1,216 research outputs found
SDSS AGNs with X-ray Emission from ROSAT PSPC Pointed Observations
We present a sample of 1744 type 1 active galactic nuclei (AGNs) from the
Sloan Digital Sky Survey (SDSS DR4) spectroscopic catalog with X-ray
counterparts in the White-Giommi-Angelini Catalog (WGACAT) of ROSAT PSPC
pointed observations. Of 1744 X-ray sources, 1410 (80.9%) are new AGN
identifications. Of 4574 SDSS DR4 AGNs for which we found radio matches in the
catalog of radio sources from the FIRST catalog, 224 turned up in our sample of
SDSS X-ray AGNs. The sample objects are given in a catalog that contains
optical and X-ray parameters along with radio emission parameters where
available. We illustrate the content of our catalog and its potential for AGN
science by providing statistical relationships for the catalog data. The
potential of the morphological information is emphasized by confronting the
statistics of optically resolved and unresolved AGNs. The immediate properties
of the catalog objects include significant correlation of X-ray and optical
fluxes, which is consistent with expectations. Also expected is the decrease of
X-ray flux toward higher redshifts. The X-ray to optical flux ratio for the
unresolved AGNs exhibits a decline toward higher redshifts, in agreement with
previous results. The resolved AGNs, however, display the opposite trend. At a
given optical brightness, X-ray fluxes of radio-quiet AGNs by a factor of 2. We
caution, however, that because of the variety of selection effects present in
both the WGACAT and the SDSS, the interpretation of any relationships based on
our sample of X-ray AGNs requires a careful analysis of these effects.Comment: 34 pages, 18 figure
Detection of [OI] 6300 and Other Diagnostic Emission Lines in the Diffuse Ionized Gas of M33 with Gemini-North
We present spectroscopic observations of diffuse ionized gas (DIG) in M33
near the HII region NGC 604. We present the first detection of [OI] 6300 in the
DIG of M33, one of the critical lines for distinguishing photo- from shock
ionization models. We measure [OI]/Ha in the range of 0.04 to 0.10 and an
increase in this ratio with decreasing emission measure. Our measurements of
[SII]/Ha and [NII]/Ha also rise with decreasing emission measure, while our
[OIII]/Hb measurements remain fairly constant. We have one tentative detection
of He I in the region of brightest emission measure, with a ratio of He I/Ha =
0.033 +- 0.019, indicating that the helium is at least partially ionized. We
compare our observed emission line ratios to photoionization models and find
that field star ionization models do not fit our data well. Leaky HII region
models are consistent with our data, without the need to invoke additional
ionization mechanisms to fit our [OI] or [OIII] measurements. The closest large
HII region is NGC 604 and is therefore a likely candidate for the source of the
ionizing photons for the gas in this region.Comment: 12 pages, 4 figures, accepted by ApJ
Optical variability of the strong-lined and X-ray bright source 1WGA J0447.9-0322
We present the historic light curve of 1WGA J0447.9-0322, spanning the time
interval from 1962 to 1991, built using the Asiago archive plates. The source
shows small fluctuations of about 0.3 mag around B=16 until 1986 and a fast
dimming of its average level by about 0.5 mag after that date, again with small
short term variations. The variability pattern is within the values shown by
other QSOs with long term monitoring, notwithstanding its high X-ray/optical
ratio. We present also its overall SED using literature data and recent
UV-optical SWIFT observations.Comment: 17 pages, 4 figures, accepted by The Astronomical Journal. Table 2
available upon reques
A ROSAT Survey of Contact Binary Stars
Contact binary stars are common variable stars which are all believed to emit
relatively large fluxes of x-rays. In this work we combine a large new sample
of contact binary stars derived from the ROTSE-I telescope with x-ray data from
the ROSAT All-Sky Survey (RASS) to estimate the x-ray volume emissivity of
contact binary stars in the galaxy. We obtained x-ray fluxes for 140 contact
binaries from the RASS, as well as 2 additional stars observed by the
XMM-Newton observatory. From these data we confirm the emission of x-rays from
all contact binary systems, with typical luminosities of approximately 1.0 x
10^30 erg s^-1. Combining calculated luminosities with an estimated contact
binary space density, we find that contact binaries do not have strong enough
x-ray emission to account for a significant portion of the galactic x-ray
background.Comment: 19 pages, 5 figures, accepted by A
Risk and resiliency factors in posttraumatic stress disorder
BACKGROUND: Not everyone who experiences a trauma develops posttraumatic stress disorder (PTSD). The aim of this study was to determine the risk and resiliency factors for this disorder in a sample of people exposed to trauma. METHOD: Twenty-five people who had developed PTSD following a trauma and 27 people who had not were asked to complete the Posttraumatic Stress Diagnostic Scale, the Coping Inventory for Stressful Situations, and the State-Trait Anxiety Inventory. In addition, they completed a questionnaire to provide information autobiographic and other information. ANALYSIS: Five variables that discriminated significantly between the two groups using chi-square analysis or t-tests were entered into a logistic regression equation as predictors, namely, being female, perceiving a threat to one's life, having a history of sexual abuse, talking to someone about the event, and the "intentionality" of the trauma. RESULTS: Only being female and perceiving a threat to one's life were significant predictors of PTSD. Taking base rates into account, 96.0% of participants with PTSD were correctly classified as having the disorder and 37.0% of participants without PTSD were correctly classified as not having the disorder, for an overall success rate of 65.4% CONCLUSIONS: Because women are more likely than men to develop PTSD, more preventive measures should be directed towards them. The same is true for trauma victims (of both sexes) who feel that their life was in dange
Source Matching in the SDSS and RASS: Which Galaxies are Really X-ray Sources?
The current view of galaxy formation holds that all massive galaxies harbor a
massive black hole at their center, but that these black holes are not always
in an actively accreting phase. X-ray emission is often used to identify
accreting sources, but for galaxies that are not harboring quasars
(low-luminosity active galaxies), the X-ray flux may be weak, or obscured by
dust. To aid in the understanding of weakly accreting black holes in the local
universe, a large sample of galaxies with X-ray detections is needed. We
cross-match the ROSAT All Sky Survey (RASS) with galaxies from the Sloan
Digital Sky Survey Data Release 4 (SDSS DR4) to create such a sample. Because
of the high SDSS source density and large RASS positional errors, the
cross-matched catalog is highly contaminated by random associations. We
investigate the overlap of these surveys and provide a statistical test of the
validity of RASS-SDSS galaxy cross-matches. SDSS quasars provide a test of our
cross-match validation scheme, as they have a very high fraction of true RASS
matches. We find that the number of true matches between the SDSS main galaxy
sample and the RASS is highly dependent on the optical spectral classification
of the galaxy; essentially no star-forming galaxies are detected, while more
than 0.6% of narrow-line Seyferts are detected in the RASS. Also, galaxies with
ambiguous optical classification have a surprisingly high RASS detection
fraction. This allows us to further constrain the SEDs of low-luminosity active
galaxies. Our technique is quite general, and can be applied to any
cross-matching between surveys with well-understood positional errors.Comment: 10 pages, 10 figures, submitted to The Astronomical Journal on 19
June 200
Cluster Evolution in the ROSAT North Ecliptic Pole Survey
The deepest region of the ROSAT All-Sky Survey, at the North Ecliptic Pole,
has been studied to produce a complete and unbiased X-ray selected sample of
clusters of galaxies. This sample is used to investigate the nature of cluster
evolution and explore potential implications for large-scale structure models.
The survey is 99.6% optically identified. Spectroscopic redshifts have been
measured for all the extragalactic identifications. In this Letter, first
results on cluster evolution are presented based on a comparison between the
number of the observed clusters in the North Ecliptic Pole survey and the
number of expected clusters assuming no-evolution models. At z>0.3 there is a
deficit of clusters with respect to the local universe which is significant at
> 4.7sigma. The evolution appears to commence at L_{0.5-2.0} > 1.8x10^{44} erg
s^{-1} in our data. The negative evolution goes in the same direction as the
original EMSS result, the results from the 160 deg^{2} survey by Vikhlinin et
al. (1998) and the recent results from the RDCS (Rosati et al. 2000). At lower
redshifts there is no evidence for evolution, a result in agreement with these
and other cluster surveys.Comment: 17 pages, 3 figures. Accepted for publication in ApJ Letter
The LuckyCam Survey for Very Low Mass Binaries II: 13 new M4.5-M6.0 Binaries
We present results from a high-angular-resolution survey of 78 very low mass
(VLM) binary systems with 6.0 = 0.15
arcsec/yr. 21 VLM binaries were detected, 13 of them new discoveries. The new
binary systems range in separation between 0.18 arcsec and 1.3 arcsec. The
distance-corrected binary fraction is 13.5% (+6.5%/-4%), in agreement with
previous results. 9 of the new binary systems have orbital radii > 10 AU,
including a new wide VLM binary with 27 AU projected orbital separation. One of
the new systems forms two components of a 2300 AU separation triple system. We
find that the orbital radius distribution of the binaries with V-K < 6.5 in
this survey appears to be different from that of redder (lower-mass) objects,
suggesting a possible rapid change in the orbital radius distribution at around
the M5 spectral type. The target sample was also selected to investigate X-ray
activity among VLM binaries. There is no detectable correlation between excess
X-Ray emission and the frequency and binary properties of the VLM systems.Comment: 11 pages, 8 figures. Submitted to MNRA
The Asiago-ESO/RASS QSO Survey II. The Southern Sample
This is the second paper of a series describing the Asiago-ESO/RASS QSO
survey, a project aimed at the construction of an all-sky statistically
well-defined sample of very bright QSOs (B_J < 15). Such a survey is required
to remove the present uncertainties about the properties of the local QSO
population and constitutes an homogeneous database for detailed evolutionary
studies of AGN. We present here the complete Southern Sample, which comprises
243 bright (12.60 < B_J < 15.13) QSO candidates at high galactic latitudes
(|b_{gal}| > 30^{\circ}). The area covered by the survey is 5660 sq. deg.
Spectroscopy for the 137 still unidentified objects has been obtained. The
total number of AGN turns out to be 111, 63 of which are new identifications.
The properties of the selection are discussed. The completeness and the success
rate for this survey at the final stage are 63% and 46%, respectively.Comment: 36 pages Latex, with 15 PostScript figures. Accepted for publication
in Astronomical Journa
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