physics and astronomy. We are quite sure that GW observations will develop a lot both in precision and in number owing to the continuous works for the improvement of detectors, including the expectation to the newly joined detector, KAGRA, and the planned detector, LIGO-India. In this occasion, we review the fundamental outcomes and prospects of gravitational wave physics and astronomy. We survey the development focusing on representative sources of gravitational waves: binary black holes, binary neutron stars, and supernovae. We also summarize the role of gravitational wave observations as a probe of new physics.

gr-qc↗

arXiv2021

Variability of Late-time Radio Emission in SLSN PTF10hgi

We report the time variability of the late-time radio emission in a Type-I superluminous supernova (SLSN), PTF10hgi, at z = 0.0987. The Karl G. Jansky Very Large Array 3 GHz observations at 8.6 and 10 years after the explosion both detected radio emission with a ~40% decrease in flux density in the second epoch. This is the first report of a significant variability of the late-time radio light curve in a SLSN. Through combination with previous measurements in two other epochs, we constrained both the rise and decay phases of the radio light curve over three years, peaking at approximately 8-9 years after the explosion with a peak luminosity of L(3GHz) = 2 x 10^21 W/Hz. Possible scenarios for the origin of the variability are an active galactic nucleus (AGN) in the host galaxy, an afterglow caused by the interaction between an off-axis jet and circumstellar medium, and a wind nebula powered by a newly-born magnetar. Comparisons with models show that the radio light curve can be reproduced by both the afterglow model and magnetar wind nebula model. Considering the flat radio spectrum at 1-15 GHz and an upper limit at 0.6 GHz obtained in previous studies, plausible scenarios are a low-luminosity flat-spectrum AGN or a magnetar wind nebula with a shallow injection spectral index.

astro-ph.HE↗

arXiv2021

Observational properties of a general relativistic instability supernova from a primordial supermassive star

We present the expected observational properties of a general relativistic instability supernova (GRSN) from the 55,500 Msun primordial (Population III) star. Supermassive stars exceeding 1e4 Msun may exist in the early Universe. They are generally considered to collapse through the general relativistic instability to be seed black holes to form supermassive (~ 1e9 Msun) black holes observed as high-redshift quasars. Some of them, however, may explode as GRSNe if the explosive helium burning unbinds the supermassive stars following the collapse triggered by the general relativistic instability. We perform the radiation hydrodynamics simulation of the GRSN starting shortly before the shock breakout. We find that the GRSN is characterized by a long-lasting (550 d) luminous (1.5e44 erg/s) plateau phase with the photospheric temperature of around 5000 K in the rest frame. The plateau phase lasts for decades when it appears at high redshifts and it will likely be observed as a persistent source in the future deep near-infrared imaging surveys. Especially, the near-infrared images reaching 29 AB magnitude that can be obtained by Galaxy and Reionization EXplorer (G-REX) and James Webb Space Telescope (JWST) allow us to identify GRSNe up to z ~ 15. Deeper images enable us to discover GRSNe at even higher redshifts. Having extremely red color, they can be distinguished from other persistent sources such as high-redshift galaxies by using color information. We conclude that the deep near-infrared images are able to constrain the existence of GRSNe from the primordial supermassive stars in the Universe even without the time domain information.

astro-ph.HE↗

arXiv2021

Relationship between Radar Cross Section and Optical Magnitude based on Radar and Optical Simultaneous Observations of Faint Meteors

Radar and optical simultaneous observations of meteors are important to understand the size distribution of the interplanetary dust. However, faint meteors detected by high power large aperture radar observations, which are typically as faint as 10 mag. in optical, have not been detected until recently in optical observations, mainly due to insufficient sensitivity of the optical observations. In this paper, two radar and optical simultaneous observations were organized. The first observation was carried out in 2009 to 2010 using Middle and Upper Atmosphere Radar (MU radar) and an image-intensified CCD camera. The second observation was carried out in 2018 using the MU radar and a mosaic CMOS camera, Tomo-e Gozen, mounted on the 1.05-m Kiso Schmidt Telescope. In total, 331 simultaneous meteors were detected. The relationship between radar cross sections and optical V-band magnitudes was well approximated by a linear function. A transformation function from the radar cross section to the V-band magnitude was derived for sporadic meteors. The transformation function was applied to about 150,000 meteors detected by the MU radar in 2009--2015, large part of which are sporadic, and a luminosity function was derived in the magnitude range of $-1.5$ to $9.5$ mag. The luminosity function was well approximated by a single power-law function with the population index of $r = 3.52{\pm}0.12$. The present observation indicates that the MU radar has capability to detect interplanetary dust of $10^{-5}$ to $10^{0}$ g in mass as meteors.

astro-ph.EP↗

arXiv2021

Optical follow-up observation for GW event S190510g using Subaru/Hyper Suprime-Cam

A gravitational wave event, S190510g, which was classified as a binary-neutron-star coalescence at the time of preliminary alert, was detected by LIGO/Virgo collaboration on May 10, 2019. At 1.7 hours after the issue of its preliminary alert, we started a target-of-opportunity imaging observation in Y-band to search for its optical counterpart using the Hyper Suprime-Cam (HSC) on the Subaru Telescope. The observation covers a 118.8 deg$^2$ sky area corresponding to 11.6% confidence in the localization skymap released in the preliminary alert and 1.2% in the updated skymap. We divided the observed area into two fields based on the availability of HSC reference images. For the fields with the HSC reference images, we applied an image subtraction technique; for the fields without the HSC reference images, we sought individual HSC images by matching a catalog of observed objects with the PS1 catalog. The search depth is 22.28 mag in the former method and the limit of search depth is 21.3 mag in the latter method. Subsequently, we performed visual inspection and obtained 83 candidates using the former method and 50 candidates using the latter method. Since we have only the 1-day photometric data, we evaluated probability to be located inside the 3D skymap by estimating their distances with photometry of associated extended objects. We found three candidates are likely located inside the 3D skymap and concluded they could be an counterpart of S190510g, while most of 133 candidates were likely to be supernovae because the number density of candidates was consistent with the expected number of supernova detections. By comparing our observational depth with a light curve model of such a kilonova reproducing AT2017gfo, we show that early-deep observations with the Subaru/HSC can capture the rising phase of blue component of kilonova at the estimated distance of S190510g (~230 Mpc).

astro-ph.HE↗