Excess close burst pairs in FRB 121102

The repeating FRB 121102 emitted a pair of apparently discrete bursts separated by 37 ms and another pair, 131 d later, separated by 34 ms, during observations that detected bursts at a mean rate of ~2 × 10^-4 s^-1. While FRB 121102 is known to produce multipeaked bursts, here I assume that these 'burst pairs' are truly separate bursts and not multicomponent single bursts, and consider the implications of that assumption. Their statistics are then non- Poissonian. Assuming that the emission comes from a narrow range of rotational phase, then the measured burst intervals constrain any possible periodic modulation underlying the highly episodic emission. If more such short intervals are measured a period may be determined or periodicity may be excluded. The excess of burst intervals much shorter than their mean recurrence time may be explained if FRB emit steady but narrow beams that execute a random walk in direction, perhaps indicating origin in a black hole's accretion disc.