arXiv subjects

Q. Daniel Wang

Publications and source records attributed to Q. Daniel Wang.

At least 181 records · Page 10Linked to original sources

arXiv2006

Predicting the Starquakes in PSR J0537-6910

We report on more than 7 years of monitoring of PSR J0537-6910, the 16 ms pulsar in the Large Magellanic Cloud, using data acquired with the RXTE. During this campaign the pulsar experienced 23 sudden increases in frequency (``glitches'') amounting to a total gain of over six ppm of rotation frequency superposed on its gradual spindown of d(nu)/d(t) = -2e-10 Hz/s. The time interval from one glitch to the next obeys a strong linear correlation to the amplitude of the first glitch, with a mean slope of about 400 days ppm (6.5 days per uHz), such that these intervals can be predicted to within a few days, an accuracy which has never before been seen in any other pulsar. There appears to be an upper limit of ~40 uHz for the size of glitches in_all_ pulsars, with the 1999 April glitch of J0537 as the largest so far. The change in the spindown of J0537 across the glitches, Delta(d(nu)/d(t)), appears to have the same hard lower limit of -1.5e-13 Hz/s, as, again, that observed in all other pulsars. The spindown continues to increase in the long term, d(d(nu)/d(t))/d(t) = -1e-21 Hz/s/s, and thus the timing age of J0537 (-0.5 nu d(nu)/d(t)) continues to decrease at a rate of nearly one year every year, consistent with movement of its magnetic moment away from its rotational axis by one radian every 10,000 years, or about one meter per year. J0537 was likely to have been born as a nearly-aligned rotator spinning at 75-80 Hz, with a |d(nu)/d(t)| considerably smaller than its current value of 2e-10 Hz/s. The pulse profile of J0537 consists of a single pulse which is found to be flat at its peak for at least 0.02 cycles.

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arXiv2006

CHANDRA ACIS Spectroscopy of N157B -- A Young Composite Supernova Remnant in a Superbubble

We present Chandra ACIS observations of N157B, a young supernova remnant located in the 30 Doradus star-formation region of the LMC. This remnant contains the most energetic pulsar known (PSR J0537-6910), which is surrounded by a bright nonthermal nebula that likely represents a toroidal pulsar wind terminal shock observed edge-on. We confirm the non-thermal nature of the comet-shaped X-ray emission feature and show that the spectral steepening of this feature away from the pulsar is quantitatively consistent with synchrotron cooling of shocked pulsar wind particles flowing downstream at a bulk velocity close to the speed of light. Around the cometary nebula we unambiguously detect a thermal component, which accounts for about 1/3 of the total 0.5 - 10 keV flux from the remnant. This thermal component is distributed among various clumps of metal-enriched plasma embedded in the low surface brightness X-ray-emitting diffuse gas. The relative metal enrichment pattern suggests that the mass of the supernova progenitor is >~ 20M_sun. A comparison of the X-ray data with HST optical images suggests that the explosion site is close to a dense cloud, against which a reflection shock is launched. The interaction between this reflection shock and the nebula has likely produced both its cometary shape and the surrounding thermal emission enhancement. SNR N157B is apparently expanding into the hot low-density interior of a surrounding superbubble formed by the young OB association LH99, as revealed by Spitzer mid-infrared images. This scenario naturally explains the exceptionally large sizes of both the thermal and nonthermal components as well as the lack of an outer shell of the SNR. These results provide a rare glimpse into the SNR structure and evolution in a region of recent star-formation.

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arXiv2006

Non-Equilibrium Ionization Model for Stellar Cluster Winds and its Application

We have developed a self-consistent physical model for super stellar cluster winds based on combining a 1-D steady-state adiabatic wind solution and a non-equilibrium ionization calculation. Comparing with the case of collisional ionization equilibrium, we find that the non-equilibrium ionization effect is significant in the regime of a high ratio of energy to mass input rate and manifests in a stronger soft X-ray flux in the inner region of the star cluster. Implementing the model in X-ray data analysis softwares (e.g., XSPEC) directly facilitates comparisons with X-ray observations. Physical quantities such as the mass and energy input rates of stellar winds can be estimated by fitting observed X-ray spectra. The fitted parameters may then be compared with independent measurements from other wavelengths. Applying our model to the star cluster NGC 3603, we find that the wind accounts for no more than 50% of the total "diffuse" emission, and the derived mass input rate and terminal velocity are comparable to other empirical estimates. The remaining emission most likely originate from numerous low-mass pre-main-sequence stellar objects.

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arXiv2006

An XMM-Newton Observation of the Massive Edge-on Sb Galaxy NGC 2613

We present an XMM-Newton observation of the massive edge-on Sb galaxy NGC 2613. We discover that this galaxy contains a deeply embedded active nucleus with a 0.3-10 keV luminosity of 3.3x10^40 erg/s and a line-of-sight absorption column of 1.2x10^23 cm^-2. Within the 25 mag/arcsec^2 optical B-band isophote of the galaxy, we detect an additional 4 sources with an accumulated luminosity of 4.3x10^39 erg/s. The bulk of the unresolved X-ray emission spatially follows the near-infrared (NIR) K-band surface brightness distribution; the luminosity ratio L_X/L_K ~ 8x10^-4 is consistent with that inferred from galactic discrete sources. This X-ray-NIR association and the compatibility of the X-ray spectral fit with the expected spectrum of a population of discrete sources suggest that low-mass X-ray binaries (LMXBs) are the most likely emitters of the unresolved emission in the disk region. The remaining unresolved emission is primarily due to extraplanar hot gas. The luminosity of this gas is at least a factor of 10 less than that predicted by recent simulations of intergalactic gas accretion by such a massive galaxy with a circular rotation speed V_c ~ 304 km/s^2 (Toft et al. 2002). Instead, we find that the extraplanar hot gas most likely represents discrete extensions away from the disk, including two ``bubble-like'' features on either side of the nucleus. These extensions appear to correlate with radio continuum emission and, energetically, can be easily explained by outflows from the galactic disk.

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arXiv2006

Chandra Observations of Galactic Center: High Energy Processes at Arcsecond Resolution

About 2 million seconds of Chandra observing time have been devoted to the Galactic center (GC), including large-scale surveys and deep pointings. These observations have led to the detection of about 4000 discrete X-ray sources and the mapping of diffuse X-ray emission in various energy bands. In this review, I first summarize general results from recent studies and then present close-up views of the three massive star clusters (Arches, Quintuplet, and GC) and their interplay with the Galactic nuclear environment.

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arXiv2006

The Interplay between Star Formation and the Nuclear Environment of our Galaxy: Deep X-ray Observations of the Galactic Center Arches and Quintuplet Clusters

The Galactic center (GC) provides a unique laboratory for a detailed examination of the interplay between massive star formation and the nuclear environment of our Galaxy. Here, we present an 100 ks Chandra ACIS observation of the Arches and Quintuplet star clusters. We also report on a complementary mapping of the dense molecular gas near the Arches cluster made with the Owens Valley Millimeter Array. The number-flux relation of point-like X-ray sources indicates an over-population of relatively bright X-ray sources, which are apparently associated with the clusters. The sources in the core of the Arches and Quintuplet clusters are most likely extreme colliding wind massive star binaries. The diffuse X-ray emission from the core of the Arches cluster has a spectrum showing a 6.7-keV emission line and a surface intensity profile declining steeply with radius, indicating an origin in a cluster wind. In the outer regions near the Arches cluster, the overall diffuse X-ray enhancement demonstrates a bow shock morphology and is prominent in the Fe Kalpha 6.4-keV line emission. Much of this enhancement may result from an ongoing collision between the cluster and the adjacent molecular cloud. The older and less compact Quintuplet cluster contains much weaker X-ray sources and diffuse emission, probably originating from low-mass stellar objects as well as a cluster wind. However, the overall population of these objects, constrained by the observed total diffuse X-ray luminosities, is substantially smaller than expected for both clusters. This deficiency of low-mass objects may be a manifestation of the unique star formation environment of the Galactic center.

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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

An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources

We have studied the stellar and interstellar environments of two luminous X-ray sources and five ultraluminous X-ray sources (ULXs) in order to gain insight into their nature. Archival Hubble Space Telescope images were used to identify the optical counterparts of the ULXs Ho IX X-1 and NGC 1313 X-2, and to make photometric measurements of the local stellar populations of these and the luminous source IC 10 X-1. We obtained high-dispersion spectroscopic observations of the nebulae around these seven sources to search for He II lambda-4686 emission and to estimate the expansion velocities and kinetic energies of these nebulae. Our observations did not detect nebular He II emission from any source, with the exception of LMC X-1; this is either because we missed the He III regions or because the nebulae are too diffuse to produce He II surface brightnesses that lie within our detection limit. We compare the observed ionization and kinematics of the supershells around the ULXs Ho IX X-1 and NGC 1313 X-2 with the energy feedback expected from the underlying stellar population to assess whether additional energy contributions from the ULXs are needed. In both cases, we find insufficient UV fluxes or mechanical energies from the stellar population; thus these ULXs may be partially responsible for the ionization and energetics of their supershells. All seven sources we studied are in young stellar environments and six of them have optical counterparts with masses >~7 M_sun; thus, these sources are most likely high-mass X-ray binaries.

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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

An XMM-Newton and Chandra Study of the Starburst Galaxy IC 10

We present an X-ray study of our nearest starburst galaxy IC 10, based on XMM-Newton and Chandra observations. A list of 73 XMM-Newton and 28 Chandra detections of point-like X-ray sources in the field is provided; a substantial fraction of them are likely stellar objects in the Milky Way due to the low Galactic latitude location of IC 10. The brightest source in the IC 10 field, X-1, shows a large variation by a factor of up to $\sim 6$ on time scales during the XMM-Newton observation. The X-ray spectra of the source indicate the presence of a multi-color blackbody accretion disk with an inner disk temperature T_{in} \approx 1.1 keV. These results are consistent with the interpretation of the source as a stellar mass black hole, probably accreting from a Wolf-Rayet star companion. We infer the mass of this black hole to be about 4 Msun if it is not spinning, or a factor of up to about 6 higher if there is significant spinning. We also detect an apparent diffuse soft X-ray emission component of IC 10. An effective method is devised to remove the X-ray CCD-readout streaks of X-1 that strongly affect the study of the diffuse component in the XMM-Newton and Chandra observations. We find that the diffuse X-ray morphology is oriented along the optical body of the galaxy and is chiefly associated with starburst regions. The diffuse component can be characterized by an optically thin thermal plasma with a mean temperature of $\sim 4 \times 10^6$ K and a 0.5-2 keV luminosity of $\sim 8 \times 10^{37} {\rm erg s^{-1}}$, representing only a small fraction of the expected mechanical energy inputs from massive stars in the galaxy. There is evidence that the hot gas is driving outflows from the starburst regions; therefore, the bulk of the energy inputs may be released in a galactic wind.

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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

Highly Clumpy Structure of the Thermal Composite Supernova Remnant 3C391 Unveiled by Chandra

The nature of the internal thermal X-ray emission seen in ``thermal composite" supernova remnants is still uncertain. Chandra observation of the 3C391 shows a southeast-northwest elongated morphology and unveils a highly clumpy structure of the remnant. Detailed spatially resolved spectral analysis for the small-scale features reveals normal metal abundance and uniform temperature for the interior gas. The properties of the hot gas comparatively favor the cloudlet evaporation model as a main mechanism for the ``thermal composite" X-ray appearance, though radiative rim and thermal conduction may also be effective. A faint protrusion is found in Si and S lines out of the southwest radio border.

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arXiv2005

Supernova Blastwaves in Low-density Hot Media: a Mechanism for Spatially Distributed Heating

Most supernovae are expected to explode in low-density hot media, particularly in galactic bulges and elliptical galaxies. The remnants of such supernovae, though difficult to detect individually, can be profoundly important in heating the media on large scales. We characterize the evolution of this kind of supernova remnants, based on analytical approximations and hydrodynamic simulations. We generalize the standard Sedov solution to account for both temperature and density effects of the ambient media. Although cooling can be neglected, the expansion of such a remnant deviates quickly from the standard Sedov solution and asymptotically approaches the ambient sound speed as the swept-up thermal energy becomes important. The relatively steady and fast expansion of the remnants over large volumes provides an ideal mechanism for spatially distributed heating, which may help to alleviate the over-cooling problem of hot gas in groups and clusters of galaxies as well as in galaxies themselves. The simulations were performed with the FLASH code.

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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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arXiv2005

Diffuse X-rays from the Arches and Quintuplet clusters

The origin and initial mass function of young stellar clusters near the Galactic center are still poorly understood. Two of the more prominent ones, the Arches and Quintuplet clusters, may have formed from a shock-induced burst of star formation, given their similar age and proximity to each other. Their unusual mass distribution, however, may be evidence of a contributing role played by other factors, such as stellar capture from regions outside the clusters themselves. Diffuse X-ray emission from these sources provides us with a valuable, albeit indirect, measure of the stellar mass-loss rate from their constituents. Using recent data acquired with Chandra, we can study the nature and properties of the outflow to not only probe the pertinent physical conditions, such as high metallicity, the magnetic field, and so forth, but also to better constrain the stellar distribution within the clusters, in order to identify their formative history. In this paper, we present a set of three-dimensional smoothed particle hydrodynamics simulations of the wind-wind interactions in both the Arches and Quintuplet clusters. We are guided primarily by the currently known properties of the constituent stars, though we vary the mass-loss rates in order to ascertain the dependence of the measured X-ray flux on the assumed stellar characteristics. Our results are compared with the latest observations of the Arches cluster. Our analysis of the Quintuplet cluster may be used as a basis for comparison with future X-ray observations of this source.

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arXiv2004

Unusual X-ray transients in the Galactic Centre

We report the discovery in the Galactic Centre region of two hard X-ray sources, designated as XMM J174457-2850.3 and XMM J174544-2913.0, which exhibited flux variations in the 2--10 keV band in excess of a factor of 100 in observations spanning roughly a year. In both cases the observed hydrogen column density is consistent with a location near to the Galactic Centre, implying peak X-ray luminosities of ~5 x 10^34 erg/s. These objects may represent a new population of transient source with very different properties to the much more luminous Galactic Centre transients associated with neutron star and black-hole binary systems. Spectral analysis shows that XMM J174457-2850.3 has relatively weak iron-line emission set against a very hard continuum. XMM J174544-2913.0, on the other hand, has an extremely strong K-line from helium-like iron with an equivalent width of ~2.4keV. The nature of the latter source is of particular interest. Does it represent an entirely new class of object or does it correspond to a known class of source in a very extreme configuration?

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arXiv2004

Extra-planar Diffuse Hot Gas Around Normal Disk Galaxies

I review results from {\sl Chandra} observations of nearby normal edge-on galaxies (Sd to Sa types). These galaxies have a broad range of star formation rate, but none of them is dominated by a nuclear starburst. The galaxies are all in directions of low Galactic foreground absorption. Extra-Planar diffuse soft X-ray emission is detected unambiguously from all the galaxies, except for N4244 (Sd), which is low in both the stellar mass and the star formation rate. The thermal nature of the X-ray-emitting gas is well established, although its chemical and ionization states remain largely uncertain. The X-ray luminosity of the gas is proportional to the star formation rate and to the stellar mass of the galaxies. But the luminosity accounts for at most a few percent of the expected supernova mechanical energy input. Therefore, there is a ``missing'' energy problem for spiral galaxies. Much of the energy in late-type spirals may be converted and radiated in lower energy bands. But early-type ones most likely have outflows, which are powered primarily by Type Ia supernovae in galactic bulges. These galactic outflows may strongly affect both the dynamics and cooling of the intergalactic gas accretion, hence the evolution of the galaxies.

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arXiv2004

A Chandra ACIS view of the Thermal Composite Supernova Remnant 3C391

We present a 60 ks Chandra ACIS-S observation of the thermal composite supernova remnant 3C391. The southeast-northwest elongated morphology is similar to that previously found in radio and X-ray studies. This observation unveils a highly clumpy structure of the remnant. Detailed spatially resolved spectral analysis for the small-scale features reveals that the interior gas is generally of normal metal abundance and has approached or basically reached ionization equilibrium. The hydrogen column density increases from southeast to northwest. Three mechanisms, radiative rim, thermal conduction, and cloudlet evaporation, may all play roles in the X-ray appearance of 3C391 as a "thermal composite" remnant, but there are difficulties with each of them in explaining some physical properties. Comparatively, the cloudlet evaporation model is favored by the main characteristics such as the highly clumpy structure and the uniform temperature and density distribution over most of the remnant. The directly measured postshock temperature also implies a young age, about 4 kyr, for the remnant. The postshock gas pressure derived from the NE and SW rims, which harbor maser spots, is consistent with the estimate for the maser regions. An unresolved X-ray source is observed on the northwest border and its spectrum is best fitted by a power-law.

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