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We present Low-Frequency Array (LOFAR) 143.5-MHz radio observations offlaring activity during 2019 May from the X-ray binary Cygnus X-3. Similar toradio observations of previous outbursts from Cygnus X-3, we find that thissource was significantly variable at low frequencies, reaching a maximum fluxdensity of about 5.8 Jy. We compare our LOFAR light curve with contemporaneousobservations taken at 1.25 and 2.3 GHz with the RATAN-600 telescope, and at 15GHz with the Arcminute Microkelvin Imager (AMI) Large Array. The initial143.5-MHz flux density level, $\sim$2 Jy, is suggested to be the delayed andpossibly blended emission from at least some of the flaring activity that hadbeen detected at higher frequencies before our LOFAR observations had begun.There is also evidence of a delay of more than four days between a bright flarethat initially peaked on May 6 at 2.3 and 15 GHz, and the corresponding peak($\gtrsim$ 5.8 Jy) at 143.5 MHz. From the multi-frequency light curves, weestimate the minimum energy and magnetic field required to produce this flareto be roughly 10$^{44}$ erg and 40 mG, respectively, corresponding to a minimummean power of $\sim$10$^{38}$ erg s$^{-1}$. Additionally, we show that thebroadband radio spectrum evolved over the course of our observing campaign; inparticular, the two-point spectral index between 143.5 MHz and 1.25 GHztransitioned from being optically thick to optically thin as the flaresimultaneously brightened at 143.5 MHz and faded at GHz frequencies.

Strong low-frequency radio flaring from Cygnus X-3 observed with LOFAR