Showing papers by "Margarita Karovska published in 2004"

Chandra Multiwavelength Project. I. First X‐Ray Source Catalog

Abstract: The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei, high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-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 less than 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.

Chandra Multiwavelength Project. II. First Results of X-Ray Source Properties

Abstract: The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei [AGNs] and high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-1 cm-2) intermediate between the Chandra Deep Field surveys and previous missions. We present the first results of 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 X-ray-bright 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 Field 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 noncluster fields and also search for field-to-field variation, both of which have been previously reported. We found no significant field-to-field cosmic variation in either test within the statistics (~1 σ) 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 × 1020 cm2, suggesting that they are typical broadline 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 that 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.

Molecular Hydrogen Emission Lines in Far Ultraviolet Spectroscopic Explorer Observations of Mira B

Abstract: We present new Far Ultraviolet Spectroscopic Explorer (FUSE) observations of Mira A's wind-accreting companion star, Mira B. We find that the strongest lines in the FUSE spectrum are H2 lines fluoresced by H I Lyα. A previously analyzed Hubble Space Telescope (HST) spectrum also shows numerous Lyα-fluoresced H2 lines. The HST lines are all Lyman band lines, while the FUSE H2 lines are mostly Werner band lines, many of them never before identified in an astrophysical spectrum. We combine the FUSE and HST data to refine estimates of the physical properties of the emitting H2 gas. We find that the emission can be reproduced by an H2 layer with a temperature and column density of T = 3900 K and log N(H2) = 17.1, respectively. Another similarity between the HST and FUSE data, besides the prevalence of H2 emission, is the surprising weakness of the continuum and high-temperature emission lines, suggesting that accretion onto Mira B has weakened dramatically. The UV fluxes observed by HST on 1999 August 2 were previously reported to be over an order of magnitude lower than those observed by HST and the International Ultraviolet Explorer (IUE) from 1979-1995. Analysis of the FUSE data reveals that Mira B was still in a similarly low state on 2001 November 22.

An Error Analysis of the Geometric Baade-Wesselink Method

Abstract: We derive an analytic solution for the minimization problem in the geometric Baade-Wesselink method. This solution allows deriving the distance and mean radius of a pulsating star by fitting its velocity curve and angular diameter measured interferometrically. The method also provides analytic solutions for the confidence levels of the best-fit parameters and accurate error estimates for the Baade-Wesselink solution. Special care is taken in the analysis of the various error sources in the final solution, among which are the uncertainties due to the projection factor, the limb darkening, and the velocity curve. We also discuss the importance of the phase shift between the stellar light curve and the velocity curve as a potential error source in the geometric Baade-Wesselink method. We finally discuss the case of the classical Cepheid ζ Gem, applying our method to the measurements derived with the Palomar Testbed Interferometer. We show how a careful treatment of the measurement errors can be used to discriminate between different models of limb darkening by using interferometric techniques.

Calibration of Chandra's near on-axis optical performance

Abstract: The Chandra X-Ray telescope has excellent angular resolution for on-axis sources. While the Wolter type I design optimizes on-axis performance, there is a relatively large region in which 0.5" to 1" imaging is possible. The Chandra PSF was first characterized during ground calibration and, most recently, from actual on-orbit measurements. The ground calibration provided data with the highest signal to noise but, because of gravity-induced distortions of the optics, could not completely characterize their performance. We present the results of on-orbit calibrations of the optics' performance, focusing on the near on-axis field of view. We present for the first time an analysis of the energy dependence of the on-orbit PSF.

Quasar Winds as Dust Factories at High Redshift

Abstract: Winds from AGN and quasars will form large amounts of dust, as the cool gas in these winds passes through the (pressure, temperature) region where dust is formed in AGB stars. Conditions in the gas are benign to dust at these radii. As a result quasar winds may be a major source of dust at high redshifts, obviating a difficulty with current observations, and requiring far less dust to exist at early epochs.