662 research outputs found
A Chandra Observation of the Diffuse Emission in the Face-on Spiral NGC 6946
This paper describes the {\it Chandra} observation of the diffuse emission in
the face-on spiral NGC 6946. Overlaid on optical and H images, the
diffuse emission follows the spiral structure of the galaxy. An overlay on a 6
cm polarized radio intensity map confirms the phase offset of the polarized
emission. We then extract and fit the spectrum of the unresolved emission with
several spectral models. All model fits show a consistent continuum thermal
temperature with a mean value of 0.250.03 keV. Additional degrees of
freedom are required to obtain a good fit and any of several models satisfy
that need; one model uses a second continuum component with a temperature of
0.700.10 keV. An abundance measure of 3 for Si differs
from the solar value at the 90% confidence level; the net diffuse spectrum
shows the line lies above the instrumental Si feature. For Fe, the abundance
measure of 0.670.13 is significant at 99%. Multiple gaussians also provide
a good fit. Two of the fitted gaussians capture the O VII and O VIII emission;
the fitted emission is consistent with an {\it XMM-Newton} RGS spectrum of
diffuse gas in M81. The ratio of the two lines is 0.6-0.7 and suggests the
possibility of non-equilibrium ionization conditions exist in the ISM of NGC
6946. An extrapolation of the point source luminosity distribution shows the
diffuse component is not the sum of unresolved point sources; their
contribution is at most 25%.Comment: accepted for ApJ; 16 pages; 12 figs; to meet Archive size limits,
most converted to jpe
Absorption of the Linear Polarization of the Galactic Background Radiation by the 21-cm Line of HI
Absorption lines at 21-cm are detected in the Stokes Q and U
components of the Galactic synchrotron background. The lower limit distance
implied for the emission region is 2 kpc in the direction (l,b) =
(329.5,+1.15). The Australia Telescope Compact Array has
the capability of mapping this absorption over large areas of the Galactic
plane. Observations like these have the potential to reveal the three
dimensional structure of the Galactic synchrotron emission throughout the Milky
Way disk.Comment: 13 pages, 3 figures, Latex, see
http://ast1.spa.umn.edu/john/Galpol.html to be published in Astrophysical
Journal, 1997 October 1
AGN behind the SMC selected from radio and X-ray surveys
The XMM-Newton survey of the Small Magellanic Cloud (SMC) revealed 3053 X-ray
sources with the majority expected to be active galactic nuclei (AGN) behind
the SMC. However, the high stellar density in this field often does not allow
assigning unique optical counterparts and hinders source classification. On the
other hand, the association of X-ray point sources with radio emission can be
used to select background AGN with high confidence, and to constrain other
object classes like pulsar wind nebula. To classify X-ray and radio sources, we
use clear correlations of X-ray sources found in the XMM-Newton survey with
radio-continuum sources detected with ATCA and MOST. Deep radio-continuum
images were searched for correlations with X-ray sources of the XMM-Newton
SMC-survey point-source catalogue as well as galaxy clusters seen with extended
X-ray emission. Eighty eight discrete radio sources were found in common with
the X-ray point-source catalogue in addition to six correlations with extended
X-ray sources. One source is identified as a Galactic star and eight as
galaxies. Eight radio sources likely originate in AGN that are associated with
clusters of galaxies seen in X-rays. One source is a PWN candidate. We obtain
43 new candidates for background sources located behind the SMC. A total of 24
X-ray sources show jet-like radio structures.Comment: 9 pages, 6 figures, accepted for publication in A&
One step multiderivative methods for first order ordinary differential equations
A family of one-step multiderivative methods based on Padé approximants to the exponential function is developed.
The methods are extrapolated and analysed for use in PECE mode.
Error constants and stability intervals are calculated and the combinations compared with well known linear multi-step combinations and combinations using high accuracy Newton-Cotes quadrature formulas as correctors.
w926020
A Catalog of Candidate Intermediate-luminosity X-ray Objects
ROSAT, and now Chandra, X-ray images allow studies of extranuclear X-ray
point sources in galaxies other than our own. X-ray observations of normal
galaxies with ROSAT and Chandra have revealed that off-nuclear, compact,
Intermediate-luminosity (Lx[2-10 keV] >= 1e39 erg/s) X-ray Objects (IXOs,
a.k.a. ULXs [Ultraluminous X-ray sources]) are quite common. Here we present a
catalog and finding charts for 87 IXOs in 54 galaxies, derived from all of the
ROSAT HRI imaging data for galaxies with cz <= 5000 km/s from the Third
Reference Catalog of Bright Galaxies (RC3). We have defined the cutoff Lx for
IXOs so that it is well above the Eddington luminosity of a 1.4 Msun black hole
(10^38.3 erg/s), so as not to confuse IXOs with ``normal'' black hole X-ray
binaries. This catalog is intended to provide a baseline for follow-up work
with Chandra and XMM, and with space- and ground-based survey work at
wavelengths other than X-ray. We demonstrate that elliptical galaxies with IXOs
have a larger number of IXOs per galaxy than non-elliptical galaxies with IXOs,
and note that they are not likely to be merely high-mass X-ray binaries with
beamed X-ray emission, as may be the case for IXOs in starburst galaxies.
Approximately half of the IXOs with multiple observations show X-ray
variability, and many (19) of the IXOs have faint optical counterparts in DSS
optical B-band images. Follow-up observations of these objects should be
helpful in identifying their nature.Comment: 29 pages, ApJS, accepted (catalog v2.0) (full resolution version of
paper and future releases of catalog at http://www.xassist.org/ixocat_hri
Chandra View of the Dynamically Young Cluster of Galaxies A1367 I. Small-Scale Structures
The 40 ks \emph{Chandra} ACIS-S observation of A1367 provides new insights
into small-scale structures and point sources in this dynamically young
cluster. Here we concentrate on small-scale extended structures. A ridge-like
structure around the center (``the ridge'') is significant in the \chandra\
image. The ridge, with a projected length of 8 arcmin (or 300
h kpc), is elongated from northwest (NW) to southeast (SE), as is
the X-ray surface brightness distribution on much larger scales ( 2
h Mpc). The ridge is cooler than its western and southern
surroundings while the differences from its eastern and northern surroundings
are small. We also searched for small-scale structures with sizes
arcmin. Nine extended features, with sizes from 0.5 to 1.5, were
detected at significance levels above 4 . Five of the nine features are
located in the ridge and form local crests. The nine extended features can be
divided into two types. Those associated with galaxies (NGC 3860B, NGC 3860 and
UGC 6697) are significantly cooler than their surroundings (0.3 - 0.9 keV vs. 3
- 4.5 keV). The masses of their host galaxies are sufficient to bind the
extended gas. These extended features are probably related to thermal halos or
galactic superwinds of their host galaxies. The existence of these relatively
cold halos imply that galaxy coronae can survive in cluster environment (e.g.,
Vikhlinin et al. 2001). Features of the second type are not apparently
associated with galaxies. Their temperatures may not be significantly different
from those of their surroundings. This class of extended features may be
related to the ridge. We consider several possibilities for the ridge and the
second type of extended features. The merging scenario is preferred.Comment: To appear in ApJ, Vol 576, 2002, Sep., a high-resolution version is
in http://cfa160.harvard.edu/~sunm/a1367_a.ps.g
The population of X-ray supernova remnants in the Large Magellanic Cloud
We present a comprehensive X-ray study of the population of supernova
remnants (SNRs) in the LMC. Using primarily XMM-Newton, we conduct a systematic
spectral analysis of LMC SNRs to gain new insights on their evolution and the
interplay with their host galaxy. We combined all the archival XMM observations
of the LMC with those of our Very Large Programme survey. We produced X-ray
images and spectra of 51 SNRs, out of a list of 59. Using a careful modelling
of the background, we consistently analysed all the X-ray spectra and measure
temperatures, luminosities, and chemical compositions. We investigated the
spatial distribution of SNRs in the LMC and the connection with their
environment, characterised by various SFHs. We tentatively typed all LMC SNRs
to constrain the ratio of core-collapse to type Ia SN rates in the LMC. We
compared the X-ray-derived column densities to HI maps to probe the
three-dimensional structure of the LMC. This work provides the first
homogeneous catalogue of X-ray spectral properties of LMC SNRs. It offers a
complete census of LMC SNRs exhibiting Fe K lines (13% of the sample), or
revealing contribution from hot SN ejecta (39%). Abundances in the LMC ISM are
found to be 0.2-0.5 solar, with a lower [/Fe] than in the Milky Way.
The ratio of CC/type Ia SN in the LMC is , lower than in local SN surveys and galaxy clusters.
Comparison of X-ray luminosity functions of SNRs in Local Group galaxies
reveals an intriguing excess of bright objects in the LMC. We confirm that 30
Doradus and the LMC Bar are offset from the main disc of the LMC, to the far
and near sides, respectively. (abridged)Comment: Accepted for publication in Astronomy and Astrophysics. 54 pages, 18
figures, 12 tables. The resolution of the figures has been reduced compared
to the journal version; v2: New title, minor text edits; v3: Correct version
The XMM-Newton survey of the Small Magellanic Cloud: The X-ray point-source catalogue
Local-Group galaxies provide access to samples of X-ray source populations of
whole galaxies. The XMM-Newton survey of the Small Magellanic Cloud (SMC)
completely covers the bar and eastern wing with a 5.6 deg^2 area in the
(0.2-12.0) keV band. To characterise the X-ray sources in the SMC field, we
created a catalogue of point sources and sources with moderate extent. Sources
with high extent (>40") have been presented in a companion paper. We searched
for point sources in the EPIC images using sliding-box and maximum-likelihood
techniques and classified the sources using hardness ratios, X-ray variability,
and their multi-wavelength properties. The catalogue comprises 3053 unique
X-ray sources with a median position uncertainty of 1.3" down to a flux limit
for point sources of ~10^-14 erg cm^-2 s^-1 in the (0.2-4.5) keV band,
corresponding to 5x10^33 erg s^-1 for sources in the SMC. We discuss
statistical properties, like the spatial distribution, X-ray colour diagrams,
luminosity functions, and time variability. We identified 49 SMC high-mass
X-ray binaries (HMXB), four super-soft X-ray sources (SSS), 34 foreground
stars, and 72 active galactic nuclei (AGN) behind the SMC. In addition, we
found candidates for SMC HMXBs (45) and faint SSSs (8) as well as AGN (2092)
and galaxy clusters (13). We present the most up-to-date catalogue of the X-ray
source population in the SMC field. In particular, the known population of
X-ray binaries is greatly increased. We find that the bright-end slope of the
luminosity function of Be/X-ray binaries significantly deviates from the
expected universal high-mass X-ray binary luminosity function.Comment: 32 pages, 18 figures, accepted for publication in A&A, catalog will
be available at CD
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