arXiv subjects

Yangsen Yao

Publications and source records attributed to Yangsen Yao.

36 records · Page 2Linked to original sources

arXiv2007

Limits on Hot Galactic Halo Gas from X-ray Absorption Lines

Although the existence of large-scale hot gaseous halos around massive disk galaxies have been theorized for a long time, there is yet very little observational evidence. We report the Chandra and XMM-Newton grating spectral detection of OVII and NeIX Kalpha absorption lines along the sight-line of 4U 1957+11. The line absorption is consistent with the interstellar medium in origin. Attributing these line absorptions to the hot gas associated with the Galactic disk, we search for the gaseous halo around the Milky Way by comparing this sight-line with more distant ones (toward X-ray binary LMC X-3 and the AGN Mrk 421). We find that all the line absorptions along the LMC X-3 and Mrk 421 sight-lines are attributable to the hot gas in a thick Galactic disk, as traced by the absorption lines in the spectra of 4U~1957+11 after a Galactic latitude dependent correction. We constrain the OVII column density through the halo to be N(OVII) < 5E15 cm^{-2} (95% confidence limit), and conclude that the hot gas contribution to the metal line absorptions, if existing, is negligible.

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arXiv2007

Confirming the Detection of an Intergalactic X-ray Absorber Toward PKS 2155-304

We present new observations on PKS 2155-304 with the Chandra Low Energy Transmission Grating Spectrometer (LETG), using the Advanced CCD Imaging Spectrometer (ACIS). We confirm the detection of an absorption line plausibly identified as OVIII Ly-alpha from the warm-hot intergalactic medium associated with a small group of galaxies along the line of sight, as originally reported by Fang et al. 2002 (here after FANG02). Combining the previous observations in FANG02 and five new, long observations on the same target, we increase the total exposure time by a factor of three, and the total counts per resolution element by a factor of five. The measured line equivalent width is smaller than that observed in FANG02, but still consistent at 90% confidence. We also analyze the XMM-Newton observations on the same target, as well as observations using the Chandra LETG and the High Resolution Camera (HRC) combination. These observations have been used to challenge our reported detection. While no line is seen in either the XMM-Newton and the Chandra LETG+HRC data, we find that our result is consistent with the upper limits from both data sets. We attribute the non-detection to (1) higher quality of the Chandra LETG+ACIS spectrum, and (2) the rather extended wings of the line spread functions of both the XMM RGS and the Chandra LETG+HRC. We discuss the implication of our observation on the temperature and density of the absorber. We also confirm the detection of z ~ 0 OVII absorption and, comparing with previous Chandra analysis, we obtain much tighter constraints on the line properties.

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arXiv2007

The Galactic Central Diffuse X-ray Enhancement: A Differential Absorption/Emission Analysis

The soft X-ray background shows a general enhancement toward the inner region of the Galaxy. But whether this enhancement is a local feature (e.g., a superbubble within a distance of <= 200 pc) and/or a phenomenon related to energetic outflows from the Galactic center/bulge remains unclear. Here we report a comparative X-ray emission and absorption study of diffuse hot gas along the sight lines toward 3C 273 and Mrk 421, on and off the enhancement, but at similar Galactic latitudes. The diffuse 3/4-keV emission intensity, as estimated from the ROSAT All Sky Survey, is about three times higher toward 3C 273 than toward Mrk 421. Based on archival \chandra grating observations of these two AGNs, we detect X-ray absorption lines (e.g., OVII Kalpha, Kbeta, and OVIII Kalpha transitions at z~0) and find that the mean hot gas thermal and kinematic properties along the two sight lines are significantly different. By subtracting the foreground and background contribution, as determined along the Mrk 421 sight line, we isolate the net X-ray absorption and emission produced by the hot gas associated with the enhancement in the direction of 3C 273. From a joint analysis of these differential data sets, we obtain the temperature, dispersion velocity, and hydrogen column density as 2.0(1.6, 2.3)E6 K, 216(104,480) km/s, and 2.2(1.4, 4.1)E19 cm^{-2}, respectively (90% confidence intervals), assuming that the gas is approximately isothermal, solar in metal abundances, and equilibrium in collisional ionization. We also constrain the effective line-of-sight extent of the gas to be 3.4(1.0, 10.1) kpc, strongly suggesting that the enhancement most likely represents a Galactic central phenomenon.

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arXiv2007

GRO J1655-40: from ASCA and XMM-Newton Observations

We have analysed four ASCA observations (1994--1995, 1996--1997) and three XMM-Newton observations (2005) of this source, in all of which the source is in high/soft state. We modeled the continuum spectra with relativistic disk model kerrbb, estimated the spin of the central black hole, and constrained the spectral hardening factor f_col and the distance. If kerrbb model applies, for normally used value of f_col, the distance cannot be very small, and f_col changes with observations.

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arXiv2006

The Non-isothermality and Extent of Galactic Diffuse Hot Gas Toward Mrk~421

Diffuse hot gas can be traced effectively by its X-ray absorption and emission. We present a joint-analysis of these tracers to characterize the spatial and temperature distributions of the Galactic hot gas along the sight-line toward the nearby bright active galactic nucleus Mrk 421. We also complement this analysis with far-UV OVI absorption observations. We find that the observed absorption line strengths of OVII and OVIII are inconsistent with the diffuse background emission line ratio of the same ions, if the gas is assumed to be isothermal in a collisional ionization equilibrium state. But all these lines as well as the diffuse 3/4-keV broad-band background intensity in the field can be fitted with a plasma with a power law temperature distribution. We show that this distribution can be derived from a hot gaseous disk model with the gas temperature and density decreasing exponentially with the vertical distance from the Galactic plane. The joint fit gives the exponential scale heights as ~1.0 kpc and ~1.6 kpc and the middle plane values as 2.8E6 K and 2.4E-3 cm^{-3} for the temperature and density, respectively. These values are consistent with those inferred from X-ray observations of nearby edge-on galaxies similar to our own.

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arXiv2006

Chandra Detection of Fe XVII in Absorption: Iron Abundance in the Hot Gaseous Interstellar Medium

The iron depletion level and the gas-phase iron abundance in the hot ~10^6 K interstellar medium (ISM) is critical to the understanding of its energy balance as well as the thermal sputtering, cooling, and heating processes of dust grains. Here we report on the first detection of the Fe XVII absorption line at 15.02 A from the hot ISM in the spectrum of the low mass X-ray binary 4U~1820-303 observed with Chandra X-Ray Observatory. By jointly analyzing this absorption line with those from OVII, OVIII, and NeIX ions in the same spectrum, we obtain an abundance ratio as Fe/Ne=0.8(0.4, 2.1) in units of the Anders & Grevesse solar value (90% confidence intervals). We find that the result is robust with respect to different assumed gas temperature distributions. The obtained Fe/Ne abundance ratio, albeit with large uncertainties, is consistent with the solar value, indicating that there is very little or no iron depleted into dust grains, i.e., most of or all of the dust grains have been destroyed in the hot ISM.

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arXiv2005

X-ray Absorption Spectroscopy of the Multi-phase Interstellar Medium: Oxygen and Neon Abundances

X-ray absorption spectroscopy provides a potentially powerful tool in determining the metal abundances in various phases of the interstellar medium (ISM). We present a case study of the sight line toward 4U 1820-303 (Galactic coordinates l, b=2.79, -7.91 and distance = 7.6 kpc), based on Chandra Grating observations. The detection of OI, OII, OIII, OVII, OVIII, and NeIX Kalpha absorption lines allows us to measure the atomic column densities of the neutral, warm ionized, and hot phases of the ISM through much of the Galactic disk. By comparing these measurements with the 21 cm hydrogen emission and with the pulsar dispersion measure along the same sight line, we estimate the mean oxygen abundances in the neutral and total ionized phases as 0.3(0.2, 0.6) and 2.2(1.1, 3.5) in units of Anders & Grevesse (1989) solar value. This significant oxygen abundance difference is apparently a result of molecule/dust grain destruction and recent metal enrichment in the warm ionized and hot phases. We also measure the column density of neon from its absorption edge and obtain the Ne/O ratio of the neutral plus warm ionized gas as 2.1(1.3, 3.5) solar. Accounting for the expected oxygen contained in molecules and dust grains would reduce the Ne/O ratio by a factor of ~1.5. From a joint-analysis of the OVII, OVIII, and NeIX lines, we obtain the Ne/O abundance ratio of the hot phase as 1.4(0.9, 2.1) solar, which is not sensitive to the exact temperature distribution assumed in the absorption line modeling. These comparable ISM Ne/O ratios for the hot and cooler gas are thus considerably less than the value (2.85+-0.07; 1sigma) recently inferred from corona emission of solar-like stars (Drake & Testa 2005). (abridged)

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arXiv2005

Black hole X-ray binaries LMC X-1 and X-3: observations confront spectral models

We present a comprehensive spectral analysis of black hole X-ray binaries, LMC X-1 and LMC X-3, based on BeppoSAX observations. We test both the multi-color disk plus power law (MCD+PL) model and a newly-developed Monte-Carlo simulation- based model for a Comptonized MCD (CMCD) with either a spherical or a slab-like corona, by comparing the inferred parameters with independent direct measurements. While all models give an adequate description of the spectra, we find a significant discrepancy between the MCD+PL inferred X-ray-absorbing gas column density and the absorption-edge measurement based on dispersed X-ray spectra. The MCD+PL fits to the LMC X-1 spectra also require a change in the inner disk radius during the BeppoSAX observation, which may be due to the nonphysical effects inherited in the model. In contrast, the CMCD model with the spheric corona gives the predictions of both the disk inclination angle and the absorption that are consistent with the direct measurements, and only slightly under-predicts the black hole mass of LMC X-3. The model explains the spectral state evolution of LMC X-1 within the BeppoSAX observation as a change in the accretion rate, which leads to an increase in both the inner disk temperature and the Comptonization opacity. On the other hand, the CMCD model with the slab-like corona is more problematic in the test and is thus not recommended.

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arXiv2005

Probing the Local Group Medium Toward Mkn 421 with Chandra and FUSE

We report the detection of highly-ionized gas at z~0 seen in resonant UV and X-ray absorption lines toward the z=0.03 blazar Mkn 421. A total of 13 X-ray and 3 UV lines were measured (or upper limits derived), including three lines in the OVII K-series and Kαtransitions from neon, carbon, and nitrogen. From the three OVII lines we derive a 2σDoppler parameter constraint of 24<b<55 km/s. The FUSE spectrum shows strong Galactic low--velocity OVI 1032A absorption as well as a possible weak OVI high-velocity component (HVC). The Doppler parameter of the low-velocity OVI measured with FUSE is ~3σhigher than that derived from the OVII line ratios, indicating that the OVII and Galactic OVI arise in different phases. This velocity dispersion, along with limits on the gas temperature and density from the X-ray line ratios (assuming a single phase with collisional ionization equilibrium plus photoionization) are all consistent with an extragalactic absorber. However, the OVII Doppler parameter is inconsistent with the high temperature required to produce the observed OVI_HVC/OVII ratio, implying that the HVC is probably not related to the OVII. In addition, the OVI_Kαline detected by Chandra implies a column density ~4 times higher than the 1032A absorption. Although an extragalactic absorber is fully consistent with the measured column density ratios, a Galactic origin cannot be ruled out given the uncertainties in the available data.

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arXiv2005

Probing the Spatial Distribution of the Interstellar Dust Medium by High Angular Resolution X-ray Halos of Point Sources

We studied the X-ray dust scattering halos around 17 bright X-ray point sources using {\it Chandra} data. We use an iterative method to resolve the halos at small scattering angles from the zeroth order data in CC-mode or the first order data in TE-mode which is not or less piled-up. Using the halo models of Weingartner $&$ Draine (2001, WD01) and Mathis, Rumpl $&$ Nordsieck (1977, MRN) to fit the halo profiles, we get the hydrogen column densities and the spatial distributions of the scattering dust grains along the line of sights (LOS) to these sources. We find that the scattering dust density very close to these sources is much higher than the normal interstellar medium. For X-ray pulsars GX 301-2 and Vela X-1 with companions of strong stellar winds, the X-ray absorption column densities are much higher than the derived scattering column densities, because of the dense media around the X-ray sources produce extremely small angle scatterings which are indistinguishable from the point sources even with \textit{Chandra}'s angular resolution. For LMC X-1, most of the scattering and absorption occur in Large Magellanic Cloud, rather than in the Milky Way. From the obtained X-rays spectra, the cold gas absorption and thus the quivalent hydrogen column is determined. We have obtained the linear correlation between $N_{H}$ derived from spectral fits and the one derived from the halo model WD01 and MRN (except for GX 301-2 and Vela X-1): $N_{H,WD01} = (0.720\pm0.009) \times N_{H,abs} + (0.051\pm0.013)$ and $N_{N, MRN} = (1.156\pm0.016) \times N_{H,abs} + (0.062\pm0.024)$ in the units $10^{22}$ cm$^{-2}$. High angular resolution X-ray dust scattering halos offer an excellent potential for probing the spatial distributions of interstellar dust medium.

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arXiv2005

Study the X-ray Dust Scattering Halos with Chandra Observations of Cygnus X-3 and Cygnus X-1

We improve the method proposed by Yao \emph{et al} (2003) to resolve the X-ray dust scattering halos of point sources. Using this method we re-analyze the Cygnus X-1 data observed with {\it Chandra} (ObsID 1511) and derive the halo radial profile in different energy bands and the fractional halo intensity (FHI) as $I(E)=0.402\times E_{\rm keV}^{-2}$. We also apply the method to the Cygnus X-3 data ({\it Chandra} ObsID 425) and derive the halo radial profile from the first order data with the {\it Chandra} ACIS+HETG. It is found that the halo radial profile could be fit by the halo model MRN (Mathis, Rumpl $&$ Nordsieck, 1977) and WD01 (Weingartner $&$ Draine, 2001); the dust clouds should be located at between 1/2 to 1 of the distance to Cygnus X-1 and between 1/6 to 3/4 (from MRN model) or 1/6 to 2/3 (from WD01 model) of the distance to Cygnus X-3, respectively.

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arXiv2005

X-ray Absorption Line Spectroscopy of the Galactic Hot Interstellar Medium

We present an X-ray absorption line spectroscopic study of the large-scale hot interstellar medium (HISM) in the Galaxy. We detect Ne IX K$_α$ absorption lines in the {\sl Chandra} grating spectra of seven Galactic low-mass X-ray binaries. Three of these sources also show absorption of O VII K$_α$, O VII K$_β$, and/or O VIII K$_α$. Both centroid and width of the lines are consistent with a Galactic HISM origin of the absorption. By jointly fitting the multiple lines, accounting for line saturation and assuming the collisional ionization equilibrium, we estimate the average absorbing gas temperature as $\sim 2.4\pm0.3\times10^6$ K (90% confidence errors). We further characterize the spatial density distribution of the gas as ${\rm 6.4_{-1.7}^{+2.4} exp(-|z|/1.2_{-0.5}^{+1.0} kpc) \times10^{-3}{\rm cm^{-3}}} $ (a disk morphology) or ${\rm 6.2_{-2.1}^{+3.8} [ 1 + (R/2.3_{-1.1}^{+1.6} kpc)^2]^{-1} \times10^{-3} {\rm cm^{-3}}}$ (a sphere morphology), where $z$ and $R$ are the distances from the Galactic plane and Galactic center (GC) respectively. Since nearly all the sight-lines with significant absorption lines detected are somewhat toward GC and at low Galactic latitudes, these results could be severely biased. More observations toward off-GC sight-lines and at high latitudes are urgently needed to further the study. Nevertheless, the results demonstrate the excellent potential of X-ray absorption line spectroscopy in the study of the HISM.

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arXiv2004

Studying the Properties of Accretion Disks and Coronae in Black Hole X-ray Binaries with Monte-Carlo Simulations

Understanding the properties of the hot corona is important for studying the accretion disks in black hole X-ray binary systems. Using the Monte-Carlo technique to simulate the inverse Compton scattering between photons emitted from the cold disk and electrons in the hot corona, we have produced two table models in the $XSPEC$ format for the spherical corona case and the disk-like (slab) corona case. All parameters in our table models are physical properties of the system and can be derived from data fitting directly. Applying the models to broad-band spectra of the black hole candidate XTE J2012+381 observed with BeppoSAX, we estimated the size of the corona and the inner radius of the disk. The size of the corona in this system is several tens of gravitational radius, and the substantial increase of the inner disk radius during the transit from hard-state to soft-state is not found.

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arXiv2004

XMM-Newton Spectra of Intermediate-Mass Black Hole Candidates: Application of a Monte-Carlo Simulated Model

We present a systematic spectral analysis of six ultraluminous X-ray sources (NGC1313 X-1/X-2, IC342 X-1, HoIX X-1, NGC5408 X-1 and NGC3628 X-1) observed with XMM-Newton Observatory. These extra-nuclear X-ray sources in nearby late-type galaxies have been considered as intermediate-mass black hole candidates. We have performed Monte-Carlo simulations of Comptonized multi-color black-body accretion disks. This unified and self-consistent spectral model assumes a spherically symmetric, thermal corona around each disk and accounts for the radiation transfer in the Comptonization. We find that the model provides satisfactory fits to the XMM-Newton spectra of thesources. The characteristic temperatures of the accretion disks (T_in), for example, are in the range of ~ 0.05-0.3 keV, consistent with the intermediate-mass black hole interpretation. We find that the black hole mass is typically about a few times 10^3 M_\odot and has an accretion rate ~ 10^{-6} - 10^{-5} M_\odot yr^{-1}. For the spectra considered here, we find that the commonly used multi-color black-body accretion disk model with an additive power law component, though not physical, provides a good mathematical approximation to the Monte-Carlo simulated model. However, the latter model provides additional constraints on the properties of the accretion systems, such as the disk inclination angles and corona optical depths.

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arXiv2002

Relativistic Effects on the Appearance of a Clothed Black Hole

For an accretion disk around a black hole, the strong relativistic effects affect every aspect of the radiation from the disk, including its spectrum, light-curve, and image. This work investigates in detail how the images of a thin disk around a black hole will be distorted, and what the observer will see from different viewing angles and in different energy bands.

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arXiv2001

Studying the Accretion Disks in Black Hole X-ray Binaries with Monte-Carlo Simulations

Understanding the properties of the hot corona is very important for studying the accretion disks in black hole X-ray binary systems. Using the Monte-Carlo technique to simulate the inverse Compton scattering process between disk photons and electrons in the hot corona, we have produced two table models in the XSPEC package. Applying the models to the broad-band BeppoSAX observations of the black hole candidate XTE J2012+381, we demonstrate the power of this table model. Our results indicate that the electron distribution in the corona has a powerlaw shape with a spectral index around 4 and the size of the corona is just several tens of gravitational radius and the inclination angle of the disk is around 60 degrees.

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arXiv2001

Relativistic Effects on X-ray Emissions from Accretion Disks around Black Holes

Special and general relativistic effects on the X-ray emissions, especially the continuum spectra, from the accretion disks around black holes are investigated using the ray-tracing method. Because both the special and general relativistic effects are more important at distances closer to the black hole, the relativistic modifications to the emitted X-ray spectra are more significant at higher energies. In a simple accretion disk precession model, the relativistic effects can account for the energy dependence of the QPO amplitude and phase-lags observed in several black hole binaries. The dramatic QPO phase-lag transitions only show up when the central black hole is spinning rapidly. The narrow distribution of the observed system inclination angles of black hole binaries may be due to the selection effect caused by the relativistic effects around black holes.

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arXiv2000

Three-Layered Atmospheric Structure in Accretion Disks Around Stellar-Mass Black Holes

Modeling of the x-ray spectra of the Galactic superluminal jet sources GRS 1915+105 and GRO J1655-40 reveal a three-layered atmospheric structure in the inner region of their accretion disks. Above the cold and optically thick disk of a temperature 0.2-0.5 keV, there is a warm layer with a temperature of 1.0-1.5 keV and an optical depth around 10. Sometimes there is also a much hotter, optically thin corona above the warm layer, with a temperature of 100 keV or higher and an optical depth around unity. The structural similarity between the accretion disks and the solar atmosphere suggest that similar physical processes may be operating in these different systems.

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