Optical Counterparts of X-ray Sources in the Small Magellanic Cloud

Abstract



We searched for optical counterparts to X-ray sources in the SMC. We looked at thirteen fields observed with the CTIO 1.5-m telescope for potential canidates. These fields were selected from a ROSAT survey of the SMC that found about 250 X-ray sources. These thirteen fields were chosen because of their weak, hard X-ray emissions, which might correspond to cataclysmic variables. These fields also had the smallest pointing errors from the X-ray satellite. Each field was observed in the optical UBVRI passbands.

A region approximately twice as large as the pointing error given by the satellite was studied in each field. Standard magnitudes were obtained for every star in this region, from the standard stars which were viewed each night of the run. These standard magnitudes were then used in a ``poor man's spectrum'' by graphing them against their central wavelength. Cataclysmic variables should appear brightest in the blue, but also have substantial red flux. All of the stars that are strongest in the blue are considered to be candidates for the X-ray source. In the future, these candidate stars will be re-observed in order to determine which one is producing the X-rays in each field.

Figures:

Figure 1: A representative field we observed. The green circle represents the pointing error of the ROSAT satellite. (The linear feature is an unfortunately-located satellite trail.) JPG (63 kb) or Gzipped Postscript (510 kb)

Figure 2: X-ray sources are well-known for being located just outside the pointing error box, so this image shows the stars we actually analyzed. JPG (59 kb) or Gzipped Postscript (465 kb)

Figure 3: In order to form a ``poor man's spectrum,'' the wavelengths of the filters are graphed against the calibrated magnitudes for each star. This is the poor man's spectrum of four example stars. The orange star stays fairly constant over the spectrum. The red star is brightest in the red wavelengths. The purple star is high in the blue, however it is even higher in the red. The blue star is fairly constant across the spectrum, then goes up sharply in the blue wavelengths. This is the best candidate in this example. JPG (31 kb) or Gzipped Postscript (274 kb)

Figure 4: The locations of the four stars in the field. JPG (64 kb) or Gzipped Postscript (520 kb)



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