Issue |
A&A
Volume 418, Number 1, April IV 2004
|
|
---|---|---|
Page(s) | 143 - 150 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20040059 | |
Published online | 02 April 2004 |
The heliospheric soft X-ray emission pattern during the ROSAT survey: Inferences on Local Bubble hot gas
Service d'Aéronomie du CNRS, 91371 Verrières-le-Buisson, France
Corresponding author: rosine.lallement@aerov.jussieu.fr
Received:
17
April
2003
Accepted:
13
January
2004
Solar wind interaction with neutral gas has been shown to generate
soft X-rays, through the charge-exchange of highly ionized ions
with neutral atoms and molecules in
the interplanetary space and in the geocorona.
The resulting diffuse emission
can explain the non-cosmic long term enhancements (LTE) of ROSAT soft X-ray
observations,
which are produced by strong solar wind flux increases.
This paper focuses on the emission pattern
resulting from the impact on the interstellar gas of the average, “quiet” solar
wind, i.e., the non variable signal “below” the LTEs. A sky
map of the heliospheric emission is calculated, based on realistic hydrogen
and helium atom distributions under the influence of solar conditions such
as those prevailing during the ROSAT survey. It is
shown that parallax effects linked to the ROSAT observing strategy have a
strong influence on the resultant emission pattern. For a stationary solar
wind the modeled emission has an anisotropy factor
of 1.8, and the emission maxima do not coincide with the interstellar wind
axis direction. This emission is compared with the fraction
of the ROSAT 0.25 keV emission assigned to hot (
K) gas in the
Local Interstellar Bubble and with the
dense gas contour maps of the Bubble drawn from NaI absorption.
Assuming constancy of the global pattern, but allowing for a
variable scaling factor, the maximum heliospheric contribution is derived and
this allows the placement of lower limits on the truly
interstellar emission in many directions. After subtraction of the
heliospheric emission, there is significant interstellar
0.25 keV emission from the Local Bubble “openings” to the lower
halo, as expected, but also to neighbouring interstellar “bubbles”, as well as towards
a number of regions along the galactic plane. There is evidence
that there is hot gas
emission attributable to our local cavity and generated throughout its volume,
including along the galactic plane, but significantly
weaker than initially thought. This would attenuate the
discrepancy by a factor of about 4–5 between the hot gas pressure initially
derived without heliospheric decontamination and the
pressure of the embedded local diffuse clouds.
Key words: ISM: clouds / ISM: atoms / Galaxy: solar neighborhood
© ESO, 2004
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