arXiv subjects

Shuinai Zhang

Publications and source records attributed to Shuinai Zhang.

15 recordsLinked to original sources

arXiv2024

A Monte-Carlo Simulation on Resonant Scattering of X-ray Line Emission in Supernova Remnants

Resonant scattering (RS) of X-ray line emission in supernova remnants (SNRs) may modify the observed line profiles and fluxes and has potential impact on estimating the physical properties of the hot gas and hence on understanding the SNR physics, but has not been theoretically modeled ever. Here we present our Monte-Carlo simulation of RS effect on X-ray resonant-line emission, typified by O VII He$α$ r line, from SNRs. We employ the physical conditions characterized by the Sedov-Taylor solution and some basic parameters similar to those in Cygnus Loop. We show that the impact of RS effect is most significant near the edge of the remnant. The line profiles are predicted to be asymmetric because of different temperatures and photon production efficiencies of the expanding gas at different radii. We also predict the surface brightness of the line emission would decrease in the outer projected region but is slightly enhanced in the inner. The G-ratio of the OVII He$α$ triplet can be effectively elevated by RS in the outer region. We show that RS effect of the O VII He$α$ r line in the southwestern boundary region of Cygnus Loop is non-negligible. The observed OVII G-ratio $\sim$1.8 of the region could be achieved with RS taken into account for properly elevated O abundance from the previous estimates. Additional simulation performed for the SNRs in ejecta-dominated phase like Cas A shows that RS in the shocked ejecta may have some apparently effects on the observational properties of oxygen resonant lines.

astro-ph.HE↗

arXiv2023

Scientific Objectives of the Hot Universe Baryon Surveyor (HUBS) Mission

The Hot Universe Baryon Surveyor (HUBS) is a proposed space-based X-ray telescope for detecting X-ray emissions from the hot gas content in our universe. With its unprecedented spatially-resolved high-resolution spectroscopy and large field of view, the HUBS mission will be uniquely qualified to measure the physical and chemical properties of the hot gas in the interstellar medium, the circumgalactic medium, the intergalactic medium, and the intracluster medium. These measurements will be valuable for two key scientific goals of HUBS, namely to unravel the AGN and stellar feedback physics that governs the formation and evolution of galaxies, and to probe the baryon budget and multi-phase states from galactic to cosmological scales. In addition to these two goals, the HUBS mission will also help us solve some problems in the fields of galaxy clusters, AGNs, diffuse X-ray backgrounds, supernova remnants, and compact objects. This paper discusses the perspective of advancing these fields using the HUBS telescope.

astro-ph.GA↗

arXiv2022

X-ray Spectroscopic Evidence of Charge Exchange Emission in the Disk of M51

In the disks of spiral galaxies, diffuse soft X-ray emission is known to be strongly correlated with star-forming regions. However, this emission is not simply from a thermal-equilibrium plasma and its origin remains greatly unclear. In this work, we present an X-ray spectroscopic analysis of the emission from the northern hot spot; a region with enhanced star-formation off the nucleus of M51. Based on the high spectral resolution data from XMM-Newton/RGS observations, we unambiguously detect a high $G$ ratio ($3.2^{+6.9}_{-1.5}$) of the OVII He$α$ triplet. This high $G$ ratio is also spatially confirmed by oxygen emission-line maps from the same data. A physical model consisting of a thermal plasma and its charge exchange (CX) with neutral cool gas gives a good explanation for the $G$ ratio and the entire RGS spectra. This model also gives a satisfactory characterization of the complementary Chandra/ACIS-S data, which enables a direct imaging of the diffuse emission, tracing the hot plasma across the galaxy. The hot plasma has a similar characteristic temperature of ~0.34 keV and an approximately solar metallicity. The CX contributes ~50% to the diffuse emission in the 0.4-1.8 keV band, suggesting an effective hot/cool gas interface area about five times the geometric area of the M51 disk. Therefore, the CX appears to play a major role in the soft X-ray production and may be used as a powerful tool to probe the interface astrophysics, important for studying galactic ecosystems.

astro-ph.GA↗

arXiv2022

Deep Near-Infrared Survey Toward the W40 and Serpens South Region in Aquila Rift: A Comprehensive Catalog of Young Stellar Objects

Active star forming regions are excellent laboratories for studying the origins and evolution of young stellar object (YSO) clustering. The W40 - Serpens South region is such a region, and we compile a super near-and-mid-infrared catalog of point sources in it, based on deep NIR observations of CFHT in combination with 2MASS, UKIDSS, and Spitzer catalogs. From this catalog, we identify 832 YSOs, and classify 15, 135, 647, and 35 of them to be the deeply embedded sources, Class I, Class II YSOs, and transition disk sources, respectively. In general, these YSOs are well correlated with the filamentary structures of molecular clouds, especially the deeply embedded sources and the Class I YSOs. The W40 central region is dominated by Class II YSOs, but in the Serpens South region, a half of the YSOs are Class I. We further generate a minimum spanning tree (MST) for all the YSOs. Around the W40 cluster, there are eight prominent MST branches that may trace vestigial molecular gas filaments that once fed gas to the central natal gas clump. Of the eight, only two now include detectable filamentary gas in Herschel data and corresponding Class I YSOs, while the other six are exclusively populated with Class II. Four MST branches overlap with the Serpens South main filament, and where they intersect, molecular gas "hubs" and more Class I YSOs are found. Our results imply a mixture of YSO distributions composed of both primordial and somewhat evolved YSOs in this star forming region.

astro-ph.SR↗

arXiv2022

Hierarchical Structure of YSO Clusters in the W40 and Serpens South Region: Group Extraction and Comparison with Fractal Clusters

Young stellar clusters are believed to inherit the spatial distribution like hierarchical structures of their natal molecular cloud during their formation. However, the change of the structures between the cloud and the young clusters is not well constrained observationally. We select the W40 - Serpens South region (~ 7 $\times$ 9 pc$^{2}$) of the Aquila Rift as a testbed and investigate hierarchical properties of spatial distribution of young stellar objects (YSOs) in this region. We develop a minimum spanning tree (MST)-based method to group stars into several levels by successively cutting down edges longer than an algorithmically determined critical value. A total of 832 YSOs are divided into 5 levels with 23 groups. For describing the hierarchical properties in a controlled way, we construct a set of synthetic source distributions at various fractal dimensions, and apply the same method to explore their group characters. By comparing the $Q$ parameter and the surface density profiles of the observed and the synthetic data, we find that the YSO observation matches spatial patterns from multi-fractal dimensions. In the periphery region where the molecular clouds are more diffuse, the YSO structure is close to a fractal dimension of 2.0, while in the core regions the fractal dimensions are close to 1.6 and 1.4 for the W40 and the Serpens South regions, respectively. Therefore, the YSOs may inherit the fractal pattern of the dense part of the molecular clouds, but such pattern dissipates slowly in several Myr.

astro-ph.SR↗

arXiv2021

TeV cosmic ray nuclei acceleration in shell-type supernova remnants with hard $γ$-ray spectra

The emission mechanism for hard $γ$-ray spectra from supernova remnants (SNRs) is still a matter of debate. Recent multi-wavelength observations of TeV source HESS J1912+101 show that it is associated with an SNR with an age of $\sim 100$ kyrs, making it unlikely produce the TeV $γ$-ray emission via leptonic processes. We analyzed Fermi observations of it and found an extended source with a hard spectrum. HESS J1912+101 may represent a peculiar stage of SNR evolution that dominates the acceleration of TeV cosmic rays. By fitting the multi-wavelength spectra of 13 SNRs with hard GeV $γ$-ray spectra with simple emission models with a density ratio of GeV electrons to protons of $\sim 10^{-2}$, we obtain reasonable mean densities and magnetic fields with a total energy of $\sim 10^{50}$ ergs for relativistic ions in each SNR. Among these sources, only two of them, namely SN 1006 and RCW 86, favor a leptonic origin for the $γ$-ray emission. The magnetic field energy is found to be comparable to that of the accelerated relativistic ions and their ratio has a tendency of increase with the age of SNRs. These results suggest that TeV cosmic rays mainly originate from SNRs with hard $γ$-ray spectra.

astro-ph.HE↗

arXiv2020

Mapping Diffuse Emission in Lyman UV band

The CAFE (Census of warm-hot intergalactic medium, Accretion, and Feedback Explorer) and LyRIC (Lyman UV Radiation from Interstellar medium and Circum-galactic medium) have been proposed to the space agencies in China respectively. CAFE was first proposed in 2015 as a joint scientific CAS-ESA small space mission. LyRIC was proposed in 2019 as an independent external payload operating on the Chinese Space Station. Both missions are dedicated to mapping the Lyman UV emission (ionized oxygen (O VI) resonance lines at 103.2 and 103.8 nm, and Lyman series) for the diffuse sources in our Galaxy and the circum-galactic mediums of the nearby galaxies. We present the primary science objectives, mission concepts, the enabling technologies, as well as the current status.

astro-ph.IM↗

arXiv2020

Spectral Modeling of Charge Exchange in the Central Region of M51

Charge exchange (CX) emission reveals the significant interaction between neutral and ionized interstellar medium (ISM) components of the dense, multiphase, circumnuclear region of a galaxy. We use a model including a thermal and a CX components to describe the high-resolution XMM-Newton/RGS spectrum of the diffuse emission in the central region of M51. Representative signatures of CX emission -- especially the prominent OVII forbidden line and the excess emission in the OVIII Ly$γ$ lines -- can be well explained by the model. Combined with the Chandra images in the OVIII and the OVII bands, we find the soft X-ray emission is dominated by the jet-driven outflow and its interaction with the ambient neutral material. The jet-driven outflow itself is likely a thermal plasma of $\sim 0.59$ keV, with mostly sub-solar abundances. It runs into the ambient neutral gas, and produces significant CX emission that accounts for one-fifth of the diffuse X-ray emission in the 7--28 Å band. The effective interface area in the CX process is one order of magnitude greater than the geometrical surface area of the jet-driven outflow. The tenuous outflow driven by the nuclear star formation may also contribute a small portion to both the diffuse thermal and CX emission. The photoionization by the active galactic nuclei (AGNs) and the resonance scattering by the hot gas itself are disfavored, though the effects from past AGN events may not be ruled out.

astro-ph.GA↗

arXiv2019

Charge exchange emission and cold clumps in multi-phase galactic outflows

Large-scale outflows from starburst galaxies are multi-phase, multi-component fluids. Charge-exchange lines which originate from the interfacing surface between the neutral and ionised components are a useful diagnostic of the cold dense structures in the galactic outflow. From the charge-exchange lines observed in the nearby starburst galaxy M82, we conduct surface-to-volume analyses and deduce that the cold dense clumps in its galactic outflow have flattened shapes, resembling a hamburger or a pancake morphology rather than elongated shapes. The observed filamentary H$α$ features are therefore not prime charge-exchange line emitters. They are stripped material torn from the slow moving dense clumps by the faster moving ionised fluid which are subsequently warmed and stretched into elongated shapes. Our findings are consistent with numerical simulations which have shown that cold dense clumps in galactic outflows can be compressed by ram pressure, and also progressively ablated and stripped before complete disintegration. We have shown that some clumps could survive their passage along a galactic outflow. These are advected into the circumgalactic environment, where their remnants would seed condensation of the circumgalactic medium to form new clumps. The infall of these new clumps back into the galaxy and their subsequent re-entrainment into the galactic outflow form a loop process of galactic material recycling.

astro-ph.GA↗

arXiv2019

XMM-Newton RGS spectroscopy of the M31 bulge. I: Evidences for a past AGN half a million years ago

Existing analysis based on XMM-Newton/RGS spectra already shows that the G-ratio of the OVII He$α$ triplet in the inner bulge of M31 is too high to be consistent with a pure optically thin thermal plasma in collisional ionization equilibrium (CIE). Different processes that may affect properties of diffuse hot plasma were proposed, such as resonance scattering (RS) and charge exchange (CX) with cold gas. To determine which physical process(es) may be responsible for this inconsistency, we present a systematic spectroscopic analysis based on 0.8 Ms XMM-Newton/RGS data, together with complementary Chandra/ACIS-S images. The combination of these data enables us to reveal multiple non-CIE spectroscopic diagnostics, including but not limited to the large G-ratios of He$α$ triplets (OVII, NVI, and NeIX) and the high Lyman series line ratios (OVIII Ly$β$/Ly$α$ and Ly$γ$/Ly$α$, and NVII Ly$β$/Ly$α$), which are not expected for a CIE plasma, and the high iron line ratios (FeXVIII 14.2 Å/FeXVII~17 Åand FeXVII~15 Å/17 Å), which suggest much higher temperatures than other line ratios, as well as their spatial variations. Neither CX nor RS explains all these spectroscopic diagnostics satisfactorily. Alternatively, we find that an active galactic nucleus (AGN) relic scenario provides a plausible explanation for virtually all the signatures. We estimate that an AGN was present at the center of M31 about half a million years ago and that the initial ionization parameter $ξ$ of the relic plasma is in the range of 3-4.

astro-ph.GA↗

arXiv2018

Resonant scattering effect on the soft X-ray line emission from the hot interstellar medium: I. galactic bulges

Diffuse soft X-ray line emission is commonly used to trace the thermal and chemical properties of the hot interstellar medium, as well as its content, in nearby galaxies. Although resonant line scattering complicates the interpretation of the emission, it also offers an opportunity to measure the kinematics of the medium. We have implemented a direct Monte Carlo simulation scheme that enables us to account for resonant scattering effect in the medium, in principle, with arbitrary spatial, thermal, chemical, and kinematic distributions. Here we apply this scheme via dimensionless calculation to an isothermal, chemically uniform, and spherically symmetric medium with a radial density distribution characterized by a $β$-model. This application simultaneously account for both optical depth-dependent spatial distortion and intensity change of the resonant line emission due to the scattering, consistent with previous calculations. We further apply the modeling scheme to the OVII and OVIII emission line complex observed in the XMM-Newton RGS spectrum of the M31 bulge. This modeling, though with various limitations due to its simplicity, shows that the resonant scattering could indeed account for much of the spatial distortion of the emission, as well as the relative strengths of the lines, especially the large forbidden to resonant line ratio of the OVII He$α$ triplet. We estimate the isotropic turbulence Mach number of the medium in M31 as $\sim0.17$ for the first time and the line-emitting gas temperature as $\sim2.3\times10^6$ K. We conclude that the resonant scattering may in general play an important role in shaping the soft X-ray spectra of diffuse hot gas in normal galaxies.

astro-ph.GA↗

arXiv2016

Evolution of High-Energy Particle Distribution in Mature Shell-Type Supernova Remnants

Multi-wavelength observations of mature supernova remnants (SNRs), especially with recent advances in gamma-ray astronomy, make it possible to constrain energy distribution of energetic particles within these remnants. In consideration of the SNR origin of Galactic cosmic rays and physics related to particle acceleration and radiative processes, we use a simple one-zone model to fit the nonthermal emission spectra of three shell-type SNRs located within 2 degrees on the sky: RX J1713.7-3946, CTB 37B, and CTB 37A. Although radio images of these three sources all show a shell (or half-shell) structure, their radio, X-ray, and gamma-ray spectra are quite different, offering an ideal case to explore evolution of energetic particle distribution in SNRs. Our spectral fitting shows that 1) the particle distribution becomes harder with aging of these SNRs, implying a continuous acceleration process, and the particle distributions of CTB 37A and CTB 37B in the GeV range are harder than the hardest distribution that can be produced at a shock via the linear diffusive shock particle acceleration process, so spatial transport may play a role; 2) the energy loss timescale of electrons at the high-energy cutoff due to synchrotron radiation appears to be always a bit (within a factor of a few) shorter than the age of the corresponding remnant, which also requires continuous particle acceleration; 3) double power-law distributions are needed to fit the spectra of CTB 37B and CTB 37A, which may be attributed to shock interaction with molecular clouds.

astro-ph.HE↗

arXiv2014

Spectral Modeling of the Charge-Exchange X-ray Emission from M82

It has been proposed that the charge exchange (CX) process at the interface between hot and cool interstellar gases could contribute significantly to the observed soft X-ray emission in star forming galaxies. We analyze the XMM-Newton/RGS spectrum of M82, using a newly developed CX model combined with a single-temperature thermal plasma to characterize the volume-filling hot gas. The CX process is largely responsible for not only the strongly enhanced forbidden lines of the K$α$ triplets of various He-like ions, but also good fractions of the Ly$α$ transitions of C VI (~87%), O VIII and N VII ($\gtrsim$50%) as well. In total about a quarter of the X-ray flux in the RGS 6-30 Å band originates in the CX. We infer an ion incident rate of $3\times10^{51}\,\rm{s^{-1}}$ undergoing CX at the hot and cool gas interface, and an effective area of the interface as $\sim2\times10^{45}\,{\rm cm^2}$ that is one order of magnitude larger than the cross section of the global biconic outflow. With the CX contribution accounted for, the best fit temperature of the hot gas is 0.6 keV, and the metal abundances are approximately solar. We further show that the same CX/thermal plasma model also gives an excellent description of the EPIC-pn spectrum of the outflow Cap, projected at 11.6 kpc away from the galactic disk of M82. This analysis demonstrates that the CX is potentially an important contributor to the X-ray emission from starburst galaxies and also an invaluable tool to probe the interface astrophysics.

astro-ph.GA↗

arXiv2011

Multi-Phase Dusty Gas in the Center of NGC 4278

We present the Spitzer spectroscopic mapping observations toward the central kpc region of the nearby elliptical LINER galaxy, NGC 4278, by using the Infrared Spectrograph (IRS). These observations reveal rich mid-IR emission features of extended ionized gas, warm molecular hydrogen and dust. Different phases of gas and dust are closely related and belong to a same elongated feature. We further study properties of multi-phase dusty gas to uncover the underlying mechanism of ionization and excitation. The band ratio and intensity of PAH features in the central region might reflect modified size distribution resulted from selective destruction. H$_{2}$ S(0)-S(7) pure rotational lines of molecular hydrogen show excessive intensity and moderately high excitation temperature comparing with photon dissociation region (PDR). A strong and extended [SiII] emission line is detected, which could be a sign of reduced depletion of silicon in interstellar dust. We also discover an extended high ionization region associated with enhanced H$_{2}$ S(1) emission. We conclude that a shock-heating component is required to account for observed emission characteristics, which could be triggered by cloud-cloud interactions during accretion of cold gas from the large HI disk.

astro-ph.CO↗

arXiv2010

Chandra and XMM-Newton view of the Warm Absorbing Gas in Mrk 290

We present a detailed analysis of the Chandra HETGS and XMM-Newton high resolution spectra of the bright Seyfert 1 galaxy, Mrk 290. The Chandra spectra reveal complex absorption features that can be best described by a combination of three ionized absorbers. The outflow velocities of these warm absorbers are about 450 km/s, consistent with the three absorption components found in a previous far UV study. The ionizing continuum of Mrk 290 fluctuated by a factor of 1.4 during Chandra observations on a time scale of 17 days. Thus, we put a lower limit on the distance from the ionizing source of 0.9 pc for the medium ionized absorber and an upper limit on distance of 2.5 pc for the lowest ionized absorber. The three ionization components lie on the stable branch of the thermal equilibrium curve, indicating that the torus is most likely the origin of warm absorbing gas in Mrk 290. During the XMM-Newton observation, the ionizing luminosity was 50% lower compared to the one in the Chandra observation. Neither the ionization parameter nor the column density of the two absorbing components varied significantly, compared to the results from Chandra observations. However, the outflow velocities of both components were 1260 km/s. We suggest that an entirely new warm absorber from the torus passed through our line of sight. Assuming the torus wind model, the estimated mass outflow rate is about 1 Solar mass per year.

astro-ph.CO↗