149 research outputs found
Atmospheric electrical modeling in support of the NASA F106 Storm Hazards Project
With the use of composite (non-metallic) and microelectronics becoming more prevalent in the construction of both military and commercial aircraft, the control systems have become more susceptible to damage or failure from electromagnetic transients. One source of such transients is the lightning discharge. In order to study the effects of the lightning discharge on the vital components of an aircraft, NASA Langley Research Center has undertaken a Storm Hazards Program in which a specially instrumented F106B jet aircraft is flown into active thunderstorms with the intention of being struck by lightning. One of the specific purposes of the program is to quantify the environmental conditions which are conductive to aircraft lightning strikes
The intragroup medium in loose groups of galaxies
We have used the ROSAT PSPC to study the properties of a sample of 24 X-ray
bright galaxy groups, representing the largest sample examined in detail to
date. Hot plasma models are fitted to the spectral data to derive temperatures,
and modified King models are used to characterise the surface brightness
profiles. In agreement with previous work, we find evidence for the presence of
two components in the surface brightness profiles. The extended component is
generally found to be much flatter than that observed in galaxy clusters, and
there is evidence that the profiles follow a trend with system mass. We derive
relationships between X-ray luminosity, temperature and optical velocity
dispersion. The relation between X-ray luminosity and temperature is found to
be L_X \propto T^{4.9}, which is significantly steeper than the same relation
in galaxy clusters. These results are in good agreement with preheating models,
in which galaxy winds raise the internal energy of the gas, inhibiting its
collapse into the shallow potential wells of poor systems.Comment: 17 pages, 10 figures. Accepted for publication in MNRA
X-ray bright groups and their galaxies
Combining X-ray data from the ROSAT PSPC and optical data drawn from the
literature, we examine in detail the relationship between the X-ray and optical
properties of X-ray bright galaxy groups. We find a relationship between
optical luminosity and X-ray temperature consistent with that expected from
self-similar scaling of galaxy systems, L_B \propto T^{1.6 +/- 0.2}. The
self-similar form and continuity of the L_B : T relation from clusters to
groups and the limited scatter seen in this relation, implies that the star
formation efficiency is rather similar in all these systems. We find that the
bright extended X-ray components associated with many central galaxies in
groups appear to be more closely related to the group than the galaxy itself,
and we suggest that these are group cooling flows rather than galaxy halos. In
addition we find that the optical light in these groups appears to be more
centrally concentrated than the light in clusters. We also use the optical and
X-ray data to investigate whether early or late type galaxies are primarily
responsible for preheating in groups. Using three different methods, we
conclude that spiral galaxies appear to play a comparable role to early types
in the preheating of the intragroup medium. This tends to favour models in
which the preheating arises primarily from galaxy winds rather than AGN, and
implies that spirals have played a significant role in the metal enrichment of
the intragroup medium.Comment: 17 pages, accepted for publication in MNRA
A cloud, precipitation and electrification modeling effort for COHMEX
In mid-1987, the Modeling Group of the Institute of Atmospheric Sciences (IAS) began to simulate and analyze cloud runs that were made during the Cooperative Huntsville Meteorological Experiment (COHMEX) Project and later. The cloud model was run nearly every day during the summer 1986 COHMEX Project. The Modeling Group was then funded to analyze the results, make further modeling tests, and help explain the precipitation processes in the Southeastern United States. The main science objectives of COHMEX were: (1) to observe the prestorm environment and understand the physical mechanisms leading to the formation of small convective systems and processes controlling the production of precipitation; (2) to describe the structure of small convective systems producing precipitation including the large and small scale events in the environment surrounding the developing and mature convective system; (3) to understand the interrelationships between electrical activity within the convective system and the process of precipitation; and (4) to develop and test numerical models describing the boundary layer, tropospheric, and cloud scale thermodynamics and dynamics associated with small convective systems. The latter three of these objectives were addressed by the modeling activities of the IAS. A series of cloud modes were used to simulate the clouds that formed during the operational project. The primary models used to date on the project were a two dimensional bulk water model, a two dimensional electrical model, and to a lesser extent, a two dimensional detailed microphysical cloud model. All of the models are based on fully interacting microphysics, dynamics, thermodynamics, and electrical equations. Only the 20 July 1986 case was analyzed in detail, although all of the cases run during the summer were analyzed as to how well they did in predicting the characteristics of the convection for that day
Atmospheric Electrical Modeling in Support of the NASA F-106 Storm Hazards Project
A recently developed storm electrification model (SEM) is used to investigate the operating environment of the F-106 airplane during the NASA Storm Hazards Project. The model is 2-D, time dependent and uses a bulkwater microphysical parameterization scheme. Electric charges and fields are included, and the model is fully coupled dynamically, microphysically and electrically. One flight showed that a high electric field was developed at the aircraft's operating altitude (28 kft) and that a strong electric field would also be found below 20 kft; however, this low-altitude, high-field region was associated with the presence of small hail, posing a hazard to the aircraft. An operational procedure to increase the frequency of low-altitude lightning strikes was suggested. To further the understanding of lightning within the cloud environment, a parameterization of the lightning process was included in the SEM. It accounted for the initiation, propagation, termination, and charge redistribution associated with an intracloud discharge. Finally, a randomized lightning propagation scheme was developed, and the effects of cloud particles on the initiation of lightning investigated
Investigations into the F-106 lightning strike environment as functions of altitude and storm phase
Work accomplished during this period centered on the completion of the first order parameterization scheme for the intracloud lightning discharge and its incorporation within the framework of the Storm Electrification Model (SEM)
The unusual morphology of the intragroup medium in NGC 5171
We present the results of a 24 ks XMM-Newton observation of the NGC 5171
group of galaxies. NGC 5171 is unusual in that it is an X-ray bright group (L >
10^42 erg/s), with irregular contours which are not centred on a bright galaxy.
The global spectrum is adequately described by a single temperature APEC model
with T = 0.96 +/- 0.04 keV, and Z = 0.13 +/- 0.02 Zsol, in good agreement with
previous ROSAT data. We find the X-ray contours are centred on a bright ridge
of emission stretching from the BGG to a nearby galaxy. Spectral mapping
reveals this ridge to be both cool (T ~ 1.1 keV) and metallic (Z ~ 0.4 Zsol) in
comparison to its surrounding, suggesting it is the result of a tidal
interaction between the two galaxies. Optical data reveals the member galaxies
to have a high velocity dispersion (sigma = 494 +/- 99 km/s), and a
significantly non-Gaussian velocity distribution, suggesting the group is in
the process of merging. A region of hot gas with T = 1.58 +/- 0.36 keV is found
to the West of the bright central ridge, and we interpret this as shock-heating
resulting from the merging. A further region of emission to the South-East of
the bright central ridge, with T = 1.14 +/- 0.13 keV, is probably associated
with a background group, four times more distant.Comment: 10 pages, 8 figures, to be published in MNRA
Are X-ray properties of loose groups different from those of compact groups?
We compare the X-ray properties of loose and compact galaxy groups, using a
combined sample of 42 groups. We find that we are unable to separate loose and
compact groups on the luminosity-temperature relation, the luminosity-velocity
dispersion relation or the velocity dispersion-temperature relation using
equally weighted errors. This suggests that the distinction between compact and
loose groups is not a fundamental one, and we argue that a more useful
distinction is that between X-ray bright and X-ray faint systems. Given their
similarity in X-ray properties, we combine the loose and compact subsamples to
derive relations based on the full sample. This provides the highest
statistical quality results to date on the way in which the correlations in
X-ray properties of low mass systems depart from those seen in rich clusters.Comment: 6 pages, 6 figures. Accepted for publication in MNRA
Chandra Observations of low velocity dispersion groups
Deviations of galaxy groups from cluster scaling relations can be understood
in terms of an excess of entropy in groups. The main effect of this excess is
to reduce the density and thus luminosity of the intragroup gas. Given this,
groups should also should show a steep relationship between X-ray luminosity
and velocity dispersion. However, previous work suggests that this is not the
case with many measuring slopes flatter than the cluster relation.
Examining the group L_X:\sigma relation shows that much of the flattening is
caused by a small subset of groups which show very high X-ray luminosities for
their velocity dispersions (or vice versa).
Detailed Chandra study of two such groups shows that earlier ROSAT results
were subject to significant (~30-40%) point source contamination, but confirm
that a significant hot IGM is present in these groups, although these are two
of the coolest systems in which intergalactic X-ray emission has been detected.
Their X-ray properties are shown to be broadly consistent with those of other
galaxy groups, although the gas entropy in NGC 1587 is unusually low, and its
X-ray luminosity correspondingly high for its temperature, compared to most
groups.
This leads us to suggest that the velocity dispersion in these systems has
been reduced in some way, and we consider how this might have come about.Comment: Accepted for publication in Ap
Preheating in the Universe Suppressing High Energy Gamma-rays from Structure Formation
Structure formation in the universe can produce high energy gamma-rays from
shock-accelerated electrons, and this process may be the origin of the
extragalactic gamma-ray background (EGRB) as well as a part of the unidentified
sources detected by EGRET in the GeV band, if about 5% of the kinetic energy of
the shock is going into electron acceleration. However, we point out that the
production of gamma-rays may be severely suppressed if the collapsing matter
has been preheated by external entropy sources at the time of gravitational
collapse, as can be inferred from the luminosity-temperature (LT) relation of
galaxy clusters and groups. We also make a rough estimate of this effect by a
simple model, showing that the EGRB flux may be suppressed by a factor of about
30. Hence structure formation is difficult to be the dominant origin of EGRB if
preheating is actually responsible for the observed anomary in the LT relation.
The detectable number of gamma-ray clusters is also reduced, but about 5-10
forming clusters should still be detectable by EGRET all sky, and this number
is similar to that of the steady and high-latitude unidentified sources in the
EGRET catalog. The future GLAST mission should detect 10^2-10^3 gamma-ray
clusters of galaxies even if the intergalactic medium has been preheated.Comment: References added for relevant work. 5 pages, 4 figures, accepted in
Astroparticle Physic
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