507 research outputs found
Development of advanced digital techniques for data acquisition processing and communication Interim scientific report
Image correlation and computerized simulation applied to data acquisition and imaging technique
PCM telemetry data compression study, phase II Quarterly report, 25 Nov. 1965 - 25 Feb. 1966
Model analyses and computer simulations used in data compression study for improved pulse code modulation telemetry link
PCM telemetry data compression study, phase 1 Final report, 15 Sep. 1964 - 15 Aug. 1965
Pulse Code Modulation /PCM/ telemetry data compression study using S-6 Explorer XVII DAT
The development of advanced digital techniques for data acquisition processing and communication Interim scientific report
Digital techniques for characterization of image sources, second order interpolation, sensitivity of compressed pictures to error, and simulation of mapping technique
Evolution of the X-ray Emission of Radio-Quiet Quasars
We report new Chandra observations of seven optically faint, z \sim 4
radio-quiet quasars. We have combined these new observations with previous
Chandra observations of radio-quiet quasars to create a sample of 174 sources.
These sources have 0.1 < z < 4.7, and 10^{44} ergs s^{-1} < nu L_{nu} (2500
\AA) < 10^{48} ergs s^{-1}. The X-ray detection fraction is 90%. We find that
the X-ray loudness of radio-quiet quasars decreases with UV luminosity and
increases with redshift. The model that is best supported by the data has a
linear dependence of optical-to-X-ray ratio, alpha_{ox}, on cosmic time, and a
quadratic dependence of alpha_{ox} on log L_{UV}, where alpha_{ox} becomes
X-ray quiet more rapidly at higher log L_{UV}. We find no significant evidence
for a relationship between the X-ray photon index, Gamma_X, and the UV
luminosity, and we find marginally significant evidence that the X-ray
continuum flattens with increasing z (2 sigma). The Gamma_X-z anti-correlation
may be the result of X-ray spectral curvature, redshifting of a Compton
reflection component into the observed Chandra band, and/or redshifting of a
soft excess out of the observed Chandra band. Using the results for Gamma_X, we
show that the alpha_{ox}-z relationship is unlikely to be a spurious result
caused by redshifting of the observable X-ray spectral region. A correlation
between alpha_{ox} and z implies evolution of the accretion process. We present
a qualitative comparison of these new results with models for accretion disk
emission.Comment: Accepted by ApJ, 48 pages, 10 figures, 5 table
Chandra Discovery of a 300 kpc X-ray Jet in the GPS Quasar PKS1127-145
We have discovered an X-ray jet with Chandra imaging of the z=1.187
radio-loud quasar PKS1127-145. In this paper we present the Chandra X-ray data,
follow-up VLA observations, and optical imaging using the HST WFPC2. The X-ray
jet contains 273+/-5 net counts in 27ksec and extends ~30 arcsec, from the
quasar core, corresponding to a minimum projected linear size of ~330/h_50 kpc.
The evaluation of the X-ray emission processes is complicated by the observed
offsets between X-ray and radio brightness peaks. We discuss the problems posed
by these observations to jet models. In addition, PKS1127-145 is a Giga-Hertz
Peaked Spectrum radio source, a member of the class of radio sources suspected
to be young or ``frustrated'' versions of FRI radio galaxies. However the
discovery of an X-ray and radio jet extending well outside the host galaxy of
PKS1127-145 suggests that activity in this and other GPS sources may be
long-lived and complex.Comment: 22 pages, 11 ps figures, 1 figure in a JPG file, 3 tables. AASTEX.
Accepted by The Astrophysical Journa
Observational Constraints on the Dependence of Radio-Quiet Quasar X-ray Emission on Black Hole Mass and Accretion Rate
In this work we use a sample of 318 radio-quiet quasars (RQQ) to investigate
the dependence of the ratio of optical/UV flux to X-ray flux, alpha_ox, and the
X-ray photon index, Gamma_X, on black hole mass, UV luminosity relative to
Eddington, and X-ray luminosity relative to Eddington. Our sample is drawn from
the SDSS, with X-ray data from ROSAT and Chandra, and optical data mostly from
the SDSS; 153 of these sources have estimates of Gamma_X from Chandra. We
estimate M_BH using standard estimates derived from the Hbeta, Mg II, and C IV
broad emission lines. Our sample spans a broad range in black hole mass (10^6 <
M_BH / M_Sun < 10^10) and redshift (z < 4.8). We find that alpha_ox increases
with increasing M_BH and L_UV / L_Edd, and decreases with increasing L_X /
L_Edd. In addition, we confirm the correlation seen in previous studies between
Gamma_X and M_BH and both L_UV / L_Edd and L_X / L_Edd; however, we also find
evidence that the dependence of Gamma_X of these quantities is not monotonic,
changing sign at M_BH ~ 3 x 10^8 M_Sun. We argue that the alpha_ox correlations
imply that the fraction of bolometric luminosity emitted by the accretion disk,
as compared to the corona, increases with increasing accretion rate relative to
Eddington. In addition, we argue that the Gamma_X trends are caused by a
dependence of X-ray spectral index on accretion rate. We discuss our results
within the context of accretion models with comptonizing corona, and discuss
the implications of the alpha_ox correlations for quasar feedback. To date,
this is the largest study of the dependence of RQQ X-ray parameters on black
hole mass and related quantities, and the first to attempt to correct for the
large statistical uncertainty in the broad line mass estimates.Comment: Accepted by ApJ, 23 pages, 15 figures, emulateapj styl
LBT Discovery of a Yellow Supergiant Eclipsing Binary in the Dwarf Galaxy Holmberg IX
In a variability survey of M81 using the Large Binocular Telescope we have
discovered a peculiar eclipsing binary (MV ~ -7.1) in the field of the dwarf
galaxy Holmberg IX. It has a period of 272 days and the light curve is well-fit
by an overcontact model in which both stars are overflowing their Roche lobes.
It is composed by two yellow supergiants (V-I ~ 1 mag, T_eff = 4800 K), rather
than the far more common red or blue supergiants. Such systems must be rare.
While we failed to find any similar systems in the literature, we did, however
note a second example. The SMC F0 supergiant R47 is a bright (MV ~ -7.5)
periodic variable whose All Sky Automated Survey (ASAS) light curve is well-fit
as a contact binary with a 181 day period. We propose that these systems are
the progenitors of supernovae like SN 2004et and SN 2006ov, which appeared to
have yellow progenitors. The binary interactions (mass transfer, mass loss)
limit the size of the supergiant to give it a higher surface temperature than
an isolated star at the same core evolutionary stage. We also discuss the
possibility of this variable being a long-period Cepheid.Comment: ApJ Letters, accepted, 15 pages, 4 figures, 1 table, and 1 photometry
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