https://scholarworks.unist.ac.kr/handle/201301/74 2026-04-19T03:17:54Z https://scholarworks.unist.ac.kr/handle/201301/91306 Title: Ring Asymmetry and Spin in M87* Author(s): Bernshteyn, Vadim; Conroy, Nicholas S.; Bauböck, Michi; Tiede, Paul; Joshi, Abhishek V.; Prather, Ben S.; Gammie, Charles F.; Kim, Jae-Young Abstract: Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetricring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring’s peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin a∗. We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor |a∗| ≲ 0.2. This is consistent with the Blandford-Znajek model for M87’s jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints, and discuss how improved spin constraints could distinguish between differing jetlaunching mechanisms and black hole growth scenarios. 2026-02-28T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91278 Title: Demonstration of time-resolved Fe K-edge XANES with a self-seeded X-ray free-electron laser at PAL-XFEL Author(s): Ma, Rory; Kim, Yujin; Nam, Inhyuk; Kim, Gyujin; Kim, Minseok; Park, Gisu; Lee, Jae Hyuk Abstract: We demonstrate, for the first time, time-resolved X-ray absorption near-edge structure (XANES) spectroscopy at the Fe K-edge using a self-seeded X-ray free-electron laser (XFEL) beam at the FXL endstation in Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL). Focusing on the application of self-seeded XFEL for time-resolved XANES, we show advantages in photon flux, measurement speed, and signal-to-noise ratio (S/N). Using the high-stability, narrow-bandwidth self-seeded mode, we achieved an incident X-ray bandwidth of approximately 0.71 eV and improved spectral purity compared to conventional self-amplified spontaneous emission (SASE) operation. A 50 mM aqueous solution of iron(II) tris(1,10-phenanthroline) dichloride was photo-excited by a 400 nm femtosecond laser, and ultrafast electronic dynamics were probed by synchronized XFEL pulses. The enhanced spectral purity and energy stability enabled clear detection of transient features obscured by SASE pulse bandwidth and energy jitter. The results highlight the clear benefits of the self-seeded XFEL source, showing a 25% improvement in signal-to-noise ratios compared to the SASE with double crystal monochromator (DCM) mode, particularly for time-resolved XANES experiments. This work lays the foundation for advanced exploration of chemical and biological dynamics with improved spectral accuracy in complex environments. 2025-11-30T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91246 Title: VLBI Detections of Compact Nuclei in Spiral-hosted Double-lobed Radio-loud Active Galactic Nuclei (DRAGNs): Evidence for Weak Parsec-Scale Jet Activity Author(s): Ryu, Mingyu; Kim, Jae-Young Abstract: We report milliarcsecond-scale VLBI detections of compact radio nuclei in four spiral-hosted, doublelobed radio-loud AGNs (spiral DRAGNs), a rare class that challenges the traditional association of powerful jets with elliptical hosts. Using public VLBI data archives, we identify compact cores in four sources and resolve parsec-scale jets in two of them. The VLBI components show low brightness temperatures (Tb ≈ 109 K in the core) and jet-to-counterjet ratios consistent with only mildly relativistic intrinsic speeds (β ≲ 0.6 for inclinations θ ≲ 80◦ ), indicating weakly powered pc-scale outflows. The low radio-Eddington ratios log(LR,1.4 GHz/LEdd) ≈ −5 to −8 support this interpretation. Three objects lie on the fundamental plane of black hole activity, implying that global accretion-jet coupling in spiral DRAGNs is similar to that in other AGNs. Comparison with recent GRMHD simulations of thin-disk jets suggests that the VLBI-scale cores in spiral DRAGNs may trace an early or intermittently magnetized phase of jet launching. The coexistence of weak pc-scale jets and large kpc-scale lobes implies recurrent or long-duty-cycle jet activity in these late-type hosts. 2026-03-31T15:00:00Z https://scholarworks.unist.ac.kr/handle/201301/91195 Title: Reconfigurable second harmonic generation via plasmonic nanoslits counteracting strain-induced suppression in monolayer MoS2 Author(s): Sharma, Sobhagyam; Behera, Satyabrat; Son, Byung Hee; Ji, Gangseon; Park, Daehwan; Alagurasu, Arunkumar; Moghaddam, Mahsa H.; Kim, Dai-Sik; Namgung, Seon; Ahn, Yeong Hwan; Park, Hyeong‐Ryeol Abstract: Second-harmonic generation (SHG) in monolayer transition-metal dichalcogenides is highly sensitive to mechanical strain, often leading to signal suppression under deformation. Here, we demonstrate reconfigurable SHG in monolayer MoS2 integrated with plasmonic nanoslits, where localized plasmonic fields counteract strain-induced suppression. The plasmonic nanoslits induce strong spatially localized field enhancement, yielding an SHG enhancement of up to 8,000, estimated by normalizing the signal to the illumination area, relative to MoS2 on unpatterned Au films. Applying 1.2 % compressive strain increases the SHG intensity by approximately three-fold relative to the unstrained state, demonstrating effective strain-enabled modulation. Upon mechanical bending, the SHG response is reversibly modulated and remains stable after an initial pre-conditioning regime, retaining more than 95% of its initial intensity over repeated bending cycles. This strain-adaptive platform demonstrates robust cycling stability and provides a novel strategy for dynamically reconfigurable nonlinear metasurfaces, enabling applications in wearable sensors, tunable modulators, and compact frequency converters. 2026-03-31T15:00:00Z