Jet Forecast: Timing of Discrete Jet Ejections of Stellar-mass Black Holes Anticipated via X-Ray Properties

The radio–soft γ -ray data of the Galactic black hole binary XTE J1859+226 showed that discrete jets are launched when the inner radius of the accretion disk ( R in ) rapidly approaches the innermost stable circular orbit (ISCO). Given the correlations among the X-ray fractional variability amplitude (rms), R in , and the X-ray hardness, previous phenomenological findings related to jet ejections (“jet line” and sharp drops in rms) also indicate that the rapid shrinkage of R in down to the ISCO turns on the jet formation. In order to make clear how these properties appear in other objects, we investigated the X-ray and radio data of XTE J1550−564, GX 339−4, and MAXI J1820+070. We found that all the four sources show drops in R in and rms 1–10 days ahead of a jet activity, and exhibit no huge jumps in the accretion rates prior to jet ejections. XTE J1859 and MAXI J1820 show the jet activities under sub-Eddington accretion. Models for the jet production that require an enhancement in M ̇ or a super-Eddington accretion rate are unlikely. Another behavior common to multiple sources is the curved shape of the rms– R in correlation. Therefore, monitoring of R in seems to be suitable to predict the timing of jet ejections. When both R in and rms continuously decrease and when R in is about to reach the ISCO, alerts can be circulated as a “jet forecast” for triggering Target-of-Opportunity observations.