Gamma-Ray and X-Ray Observations of the Periodic-Repeater FRB 180916 During Active Phases

FRB 180916 is a most intriguing source capable of producing repeating fast radio bursts with a periodic 16.3 day temporal pattern. The source is well positioned in a star-forming region in the outskirts of a nearby galaxy at 150 Mpc distance. In this Letter we report on the X-ray and gamma-ray observations of FRB 180916 obtained by AGILE and Swift. We focused especially on the recurrent 5 day time intervals of enhanced radio bursting. In particular, we report on the results obtained in the time intervals 2020 February 3-8, 2020 February 25, 2020 March 5-10, and 2020 March 22-28 during a multiwavelength campaign involving high-energy and radio observations of FRB 180916. We also searched for temporal coincidences at millisecond timescales between the 32 known radio bursts of FRB 180916 and X-ray and MeV events detectable by AGILE. We do not detect any simultaneous event or any extended X-ray and gamma-ray emission on timescales of hours/days/weeks. Our cumulative X-ray (0.3-10 keV) flux upper limit of 5 x 10(-14) erg cm(-2) s(-1) (obtained during 5 day active intervals from several 1-2 ks integrations) translates into an isotropic luminosity upper limit of L-X,L-UL similar to 1.5 x 10(41) erg s(-1). Deep gamma-ray observations above 100 MeV over a many-year timescale provide an average luminosity upper limit one order of magnitude larger. These results provide the so-far most stringent upper limits on high-energy emission from the FRB 180916 source. Our results constrain the dissipation of magnetic energy from a magnetar-like source of radius R-m, internal magnetic field B-m, and dissipation timescale tau(d) to satisfy the relation where R-m,R-6 is R-m in units of 10(6) cm, B-m,B-16 is B-m in units of 10(16) G, and tau(d,8) in units of 10(8) s.