515 research outputs found
Nimbus-7 (-G) post launch report: Mission success
Nimbus-7, the last of the Nimbus series satellites, was launched from the Space and Missile Test Center at Vandenberg Air Force Base, California on October 24, 1978. The purpose of the mission was to collect global data of the Earth's atmosphere, oceans and polar ice with a payload of eight interdisciplinary research experiments. These experiments represent both domestic and international, scientific and governmental communities
Supercomputer networking for space science applications
The initial design of a supercomputer network topology including the design of the communications nodes along with the communications interface hardware and software is covered. Several space science applications that are proposed experiments by GSFC and JPL for a supercomputer network using the NASA ACTS satellite are also reported
Simultaneous X-ray and Ultraviolet spectroscopy of the Seyfert galaxy NGC 5548. III. X-ray time variability
The Seyfert 1 galaxy NGC 5548 was observed for a week by Chandra using both
the HETGS and LETGS spectrometers. In this paper we study the time variability
of the continuum radiation. During our observation, the source showed a gradual
increase in flux over four days, followed by a rapid decrease and flattening of
the light curve afterwards. Superimposed upon these relatively slow variations
several short duration bursts or quasi-periodic oscillations occured with a
typical duration of several hours and separation between 0.6-0.9 days. The
bursts show a delay of the hard X-rays with respect to the soft X-rays of a few
hours. We interprete these bursts as due to a rotating, fluctuating hot spot at
approximately 10 gravitational radii; the time delay of the hard X-rays from
the bursts agree with the canonical picture of Inverse Compton scattering of
the soft accretion disk photons on a hot medium that is relatively close to the
central black hole.Comment: 6 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. I. ASCA Observations and the Variability of the X-ray Spectral Components
We present a 35 day ASCA observation of the NLS1 Akn 564, which was part of a
multiwavelength AGN Watch monitoring campaign. Akn 564 shows a photon index
varying across the range 2.45--2.72. The presence of the soft hump component
below 1 keV, previously detected in ASCA data, is confirmed. Time-resolved
spectroscopy with ~daily sampling reveals a distinction in the variability of
the soft hump and power-law components over a timescale of weeks, with the hump
varying by a factor of 6 across the 35-day observation compared to a factor 4
in the power-law. Flux variations in the power-law component are measured down
to a timescale of ~1000s and accompanying spectral variability suggests the
soft hump is not well-correlated with the power-law on such short timescales.
We detect Fe Ka and a blend of Fe Kb plus Ni Ka, indicating an origin in highly
ionized gas. Variability measurements constrain the bulk of the Fe Ka to
originate within a light week of the nucleus. The large EW of the emission
lines may be due to high metallicity in NLS1s, supporting some evolutionary
models for AGN.Comment: 41 pages, 15 figures. Accepted for publication in the Astrophysical
Journal (v3 has final fixes for publication
A search for periodicity in the light curves of selected blazars
We present an analysis of multifrequency light curves of the sources 2223-052
(3C 446), 2230+114 (CTA 102), and 2251+158 (3C 454.3), which had shown evidence
of quasi-periodic activity. The analysis made use of data from the University
of Michican Radio Astronomy Observatory (USA) at 4.8, 8, and 14.5 GHz, as well
as the Metsahovi Radio Astronomy Observatory (Finland) at 22 and 37 GHz.
Application of two different methods (the discrete autocorrelation function and
the method of Jurkevich) both revealed evidence for periodicity in the flux
variations of these sources at essentially all frequencies. The periods derived
for at least two of the sources -- 2223-052 and 2251+158-- are in good
agreement with the time interval between the appearance of successive VLBI
components. The derived periods for 2251+158 (P = 12.4 yr and 2223-052 (P = 5.8
yr) coincide with the periods found earlier by other authors based on optical
light curves.Comment: 27 pages, 11 figures, accepted for publication in Astronomy Report
New constraints on the continuum-emission mechanism of AGN: Intensive monitoring of NGC 7469 in the X-ray and ultraviolet
We have undertaken near-continuous monitoring of the Seyfert 1 galaxy NGC
7469 in the X-ray with RXTE over a ~30d baseline. The source shows strong
variability with a root-mean-square (rms) amplitude of ~16 per cent, and
peak-to-peak variations of a factor of order 2. Simultaneous data over this
period were obtained in the ultraviolet (UV) using IUE, making this the most
intensive X-ray UV/X-ray variability campaign performed for any active galaxy.
Comparison of the continuum light curves reveals very similar amplitudes of
variability, but different variability characteristics, with the X-rays showing
much more rapid variations. The data are not strongly correlated at zero lag.
The largest absolute value of the correlation coefficient occurs for an
anticorrelation between the two bands, with the X-ray variations leading the UV
by ~4d. The largest positive correlation is for the ultraviolet to lead the
X-rays by ~4d. Neither option appears to be compatible with any simple
interband transfer function. The peak positive correlation at ~4d occurs
because the more prominent peaks in the UV light curve appear to lead those in
the X-rays by this amount. However, the minima of the light curves are
near-simultaneous. These observations provide new constraints on theoretical
models of the central regions of active galactic nuclei. Models in which the
observed UV emission is produced solely by re-radiation of absorber X-rays are
ruled out by our data, as are those in which the X-rays are produced solely by
Compton upscattering of the observed UV component by a constant distribution of
particles.Comment: 33 pages, 8 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journal. Also available via
http://lheawww.gsfc.nasa.gov/users/nandra/pubs/7469/abstract.htm
A Spectroscopic and Photometric Study of Short-Timescale Variability in NGC5548
Results of a ground-based optical monitoring campaign on NGC5548 in June 1998
are presented. The broad-band fluxes (U,B,V), and the spectrophotometric
optical continuum flux F_lambda(5100 A) monotonically decreased in flux while
the broad-band R and I fluxes and the integrated emission-line fluxes of Halpha
and Hbeta remained constant to within 5%. On June 22, a short continuum flare
was detected in the broad band fluxes. It had an amplitude of about ~18% and it
lasted only ~90 min. The broad band fluxes and the optical continuum
F_lambda(5100 A) appear to vary simultaneously with the EUV variations. No
reliable delay was detected for the broad optical emission lines in response to
the EUVE variations. Narrow Hbeta emission features predicted as a signature of
an accretion disk were not detected during this campaign. However, there is
marginal evidence for a faint feature at lambda = 4962 A with FWHM=~6 A
redshifted by Delta v = 1100 km/s with respect to Hbeta_narrow.Comment: 12 pages, 7 figures, accepted for publishing in A&
Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. II. Ultraviolet Continuum and Emission-line Variability
We present results of an intensive two-month campaign of approximately daily
spectrophotometric monitoring of the narrow-line Seyfert 1 galaxy Akn 564 with
HST. The fractional variability amplitude of the continuum variations between
1365-3000 A is ~6%, about a factor 3 less than that found in typical Seyfert 1
galaxies over a similar period of time. However, large amplitude, short
time-scale flaring behavior is evident, with trough-to-peak flux changes of
about 18% in approximately 3 days. We present evidence for wavelength-dependent
continuum time delays, with the variations at 3000 A lagging behind those at
1365 A by about 1 day. These delays may be interpreted as evidence for a
stratified continuum reprocessing region, possibly an accretion-disk structure.
The Lyman-alpha 1216 emission-line exhibits flux variations of about 1%
amplitude.Comment: 27 pages, 14 figures. Accepted by Astrophysical Journa
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