arXiv subjects

Li Ji

Publications and source records attributed to Li Ji.

At least 55 records · Page 3Linked to original sources

arXiv2021

The BIG X-ray tail

Galaxy clusters grow primarily through the continuous accretion of group-scale haloes. Group galaxies experience preprocessing during their journey into clusters. A star-bursting compact group, the Blue Infalling Group (BIG), is plunging into the nearby cluster A1367. Previous optical observations reveal rich tidal features in the BIG members, and a long H$α$ trail behind. Here we report the discovery of a projected $\sim 250$ kpc X-ray tail behind the BIG using Chandra and XMM-Newton observations. The total hot gas mass in the tail is $\sim 7\times 10^{10}\ {\rm M}_\odot$ with an X-ray bolometric luminosity of $\sim 3.8\times 10^{41}$ erg s$^{-1}$. The temperature along the tail is $\sim 1$ keV, but the apparent metallicity is very low, an indication of the multi-$T$ nature of the gas. The X-ray and H$α$ surface brightnesses in the front part of the BIG tail follow the tight correlation established from a sample of stripped tails in nearby clusters, which suggests the multiphase gas originates from the mixing of the stripped interstellar medium (ISM) with the hot intracluster medium (ICM). Because thermal conduction and hydrodynamic instabilities are significantly suppressed, the stripped ISM can be long lived and produce ICM clumps. The BIG provides us a rare laboratory to study galaxy transformation and preprocessing.

astro-ph.GA↗

arXiv2021

What causes the absence of pulsations in Central Compact Objects in Supernova Remnants?

Most young neutron stars belonging to the class of Central Compact Objects in supernova remnants (CCOs) do not have known periodicities. We investigated seven such CCOs to understand the common reasons for the absence of detected pulsations. Making use of XMM-Newton, Chandra, and NICER observations, we perform a systematic timing and spectral analysis to derive updated sensitivity limits for both periodic signals and multi-temperature spectral components that could be associated with radiation from hotspots on the neutron star surface. Based on these limits, we then investigated for each target the allowed viewing geometry that could explain the lack of pulsations. We estimate it is unlikely ($< 10^{-6}$) to attribute that we do not see pulsations to an unfavorable viewing geometry for five considered sources. Alternatively, the carbon atmosphere model, which assumes homogeneous temperature distribution on the surface, describes the spectra equally well and provides a reasonable interpretation for the absence of detected periodicities within current limits. The unusual properties of CCOs with respect to other young neutron stars could suggest a different evolutionary path, as that proposed for sources experiencing episodes of significant fallback accretion after the supernova event.

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

A Study of Opacity Ranges for Transparent Overlays in 3D Landscapes

{When visualizing data in a realistically rendered 3D virtual environment, it is often important to represent not only the 3D scene but also overlaid information about additional, abstract data. These overlays must be usefully visible, i.e. be readable enough to convey the information they represent, but remain unobtrusive to avoid cluttering the view. We take a step toward establishing guidelines for designing such overlays by studying the relationship between three different patterns (filled, striped and dotted patterns), two pattern densities, the presence or not of a solid outline, two types of background (blank and with trees), and the opacity of the overlay. For each combination of factors, participants set the faintest and the strongest acceptable opacity values. Results from this first study suggest that i) ranges of acceptable opacities are around 20-70%, that ii) ranges can be extended by 5% by using an outline, and that iii) ranges shift based on features like pattern and density.

cs.GR↗

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

Probing the dynamical state, baryon content, and multiphase nature of galaxy clusters with bright background QSOs

We have initiated a programme to study the physical/dynamical state of gas in galaxy clusters and the impact of the cluster environment on gaseous halos of individual galaxies using X-ray imaging and UV absorption line spectroscopy of background QSOs. Here we report results from the analysis Chandra and XMM-Newton archival data of five galaxy clusters with such QSOs, one of which has an archival UV spectrum. We characterize the gravitational masses and dynamical states, as well as the hot intracluster medium (ICM) properties of these clusters. Most clusters are dynamically disturbed clusters based on the X-ray morphology parameters, the X-ray temperature profiles, the large offset between X-ray peak and brightest cluster galaxy (BCG). The baryon contents in the hot ICM and stars of these clusters within $r_{500}$ are lower than the values expected from the gravitational masses, according to the standard cosmology. We also estimate column densities of the hot ICM along the sightlines toward the background QSOs as well as place upper limits on the warm-hot phase for the one sightline with existing UV observations. These column densities, compared with those of the warm and warm-hot ICM to be measured with UV absorption line spectroscopy, will enable us to probe the relationship among various gaseous phases and their connection to the heating/cooling and dynamical processes of the clusters. Furthermore, our analysis of the archival QSO spectrum probing one cluster underscores the need for high quality, targeted UV observations to robustly constrain the 10$^{5-6}$ K gas phase.

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↗

arXiv2017

Joint Fit of Warm Absorbers in COS & HETG Spectra of NGC 3783

Warm Absorbers (WAs), as an important form of AGN outflows, show absorption in both UV and X-ray band. Using XSTAR generated photoionization models, for the first time we present a joint fit to the simultaneous observations of HST/COS and Chandra/HETGS on NGC 3783. A total of five WAs explain well all absorption features from the AGN outflows, which are spread over a wide range of parameters: ionization parameter logξ from 0.6 to 3.8, column density log N_H from 19.5 to 22.3 cm^{-2}, velocity v from 380 to 1060 km s^{-1}, and covering factor from 0.33 to 0.75. Not all the five WAs are consistent in pressure. Two of them are likely different parts of the same absorbing gas, and two of the other WAs may be smaller discrete clouds that are blown out from the inner region of the torus at different periods. The five WAs suggest a total mass outflowing rate within the range of 0.22-4.1 solar mass per year.

astro-ph.HE↗

arXiv2017

Disc-corona interaction in the heartbeat state of GRS 1915+105

Timing analysis provides information about the dynamics of matter accreting on to neutron stars and black holes, and hence is crucial for studying the physics of the accretion flow around these objects. It is difficult, however, to associate the different variability components with each of the spectral components of the accretion flow. We apply several new methods to two Rossi X-ray Timing Explorer observations of the black hole binary GRS 1915+105 during its heartbeat state to explore the origin of the X-ray variability and the interactions of the accretion-flow components. We offer a promising window into the disc--corona interaction through analysing the formation regions of the disc aperiodic variabilities with different time-scales via comparing the corresponding transition energies of the amplitude-ratio spectra. In a previous paper, we analysed the Fourier power density as a function of energy and frequency to study the origin of the aperiodic variability, and combined that analysis with the phase lag as a function of frequency to derive a picture of the disc--corona interaction in this source. We here, for the first time, investigate the phase lag as a function of energy and frequency, and display some interesting details of the disc--corona interaction. Besides, the results from the shape of amplitude-ratio spectrum and from several other aspects suggest that the quasi-periodic oscillation originates from the corona.

astro-ph.HE↗

arXiv2016

A timing view of the heartbeat state of GRS 1915+105

We present a timing analysis of two Rossi X-ray Timing Explorer observations of the microquasar GRS 1915+105 during the heartbeat state. The phase-frequency-power maps show that the intermediate-frequency aperiodic X-ray variability weakens as the source softens in the slow rise phase, and when the quasi-periodic oscillation disappears in the rise phase of the pulse of the double-peaked class its sub-harmonic is still present with a hard phase lag. In the slow rise phase, the energy-frequency-power maps show that most of the aperiodic variability is produced in the corona, and may also induce the aperiodic variability observed at low energies from an accretion disk, which is further supported by the soft phase lag especially in the intermediate-frequency range (with a time delay up to 20 ms). In the rise phase of the pulse, the low-frequency aperiodic variability is enhanced significantly and there is a prominent hard lag (with a time delay up to 50 ms), indicating that the variability is induced by extension of the disk toward small radii as implied by the increase in flux and propagates into the corona. However, during the hard pulse of the double-peaked class, the variability shows no significant lag, which may be attributed to an optically thick corona. These timing results are generally consistent with the spectral results presented by Neilsen et al. (2011, 2012) which indicated that the slow rise phase corresponds to a local Eddington limit and the rise phase of the pulse corresponds to a radiation pressure instability in the disk.

astro-ph.HE↗

arXiv2016

Baryon content and dynamic state of galaxy clusters: XMM-Newton observations of A1095 and A1926

We have initiated a program to study the baryon content and dynamic state of galaxy clusters. Here we present results primarily from XMM-Newton observations of two optically-selected galaxy clusters, A1095 ($z \simeq 0.210$) and A1926 ($z \simeq 0.136$). We find that both of them are actually cluster pairs at similar redshifts. We characterize the temperatures of these individual clusters through X-ray spectral fits and then estimate their gravitational masses. We show a rich set of substructures, including large position offsets between the diffuse X-ray centroids and the brightest galaxies of the clusters, which suggests that they are dynamically young. For both A1095 and A1926, we find that the mass required for the cluster pairs to be bound is smaller than the total gravitational mass. Thus both cluster pairs appear to be ongoing major mergers. Incorporating SDSS and NVSS/FIRST data, we further examine the large-scale structure environment and radio emission of the clusters to probe their origins, which also leads to the discovery of two additional X-ray-emitting clusters ($z \simeq 0.097$ and $z \simeq 0.147$) in the field of A1926. We estimate the hot gas and stellar masses of each cluster, which compared with the expected cosmological baryonic mass fraction, leave ample room for warm gas.

astro-ph.GA↗

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↗

arXiv2012

Three dimensional ring vortex solitons and their stability in Bose-Einstein condensates under magnetic confinement

The three-dimensional study of the ring vortex solitons is conducted for both attractive and repulsive BECs subject to harmonic potential confinement. A family of stationary ring vortex solitons, which is defined by the radial excitation number and the winding number of the intrinsic vorticity, are obtained numerically for a given atomic interaction strength. We find that stabilities of the ground and radially excited states of the ring vortex soliton are dependent on the winding number differently. The ground state of the ring vortex soliton with the large winding number is unstable dynamically against random perturbation. The radially excited state of the ring vortex soliton with large winding number corresponds to the increased collapse threshold, and therefore can be made stable for sufficiently small atomic interaction strengths. The ground and radially excited states also demonstrate different dynamical evolutions under large atomic interaction strengths. The former exhibits simultaneously symmetrical splitting in the transverse plane, while the latter displays periodically expand-merge cycles in the longitudinal direction.

cond-mat.quant-gas↗

arXiv2011

Serendipitous Discovery of Warm Absorbers in the Seyfert 2 Galaxy IRAS 18325-5926

Warm absorption is a common phenomenon in Seyfert 1s and quasars, but rare in Seyfert 2s. In this paper, we report the detection of warm absorbers with high energy resolution in the Seyfert 2 galaxy IRAS 18325-5926 for the first time with Chandra HETGS spectra. An intrinsic absorbing line system with an outflow velocity ~400 km/s was found, which is contributed by two warm absorbers with FWHM of 570 km/s and 1360 km/s, respectively. The two absorbers were adjacent, and doing a transverse motion across our line of sight. We constrained the distance of the absorbers to a small value, suggesting that the absorbers may originate from the highly ionized accretion disk wind. The perspective of this type 2 Seyfert provides the best situation to investigate the vertical part of the funnel-like outflows. Another weak absorbing line system with zero redshift was also detected, which could be due to Galactic absorption with very high temperature, or an intrinsic outflow with very high velocity ~6000 km/s.

astro-ph.CO↗

arXiv2010

X-ray Signatures of Non-Equilibrium Ionization Effects in Galaxy Cluster Accretion Shock Regions

The densities in the outer regions of clusters of galaxies are very low, and the collisional timescales are very long. As a result, heavy elements will be under-ionized after they have passed through the accretion shock. We have studied systematically the effects of non-equilibrium ionization for relaxed clusters in the LambdaCDM cosmology using one-dimensional hydrodynamic simulations. We found that non-equilibrium ionization effects do not depend on cluster mass but depend strongly on redshift which can be understood by self-similar scaling arguments. The effects are stronger for clusters at lower redshifts. We present X-ray signatures such as surface brightness profiles and emission lines in detail for a massive cluster at low redshift. In general, soft emission (0.3-1.0 keV) is enhanced significantly by under-ionization, and the enhancement can be nearly an order of magnitude near the shock radius. The most prominent non-equilibrium ionization signature we found is the O VII and O VIII line ratio. The ratios for non-equilibrium ionization and collisional ionization equilibrium models are different by more than an order of magnitude at radii beyond half of the shock radius. These non-equilibrium ionization signatures are equally strong for models with different non-adiabatic shock electron heating efficiencies. We have also calculated the detectability of the O VII and O VIII lines with the future International X-ray Observatory (IXO). Depending on the line ratio measured, we conclude that an exposure of ~130-380 ksec on a moderate-redshift, massive regular cluster with the X-ray Microcalorimeter Spectrometer (XMS) on the IXO will be sufficient to provide a strong test for the non-equilibrium ionization model.

astro-ph.HE↗

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↗