148 research outputs found
Technology needs of advanced Earth observation spacecraft
Remote sensing missions were synthesized which could contribute significantly to the understanding of global environmental parameters. Instruments capable of sensing important land and sea parameters are combined with a large antenna designed to passively quantify surface emitted radiation at several wavelengths. A conceptual design for this large deployable antenna was developed. All subsystems required to make the antenna an autonomous spacecraft were conceptually designed. The entire package, including necessary orbit transfer propulsion, is folded to package within the Space Transportation System (STS) cargo bay. After separation, the antenna, its integral feed mast, radiometer receivers, power system, and other instruments are automatically deployed and transferred to the operational orbit. The design resulted in an antenna with a major antenna dimension of 120 meters, weighing 7650 kilograms, and operating at an altitude of 700 kilometers
Operational fitness of box truss antennas in response to dynamic slewing
A parametric study was performed to define slewing capability of large satellites along with associated system changes or subsystem weight and complexity impacts. The satellite configuration and structural arrangement from the Earth Observation Spacecraft (EOS) study was used as the baseline spacecraft. Varying slew rates, settling times, damping, maneuver frequencies, and attitude hold times provided the data required to establish applicability to a wide range of potential missions. The key elements of the study are: (1) determine the dynamic transient response of the antenna system; (2) calculate the system errors produced by the dynamic response; (3) determine if the antenna has exceeded operational requirements at completion of the slew, and if so; (4) determine when the antenna has settled to the operational requirements. The slew event is not considered complete until the antenna is within operational limits
X-ray Observations of Gravitationally Lensed Quasars; Evidence for a Hidden Quasar Population
The large magnification factors of gravitationally lensed (GL) systems allow
us to investigate the properties of quasars with X-ray luminosities that are
substantially lower than those of unlensed ones and also provide an independent
means of estimating the contribution of faint quasars to the hard X-ray
component of the cosmic X-ray background. Our spectral analysis indicate a
flattening of the spectral index in the hard band (4-20keV restframe) for 2
radio-loud quasars in the GL quasar sample for which the data have moderate
signal-to-noise ratio. We have identified a large fraction of Broad Absorption
Line (BAL) quasars amongst the GL quasar population. We find that approximately
35% of radio-quiet GL quasars contain BAL features which is significantly
larger than the 10% fraction of BAL quasars presently found in optically
selected flux limited quasar samples. We present a simple model that estimates
the effects of attenuation and lens magnification on the luminosity function of
quasars and that explains the observed fraction of GL BAL quasars. These
observations suggest that a large fraction of BAL quasars are missed from flux
limited optical surveys. Modeling of several X-ray observations of the GL BAL
quasar PG1115+080 suggests that the observed large X-ray variability may be
caused in part by a variable intrinsic absorber consistent with previously
observed variability of the BAL troughs in the UV band. The observed large
X-ray flux variations in PG1115+080 offer the prospect of considerably reducing
errors in determining the time delay with future X-ray monitoring of this
system and hence constraining the Hubble constant H.Comment: 19 pages, 10 figures, 9 Tables, accepted for publication in Ap
BATSE Observations of the Piccinotti Sample of AGN
BATSE Earth occultation data have been used to search for emission in the
20-100 keV band from all sources in the Piccinotti sample, which represents to
date the only complete 2-10 keV survey of the extragalactic sky down to a
limiting flux of 3.1 x 10^(-11) erg cm^(-2)$ s^(-1). Nearly four years of
observations have been analyzed to reach a 5sigma sensitivity level of about
7.8x 10^(-11) erg cm^(-2) s^(-1) in the band considered. Of the 36 sources in
the sample, 14 have been detected above 5sigma confidence level while marginal
detection (3<sigma<5) can be claimed for 13 sources; for 9 objects 2sigma upper
limits are reported. Comparison of BATSE results with data at higher energies
is used to estimate the robustness of our data analysis: while the detection
level of each source is reliable, the flux measurement maybe overestimated in
some sources by as much as 35%, probably due to incomplete data cleaning.
Comparison of BATSE fluxes with X-ray fluxes, obtained in the 2-10 keV range
and averaged over years, indicates that a canonical power law of photon index
1.7 gives a good description of the broad band spectra of bright AGNs and that
spectral breaks preferentially occur above 100 keV.Comment: 18 pages, 1 figure. Accepted for publication on Apj
A Huge Drop in X-ray Luminosity of the Non-Active Galaxy RXJ1242.6-1119A, and First Post-Flare Spectrum - Testing the Tidal Disruption Scenario
It has been suggested that an unavoidable consequence of the existence of
supermassive black holes, and the best diagnostic of their presence in
non-active galaxies, would be occasional tidal disruption of stars captured by
the black holes. These events manifest themselves in form of luminous flares
powered by accretion of debris from the disrupted star into the black hole.
Candidate events among optically non-active galaxies emerged in the past few
years. For the first time, we have looked with high spatial and spectral
resolution at one of these most extreme variability events ever recorded among
galaxies. Here, we report measuring a factor ~200 drop in luminosity of the
X-ray source RXJ 1242-1119 with the X-ray observatories Chandra and XMM-Newton,
and perform key tests of the favored outburst scenario, tidal disruption of a
star by a supermassive black hole. We show that the detected `low-state'
emission has properties such that it must still be related to the flare. The
power-law shaped post-flare X-ray spectrum indicates a `hardening' compared to
outburst. The inferred black hole mass, the amount of liberated energy, and the
duration of the event favor an accretion event of the form expected from the
(partial or complete) tidal disruption of a star (abstract abbreviated).Comment: to appear in March 1 issue of ApJ Letters (submitted Nov. 10,
accepted in Dec. 2003); background information available at
http://www.xray.mpe.mpg.de/~skomossa
Differences Between The Optical/Uv Spectra Of X-Ray Bright And X-Ray Faint QSOs
We contrast measurements of composite optical and ultraviolet (UV) spectra
constructed from samples of QSOs defined by their soft X-ray brightness. X-ray
bright (XB) composites show stronger emission lines in general, but
particularly from the narrow line region. The difference in the [OIII]/Hbeta
ratio is particularly striking, and even more so when blended FeII emission is
properly subtracted. The correlation of this ratio with X-ray brightness were
principal components of QSO spectral diversity found by Boroson & Green (1992).
We find here that other, much weaker narrow optical forbidden lines ([OII] and
NeV) are enhanced by factors of 2 to 3 in our XB composites, and that narrow
line emission is also strongly enhanced in the XB UV composite. Broad permitted
line fluxes are slightly larger for all XB spectra, but the narrow/broad line
ratio stays similar or increases strongly with X-ray brightness for all strong
permitted lines except Hbeta.
Spectral differences between samples divided by their relative X-ray
brightness (as measured by alpha_{ox}) exceed those seen between complementary
samples divided by luminosity or radio loudness. We propose that the Baldwin
effect may be a secondary correlation to the primary relationship between
alpha_{ox} and emission line equivalent width. We conclude that either 1)
equivalent width depends strongly on the SHAPE of the ionizing continuum, as
crudely characterized here by alpha_{ox} or 2) both equivalent width and
alpha_{ox} are related to some third parameter characterizing the QSO physics.
One such possibility is intrinsic warm absorption; a soft X-ray absorber
situated between the broad and narrow line regions can successfully account for
many of the properties observed.Comment: 16 pages including 3 figures, AAS latex, plus 4 tables totaling 5
pages, to appear in ApJ Vol. 498, May 1, 199
Detection of a possible multiphase ultra-fast outflow in IRAS 13349+2438 with NuSTAR and XMM-Newton
We present joint NuSTAR and XMM-Newton observations of the bright, variable
quasar IRAS 13349+2438. This combined dataset shows two clear iron absorption
lines at 8 and 9 keV, which are most likely associated with two layers of
mildly relativistic blueshifted absorption, with velocities of 0.14c and 0.27c.
We also find strong evidence for a series of Ly absorption lines at
intermediate energies in a stacked XMM-Newton EPIC-pn spectrum, at the same
blueshift as the lower velocity iron feature. This is consistent with a
scenario where an outflowing wind is radially stratified, so faster, higher
ionization material is observed closer to the black hole, and cooler, slower
material is seen from streamlines at larger radii.Comment: 5 pages, 2 figures, accepted for publication in MNRAS letter
XMM-Newton observations of the Narrow-Line Seyfert 1 galaxy Mrk 335 in an historical low X-ray flux state
We report the discovery of strong soft X-ray emission lines and a hard
continuum above 2 keV in the Narrow-Line Seyfert 1 galaxy Mrk 335 during an
extremely low X-ray flux state. Mrk 335 was observed for 22 ks by XMM-Newton in
July 2007 as a Target of Opportunity to examine it in its X-ray low-flux state,
which was discovered with Swift. Long-term light curves suggest that this is
the lowest flux state this AGN has ever been seen in. However, Mrk 335 is still
sufficiently bright that its X-ray properties can be studied in detail. The
X-ray continuum spectrum is very complex and requires several components to
model. Statistically, partial covering and blurred reflection models work well.
We confirm the presence of a strong narrow Fe line at 6.4 keV. High-resolution
spectroscopy with the XMM-RGS reveals strong, soft X-ray emission lines not
detected in previous, higher signal-to-noise, XMM-Newton observations, such as:
highly ionized Fe lines, O VII, Ne IX and Mg XI lines. The optical/UV fluxes
are similar to those previously measured with Swift. Optical spectroscopy taken
in 2007 September do not show any changes to optical spectra obtained 8 years
earlier.Comment: Accepted for publication in ApJ; 26 pages, 10 figues, in press, ApJ
681 (July 01, 2008); Updated version with corrections made by the edito
Evidence of thick obscuring matter revealed in the X-ray spectrum of the Z=4.28 quasar RXJ1028.6-0844
We report the discovery of an unambiguous, substantial low-energy cutoff in
the broad band X-ray spectrum of the radio quasar RXJ1028.6-0844 at a redshift
of 4.276 obtained with the ASCA satellite, which we preferably explained as
indication of excess X-ray absorption. The equivalent hydrogen column density
of the absorbing matter, depending on the redshift and metallicity, ranges from
2.5x10**21 cm**(-2) for local absorption up to 2.1x10**23 cm**(-2) (solar
metallicity) or 1.6x10**24 cm**(-2) (10% solar metallicity) for absorption at
the quasar redshift. Such a value is among the largest found for high-redshift
radio quasars. The absorption, if interpreted as being produced close to the
quasar, may indicate the presence of a remarkably large amount of obscuring
matter in the quasar environment in the early universe. Implications of the
result for the possible origins of the absorbing matter are discussed,
concerning especially galactic intervening matter, cool intracluster gas, and
ambient medium around the quasar jet. The quasar itself has an enormous
apparent luminosity of at least about 2.6x10**47 erg/s (H0=50, q0=0.5) and a
power law photon index of 1.67(+0.07,-0.04) in the 2-50keV band in the source
rest frame.Comment: Accepted by ApJ, 11 pages using emulateapj5.sty; minor changes (12
pages), subsection 3.4 added, references adde
A Large, Uniform Sample of X-ray Emitting AGN from the ROSAT All-Sky and Sloan Digital Sky Surveys: the Data Release 5 Sample
We describe further results of a program aimed to yield ~10^4 fully
characterized optical identifications of ROSAT X-ray sources. Our program
employs X-ray data from the ROSAT All-Sky Survey (RASS), and both optical
imaging and spectroscopic data from the Sloan Digital Sky Survey (SDSS).
RASS/SDSS data from 5740 deg^2 of sky spectroscopically covered in SDSS Data
Release 5 (DR5) provide an expanded catalog of 7000 confirmed quasars and other
AGN that are probable RASS identifications. Again in our expanded catalog, the
identifications as X-ray sources are statistically secure, with only a few
percent of the SDSS AGN likely to be randomly superposed on unrelated RASS
X-ray sources. Most identifications continue to be quasars and Seyfert 1s with
15<m<21 and 0.01<z<4; but the total sample size has grown to include very
substantial numbers of even quite rare AGN, e.g., now including several
hundreds of candidate X-ray emitting BL Lacs and narrow-line Seyfert 1
galaxies. In addition to exploring rare subpopulations, such a large total
sample may be useful when considering correlations between the X-ray and the
optical, and may also serve as a resource list from which to select the "best"
object (e.g., X-ray brightest AGN of a certain subclass, at a preferred
redshift or luminosity) for follow-on X-ray spectral or alternate detailed
studies.Comment: Accepted for publication in AJ; 32 pages, including 11 figures, and 6
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