arXiv subjects

Yuji Urata

Publications and source records attributed to Yuji Urata.

At least 19 recordsLinked to original sources

arXiv2026

A pilot submillimeter search for IceCube neutrino counterparts: JCMT follow-up and dusty-galaxy catalog associations

The submillimeter band offers a complementary route to identifying electromagnetic counterparts of high-energy neutrinos, tracing relativistic transients and persistent dust-obscured galaxies. We present a pilot search combining JCMT/SCUBA-2 follow-up of IceCube events with catalog searches for bright high-redshift dusty systems. Among nine rapidly observed IceCube fields, sources of particular interest were found in three: JCMT0402-0424 ("Shadow Blaster") in IC 210922A, a transient-like source following IC 220115A, and two known blazars in IC 240105A. ALMA resolved the Shadow Blaster into a strongly lensed dusty star-forming galaxy at z=2.988 with a compact gas-rich starburst core. Motivated by this result, we cross-matched IceCube localizations with the Herschel Bright Sources (HerBS) and Planck High-Redshift Source Candidate (PHZ) catalogs. Among 51 historical IceCube events with A_bbox,90 <= pi deg^2, four fields contain at least one HerBS source, compared with 0.723 expected from right-ascension scrambling (fixed-cut p=4.18e-3). Scanning the localization-area threshold gives p_global,A=0.0135. Applying the same IceCube selection to PHZ gives four associated fields versus 3.544 expected (p=0.478). Because the focused HerBS selection followed a broader exploratory analysis with trials-corrected p_global=0.130, the focused result is suggestive rather than definitive. The four well-localized HerBS fields contain six high-redshift DSFGs with apparent L_IR~10^13 L_sun. ALMA imaging and published spectroscopy reveal compact or multiple-component morphologies, extreme molecular-gas kinematics, and possible obscured nuclear activity. These results support a hierarchical strategy: identify luminous submillimeter candidates through wide-field imaging and catalogs, then characterize them at high angular resolution to test for compact gas-rich calorimetric conditions and obscured AGN activity.

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arXiv2026

Compact dusty starbursts at cosmic noon linked to high-energy neutrinos

The origin of high-energy astrophysical neutrinos remains unresolved, and secure electromagnetic counterparts to individual events are rare despite rapid follow-up. Dusty star-forming galaxies (DSFGs) at cosmic noon (z ~ 1-4) are natural cosmic-ray calorimeters, yet observational links between DSFGs and neutrinos have remained elusive. Here we report a compact-core DSFG within an IceCube localization, JCMT0402-0424, a quadruply lensed galaxy at z = 2.988 located inside the 90% containment region of the IceCube event IC 210922A. ALMA imaging and lens modeling resolve a highly magnified, compact starburst with no bright gamma-ray or X-ray counterpart above current sensitivity limits. Considering the positional agreement, the low chance-coincidence probability (less than about 1%) for such an extreme submillimeter source, the absence of equally plausible alternatives in the field, and the compact, gas-rich core revealed by ALMA, JCMT0402-0424 is the most plausible electromagnetic counterpart candidate within the IC 210922A localization. In a population context, compact-core starbursts at cosmic noon can provide a non-negligible population-level contribution to the diffuse high-energy neutrino background, even though the neutrino yield from any single DSFG is modest. This result connects high-energy neutrino production to the peak epoch of cosmic star formation, opening a new avenue to probe galaxy evolution and cosmic-ray acceleration across cosmic time.

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arXiv2024

Multiwavelength Afterglow Analysis of GRB 221009A: Unveiling the Evolution of a Cooling Break in a Wind-like Medium

Gamma-ray bursts (GRBs) are the most energetic explosions in the universe, and their afterglow emission provides an opportunity to probe the physics of relativistic shock waves in an extreme environment. Several key pieces for completing the picture of the GRB afterglow physics are still missing, including jet properties, emission mechanism, and particle acceleration. Here we present a study of the afterglow emission of GRB 221009A, the most energetic GRB ever observed. Using optical, X-ray, and gamma-ray data up to approximately two days after the trigger, we trace the evolution of the multi-wavelength spectrum and the physical parameters behind the emission process. The broadband spectrum is consistent with the synchrotron emission emitted by relativistic electrons with its index of $p = 2.29\pm 0.02$. We identify a break energy at keV and an exponential cutoff at GeV in the observed multi-wavelength spectrum. The break energy increases in time from $16.0_{-4.9}^{+7.1}$ keV at 0.65 days to $46.8_{-15.5}^{+25.0}$ keV at 1.68 days, favoring a stellar wind-like profile of the circumburst medium with $k=2.4\pm0.1$ as in $\rho (r) \propto r^{-k}$. The high-energy attenuation at around 0.4 to 4 GeV is attributed to the maximum of the particle acceleration in the relativistic shock wave. This study confirms that the synchrotron process can explain the multi-wavelength afterglow emission and its evolution.

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arXiv2022

Simultaneous Radio and Optical Polarimetry of GRB 191221B Afterglow

Gamma-ray bursts (GRBs) are the most luminous transients in the universe and are utilized as probes of early stars, gravitational wave counterparts, and collisionless shock physics. In spite of studies on polarimetry of GRBs in individual wavelengths that characterized intriguing properties of prompt emission and afterglow, no coordinated multi-wavelength measurements have yet been performed. Here, we report the first coordinated simultaneous polarimetry in the optical and radio bands for the afterglow associated with the typical long GRB 191221B. Our observations successfully caught the radio emission, which is not affected by synchrotron self-absorption, and show that the emission is depolarized in the radio band compared to the optical one. Our simultaneous polarization angle measurement and temporal polarization monitoring indicate the existence of cool electrons that increase the estimate of jet kinetic energy by a factor of $>$ 4 for this GRB afterglow. Further coordinated multi-wavelength polarimetric campaigns would improve our understanding of the total jet energies and magnetic field configurations in the emission regions of various types of GRBs, which are required to comprehend the mass scales of their progenitor systems and the physics of collisionless shocks.

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arXiv2021

ALMA Host galaxy observation of the off-axis Gamma-Ray Burst XRF020903

We investigated the radio properties of the host galaxy of X-ray flash, XRF020903, which is the best example for investigating of the off-axis origin of gamma-ray bursts(GRBs). Dust continuum at 233 GHz and CO are observed using the Atacama Large millimeter/submillimeter array. The molecular gas mass derived by applying the metalicity-dependent CO-to-H$_{2}$ conversion factor matches the global trend along the redshift and stellar mass of the GRB host galaxies. The estimated gas depletion timescale (pertaining to the potential critical characteristics of GRB host galaxies) is equivalent to those of GRBs and super-luminous supernova hosts in the same redshift range. These properties of the XRF020903 host galaxy observed in radio resemble those of GRB host galaxies, thereby supporting the identical origin of XRF020903 and GRBs.

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arXiv2021

Multi-Messenger Astrophysics with THESEUS in the 2030s

Multi-messenger astrophysics is becoming a major avenue to explore the Universe, with the potential to span a vast range of redshifts. The growing synergies between different probes is opening new frontiers, which promise profound insights into several aspects of fundamental physics and cosmology. In this context, THESEUS will play a central role during the 2030s in detecting and localizing the electromagnetic counterparts of gravitational wave and neutrino sources that the unprecedented sensitivity of next generation detectors will discover at much higher rates than the present. Here, we review the most important target signals from multi-messenger sources that THESEUS will be able to detect and characterize, discussing detection rate expectations and scientific impact.

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arXiv2020

Orphan GRB afterglow searches with the Pan-STARRS1 COSMOS survey

We present the result of a search for orphan Gamma-Ray Burst (GRB) afterglows in the Pan-STARRS1 (PS1) COSMOS survey. There is extensive theoretical and observational evidence suggesting that GRBs are collimated jets; the direct observation of orphan GRB afterglows would further support this model. An optimal survey strategy is designed by coupling the PS1 survey with the Subaru/Hyper-Suprime-Cam (HSC) survey. The PS1 COSMOS survey, one of the survey fields in the PS1 Medium Deep Survey (PS1/MDS), searches a field of 7 deg$^2$ from December 2011 to January 2014, reaching a limiting magnitude R $\sim$ 23. The dense cadence of PS1/MDS is crucial for identifying transients, and the deep magnitude reached by the HSC survey (R $\sim$ 26) is important for evaluating potential GRB hosts. A transient classification method is employed to select potential orphan GRB afterglow candidates. After a thorough analysis of the transient and host galaxy properties, we conclude that there are no candidates in this survey field. The null result implies that the consideration of jet structures is essential for further orphan GRB afterglow surveys.

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arXiv2020

Two Component Jets of GRB160623A as Shocked Jet cocoon afterglow

Two components of jets associated with the afterglow of the gamma-ray burst GRB 160623A were observed with multi-frequency observations including long-term monitoring in a sub-millimetre range (230 GHz) using the SMA. The observed light curves with temporal breaks suggests on the basis of the standard forward-shock synchrotron radiation model that the X-ray radiation is narrowly collimated with an opening angle $\theta_{n,j}<\sim6^{\circ}$ whereas the radio radiation originated from wider jets ($\sim27^{\circ}$). The temporal and spectral evolutions of the radio afterglow agree with those expected from a synchrotron radiation modelling with typical physical parameters except for the fact that the observed wide jet opening angle for the radio emission is significantly larger than the theoretical maximum opening angle. By contrast, the opening angle of the X-ray afterglow is consistent with the typical value of GRB jets. Since the theory of the relativistic cocoon afterglow emission is similar to that of a regular afterglow with an opening angle of $\sim30^{\circ}$, the observed radio emission can be interpreted as the shocked jet cocoon emission. This result therefore indicates that the two components of the jets observed in the GRB 160623A afterglow is caused by the jet and the shocked jet cocoon afterglows.

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arXiv2016

ALMA and RATIR observations of GRB131030A

We report on the first open-use based Atacama Large Millimeter/submm Array (ALMA) 345-GHz observation for the late afterglow phase of GRB131030A. The ALMA observation constrained a deep limit at 17.1 d for the afterglow and host galaxy. We also identified a faint submillimeter source (ALMAJ2300-0522) near the GRB131030A position. The deep limit at 345 GHz and multifrequency observations obtained using {\it Swift} and RATIR yielded forward shock modeling with a two-dimensional relativistic hydrodynamic jet simulation and described X-ray excess in the afterglow. The excess was inconsistent with the synchrotron self-inverse Compton radiation from the forward shock. The host galaxy of GRB131030A and optical counterpart of ALMAJ2300-0522 were also identified in the SUBARU image. Based on the deep ALMA limit for the host galaxy, the 3-$\sigma$ upper limits of IR luminosity and the star formation rate (SFR) is estimated as $L_{IR}<1.11\times10^{11} L_{\odot}$ and SFR$<18.7$ ($M_{\odot}$~yr$^{-1}$), respectively. Although the separation angle from the burst location (3.5 arcsec) was rather large, ALMAJ2300-0522 may be one component of the GRB131030A host galaxy, according to previous host galaxy cases.

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arXiv2016

Hard X-ray Spectral Investigations of Gamma-ray Bursts, 120521C and 130606A, at High-redshift z~6

This study presents the temporal and spectral analysis of the prompt emission of two high-redshift gamma-ray bursts (GRBs), 120521C at $z\sim6$ and 130606A at $z\sim5.91$, which were performed using the Swift-XRT/BAT and the Suzaku-WAM simultaneously. Based on follow-up XRT observations, the longest durations of the prompt emissions were approximately $80$ s (120521C) and $360$ s (130606A) in the rest frame of each GRB, which are categorized as long-duration GRBs, but are insufficiently long compared with the predicted duration of GRBs that originate from first-generation stars. Because of the wide bandpass of the instruments covering the ranges of 15 keV--5 MeV (BAT-WAM) and 0.3 keV--5.0 MeV (XRT-BAT-WAM), we successfully determined the $\nu F_{\nu}$ peak energies $E_{\rm peak}^{\rm src}$ of $682^{+845}_{-207}$ keV and $1209^{+553}_{-304}$ keV in the rest frame, and the isotropic-equivalent radiated energies $E_{\rm iso}$ of $(8.25^{+2.24}_{-1.96})\times10^{52}$ erg and $(2.82^{+0.17}_{-0.71})\times10^{53}$ erg, respectively. These obtained characteristic parameters are in accordance with the well-known relation between $E_{\rm peak}^{\rm src}$ and $E_{\rm iso}$ (Amati relation). In addition, we examined the relations between $E_{\rm peak}^{\rm src}$ and the 1-s peak luminosity, $L_{\rm p}$, or the geometrical corrected radiated energy, $E_{\gamma}$, and confirmed the $E_{\rm peak}^{\rm src}$-$L_{\rm p}$ (Yonetoku) and $E_{\rm peak}^{\rm src}$-$E_{\gamma}$ (Ghirlanda) relations. The results implied that these high-redshift GRBs at $z\sim6$, expected as having radiated from the reionization epoch, have similar properties as that of X-ray prompt emission with low-redshift GRBs.

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arXiv2016

GRB 110530A: Peculiar Broad Bump and Delayed Plateau in Early Optical Afterglows

We report our very early optical observations of GRB 110530A and investigate its jet properties together with its X-ray afterglow data. A peculiar broad onset bump followed by a plateau is observed in its early R band afterglow lightcurve. The optical data in the other bands and the X-ray data are well consistent with the temporal feature of the R band lightcurve. Our joint spectral fits of the optical and X-ray data show that they are in the same regime, with a photon index of $\sim 1.70$. The optical and X-ray afterglow lightcurves are well fitted with the standard external shock model by considering a delayed energy injection component. Based on our modeling results, we find that the radiative efficiency of the GRB jet is $\sim 1\%$ and the magnetization parameter of the afterglow jet is $<0.04$ with the derived extremely low $\epsilon_B$ (the fraction of shock energy to magnetic field) of $(1.64\pm 0.25)\times 10^{-6}$. These results indicate that the jet may be matter dominated. Discussion on delayed energy injection from accretion of late fall-back material of its pre-supernova star is also presented.

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arXiv2015

Multi-Wavelength Observations of GRB 111228A and Implications for the Fireball and its environment

Observations of very early multi-wavelength afterglows are critical to reveal the properties of the radiating fireball and its environment as well as the central engine of gamma-ray bursts (GRBs). We report our optical observations of GRB 111228A from 95 sec to about 50 hours after the burst trigger and investigate its properties of the prompt gamma-rays and the ambient medium using our data and the data observed with {\em Swift} and {\em Fermi} missions. Our joint optical and X-ray spectral fits to the afterglow data show that the ambient medium features as low dust-to-gas ratio. Incorporating the energy injection effect, our best fit to the afterglow lightcurves with the standard afterglow model via the Markov Chain Monte Carlo (MCMC) technique shows that $\epsilon_e=(6.9\pm 0.3)\times 10^{-2}$, $\epsilon_B=(7.73\pm 0.62)\times 10^{-6}$, $E_{\rm K}=(6.32\pm 0.86)\times 10^{53}\rm erg$, $n=0.100\pm 0.014$ cm$^{-3}$. The low medium density likely implies that the afterglow jet may be in a halo or in a hot ISM. Achromatic shallow decay segment observed in the optical and X-ray bands is well explained with the long-lasting energy injection from the central engine, which would be a magnetar with a period of about 1.92 ms inferred from the data. The $E_p$ of its time-integrated prompt gamma-ray spectrum is $\sim 26$ KeV. Using the initial Lorentz factor ($\Gamma_0=476^{+225}_{-237}$) derived from our afterglow model fit, it is found that GRB 111228A satisfies the $L_{\rm iso}-E_{\rm p,z}-\Gamma_0$ relation and bridges the typical GRBs and low luminosity GRBs in this relation.

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arXiv2015

First-generation Science Cases for Ground-based Terahertz Telescopes

Ground-based observations at terahertz (THz) frequencies are a newly explorable area of astronomy for the next ten years. We discuss science cases for a first-generation 10-m class THz telescope, focusing on the Greenland Telescope as an example of such a facility. We propose science cases and provide quantitative estimates for each case. The largest advantage of ground-based THz telescopes is their higher angular resolution (~ 4 arcsec for a 10-m dish), as compared to space or airborne THz telescopes. Thus, high-resolution mapping is an important scientific argument. In particular, we can isolate zones of interest for Galactic and extragalactic star-forming regions. The THz windows are suitable for observations of high-excitation CO lines and [N II] 205 um lines, which are scientifically relevant tracers of star formation and stellar feedback. Those lines are the brightest lines in the THz windows, so that they are suitable for the initiation of ground-based THz observations. THz polarization of star-forming regions can also be explored since it traces the dust population contributing to the THz spectral peak. For survey-type observations, we focus on ``sub-THz'' extragalactic surveys, whose uniqueness is to detect galaxies at redshifts z ~ 1--2, where the dust emission per comoving volume is the largest in the history of the Universe. Finally we explore possibilities of flexible time scheduling, which enables us to monitor active galactic nuclei, and to target gamma-ray burst afterglows. For these objects, THz and submillimeter wavelength ranges have not yet been explored.

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arXiv2015

Host Galaxy Properties and Black Hole Mass of Swift J164449.3+573451 from Multi-Wavelength Long-Term Monitoring and HST Data

We study the host galaxy properties of the tidal disruption object, Swift J164449.3+573451 using long-term optical to near-infrared (NIR) data. First, we decompose the galaxy surface brightness distribution and analyze the morphology of the host galaxy using high resolution \emph{HST} WFC3 images. We conclude that the host galaxy is a bulge-dominant galaxy that is well described by a single S\'{e}rsic model with S\'{e}rsic index $n=3.43\pm0.05$. Adding a disk component, the bulge to total host galaxy flux ratio (B/T) is $0.83\pm0.03$, which still indicates a bulge-dominant galaxy. Second, we estimate multi-band fluxes of the host galaxy through long-term light curves. Our long-term NIR light curves reveal the pure host galaxy fluxes $\sim500$ days after the burst. We fit spectral energy distribution (SED) models to the multi-band fluxes from the optical to NIR of the host galaxy and determine its properties. The stellar mass, the star formation rate, and the age of stellar population are $\log(M_{\star}/M_{\odot}) = 9.14^{+0.13}_{-0.10}$, $0.03^{+0.28}_{-0.03}\, M_{\odot}$/yr, and $0.63^{+0.95}_{-0.43}$ Gyr. Finally, we estimate the mass of the central super massive black hole which is responsible for the tidal disruption event. The black hole mass is estimated to be $10^{6.7\pm0.4}\, M_{\odot}$ from $M_{\mathrm{BH}}$ - $M_{\star,\mathrm{bul}}$ and $M_{\mathrm{BH}}$ - $L_{\mathrm{bul}}$ relations for the $K$ band, although a smaller value of $\sim10^5\, M_{\odot}$ cannot be excluded convincingly if the host galaxy harbors a pseudobulge.

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arXiv2015

The Accreting Black Hole Swift J1753.5-0127 from Radio to Hard X-Ray

(abridged) We report on multi-wavelength measurements of Swift J1753.5-0127 in the hard state at L=2.7e36 erg/s (assuming d=3 kpc) in 2014. The radio emission is optically thick synchrotron, presumably from a compact jet. We take advantage of the low extinction and model the near-IR to UV emission with a multi-temperature disk model. Assuming a BH mass of M_BH=5 Msun and a system inclination of 40 deg, the fits imply an inner radius for the disk of Rin/Rg>212 d_3 (5Msun/M_BH). The outer radius is R_out/R_g=90,000 d_3 (5Msun/M_BH), which corresponds to 6.6e10 d_3 cm, consistent with the expected size of the disk. The 0.5-240 keV spectrum measured by Swift/XRT, Suzaku, and NuSTAR is relatively well characterized by a power-law with a photon index of Gamma=1.722+/-0.003, but a significant improvement is seen when a second continuum component is added. Reflection is a possibility, but no iron line is detected, implying a low iron abundance. We are able to fit the entire SED with a multi-temperature disk component, a Comptonization component, and a broken power-law, representing the emission from the compact jet. The broken power-law cannot significantly contribute to the soft X-ray emission, and this may be related to why Swift J1753.5-0127 is an outlier in the radio/X-ray correlation. The broken power-law might dominate above 20 keV, which would constrain the break frequency to be between 2.4e10 Hz and 3.6e12 Hz. Although the fits to the full SED do not include significant thermal emission in the X-ray band, previous observations have consistently seen such a component, and we find that there is evidence at the 3.1-sigma level for a disk-blackbody component with a temperature of 150(+30)(-20) eV and an inner radius of 5-14 R_g. If this component is real, it might imply the presence of an inner accretion disk in addition to the strongly truncated (R_in>212 R_g) disk.

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arXiv2015

Extremely Soft X-ray Flash as the indicator of off-axis orphan GRB afterglow

We verified the off-axis jet model of X-ray flashes (XRFs) and examined a discovery of off-axis orphan gamma-ray burst (GRBs) afterglows. The XRF sample was selected on the basis of the following three factors: (1) a constraint on the lower peak energy of the prompt spectrum $E^{src}_{obs}$, (2) redshift measurements, and (3) multi-color observations of an earlier (or brightening) phase. XRF020903 was the only sample selected basis of these criteria. A complete optical multi-color afterglow light curve of XRF020903 obtained from archived data and photometric results in literature showed an achromatic brightening around 0.7 days. An off-axis jet model with a large observing angle (0.21 rad, which is twice the jet opening half-angle, $\theta_{jet}$) can naturally describe the achromatic brightening and the prompt X-ray spectral properties. This result indicates the existence of off-axis orphan GRB afterglow light curves. Events with a larger viewing angle ($>\sim2\theta_{jet}$) could be discovered using an 8-m class telescope with wide field imagers such as Subaru Hyper-Suprime-Cam and the Large Synoptic Survey Telescope.

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arXiv2015

iPTF14yb: The First Discovery of a GRB Afterglow Independent of a High-Energy Trigger

We report here the discovery by the Intermediate Palomar Transient Factory (iPTF) of iPTF14yb, a luminous ($M_{r}\approx-27.8$ mag), cosmological (redshift 1.9733), rapidly fading optical transient. We demonstrate, based on probabilistic arguments and a comparison with the broader population, that iPTF14yb is the optical afterglow of the long-duration gamma-ray burst GRB 140226A. This marks the first unambiguous discovery of a GRB afterglow prior to (and thus entirely independent of) an associated high-energy trigger. We estimate the rate of iPTF14yb-like sources (i.e., cosmologically distant relativistic explosions) based on iPTF observations, inferring an all-sky value of $\Re_{\mathrm{rel}}=610$ yr$^{-1}$ (68% confidence interval of 110-2000 yr$^{-1}$). Our derived rate is consistent (within the large uncertainty) with the all-sky rate of on-axis GRBs derived by the Swift satellite. Finally, we briefly discuss the implications of the nondetection to date of bona fide "orphan" afterglows (i.e., those lacking detectable high-energy emission) on GRB beaming and the degree of baryon loading in these relativistic jets.

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arXiv2015

A new era of sub-millimeter GRB afterglow follow-ups with the Greenland Telescope

A planned rapid submillimeter (submm) Gamma Ray Burst (GRBs) follow-up observations conducted using the Greenland Telescope (GLT) is presented. The GLT is a 12-m submm telescope to be located at the top of the Greenland ice sheet, where the high-altitude and dry weather porvides excellent conditions for observations at submm wavelengths. With its combination of wavelength window and rapid responding system, the GLT will explore new insights on GRBs. Summarizing the current achievements of submm GRB follow-ups, we identify the following three scientific goals regarding GRBs: (1) systematic detection of bright submm emissions originating from reverse shock (RS) in the early afterglow phase, (2) characterization of forward shock and RS emissions by capturing their peak flux and frequencies and performing continuous monitoring, and (3) detections of GRBs as a result of the explosion of first-generation stars result of GRBs at a high redshift through systematic rapid follow ups. The light curves and spectra calculated by available theoretical models clearly show that the GLT could play a crucial role in these studies.

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