Yohsuke Takamori

Perturbative analysis of a stationary magnetosphere in an extreme black hole space–time: on the Meissner-like effect of an extreme black hole

It is known that the Meissner-like effect is seen in the vacuum solutions of blackhole magnetosphere: no non-monopole component of magnetic flux penetrates the event horizon if the black hole is extreme. In this article, in order to see the effects of charge currents, we study the force-free magnetic field on the extreme ReissnerNordstrom background. In this case, we should solve one elliptic differential equation called the Grad-Shafranov equation which has singular points called light surfaces. Due to the surfaces, it is difficult to solve the equation in the region from the event horizon to infinity. In order to see the Meissner effect, we consider the region near the event horizon and try to solve the equation by Taylor expansion about the event horizon. Moreover, we assume that the small rotational velocity of the magnetic field, and then, we construct a perturbative method to solve the Grad-Shafranov equation considering the effect of the inner light surface and study the behavior of the magnetic field near the event horizon.

Stable Bound Orbits of Massless Particles around a Black Ring

We study the geodesic motion of massless particles in singly rotating black ring spacetimes. We find stable stationary orbits of massless particles in toroidal spiral shape in the case that the thickness parameter of a black ring is less than a critical value. Furthermore, there exist nonstationary massless particles bounded in a finite region outside the horizon. This is the first example of stable bound orbits of massless particles around a black object.

Radial velocity measurements of an orbiting star around Sgr A

During the next closest approach of the orbiting star S2/S0-2 to the Galactic supermassive black hole (SMBH), it is estimated that RV uncertainties of ~ 10 km/s allow us to detect post-Newtonian effects throughout 2018. To evaluate an achievable uncertainty in RV and its stability, we have carried out near-infrared, high resolution (R ~ 20,000) spectroscopic monitoring observations of S2 using the Subaru telescope and the near-infrared spectrograph IRCS from 2014 to 2016. The Br-gamma absorption lines are used to determine the RVs of S2. The RVs we obtained are 497 km/s, 877 km/s, and 1108 km/s in 2014, 2015, and 2016, respectively. The statistical uncertainties are derived using the jackknife analysis. The wavelength calibrations in our three-year monitoring are stable: short-term (hours to days) uncertainties in RVs are < 0.5 km/s, and a long-term (three years) uncertainty is 1.2 km/s. The uncertainties from different smoothing parameter, and from the partial exclusion of the spectra, are found to be a few km/s. The final results using the Br-gamma line are 497 +- 17 (stat.) +- 3 (sys.) km/s in 2014, 877 +- 15 (stat.) +- 4 (sys.) km/s in 2015, and 1108 +- 12 (stat.) +- 4 (sys.) km/s in 2016. When we use two He I lines at 2.113\mum in addition to Br-gamma, the mean RVs are 513 km/s and 1114 km/s for 2014 and 2016, respectively. The standard errors of the mean are 16.2 km/s (2014) and 5.4 km/s (2016), confirming the reliability of our measurements. The difference between the RVs estimated by Newtonian mechanics and general relativity will reach about 200 km/s near the next pericenter passage in 2018. Therefore our RV uncertainties of 13 - 17 km/s with Subaru enable us to detect the general relativistic effects in the RV measurements with more than 10 sigma in 2018.

Stable Bound Orbits around Black Rings

We study stable bound orbits of a free particle around a black ring. Unlike the higher-dimensional black hole case, we find that there exist stable bound orbits in toroidal spiral shape near the ring axis and stable circular orbits on the axis. In addition, radii of stable bound orbits can be infinitely large if the ring thickness is less than a critical value.