SIMBAD references

As part of a large-scale search for coherent pulsations from low-mass X-ray binaries (LMXBs) in the Rossi X-Ray Timing Explorer (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. This binary's short period implies that any coherent signal would be highly accelerated, so we used phase modulation searches, orbital-parameter-fitting coherent searches, and standard acceleration searches to give significant sensitivity to millisecond pulsations. At a range of luminosities and in four energy bands, we searched a total of 34 archival RXTE observations, 32 of which had on-source integration times longer than 10 ks and some of which were made consecutively, which allowed us to combine them. We found no pulsations. Using our phase modulation search technique, which we ran on all 34 observations, we have been able to place the first stringent (95% confidence) pulsed fraction limits of ≲0.8% on all realistic spin frequencies (i.e., ≲2 kHz) and likely companion masses (0.02M_☉≤M_c≤0.3M_☉). Using our orbital-parameter-fitting coherent search, which we ran on only 11 selected observations, we have placed a pulsed fraction limit of ≲0.3% on spin frequencies ≲1.25 kHz and companion masses M_c≤0.106 M_☉. By contrast, all five LMXBs known to emit coherent pulsations have intrinsic pulsed fractions in the range 3%-7% when pulsations are observed. Our searches therefore rule out even pulsations with significantly lower pulsed fractions than those already observed.