261 research outputs found
A window to the Galactic X-ray halo: The ISM towards the Lockman hole
We present a combined X-ray/HI-analysis of the ISM towards the Lockman hole.
This sky region is considered as the "window to the distant universe" because
of its absolute lowest HI column density on the whole sky. The Lockman hole
appears to be not as transparent as the HI data suggest. We propose that about
half of the ISM towards the Lockman hole is in form of ionized hydrogen rather
than HI.Comment: Proceedings, International Workshop on X-RAY SURVEYS, in the light of
new observatories Santander (Spain), 4-6 September 2000 (accepted
Anisotropies in the HI gas distribution toward 3C196
The local Galactic HI gas was found to contain cold neutral medium (CNM)
filaments that are aligned with polarized dust emission. These filaments appear
to be dominated by the magnetic field and in this case turbulence is expected
to show distinct anisotropies. We use the Galactic Effelsberg--Bonn HI Survey
(EBHIS) to derive 2D turbulence spectra for the HI distribution in direction to
3C196 and two more comparison fields. Prior to Fourier transform we apply a
rotational symmetric 50% Tukey window to apodize the data. We derive average as
well as position angle dependent power spectra. Anisotropies in the power
distribution are defined as the ratio of the spectral power in orthogonal
directions. We find strong anisotropies. For a narrow range in position angle,
in direction perpendicular to the filaments and the magnetic field, the
spectral power is on average more than an order of magnitude larger than
parallel. In the most extreme case the anisotropy reaches locally a factor of
130. Anisotropies increase on average with spatial frequency as predicted by
Goldreich and Sridhar, at the same time the Kolmogorov spectral index remains
almost unchanged. The strongest anisotropies are observable for a narrow range
in velocity and decay with a power law index close to --8/3, almost identical
to the average isotropic spectral index of . HI
filaments, associated with linear polarization structures in LOFAR observations
in direction to 3C196, show turbulence spectra with marked anisotropies.
Decaying anisotropies appear to indicate that we witness an ongoing shock
passing the HI and affecting the observed Faraday depth.Comment: minor errors corrected, 15 pages, 29 figures, accepted for
publication by A&
The soft X-ray background towards the northern sky. A detailed analysis of the Milky Way halo
We present a correlation analysis of the diffuse X-ray background emission of
the ROSAT all-sky survey with the Leiden/Dwingeloo 21-cm HI line survey. We
derive a consistent model for the diffuse X-ray background emission over about
50% of the sky. Only three diffuse X-ray components are necessary to fit the
ROSAT data from 0.1 keV to 2.4 keV: a) the Local Hot Bubble, b) the Milky Way
Halo, and c) the extragalactic X-ray background. Only one temperature of the
hot coronal gas in the Milky Way Halo is needed. Our model predicts, that a
major fraction of the 1/4 keV and about 50% of the 3/4 keV diffuse X-ray
emission originates from the Milky Way Halo. We detect a difference between the
intensities towards the Galactic center and its anti-center, which is
consistent with the electron density distribution independently derived from
pulsar dispersion measurements.Comment: Astron. Nachr. in press, issue dedicated to the proceedings of the
workshop "X-ray Surveys in the light of New Observatories", Sep. 2002,
Santander, Spai
HI anisotropies associated with radio-polarimetric filaments. Steep power spectra associated with cold gas
LOFAR detected toward 3C 196 linear polarization structures which were found
subsequently to be closely correlated with cold filamentary HI structures. The
derived direction-dependent HI power spectra revealed marked anisotropies for
narrow ranges in velocity, sharing the orientation of the magnetic field as
expected for magneto hydrodynamical turbulence. Using the Galactic portion of
the Effelsberg-Bonn HI Survey we continue our study of such anisotropies in the
HI distribution in direction of two WSRT fields, Horologium and Auriga; both
are well known for their prominent radio-polarimetric depolarization canals. At
349 MHz the observed pattern in total intensity is insignificant but polarized
intensity and polarization angle show prominent ubiquitous structures with so
far unknown origin. Apodizing the HI survey data by applying a rotational
symmetric 50 percent Tukey window, we derive average and position angle
dependent power spectra. We fit power laws and characterize anisotropies in the
power distribution. We use a Gaussian analysis to determine relative abundances
for the cold and warm neutral medium. For the analyzed radio-polarimetric
targets significant anisotropies are detected in the HI power spectra; their
position angles are aligned to the prominent depolarization canals, initially
detected by WSRT. HI anisotropies are associated with steep power spectra.
Steep power spectra, associated with cold gas, are detected also in other
fields. Radio-polarimetric depolarization canals are associated with
filamentary HI structures that belong to the cold neutral medium (CNM).
Anisotropies in the CNM are in this case linked to a steepening of the
power-spectrum spectral index, indicating that phase transitions in a turbulent
medium occur on all scales. Filamentary HI structures, driven by thermal
instabilities, and radio-polarimetric filaments are associated with each other.Comment: Accepted for publication by A&A, 28 pages, 41 figures, minor updates
in styl
The Composition of the Interstellar Medium towards the Lockman Hole. HI, UV and X-ray observations
The Lockman Hole is well known as the region with the lowest neutral atomic
hydrogen colum density on the entire sky. We present an analysis of the soft
X-ray background radiation towards the Lockman Hole using ROSAT all-sky survey
data. This data is correlated with the Leiden/Dwingeloo survey (Galactic HI
21cm-line emission) in order to model the soft X-ray background by using
radiative transfer calculations for four ROSAT energy bands simultaneously. It
turns out, that an important gas fraction, ranging between 20-50%,of the X-ray
absorbing material is not entirely traced by the HI but is in the form of
ionized hydrogen. Far-ultraviolet absorption line measurements by FUSE are
consistent with this finding and support an ionized hydrogen component towards
the Lockman Hole.Comment: 11 pages, 11 figures, Accepted for Publication in Astronomy &
Astrophysics, For full resolution images, see
http://www.astro.uni-bonn.de/~mkappes/pub/ms3506.pd
- …