Tatsuya Matsumoto

The gravitational waves from the neutron star merger event GW170817 were accompanied by an unusually weak short gamma-ray burst (sGRB) 170817A, by an optical/IR macronova/kilonova and by a long-lasting radio to X-rays counterpart. While association of sGRBs with mergers was predicted a long time ago, the luminosity of this prompt γ-ray emission was weaker by a few orders of magnitude than all known previous sGRBs and it was softer than typical sGRBs. This raises the question whether the γ-rays t

Abstract The process of unstable mass transfer in a stellar binary can result in either a complete merger of the stars or successful removal of the donor envelope leaving a surviving more compact binary. Luminous red novae (LRNe) are the class of optical transients believed to accompany such merger/common envelope events. Past works typically model LRNe using analytic formulae for supernova light curves that make assumptions (e.g., radiation-dominated ejecta, neglect of hydrogen recombination en

A $\gamma$-ray source must have a limited optical depth to pair production. This simple condition, called compactness, implies that gamma-ray bursts (GRBs) must involve a highly relativistic motion ($\Gamma \gtrsim 100$) giving the first and most important clue on their nature. So far, this condition has been discussed under the assumption that the $\gamma$-ray sources are viewed on-axis, that is, by an observer within the beaming cone of the relativistic source. Recently, following the detectio

Abstract A growing number of core-collapse supernovae (SNe) that show evidence for interaction with dense circumstellar medium (CSM) are accompanied by “precursor” optical emission rising weeks to months prior to the explosion. The precursor luminosities greatly exceed the Eddington limit of the progenitor star, implying that they are accompanied by substantial mass loss. Here, we present a semi-analytic model for SN precursor light curves, which we apply to constrain the properties and mechanis

ABSTRACT The Gravity Collaboration detected a near-infrared hotspot moving around Sgr A* during the 2018 July 22 flare. They fitted the partial loop the hotspot made on the sky with a circular Keplerian orbit of radius $\simeq 7.5\, r_{\rm g}$ around the supermassive black hole (BH), where rg is the gravitational radius. However, because the hotspot traversed the loop in a short time, models in which the hotspot tracks the motion of some fluid element tend to produce a best-fitting trajectory sm

Abstract The gravitational wave event GW170817 from a binary neutron star merger is accompanied by electromagnetic counterparts, and the optical and near-infrared emission is called a macronova (or kilonova). Although the radioactivity of synthesized r -process elements is widely discussed as an energy source, its decisive evidence is not clearly shown yet. We discuss a macronova powered by central engine activities such as jet activities and X-rays from the matter fallback and show that the eng

Reaction of gamma-silyl-beta,gamma-epoxybutanenitrile with a base generates an alpha-nitrile carbanion derivative of 4-siloxybut-3-enenitrile, which undergoes reaction with bis-electrophiles such as 1,omega-dihaloalkanes, omega-bromo-alpha,beta-unsaturated esters, and bisenoates to provide highly functionalized carbocycles.

ABSTRACT Radio flares from tidal disruption events (TDEs) are generally interpreted as synchrotron emission arising from the interaction of an outflow with the surrounding circumnuclear medium (CNM). We generalize the common equipartition analysis to be applicable in cases lacking a clear spectral peak or even with just an upper limit. We show that, for detected events, there is a lower limit on the combination of the outflow’s velocity v and solid angle Ω, ≃vΩa (with a ≃ 0.5). Considering sever

ABSTRACT The equipartition analysis yields estimates of the radius and energy of synchrotron self-absorbed radio sources. Here we generalize this method to relativistic off-axis viewed emitters. We find that the Lorentz factor Γ and the viewing angle θ cannot be determined independently but become degenerate along a trajectory of minimal energy solutions. The solutions are divided into on-axis and off-axis branches, with the former reproducing the classical analysis. A relativistic source viewed

The tidal disruption event AT2019dsg was observed from radio to X-rays and was possibly accompanied by a high-energy neutrino. Previous interpretations have focused on continued injection by a central engine as the source of energy for radio emission. We show that continuous energy injection is unnecessary; the radio data can be explained by a single ejection of plasma that supplies all the energy needed. To support this assertion, we analyze the synchrotron self-absorbed spectra in terms of the

ABSTRACT The discovery of optical/UV (ultraviolet) tidal disruption events (TDEs) was surprising. The expectation was that, upon returning to the pericentre, the stellar-debris stream will form a compact disc that will emit soft X-rays. Indeed, the first TDEs were discovered in this energy band. A common explanation for the optical/UV events is that surrounding optically thick matter reprocesses the disc’s X-ray emission and emits it from a large photosphere. If accretion follows the super-Eddin

Almost all black holes (BHs) and BH candidates in our Galaxy have been discovered as soft X-ray transients, so-called X-ray novae. X-ray novae are usually considered to arise from binary systems. Here we propose that X-ray novae are also caused by isolated single BHs. We calculate the distribution of the accretion rate from interstellar matter to isolated BHs, and find that BHs in molecular clouds satisfy the condition of the hydrogen-ionizaiton disk instability, which results in X-ray novae. Th

ABSTRACT AT 2022cmc is a luminous optical transient (νLν ≳ 1045 erg s−1) accompanied by decaying non-thermal X-rays (peak duration tX ≲ days and isotropic energy EX,iso ≳ 1053 erg) and a long-lived radio/mm synchrotron afterglow, which has been interpreted as a jetted tidal disruption event (TDE). Both an equipartition analysis and a detailed afterglow model reveal the radio/mm emitting plasma to be expanding mildly relativistically (Lorentz factor $\Gamma \gtrsim \, \mathrm{ few}$ ) with an ope

Abstract Radio monitoring unveiled late (hundreds to a thousand days) radio flares in a significant fraction of tidal disruption events. We propose that these late-time radio flares are a natural outcome if the surrounding density profile flattens outside the Bondi radius. At the Bondi radius, the outflow is optically thin (above a few GHz) to synchrotron self-absorption. As more and more material is swept up, the radio emission rises asymptotically as ∝ t 3 until the outflow begins to decelerat

ABSTRACT GRB 200522A is a short duration gamma-ray burst (GRB) at redshift z= 0.554 characterized by a bright infrared counterpart. A possible, although not unambiguous, interpretation of the observed emission is the onset of a luminous kilonova powered by a rapidly rotating and highly magnetized neutron star, known as magnetar. A bright radio flare, arising from the interaction of the kilonova ejecta with the surrounding medium, is a prediction of this model. Whereas the available data set rema