We describe an online system for automated classification of X-ray sources, ClassX, and we present preliminary results of classification of the three major catalogs of ROSAT sources, ROSAT All-Sky ...Survey (RASS) Bright Source Catalog, RASS Faint Source Catalog, and WGACAT, into six class categories: stars, white dwarfs, X-ray binaries, galaxies, active galactic nuclei, and clusters of galaxies. ClassX is based on a machine-learning technology. It represents a system of classifiers, each classifier consisting of a considerable number of oblique decision trees. These trees are built as the classifier is "trained" to recognize various classes of objects using a training sample of sources of known object types. Each source is characterized by a preselected set of parameters, or attributes; the same set is then used as the classifier conducts classification of sources of unknown identity. The ClassX pipeline features an automatic search for X-ray source counterparts among heterogeneous data sets in online data archives using Virtual Observatory protocols; it retrieves from those archives all the attributes required by the selected classifier and inputs them to the classifier. The user input to ClassX is typically a file with target coordinates, optionally complemented with target IDs. The output contains the class name, attributes, and class probabilities for all classified targets. We discuss ways to characterize and assess the classifier quality and performance, and we present the respective validation procedures. On the basis of both internal validation and external verification, we conclude that the ClassX classifiers yield reasonable and reliable classifications for ROSAT sources and have the potential to broaden class representation significantly for rare object types.
We present X-ray and optical observations of the X-ray bright AGN 1ES 1927+654. The X-ray observations obtained with ROSAT and Chandra reveal persistent, rapid and large scale variations, as well ...as steep 0.1–2.4 keV ($\rm \Gamma = 2.6 \pm 0.3$) and 0.3–7.0 keV ($\rm \Gamma = 2.7 \pm 0.2$) spectra. The measured intrinsic neutral X-ray column density is approximately $\rm 7 \times 10^{20}\ cm^{-2}$. The X-ray timing properties indicate that the strong variations originate from a region, a few hundred light seconds from the central black hole, typical for type 1 AGN. High quality optical spectroscopy reveals a typical Seyfert 2 spectrum with some host galaxy contamination and no evidence of Fe II multiplets or broad hydrogen Balmer wings. The intrinsic optical extinction derived from the BLR and NLR are $A_V \ge 3.7$ and AV=1.7, respectively. The X-ray observations give an AV value of less than 0.58, in contrast to the optical extinction values. We discuss several ideas to explain this apparent difference in classification including partial covering, an underluminous BLR or a high dust to gas ratio.
We present the discovery of seven quasars at z > 5.7, selected from ~2000 deg2 of multicolor imaging data of the Sloan Digital Sky Survey. The new quasars have redshifts z from 5.79 to 6.13. Five are ...selected as part of a complete flux-limited sample in the SDSS northern Galactic cap; two have larger photometric errors and are not part of the complete sample. One of the new quasars, SDSS J1335+3533 (z = 5.93), exhibits no emission lines; the 3 s limit on the rest-frame equivalent width of the Lya+N V line is 5 A. It is the highest redshift lineless quasar known and could be a gravitational lensed galaxy, a BL Lac object, or a new type of quasar. Two new z > 6 quasars, SDSS 1250+3130 (z = 6.13) and SDSS J1137+3549 (z = 6.01), show deep Gunn-Peterson absorption gaps in Lya. These gaps are narrower than the complete Gunn-Peterson absorption troughs observed among quasars at z > 6.2 and do not have complete Lyb absorption.
We present the C4 Cluster Catalog, a new sample of 748 clusters of galaxies identified in the spectroscopic sample of the Second Data Release (DR2) of the Sloan Digital Sky Survey (SDSS). The C4 ...cluster-finding algorithm identifies clusters as overdensities in a seven-dimensional position and color space, thus minimizing projection effects that have plagued previous optical cluster selection. The present C4 catalog covers ~2600 deg2 of sky and ranges in redshift from z = 0.02 to 0.17. The mean cluster membership is 36 galaxies (with measured redshifts) brighter than r = 17.7, but the catalog includes a range of systems, from groups containing 10 members to massive clusters with over 200 cluster members with measured redshifts. The catalog provides a large number of measured cluster properties including sky location, mean redshift, galaxy membership, summed r-band optical luminosity (Lr), and velocity dispersion, as well as quantitative measures of substructure and the surrounding large-scale environment. We use new, multicolor mock SDSS galaxy catalogs, empirically constructed from the CDM Hubble Volume (HV) Sky Survey output, to investigate the sensitivity of the C4 catalog to the various algorithm parameters (detection threshold, choice of passbands, and search aperture), as well as to quantify the purity and completeness of the C4 cluster catalog. These mock catalogs indicate that the C4 catalog is 90% complete and 95% pure above M200 = 1 X 1014 h-1 M and within 0.03 <= z <= 0.12. Using the SDSS DR2 data, we show that the C4 algorithm finds 98% of X-ray-identified clusters and 90% of Abell clusters within 0.03 <= z <= 0.12. Using the mock galaxy catalogs and the full HV dark matter simulations, we show that the Lr of a cluster is a more robust estimator of the halo mass (M200) than the galaxy line-of-sight velocity dispersion or the richness of the cluster. However, if we exclude clusters embedded in complex large-scale environments, we find that the velocity dispersion of the remaining clusters is as good an estimator of M200 as Lr. The final C4 catalog will contain 2500 clusters using the full SDSS data set and will represent one of the largest and most homogeneous samples of local clusters.
The Swift Gamma-Ray Burst Mission Gehrels, N; Chincarini, G; Giommi, P ...
The Astrophysical journal,
08/2004, Letnik:
611, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient ...astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr super(-1) and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z > 10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a new-generation wide-field gamma-ray (15-150 keV) detector that will detect bursts, calculate 1arcmin-4arcmin positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5arc sec positions and perform spectroscopy in the 0.2-10 keV band; and a narrow-field UV/optical telescope that will operate in the 170-600 nm band and provide 0!!3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of approx1 mcrab (approx2 x 10 super(-11) ergs cm super(-2) s super(-1) in the 15-150 keV band), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. Swift transient data will be rapidly distributed to the astronomical community, and all interested observers are encouraged to participate in follow- up measurements. A Guest Investigator program for the mission will provide funding for community involvement. Innovations from the Swift program applicable to the future include (1) a large-area gamma-ray detector using the new CdZnTe detectors, (2) an autonomous rapid-slewing spacecraft, (3) a multiwavelength payload combining optical, X-ray, and gamma-ray instruments, (4) an observing program coordinated with other ground-based and space-based observatories, and (5) immediate multiwavelength data flow to the community. The mission is currently funded for 2 yr of operations, and the spacecraft will have a lifetime to orbital decay of approx8 yr.
An x-ray survey of the central region of the Large Magellanic Cloud reveals 45 individual astrophysical objects, 15 of which are new discoveries. The brightest object is a new and strongly variable ...low-mass x-ray binary.
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