427 research outputs found
Comment on "1/f noise in the Bak-Sneppen model"
Contrary to the recently published results by Daerden and Vanderzande [Phys.
Rev. E 53, 4723 (1996)], we show that the time correlation function in the
random-neighbor version of the Bak-Sneppen model can be well approximated by an
exponential giving rise to a 1/f2 power spectrum.Comment: 2 pages, 2 figure
Far-Ultraviolet Observations of NGC 3516 using the Hopkins Ultraviolet Telescope
We observed the Seyfert 1 galaxy NGC 3516 twice during the flight of Astro-2
using the Hopkins Ultraviolet Telescope in March 1995. Simultaneous X-ray
observations were performed with ASCA. Our far-ultraviolet spectra cover the
spectral range 820-1840 A with a resolution of 2-4 A. No significant variations
were found between the two observations. The total spectrum shows a red
continuum, , with an observed flux of at 1450 A, slightly above the historical
mean. Intrinsic absorption in Lyman is visible as well as absorption
from O~vi 1032,1038, N~v 1239,1243, Si~iv 1394,1403, and C~iv 1548,1551. The UV
absorption lines are far weaker than is usual for NGC~3516, and also lie closer
to the emission line redshift rather than showing the blueshift typical of
these lines when they are strong. The neutral hydrogen absorption, however, is
blueshifted by relative to the systemic velocity, and it is
opaque at the Lyman limit. The sharpness of the cutoff indicates a low
effective Doppler parameter, . For
the derived intrinsic column is . As in
NGC~4151, a single warm absorber cannot produce the strong absorption visible
over the wide range of observed ionization states. Matching both the UV and
X-ray absorption simultaneously requires absorbers spanning a range of
in both ionization parameter and column density.Comment: 18 pages, 4 PostScript figures, uses aaspp4.sty To appear in the
August 20, 1996, issue of The Astrophysical Journa
Network of Earthquakes and Recurrences Therein
We quantify the correlation between earthquakes and use the same to
distinguish between relevant causally connected earthquakes. Our correlation
metric is a variation on the one introduced by Baiesi and Paczuski (2004). A
network of earthquakes is constructed, which is time ordered and with links
between the more correlated ones. Data pertaining to the California region has
been used in the study. Recurrences to earthquakes are identified employing
correlation thresholds to demarcate the most meaningful ones in each cluster.
The distribution of recurrence lengths and recurrence times are analyzed
subsequently to extract information about the complex dynamics. We find that
the unimodal feature of recurrence lengths helps to associate typical rupture
lengths with different magnitude earthquakes. The out-degree of the network
shows a hub structure rooted on the large magnitude earthquakes. In-degree
distribution is seen to be dependent on the density of events in the
neighborhood. Power laws are also obtained with recurrence time distribution
agreeing with the Omori law.Comment: 17 pages, 5 figure
Experimental evidence of accelerated seismic release without critical failure in acoustic emissions of compressed nanoporous materials
The total energy of acoustic emission (AE) events in externally stressed
materials diverges when approaching macroscopic failure. Numerical and
conceptual models explain this accelerated seismic release (ASR) as the
approach to a critical point that coincides with ultimate failure. Here, we
report ASR during soft uniaxial compression of three silica-based (SiO)
nanoporous materials. Instead of a singular critical point, the distribution of
AE energies is stationary and variations in the activity rate are sufficient to
explain the presence of multiple periods of ASR leading to distinct brittle
failure events. We propose that critical failure is suppressed in the AE
statistics by dissipation and transient hardening. Some of the critical
exponents estimated from the experiments are compatible with mean field models,
while others are still open to interpretation in terms of the solution of
frictional and fracture avalanche models.Comment: preprint, Main article: 7 pages, 3 figures. Supplementary material
included in \anc folder: 6 pages, 3 figure
Far-UV Observations of NGC 4151 during the ORFEUS-SPAS II Mission
We observed the Seyfert 1 galaxy NGC 4151 on eleven occasions at 1-2 day
intervals using the Berkeley spectrometer during the ORFEUS-SPAS II mission in
1996 November. The mean spectrum covers 912-1220 A at ~0.3 A resolution with a
total exposure of 15,658 seconds. The mean flux at 1000 A was 4.7e-13
erg/cm^2/s/A. We identify the neutral hydrogen absorption with a number of
components that correspond to the velocity distribution of \ion{H}{1} seen in
our own Galaxy as well as features identified in the CIV 1549 absorption
profile by Weymann et al. The main component of neutral hydrogen in NGC 4151
has a total column density of log N_HI = 18.7 +/- 1.5 cm^{-2} for a Doppler
parameter b=250 +/- 50 km/s, and it covers 84 +/- 6% of the source. This is
consistent with previous results obtained with the Hopkins Ultraviolet
Telescope. Other intrinsic far-UV absorption features are not resolved, but the
CIII* 1176 absorption line has a significantly higher blueshift relative to NGC
4151 than the CIII 977 resonance line. This implies that the highest velocity
region of the outflowing gas has the highest density. Variations in the
equivalent width of the CIII* 1176 absorption line anticorrelate with continuum
variations on timescales of days. For an ionization timescale <1 day, we set an
upper limit of 25 pc on the distance of the absorbing gas from the central
source. The OVI 1034 and HeII 1085 emission lines also vary on timescales of
1-2 days, but their response to the continuum variations is complex. For some
continuum variations they show no response, while for others the response is
instantaneous to the limit of our sampling interval.Comment: 4 pages, 2 PostScript figures, uses emulateapj.sty, apjfonts.sty. To
appear in the Astrophysical Journal (Letters) special issue for ORFEU
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