Simultaneous X-Ray and Radio Observations of the Repeating Fast Radio Burst FRB ∼ 180916.J0158+65

We report on simultaneous radio and X-ray observations of the repeating fast radio burst source FRB 180916.J0158+65 using the Canadian Hydrogen Intensity Mapping Experiment (CHIME), Effelsberg, and Deep Space Network (DSS-14 and DSS-63) radio telescopes and the Chandra X-ray Observatory. During 33 ks of Chandra observations, we detect no radio bursts in overlapping Effelsberg or Deep Space Network observations and a single burst during CHIME/FRB source transits. We detect no X-ray events in excess of the background during the Chandra observations. These non-detections imply a 5σ limit of <5 × 10^-10 erg cm^-2 for the 0.5-10 keV fluence of prompt emission at the time of the radio burst and 1.3 × 10^-9 erg cm^-2 at any time during the Chandra observations. Given the host-galaxy redshift of FRB 180916.J0158+65 (z ∼ 0.034), these correspond to energy limits of <1.6 × 10⁴⁵ erg and <4 × 10⁴⁵ erg, respectively. We also place a 5σ limit of <8 × 10^-15 erg s^-1 cm^-2 on the 0.5-10 keV absorbed flux of a persistent source at the location of FRB 180916.J0158+65. This corresponds to a luminosity limit of <2 × 10⁴⁰ erg s^-1. Using an archival set of radio bursts from FRB 180916.J0158+65, we search for prompt gamma-ray emission in Fermi/GBM data but find no significant gamma-ray bursts, thereby placing a limit of 9 × 10^-9 erg cm^-2 on the 10-100 keV fluence. We also search Fermi/LAT data for periodic modulation of the gamma-ray brightness at the 16.35 days period of radio burst activity and detect no significant modulation. We compare these deep limits to the predictions of various fast radio burst models, but conclude that similar X-ray constraints on a closer fast radio burst source would be needed to strongly constrain theory.