The scientists looked at a specific feature called the “dispersion measure” — which represents the time differential between the detection of a burst’s high frequencies and its low frequencies. (Low frequencies travel more slowly through space dust, and thus take longer than high frequencies to reach Earth.)
To their surprise, they found that the dispersion measure of every pulse was a multiple of the number 187.5.
Hippke, Domainko, Learned, 30 March 2015: http://arxiv.org/pdf/1503.05245.pdf