Einstein observations of ultrasoft X-ray sources in the Magellanic Clouds

We have analyzed all of the archival Einstein Observatory data on three ultrasoft X-ray-emitting compact sources in the Magellanic Clouds. Two of the sources, Columbia Astrophysics Laboratory (CAL) 83 and CAL 87, are thought to be low-mass X-ray binaries (LMXBs) located in the LMC, while the other is believed to be associated with one of three optical counterparts in the Small Magellanic Cloud (SMC), the most likely candidate being the planetary nebula N67. The LMXBs are particularly unusual because they do not emit X-rays between 0.5 and 4 keV, in startling contrast to other known LMXBs. If the SMC source is N67, then it is very unusual as well, since its total X-ray luminosity compared to all other known planetaries is exceptionally high. CAL 83 and CAL 87 have X-ray light curves which appear to exhibit random fluctuations in intensity, while the SMC source appears to exhibit fluctuations which might be nonrandom. All three of the sources can be modeled with extremely low equivalent blackbody temperatures: the best-fit value of kT for CAL 87 is 63 eV, and for the SMC source is 38 eV, adopting the Galactic values for the equivalent hydrogen column densities in the lines of sight to these two objects. Two different observations of CAL 83 revealed slightly different blackbody temperatures of 13 and 9 eV, although this difference is not significant within the errors of measurement.