arXiv subjects

Junfeng Wang

Publications and source records attributed to Junfeng Wang.

At least 235 records · Page 13Linked to original sources

arXiv2011

A Deep Chandra ACIS Study of NGC 4151. I. the X-ray Morphology of the 3 kpc-diameter Circum-nuclear Region and Relation to the Cold Interstellar Medium

We report on the imaging analysis of 200 ks sub-arcsecond resolution Chandra ACIS-S observations of the nearby Seyfert 1 galaxy NGC 4151. Bright, structured soft X-ray emission is observed to extend from 30 pc to 1.3 kpc in the south-west from the nucleus, much farther than seen in earlier X-ray studies. The terminus of the north-eastern X-ray emission is spatially coincident with a CO gas lane, where the outflow likely encounters dense gas in the host galactic disk. X-ray emission is also detected outside the boundaries of the ionization cone, which indicates that the gas there is not completely shielded from the nuclear continuum, as would be the case for a molecular torus collimating the bicone. In the central r<200 pc region, the subpixel processing of the ACIS data recovers the morphological details on scales of <30~pc (<0.5") first discovered in Chandra HRC images. The X-ray emission is more absorbed towards the boundaries of the ionization cone, as well as perpendicular to the bicone along the direction of a putative torus in NGC 4151. The innermost region where X-ray emission shows the highest hardness ratio, is spatially coincident with the near-infrared resolved H_2 emission and dusty spirals we find in an HST V-H color image. The agreement between the observed H_2 line flux and the value predicted from X-ray-irradiated molecular cloud models supports photo-excitation by X-rays from the active nucleus as the origin of the H_2 line, although contribution from UV fluorescence or collisional excitation cannot be fully ruled out with current data. The discrepancy between the mass of cold molecular gas inferred from recent CO and near-infrared H_2 observations may be explained by the anomalous CO abundance in this X-ray dominated region. The total H_2 mass derived from the X-ray observation agrees with measurement in Storchi-Bergmann et al.

astro-ph.CO↗

arXiv2010

Magnetization and Magnetoresistance of CeRu2Al10 under High Magnetic Fields along c-Axis

We have studied the magnetization and magnetoresistance of CeRu2Al10 in the applied magnetic field H along the c-axis up to ~ 55 T. The magnetization M at low temperatures shows an H-linear increase with a small slope of M/H than that for H // a-axis up to ~ 55 T after showing a small anomaly at H ~ 4 T, which indicates that the critical field to the paramagnetic phase H_c^p is higher than 55 T for H // c-axis. The magnetization curves for H // a- and c-axes below the antiferro magnetic (AFM) transition temperature T0 behave as if the magnetic anisotropy in the AFM-ordered phase is small, although there exists a large magnetic anisotropy in the paramagnetic phase, which favors the easy magnetization axis along the a-axis. On the other hand, very recently, Khalyavin et al. have reported that the AFM order where the magnetic moment is parallel to the c-axis takes place below T0. These results indicate that the AFM order in this compound is not a simple one. The longitudinal magnetoresistance for H // c-axis at low temperatures shows no anomaly originating from the phase transition, but shows oscillations below 4.2 K. This oscillatory behavior below 4.2 K originates from the Shubnikov-de Haas oscillations, from which the cross section of the Fermi surface normal to the c-axis is estimated to be 1.0*10^14 cm-2, with no large effective mass. This is the first direct evidence of the existence of the Fermi surface below T0.

cond-mat.str-el↗

arXiv2010

High Dust Depletion in Two Intervening Quasar Absorption Line Systems with the 2175-Å Extinction Bump at $z\sim1.4$

We present the column densities of heavy-elements and dust depletion studies in twostrong Mg~II absorption systems at $z\sim1.4$ displaying the 2175-Å dust extinction feature. Column densities are measured from low-ionization absorption lines using Apparent Optical Depth Method on the Keck/ESI spectra. We find the dust depletion patterns resemble to that of cold diffuse clouds in the Milky Way (MW). The values, [Fe/Zn]$\approx -1.5$ and [Si/Zn]$<-0.67$, are among the highest dust depletion measured for quasar absorption line systems. In another 2175-Å absorber at $z$=1.64 toward the quasar SDSS J160457.50+220300.5, Noterdaeme et al. (2009) reported a similar dust depletion measurement ([Fe/Zn]=$-1.47$ and [Si/Zn]=$-1.07$) and detected C~I and CO absorption lineson its VLT/UVES spectrum. We conclude that heavy dust depletion (i.e. a characteristic of cold dense clouds in MW) is required to produce a pronounced 2175-Å extinction bump.

astro-ph.CO↗

arXiv2010

The Hot Interstellar Medium of the Interacting Galaxy NGC 4490

We present an analysis of the hot interstellar medium (ISM) in the spiral galaxy NGC 4490, which is interacting with the irregular galaxy NGC 4485, using ~100ks of Chandra ACIS-S observations. The high angular resolution of Chandra enables us to remove discrete sources and perform spatially resolved spectroscopy for the star forming regions and associated outflows, allowing us to look at how the physical properties of the hot ISM such as temperature, hydrogen column density and metal abundances vary throughout these galaxies. We find temperatures of >0.41 keV and 0.85 +0.59/-0.12 keV, electron densities of >1.87 eta^(-1/2) x 10^(-3) cm^(-3) and 0.21 +0.03/-0.04 eta^(-1/2) x 10^(-3) cm^(-3), and hot gas masses of >1.1 eta^(1/2) x 10^7 M_{\odot} and ~3.7 eta^(1/2) x 10^7 M_{\odot} in the plane and halo of NGC 4490 respectively, where eta is the filling factor of the hot gas. The abundance ratios of Ne, Mg and Si with respect to Fe are found to be consistent with those predicted by theoretical models of type II supernovae. The thermal energy in the hot ISM is ~5% of the total mechanical energy input from supernovae, so it is likely that the hot ISM has been enriched and heated by type II supernovae. The X-ray emission is anticorrelated with the H-alpha and mid-infrared emission, suggesting that the hot gas is bounded by filaments of cooler ionized hydrogen mixed with warm dust.

astro-ph.GA↗

arXiv2010

Extended X-ray Emission in the HI Cavity of NGC 4151: Galaxy-scale AGN Feedback?

We present the Chandra discovery of soft diffuse X-ray emission in NGC 4151 (L[0.5-2keV]~10^{39} erg s$^{-1}$), extending ~2 kpc from the active nucleus and filling in the cavity of the HI material. The best fit to the X-ray spectrum requires either a kT~0.25 keV thermal plasma or a photoionized component. In the thermal scenario, hot gas heated by the nuclear outflow would be confined by the thermal pressure of the HI gas and the dynamic pressure of inflowing neutral material in the galactic disk. In the case of photoionization, the nucleus must have experienced an Eddington limit outburst. For both scenarios, the AGN-host interaction in NGC 4151 must have occured relatively recently (some 10^4 yr ago). This very short timescale to the last episode of high activity phase may imply such outbursts occupy $\gtrsim$1% of AGN lifetime.

astro-ph.GA↗

arXiv2010

High-Field Magnetization of CeRu2Al10

The magnetization measurements of CexLa1-xRu2Al10 (x = 1, 0.75) under the high magnetic field were performed in order to obtain the information for the long-range order (LRO) in CeRu2Al10. We successfully obtained the magnetic phase diagram of these two compounds for the applied magnetic field along the a-axis which is the magnetization easy axis, and found that the LRO for x = 1 disappears at ~50 T which is the critical field to the paramagnetic phase. For x = 0.75, the critical magnetic field decreases to ~37 T by La substitution. The magnetic phase diagram and magnetization curve are qualitatively consistent with the recent Hanzawa's mean field calculation results obtained by assuming the dimer of Ce ions whose crystalline electric field ground state has a large magnetic anisotropy. These results support the singlet pair formation scenario recently proposed by Tanida et al.. We also pointed out the possibility of the appearance of the field-induced magnetic phase between ~40 T and ~50 T for x = 1.

cond-mat.str-el↗

arXiv2010

A Chandra Study of the Rosette Star-Forming Complex. III. The NGC 2237 Cluster and the Region's Star Formation History

We present Chandra X-ray images of the NGC 2237 young star cluster on the periphery of the Rosette Nebula. We detect 168 X-ray sources, 80% of which have counterparts in USNO, 2MASS, and deep FLAMINGOS images. These constitute the first census of the cluster members with 0.2<~M<~2 Msun. Star locations in near-infrared color-magnitude diagrams indicate a cluster age around 2 Myr with a visual extinction of 1<Av<3 at 1.4 kpc, the distance of the Rosette Nebula's main cluster NGC 2244. We derive the K-band luminosity function and the X-ray luminosity function of the cluster, which indicate a population ~400-600 stars. The X-ray-selected sample shows a K-excess disk frequency of 13%. The young Class II counterparts are aligned in an arc ~3 pc long suggestive of a triggered formation process induced by the O stars in NGC 2244. The diskless Class III sources are more dispersed. Several X-ray emitting stars are located inside the molecular cloud and around gaseous pillars projecting from the cloud. These stars, together with a previously unreported optical outflow originating inside the cloud, indicate that star formation is continuing at a low level and the cluster is still growing. This X-ray view of young stars on the western side of the Rosette Nebula complements our earlier studies of the central cluster NGC 2244 and the embedded clusters on the eastern side of the Nebula. The large scale distribution of the clusters and molecular material is consistent with a scenario in which the rich central NGC 2244 cluster formed first, and its expanding HII region triggered the formation of the now-unobscured clusters RMC XA and NGC 2237. A large swept-up shell material around the HII region is now in a second phase of collect-and-collapse fragmentation, leading to the recent formation of subclusters. Other clusters deeper in the molecular cloud appear unaffected by the Nebula expansion.

astro-ph.SR↗

arXiv2010

Revisit Short Term X-ray Spectral Variability of NGC 4151 with Chandra

We present new X-ray spectral data for the Seyfert 1 nucleus in NGC 4151 observed with Chandra for 200 ks. A significant ACIS pileup is present, resulting in a non-linear count rate variation during the observation. With pileup corrected spectral fitting, we are able to recover the spectral parameters and find consistency with those derived from unpiled events in the ACIS readout streak and outer region from the bright nucleus. The absorption corrected 2-10 keV flux of the nucleus varied between 6E-11 and 1E-10 erg s^{-1} cm^{-2}. Similar to earlier Chandra studies of NGC 4151 at a historical low state, the photon indices derived from the same absorbed power-law model are Γ~0.7-0.9. However, we show that Γis highly dependent on the adopted spectral models. Fitting the power-law continuum with a Compton reflection component gives Γ~1.1. By including passage of non-uniform X-ray obscuring clouds, we can reproduce the apparent flat spectral states with Γ~1.7, typical for Seyfert 1 AGNs. The same model also fits the hard spectra from previous ASCA "long look" observation of NGC 4151 in the lowest flux state. The spectral variability during our observation can be interpreted as variations in intrinsic soft continuum flux relative to a Compton reflection component that is from distant cold material and constant on short time scale, or variations of partially covering absorber in the line of sight towards the nucleus. An ionized absorber model with ionization parameter \logξ~ 0.8-1.1 can also fit the low-resolution ACIS spectra. If the partial covering model is correct, adopting a black hole mass M_{BH} ~ 4.6E+7 Msun we constrain the distance of the obscuring cloud from the central black hole to be r<~9 light-days, consistent with the size of broad emission line region of NGC 4151 from optical reverberation mapping.

astro-ph.CO↗

arXiv2009

The Highest Resolution Chandra View of Photoionization and Jet-Cloud Interaction in the Nuclear Region of NGC 4151

We report high resolution imaging of the nucleus of the Seyfert 1 galaxy NGC 4151 obtained with a 50 ks Chandra HRC observation. The HRC image resolves the emission on spatial scales of 0.5", ~30 pc, showing an extended X-ray morphology overall consistent with the narrow line region (NLR) seen in optical line emission. Removal of the bright point-like nuclear source and image deconvolution techniques both reveal X-ray enhancements that closely match the substructures seen in the Hubble Space Telescope [OIII] image and prominent knots in the radio jet. We find that most of the NLR clouds in NGC 4151 have [OIII] to soft X-ray ratio ~10, despite the distance of the clouds from the nucleus. This ratio is consistent with the values observed in NLRs of some Seyfert 2 galaxies, which indicates a uniform ionization parameter even at large radii and a density decreasing as $r^{-2}$ as expected for a nuclear wind scenario. The [OIII]/X-ray ratios at the location of radio knots show an excess of X-ray emission, suggesting shock heating in addition to photoionization. We examine various mechanisms for the X-ray emission and find that, in contrast to jet-related X-ray emission in more powerful AGN, the observed jet parameters in NGC 4151 are inconsistent with synchrotron emission, synchrotron self-Compton, inverse Compton of CMB photons or galaxy optical light. Instead, our results favor thermal emission from the interaction between radio outflow and NLR gas clouds as the origin for the X-ray emission associated with the jet. This supports previous claims that frequent jet-ISM interaction may explain why jets in Seyfert galaxies appear small, slow, and thermally dominated, distinct from those kpc scale jets in the radio galaxies.

astro-ph.GA↗

arXiv2009

AGN/starburst connection in action: the half million second RGS spectrum of NGC1365

We present a deep (~5.8 days) 0.3-2 keV high-resolution spectrum of NGC1365, collected with the reflection grating spectrometer (RGS) on board XMM-Newton. The spectrum is dominated by strong recombination lines of He- and H-like transitions from carbon to silicon, as well as by L transitions from FeXVII. The continuum is strong, especially in the 10 to 20 Angstrom, range. Formal fits require two optically thin, collisionally ionised plasma components, with temperatures ~300 and ~640 eV. However, they leave the bulk of the forbidden components of the He-alpha OVII and NVI triplets unaccounted for. These features can be explained as being produced by photoionised gas. NGC1365 is therefore the first obscured AGN, whose high-resolution X-ray spectrum requires both collisional ionisation and photoionisation. The relative weakness of photoionisation does not stem from the intrinsic weakness of its AGN, whose X-ray luminosity is ~10^{42} erg/s. We suggest that it may instead come from the line-of-sight from the active nucleus to the NLR being blocked by optically thick matter in the broad line region, at the same time responsible for the large observed variation of the column density obscuring the X-ray active nucleus. Alternatively, NGC1365 could host a remarkably luminous nuclear starburst when compared to the AGN accretion power [abriged].

astro-ph.CO↗

arXiv2009

Protoplanetary Disk Evolution around the Triggered Star Forming Region Cepheus B

The Cepheus B (CepB) molecular cloud and a portion of the nearby CepOB3b OB association, one of the most active regions of star formation within 1 kpc, have been observed with the IRAC detector on board the Spitzer Space Telescope. The goals are to study protoplanetary disk evolution and processes of sequential triggered star formation in the region. Out of ~400 pre-main sequence (PMS) stars selected with an earlier Chandra X-ray Observatory observation, 95% are identified with mid-infrared sources and most of these are classified as diskless or disk-bearing stars. The discovery of the additional >200 IR-excess low-mass members gives a combined Chandra+Spitzer PMS sample complete down to 0.5 Mo outside of the cloud, and somewhat above 1 Mo in the cloud. Analyses of the nearly disk-unbiased combined Chandra+Spitzer selected stellar sample give several results. Our major finding is a spatio-temporal gradient of young stars from the hot molecular core towards the primary ionizing O star HD 217086. This strongly supports the radiation driven implosion (RDI) model of triggered star formation in the region. The empirical estimate for the shock velocity of 1 km/s is very similar to theoretical models of RDI in shocked molecular clouds...ABRIDGED... Other results include: 1. agreement of the disk fractions, their mass dependency, and fractions of transition disks with other clusters; 2. confirmation of the youthfulness of the embedded CepB cluster; 3. confirmation of the effect of suppression of time-integrated X-ray emission in disk-bearing versus diskless systems.

astro-ph.SR↗

arXiv2009

A Chandra Study of the Rosette Star-Forming Complex. II. Clusters in the Rosette Molecular Cloud

We explore here the young stellar populations in the Rosette Molecular Cloud (RMC) region with high spatial resolution X-ray images from the Chandra X-ray Observatory, which are effective in locating weak-lined T Tauri stars as well as disk-bearing young stars. A total of 395 X-ray point sources are detected, 299 of which (76%) have an optical or near-infrared (NIR) counterpart identified from deep FLAMINGOS images. From X-ray and mass sensitivity limits, we infer a total population of about 1700 young stars in the survey region. Based on smoothed stellar surface density maps, we investigate the spatial distribution of the X-ray sources and define three distinctive structures and substructures within them. Structures B and C are associated with previously known embedded IR clusters, while structure A is a new X-ray-identified unobscured cluster. A high mass protostar RMCX #89 = IRAS 06306+0437 and its associated sparse cluster is studied. The different subregions are not coeval but do not show a simple spatial-age pattern. Disk fractions vary between subregions and are generally 20% of the total stellar population inferred from the X-ray survey. The data are consistent with speculations that triggered star formation around the HII region is present in the RMC, but do not support a simple sequential triggering process through the cloud interior. While a significant fraction of young stars are located in a distributed population throughout the RMC region, it is not clear they originated in clustered environments.

astro-ph.SR↗

arXiv2009

Young stellar clusters and star formation throughout the Galaxy

Most stars are born in rich young stellar clusters (YSCs) embedded in giant molecular clouds. The most massive stars live out their short lives there, profoundly influencing their natal environments by ionizing HII regions, inflating wind-blown bubbles, and soon exploding as supernovae. Thousands of lower-mass pre-main sequence stars accompany the massive stars, and the expanding HII regions paradoxically trigger new star formation as they destroy their natal clouds. While this schematic picture is established, our understanding of the complex astrophysical processes involved in clustered star formation have only just begun to be elucidated. The technologies are challenging, requiring both high spatial resolution and wide fields at wavelengths that penetrate obscuring molecular material and remove contaminating Galactic field stars. We outline several important projects for the coming decade: the IMFs and structures of YSCs; triggered star formation around YSC; the fate of OB winds; the stellar populations of Infrared Dark Clouds; the most massive star clusters in the Galaxy; tracing star formation throughout the Galactic Disk; the Galactic Center region and YSCs in the Magellanic Clouds. Programmatic recommendations include: developing a 30m-class adaptive optics infrared telescope; support for high-resolution and wide field X-ray telescopes; large-aperture sub-millimeter and far-infrared telescopes; multi-object infrared spectrographs; and both numerical and analytical theory.

astro-ph.GA↗

arXiv2009

Imaging the Circumnuclear Region of NGC 1365 with Chandra

We present the first Chandra/ACIS imaging study of the circumnuclear region of the nearby Seyfert galaxy NGC 1365. The X-ray emission is resolved into point-like sources and complex, extended emission. The X-ray morphology of the extended emission shows a biconical soft X-ray emission region extending ~5 kpc in projection from the nucleus, coincident with the high excitation outflow cones seen in optical emission lines particularly to the northwest. Harder X-ray emission is detected from a kpc-diameter circumnuclear ring, coincident with the star-forming ring prominent in the Spitzer mid-infrared images; this X-ray emission is partially obscured by the central dust lane of NGC 1365. Spectral fitting of spatially separated components indicates a thermal plasma origin for the soft extended X-ray emission (kT=0.57 keV). Only a small amount of this emission can be due to photoionization by the nuclear source. Detailed comparison with [OIII]5007 observations shows the hot interstellar medium (ISM) is spatially anticorrelated with the [OIII] emitting clouds and has thermal pressures comparable to those of the [OIII] medium, suggesting that the hot ISM acts as a confining medium for the cooler photoionized clouds. The abundance ratios of the hot ISM are fully consistent with the theoretical values for enrichment from Type II supernovae, suggesting that the hot ISM is a wind from the starburst circumnuclear ring. X-ray emission from a ~450 pc long nuclear radio jet is also detected to the southeast.

astro-ph.GA↗

arXiv2007

A Chandra Study of the Rosette Star-Forming Complex. I. The Stellar Population and Structure of the Young Open Cluster NGC 2244

We present the first high spatial resolution Chandra X-ray study of NGC 2244, the 2 Myr old stellar cluster immersed in the Rosette Nebula. Over 900 X-ray sources are detected; 77% have optical or FLAMINGOS near-infrared (NIR) stellar counterparts and are mostly previously uncatalogued young stellar cluster members. All known OB stars with spectral type earlier than B1 are detected and the X-ray selected stellar population is estimated to be nearly complete between 0.5 and 3 Msun. The X-ray luminosity function (XLF) ranges from 29.4<logLx<32 ergs/s in the hard (2-8keV) band. By comparing the NGC 2244 and Orion Nebula Cluster XLFs, we estimate a total population of 2000 stars in NGC 2244. A number of further results emerge from our analysis: The XLF and the associated K-band luminosity function indicate a normal Salpeter initial mass function (IMF) for NGC 2244. This is inconsistent with the top-heavy IMF reported from earlier optical studies that lacked a good census of <4Msun stars. The spatial distribution of X-ray stars is strongly concentrated around the central O5 star, HD 46150. The other early O star, HD 46223, has few companions. The cluster's stellar radial density profile shows two distinctive structures. This double structure, combined with the absence of mass segregation, indicates that this cluster is not in dynamical equilibrium. The spatial distribution of X-ray selected K-excess disk stars and embedded stars is asymmetric with an apparent deficit towards the north. The fraction of X-ray-selected cluster members with K-band excesses caused by inner protoplanetary disks is 6%, slightly lower than the 10% disk fraction estimated from the FLAMINGOS study based on the NIR-selected sample. This is due to the high efficiency of X-ray surveys in locating disk-free T Tauri stars.[Abridged]

astro-ph↗

arXiv2006

The young stellar population in M17 revealed by Chandra

We report here results from a Chandra ACIS observation of the stellar populations in and around the Messier 17 H II region. The field reveals 886 sources, 771 of which have stellar counterparts in infrared images. In addition to comprehensive tables of X-ray source properties, several results are presented: * The X-ray Luminosity Function is calibrated to that of the Orion Nebula Cluster population to infer a total population of roughly 8000--10,000 stars * About 40% of the ACIS sources are heavily obscured with A_V > 10 mag. Some are concentrated around well-studied star-forming regions but most are distributed across the field. X-ray emission is detected from 64 of the hundreds of Class I protostar candidates that can be identified by near- and mid-infrared colors. These constitute the most likely protostar candidates known in M17. * The spatial distribution of X-ray stars is complex: we find a new embedded cluster, a 2 pc-long arc of young stars along the southwest edge of the M17 H II region, and 0.1 pc substructure within various populations. These structures may indicate that the populations are dynamically young. * All (14/14) of the known O stars but only about half (19/34) of the known B0--B3 stars in the M17 field are detected. Six of these stars exhibit very hard thermal plasma components (kT>4 keV) that may be due to colliding wind binaries. More than 100 candidate new OB stars are found, including 28 X-ray detected intermediate- and high-mass protostar candidates with infrared excesses. * Only a small fraction (perhaps 10%) of X-ray selected high- and intermediate- mass stars exhibit K-band emitting protoplanetary disks, providing further evidence that inner disks evolve very rapidly around more massive stars.

astro-ph↗

arXiv2006

X-ray Study of Triggered Star Formation and Protostars in IC 1396N

The IC 1396N cometary globule within the large nearby HII region IC 1396 has been observed with the ACIS detector on board the Chandra X-ray Observatory. We detect 117 X-ray sources, of which ~50-60 are likely members of the young open cluster Trumpler~37 dispersed throughout the HII region, and 25 are associated with young stars formed within the globule. Spitzer/2MASS photometry shows the X-ray population is very young: 3 older Class III stars, 16 classical T Tauri stars, 6 protostars including a Class 0/I system. We infer a total T Tauri population of ~30 stars in the globule, including the undetected population, with a star formation efficiency of 1-4%. An elongated source spatial distribution with an age gradient oriented towards the exciting star is discovered in the X-ray population of IC 1396N, supporting similar findings in other cometary globules. The geometric and age distribution is consistent with the RDI model for triggered star formation in CGs by HII region shocks. The inferred velocity of the shock front propagating into the globule is ~0.6km/s. The large number of X-ray-luminous protostars in the globule suggests either an unusually high ratio of Class I/0 vs. Class II/III stars, or a non-standard IMF favoring higher mass stars by the triggering process. The Chandra source associated with the luminous Class 0/I protostar IRAS 21391+5802 is one of the youngest stars ever detected in the X-ray band. We also establish for the first time that the X-ray absorption in protostars arises from the local infalling envelopes rather than ambient molecular cloud material.

astro-ph↗

arXiv2006

An X-ray Census of Young Stars in the Massive Southern Star-Forming Complex NGC 6357

We present the first high spatial resolution X-ray study of the massive star forming region NGC 6357, obtained in a 38 ks Chandra/ACIS observation. Inside the brightest constituent of this large HII region complex is the massive open cluster Pismis 24. It contains two of the brightest and bluest stars known, yet remains poorly studied; only a handful of optically bright stellar members have been identified. We investigate the cluster extent and Initial Mass Function and detect ~800 X-ray sources with a limiting sensitivity of 10^{30} ergs s^{-1}; this provides the first reliable probe of the rich intermediate-mass and low-mass population of this massive cluster, increasing the number of known members from optical study by a factor of ~50. The high luminosity end (log L_h[2-8 keV]\ge 30.3 ergs s^{-1}) of the observed X-ray luminosity function in NGC 6357 is clearly consistent with a power law relation as seen in the Orion Nebula Cluster and Cepheus B, yielding the first estimate of NGC 6357's total cluster population, a few times the known Orion population. We investigate the structure of the cluster, finding small-scale substructures superposed on a spherical cluster with 6 pc extent, and discuss its relationship to the nebular morphology. The long-standing Lx - 10^{-7}L_{bol} correlation for O stars is confirmed. Twenty-four candidate O stars and one possible new obscured massive YSO or Wolf-Rayet star are presented. Many cluster members are estimated to be intermediate-mass stars from available infrared photometry (assuming an age of 1 Myr), but only a few exhibit K-band excess. We report the first detection of X-ray emission from an Evaporating Gaseous Globule at the tip of a molecular pillar; this source is likely a B0-B2 protostar.

astro-ph↗