Local Hot Bubble Science
Does it Exist?
Steve Snowden
NASA/GSFC
What makes up the soft X-ray background?
RASS ¼ keV Band
RASS ¾ keV Band
From Near to Far:
1) Geocoronal emission from scattered
solar X-rays and solar wind charge
exchange, and auroral X-rays
2) Heliospheric solar wind charge exchange
(SWCX) emission
3) Local Hot Bubble (LHB)
4) Nearby supernova remnants (SNRs) and
superbubbles (SBs)
5) More distant SNRs and SBs
6) Stars (unresolved so “diffuse”)
7) Galactic Bulge
8) Galactic Ridge
9) Galactic Halo, part 1 (cooler)
10) Galactic Halo, part 2 (hotter)
11) Local Group
12) Extragalactic power law (unresolved
AGN and so also “diffuse”)
13) Clusters and structure of the Universe
Nearly all diffuse emission is thermal in
origin, and therefore dominated by line
emission.
The ¼ keV Background
The ¼ keV diffuse background was first observed
in the late 1960’s using sounding rocket data.
IRAS 100 mm
During the 1970’s and early 1980’s the sky was
mapped using both sounding rockets and satellite
observatories.
All maps agreed quite well and showed a distinct
negative correlation between the ¼ keV
background and the column density of Galactic
material.
Mapping culminated in the 1990’s with the
ROSAT All-Sky Survey and the IRAS 100 mm
data.
The negative correlation was first interpreted as
an extragalactic background absorbed by
Galactic material.
Next it was realized that there was a real
background in the Galactic plane, and that due to
the short MFP at ¼ keV it had to originate locally
RASS ¼ keV
The Local Hot Bubble
This led to the concept of the Local Hot Bubble
Local - meaning surrounding the Sun with an
extent of ~50 to ~150 pc
Hot - meaning T~0.1 keV
Bubble - meaning there isn’t much else within the
bubble except for the hot plasma and a few
partially ionized clouds (e.g.,the Local Fluff)
21 cm observations
ISM absorption line studies
No significant communication with Loop I
(different temperatures)
Emission appears to be bounded
Supernova origin (deep sea floor sediment, OB
association movement through the ISM)
In the late 1980’s it was suggested that nearly all of the
¼ keV background could be produced by the LHB. This
model was “Not inconsistent with any of the available
data” (Dan McCammon). This was actually a step
forward.
But the RASS changed all that, and the Galaxy got a
whole lot more complicated.
Nearby Supernova Remnants and Superbubbles
North Polar Spur/Loop I
Cygnus
Superbubble
Vela/Puppis SNRs
Monogem SNR
Eridanus
Superbubble
Cygnus Loop
RASS ¼ keV Band
Galactic Bulge
LMC
Cartoon of the ISM
The solar neighborhood.
The locations and
extents of the various
objects have come from
a variety of sources.
Solar System Emission
SWCX emission occurs when highly
ionized solar wind particles charge
exchange with either exospheric or
ISM material. Carbon, oxygen, neon,
and magnesium emission is common.
Most striping from SWCX
The SWCX is both distributed
throughout the solar system and
associated with Earth’s magnetosheath.
There is, perhaps, additional emission
at the heliopause.
Strongly time variable
O VII and O VIII emission is
particularly problematic for
astrophysical observations.
XMM-Newton SWCX emission spectrum
Solar System Emission
SWCX emission can also be
problematic to detect with
relatively constant fluxes over
extended periods (tens of ks).
Multiple observations of the same
direction can give some idea of the
level of contamination.
Abnormally strong O VII, O VIII,
and Mg XI can also suggest the
presence of SWCX emission
XMM-Newton SWCX emission and
associated light curves
But, space astronomy folks actually
care about SWCX emission as it
can possibly provide remote
sensing on phenomena which
currently rely upon in-situ
measurements.
Suzaku
SWCX emission is relatively common
Expected from the RASS LTEs
However, we can only easily identify the brighter episodes
=> What is the zero level of SWCX emission
Currently we are only able to observe at E>0.5 keV
=> Not in general the same spectrum as the LHB
Pros and Cons
LHB
Pro
Explains many observational issues well
Negative correlation between the ¼ keV intensity and HI column density
Lack of spectral hardness variation with intensity
Con
Requires a high thermal pressure inconsistent with local ISM
SWCX
Pro
We know it exists, it has to contribute some to the observed flux
Models seem to predict the observed amount of O VII and O VIII emission
Con
Good agreement between surveys with radically different geometries
SWCX cross sections uncertain at lower energies
Odd geometry at higher latitudes, two separate emission regions
ROSAT Spectra
The ROSAT All-Sky Survey gave us the best available view of the
¼ keV diffuse background. It used a proportional counter and so
Had very poor spectral resolution.
LHB Emission
Model vs Observed
Model Spectrum
DXS Spectrum
The observed spectrum (DXS) is dominated by line
emission.
The spectrum, though, is completely different from
what has been used for modeling.
The spectrum is not from a thermal equilibrium
spectrum, strange abundances and non-equilibrium
ionization states.
SWCX Emission
Model vs Observed
DXS Spectrum
DXS Data vs. Model SWCX Emission
Not many similarities here either
Model SWCX