2019ApJ...885...55S

The nature of fast radio bursts (FRBs), which occur on millisecond timescales in the radio band, is well-understood. Among their unknown observational properties are their broadband spectra, and persistent and transient multiwavelength counterparts. Well-localized FRBs provide the opportunity to address these issues in archival observations. We performed searches for 15-150 keV hard X-ray bursts on timescales as short as 1 ms in the direction of the repeating FRB 121102 (with a spacial resolution of a few arcminutes) in the archival Swift/BAT data between 2016 October and 2017 September. We found no significant (5σ) hard X-ray bursts in the direction of the repeating FRB. We derived an upper limit of the hard X-ray (15-150 keV) flux of any X-ray bursts on a 1 ms timescale of around 1.01 x 10^–7 erg cm^–2 s^–1, if assuming a photoindex of 2 for potential X-ray flares in the X-ray band. A plausible scenario for the repeating FRB as being associated with a magnetar giant flare is still far below the upper limit.