We report a series of joint detections of FRB 180916.J0158+65, recently discovered to exhibit a 16.35-day periodicity with a ~4-day active phase window, between the GBT (600-1000 MHz), uGMRT (300-500 MHz), and CHIME (400-800 MHz) radio observatories. We observed a total of 7, 4, and 1 bursts from the source, respectively, which collectively spanned the 700 MHz bandwidth over the course of ~3 days, with 1 hour of overlapping observations. The GBT detections were made earlier in the active phase window and exhibited a significantly higher burst-rate compared to those made by uGMRT, hinting at a likely frequency dependence of burst activity phase. In both GBT and uGMRT detections, we observe clear downward drifting emissions and complex multi-component time-frequency modulations at narrow scales, which motivate the use of baseband data for investigations of FRB sub-structure.
We performed our analysis using the transient detection pipeline SPANDAK and analysis pipeline FLITS, which currently run on the 64-node Breakthrough Listen Digital Instrument at the GBT. SPANDAK uses SIGPROC filterbank data products to search for transient signals (FRBs and ETIs). FLITS successively extracts the baseband raw voltages, stored at the GBT native time and frequency resolutions, which enable coherent dedispersion, calibration with full-Stokes parameters, and comprehensive offline analysis. We leverage these analysis techniques for both GBT and uGMRT bursts to fully resolve spectral, temporal, and polarization angle variability in emission signatures. In this process, we employ a DM metric that maximizes frequency-averaged pulse structure, rather than signal-to-noise. Furthermore, to constrain potential source models, we consider features which could be intrinsic to the emission mechanism, and how they might reasonably differ from those superimposed by propagation effects in the surrounding medium.