Monitoring and Discovering X-ray Pulsars in the Small Magellanic Cloud

Regular monitoring of the SMC with RXTE has revealed a huge number of X-ray pulsars. Together with discoveries from other satellites at least 45 SMC pulsars are now known. One of these sources, a pulsar with a period of approximately 7.8 seconds, was first detected in early 2002 and since discovery it has been found to be in outburst nine times. The outburst pattern clearly shows a period of 45.1 ± 0.4 d which is thought to be the orbital period of this system. Candidate outburst periods have also been obtained for nine other pulsars and continued monitoring will enable us to confirm these. This large number of pulsars, all located at approximately the same distance, enables a wealth of comparative studies. In addition, the large number of pulsars found (which vastly exceeds the number expected simply by scaling the relative mass of the SMC and the Galaxy) reveals the recent star formation history of the SMC which has been influenced by encounters with both the LMC and the Galaxy. THE EARLY HISTORY OF SMC X-RAY PULSARS The first known X-ray pulsar in the SMC was the persistent supergiant system SMC X-1. Two luminous transients (SMC X-2, SMC X-3) were discovered with SAS3. [1]. These were thought to be transient Be/neutron star systems although pulsations were not detected due to the low sensitivity of SAS-3. It was hypothesized that SMC pulsars were exceptionally luminous, possibly related to the low metallicity of the SMC. [2] This was later to be disproved and an alternative explanation found for the high luminosity of the first few SMC X-ray pulsars to be discovered. Over the years a few pulsars were also found with satellites such as ROSAT. [3] SMC X-RAY PULSARS WITH RXTE Serendipitous RXTE slew observations in 1997 showed a possible outburst from the vicinity of SMC X-3. Follow up target of opportunity pointed RXTE observation showed a complicated power spectrum with several peaks that were not all harmonically related to each other. Imaging ASCA observations were next made which showed two separate pulsars, however neither was found to be located at the position of SMC X-3. A more detailed look at RXTE power spectrum showed that in fact three pulsars were simultaneously active [4]. These observations were the first sign of the existence of a very large SMC X-ray pulsar population. THE RXTE MONITORING PROGRAM RXTE has been regularly monitoring the SMC since 1997. We have discovered very many transient X-ray pulsars. For those sources where optical counterparts have been identified they are all found to be Be stars. We primarily make weekly observations of one particularly active region near SMC X-3. Other SMC regions have been monitored monthly depending on the amount of time awarded in a particular observing cycle. We use power spectrum to extract pulsed flux from any pulsars. In this way, although the PCA is not an imaging instrument, the pulsed flux from multiple sources can be monitored independently. When new sources are identified we use cross scans in Right Ascension and declination to localize the position of the new sources. Initially scans were done as Target of Opportunity observations. However, this was not always successful if the target had faded before the TOO could be performed. Therefore we currently include R.A./dec. scans in all our SMC observations. For new sources we thus obtain at least minimal positional information. However, position determination can be problematic if too many pulsars are simultaneously active. A log of known X-ray pulsars in the SMC is currently maintained at http://http://lheawww. gsfc.nasa.gov/~corbet/pulsars/.