The nature of fast radio bursts ( FRBs ) , which occurs on millisecond time scales in the radio band , has not been well-understood .
Among their unknown observational properties are their broadband spectra and persistent and transient multi-wavelength counterparts .
Well-localized FRBs provide us the opportunity to address these issues in archival observations .
We have performed searches for 15-150 keV hard X-ray bursts on time scales as short as { 10 } milliseconds in the direction of the repeating FRB 121102 ( with a spacial resolution of a few arcminutes ) in the archival Swift /BAT data during the period between October 2016 and September 2017 .
We have found no significant ( 5 \sigma ) hard X-ray bursts in the direction of the repeating FRB .
We have derived an upper limit of the hard X-ray ( 15–150 keV ) flux of any X-ray bursts on 1 ms time scale of around 1.01 \times 10 ^ { -7 } erg cm ^ { -2 } s ^ { -1 } , if assuming a photo-index of 2 for potential X-ray flares in X-ray band .
A plausible scenario for the repeating FRB as being associated with magnetar giant flare is still far below the upper limit .