1,151 research outputs found
ROSAT HRI observations of Centaurus A
We present results from a sensitive high-resolution X-ray observation of the
nearby active galaxy Centaurus A (NGC 5128) with the ROSAT HRI. The 65~ksec
X-ray image clearly distinguishes different components of the X-ray emission
from Cen A: the nucleus and the jet, the diffuse galaxy halo, and a number of
individual sources associated with the galaxy. The luminosity of the nucleus
increased by a factor of two compared to an earlier ROSAT observation in 1990.
The high spatial resolution of the ROSAT HRI shows that most of the knots in
the jet are extended both along and perpendicular to the jet axis. We report
the detection of a new X-ray feature, at the opposite side of the X-ray jet
which is probably due to compression of hot interstellar gas by the expanding
southwestern inner radio lobe.Comment: To be published in Astrophys. Journal Letters. 4 pages, 3 plate
Magnetic fields of our Galaxy on large and small scales
Magnetic fields have been observed on all scales in our Galaxy, from AU to
kpc. With pulsar dispersion measures and rotation measures, we can directly
measure the magnetic fields in a very large region of the Galactic disk. The
results show that the large-scale magnetic fields are aligned with the spiral
arms but reverse their directions many times from the inner-most arm (Norma) to
the outer arm (Perseus). The Zeeman splitting measurements of masers in HII
regions or star-formation regions not only show the structured fields inside
clouds, but also have a clear pattern in the global Galactic distribution of
all measured clouds which indicates the possible connection of the large-scale
and small-scale magnetic fields.Comment: 9 pages. Invited Talk at IAU Symp.242, 'Astrophysical Masers and
their Environments', Proceedings edited by J. M. Chapman & W. A. Baa
The Exceptionally Soft X-ray Spectrum of the Low-mass Starburst Galaxy NGC 1705
NGC 1705 is one of the optically brightest and best studied dwarf galaxies.
It appears to be in the late stage of a major starburst and contains a young
super star cluster. Type II supernovae are therefore likely to have been a
major effect in the recent evolution of this galaxy and are likely to have
produced a superbubble whose affects on the low-density ambient interstellar
medium can be ideally studied. ROSAT PSPC observations of this galaxy reveal
two striking blobs of X-ray emission embedded in \Ha loops which can be
interpreted as both sides of the upper plumes of the same superbubble. These
sources are a surprise. They are much softer than those observed from other
starburst dwarf galaxies, and are so soft that they should have been blocked if
the observed Galactic HI column density were uniformly distributed across NGC
1705 or if the sources were embedded in the HI disk of NGC 1705. In addition,
the total X-ray luminosity in the ROSAT energy band of 1.2x10^{38} erg s^{-1}
is low in comparison to similar objects. We discuss possible models for the two
X-ray peaks in NGC 1705 and find that the sources most likely originate from
relatively cool gas of one single superbubble in NGC 1705. The implications of
the exceptional softness of these sources are addressed in terms of intrinsic
properties of NGC 1705 and the nature of the foreground Galactic absorption.Comment: 7 pages, 2 ps-figures, LATEX-file; accepted for publication in
ApJ.Letter
Active Galactic Nuclei with Starbursts: Sources for Ultra High Energy Cosmic Rays
Ultra high energy cosmic ray events presently show a spectrum, which we
interpret here as galactic cosmic rays due to a starburst in the radio galaxy
Cen A pushed up in energy by the shock of a relativistic jet. The knee feature
and the particles with energy immediately higher in galactic cosmic rays then
turn into the bulk of ultra high energy cosmic rays. This entails that all
ultra high energy cosmic rays are heavy nuclei. This picture is viable if the
majority of the observed ultra high energy events come from the radio galaxy
Cen A, and are scattered by intergalactic magnetic fields across most of the
sky.Comment: 4 pages, 1 figure, proceedings of "High-Energy Gamma-rays and
Neutrinos from Extra-Galactic Sources", Heidelber
A molecular shell with star formation toward the supernova remnant G349.7+0.2
A field of ~38'x38' around the supernova remnant (SNR) G349.7+0.2 has been
surveyed in the CO J=1-0 transition with the 12 Meter Telescope of the NRAO,
using the On-The-Fly technique. The resolution of the observations is 54". We
have found that this remnant is interacting with a small CO cloud which, in
turn, is part of a much larger molecular complex, which we call the ``Large CO
Shell''. The Large CO Shell has a diameter of about 100 pc, an H_2 mass of
930,000 solar masses, and a density of 35 cm-3. We investigate the origin of
this structure and suggest that an old supernova explosion ocurred about 4
million years ago, as a suitable hypothesis. Analyzing the interaction between
G349.7+0.2 and the Large CO Shell, it is possible to determine that the shock
front currently driven into the molecular gas is a non-dissociative shock
(C-type), in agreement with the presence of OH 1720 MHz masers. The positional
and kinematical coincidence among one of the CO clouds that constitute the
Large CO Shell, an IRAS point-like source and an ultracompact H II region,
indicate the presence of a recently formed star. We suggest that the formation
of this star was triggered during the expansion of the Large CO Shell, and
suggest the possibility that the same expansion also created the progenitor
star of G349.7+0.2. The Large CO Shell would then be one of the few
observational examples of supernova-induced star formation.Comment: accepted in Astronomical Journal, corrected typo in the abstract (in
first line, 38' instead of 38"
Spitzer Observations of Centaurus A: Infrared Synchrotron Emission from the Northern Lobe
We present measurements obtained with the Spitzer Space Telescope in five
bands from 3.6-24 microns of the northern inner radio lobe of Centaurus A, the
nearest powerful radio galaxy. We show that this emission is synchrotron in
origin. Comparison with ultraviolet observations from GALEX shows that diffuse
ultraviolet emission exists in a smaller region than the infrared but also
coincides with the radio jet. We discuss the possibility, that synchrotron
emission is responsible for the ultraviolet emission and conclude that further
data are required to confirm this.Comment: 4 pages, accepted by ApJ
A Highly Ordered Faraday-Rotation Structure in the Interstellar Medium
We describe a Faraday-rotation structure in the Interstellar Medium detected
through polarimetric imaging at 1420 MHz from the Canadian Galactic Plane
Survey (CGPS). The structure, at l=91.8, b=-2.5, has an extent of ~2 degree,
within which polarization angle varies smoothly over a range of ~100 degree.
Polarized intensity also varies smoothly, showing a central peak within an
outer shell. This region is in sharp contrast to its surroundings, where
low-level chaotic polarization structure occurs on arcminute scales. The
Faraday-rotation structure has no counterpart in radio total intensity, and is
unrelated to known objects along the line of sight, which include a Lynds
Bright Nebula, LBN 416, and the star cluster M39 (NGC7092). It is interpreted
as a smooth enhancement of electron density. The absence of a counterpart,
either in optical emission or in total intensity, establishes a lower limit to
its distance. An upper limit is determined by the strong beam depolarization in
this direction. At a probable distance of 350 +/- 50 pc, the size of the object
is 10 pc, the enhancement of electron density is 1.7 cm-3, and the mass of
ionized gas is 23 M_sun. It has a very smooth internal magnetic field of
strength 3 microG, slightly enhanced above the ambient field. G91.8-2.5 is the
second such object to be discovered in the CGPS, and it seems likely that such
structures are common in the Magneto-Ionic Medium.Comment: 16 pages, 5 figures, ApJ accepte
New evidence on the origin of the microquasar GRO J1655-40
Aims. Motivated by the new determination of the distance to the microquasar
GRO J1655-40 by Foellmi et al. (2006), we conduct a detailed study of the
distribution of the atomic and molecular gas, and dust around the open cluster
NGC 6242, the possible birth place of the microquasar. The proximity and
relative height of the cluster on the galactic disk provides a unique
opportunity to study SNR evolution and its possible physical link with
microquasar formation. Methods. We search in the interstellar atomic and
molecular gas around NGC 6242 for traces that may have been left from a
supernova explosion associated to the formation of the black hole in GRO
J1655-40. Furthermore, the 60/100 mu IR color is used as a tracer of
shocked-heated dust. Results. At the kinematical distance of the cluster the
observations have revealed the existence of a HI hole of 1.5*1.5 degrees in
diameter and compressed CO material acumulated along the south-eastern internal
border of the HI cavity. In this same area, we found extended infrared emission
with characteristics of shocked-heated dust. Based on the HI, CO and FIR
emissions, we suggest that the cavity in the ISM was produced by a supernova
explosion occured within NGC 6242. The lower limit to the kinematic energy
transferred by the supernova shock to the surrounding interstellar medium is ~
10^{49} erg and the atomic and molecular mass displaced to form the cavity of ~
16.500 solar masses. The lower limit to the time elapsed since the SN explosion
is ~ 2.2*10^{5} yr, which is consistent with the time required by GRO J1655-40
to move from the cluster up to its present position. The observations suggest
that GRO J1655-40 could have been born inside NGC 6242, being one of the
nearest microquasars known so far.Comment: 6 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysic
Dynamics in the O(2 × 1) adlayer on Ru(0001): bridging timescales from milliseconds to minutes by scanning tunneling microscopy
The dynamics within an O(2 × 1) adlayer on Ru(0001) is studied by density functional theory and high-speed scanning tunneling microscopy. Transition state theory proposes dynamic oxygen species in the reduced O(2 × 1) layer at room temperature. Collective diffusion processes can result in structural reorientations of characteristic stripe patterns. Spiral high-speed scanning tunneling microscopy measurements reveal this reorientation as a function of time in real space. Measurements, ranging over several minutes with constantly high frame rates of 20 Hz resolved the gradual reorientation. Moreover, reversible fast flipping events of stripe patterns are observed. These measurements relate the observations of long-term atomic rearrangements and their underlying fast processes captured within several tens of milliseconds
Spiral high-speed scanning tunneling microscopy: Tracking atomic diffusion on the millisecond timescale
Scanning tunneling microscopy (STM) is one of the most prominent techniques to resolve atomic structures of flat surfaces and thin films. With the scope to answer fundamental questions in physics and chemistry, it was used to elucidate numerous sample systems at the atomic scale. However, dynamic sample systems are difficult to resolve with STM due to the long acquisition times of typically more than 100 s per image. Slow electronic feedback loops, slow data acquisition, and the conventional raster scan limit the scan speed. Raster scans introduce mechanical noise to the image and acquire data discontinuously. Due to the backward and upward scan or the flyback movement of the tip, image acquisition times are doubled or even quadrupled. By applying the quasi-constant height mode and by using a combination of high-speed electronics for data acquisition and innovative spiral scan patterns, we could increase the frame rate in STM significantly. In the present study, we illustrate the implementation of spiral scan geometries and focus on the scanner input signal and the image visualization. Constant linear and constant angular velocity spirals were tested on the Ru(0001) surface to resolve chemisorbed atomic oxygen. The spatial resolution of the spiral scans is comparable to slow raster scans, while the imaging time was reduced from ~100 s to ~8 ms. Within 8 ms, oxygen diffusion processes were atomically resolved
- …