An RXTE archival search for coherent X-ray pulsations in LMXB 4U 1820-30

As part of a large-scale search for coherent pulsations from LMXBs in the RXTE archive, we have completed a detailed series of searches for coherent pulsations of 4U 1820-30 - an ultracompact LMXB with a binary period of 11.4 minutes located in the globular cluster NGC 6624. The small binary period leads to a very high acceleration, so we used phase modulation searches as well as acceleration searches to give significant sensitivity to millisecond pulsations. We searched a total of 34 archival RXTE observations, 32 of which had an on-source integration time longer than 10 ks, and some of which were made consecutively which allowed us to combine them. While we found no pulsations, we have been able to place the first stringent (95% confidence) pulsed fraction limits of less than or similar to8% for all realistic spin frequencies (i.e. less than or similar to1 kHz) and likely companion masses (0.02 M-circle dot less than or equal to M-c less than or equal to 0.3 M-circle dot). By contrast all five LMXBs known to emit coherent pulsations have intrinsic pulsed fractions in the range 3% to 7% when pulsations are observed.