Thinner Disks Struggle to Launch Jets: GRMHD Simulations of Magnetic Field Generation and Transport

Abstract

Relativistic jets are observed in some accreting black-hole systems but not others. In X-ray binaries, jet production is closely associated with geometrically thicker accretion states, suggesting that disk thickness may influence the conditions for jet launching. We investigate this connection using three-dimensional GRMHD simulations of disks with $h/r=0.1$--$0.3$, initialized with a toroidal magnetic field with plasma $β=5$. We find that the magnetorotational instability (MRI) generates large-scale poloidal fields through non-axisymmetric shearing waves, with an additional contribution from axisymmetric induction through stretching by channel-like modes. The resulting mean fields become progressively weaker as the disk becomes thinner. Because these fields regulate MRI turbulence, thinner disks exhibit weaker angular-momentum transport and accrete more slowly than the standard $v_{\rm acc}\propto(h/r)^2$ thin-disk scaling would predict. The magnetic-flux transport velocity remains comparable to the accretion velocity, so weaker field generation and slower transport combine to strongly suppress black-hole flux accumulation at smaller $h/r$. Correspondingly, the thinnest disk ($h/r=0.1$) produces an electromagnetic jet efficiency of only $η_{\rm{EM}}\sim1\%$, while the thickest disk reaches the magnetically arrested disk (MAD) state and attains $η_{\rm{EM}}\geq100\%$. Using the measured large-scale magnetic flux distribution and transport velocities, we estimate the time required to accumulate MAD-level magnetic flux. Only the $h/r\sim0.3$ disk reaches MAD in the duration of our simulations, at $t\simeq2\times10^4\,r_g/c$, whereas for $h/r=0.1$ we estimate a timescale of $\sim10^8\,r_g/c$. These results link disk geometry to magnetic-flux accumulation and jet launching, with direct implications for the timescales over which jets can emerge in transient accreting systems.