Thermal Stability of Radiation-Pressure-Dominated Accretion Disks Threaded by Net Vertical Magnetic Flux

The classical radiation-pressure instability predicts strong thermal variability in luminous black-hole accretion disks, whereas most disk-dominated X-ray binary soft states remain comparatively stable. We examine whether net vertical magnetic flux can weaken this instability through its contribution to the radial stress. The stability depends not only on the equilibrium magnetic stress but also on how that stress changes during a thermal perturbation. We write the local stability condition in terms of the logarithmic heating response, $q_+<q_{+,\rm crit}$, which avoids specifying how the turbulent stress is divided into reference and net-flux components. For the illustrative closure $δ=ζ\sqrt{\bpol\btor}/α$, $\etad=-d\lnδ/d\ln H$ describes the response of the fractional stress correction, while $\Gnf=d\ln W_{\rm nf}/d\ln H$ describes the response of the additional net-flux stress itself. For an accretion disk around a $10M_\odot$ black hole at $R=20\rg$ and $\Mdot=0.5\MEdd$, we adopt fixed local mass flux on the thermal timescale, $(γ,\gp)=(1,0)$, and $d\lnζ/d\ln H=0$. Marginal stability then occurs at $\bpol^{\rm crit}=0.0176$ for $ζ=1$ and $0.1338$ for $ζ=0.25$. These thresholds depend on the adopted stress closure and field-response prescription and should not be interpreted as universal magnetic-pressure fractions. Fixed-$B_z$ equilibrium sequences, using a separate relation for the variation of $B_\varphi$ between steady states, show that increasing vertical field narrows the thermally unstable accretion-rate interval but does not eliminate it for either $H/R<0.1$ or $H/R<0.2$. The relevant quantity for stabilization is therefore the thermal response of the stress associated with net vertical flux rather than the vertical magnetic-pressure fraction alone.