arXiv subjects

D. -W. Kim

Publications and source records attributed to D. -W. Kim.

71 records · Page 4Linked to original sources

arXiv2005

The X-ray binary population in M33: I. Source list and luminosity function

In this paper we present the source list for 3 Chandra observations of the Local Group galaxy M33. The observations are centered on the nucleus and on the star forming region NGC 604. We detect a total of 261 sources in an area of \~0.2 square degree down to a flux limit of 3*10^{-16} erg/s/cm^2 which corresponds to a luminosity of ~2*10^34 erg/s at a distance of 840 kpc. From the source list we construct the luminosity functions of sources observed in M33. Taking into account background contamination the luminosity functions are consistent with those of other star forming galaxies. In addition, the combination of X-ray color analysis and the existence of ``blue'' optical counterparts strongly indicate that the X-ray point source population in M33 consists of young objects. Above 3*10^35 erg/s there are few X-ray sources in the locus of the X-ray hardness ratio diagram that is generally populated by LMXBs.

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arXiv2005

Co-moving space density of X-ray-selected Active Galactic Nuclei

For measurement of the AGN luminosity function and its evolution, X-ray selection samples all types of AGN and provides reduced obscuration bias in comparison with UV-excess or optical surveys. The apparent decline in optically-selected quasars above z~3 may be strongly affected by such a bias. The Chandra Multiwavelength Project (ChaMP) is characterizing serendipitously detected X-ray sources in a large number of fields with archival Chandra imaging. We present a preliminary measure of the co-moving space density using a sample of 311 AGN found in 23 ChaMP fields (~1.8 deg^2) supplemented with 57 X-ray bright AGN from the CDF-N and CDF-S. Within our X-ray flux (f(0.3-8.0 keV) > 4x10^-15 erg cm^-2 s^-1) and optical magnitude (r' < 22.5) limits, our sample includes 14 broad emission line AGN at z > 3. Using this X-ray selected sample, we detect a turnover in the co-moving space density of luminous type 1 AGN (log Lx > 44.5; units erg s^-1; measured in the 0.3-8.0 keV band and corrected for Galactic absorption) at z > 2.5. Our X-ray sample is the first to show a behavior similar to the well established evolution of the optical quasar luminosity function. A larger sample of high redshift AGN and with a greater fraction of identified sources, either spectroscopic or photometric, at faint optical magnitudes (r' > 22.5) are required to remove the remaining uncertainty in our measure of the X-ray luminosity function. We confirm that for z < 1, lower luminosity AGN (log Lx < 44.5) are more prevalent by more than an order of magnitude than those with high luminosity. We have combined the Chandra sample with AGN from the ROSAT surveys to present a measure of the space density of luminous type 1 AGN in the soft X-ray band (0.5-2.0 keV) which confirms the broad band turnover described above.

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arXiv2004

Discovery of a Jet-Like Structure at the High Redshift QSO CXOMP J084128.3+131107

The Chandra Multiwavelength Project (ChaMP) has discovered a jet-like structure associated with a newly recognized QSO at redshift z=1.866. The system was 9.4 arcmin off-axis during an observation of 3C 207. Although significantly distorted by the mirror PSF, we use both a raytrace and a nearby bright point source to show that the X-ray image must arise from some combination of point and extended sources, or else from a minimum of three distinct point sources. We favor the former situation, as three unrelated sources would have a small probability of occurring by chance in such a close alignment. We show that interpretation as a jet emitting X-rays via inverse Compton (IC) scattering on the cosmic microwave background (CMB) is plausible. This would be a surprising and unique discovery of a radio-quiet QSO with an X-ray jet, since we have obtained upper limits of 100 microJy on the QSO emission at 8.46 GHz, and limits of 200 microJy for emission from the putative jet.

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arXiv2003

The Chandra Multiwavelength Project: Optical Followup of Serendipitous Chandra Sources

We present followup optical g', r', and i', imaging and spectroscopy of serendipitous X-ray sources detected in 6 archival Chandra, images included in the Chandra, Multiwavelength Project (ChaMP). Of the 486 X-ray sources detected between 3e-16 and 2e-13 (with a median flux of 3e-15 erg cm-2 s-1, we find optical counterparts for 377 (78%), or 335 (68%) counting only unique counterparts. We present spectroscopic classifications for 125 objects, representing 75% of sources with r<21 optical counterparts (63% to r=22). Of all classified objects, 63 (50%) are broad line AGN, which tend to be blue in g-r colors. X-ray information efficiently segregates these quasars from stars, which otherwise strongly overlap in these SDSS colors until z>3.5. We identify 28 sources (22%) as galaxies that show narrow emission lines, while 22 (18%) are absorption line galaxies. Eight galaxies lacking broad line emission have X-ray luminosities that require they host an AGN (logL_X>43). Half of these have hard X-ray emission suggesting that high gas columns obscure both the X-ray continuum and the broad emission line regions. We find objects in our sample that show signs of X-ray or optical absorption, or both, but with no strong evidence that these properties are coupled. ChaMP's deep X-ray and optical imaging enable multiband selection of small and/or high-redshift groups and clusters. In these 6 fields we have discovered 3 new clusters of galaxies, two with z>0.4, and one with photometric evidence that it is at a similar redshift.

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arXiv2003

Chandra Multi-wavelength Project (ChaMP). I. First X-ray Source Catalog

The Chandra Multi-wavelength Project (ChaMP) is a wide-area (~14 deg^2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed AGNs, high z clusters of galaxies) at flux levels (fX ~ 10^-15 - 10^-14 erg sec-1 cm-2) intermediate between the Chandra Deep surveys and previous missions. We present the first ChaMP catalog, which consists of 991 near on-axis, bright X-ray sources obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. To assess source reliability and positional uncertainty, we perform a series of simulations and also use Chandra data to complement the simulation study. The false source detection rate is found to be as good as or better than expected for a given limiting threshold. On the other hand, the chance of missing a real source is rather complex, depending on the source counts, off-axis distance (or PSF), and background rate. The positional error (95% confidence level) is usually < 1" for a bright source, regardless of its off-axis distance while it can be as large as 4" for a weak source (~20 counts) at a large off-axis distance (Doff-axis > 8'). We have also developed new methods to find spatially extended or temporary variable sources and those sources are listed in the catalog.

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arXiv2003

Chandra Multi-wavelength Project (ChaMP). II. First Results of X-ray Source Properties

We present the first results of ChaMP X-ray source properties obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. Utilizing only near on-axis, bright X-ray sources (to avoid problems caused by incompleteness and the Eddington bias), we derive the Log(N)-Log(S) relation in soft (0.5-2 keV) and hard (2-8 keV) energy bands. The ChaMP data are consistent with previous results of ROSAT, ASCA and Chandra deep surveys. In particular, our data nicely fill in the flux gap in the hard band between the Chandra Deep Field data and the previous ASCA data. We check whether there is any systematic difference in the source density between cluster and non-cluster fields and also search for field-to-field variations, both of which have been previously reported. We found no significant field-to-field cosmic variation in either test within the statistics (~1 sigma) across the flux levels included in our sample. In the X-ray color-color plot, most sources fall in the location characterized by photon index = 1.5-2 and NH = a few x 10^20 cm^2, suggesting that they are typical broad-line AGNs. There also exist a considerable number of sources with peculiar X-ray colors (e.g., highly absorbed, very hard, very soft). We confirm a trend that on average the X-ray color hardens as the count rate decreases. Since the hardening is confined to the softest energy band (0.3-0.9 keV), we conclude it is most likely due to absorption. We cross-correlate the X-ray sources with other catalogs and describe their properties in terms of optical color, X-ray-to-optical luminosity ratio and X-ray colors.

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arXiv2003

An Unusual Discontinuity in the X-ray Surface Brightness Profile of NGC 507: Evidence of an Abundance Gradient?

We present results from a 45 ks Chandra/ACIS-I observation of the nearby ($z=0.01646$) NGC 507 group. The X-ray surface brightness profile of the outer region of the group is well described by an isothermal $β$-profile consistent with earlier ROSAT observations. We find a sharp edge or discontinuity in the radial surface brightness profile 55 kpc E and SE of NGC 507 covering a $\sim125^\circ$ arc. At the sharpest part of the discontinuity, the surface brightness declines by a factor of $\sim$2 over a distance of 6.9 kpc ($\sim 15''$). The inner and outer gas temperatures across the discontinuity differ by only about 0.2 keV (interior and exterior gas temperatures are 1.2 and 1.3 keV, respectively). Spectral analysis indicates that there is a large gradient in the elemental abundance across the discontinuity, and comparison with the low resolution NVSS radio maps suggests that the discontinuity is aligned with a low surface brightness radio lobe. We conclude that the appearance of this discontinuity is the result of the transport of high abundance material from the center of the galaxy by the expanding radio lobe, similar to structures seen in {\em Chandra} observations of Perseus A and Hydra A.

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arXiv2003

Chandra Temperature Map of Abell 754 and Constraints on Thermal Conduction

We use Chandra data to derive a detailed gas temperature map of the nearby, hot, merging galaxy cluster A754. Combined with the X-ray and optical images, the map reveals a more complex merger geometry than previously thought, possibly involving more than two subclusters or a cool gas cloud sloshing independently from its former host subcluster. In the cluster central region, we detect spatial variations of the gas temperature on all linear scales, from 100 kpc (the map resolution) and up, which likely remain from a merger shock passage. These variations are used to derive an upper limit on effective thermal conductivity on a 100 kpc scale, which is at least an order of magnitude lower than the Spitzer value. This constraint pertains to the bulk of the intracluster gas, as compared to the previously reported estimates for cold fronts (which are rather peculiar sites). If the conductivity in a tangled magnetic field is at the recently predicted higher values (i.e., about 1/5 of the Spitzer value), the observed suppression can be achieved, for example, if the intracluster gas consists of magnetically isolated domains.

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arXiv2002

AGNs among ROSAT Blank Field Sources

We selected a sample of sources with high X-ray over optical flux ratio from the ROSAT PSPC archives. Out of the 16 sources in our sample, one is a (possibly extreme) BL Lac and two are AGNs. Six more sources are tentatively classified as AGNs on the basis of their X-ray properties and of our optical-IR imaging. For one of the AGNs and for the AGN-like blanks we find a discrepancy between the lack of absorption indicated by the X-ray spectra and the presence of red counterparts in their error circles. We discuss various possibilities to reconcile the X-ray and optical information.

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arXiv2002

ROSAT-HRI investigation of the NGC 507 X-ray halo

We present an X-ray investigation of the elliptical galaxy NGC 507. We make use of archival Rosat HRI, PSPC and Chandra data, to connect the large-scale structure of the halo to the core morphology. Our analysis shows that the halo core (r<2-3 r_e) and the external halo (r>3 r_e) are characterised by different dynamical properties and suggests a different origin of the two components. The halo core has a complex morphology with a main X-ray emission peak, coincident with the center of the optical galaxy, and several secondary peaks. The spatial and spectral analysis of the central peak shows that this feature is produced by denser hot gas in the galaxy core. Our data support the scenario where the gas is kinetically heated by stellar mass losses. Comparison with previously published studies suggest that the core of X-ray extended galaxies is associated to the stellar distribution and has similar properties to the X-ray halo of compact galaxies. The secondary peaks are due instead to interactions between the radio-emitting plasma and the surrounding ISM. We found that the energy input by the central radio source in the ISM may be large enough to prevent gas from cooling. The total mass profile shows the presence of a dark matter halo extending on cluster scales which is likely to be associated with the whole cluster rather than with NGC 507. This structure is typical of many X-ray bright Early-Type galaxies and may explain the spatial and spectral differences with X-ray compact galaxies largely debated in literature. Finally we discuss the nature of the point sources detected in the HRI FOV.

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arXiv2001

Deep ROSAT-HRI observations of the NGC 1399/NGC 1404 region: morphology and structure of the X-ray halo

We present the analysis of a deep (167 ks) ROSAT HRI observation of the cD galaxy NGC 1399 in the Fornax cluster. Using both HRI and, at larger radii, archival PSPC data, we find that the radial behavior of the X-ray surface brightness profile is not consistent with a simple Beta model and suggests instead three distinct components. We use a multi-component bidimensional model to study in detail these three components that we identify respectively with the cooling flow region, the galactic and the cluster halo. From these data we derive a binding mass distribution in agreement with that suggested by optical dynamical indicators, with an inner core dominated by luminous matter and an extended dark halo differently distributed on galactic and cluster scales. The HRI data and a preliminary analysis of Chandra public data, allow us to detect significant density fluctuations in the halo. We discuss possible non-equilibrium scenarios to explain the hot halo structure, including tidal interactions with neighboring galaxies, ram stripping from the intra-cluster medium and merging events. In the innermost region of NGC 1399, the comparison between the X-ray and radio emission suggests that the radio emitting plasma is displacing and producing shocks in the hot X-ray emitting gas. We found that the NGC 1404 halo is well represented by a single symmetric Beta model and follows the stellar light profile within the inner 8 kpc. The mass distribution is similar to the `central' component of the NGC 1399 halo. At larger radii ram pressure stripping from the intra-cluster medium produces strong asymmetries in the gas distribution. Finally we discuss the properties of the point source population finding evidence of correlation between the source excess and NGC 1399.

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arXiv2001

Deep ROSAT-HRI observation of the elliptical galaxy NGC 1399

We present the preliminary results of a deep (167 ks) ROSAT HRI observation of the cD galaxy NGC1399 in the Fornax cluster. We find, in agreement with previous observations, an extended (41 Kpc adopting a distance of 19 Mpc) gaseous halo with a luminosity of L_X=(4.41\pm 0.04)x10^{41} erg/s. The 5 arcsec resolution of the data allows us to detect a very complex and asymmetric structure of the halo with respect to the optical galaxy. Moreover the analysis of the radial structure reveals the presence of a multi-component profile not consistent with a simple King model over the whole 40 Kpc. We do not detect the presence of a central source and pose an upper limit to the luminosity of a possible active nucleus. Due to the length of the observation, comparable to that of a deep survey, we detect a large number of sources within the HRI FOV, in slight excess with respect to the estimates based on previous surveys. We study the flux distribution of the sources, their temporal behaviour and their spatial distribution with respect to the central galaxy.

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arXiv2001

1WGAJ1226.9+3332: a high redshift cluster discovered by Chandra

We report the detection of 1WGAJ1226.9+3332 as an arcminute scale extended X-ray source with the Chandra X-ray Observatory. The Chandra observation and R and K band imaging strongly support the identification of 1WGAJ1226.9+3332 as a high redshift cluster of galaxies, most probably at z=0.85 +- 0.15, with an inferred temperature kT =10 (+4;-3) keV and an unabsorbed luminosity (in a r=120" aperture) of 1.3 (+0.16;-0.14) x 1e45 erg/s (0.5-10 keV). This indication of redshift is also supported by the K and R band imaging, and is in agreement with the spectroscopic redshift of 0.89 found by Ebeling et al. (2001). The surface brightness profile is consistent with a beta-model with beta=0.770 +- 0.025, rc=(18.1 +-0.9)" (corresponding to 101 +- 5 kpc at z=0.89), and S(0)=1.02 +- 0.08 counts/arcsec**2. 1WGAJ1226.9+3332 was selected as an extreme X-ray loud source with FX/FV>60; this selection method, thanks to the large area sampled, seems to be a highly efficient method for finding luminous high z clusters of galaxies.

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arXiv2001

Clusters of galaxies among ROSAT blank field sources

We present here an efficient method for selecting high luminosity and massive high redshift clusters of galaxies, crucially important tools in cosmology. By selecting bright and extremely X-ray loud (high F(X)/F(V)) sources, we were able to identify 2 high redshift (z=0.45 and z=0.89) clusters so far and we have evidence for at least one more candidate at 0.4<z<1.1 out of a total of 16 selected sources.

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arXiv2000

Chandra High-Resolution Camera Observations of the Luminous X-Ray Source in the Starburst Galaxy M82

We have analyzed Chandra High Resolution Camera observations of the starburst galaxy M82, concentrating on the most luminous x-ray source. We find a position for the source of $\rm R.A. = 09^h 55^m 50^s.2, decl. = +69^{\circ} 40\arcmin 46\arcsec.7$ (J2000) with a $1σ$ radial error of $0.7\arcsec$. The accurate x-ray position shows that the luminous source is not at the dynamical centre of M82 nor coincident with any suggested radio AGN candidate. The source is highly variable between observations, which suggests that the source is a compact object and not a supernova or remnant. There is no significant short term variability within the observations. Dynamical friction and the off-center position place an upper bound of $10^{5} - 10^{6} M_{\odot}$ on the mass of the object, depending on its age. The x-ray luminosity suggests a compact object mass of at least $500 M_{\odot}$. Thus, the luminous source in M82 may represent a new class of compact object with a mass intermediate between those of stellar mass black hole candidates and supermassive black holes.

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arXiv2000

The Chandra Multi-wavelength Project (ChaMP): a serendipitous survey with Chandra archival data

The launch of the Chandra X-ray Observatory in July 1999 opened a new era in X-ray astronomy. Its unprecedented, <0.5" spatial resolution and low background are providing views of the X-ray sky 10-100 times fainter than previously possible. We have initiated a serendipitous survey (ChaMP) using Chandra archival data to flux limits covering the range between those reached by current satellites and those of the small area Chandra deep surveys. We estimate the survey will cover ~5 sq.deg./year to X-ray fluxes (2-10 keV) in the range 1E(-13)-6E(-16) erg/cm^2/s discovering ~2000 new X-ray sources, ~80% of which are expected to be AGN. The ChaMP has two parts, the extragalactic survey (ChaMP) and the galactic plane survey (ChaMPlane). ChaMP promises profoundly new science return on a number of key questions at the current frontier of many areas of astronomy including (1) locating and studying high redshift clusters and so constraining cosmological parameters (2) defining the true population of AGN, including those that are absorbed, and so constraining the accretion history of the universe, (3) filling in the gap in the luminosity/redshift plane between Chandra deep and previous surveys in studying the CXRB, (4) studying coronal emission from late-type stars and (5) search for CVs and quiescent Low-Mass X-ray Binaries (qLXMBs) to measure their luminosity functions. In this paper we summarize the status, predictions and initial results from the X-ray analysis and optical imaging.

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arXiv2000

Chandra Observation of Abell 2142: Survival of Dense Subcluster Cores in a Merger

We use Chandra data to map the gas temperature in the central region of the merging cluster A2142. The cluster is markedly nonisothermal; it appears that the central cooling flow has been disturbed but not destroyed by a merger. The X-ray image exhibits two sharp, bow-shaped, shock-like surface brightness edges or gas density discontinuities. However, temperature and pressure profiles across these edges indicate that these are not shock fronts. The pressure is reasonably continuous across these edges, while the entropy jumps in the opposite sense to that in a shock (i.e. the denser side of the edge has lower temperature, and hence lower entropy). Most plausibly, these edges delineate the dense subcluster cores that have survived a merger and ram pressure stripping by the surrounding shock-heated gas.

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