STRUCTURE OF THE SOFT X-RAY BACKGROUND IN THE GALACTIC PLANE

Sangwook Park
Department of Physics
Purdue University

The origin and structure of the soft X-ray background (SXRB) has long eluded a satisfactory explanation for the last three decades primarily due to technical difficulties of the observations: i.e., identification and subtraction of the non-cosmic contaminations and good statistics with large (a few degrees or more) fields of view. The wealth of the archival ROSAT PSPC pointed observations, however, provides an ideal tool for the study of the SXRB. I completed the data reduction for 54 ROSAT PSPC pointed observations in the first quadrant (l = 4 - 26 degrees) of the Galactic plane and produced mosaics of these individual observations. All the identified non-cosmic contamination (Snowden et al. 1994) has been modeled and subtracted and the relative offsets in zero-level between all pairs of the overlapping pointings were determined. The produced mosaics provide ~100 sq. degree field of view with ~10 ks average exposure yielding ~10% statistics per pixel with 5' binning.

In the 0.5 - 2.0 keV band, I studied the 3-D spatial distribution of the mysterious Galactic X-ray emitting gas beyond the Local Hot Bubble (LHB) in the plane by searching for X-ray shadows cast by absorbing foreground molecular clouds. IRAS 100 um and CO data have been used as a tracer for such molecular clouds. Band-fraction and hardness ratio analysis in comparison with the ROSAT all-sky survey data was utilized to study spectral characteristics of the X-ray emission. The results of these studies suggest the existence of multiple components of Galactic gas beyond the LHB in the Galactic plane, including striking evidence for the presence of an X-ray emitting Galactic bulge (Park et al. 1997; Park, Finley, & Dame 1998). The estimated plasma temperature (logT=6.6 K) and spatial sturucture of the Galactic X-ray bulge is in good agreement with the ROSAT all-sky survey data (Snowden et al. 1997).

In the 0.1 - 0.3 keV (1/4 keV) band, I detected degree scale X-ray intensity variations, which are likely due to the shell-like structure at the boundary of the LHB. The detected magnitude and angular scales of the intensity variation and the hardness ratio of the spectrum support a significant contribution from a clumpy boundary structure of the LHB to the observed 0.1 - 0.3 keV band diffuse X-ray background in the Galactic plane (Park, Finley & Snowden 1997). The possible origins of such clumpy boundary structure include the presence of a magnetic Rayleigh-Taylor instability and interaction of the LHB with inhomogeneous ambient ISM at the boundary layer.

This project produced the best-ever observational data sets for the study of the soft X-ray background in the Galactic plane and is a good first step into the study of the detailed 3-D structure of the soft X-ray background. Follow up studies utilizing upcoming facitities with better spatial/spectral capability will be essential for understanding the origin and structure of the Galactic diffuse X-ray emission and its implications on the Galactic structure and evolution.

ROSAT ALL SKY SURVEY MAPS (Snowden et al. 1995)

1/4 keV 3/4 keV 1.5 keV
1/4 keV band X-ray intensity variations in the Galactic Plane
where 4 < l < 26 degree (Park, Finley, & Snowden 1997)


3/4 keV & 1.5 keV band X-ray Shadows in (l, b=10,0 deg) ( Park et al. 1997)


3/4 keV & 1.5 keV band X-ray Shadows in (l, b=25,0.5 deg) (Park, Finley, & Dame 1998)


3/4 keV band X-ray Shadows in (l, b= 8,-8 deg) (Park 1998)



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