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EXPLOliy^G THE TOf bSlcf IC HAm 


Grant No. NAG5-3197 


For the Period from 15 May 1996 through 14 May 2001 

Principal Investigator 
D r . M . Juda 

December 2001 

Prepared for: 

NationaJ Aerpnautic^^ Space Administ^atio^^ 

Goddard_Space FUght Center 

- ::: Greenbelt, Maryland 2"077r " 

Smithsonian Institution 
AstrophysicaT Observatory 

Cambridge, Massachusetts 02138 

The Smithsonian Astrophysical Observatory 

is a member of the 
Harvard-Smithsonian Center for Astrophysics 

The NASA Technical Officer for this grant is Dr. Robert Petre, Code 662, Laboratory for High 
Energ)' Astrophysics, NASA/Goddard "Space Flight Center, Greenbelt, Mar>'land 20771. 

The objective of this proposal was to measure variations in the 1/4 keV emission 
from the galactic halo, using ROSAT PSPC observations toward known enhancements 
in the absorbing column density along the line-of -sight out of the Galaxy. 
Target directions were selected to have a low total hydrogen column density but 
to also show significant gradients in the amount of absorbing material, as 
traced by IRAS 100 micron emission, on angular scales that would be contained 
within the PSPC field of view. In addition, we restricted the galactic latitude 
of the target directions to be greater than 60 degrees or less than -60 degrees 
in order to enable a cleaner separation of Galactic halo emission from that of 
the Galactic disk. The observations would also provide a measurement of the 
brightness of the emission from the Local Bubble. 

Five of six targets from the proposal were granted and received o'^^^'^i^f^!,^'^^;,^ 
we supplemented these observations with additional pointings from the ROSAT PSPC 
archive The data were filtered to remove times of high rates from particle or 
solar backgrounds and brightness images in the 1/4 keV band were produced. 
Estimated contributions from internal and external non-x-ray backgrounds and 
scattered solar x-rays were subtracted from the brightness images. Finally, 
point sources and discrete extended sources were removed from the images to 
produce images of the 1/4 keV brightness of the diffuse emission. 

Initially the 1/4 keV sky brightness images were correlated with IRAS 100 micron 
images obtained through the HEASARC. Most of the target directions showed an 
anticorrelation between the 1/4 kev brightness and the 100 micron brightness 
indicating the presence of emission form behind the material responsible for the 
100 micron emission. By assuming an extremely simple model for the distribution 
of the material responsible of the 1/4 kev emission and absorption, a local 
emitting region and a single distant emitting region separated by all the 
absorbing material, fits to the anticorrelation could be made. Unfortunately, 
while the relative brightness of the IRAS 100 micron emission on small angular 
scales was reasonably well calibrated, the absolute level was not. The 
brightness of the distant, absorbed 1/4 keV emission could only be determined by 
adding or subtracting a constant addition term in the absorption column density; 
where the constant was calculated to make up the difference between the scaled 
IRAS 100 micron brightness and the hydrogen column density derived from lower 
angular resolution 21 cm measurements. Later, a rescaled version of the IRAS 100 
micron data from Schlegel, Finkbeiner, and Davis, (1998, ApJ, 500, 525) allowed 
the anticorrelation fits to be performed without this added uncertainty. On 
average 60% of the total observed 1/4 keV diffuse emission is generated by the 
local, unabsorbed emission region (the Local Bubble) . The distant, absorbed 
component (the local Galactic halo) is brighter at the northern Galactic pole 
than the southern. In the northern directions we examined that are away from the 
North Polar Spur the halo brightness varies by a factor of four from faintest to 
brightest. The average northern brightness is 0.001 counts/s/square-arcmin; m 
the south the average halo brightness is about 2/3 this value. 

There are a few limitations and complications to this work. The distribution of 
the emitting and absorbing material that we assumed is quite simple. More _ 
complex arrangements with several emitting regions separated by absorbers having 
gradients in different directions across the field or with intermixed absorbing 
and emitting gas could produce an anticorrelation between the 1/4 keV and 100 
micron brightnesses but it would be significantly more difficult to interpret. 
We do not know the distance to the absorption enhancements traced by the 100 
micron emission and it is only the gradient in it that generates the 
anticorrelation we observe. Obtaining distances to these clouds would help in 

establishing the validity of the assumed geometry. Even in our simple geometry 
the distant emitting component is not exclusively due to emission from the halo, 
a contribution should come from the extragalactic cosmic x-ray background. The 
extragalactic contribution is of unknown size and is not likely to be constant 
within the PSPC field of view. 

Results of these analyses were presented at scientific meetings in the following 
papers : 

• "ROSAT PSPC Observations of Clouds at High Galactic Latitude", M. Juda, D. 
McCammon, W. T. Sanders, and S. L. Snowden, 182th Meeting of the A.A.S., 
Berkeley, CA, June 1993. 

• "Emission from the Local Galactic Halo in the 1/4 keV Band", M. Juda, 185th 
Meeting of the A.A.S., Tucson, AZ, January 1995. 

• "Mapping the Distribution of 10"6 K Gas Toward the Galactic Poles", M.Juda, 
188th Meeting of the A.A.S., Madison, WI, June 1996. 

• "Intensity of the 1/4 keV Emission from the Local Galactic Halo", M. Juda, 
Meeting of the High Energy Astrophysics Division of the A.A.S., Estes Park, 
CO, November 1997. 

• A draft of a paper for submission for publication in the Astrophysical 
Journal is in work but progress has been slowed by outside pressures.