arXiv subjects

Liyi Gu

Publications and source records attributed to Liyi Gu.

At least 55 records · Page 3Linked to original sources

arXiv2025

Constraining gas motion and non-thermal pressure beyond the core of the Abell 2029 galaxy cluster with XRISM

We report a detailed spectroscopic study of the gas dynamics and hydrostatic mass bias of the galaxy cluster Abell 2029, utilizing high-resolution observations from XRISM Resolve. Abell 2029, known for its cool core and relaxed X-ray morphology, provides an excellent opportunity to investigate the influence of gas motions beyond the central region. Expanding upon prior studies that revealed low turbulence and bulk motions within the core, our analysis covers regions out to the scale radius $R_{2500}$ (670~kpc) based on three radial pointings extending from the cluster center toward the northern side. We obtain accurate measurements of bulk and turbulent velocities along the line of sight. The results indicate that non-thermal pressure accounts for no more than 2% of the total pressure at all radii, with a gradual decrease outward. The observed radial trend differs from many numerical simulations, which often predict an increase in non-thermal pressure fraction at larger radii. These findings suggest that deviations from hydrostatic equilibrium are small, leading to a hydrostatic mass bias of around 2% across the observed area.

astro-ph.CO↗

arXiv2025

Mapping Cassiopeia A's silicon/sulfur Doppler velocities with XRISM-Resolve

Young supernova remnants (SNRs) provide crucial insights into explosive nucleosynthesis products and their velocity distribution soon after the explosion. However, these velocities are influenced by the dynamics of the circumstellar medium (CSM), which originates from the progenitor's late-phase mass loss. Cas A, the youngest known Galactic core-collapse SNR, was studied to analyze the spatial distribution of Si and S radial velocities using two high-spectral resolution observations from the XRISM-Resolve imaging spectrometer.Resolve's capabilities enabled the detailed characterization of Si XIII, Si XIV, S XV, and S XVI lines, whose line shapes can be resolved and modeled using Gaussian radial-velocity components. The radial velocities measured generally align with previous CCD-based results, confirming that they were not artifacts caused by blended lines or ionization variations. Modeling line profiles with two-component Gaussians improved fits in some regions, revealing distinct redshifted (backside) and blueshifted (frontside) components only in a few specific areas. In most regions, however, both components were either both redshifted (northwest) or both blueshifted (southeast), consistent with the patchy ejecta shell morphology seen in optically emitting fast-moving knots. The individual line components revealed a line broadening ranging from $σ_v \approx 200$ to $σ_v \approx 2000$ km/s. Components with $1000 \lesssim σ_v \lesssim 2000$km/s are consistent with previously determined reverse shock velocities, suggesting non-equilibrated or partially equilibrated ion temperatures. Narrow components with small radial velocities found near Cas A's projected center likely originate from shocked CSM plasma. But the low radial velocity and small $σ_v$ defies identifying these components with either the frontside or backside of the SNR, or both.

astro-ph.HE↗

arXiv2025

XRISM forecast for the Coma cluster: stormy, with a steep power spectrum

The XRISM Resolve microcalorimeter array measured the velocities of hot intracluster gas at two positions in the Coma galaxy cluster: 3'x3' squares at the center and at 6' (170 kpc) to the south. We find the line-of-sight velocity dispersions in those regions to be sigma_z=208+-12 km/s and 202+-24 km/s, respectively. The central value corresponds to a 3D Mach number of M=0.24+-0.015 and the ratio of the kinetic pressure of small-scale motions to thermal pressure in the intracluster plasma of only 3.1+-0.4%, at the lower end of predictions from cosmological simulations for merging clusters like Coma, and similar to that observed in the cool core of the relaxed cluster A2029. Meanwhile, the gas in both regions exhibits high line-of-sight velocity differences from the mean velocity of the cluster galaxies, Delta v_z=450+-15 km/s and 730+-30 km/s, respectively. A small contribution from an additional gas velocity component, consistent with the cluster optical mean, is detected along a sightline near the cluster center. The combination of the observed velocity dispersions and bulk velocities is not described by a Kolmogorov velocity power spectrum of steady-state turbulence; instead, the data imply a much steeper effective slope (i.e., relatively more power at larger linear scales). This may indicate either a very large dissipation scale resulting in the suppression of small-scale motions, or a transient dynamic state of the cluster, where large-scale gas flows generated by an ongoing merger have not yet cascaded down to small scales.

astro-ph.HE↗

arXiv2025

Measuring the asymmetric expansion of the Fe ejecta of Cassiopeia A with XRISM/Resolve

The expansion structure of supernova remnants (SNRs) is important for understanding not only how heavy elements are distributed into space, but also how supernovae explode. The ejecta expansion structure of the young core-collapse SNR Cas A is investigated, with Doppler parameter mapping of the Fe-K complex by the Resolve microcalorimeter onboard the X-ray Imaging and Spectroscopy Mission, XRISM. It is found that the Fe ejecta are blueshifted in the southeast (SE) and redshifted in the northwest (NW), indicating an incomplete shell structure, similar to the intermediate mass elements (IMEs), such as Si and S. The Fe has a velocity shift of $\sim1400$ km~s$^{-1}$ in the NW and $\sim2160$ km~s$^{-1}$ in the SE region, with the error range of a few 100s km~s$^{-1}$. These values are consistent with those for the IMEs in the NW region, whereas larger than those for the IMEs in the SE region, although the large error region prevented us from concluding which component has significantly higher velocity. The line broadening is larger in the center with values of $\sim$2000--3000~km~s$^{-1}$, and smaller near the edges of the remnant. The radial profiles of the Doppler shift and broadening of the IMEs and Fe indicate that the Fe ejecta may expand asymmetrically as IME ejacta, although the large error regions do not allow us to conclude it. Moreover, we see little bulk Doppler broadening of the Fe lines in the northeastern jet region whereas the IME lines exhibit significant broadening. No such narrow lines are detected in the NW region. These findings suggest an asymmetric expansion of the ejecta potentially driven by large-scale asymmetries originating from the supernova explosion. This interpretation aligns with the large-scale asymmetries predicted by models of neutrino-driven supernova explosions.

astro-ph.HE↗

arXiv2025

Spectroscopic diagnostics of high-temperature plasma in stellar corona using Fe XXIV--XXVI K-shell lines with XRISM

The RS CVn type binary star GT Mus was observed during its quiescence using the Resolve X-ray microcalorimeter spectrometer onboard XRISM. The main and satellite lines of the Fe XXIV--XXVI K-shell transitions were resolved for the first time from stellar sources. We conducted line ratio analysis to investigate any deviations from collisional onization equilibrium (CIE) and Maxwell electron energy distribution with a single-temperature. By using five combinations of direct excitation lines and dielectronic recombination satellite lines in three line complexes (Fe He$α$, Ly$α$, and He$β$), we found that the plasma is well characterized by two-temperature thermal plasmas with temperatures of 1.7 and 4.3 keV, which is consistent with a thermal broadening of Fe XXV and the broadband fitting results in the 1.7--10 keV band. Other forms of deviation from a single-temperature plasma, such as different ionization and electron temperatures or the $κ$ distribution for the electron energy distributions, are not favored, which is reasonable for stellar coronae at quiescence. This study demonstrates the utility of the Fe K-shell line ratio diagnostics to probe plasma conditions using X-ray microcalorimeters.

astro-ph.HE↗

arXiv2025

Evidence for Charge Exchange Emission in Supernova Remnant N132D from XRISM/Resolve Observations

XRISM has delivered one of its first light observations on N132D, the X-ray brightest supernova remnant in the Large Magellanic Cloud. Utilizing 193 ks of high-resolution X-ray spectroscopy data, we conduct a comprehensive search for charge exchange emission. By incorporating a charge exchange model into our spectral analysis, we observe an improvement in the fits of two weak features at 2.41 keV and 2.63 keV. These features, with a combined significance of 99.6%, are consistent with transitions from highly ionized silicon ions in high Rydberg states, which are unique indicators of charge exchange. Our analysis constrains the charge exchange flux to no more than 4% of the total source flux within the 1.7-3.0 keV band, and places an upper limit on the charge exchange interaction velocity at 450 km/s. This result supports ongoing shock-cloud interactions within N132D and highlights the unique capabilities of XRISM to probe the complex physical processes at play.

astro-ph.HE↗

arXiv2025

A failed wind candidate in NGC 3783 from the 2001 year campaign with Chandra/HETGS

We reanalyze the Chandra/HETGS observations of NGC 3783 from the campaign in the year 2001, identifying significant spectral variations in the Fe unresolved transition array (UTA) over timescales of weeks to months. These changes correlate with a $1.4-2$ fold increase in the ionizing continuum and exceed $10 \, σ$ significance. The variations primarily originate from a low-ionization state ($\rm log ξ= 1.65$) component of the warm absorber. Time-dependent photoionization modelling confirms the sensitivity of this low-ionization component to continuum variations within the Fe UTA band. Local fitting indicates a lower density limit of $>10^{12.3} \, \rm m^{-3}$ at $3 \, σ$ statistical uncertainty, with the component located within $0.27 \, \rm pc$. Our findings suggest that this low-ionization component is a potential failed wind candidate.

astro-ph.HE↗

arXiv2024

Is the low-energy tail of shock-accelerated protons responsible for over-ionized plasma in supernova remnants?

Over-ionized, recombining plasma is an emerging class of X-ray bright supernova remnants (SNRs). This unique thermal state where the ionization temperature ($T_{\rm z}$) is significantly higher than the electron temperature ($T_{\rm e}$) is not expected from the standard evolution model assuming a point explosion in a uniform interstellar medium, requiring a new scenario for the dynamical and thermal evolution. A recently proposed idea attributes the over-ionization state to additional ionization contribution from the low-energy tail of shock-accelerated protons. However, this new scenario has been left untested, especially from the atomic physics point of view. We report calculation results of the proton impact ionization rates of heavy-element ions in ejecta of SNRs. We conservatively estimate the requirement for accelerated protons, and find that their relative number density to thermal electrons needs to be higher than $5~(T_{\rm e}/{\rm 1~keV})\%$ in order to explain the observed over-ionization degree at $T_{\rm z}/T_{\rm e} \ge 2$ for K-shell emission. We conclude that the proton ionization scenario is not feasible because such a high abundance of accelerated protons is prohibited by the injection fraction from thermal to non-thermal energies, which is expected to be $\sim 1\%$ at largest.

astro-ph.HE↗

arXiv2024

First High-Resolution Spectroscopy of X-ray Absorption Lines in the Obscured State of NGC 5548

Multi-wavelength spectroscopy of NGC 5548 revealed remarkable changes due to presence of an obscuring wind from the accretion disk. This broadened our understanding of obscuration and outflows in AGN. Swift monitoring of NGC 5548 shows that over the last 10 years the obscuration has gradually declined. This provides a valuable opportunity for analyses that have not been feasible before because of too much obscuration. The lowered obscuration, together with the high energy spectral coverage of Chandra HETG, facilitate the first study of X-ray absorption lines in the obscured state. The comparison of the lines (Mg XI, Mg XII, Si XIII, and Si XIV) between the new and historical spectra reveals interesting changes, most notably the He-like absorption being significantly diminished in 2022. Our study finds that the changes are caused by an increase in both the ionization parameter and the column density of the warm-absorber outflow in the obscured state. This is contrary to the shielding scenario that is evident in the appearance of the UV lines, where the inner obscuring wind shields outflows that are located further out, thus lowering their ionization. The X-ray absorption lines in the HETG spectra appear to be unaffected by the obscuration. The results suggest that the shielding is complex since various components of the ionized outflow are impacted differently. We explore various possibilities for the variability behavior of the X-ray absorption lines and find that the orbital motion of a clumpy ionized outflow traversing our line of sight is the most likely explanation.

astro-ph.HE↗

arXiv2023

Time-dependent photoionization spectroscopy of the Seyfert galaxy NGC 3783

We present an investigation into the spectroscopic properties of non-equilibrium photoionization processes operating in a time-evolving mode. Through a quantitative comparison between equilibrium and time-evolving models, we find that the time-evolving model exhibits a broader distribution of charge states compared to the equilibrium model, accompanied by a slight shift in the peak ionization state depending on the source variability and gas density. The time-evolving code, tpho in SPEX, has been successfully employed to analyze the spectral properties of warm absorbers in the Seyfert galaxy NGC 3783. The incorporation of variability in the tpho model improves the fits of the time-integrated spectra, providing more accurate descriptions to the average charge states of several elements, in particular for Fe which is peaked around Fe XIX. The inferred densities and distances of the relevant X-ray absorber components are estimated to be approximately a few 1E11 per cubic meter and less than 1 pc, respectively. Furthermore, the updated fit suggests a potential scenario in which the observed absorbers are being expelled from the central AGN at the escape velocities. This implies that these absorbers might not play a significant role in the AGN feedback mechanism.

astro-ph.HE↗

arXiv2023

Density calculations of NGC 3783 warm absorbers using a time-dependent photoionization model

Outflowing wind as one type of AGN feedback, which involves noncollimated ionized winds prevalent in Seyfert-1 AGNs, impacts their host galaxy by carrying kinetic energy outwards. However, the distance of the outflowing wind is poorly constrained due to a lack of direct imaging observations, which limits our understanding of their kinetic power and therefore makes the impact on the local galactic environment unclear. One potential approach involves a determination of the density of the ionized plasma, making it possible to derive the distance using the ionization parameter ξ, which can be measured based on the ionization state. Here, by applying a new time-dependent photoionization model, tpho, in SPEX, we define a new approach, the tpho-delay method, to calculate/predict a detectable density range for warm absorbers of NGC 3783. The tpho model solves self-consistently the time-dependent ionic concentrations, which enables us to study delayed states of the plasma in detail. We show that it is crucial to model the non-equilibrium effects accurately for the delayed phase, where the non-equilibrium and equilibrium models diverge significantly. Finally, we calculate the crossing time to consider the effect of the transverse motion of the outflow on the intrinsic luminosity variation. It is expected that future spectroscopic observations with more sensitive instruments will provide more accurate constraints on the outflow density, and thereby on the feedback energetics.

astro-ph.HE↗

arXiv2023

Charge exchange in X-ray astrophysics

Charge exchange is an atomic process that primarily occurs at interfaces between the neutral and ionized gas. The study of the process has been carried out on three levels: the theoretical calculation of the cross sections, the laboratory measurements of reaction rates and line strengths, and the observational constraints using celestial objects. For a long time in the past, the status of astrophysical observations in the X-ray band lagged behind the other two aspects until the discovery of X-ray from a comet was made in 1996, which changed the research landscape. Recent observational evidence suggests that charge exchange has been seen or can be expected from a surprisingly broad range of locations, from the Earth's exosphere to the large-scale structures of the Universe. The rapid development of high-resolution X-ray spectroscopy, in particular the non-dispersive micro-calorimeters, is paving the way to revolutionary new science possibilities both in the laboratory and astrophysics. This chapter summarizes the current knowledge of charge exchange and its relevance on astrophysics, especially X-ray spectroscopy.

astro-ph.IM↗

arXiv2022

Diffuse radio source candidate in CIZA J1358.9-4750

We report on results of our upgraded Giant Metrewave Radio Telescope (uGMRT) observations for an early-stage merging galaxy cluster, CIZA J1358.9-4750 (CIZA1359), in Band-3 (300--500 MHz). We achieved the image dynamic range of $\sim 38,000$ using the direction dependent calibration and found a candidate of diffuse radio emission at 4~$σ_{rms}$ significance. The flux density of the candidate at 400~MHz, $24.04 \pm 2.48$~mJy, is significantly positive compared to noise, where its radio power, $2.40 \times 10^{24}$~W~Hz$^{-1}$, is consistent with those of typical diffuse radio sources of galaxy clusters. The candidate is associated with a part of the X-ray shock front at which the Mach number reaches its maximum value of $\mathcal{M}\sim 1.7$. The spectral index ($F_ν\propto ν^α$) of the candidate, $α= - 1.22 \pm 0.33$, is in agreement with an expected value derived from the standard diffusive shock acceleration (DSA) model. But such a low Mach number with a short acceleration time would require seed cosmic-rays supplied from active galactic nucleus (AGN) activities of member galaxies, as suggested in some other clusters. Indeed, we found seven AGN candidates inside the diffuse source candidate. Assuming the energy equipartition between magnetic fields and cosmic-rays, the magnetic field strength of the candidate was estimated to be $2.1~μ$G. We also find head-tail galaxies and radio phoenixes or fossils near the CIZA1359.

astro-ph.HE↗

arXiv2022

R-matrix electron-impact excitation data for the H- and He-like ions with Z=6-30

Plasma models built on extensive atomic data are essential to interpreting the observed cosmic spectra. H-like Lyman series and He-like triplets observable in the X-ray band are powerful diagnostic lines to measure the physical properties of various types of astrophysical plasmas. Electron-impact excitation is a fundamental atomic process for the formation of H-like and He-like key diagnostic lines. Electron-impact excitation data adopted by the widely used plasma codes (AtomDB, CHIANTI, and SPEX) do not necessarily agree with each other. Here we present a systematic calculation of electron-impact excitation data of H-like and He-like ions with the atomic number Z=6-30 (i.e., C to Zn). Radiation damped R-matrix intermediate coupling frame transformation calculation was performed for each ion with configurations up to $n=6$. We compare the present work with the above three plasma codes and literature to assess the quality of the new data, which are relevant for current and future high-resolution X-ray spectrometers.

physics.atom-ph↗

arXiv2022

Detection of an Unidentified Soft X-ray Emission Feature in NGC 5548

NGC~5548 is an X-ray bright Seyfert 1 active galaxy. It exhibits a variety of spectroscopic features in the soft X-ray band, including in particular the absorption by the AGN outflows of a broad range of ionization states, with column densities up to 1E27 /m^2, and having speeds up to several thousand kilometers per second. The known emission features are in broad agreement with photoionized X-ray narrow and broad emission line models. We report on an X-ray spectroscopic study using 1.1 Ms XMM-Newton and 0.9 Ms Chandra grating observations of NGC 5548 spanning two decades. The aim is to search and characterize any potential spectroscopic features in addition to the known primary spectral components that are already modeled in high precision. We detect a weak unidentified excess emission feature at 18.4 Angstrom (18.1 Angstrom in the restframe). The feature is seen at >5 sigma statistical significance taking into account the look elsewhere effect. No known instrumental issues, atomic transitions, and astrophysical effects can explain this excess. The observed intensity of the possible feature seems to anti-correlate in time with the hardness ratio of the source. However, the variability might not be intrinsic, it might be caused by the time-variable obscuration by the outflows. An intriguing possibility is the line emission from charge exchange between a partially ionized outflow and a neutral layer in the same outflow, or in the close environment. Other possibilities, such as emission from a highly-ionized component with high outflowing speed, cannot be fully ruled out.

astro-ph.HE↗

arXiv2022

10-Year Transformation of the Obscuring Wind in NGC 5548

A decade ago the archetypal Seyfert-1 galaxy NGC 5548 was discovered to have undergone major spectral changes. The soft X-ray flux had dropped by a factor of 30 while new broad and blueshifted UV absorption lines appeared. This was explained by the emergence of a new obscuring wind from the accretion disk. Here we report on the striking long-term variability of the obscuring disk wind in NGC 5548 including new observations taken in 2021-2022 with the Swift Observatory and the Hubble Space Telescope's (HST) Cosmic Origins Spectrograph (COS). The X-ray spectral hardening as a result of obscuration has declined over the years, reaching its lowest in 2022 at which point we find the broad C IV UV absorption line to be nearly vanished. The associated narrow low-ionization UV absorption lines, produced previously when shielded from the X-rays, are also remarkably diminished in 2022. We find a highly significant correlation between the variabilities of the X-ray hardening and the equivalent width of the broad C IV absorption line, demonstrating that X-ray obscuration is inherently linked to disk winds. We derive for the first time a relation between the X-ray and UV covering fractions of the obscuring wind using its long-term evolution. The diminished X-ray obscuration and UV absorption are likely caused by an increasingly intermittent supply of outflowing streams from the accretion disk. This results in growing gaps and interstices in the clumpy disk wind, thereby reducing its covering fractions.

astro-ph.HE↗

arXiv2022

X-ray spectra of the Fe-L complex III: systematic uncertainties in the atomic data

There has been a growing request from the X-ray astronomy community for a quantitative estimate of systematic uncertainties originating from the atomic data used in plasma codes. Though there have been several studies looking into atomic data uncertainties using theoretical calculations, in general, there is no commonly accepted solution for this task. We present a new approach for estimating uncertainties in the line emissivities for the current models of collisional plasma, mainly based upon dedicated analysis of observed high resolution spectra of stellar coronae and galaxy clusters. We find that the systematic uncertainties of the observed lines consistently show anti-correlation with the model line fluxes, after properly accounting for the additional uncertainties from the ion concentration calculation. The strong lines in the spectra are in general better reproduced, indicating that the atomic data and modeling of the main transitions are more accurate than those for the minor ones. This underlying anti-correlation is found to be roughly independent on source properties, line positions, ion species, and the line formation processes. We further apply our method to the simulated XRISM and Athena observations of collisional plasma sources and discuss the impact of uncertainties on the interpretation of these spectra. The typical uncertainties are 1-2% on temperature and 3-20% on abundances of O, Ne, Fe, Mg, and Ni.

astro-ph.HE↗

arXiv2022

Uncertainties in Atomic Data for Modeling Astrophysical Charge Exchange Plasmas

Relevant uncertainties on theoretical atomic data are vital to determine the accuracy of plasma diagnostics in a number of areas including in particular the astrophysical study. We present a new calculation of the uncertainties on the present theoretical ion-impact charge exchange atomic data and X-ray spectra based on a set of comparisons with the existing laboratory data obtained in historical merged-beam, cold-target recoil-ion momentum spectroscopy, and electron beam ion traps experiments. The average systematic uncertainties are found to be 35-88% on the total cross sections, and 57-75% on the characteristic line ratios. The model deviation increases as the collision energy decreases. The errors on total cross sections further induce a significant uncertainty to the calculation of ionization balance for low temperature collisional plasmas. Substantial improvements of the atomic database and dedicated laboratory measurements are needed to get the current models ready for the X-ray spectra from the next X-ray spectroscopic mission.

astro-ph.HE↗