Transonic accretion flow in the mini discs of a binary black hole system

Abstract

We study the general relativistic transonic accretion flow around the primary black hole, which forms the circumprimary disc (CPD), within a binary black hole (BBH) system. The BBH spacetime is characterized by the mass ratio ($q$) and the separation distance ($z_2$) between the two black holes. We numerically solve the radial momentum and energy equations to obtain the accretion solutions. It is observed that the CPD can exhibit shock solutions, which exist for a wide range parameter space spanned by flow specific angular momentum ($λ$) and energy ($E$). We find that the shock parameter space is modified by $q$ and $z_2$. Investigations show that $q$ and $z_2$ also affect various shock properties, such as density compression and temperature compression across the shock fronts. Moreover, we calculate the spectral energy distributions (SEDs) of the CPD and examine how the SEDs are modified by $q$ and $z_2$ for both shock-free and shock-induced accretion solutions. SED is found to be nearly independent of the binary parameters. We essentially show that although $q$ and $z_2$ alter the effective horizon area of the primary black hole located at the center of the CPD, they have a minimal impact on the dynamical and spectral properties of the accretion flow around the primary black hole.

Figures (9)

• Figure 1: Schematic diagram of a binary black hole system featuring two co-linear Schwarzschild black holes. The primary and secondary black holes are located at $z_1$ and $z_2$, respectively. The thick blue lines represent the extended regions of the event horizons along the $Z$-axis. For the primary black hole, the horizon extends between $w_1$ and $w_2$, while for the secondary black hole, the horizon extends between $w_3$ and $w_4$. See the test for more details.
• Figure 2: Plot of horizon area ($A_1$) of the primary black hole with binary mass ratio ($q$) for different binary separations $z_2 = 300$, $400$, $500$, and $600$. See the text for details.
• Figure 3: Plot of specific energy ($E$) as a function of critical point location ($r_c$) for different angular momentum ($\lambda$) in panel (a), binary mass ratio ($q$) in panel (b), and binary separation ($z_2$) in panel (c). Here, solid, dotted, and dashed curves represent saddle, nodal, and spiral types critical points, respectively. The horizontal dashed lines (magenta) denote the energy values where the flow possesses multiple critical points. See the text for details.
• Figure 4: Plot of Mach number ($M = v/C_s$) as a function of radial distance ($x$) for the binary mass ratio $q = 0.01$, $0.1$, and $1$ with binary separation $z_2 = 500$ (left panels), and for $z_2 = 500$, $750$, and $1000$ with $q = 0.5$ (right panels). Here, the critical points are marked by the filled circles, and we choose $\lambda = 3.5$ and $E = 1.01$. See the text for details.
• Figure 5: Illustration of the shock-induced accretion solutions (Mach number ($M$) versus radial distance ($x$) curves (solid) in panel (a)) for binary mass ratios $q = 0.25$, $0.5$, $0.75$, and $1$ with binary separation $z_2 = 350$. The respective profiles of radial-velocity ($v$) in panel (b), mass density ($\rho_0$) in panel (c), electron temperature ($T_e$) in panel (d), frequency-integrated emissivity ($\mathcal{E}$) in panel (e), and effective optical depth ($\tau_{\text{eff}}$) in panel (f). The dashed curves denote the scenario where shock transitions have not occurred. The flow parameters for this figure are chosen as $\lambda = 3.45$ and $E = 1.01$. See the text for details.