AN HRI MOSAIC OF THE SMC
S. L. Snowden & Y.-H. Chu
USRA/Goddard Space Flight Center,
snowden@lheavx.gsfc.nasa.gov
University of Illinois, chu@astro.uiuc.edu
Abstract:
We have proposed a mosaic of HRI pointings to map the central region of the Small Magellanic Cloud. The relatively deep PSPC pointings now in the ROSAT archive not only display a large collection of intriguing point sources but also the suggestion of extended emission at 3/4 keV. The higher spatial resolution of deep HRI pointings will provide a greater sensitivity in the identification of point sources by reducing source confusion, and therefore allow better limits to be placed on any emission of truly diffuse origin. The data will lend themselves to a wide range of other scientific studies such as the determination of luminosity functions of SMC sources to ergs s and the identification of small-scale extended sources.
Introduction
The Magellanic Clouds (MCs) are the only two external galaxies near enough so that a substantial fraction of their X-ray sources can be adequately resolved, and many of their optical counterparts can be identified and studied for further astrophysical information. Compared to the Milky Way galaxy, these two galaxies have the advantages of having small inclination angles to minimize the confusion and a low foreground obscuration to maximize the visibility. Therefore, the MCs offer the unique opportunity to obtain high-resolution pictures of an entire galaxy for us to study the detailed physical properties of individual X-ray sources, as well as the global statistical properties of different types of X-ray sources.
The ROSAT PSPC mosaic of archived observations of the Small
Magellanic Cloud shows considerably different structure in three different
energy bands (1/4 keV, 3/4 keV, and 1.5 keV). At
1/4 keV, variation over the field is relatively smooth and large scale
compared to the resolution of the instrument, and is punctuated by
a limited number of bright point-like sources. At higher energies
(Figure 1 
), the appearance of the field is nearly the
opposite. The 1.5 keV band is relatively smooth with a flat background
dominated by emission from point-like sources and a few extended sources of
limited size. At 3/4 keV the situation is considerably more complex.
While in general the appearance is similar to that at 1.5 keV, there is a
suggestion of extended emission in the central region as well. The
question of whether this emission is truly diffuse in origin or is the
superposition of a number of lower luminosity point sources is of great
interest. The existence of extensive diffuse emission will have a
significant impact on our understanding of both the ISM and energy balance
of the SMC.
We have therefore proposed to continue an extensive survey of the SMC
using the ROSAT HRI. This phase will complete the coverage of the
densest region of discrete sources (See Table and
Figure 2 ) which is also the area
of the possible extended emission at 3/4 keV. The present survey will also
cover 36 of the discrete sources detected by the Einstein survey
(Wang & Wu 1991; Seward & Mitchell 1981) and all 23 candidate supernova
remnants of Winkler & LeWinter (1993). The coverage of the survey will
be extended as subsequent AOs allow. We have requested an exposure
time of 25 ks for each pointing and all other existing HRI exposures
can be incorporated as they reach the archive. This survey is complementary
to the mosaic of pointings by the PSPC and in combination with those data
will allow the widest possible range of scientific projects. Because of
this and the relatively large amount of requested time, we have waived
the normal proprietary period for the observations. While independent
of a similar survey for the LMC, the two surveys aim at a similar
limiting magnitude.
Figure 1: 0.5-2.0 keV PSPC mosaic of the SMC.
Figure 2: Scheduled and requested HRI pointings >10 ks. Solid lines
indicate existing pointings, dot-dash lines indicate AO-6 pointings, and
dotted lines indicate observations proposed for AO-7.
Scientific Motivation
The HRI survey of the SMC will detect sources down to a threshold of ergs s, comparable to a companion survey in progress for the LMC. The scientific goals are also essentially identical to those of the LMC survey. We discuss three broad categories of possible scientific projects: 1) point source survey, 2) small-scale extended emission regions, and 3) large-scale diffuse emission.
Point Source Survey
The HRI survey will permit us to detect all point sources brighter than 2 ergs s. The SMC and LMC will be the first and only galaxies where the luminosity function of point X-ray sources can be determined to this level. Furthermore, the spatial distribution of the faint X-ray sources can be compared to the distribution of the underlying stellar content in order to determine what stellar population these sources may be associated with. The SMC is near enough that individual stars can be resolved and observed photometrically, and it is possible to use the observed color-magnitude diagram to determine the age of the underlying stellar population. The higher spatial-resolution data of this HRI survey will ease the task of making optical identifications for the X-ray sources. While the PSPC mosaic of pointings has a somewhat lower source-detection threshold, the possible source confusion is greater because of the poorer spatial resolution.
As noted above, the region of the survey (including the existing HRI pointings) covers 36 of the sources identified by Wang & Wu (1991). Their minimum detected count rate (0.002 Einstein IPC counts s) is roughly a factor of three higher than the sensitivity of our proposed survey. This survey will also have a more uniform sensitivity over the sampled region.
Small Extended Sources
The HRI survey proposed here will allow the determination with greater reliability of whether the faint sources detected in PSPC mosaic are point-like or extended. This is of great importance for the identification of SNRs and the determination of X-ray emitting mechanisms in superbubbles. Since Population I SNRs and superbubbles are often found associated with active star formation regions, the high concentration of massive stars make it statistically more likely to find massive X-ray binaries. For example, two point sources exist in the core of the 30 Doradus in the LMC. One is near the cluster at R136 and the other coincident with a tight group of WR stars at R140. In a Population I environment it is also statistically likely to find nebulosity near point X-ray sources. If the point sources are not distinguished from the small extended sources, it is possible to mis-identify SNRs and misinterpret the diffuse emission in superbubbles. By overlaying an optical image on the existing X-ray map, we have found coincidences of shell-like structures a few hundred parsecs in size and apparently diffuse X-ray emission.
The region covered by the proposed survey includes all six of the SNRs identified by Inoue, Koyama, & Tanaka (1983), which will be spatially resolved by the HRI. However, it has been recently found that at least two previously identified SNRs in the Small Magellanic Cloud (SMC) actually contain point X-ray sources that are coincident with Be stars (Hughes & Smith 1994). The lack of diffuse X-ray emission in a SNR that shows otherwise perfectly normal nonthermal radio shell and enhanced [S II]/H ratio is hard to explain (Ye et al. 1995). This example illustrates the importance of distinguishing point sources from diffuse emission from SNR candidates. Of course, for the classical SNRs, the HRI observations can be further used to study the physical structure of the SNRs.
One of the great contributions of ROSAT in the study of faint diffuse X-ray sources is the detection of a large number of superbubbles, most of whose surface brightness was below Einstein's detection threshold. Again for the LMC, the optically brightest superbubbles are all detected, including N11, N44, N51, N57, N144, N154, N158, and N206. On the other hand, the optically faint superbubbles such as DEM137, DEM105/106, N9, and N198 were not detected. The rough correlation between the X-ray and optical brightness implies that the X-ray emission mechanism may be more complicated than what has been proposed for the bright X-ray superbubbles, namely, SNRs hitting the superbubble walls from the interior (Chu & Mac Low 1990; Wang & Helfand 1991). Perhaps a clumpy medium, photoevaporation, and heat conduction have played a more important role than previous models assumed. In order to model the X-ray emission from superbubbles, we must know how the X-ray emission is distributed relative to the shell morphology and how much of the X-ray emission is truly diffuse in origin. High resolution images are obviously needed for the study of similar superbubbles in the SMC.
Large-Scale Diffuse Emission
A major goal of these observations is to determine whether there exists truly diffuse 3/4 keV emission from the SMC. As opposed to the LMC where diffuse emission is readily apparent, the SMC may have little above keV (Wang 1991 claims to have discovered a gaseous halo surrounding the SMC dominated by 1/4 keV band emission). A higher resolution search for point sources will allow us to better exclude the contribution of their emission from any flux of truly diffuse origin. In this scientific objective, the HRI observations are perfectly complementary to those of the PSPC. After removal of additional sources identified by the HRI, the PSPC data can be more accurately analyzed to provide the best possible spectral information for studying the diffuse emission. This in turn will make it more likely to correctly identify the source and emission mechanisms. Visual examination of the available long archive pointings (three) shows little if any evidence for additional discrete sources sufficient to account for the extended emission, supporting the existence of a hot ISM.
References
Chu Y.-H. & Mac Low M.-M. 1990, ApJ, 365, 510
Hughes, J.P. & Smith, R.C. 1994, ApJ, in press
Inoue, H., Koyama, K., & Tanaka, Y. 1983, in IAU Symposium No. 101, Supernova Remnants and their X-ray Emission, ed. J. Danziger and P. Gorenstein (Reidel:Dordrecht), p. 535
Seward, F. D., & Mitchell, M. 1981, ApJ, 243, 736
Wang, Q. 1991, ApJL, 377, L85
Wang, Q., & Helfand, D. J. 1991, ApJ, 373, 497
Wang, Q., & Wu, X. 1991, ApJS, 78, 391
Winkler, P. F., & LeWinter, R. 1993, AAS, Phoenix, January
Ye, T., Amy, S.W., Wang, Q.D., Ball, L., & Dickel, J.R. 1995, MNRAS, 275, 1218
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