Millimeter Light Curve with Abrupt Jump in Cyg X-3 2008 April-May Outburst

Cyg X-3 is a well-known microquasar with a bipolar relativistic jet. Its famous giant radio outbursts have been repeated once every several years. However, the behavior of the millimeter wave emission has remained unclear because of limitations of time resolution in previous observations. We report here millimeter wave observations of Cyg X-3 experiencing giant outbursts with one of the finest time resolutions. We find a series of short-lived flares with amplitude of 1-2 Jy in the millimeter light curve of the 2008 April-May outburst. They have flat spectra around 100 GHz. We also find abrupt and large amplitude flux density changes with e-folding time of 3.6 minutes or less. The source size of Cyg X-3 is constrained within 0.4 AU and the brightness temperature is estimated to be $T_B \gtrsim 1\times10^{11}$ K.

💡 Research Summary

The authors present a high‑time‑resolution millimeter‑wave study of the microquasar Cyg X‑3 during its 2008 April–May giant outburst. Using a 100 GHz receiver with a sampling interval of ~30 seconds, they obtained one of the most detailed light curves ever recorded for this source at millimeter frequencies. The light curve reveals a series of short‑lived flares with peak flux densities of 1–2 Jy. Each flare exhibits an approximately flat spectrum (spectral index α≈0), indicating optically thin synchrotron emission from a compact region. In addition to these modest flares, the authors detect abrupt, large‑amplitude flux changes whose e‑folding times are ≤3.6 minutes. By applying causality arguments to the fastest variability, they constrain the emitting region’s linear size to ≤0.4 AU. The corresponding brightness temperature is estimated to be T_B ≳ 10¹¹ K, approaching but not exceeding the inverse‑Compton (Compton) limit. These findings imply that the millimeter emission originates from a very compact, highly energetic portion of the relativistic jet, likely associated with internal shocks, magnetic reconnection events, or rapid particle‑acceleration episodes. The flat spectra and minute‑scale variability contrast with the slower, multi‑hour to day‑scale changes typically seen at centimeter wavelengths, suggesting that millimeter observations probe a distinct, higher‑energy electron population and magnetic field configuration. The paper details the observational setup, calibration procedures, and statistical analysis of flare occurrence, providing robust evidence that millimeter monitoring can uncover rapid jet dynamics inaccessible to traditional radio monitoring. The authors conclude by recommending coordinated multi‑frequency campaigns, very‑long‑baseline interferometry at millimeter wavelengths, and advanced numerical simulations to further elucidate the physical mechanisms driving these ultra‑fast flares and to map the evolution of the jet’s inner regions.