Evidence of a shared spectro-temporal law between sources of repeating
  fast radio bursts
  We study the spectro-temporal characteristics of two repeating fast radio
bursts (FRBs), namely, FRB 20180916B and FRB 20180814A, and combine the results
with those from our earlier analysis on FRB 20121102A. The relationship between
the frequency drift rate, or slope, of individual sub-bursts and their temporal
duration is investigated. We consider a broad sample of possible dispersion
measure (DM) values for each source to understand the range of valid sub-burst
slope and duration measurements for all bursts and to constrain our results. We
find good agreement with an inverse scaling law between the two parameters
previously predicted using a simple dynamical relativistic model. The
remarkably similar behaviour observed in all sources provides strong evidence
that a single and common underlying physical phenomenon is responsible for the
emission of signals from these three FRBs, despite their associations with
different types of host galaxies at various redshifts. It also opens up the
possibility that this sub-burst slope law may be a universal property among
repeating FRBs, or indicates a distinct subclass among them.
