arXiv subjects

Masaomi Tanaka

Publications and source records attributed to Masaomi Tanaka.

At least 91 records · Page 5Linked to original sources

arXiv2020

Follow-up Observations for IceCube-170922A: Detection of Rapid Near-Infrared Variability and Intensive Monitoring of TXS 0506+056

We present our follow-up observations to search for an electromagnetic counterpart of the IceCube high-energy neutrino, IceCube-170922A. Monitoring observations of a likely counterpart, TXS 0506+056, are also described. First, we quickly took optical and near-infrared images of 7 flat-spectrum radio sources within the IceCube error region right after the neutrino detection and found a rapid flux decline of TXS 0506+056 in Kanata/HONIR J-band data. Motivated by this discovery, intensive follow-up observations of TXS 0506+056 are continuously done, including our monitoring imaging observations, spectroscopic observations, and polarimetric observations in optical and near-infrared wavelengths. TXS 0506+056 shows a large amplitude (~1.0 mag) variability in a time scale of several days or longer, although no significant variability is detected in a time scale of a day or shorter. TXS 0506+056 also shows a bluer-when-brighter trend in optical and near-infrared wavelengths. Structure functions of variabilities are examined and indicate that TXS 0506+056 is not a special blazar in terms of optical variability. Polarization measurement results of TXS 0506+056 are also discussed.

astro-ph.HE↗

arXiv2020

Photometric classification of HSC transients using machine learning

The advancement of technology has resulted in a rapid increase in supernova (SN) discoveries. The Subaru/Hyper Suprime-Cam (HSC) transient survey, conducted from fall 2016 through spring 2017, yielded 1824 SN candidates. This gave rise to the need for fast type classification for spectroscopic follow-up and prompted us to develop a machine learning algorithm using a deep neural network (DNN) with highway layers. This machine is trained by actual observed cadence and filter combinations such that we can directly input the observed data array into the machine without any interpretation. We tested our model with a dataset from the LSST classification challenge (Deep Drilling Field). Our classifier scores an area under the curve (AUC) of 0.996 for binary classification (SN Ia or non-SN Ia) and 95.3% accuracy for three-class classification (SN Ia, SN Ibc, or SN II). Application of our binary classification to HSC transient data yields an AUC score of 0.925. With two weeks of HSC data since the first detection, this classifier achieves 78.1% accuracy for binary classification, and the accuracy increases to 84.2% with the full dataset. This paper discusses the potential use of machine learning for SN type classification purposes.

astro-ph.IM↗

arXiv2020

Simulations of early kilonova emission from neutron star mergers

We present radiative transfer simulations for blue kilonovae hours after neutron star (NS) mergers by performing detailed opacity calculations for the first time. We calculate atomic structures and opacities of highly ionized elements (up to the tenth ionization) with atomic number Z = 20 - 56. We find that the bound-bound transitions of heavy elements are the dominant source of the opacities in the early phase (t < 1 day after the merger), and that the ions with a half-closed electron shell provide the highest contributions. The Planck mean opacity for lanthanide-free ejecta (with electron fraction of Ye = 0.30 - 0.40) can only reach around kappa ~ 0.5 - 1 cm^2 g^-1 at t = 0.1 day, whereas that increases up to kappa ~ 5 - 10 cm^2 g^-1 at t = 1 day. The spherical ejecta model with an ejecta mass of Mej = 0.05Msun gives the bolometric luminosity of ~ 2 x 10^42 erg s^-1 at t ~ 0.1 day. We confirm that the existing bolometric and multi-color data of GW170817 can be naturally explained by the purely radioactive model. The expected early UV signals reach 20.5 mag at t ~ 4.3 hours for sources even at 200 Mpc, which is detectable by the facilities such as Swift and the Ultraviolet Transient Astronomy Satellite (ULTRASAT). The early-phase luminosity is sensitive to the structure of the outer ejecta, as also pointed out by Kasen et al. (2017). Therefore, the early UV observations give strong constraints on the structure of the outer ejecta as well as the presence of a heating source besides r-process nuclei.

astro-ph.HE↗

arXiv2020

Constraint on the ejecta mass for a black hole-neutron star merger event candidate S190814bv

We derive the upper limit to the ejecta mass of S190814bv, a black hole-neutron star merger candidate, through the radiative transfer simulations for kilonovae with the realistic ejecta density profile as well as the detailed opacity and heating rate models. The limits to the ejecta mass strongly depend on the viewing angle. For the face-on observations ($\le45^\circ$), the total ejecta mass should be smaller than $0.1\,M_\odot$ for the average distance of S190814bv ($D=267$ Mpc), while larger mass is allowed for the edge-on observations. We also derive the conservative upper limits of the dynamical ejecta mass to be $0.02\,M_\odot$, $0.03\,M_\odot$, and $0.05\,M_\odot$ for the viewing angle $\le 20^\circ$, $\le 50^\circ$, and for $\le 90^\circ$, respectively. We show that the {\it iz}-band observation deeper than $22$ mag within 2 d after the GW trigger is crucial to detect the kilonova with the total ejecta mass of $0.06\,M_\odot$ at the distance of $D=300$ Mpc. We also show that a strong constraint on the NS mass-radius relation can be obtained if the future observations put the upper limit of $0.03\,M_\odot$ to the dynamical ejecta mass for a BH-NS event with the chirp mass smaller than $\lesssim 3\,M_\odot$ and effective spin larger than $\gtrsim 0.5$.

astro-ph.HE↗

arXiv2020

Systematic Opacity Calculations for Kilonovae

Coalescence of neutron stars gives rise to kilonova, thermal emission powered by radioactive decays of freshly synthesized r-process nuclei. Although observational properties are largely affected by bound-bound opacities of r-process elements, available atomic data have been limited. In this paper, we study element-to-element variation of the opacities in the ejecta of neutron star mergers by performing systematic atomic structure calculations of r-process elements for the first time. We show that the distributions of energy levels tend to be higher as electron occupation increases for each electron shell due to the larger energy spacing caused by larger effects of spin-orbit and electron-electron interactions. As a result, elements with a fewer number of electrons in the outermost shells tend to give larger contributions to the bound-bound opacities. This implies that Fe is not representative for the opacities of light r-process elements. The average opacities for the mixture of r-process elements are found to be kappa ~ 20-30 cm^2 g^{-1} for the electron fraction of Ye < 0.20, kappa ~ 3-5 cm^2 g^{-1} for Ye = 0.25-0.35, and kappa ~ 1 cm^2 g^{-1} for Ye = 0.40 at T = 5,000-10,000 K, and they steeply decrease at lower temperature. We show that, even with the same abundance or Ye, the opacity in the ejecta changes with time by one order of magnitude from 1 to 10 days after the merger. Our radiative transfer simulations with the new opacity data confirm that ejecta with a high electron fraction (Ye >~ 0.25, with no lanthanide) are needed to explain the early, blue emission in GW170817/AT2017gfo while lanthanide-rich ejecta (with a mass fraction of lanthanides ~ 5 x 10^{-3}) reproduce the long-lasting near-infrared emission.

astro-ph.HE↗

arXiv2020

Calcium-rich Transient SN 2019ehk in A Star-Forming Environment: Yet Another Candidate for An Ultra-Stripped Envelope Supernova

We present optical and near-infrared observations of SN~Ib~2019ehk. We show that it evolved to a Ca-rich transient according to its spectral properties and evolution in late phases. It, however, shows a few distinguishable properties from the canonical Ca-rich transients: a short-duration first peak in the light curve, high peak luminosity, and association with a star-forming environment. Indeed, some of these features are shared with iPTF14gqr and iPTF16hgs, which are candidates for a special class of core-collapse SNe (CCSNe): the so-called ultra-stripped envelope SNe, i.e., a relatively low-mass He (or C+O) star explosion in a binary as a precursor of double neutron star binaries. The estimated ejecta mass ($0.43 M_\odot$) and explosion energy ($1.7 \times 10^{50} $~erg) are consistent with this scenario. The analysis of the first peak suggests existence of dense circumstellar material in the vicinity of the progenitor, implying a CCSN origin. Based on these analyses, we suggest SN 2019ehk is another candidate for an ultra-stripped envelope SN. These ultra-stripped envelope SN candidates seem to form a subpopulation among Ca-rich transients, associated with young population. We propose that the key to distinguishing this population is the early first peak in their light curves.

astro-ph.HE↗

arXiv2020

Late-Phase Spectropolarimetric Observations of Superluminous Supernova SN 2017egm to Probe the Geometry of the Inner Ejecta

We present our spectropolarimetric observations of SN 2017egm, a Type I superluminous supernova (SLSN-I) in a nearby galaxy NGC 3191, with Subaru telescope at 185.0 days after the g-band maximum light. This is the first spectropolarimetric observation for SLSNe at late phases. We find that the degree of the polarization in the late phase significantly changes from that measured at the earlier phase. The spectrum at the late phase shows a strong Ca emission line and therefore we reliably estimate the interstellar polarization component assuming that the emission line is intrinsically unpolarized. By subtracting the estimated interstellar polarization, we find that the intrinsic polarization at the early phase is only ~ 0.2 %, which indicates an almost spherical photosphere, with an axial ratio ~ 1.05. The intrinsic polarization at the late phase increases to ~ 0.8 %, which corresponds to the photosphere with an axial ratio 1.2. A nearly constant position angle of the polarization suggests the inner ejecta are almost axisymmetric. By these observations, we conclude that the inner ejecta are more aspherical than the outer ejecta. It may suggest the presence of a central energy source producing aspherical inner ejecta.

astro-ph.HE↗

arXiv2020

The HSC-SSP Transient Survey: Implications from Early Photometry and Rise Time of Normal Type Ia Supernovae

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 non-degenerate companion or surrounding dense circumstellar matter but also radiation powered by radioactive decays of $^{56}$Ni 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 $^{56}$Ni 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.

astro-ph.HE↗

arXiv2020

Rapidly Evolving Transients from the Hyper Suprime-Cam SSP Transient Survey

Rapidly evolving transients form a new class of transients which show shorter timescales of the light curves than those of typical core-collapse and thermonuclear supernovae. We performed a systematic search for rapidly evolving transients using the deep data taken with the Hyper Suprime-Cam Subaru Strategic Program Transient Survey. By measuring the timescales of the light curves of 1824 transients, we identified 5 rapidly evolving transients. Our samples are found in a wide range of redshifts (0.3 $\le$ z $\le$ 1.5) and peak absolute magnitudes ($-$17 $\ge$ $M_i$ $\ge$ $-$20). The properties of the light curves are similar to those of the previously discovered rapidly evolving transients. They show a relatively blue spectral energy distribution, with the best-fit blackbody of 8,000 - 18,000 K. We show that some of the transients require power sources other than the radioactive decays of $^{56}$Ni because of their high peak luminosities and short timescales. The host galaxies of all the samples are star-forming galaxies, suggesting a massive star origin for the rapidly evolving transients. The event rate is roughly estimated to be $\sim$4,000 events yr$^{-1}$ Gpc$^{-3}$, which is about 1 $\%$ of core-collapse supernovae.

astro-ph.HE↗

arXiv2020

Extended calculations of energy levels and transition rates for singly ionized lanthanide elements I: Pr - Gd

Lanthanide elements play important roles as an opacity source in the ejected material from neutron star mergers. Accurate and complete atomic data are necessary to evaluate the opacities and to analyze the observed data. In this paper, we perform extended, ab-initio atomic calculations from Pr II (Z=59) to Gd II (Z=64). By using multiconfiguration Dirac-Hartree-Fock and relativistic configuration-interaction methods, implemented in the general-purpose relativistic atomic structure package GRASP2K, we calculate the energy levels and transition data of electric dipole transitions. These computations are based on strategies (with small variations) of Nd II published by Gaigalas et al. (2019). Accuracy of data is evaluated by comparing computed energy levels with the NIST database or other works. For the energy levels, we obtain the average relative accuracy of 8%, 12%, 6%, 8%, and 7% for Pr II, Pm II, Sm II, Eu II, and Gd II ions, respectively as compared with the NIST data. Accuracy of energy transfer to the wavelength as 3%, 14% and 11% for Pr II, Eu II and Gd II. Our computed E1 type transition probabilities are in good agreement with experimental values presented by other authors especially for strong transitions.

physics.atom-ph↗

arXiv2020

Energy level structure and transition data of Er$^{2+}$

The main aim of this paper is to present accurate energy levels of the ground [Xe]$4f^{12}$ and first excited [Xe]$4f^{11}5d$ configurations of Er$^{2+}$. The energy level structure of the Er$^{2+}$ ion was computed using the multiconfiguration Dirac-Hartree-Fock and relativistic configuration interaction (RCI) methods, as implemented in the GRASP2018 program package. The Breit interaction, self-energy and vacuum polarization corrections were included in the RCI computations. The zero-first-order approach was used in the computations. Energy levels with the identification in $LS$ coupling for all (399) states belonging to the [Xe]$4f^{12}$ and [Xe]$4f^{11}5d$ configurations are presented. Electric dipole (E1) transition data between the levels of these two configurations are computed. The accuracy of the these data are evaluated by studying the behaviour of the transition rates as functions of the gauge parameter as well as by evaluating the cancellation factors. The core electron correlations were studied using different strategies. Root-mean-square deviations obtained in this study for states of the ground and excited configurations from the available experimental or semi-empirical data are 649 cm$^{-1}$, and 747 cm$^{-1}$, respectively.

physics.atom-ph↗

arXiv2020

On the possibility of GW190425 being a black hole--neutron star binary merger

We argue that the kilonova/macronova associated with the gravitational-wave event GW190425 could have been bright enough to be detected if it was caused by the merger of a low-mass black hole and a neutron star. Although tidal disruption occurs for such a low-mass black hole as is generally expected, the masses of the dynamical ejecta are limited to <~10^{-3}M_sun, which is consistent with previous work in the literature. The remnant disk could be as massive as 0.05--0.1M_sun, and the disk outflow of ~0.01--0.03M_sun is likely to be driven by viscous or magnetohydrodynamic effects. The disk outflow may not be neutron-rich enough to synthesize an abundance of lanthanide elements, even in the absence of strong neutrino emitter, if the ejection is driven on the viscous time scale of >~0.3s. If this is the case, the opacity of the disk outflow is kept moderate, and a kilonova/macronova at the distance of GW190425 reaches a detectable brightness of 20--21mag at 1day after merger for most viewing angles. If some disk activity ejects the mass within ~0.1s, instead, lanthanide-rich outflows would be launched and the detection of emission becomes challenging. Future possible detections of kilonovae/macronovae from GW190425-like systems will disfavor the prompt collapse of binary neutron stars and a non-disruptive low-mass black hole--neutron star binary associated with a small neutron-star radius, whose mass ejection is negligible. The host-galaxy distance will constrain the viewing angle and deliver further information about the mass ejection.

astro-ph.HE↗

arXiv2020

$g$-matrix folding-model approach to reaction cross sections for scattering of Ca isotopes on a C target

We first predict the ground-state properties of Ca isotopes, using the Gogny-D1S Hartree-Fock-Bogoliubov (GHFB) with and without the angular momentum projection (AMP). We find that $^{64}$Ca is an even-dripline nucleus and $^{59}$Ca is an odd-dripline nucleus, using $A$ dependence of the one-neutron separation energy $S_{1}$ and the two-neutron separation energy, $S_{2}$. As for $S_{1}$, $S_{2}$ and the binding energies $E_{\rm B}$, our results agree with the experimental data in $^{40-58}$Ca. As other ground-state properties of $^{40-60,62,64}$Ca, we predict charge, proton, neutron, matter radii, neutron skin and deformation. As for charge radii, our results are consistent with the experimental data in $^{40-52}$Ca. For $^{48}$Ca, our results on proton, neutron, matter radii agree with the experimental data. Very lately, Tanaka et. al. measured interaction cross sections for $^{42-51}$Ca scattering on a $^{12}$C target at an incident energy per nucleon of $E_{\rm lab}=280$MeV. Secondly, we predict reaction cross sections $σ_{\rm R}$ for $^{40-60,62,64}$Ca, using a chiral $g$-matrix double-folding model (DFM). To show the reliability of the present DFM for $σ_{\rm R}$, we apply the DFM for the data on $^{12}$C scattering on $^{9}$Be, $^{12}$C, $^{27}$Al targets in $30 < E_{\rm lab} < 400 $MeV, and show that the present DFM is good in $30 < E_{\rm lab} < 100 $MeV and $250 < E_{\rm lab} < 400 $MeV. For $110 < E_{\rm lab} < 240 $MeV, our results have small errors. To improve the present DFM for $σ_{\rm R}$, we propose two prescriptions.

nucl-th↗

arXiv2019

Diversity of kilonova light curves

We perform radiative transfer simulations for kilonova in various situations, including the cases of prompt collapse to a black hole from neutron-star mergers, high-velocity ejecta possibly accelerated by magnetars, and a black hole-neutron star merger. Our calculations are done employing ejecta profiles predicted by numerical-relativity simulations and a new line list for all the r-process elements. We found that (i) the optical emission for binary neutron stars promptly collapsing to a black hole would be fainter by $\gtrsim1$-$2\,{\rm mag}$ than that found in GW170817, while the infrared emission could be as bright as that in GW170817 if the post-merger ejecta is as massive as $\approx0.01\,M_\odot$; (ii) the kilonova would be brighter than that observed in GW170817 for the case that the ejecta is highly accelerated by the electromagnetic energy injection from the remnant, but it would decline rapidly and the magnitude would become fainter than in GW170817 within a few days; (iii) the optical emission from a black hole-neutron star merger ejecta could be as bright as that observed in GW170817 for the case that sufficiently large amount of matter is ejected ($\gtrsim0.02\,M_\odot$), while the infrared brightness would be brighter by $1$-$2\,{\rm mag}$ at the same time. We show that the difference in the ejecta properties would be imprinted in the differences in the peak brightness and time of peak. This indicates that we may be able to infer the type of the central engine for kilonovae by observation of the peak in the multiple band.

astro-ph.HE↗

arXiv2019

An optical search for transients lasting a few seconds

Using a prototype of the Tomo-e Gozen wide-field CMOS mosaic camera, we acquire wide-field optical images at a cadence of 2 Hz and search them for transient sources of duration 1.5 to 11.5 seconds. Over the course of eight nights, our survey encompasses the equivalent of roughly two days on one square degree, to a fluence equivalent to a limiting magnitude about $V = 15.6$ in a 1-second exposure. After examining by eye the candidates identified by a software pipeline, we find no sources which meet all our criteria. We compute upper limits to the rate of optical transients consistent with our survey, and compare those to the rates expected and observed for representative sources of ephemeral optical light.

astro-ph.IM↗

arXiv2019

A rapidly declining transient discovered with Subaru/Hyper Suprime-Cam

We perform a high-cadence transient survey with 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 with an image subtraction technique. The rate of brightness change is $+1.28^{+0.40}_{-0.27}~{\rm mag~day^{-1}}$ ($+1.83^{+0.57}_{-0.39}~{\rm mag~day^{-1}}$) in the observer (rest) frame and the rest-frame color between $3400$ and $4400~\unicode[.8,0]{x212B}$ is $M_{\rm 3400\unicode[.8,0]{x212B}}-M_{\rm 4400\unicode[.8,0]{x212B}}=-0.4$. 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 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 Type IIb supernova 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 $25M_{\odot}$ red supergiant star with a small explosion energy of $\leq0.4\times10^{51}$ 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.

astro-ph.HE↗

arXiv2019

HSC16aayt: Slowly evolving interacting transient rising for more than 100 days

We report our observations of HSC16aayt (SN 2016jiu), which was discovered by the Subaru/Hyper Suprime-Cam (HSC) transient survey conducted as part of Subaru Strategic Program (SSP). 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.

astro-ph.HE↗

arXiv2019

The Hyper Suprime-Cam SSP Transient Survey in COSMOS: Overview

We present an overview of a deep transient survey of the COSMOS field with the Subaru Hyper Suprime-Cam (HSC). The survey was performed for the 1.77 deg$^2$ ultra-deep layer and 5.78 deg$^2$ deep layer in the Subaru Strategic Program over 6- and 4-month periods from 2016 to 2017, respectively. The ultra-deep layer shows a median depth per epoch of 26.4, 26.3, 26.0, 25.6, and 24.6 mag in $g$, $r$, $i$, $z$, and $y$ bands, respectively; the deep layer is $\sim0.6$ mag shallower. In total, 1,824 supernova candidates were identified. Based on light curve fitting and derived light curve shape parameter, we classified 433 objects as Type Ia supernovae (SNe); among these candidates, 129 objects have spectroscopic or COSMOS2015 photometric redshifts and 58 objects are located at $z > 1$. Our unique dataset doubles the number of Type Ia SNe at $z > 1$ and enables various time-domain analyses of Type II SNe, high redshift superluminous SNe, variable stars, and active galactic nuclei.

astro-ph.GA↗