16,831 research outputs found
Searching for additional heating - [OII] emission in the diffuse ionized gas of NGC891, NGC4631 and NGC3079
We present spectroscopic data of ionized gas in the disk--halo regions of
three edge-on galaxies, NGC 891, NGC 4631 and NGC 3079, covering a wavelength
range from [\ion{O}{2}] 3727\AA to [\ion{S}{2}] 6716.4\AA.
The inclusion of the [\ion{O}{2}] emission provides new constraints on the
properties of the diffuse ionized gas (DIG), in particular, the origin of the
observed spatial variations in the line intensity ratios. We used three
different methods to derive electron temperatures, abundances and ionization
fractions along the slit. The increase in the [\ion{O}{2}]/H line ratio
towards the halo in all three galaxies requires an increase either in electron
temperature or in oxygen abundance. Keeping the oxygen abundance constant
yields the most reasonable results for temperature, abundances, and ionization
fractions. Since a constant oxygen abundance seems to require an increase in
temperature towards the halo, we conclude that gradients in the electron
temperature play a significant role in the observed variations in the optical
line ratios from extraplanar DIG in these three spiral galaxies.Comment: 43 pages, 29 figure
Stabilized lanthanum sulphur compounds
Lanthanum sulfide is maintained in the stable cubic phase form over a temperature range of from 500 C to 1500 C by adding to it small amounts of calcium, barium, or strontium. This compound is an excellent thermoelectric material
The Chandra Dust Scattering Halo of Galactic Center transient Swift J174540.7-290015
We report the detection of a dust scattering halo around a recently
discovered X-ray transient, Swift J174540.7-290015, which in early February of
2016 underwent one of the brightest outbursts (F_X ~ 5e-10 erg/cm^2/s) observed
from a compact object in the Galactic Center field. We analyze four Chandra
images that were taken as follow-up observations to Swift discoveries of new
Galactic Center transients. After adjusting our spectral extraction for the
effects of detector pileup, we construct a point spread function for each
observation and compare it to the GC field before the outburst. We find
residual surface brightness around Swift J174540.7-290015, which has a shape
and temporal evolution consistent with the behavior expected from X-rays
scattered by foreground dust. We examine the spectral properties of the source,
which shows evidence that the object transitioned from a soft to hard spectral
state as it faded below L_X ~ 1e36 erg/s. This behavior is consistent with the
hypothesis that the object is a low mass X-ray binary in the Galactic Center.Comment: Accepted for publication in Ap
Use of passive scalar tagging for the study of coherent structures in the plane mixing layer
Data obtained from the numerical simulation of a 2-D mixing layer were used to study the feasibility of using the instantaneous concentration of a passive scalar for detecting the typical coherent structures in the flow. The study showed that this technique works quite satisfactorily and yields results similar to those that can be obtained by using the instantaneous vorticity for structure detection. Using the coherent events educed by the scalar conditioning technique, the contribution of the coherent events to the total turbulent momentum and scalar transport was estimated. It is found that the contribution from the typical coherent events is of the same order as that of the time-mean value. However, the individual contributions become very large during the pairing of these structures. The increase is particularly spectacular in the case of the Reynolds shear stress
Crossing Crawford’s conceptual divide: monumental linear earthworks in later prehistoric and early medieval Britain
In its early decades, Antiquity regularly featured the subject of linear earthworks that crisscross the British landscape. Subsequently, however, discussion has been largely relegated to
period-specific and local journals. As a result, interpretations of these imposing but often
poorly dated earthworks have been drawn in the contrasting research traditions of later
prehistory and the early medieval period. Here, the authors propose a comparative dialogue
as a means for reinterpreting these landscape features, and as a lens through which to explore
social complexity. Combined with advances in archaeometrical dating, this new approach
promises to reinvigorate the study of some of Britain’s largest archaeological monuments
Jet power extracted from ADAF and the applications to X-ray binaries and radio galaxy FR dichotomy
We calculate the jet power of the classical Blandford-Znajek(BZ) model and
hybrid model developed by Meier based on the global solutions of advection
dominated accretion flows (ADAFs) surrounding Kerr black holes. We find that
the jet power of the hybrid model is larger than that of the pure BZ model. The
jet power will dominate over the accretion power, and the objects will enter
into "jet-power-dominated advective systems", when the accretion rate is less
than a critical value mdot_c=Mdot_c/Mdot_Edd, where 3*10^-4 < mdot_c < 5*10^-3
is a function of black hole spin parameter. The accretion power will be
dominant when mdot<mdot_c and the objects will enter into
"accretion-power-dominated advective systems." This is roughly consistent with
that constrained from the low/hard-state black hole X-ray binaries (e.g.,
Fender et al.). We calculate the maximal jet power as a function of black hole
mass with the hybrid jet formation model, and find it can roughly reproduce the
dividing line of the Ledlow-Owen relation for FR I/FR II dichotomy in the jet
power-black hole(BH) mass plane (Q_jet-M_BH) if the dimensionless accretion
rate mdot~0.01 and BH spin parameter j~0.9-0.99 are adopted. This accretion
rate mdot~0.01 is consistent with that of the critical accretion rate for the
accretion mode transition of a standard disk to an ADAF constrained from the
state transition of X-ray binaries. Our results imply that most FR I galaxies
may be in the ADAF accretion mode similar to the low/hard-state XRBs.Comment: 6 pages, 3 color figures, ApJ in press, edited with ApJ style and
English is improved as suggested by Edito
The EXOSAT medium-energy slew survey catalog
We present a catalog of X-ray sources observed during slew maneuvers by the
Medium Energy Detector Array onboard the EXOSAT Observatory. The EXOSAT Medium
Energy slew-survey catalog (EXMS) provides a unique record of the 1--8 keV
X-ray sky between 1983 and 1986. 98% of the sky was observed, with 85%
receiving an exposure of >60 s. 1210 sources were detected. By comparing these
source positions with other catalogs, identifications are given for 992
detections (82% of the sample). These identifications consist of 250 distinct
objects, including 95 different X-ray binary systems, and 14 different AGN. A
further 58 detections have multiple candidates, while 160 detections remain
unidentified. Collimator transmission corrected 1-8 keV count rates are given
for the identified sources, together with raw count rates for the other
detections. The construction of the EXMS and the checks performed to ensure the
validity of the derived source properties are discussed. A publically available
version of this catalog is maintained on the EXOSAT database and archive system
(telnet://[email protected]).Comment: 52 pages. 22 Figures. To be published in A&AS. For more information,
see http://astro.estec.esa.nl/SA-general/Projects/Exosat/exmsintro.htm
Blob ejection from advection-dominated accretion flow: observational consequences
There is increasing evidence for the presence of an optically thin
advection-dominated accretion flow (ADAF) in low luminosity active galactic
nuclei and radio-loud quasars. The present paper is devoted to explore the fate
of a blob ejected from an ADAF, and to discuss its observational consequences.
It is inevitable for the ejected blob to drastically expand into its
surroundings. Consequently, it is expected that a group of relativistic
electrons should be accelerated, which may lead to nonthermal flares, since a
strong shock will be formed by the interaction between the blob and its
surroundings. Then the blob cools down efficiently, leading to the appearance
of recombination lines about s after its ejection from an ADAF. We apply
this model to NGC 4258 for some observational prediction, and to PKS 2149--306
for the explanation of observational evidence. Future simultaneous observations
of recombination X-ray lines and continuum emission are highly desired to test
the present model.Comment: 4 pages in emulateapj.sty, no figure. Accepted by ApJ Letter
- …