arXiv subjects

Tatsuya Kotani

Publications and source records attributed to Tatsuya Kotani.

6 recordsLinked to original sources

arXiv2026

Evidence for Multiple Orbiting Hotspots in the 340 GHz Variability of Sgr A*

We analyzed 11 epochs of archival Atacama Large Millimeter/submillimeter Array (ALMA) data to investigate flux density variability of Sgr A* at 340 GHz. In one epoch, the light curve exhibits two short-timescale components with characteristic periods of ~30 min and ~50 min. While the corresponding peaks in the periodogram are highly significant under a white-noise assumption, their significance decreases below 3 σwhen red-noise variability is taken into account, and we therefore do not regard them as statistically significant periodic detections. Nevertheless, the observed timescales are comparable to the orbital period near the innermost stable circular orbit of Sgr A*, and the light curve shows phase-dependent structure and amplitude evolution consistent with orbital modulation. We find that the variability is well described by a model involving multiple orbiting hotspots with decaying emission. This interpretation suggests that both periodic and non-periodic variability in Sgr A* may arise from a common physical origin in orbiting structures within the accretion flow, providing a unified framework for its millimeter variability.

astro-ph.HE↗

arXiv2026

Detection of a Molecular Cloud toward the Heartbeating Gamma-ray Source near the Microquasar SS 433

We report the detection of a molecular cloud, CO+40.05-2.40, positionally coincident with the "heartbeating" GeV source Fermi J1913+0515 at the northern boundary of the SS 433/W50 system. Millimeter and submillimeter spectroscopy with the Nobeyama 45 m telescope and the James Clerk Maxwell Telescope shows that the cloud has physical properties typical of quiescent dark clouds in the Galactic disk, with no evidence of shock heating or enhanced excitation. We examine possible high-energy emission mechanisms and find that the observed GeV luminosity cannot be accounted for by electron bremsstrahlung or hadronic interactions driven by relativistic particles originating from SS 433 under reasonable energetic assumptions. As an alternative, we propose that the gamma-rays may arise from a compact object embedded within the cloud and powered by Bondi-type accretion. In this framework, the reported heartbeat-like variability may reflect periodic modulation of the accretion flow by density waves induced by the precessing equatorial outflow of SS 433.

astro-ph.HE↗

arXiv2026

First Determination of the Cosmic Microwave Background Radiation Temperature at $z\!=\!0.68$ Using Molecular Absorption Lines

We analyzed millimeter-wave data toward the quasar B0218+357 observed with the Atacama Large Millimeter/submillimeter Array and obtained absorption spectra of the $J$=2-1 and $J$=3-2 rotational transitions of HCN, HCO$^{+}$, HNC, H$^{13}$CN, and H$^{13}$CO$^{+}$ at the cosmological redshift of $z\!=\!0.68$. For HCN, HCO$^{+}$, and HNC, we identified two distinct absorption components that are common to both transitions, whereas a single component was detected in the isotopologue spectra. In this paper, we accurately evaluate the excitation temperatures and their uncertainties from the absorption strengths of these components, and use them to determine the CMB temperature. Uncertainties in the continuum covering factor were propagated into the excitation temperature via Monte Carlo sampling. We further corrected the observed optical depths for biases due to column-density nonuniformity by assuming a lognormal column-density distribution. Under the assumption that the rotational levels are in radiative equilibrium with the cosmic microwave background (CMB), we derived excitation temperature profiles in the optically thin regime. Because the excitation of HCO$^+$ is biased by an additional velocity component and partial collisional excitation, this species was excluded from the final determination of the CMB temperature. From a weighted mean of the excitation temperatures obtained from HCN and HNC, we determined the CMB temperature at $z\!=\!0.68$ to be ${4.50\pm0.17\,\mathrm{K}}$. This constitutes the first measurement of the CMB temperature at $z\!=\!0.68$ based on a quasar absorption line system and represents the most precise determination at this redshift, highly consistent with the standard Big Bang cosmological model.

astro-ph.CO↗

arXiv2026

Discovery of Multiple Ultra-Broad-Velocity Molecular Features Associated with the W44 Molecular Cloud

We report the discovery of multiple compact molecular features exhibiting extremely broad velocity widths toward the W44 molecular cloud. ALMA CO $J$=3--2 data reveal eight ``Petit--Bullets'' surrounding the previously known ``Bullet.'' Each Petit--Bullet shows a distinct V-shaped structure in position--velocity space, reminiscent of the Y-shaped morphology of the Bullet, suggesting a common origin. These features are interpreted as the result of high-velocity plunges of compact gravitational objects into dense molecular gas. The spatial and kinematic properties of the Petit--Bullets suggest that the plunging material was not a single object but rather a small cluster of compact bodies. A virial mass of $1.0\!\times\! 10^{5}\, M_\odot$ inferred from their velocity dispersion is comparable to that of typical globular clusters. Momentum analysis further implies that the main Bullet likely formed by an isolated black hole. These findings provide new evidence for dynamical interactions between halo clusters and disk molecular gas.

astro-ph.GA↗

arXiv2025

A New Precise Measurement of the Cosmic Microwave Background Radiation Temperature at $z=0.89$ Toward PKS1830-211

In this study, we analyzed millimeter-wave data toward the quasar PKS1830-211 observed with the Atacama Large Millimeter/submillimeter Array to obtain absorption spectra of HCN $J$=2-1, $J$=3-2, $J$=4-3, and $J$=5-4 lines at the cosmological redshift of $z=0.89$. We confirmed multiple absorption components in each spectrum, and the two lower-$J$ lines are highly saturated at velocity centers of the two most prominent components. The effect of uncertainty in the continuum covering factor was carefully assessed using a Monte Carlo approach. We also accounted for systematic uncertainties in the HCN $J$=4-3 and $J$=5-4 absorption due to time variability during an intense flaring event of the quasar. Assuming local thermodynamic equilibrium and neglecting molecular collisions, we derived the excitation temperature profiles and their uncertainties in the optically thin regime. We determined the cosmic microwave background radiation temperature at $z = 0.89$ to be ${5.13\pm0.06\,\mathrm{K}}$ by taking a weighted average of calculated excitation temperatures; this is highly consistent with that expected from the standard model of the universe.

astro-ph.CO↗

arXiv2025

Signature of a Dark Ultra-compact Dwarf Galaxy Transiting the Milky Way Disk

We report the discovery of a vertical velocity anomaly (VVA) in the stellar component of the Galactic disk, consistent with the impact of a dark, ultra-compact dwarf galaxy (UCD)-sized object plunging into the Milky Way. The anomaly spatially coincides with a suite of gaseous disturbances -- including an \Hone\ void, a molecular shell (CO 16.134--0.553), and a vertical \Hone\ filament -- previously interpreted as signatures of a dark matter subhalo (DMSH) collision. Analysis of Gaia DR2 astrometry reveals a statistically significant vertical velocity dip co-located with these features, supporting a dynamical origin. The absence of a luminous source at the filament's terminus suggests a dark or failed UCD. These results provide rare observational evidence for a low-mass DMSH, with implications for the substructure of dark matter halos and constraints on the $Λ$CDM model.

astro-ph.GA↗