arXiv subjects

Junfeng Wang

Publications and source records attributed to Junfeng Wang.

At least 217 records · Page 12Linked to original sources

arXiv2013

Simultaneous analysis of three-dimensional percolation models

We simulate the bond and site percolation models on several three-dimensional lattices, including the diamond, body-centered cubic, and face-centered cubic lattices. As on the simple-cubic lattice [Phys. Rev. E, \textbf{87} 052107 (2013)], it is observed that in comparison with dimensionless ratios based on cluster-size distribution, certain wrapping probabilities exhibit weaker finite-size corrections and are more sensitive to the deviation from percolation threshold $p_c$, and thus provide a powerful means for determining $p_c$. We analyze the numerical data of the wrapping probabilities simultaneously such that universal parameters are shared by the aforementioned models, and thus significantly improved estimates of $p_c$ are obtained.

cond-mat.stat-mech↗

arXiv2013

High-precision Monte Carlo study of directed percolation in (d+1) dimensions

We present a Monte Carlo study of the bond and site directed (oriented) percolation models in $(d+1)$ dimensions on simple-cubic and body-centered-cubic lattices, with $2 \leq d \leq 7$. A dimensionless ratio is defined, and an analysis of its finite-size scaling produces improved estimates of percolation thresholds. We also report improved estimates for the standard critical exponents. In addition, we study the probability distributions of the number of wet sites and radius of gyration, for $1 \leq d \leq 7$.

cond-mat.stat-mech↗

arXiv2013

Two Compton-Thick Active Nuclei in Arp 220?

Narrow-band spectral imaging with sub-pixel resolution of the Chandra-ACIS archival observation of the ULIRG merger Arp 220 strongly suggests two Compton thick nuclei, spatially coincident with the infrared and radio emitting nuclear clusters, and separated by 1" (~ 365 pc at a distance of 76 Mpc). These previously undetected highly obscured AGNs - West (W) and East (E) - are imaged, and separated from neighboring sources, in the 6-7 keV band, where the Fe-K lines dominate the emission. The western nucleus is also detected at energies above 7 keV. We estimate Fe-K equivalent width ~ 1 keV or possibly greater for both sources, and observed 2-10 keV luminosities L_X < 3.2 x 10^40 erg/s (W) and <1.3 x 10^40 erg/s (E). From the observed Fe-K lines luminosities{, and assuming on the basis of the XMM-Newton spectrum that 40% of this may be from the 6.4 keV component, we evaluate 2-10 keV intrinsic luminosities L_X ~ 1 x 10^42 erg/s (W) and L_X ~ 0.4 x 10^42 erg/s (E). The inferred X-ray luminosity is at least a factor of 3 higher than that expected from a pure starburst with the bolometric luminosity of Arp 220. For a typical AGN SED the bolometric luminosities are 5.2 x 10^43 erg/s (W) and 2 x 10^42 erg/s (E).

astro-ph.HE↗

arXiv2013

The Progression of Star Formation in the Rosette Molecular Cloud

Using Spitzer Space Telescope and Chandra X-ray Observatory data, we identify YSOs in the Rosette Molecular Cloud (RMC). By being able to select cluster members and classify them into YSO types, we are able to track the progression of star formation locally within the cluster environments and globally within the cloud. We employ nearest neighbor method (NNM) analysis to explore the density structure of the clusters and YSO ratio mapping to study age progressions in the cloud. We find a relationship between the YSO ratios and extinction which suggests star formation occurs preferentially in the densest parts of the cloud and that the column density of gas rapidly decreases as the region evolves. This suggests rapid removal of gas may account for the low star formation efficiencies observed in molecular clouds. We find that the overall age spread across the RMC is small. Our analysis suggests that star formation started throughout the complex around the same time. Age gradients in the cloud appear to be localized and any effect the HII region has on the star formation history is secondary to that of the primordial collapse of the cloud.

astro-ph.SR↗

arXiv2013

The Exceptional Soft X-ray Halo of the Galaxy Merger NGC 6240

We report on a recent ~150-ks long Chandra observation of the ultraluminous infrared galaxy merger NGC 6240, which allows a detailed investigation of the diffuse galactic halo. Extended soft X-ray emission is detected at the 3-sigma confidence level over a diamond-shaped region with projected physical size of ~110x80 kpc, and a single-component thermal model provides a reasonably good fit to the observed X-ray spectrum. The hot gas has a temperature of ~7.5 million K, an estimated density of 2.5x10^{-3} cm^{-3}, and a total mass of ~10^10 M_sun, resulting in an intrinsic 0.4-2.5 keV luminosity of 4x10^41 erg s^{-1}. The average temperature of 0.65 keV is quite high to be obviously related to either the binding energy in the dark-matter gravitational potential of the system or the energy dissipation and shocks following the galactic collision, yet the spatially-resolved spectral analysis reveals limited variations across the halo. The relative abundance of the main alpha-elements with respect to iron is several times the solar value, and nearly constant as well, implying a uniform enrichment by type II supernovae out to the largest scales. Taken as a whole, the observational evidence is not compatible with a superwind originated by a recent, nuclear starburst, but rather hints at widespread, enhanced star formation proceeding at steady rate over the entire dynamical timescale (~200 Myr). The preferred scenario is that of a starburst-processed gas component gently expanding into, and mixing with, a pre-existing halo medium of lower metallicity (Z ~ 0.1 solar) and temperature (kT ~ 0.25 keV). This picture cannot be probed more extensively with the present data, and the ultimate fate of the diffuse, hot gas remains uncertain. Under some favorable conditions, at least a fraction of it might be retained after the merger completion, and evolve into the hot halo of a young elliptical galaxy.

astro-ph.CO↗

arXiv2012

X-ray absorption variability in NGC 4507

We present a complete spectral analysis of an XMM-Newton and Chandra campaign of the obscured AGN in NGC 4507, consisting of six observations spanning a period of six months, ranging from June 2010 to December 2010. We detect strong absorption variability on time scales between 1.5 and 4 months, suggesting that the obscuring material consists of gas clouds at parsec-scale distance. The lack of significant variability on shorter time scales suggests that this event is not due to absorption by broad line region clouds, which was instead found in other studies of similar sources. This shows that a single, universal structure of the absorber (either BLR clouds, or the parsec-scale torus) is not enough to reproduce the observed complexity of the X-ray absorption features of this AGN.

astro-ph.HE↗

arXiv2012

The Chandra HRC View of the Sub-arcsecond Structures in the Nuclear Region of NGC 1068

We have obtained a high spatial resolution X-ray image of the nucleus of NGC 1068 using the High Resolution Camera (HRC-I) on board the Chandra X-ray Observatory, which provides an unprecedented view of the innermost 1 arcsecond radius region of this galaxy. The HRC image resolves the narrow line region into X-ray emission clumps matching bright emission-line clouds in the HST [OIII]5007 images and allows comparison with sub-arcsecond scale radio jet for the first time. Two distinct X-ray knots are revealed at 1.3-1.4 arcsecond northeast and southwest of the nucleus. Based on the combined X-ray, [OIII], and radio continuum morphology, we identify the locations of intense radio jet-cloud interaction. The [OIII] to soft X-ray ratios show that some of these clouds are strongly affected by shock heating, whereas in other locations the jet simply thrusts through with no signs of strong interaction. This is further strengthened by the presence of a kT~1 keV collisionally ionized component in the ACIS spectrum of a shock heated cloud (HST-G). We estimate that the kinematic luminosity of the jet-driven shocks is 6x10^{38} erg/s, a negligible fraction (10^{-4}) of the estimated total jet power.

astro-ph.CO↗

arXiv2012

CHEERS results on Mrk 573: Study of deep Chandra observations

We present results on Mrk 573 obtained as part of the CHandra survey of Extended Emission-line Regions in nearby Seyfert galaxies (CHEERS). Previous studies showed that this source features a biconical emission in the soft X-ray band closely related with the Narrow Line Region as mapped by the [O iii] emission line and the radio emission, though on a smaller scale; we investigate the properties of soft X-ray emission from this source with new deep Chandra observations. Making use of the subpixel resolution of the Chandra/ACIS image and PSF-deconvolution, we resolve and study substructures in each ionizing cone. The two cone spectra are fitted with photoionization model, showing a mildly photoionized phase diffused over the bicone. Thermal collisional gas at about ~ 1.1 keV and ~ 0.8 keV appears to be located between the nucleus and the "knots" resolved in radio observations, and between the "arcs" resolved in the optical images, respectively; this can be interpreted in terms of shock interaction with the host galactic plane. The nucleus shows a significant flux decrease across the observations indicating variability of the AGN, with the nuclear region featuring higher ionization parameter with respect to the bicone region. The long exposure allows us to find extended emission up to ~ 7 kpc from the nucleus along the bicone axis. Significant emission is also detected in the direction perpendicular to the ionizing cones, disagreeing with the fully obscuring torus prescribed in the AGN unified model, and suggesting instead the presence of a clumpy structure.

astro-ph.HE↗

arXiv2012

Probing the X-Ray Binary Populations of the Ring Galaxy NGC 1291

We present Chandra studies of the X-ray binary (XRB) populations in the bulge and ring regions of the ring galaxy NGC 1291. We detect 169 X-ray point sources in the galaxy, 75 in the bulge and 71 in the ring, utilizing the four available Chandra observations totaling an effective exposure of 179 ks. We report photometric properties of these sources in a point-source catalog. There are ~40% of the bulge sources and ~25% of the ring sources showing >3σlong-term variability in their X-ray count rate. The X-ray colors suggest that a significant fraction of the bulge (~75%) and ring (~65%) sources are likely low-mass X-ray binaries (LMXBs). The spectra of the nuclear source indicate that it is a low-luminosity AGN with moderate obscuration; spectral variability is observed between individual observations. We construct 0.3-8.0 keV X-ray luminosity functions (XLFs) for the bulge and ring XRB populations, taking into account the detection incompleteness and background AGN contamination. We reach 90% completeness limits of ~1.5\times10^{37} and ~2.2\times10^{37} erg/s for the bulge and ring populations, respectively. Both XLFs can be fit with a broken power-law model, and the shapes are consistent with those expected for populations dominated by LMXBs. We perform detailed population synthesis modeling of the XRB populations in NGC 1291, which suggests that the observed combined XLF is dominated by an old LMXB population. We compare the bulge and ring XRB populations, and argue that the ring XRBs are associated with a younger stellar population than the bulge sources, based on the relative overdensity of X-ray sources in the ring, the generally harder X-ray color of the ring sources, the overabundance of luminous sources in the combined XLF, and the flatter shape of the ring XLF.

astro-ph.CO↗

arXiv2012

The X-ray reflector in NGC 4945: a time and space resolved portrait

We present a time, spectral and imaging analysis of the X-ray reflector in NGC 4945, which reveals its geometrical and physical structure with unprecedented detail. NGC 4945 hosts one of the brightest AGN in the sky above 10 keV, but it is only visible through its reflected/scattered emission below 10 keV, due to absorption by a column density of ~4\times10^24 cm-2. A new Suzaku campaign of 5 observations spanning ~6 months, together with past XMM-Newton and Chandra observations, show a remarkable constancy (within <10%) of the reflected component. Instead, Swift-BAT reveals strong intrinsic variability on time scales longer than one year. Modeling the circumnuclear gas as a thin cylinder with the axis on the plane of the sky, we show that the reflector is at a distance >30-50 pc, well within the imaging capabilities of Chandra at the distance of NGC 4945 (1"~18 pc). Accordingly, the Chandra imaging reveals a resolved, flattened, ~150 pc-long clumpy structure, whose spectrum is fully due to cold reflection of the primary AGN emission. The clumpiness may explain the small covering factor derived from the spectral and variability properties.

astro-ph.CO↗

arXiv2011

A Deep Chandra ACIS Study of NGC 4151. III. the Line Emission and Spectral Analysis of the Ionization Cone

This paper is the third in a series in which we present deep Chandra ACIS-S imaging spectroscopy of the Seyfert 1 galaxy NGC 4151, devoted to study its complex circum-nuclear X-ray emission. Emission features in the soft X-ray spectrum of the bright extended emission (L[0.3-2keV]~10^40 erg/s) at r>130 pc (2") are consistent with the brighter OVII, OVIII, and NeIX lines seen in the Chandra HETGS and XMM-Newton RGS spectra below 2 keV. We construct emission line images of these features and find good morphological correlations with the narrow line region clouds mapped in [OIII]5007A. Self-consistent photoionization models provide good descriptions of the spectra of the large scale emission, as well as resolved structures, supporting the dominant role of nuclear photoionization, although displacement of optical and X-ray features implies a more complex medium. Collisionally ionized emission is estimated to be <12% of the extended emission. Presence of both low and high ionization spectral components and extended emission in the X-ray image perpendicular to the bicone indicates leakage of nuclear ionization, likely filtered through warm absorbers, instead of being blocked by a continuous obscuring torus. The ratios of [OIII]/soft X-ray flux are approximately constant (~15) for the 1.5 kpc radius spanned by these measurements, indicating a relatively constant ionization parameter, consistent with the photoionized outflow of a wind-like density profile. Using spatially resolved features, we estimate that the mass outflow rate in NGC 4151 is ~2Msun/yr at 130 pc and the kinematic power of the ionized outflow is 1.7x10^41 erg/s, approximately 0.3% of the bolometric luminosity of NGC 4151.

astro-ph.CO↗

arXiv2011

A Deep Chandra ACIS Study of NGC 4151. II. The Innermost Emission Line Region and Strong Evidence for Radio Jet-NLR Cloud Collision

We have studied the X-ray emission within the inner 150 pc radius of NGC 4151 by constructing high spatial resolution emission line images of OVII, OVIII, and NeIX. These maps show extended structures that are spatially correlated with the radio outflow and optical [OIII] emission. We find strong evidence for jet--gas cloud interaction, including morphological correspondences with regions of X-ray enhancement, peaks of near-infrared [FeII] emission, and optical clouds. In these regions, moreover, we find evidence of elevated NeIX/OVII ratios; the X-ray emission of these regions also exceeds that expected from nuclear photoionization. Spectral fitting reveals the presence of a collisionally ionized component. The thermal energy of the hot gas suggests that >0.1% of the estimated jet power is deposited into the host interstellar medium through interaction between the radio jet and the dense medium of the circum-nuclear region. We find possible pressure equilibrium between the collisionally ionized hot gas and the photoionized line-emitting cool clouds. We also obtain constraints on the extended iron and silicon fluorescent emission. Both lines are spatially unresolved. The upper limit on the contribution of an extended emission region to the Fe Kalpha emission is <5% of the total, in disagreement with a previous claim that 65% of the Fe Kalpha emission originates in the extended narrow line region.

astro-ph.CO↗

arXiv2011

High-field magnetization and magnetic phase transition in CeOs2Al10

We have studied the magnetization of CeOs2Al10 in high magnetic fields up to 55 T for H // a and constructed the magnetic phase diagram for H // a. The magnetization curve shows a concave H dependence below T_max \sim40 K which is higher than the transition temperature T_0 \sim29 K. The magnetic susceptibility along the a-axis shows a smooth and continuous decrease down to \sim20 K below T_max \sim40 K without showing an anomaly at T_0. From these two results, a Kondo singlet is formed below T_max and coexists with the antiferro magnetic order below T_0. We also propose that the larger suppression of the spin degrees of freedom along the a-axis than along the c-axis below T_max is associated with the origin of the antiferro magnetic component.

cond-mat.str-el↗

arXiv2011

The Search for Low-mass Companions of B Stars in the Carina Nebula Cluster Trumpler 16

We have developed lists of likely B3--A0 stars (called "late B" stars) in the young cluster Trumpler 16. The following criteria were used: location within 3' of Eta Car, an appropriate V and B-V combination, and proper motion (where available). Color and magnitude cuts have been made assuming an E(B-V) =0.55 mag +/- 0.1, which is a good approximation close to the center of Trumpler 16. These lists have been cross-correlated with X-ray sources found in the Chandra Carina Complex Project (CCCP). Previous studies have shown that only very rarely (if at all) do late main sequence B stars produce X-rays. We present evidence that the X-ray detected sources are binaries with low-mass companions, since stars less massive than 1.4 Msun are strong X-ray sources at the age of the cluster. Both the median X-ray energies and X-ray luminosities of these sources are in good agreement with values for typical low-mass coronal X-ray sources. We find that 39% of the late B stars based on a list with proper motions have low-mass companions. Similarly, 32% of a sample without proper motions have low-mass companions. We discuss the X-ray detection completeness. These results on low-mass companions of intermediate mass stars are complementary to spectroscopic and interferometric results, and probe new parameter space of low mass companions at all separations. They do not support a steeply rising distribution of mass ratios to low masses for intermediate-mass (5 Msun) primaries, such as would be found by random pairing from the Initial Mass Function.

astro-ph.SR↗

arXiv2011

The Chandra Carina Complex Project View of Trumpler 16

Trumpler 16 is a well--known rich star cluster containing the eruptive supergiant $η$ Carinæ and located in the Carina star-forming complex. In the context of the Chandra Carina Complex Project, we study Trumpler 16 using new and archival X-ray data. A revised X-ray source list of the Trumpler 16 region contains 1232 X-ray sources including 1187 likely Carina members. These are matched to 1047 near-infrared counterparts detected by the HAWK-I instrument at the VLT allowing for better selection of cluster members. The cluster is irregular in shape. Although it is roughly circular, there is a high degree of sub-clustering, no noticeable central concentration and an extension to the southeast. The high--mass stars show neither evidence of mass segregation nor evidence of strong differential extinction. The derived power-law slope of the X-ray luminosity function for Trumpler 16 reveals a much steeper function than the Orion Nebula Cluster implying different ratio of solar- to higher-mass stars. We estimate the total Trumpler 16 pre-main sequence population to be > 6500 Class II and Class III X-ray sources. An overall K-excess disk frequency of ~ 8.9% is derived using the X-ray selected sample, although there is some variation among the sub-clusters, especially in the Southeastern extension. X-ray emission is detected from 29 high--mass stars with spectral types between B2 and O3.

astro-ph.SR↗

arXiv2011

A Chandra ACIS Study of the Young Star Cluster Trumpler 15 in Carina and Correlation with Near-infrared Sources

Using the highest-resolution X-ray observation of the Trumpler 15 star cluster taken by the Chandra X-ray Observatory, we estimate the total size of its stellar population by comparing the X-ray luminosity function of the detected sources to a calibrator cluster, and identify for the first time a significant fraction (~14%) of its individual members. The highest-resolution near-IR observation of Trumpler 15 (taken by the HAWK-I instrument on the VLT) was found to detect most of our X-ray selected sample of cluster members, with a K-excess disk frequency of 3.8+-0.7%. The near-IR data, X-ray luminosity function, and published spectral types of the brightest members support a cluster age estimate (5-10 Myr) that is older than those for the nearby Trumpler 14 and Trumpler 16 clusters, and suggest that high-mass members may have already exploded as supernovae. The morphology of the inner ~0.7 pc core of the cluster is found to be spherical. However, the outer regions (beyond 2 pc) are elongated, forming an `envelope' of stars that, in projection, appears to connect Trumpler 15 to Trumpler 14; this morphology supports the view that these clusters are physically associated. Clear evidence of mass segregation is seen. This study appears in a Special Issue of the ApJS devoted to the Chandra Carina Complex Project (CCCP), a 1.42 square degree Chandra X-ray survey of the Great Nebula in Carina.

astro-ph.SR↗

arXiv2011

X-ray Star Clusters in the Carina Complex

The distribution of young stars found in the Chandra Carina Complex Project (CCCP) is examined for clustering structure. X-ray surveys are advantageous for identifying young stellar populations compared to optical and infrared surveys in suffering less contamination from nebular emission and Galactic field stars. The analysis is based on smoothed maps of a spatially complete subsample of about 3000 brighter X-ray sources classified as Carina members, and about 10,000 stars from the full CCCP sample. The principal known clusters are recovered, and some additional smaller groups are identified. No rich embedded clusters are present, although a number of sparse groups are found. The CCCP reveals considerable complexity in clustering properties. The Trumpler 14 and 15 clusters have rich stellar populations in unimodal, centrally concentrated structures several parsecs across. Non-spherical internal structure is seen, and large-scale low surface density distributions surround these rich clusters. Trumpler 16, in contrast, is comprised of several smaller clusters within a circular boundary. Collinder 228 is a third type of cluster which extends over tens of parsecs with many sparse compact groups likely arising from triggered star formation processes. A widely dispersed, but highly populous, distribution of X-ray stars across the about 50 pc CCCP mosaic supports a model of past generations of star formation in the region. Collinder 234, a group of massive stars without an associated cluster of pre-main sequence stars, may be part of this dispersed population.

astro-ph.SR↗

arXiv2011

An Introduction to the Chandra Carina Complex Project

The Great Nebula in Carina provides an exceptional view into the violent massive star formation and feedback that typifies giant HII regions and starburst galaxies. We have mapped the Carina star-forming complex in X-rays, using archival Chandra data and a mosaic of 20 new 60ks pointings using the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer, as a testbed for understanding recent and ongoing star formation and to probe Carina's regions of bright diffuse X-ray emission. This study has yielded a catalog of properties of >14,000 X-ray point sources; >9800 of them have multiwavelength counterparts. Using Chandra's unsurpassed X-ray spatial resolution, we have separated these point sources from the extensive, spatially-complex diffuse emission that pervades the region; X-ray properties of this diffuse emission suggest that it traces feedback from Carina's massive stars. In this introductory paper, we motivate the survey design, describe the Chandra observations, and present some simple results, providing a foundation for the 15 papers that follow in this Special Issue and that present detailed catalogs, methods, and science results.

astro-ph.SR↗