arXiv subjects

Q. D. Wang

Publications and source records attributed to Q. D. Wang.

At least 55 records · Page 3Linked to original sources

arXiv2009

X-raying the Intergalactic OVI Absorbers

The observed intergalactic OVI absorbers at z>0 have been regarded as a significant reservoir of the ``missing baryons''. However, to fully understand how these absorbers contribute to the baryon inventory, it is crucial to determine whether the systems are collisionally ionized or photoionized (or both). Using the identified intergalactic OVI absorbers as tracers, we search for the corresponding X-ray absorption lines, which are useful for finding the missing baryons and for revealing the nature of the OVI absorbers. Stacking the Chandra grating spectra along six AGN sight lines, we obtain three spectra with signal-to-noise ratios of 32, 28, and 10 per 12.5 mA spectral bin around the expected OVII Kalpha wavelength. These spectra correspond to OVI absorbers with various dynamic properties. We find no detectable NeIX, OVII, OVIII, NVII, or CVI absorption lines in the spectra, but the high counting statistics allows us to obtain firm upper limits on the corresponding ionic column densities (in particular N(OVII)<=10 N(OVI) on average at the 95% confidence level). Jointly analyzing these non-detected X-ray lines with the averaged OVI column density, we further limit the average temperature of the OVI-bearing gas to be log[T(K)]<=5.7 in collisional ionization equilibrium. We discuss the implications of these results for physical properties of the putative warm-hot intergalactic medium and its detection in future X-ray observations.

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arXiv2008

A Catalog of X-ray Point Sources from Two Megaseconds of Chandra Observations of the Galactic Center

We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2 by 0.8 degree field around the Galactic center. We increase the number of known X-ray sources in the region by a factor of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Msec of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities >4e32 erg/s (0.5-8.0 keV; 90% confidence) over an area of one square degree, and up to an order of magnitude more sensitive in the deepest exposure (1.0 Msec) around Sgr A*. The positions of 60% of our sources are accurate to <1" (95% confidence), and 20% have positions accurate to <0.5". We search for variable sources, and find that 3% exhibit flux variations within an observation, 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7-285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically-accreting cataclysmic variable. We compare the spatial distribution of X-ray sources to a model for the stellar distribution, and find 2.8 sigma evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l~0.1 degree.

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arXiv2008

X-ray and UV spectroscopy of Galactic diffuse hot gas along the LMC X--3 sight line

We present Suzaku spectra of X-ray emission in the fields just off the LMC X-3 sight line. OVII, OVIII, and NeIX emission lines are clearly detected, suggesting the presence of an optically thin thermal plasma with an average temperature of 2.4E6. This temperature is significantly higher than that inferred from existing X-ray absorption line data obtained with Chandra grating observations of LMC X-3, strongly suggesting that the gas is not isothermal. We then jointly analyze these data to characterize the spatial and temperature distributions of the gas. Assuming a vertical exponential Galactic disk model, we estimate the gas temperature and density at the Galactic plane and their scale heights as 3.6(2.9, 4.7)E6 K and 1.4(0.3, 3.4)E-3 cm^{-3} and 1.4(0.2, 5.2) kpc and $2.8(1.0, 6.4)$ kpc, respectively. This characterization can account for all the \ovi line absorption, as observed in a FUSE spectrum of LMC X-3, but only predicts less than one tenth of the OVI line emission intensity typically detected at high Galactic latitudes. The bulk of the OVI emission most likely arises at interfaces between cool and hot gases.

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arXiv2008

Chandra Observation of the Edge-on Spiral NGC 5775: Probing the Hot Galactic Disk/Halo Connection

We study the edge-on galaxy NGC 5775, utilizing a 58.2 ks {\sl Chandra} ACIS-S observation together with complementary {\sl HST} ACS, {\sl Spitzer} IRAC and other multi-wavelength data sets. This edge-on galaxy, with its disk-wide active star formation, is particularly well-suited for studying the disk/halo interaction on sub-galactic scales. We detect 27 discrete X-ray sources within the $D_{25}$ region of the galaxy, including an ultra-luminous source with a 0.3-7 keV luminosity of $\sim7\times10^{40}\rm ergs s^{-1}$. The source-removed diffuse X-ray emission shows several prominent extraplanar features, including a $\sim10\rm kpc$ diameter ``shell-like'' feature and a ``blob'' reaching a projected distance of $\sim25\rm kpc$ from the galactic disk. The bulk of the X-ray emission in the halo has a scale height of $\sim$1.5 kpc and can be characterized by a two-temperature optically thin thermal plasma with temperatures of $\sim$ 0.2 and 0.6 keV and a total 0.3-2 keV luminosity of $\sim3.5\times10^{39}\rm ergs s^{-1}$. The high-resolution, multi-wavelength data reveal the presence of several extraplanar features around the disk, which appear to be associated with the in-disk star formation. We suggest that hot gas produced with different levels of mass loading can have different temperatures, which may explain the characteristic temperatures of hot gas in the halo. We have obtained a sub-galactic scale X-ray-intensity-star formation relation, which is consistent with the integrated version in other star forming galaxies.

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arXiv2008

Distance Determination to the Crab-like PWN G54.1+0.3 and Search for its Supernova Remnant Shell

We discover a large-scale shell G53.9+0.2 around the Crab-like pulsar wind nebula (PWN) G54.1+0.3 with 1420 MHz continuum VLA observations. This is confirmed by a new infrared image at 8 μm from the GLIMPSE Legacy Project, which reveals an intriguing infrared shell just surrounding the large radio shell. We analyze the 21 cm HI absorption spectra and 13CO emission spectra towards PWN G54.1+0.3 and bright sources on both radio and IR shells. Continuous HI absorption up to the tangent point and absence of negative HI absorption features imply that PWN G54.1+0.3 has a distance beyond the tangent point but within the Solar circle, i.e. 4.5 to 9 kpc. G54.1+0.3 is likely at distance of \simeq 6.2 kpc due to the morphological association between the PWN and a CO molecular cloud at velocity of \simeq 53 km/s, as revealed by high-resolution 13CO images. Based on the HI absorption spectrum and recombination line velocity (\simeq 40 km/s) of the bright HII region G54.09-0.06, which is on the IR shell, the IR shell is likely located at a distance of \simeq 7.3 kpc, which is also the distance of the associated large-scale radio shell. At this distance, the radio shell has a radius of ~ 30 pc. The radio shell may be thermal and lack IR emission due to dust destruction, or it may be nonthermal and part of a newly found old SNR. In either case it is located at a different distance than PWN G54.1+0.3.

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arXiv2007

Chandra detection of diffuse hot gas in and around the M31 bulge

We report the detection of diffuse hot gas in M31, using archival Chandra observations which allow us to map out a 30' by 30' field (covering a galactocentric radius up to 4.5 kpc) and to detect sources in the galaxy down to a 0.5-8 keV luminosity limit of ~10^35 ergs/s. We estimate the remaining stellar contribution from fainter X-ray sources (primarily cataclysmic variables and coronally active binaries), assuming that they spatially follow the stellar distribution. Indeed, the near-IR K-band light of the galaxy closely traces the 2-8 keV unresolved X-rays, indicating a collective stellar X-ray emissivity consistent with those determined for the Galactic ridge and M32, whereas the amount of the 0.5-2 keV unresolved emission is significantly greater than the expected stellar contribution, especially within a galactocentric radius of ~2 kpc. Morphologically, this soft X-ray excess appears substantially rounder than the bulge as seen in K-band and is elongated approximately along the minor-axis at large radii. The excess thus most likely represents the emission of diffuse hot gas in and around the galactic bulge. Furthermore, the near side of the M31 disk casts an apparent shadow against the soft X-ray excess, indicating that the hot gas extends to at least 2.5 kpc from the galactic plane. We briefly discuss the implications of these results on the energy balance in the M31 bulge and on understanding the large-scale soft X-ray enhancement observed toward the inner region of our own Galaxy.

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arXiv2007

Radio and X-ray images of SNR G18.8+0.3 interacting with molecular clouds

New HI images from the VLA Galactic Plane Survey (VGPS) show clear absorption features associated with the supernova remnant (SNR) G18.8+0.3. High-resolution $^{13}$CO images reveal that molecular clouds overlap the radio filaments of G18.8+0.3. The $^{13}$CO emission spectrum over the full velocity range in the direction of G18.8+0.3 shows two molecular components with high brightness-temperatureand three molecular components with low brightness-temperature, all with respective HI absorption. This implies that these clouds are in front of G18.8+0.3. In the HI images, the highest velocity absorption feature seen against the continuum image of G18.8+0.3 is at 129 km s$^{-1}$, which corresponds to the tangent point in this direction. This yields a lower distance limit of 6.9 kpc for G18.8+0.3. Absence of absorption at negative velocities gives an upper distance limit of 15 kpc. The broadened profile at 20$\pm$5 km s$^{-1}$ in the $^{13}$CO emission spectra is a strong indicator of a possible SNR/CO cloud interaction. Thus, G18.8+0.3 is likely to be at the distance of about 12 kpc. The upper mass limit and mean density of the giant CO cloud at 20$\pm$5 km s$^{-1}$ are \sim 6 \times 10^{5} M_{\odot} and \sim 2 \times 10^{2} cm^{-3}. We find an atomic hydrogen column density in front of G18.8+0.3 of $N_{HI}$ \sim 2 \times 10^{22} cm^{-2}. The ROSAT PSPC observations show a diffuse X-ray enhancement apparently associated with part of the radio shell of G18.8+0.3. Assuming an association, the unabsorbed flux is 2.2 \times10^{-12} erg cm^{-2} s^{-1}, suggesting an intrinsic luminosity of 3.6 \times 10^{34} erg s^{-1} for G18.8+0.3.

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arXiv2007

VLA and XMM-Newton observations of the SNR W41/TeV Gamma-ray source HESS J1834-087

The recently discovered extended TeV source HESS J1834-087 is associated with both a diffuse X-ray enhancement and a molecular cloud, projected at the center of an old radio supernova remnant G23.3-0.3 (SNR W41). New HI observations from the VLA Galactic Plane Survey (VGPS) show unambiguous structures associated with W41 in the radial velocity range of 53 to 63 km/s, so we obtain for W41 a distance of 4+-0.2 kpc. A new higher sensitivity VGPS continuum image of W41 at 1420 MHz shows faint emission in its eastern part not detected by previous observations, so we give a new angular size of 36'x30' in b-l direction (average radius of 19 pc). We estimate for W41 a Sedov age of ~10e5 yr. New XMM-Newton observation reveals diffuse X-ray emission within the HESS source and suggests an association between the X-ray and Gamma-ray emission. The high-resolution ^{13}CO images of W41 further reveal a giant molecular cloud (GMC) located at the center of W41, likely associated with W41 in the the radial velocity range of 61 to 66 km/s. Altogether, the new observations can be interpreted as providing the first evidence that an old SNR encounters a GMC to emit TeV gamma-rays in the cloud material.

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arXiv2006

A Chandra Catalog of X-ray Sources in the Central 150 pc of the Galaxy

We present the catalog of X-ray sources detected in a shallow Chandra survey of the inner 2 by 0.8 degrees of the Galaxy, and in two deeper observations of the Radio Arches and Sgr B2. The catalog contains 1352 objects that are highly-absorbed (N_H > 4e22 cm^-2 and are therefore likely to lie near the Galactic center (D~8 kpc), and 549 less-absorbed sources that lie within <6 kc of Earth. Based on the inferred luminosities of the X-ray sources and the expected numbers of various classes of objects, we suggest that the sources with L_X < 1e33 erg/s that comprise ~90% of the catalog are cataclysmic variables, and that the ~100 brighter objects are accreting neutron stars and black holes, young isolated pulsars, and Wolf-Rayet and O stars in colliding-wind binaries. We find that the spatial distribution of X-ray sources matches that of the old stellar population observed in the infrared, which supports our suggestion that most of the X-ray sources are old cataclysmic variables. However, we find that there is an apparent excess of ~10 bright sources in the Radio Arches region. That region is already known to be the site of recent star formation, so we suggest that the bright sources in this region are young high-mass X-ray binaries, pulsars, or WR/O star binaries. We briefly discuss some astrophysical questions that this catalog can be used to address.

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arXiv2005

G359.95-0.04: Pulsar Candidate Near Sgr A*

We report the discovery of a prominent nonthermal X-ray feature located near the Galactic center that we identify as an energetic pulsar wind nebula. This feature, G359.95-0.04, lies 1 lyr north of Sgr A* (in projection), is comet-like in shape, and has a power law spectrum that steepens with increasing distance from the putative pulsar. The distinct spectral and spatial X-ray characteristics of the feature are similar to those belonging the rare class of ram-pressure confined pulsar wind nebulae. The luminosity of the nebula at the distance of \sgra, consistent with the inferred X-ray absorptions, is 1 10^{34} ergs s^{-1} in the 2--10 keV energy band. The cometary tail extends back to a region centered at the massive stellar complex IRS 13 and surrounded by enhanced diffuse X-ray emission, which may represent an associated supernova remnant. Furthermore, the inverse Compton scattering of the strong ambient radiation by the nebula consistently explains the observed TeV emission from the Galactic center. We also briefly discuss plausible connections of G359.95-0.04 to other high-energy sources in the region, such as the young stellar complex IRS 13 and SNR Sgr A East.

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arXiv2005

A Deep Radio Survey of Abell 2125 III: The Cluster Core - Merging and Stripping

We use radio, near-IR, optical, and X-ray observations to examine dynamic processes in the central region of Abell 2125. In addition to the central triple, including members of both major dynamical subsystems identified from a redshift survey, this region features a galaxy showing strong evidence for ongoing gas stripping during a high-velocity passage through the gas in the cluster core. The disk galaxy C153 exhibits a plume stretching toward the cluster center seen in soft X-rays by Chandra, parts of which are also seen in [O II] emission and near-UV continuum light. HST imaging shows a distorted disk, with star-forming knots asymmetrically distributed and remnant spiral structure possibly defined by dust lanes. The stars and ionized gas in its disk are kinematically decoupled, demonstrating that pressure stripping must be important, and that tidal disruption is not the only mechanism at work. Comparison of the gas properties seen in the X-ray and optical data on the plume highlight significant features of the history of stripped gas in the intracluster medium. The nucleus of C153 also hosts an AGN, shown by the weak and distorted extended radio emission and a radio compact core. The unusual strength of the stripping signatures in this instance is likely related to the high relative velocity of the galaxy with respect to the intracluster medium, during a cluster/cluster merger, and its passage very near the core of the cluster. Another sign of recent dynamical events is diffuse starlight asymmetrically placed about the central triple in a cD envelope. Transient and extreme dynamical events as seen in Abell 2125 may be important drivers of galaxy evolution in the cores of rich clusters.

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arXiv2005

The XMM-Newton/Chandra monitoring campaign of the Galactic center region: Description of the program and preliminary results

We present the first results of our X-ray monitoring campaign on a 1.7 square degree region centered on Sgr A* using the satellites XMM-Newton and Chandra. The purpose of this campaign is to monitor the behavior (below 10 keV) of X-ray sources (both persistent and transient) which are too faint to be detected by monitoring instruments aboard other satellites currently in orbit (e.g., Rossi X-ray Timing Explorer; INTEGRAL). Our first monitoring observations (using the HRC-I aboard Chandra) were obtained on June 5, 2005. Most of the sources detected could be identified with foreground sources, such as X-ray active stars. In addition we detected two persistent X-ray binaries (1E 1743.1-2843; 1A 1742-294), two faint X-ray transients (GRS 1741.9-2853; XMM J174457-2850.3), as well as a possible new transient source at a luminosity of a few times 1E34 erg/s. We report on the X-ray results on these systems and on the non-detection of the transients in follow-up radio data using the Very Large Array. We discuss how our monitoring campaign can help to improve our understanding of the different types of X-ray transients (i.e., the very faint ones).

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arXiv2005

An Infrared Imaging Survey of the Faint Chandra Sources near the Galactic Centre

We present near-IR imaging of a sample of the faint, hard X-ray sources discovered in the 2001 Chandra ACIS-I survey towards the Galactic Centre (GC) (Wang et al. 2002). These ~800 discrete sources represent an important and previously undetected population within the Galaxy. From our VLT observations of 77 X-ray sources, we identify candidate K-band counterparts to 75% of the Chandra sources in our sample. The near-IR magnitudes and colours of the majority of candidate counterparts are consistent with highly reddened stars, indicating that most of the Chandra sources are likely to be accreting binaries at or near the GC.

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arXiv2005

Warm-Hot Gas in and around the Milky Way: Detection and Implications of OVII Absorption toward LMC X-3

X-ray absorption lines of highly-ionized species such as OVII at about zero redshift have been firmly detected in the spectra of several active galactic nuclei. However, the location of the absorbing gas remains a subject of debate. To separate the Galactic and extragalactic contributions to the absorption, we have obtained Chandra LETG-HRC and FUSE observations of the black hole X-ray binary LMC X--3. A joint analysis of the detected OVII and Ne IX Kalpha lines, together with the non-detection of the OVII Kbeta and OVIII Kalpha lines, gives the measurements of the temperature, velocity dispersion, and hot oxygen column density. The X-ray data also allow us to place a 95% confidence lower limit to the Ne/O ratio as 0.14. The OVII line centroid and its relative shift from the Galactic OI Kalpha absorption line, detected in the same observations, are inconsistent with the systemic velocity of LMC X--3 ($+310 {\rm km s^{-1}}$). The far-UV spectrum shows OVI absorption at Galactic velocities, but no OVI absorption is detected at the LMC velocity at $> 3σ$ significance. Both the nonthermal broadening and the decreasing scale height with the increasing ionization state further suggest an origin of the highly-ionized gas in a supernova-driven galactic fountain. In addition, we estimate the warm and hot electron column densities from our detected OVII Kalpha line in the LMC X--3 X-ray spectra and from the dispersion measure of a pulsar in the LMC vicinity. We then infer the O/H ratio of the gas to be $\gtrsim 8 \times 10^{-5}$, consistent with the chemically-enriched galactic fountain scenario. We conclude that the Galactic hot interstellar medium should in general substantially contribute to zero-redshift X-ray absorption lines in extragalactic sources.

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arXiv2005

Exploring the Nature of Weak Chandra Sources near the Galactic Centre

We present results from the first near-IR imaging of the weak X-ray sources discovered in the Chandra/ACIS-I survey (Wang et al. 2002) towards the Galactic Centre (GC). These ~800 discrete sources, which contribute significantly to the GC X-ray emission, represent an important and previously unknown population within the Galaxy. From our VLT observations we will identify likely IR counterparts to a sample of the hardest sources, which are most likely X-ray binaries. With these data we can place constraints on the nature of the discrete weak X-ray source population of the GC.

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arXiv2004

A Deep Radio Survey of Abell 2125 II: Accelerated Galaxy Evolution during a Cluster-Cluster Merger

Using our extensive radio, optical, near-IR and X-ray imaging and spectroscopy, we consider the reason for the unusually large number of radio detected galaxies, mostly found outside the cluster core, in Abell 2125 (z=0.2465, richness class 4). With 20-cm VLA data, we detect continuum emission from 90 cluster members. The multiwavelength properties of these galaxies suggest that most of the radio emission is due to an enhanced star-formation rate. The dynamical study of Miller et al (2004) suggests that Abell 2125 is undergoing a major cluster-cluster merger, with our view within 30 degrees of the merger axis and within 0.2 Gyr of core passage. The combination of projection effects and the physical processes at work during this special time in the cluster's evolution seem likely to be responsible for the unusual level of activity we see in the cluster. We argue that tidal effects on individual cluster members, often far from the cluster core, are responsible for the increased star formation. Our results are consistent with the idea that disk galaxies during this phase of a cluster's evolution undergo rapid evolution, through a burst of star formation, on their way to becoming S0's.

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arXiv2003

Exploring the Nature of Weak Chandra Sources near the Galactic Centre

We present early results from the first IR imaging of the weak X-ray sources discovered in a recent Chandra survey towards the Galactic Centre. From our VLT observations we will identify likely counterparts to a sample of the hardest sources in order to place constraints on the nature of this previously unknown population.

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arXiv2003

The Chandra Detection of Galactic Center X-ray Features G359.89-0.08 and G359.54+0.18

We report on the detection of two elongated X-ray features G359.89-0.08 and G359.54+0.18 in the Galactic center (GC) region using the Chandra X-ray Observatory. G359.89-0.08 is an elongated X-ray feature located $\sim$2\arcmin in projection south of the center of the Galaxy, SgrA$^*$. This X-ray feature source is partially coincident with a slightly curved (``wisp''-like) non-thermal radio source. The X-ray spectrum of G359.89-0.08 can be best characterized as non-thermal, with a photon index of 2. The morphological and spectral characteristics of the X-ray and radio emission associated with G359.89-0.08 are best interpreted as the synchrotron emission from a ram-pressure confined pulsar wind nebula. G359.54+0.18 is one of the most prominent radio non-thermal filaments (NTFs) in the GC region, located $\sim$30\arcmin in projection from SgrA$^*$. A narrow ($\sim$10\arcsec) filament of X-ray emission appears to arise from one of the two strands that comprise the radio NTF. Although the photon statistics are poor for this source, the X-ray emission is also likely to be non-thermal in nature. Several models for the production of X-ray emission in G359.54+0.18 are discussed.

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