A search for short-term hard X-ray bursts in the direction of the repeating FRB 121102

Abstract: 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 millisecond 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 \emph{magnetar giant flare} is still far below the upper limit.