2025ApJ...985...34R

We present the first study dedicated to measuring the timescales for black hole accretion and jet launching in a quasar at the edge of reionization, PSO J352.4034-15.3373 at z = 5.832 ± 0.001. Previous work presented evidence of the strong radio synchrotron emission from the jet affecting the host galaxy's dust-dominated continuum emission at ν_rest = 683 GHz (ν_obs = 100 GHz), implying a break in the synchrotron spectrum. In this work, we present quasi-simultaneous observations at 1.5 GHz-42 GHz with the Karl G. Jansky Very Large Array, and derive a frequency break at ${\nu }_{{\rm{rest}}}^{{\rm{break}}}=196.46$ GHz (${\nu }_{{\rm{obs}}}^{{\rm{break}}}=28.76$ GHz). Modeling these observations, we calculate the jet spectral aging from the cooling of electrons to be t_spec ∼ 580 yr. From this measurement, we approximate the dynamical age t_dyn to be ∼2000 yr, implying a recent jet ejection. We compare the jet timescale to the quasar's lifetime (t_Q) that indicates the duration of the latest black hole accretion event and is derived from the proximity zone size in the rest-UV/optical spectrum. However, a ghostly damped Lyα system affects this measurement yielding an upper limit of t_Q ≲ 10⁴ yr, consistent with the jet lifetime and indicative of a young quasar. This suggests that the triggering of a UV-bright quasar phase may occur within comparable timescales as the launch of a relativistic radio jet. Therefore, we may be witnessing an early stage of black hole and jet interactions in a quasar during the first billion years of the Universe.