Simulating a GEM X-ray Polarimeter

T X-ray polarimetry is one of the last remaining largely untouched frontiers in observational astronomy and provides a novel way to probe poorly understood details of high energy emission process for a variety of astrophysical sources. X-ray polarization is exceedingly difficult to measure, one solution is the Gas Pixel Detector (GPD). In a GPD an astronomical X-ray enters a gas cell, collides with an atom of gas, and emits a high energy electron. Critically, the direction of this electron corresponds to the polarization of the astronomical X-ray. This electron creates an ionization tract whose electrons are drifted by a small electric potential across the gas cell into a bottom plate consisting of a double layered conductor separated by an insulator with a strong potential difference between them. This bottom plate, called a Gas Electron Multiplier (GEM), has an array of tiny holes and the ionization tract electrons fortunate enough to pass though the holes are strongly accelerated causing them to create secondary cascades in the direction of a pixelated ASIC detector array. The cumulative pattern of secondary shower electrons across the detector pixels thus reflects the direction of the initial electron ejected from the collision with the astronomical X-ray and thus ultimately the polarization of the astrophysical X-ray source. Therefore, for effective detector operation, it is necessary to characterize the detectors response to varying physical properties such as pressure, temperature, and voltage configurations both to determine optimal parameters for launch and to understand changes in post-launch detector performance. Here I will discuss my work simulating the secondary cascade electron behavior using the Garfield++ software package, developed by CERN for modeling particle detectors with gas particle interactions such as drift chambers.

Perturbation of a natural AdS wormhole in Einstein-Born-Infeld gravity

We consider the scalar perturbations of a new AdS wormhole in (3+1)-dimensional Einstein-Born-Infeld gravity. We discuss the stability of the perturbation by numerically computing the quasi-normal modes of a massive scalar field in the wormhole background. We investigate the dependence of quasi-normal frequencies on the mass of the scalar field as well as other parameters of the wormhole. It is found that the perturbations are always stable for the wormhole geometry which has the general relativity limit when the scalar mass has a certain tachyonic mass bound analogous to the Breitenlohner-Freedman (BF) bound in the global-AdS space.

Creation and Evolution of Cosmological Horizons in FLRW Universe with Wormhole

Recently we solved the Einstein's field equations to obtain the exact solution of the cosmological model with the Morris-Thorne type wormhole. We found the apparent horizons and analyzed their geometric natures, including the causal structures. We also derived the Hawking temperature near the apparent cosmological horizon. In this paper, we investigate the dynamic properties of the apparent horizons under the matter-dominated universe and lambda-dominated universe. As a more realistic universe, we also adopt the $\Lambda$CDM universe which contains both the matter and lambda. The past light cone and the particle horizon is examined for what happens in the case of the model with wormhole. Since the spatial coordinates of the spacetime with the wormhole are limited outside the throat, the past light cone can be operated by removing the smaller-than-wormhole region. The past light cones without wormhole begin start earlier than the past light cones with wormhole in conformal time-proper distance coordinates. The light cone consists of two parts: the information from our universe and the information from other universe or far distant region through the wormhole. Therefore, the particle horizon distance determined from the observer's past light cone cannot be defined in a unique way.

Tidal deformability of neutron stars and gravitational waves

Gravitational waves (GW170817) produced in a binary neutron star inspiral have been observed followed by gamma-ray burst (GRB 170817A) and afterglows from X-ray to radio. By combining the distance obtained by gravitational waves and red shift obtained by electromagnetic waves, even Hubble constant has been estimated. This indicates the start of new era of multimessenger astronomy. In addition to the masses of inspiralling neutron stars, the tidal deformability which depends on the inner structure of neutron stars has been estimated from gravitational waves. This confirms that even strong interactions can be tested by gravitational waves. In this talk, I review the effect of tidal deformability of neutron stars to the gravitational waves produced in the inspiral process, and discuss the implications of detected tidal deformability to the neutron star equations of state.

Equation of motion for test particle in modified gravity

We calculated the equation of motion for the modified gravity theory introduced by us recently. We will study in detail for this modified gravity to see the possibility of explaining galaxy rotational curve and dark matter content.

Quantum gravitational aspects inside a black hole

We investigate inside a black hole using quantum gravitational methods. We solve the Wheeler-DeWitt equation for the interior of the Schwarzschild black hole. As a result, we can obtain interesting interpretations, where this can give the wisdom to understand the information loss paradox of black holes.