arXiv subjects

Y. Omiya

Publications and source records attributed to Y. Omiya.

3 recordsLinked to original sources

arXiv2026

Diagnosing the Fe line complex of the intracluster medium by XRISM high-resolution spectroscopy

We aim to test the validity of the CIE framework in the ICM by performing line diagnostics based mainly on resolved Fe-K emission lines. Methods. We analyse Resolve full-array spectra of 17 galaxy clusters. Prominent Fe-K line components (the Fe xxv w, x, y, z, and Fe xxvi Ly{\alpha}1,2 lines) are removed from plasma emission models and instead fitted with Gaussian profiles, enabling direct measurements of line fluxes without relying on synthetic spectral models. Some cool-core systems show w/z ratios lower than predicted by about 20 per cent, and a broader w than z, consistent with resonant scattering effects. The y/x ratios exhibit marginal deviations from model predictions for some objects, suggesting possible origins of cascade process due to electron recombination and contribution from low-ionised Fe. The Fe Ly{\alpha}2/Ly{\alpha}1 ratios are globally close to the expected value of about 0.5, and the samples with good photon statistics prefer 0.55. This subtle excess is consistent with an unresolved contribution to Ly{\alpha}2 from the magnetic-dipole (M1) transition, which is absent from one of the atomic codes considered here. More interestingly, systems at around 7 keV preferentially exhibit Ly{\alpha}2/Ly{\alpha}1 ratios above 0.55. Although the statistical significance of this trend is limited, it suggests that resolved Fe Ly{\alpha} spectroscopy may provide a sensitive probe of additional atomic processes to collisional excitation, including dielectronic and radiative recombination and polarisation effects.

astro-ph.GA↗

arXiv2024

Indications of an offset merger in Abell 3667

Abell 3667 is a nearby merging cluster with a prominent cold front and a pair of two bright radio relics. Assuming a head-on merger, the origin of the cold front is often considered to be a remnant of the cluster core stripped by its surrounding ICM. Some authors have proposed an offset merger scenario in which the subcluster core rotates after the first core crossing. This scenario can reproduce features such as the cold front and a pair of radio relics. To distinguish between these scenarios, we reanalyzed the ICM distribution and measured the line-of-sight bulk ICM velocity using the XMM-Newton PN data. In the unsharp masked image, we identify several ICM features. The notable feature is a RG1 vortex, which is a clockwise vortex-like enhancement with a radius of about 250 kpc connecting the first BCG to the radio galaxy (RG1). It is particularly enhanced near the north of the 1st BCG, which is named the BCG-N tail. The thermodynamic maps show that the ICM of the RG1 vortex has a relatively high abundance of 0.5-0.6 solar compared to the surrounding regions. The ICM of the BCG-E tail also has a high abundance and low pseudo-entropy and can be interpreted as a remnant of the cluster core's ICM. Including its arc-like shape, the RG1 vortex supports the idea that the ICM around the cluster center is rotating, which is natural for an offset merger scenario. The results of the line-of-sight bulk ICM velocity measurements show that the ICM around the BCG-N tail is redshifted with a velocity difference of 940+/-440 km/s compared to the optical redshift of the first BCG. We obtain other indications of variations in the line-of-sight velocity of the ICM and discuss these in the context of an offset merger.

astro-ph.HE↗

arXiv2022

XMM-Newton view of the shock heating in an early merging cluster, CIZA J1358.9$-$4750

CIZA J1358.9-4750 is a nearby galaxy cluster in the early phase of a major merger. The two-dimensional temperature map using XMM-Newton EPIC-PN observation confirms the existence of a high temperature region, which we call the "hot region", in the "bridge region" connecting the two clusters. The ~ 500 kpc wide region between the southeast and northwest boundaries also has higher pseudo pressure compared to the unshocked regions, suggesting the existence of two shocks. The southern shock front is clearly visible in the X-ray surface brightness image and has already been reported by Kato et al. (2015). The northern one, on the other hand, is newly discovered. To evaluate their Mach number, we constructed a three-dimensional toy merger model with overlapping shocked and unshocked components in line of sight. The unshocked and preshock ICM conditions are estimated based on those outside the interacting bridge region assuming point symmetry. The hot region spectra are modeled with two-temperature thermal components, assuming that the shocked condition follows the Rankin-Hugoniot relation with the preshock condition. As a result, the shocked region is estimated to have a line-of-sight depth of ~ 1 Mpc with a Mach number of ~ 1.3 in the southeast shock and ~ 1.7 in the northwest shock. The age of the shock waves is estimated to be ~ 260 Myr. This three dimensional merger model is consistent with the Sunyaev-Zeldovich signal obtained using the Planck observation within the CMB fluctuations. The total flow of the kinetic energy of the ICM through the southeast shock was estimated to be ~ 2.2 x $10^{42}$ erg/s. Assuming that 10 % of this energy is converted into ICM turbulence, the line-of-sight velocity dispersion is calculated to be ~ 200 km/s, which is basically resolvable via coming high spectral resolution observations.

astro-ph.GA↗