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Investigating the Soft X-Ray
Background of the Milky Way
Michelle Rosenthal, Richard Willingale
X-ray & Observational Astronomy
[email protected]
[email protected]
INTRODUCTIONu The hot, diffuse, x-ray emitting interstellar medium of the Milky Way Galaxy appears almost
featureless, unless it is observed in the soft x-ray band. In 1995, complex structures, visible only between 0.1 and 2.0keV,
were revealed by the ROSAT all-sky survey. The XMM-Newton observatory allows us to resolve more point sources than
ever before. When these are removed, spectra of the underlying soft x-ray background (SXRB) can be extracted and high
resolution spectroscopy performed.
North Polar Spur
Northern Bulge
In this investigation, XMM-Newton was used to observe and measure the spectral characteristics of the SXRB emission
present at ten positions within the Milky Way, shown by blue circles in figure 1. A series of models representing the
different features of the SXRB were then fitted to the spectra.
Southern Bulge
Loop 1 model boundary
AIMSu By combining parameters from the fitted spectra with geometric models of the interstellar structures, we hope to
further understand the origin of the interstellar medium, its chemical composition, morphology and heating mechanisms.
t
Figure 1: The ¾ KeV ROSAT All Sky
Survey map, Snowden et al., 1995.
MODELLINGu Previous investigations of the SXRB in the northern
galactic hemisphere (including Willingale et al. [MNRAS, 343, 9951001, 2003], Snowden et al. [ApJ, 485, 125-135, 1997]) showed that
the x-ray spectra can be deconvolved into emissions from four distinct
structures that lie along the line of sight. This study has mapped a fifth
component in the southern survey fields; a hard emission of at least
4keV, possibly associated with galactic bulge.
To fit the spectra of the SXRB, a plasma model was assigned to each
structure, as illustrated in figure 2. These were then fitted in order of
increasing energy.
PN Data
Mos Data
OVIII
Galactic Halo
p
Figure 2: Cartoon showing interstellar structures, their energies, and the plasma
models used to represent them.
Local Hot Bubble (LHB)
Loop1 Superbubble
nH (the Wall)
Earth
Apec
(0.1keV)
Cosmic Background
Cool Halo
Galactic Bulge
nH
nH
nH
(galactic plane) (cold column)
(galactic column)
Wabs x Vapec
(0.3keV)
Wabs x Brem
(4.0keV)
Wabs x Apec
(0.1keV)
Wabs x Bknpower
FINDINGSu We have identified a hard component in Southern Galactic Bulge
visible at energies >2 keV, which can be fitted using a either Bremsstrahlung of
T~4 keV or a power law.
Rosat Data
OVII
q
FeXII NeIX NeX
Loop1 Plasma
Figure 3: An SXRB spectrum with best fit model, showing
emission lines and residuals to the fit.
Data from both the Mos and PN EPIC cameras were
used to fit the spectra between 0.3 - 4.0keV. PSPC
count rates from the ¼ keV ROSAT All-Sky Survey
were added below 0.3keV, and fitted simultaneously
with the EPIC data, to help constrain the model.
The plasma model specified in figure 2 adequately describes the SXRB in all
fields, except those within 6˚ of the Galactic Plane. This was the case for two
fields lying in the Southern Bulge: Here, the hard component artificially raises
the ambient emission measures of both Loop1 (~8.8x10-2 cm-6 pc) and the
Galactic Halo (~1.2x10-2 cm-6 pc), increasing them by factors of 9 and 5
respectively. Also, the relative abundance (compared to Solar) of Ne, Fe and
Mg within Loop1 are significantly lower nearer the Galactic Plane than
elsewhere, but O abundance is largely unaffected. This suggests that the hard
component found in the Southern Fields is non-thermal in nature, largely
continuum based, and is rich in O VII.
The emission measure of the LHB was found to be almost constant for all fields,
varying slightly between 3 - 8x10-4 cm-6 pc. This is to be expected, since the
Solar System is embedded within it.
The Wall seemed to vanish at 12˚ latitude, indicating that the LHB and the
Loop1 Superbubble may touch near this point.
q
Figure 5: Cartoon showing a vertical slice through the Galactic Plane indicating the relative
orientation of the various interstellar structures, together with the lines of sight of the observed
fields: NPS (dotted), Northern (grey), Southern (black). Since the LHB is shaped like a bent
hourglass, two profile projections are shown.
The Wall
p
Galactic Halo
LHB (Northern fields projection)
Figure 4: All-sky map showing the distance to the edge of the
local bubble: Closest regions are coloured blue, and the most
distant, red.
A geometric model was constructed, comprising a
cool absorbing layer and a sphere. The Wall is a
region of material between the LHB and Loop1, with a
density ~25 times greater than the other cool material
around the LHB, shown in figure 4. The sphere,
centred at (-8˚,25˚), 210pc from the Sun, and with a
radius of 42˚, represents the Loop1 Superbubble.
This model was used together with the fitted spectra
to calculate various physical parameters relating to
the SXRB structures, including their emission
measures and relative orientation (Figure 5).
Cold Material
(wall)
Cold Column
Loop1 Superbubble
Galactic Bulge
LHB (Southern fields projection)