A case for a binary black hole system revealed via quasi-periodic outflows

Abstract

Binaries containing a compact object orbiting a supermassive black hole are thought to be precursors of gravitational wave events, but their identification has been extremely challenging. Here, we report quasi-periodic variability in x-ray absorption, which we interpret as quasi-periodic outflows (QPOuts) from a previously low-luminosity active galactic nucleus after an outburst, likely caused by a stellar tidal disruption. We rule out several models based on observed properties and instead show using general relativistic magnetohydrodynamic simulations that QPOuts, separated by roughly 8.3 days, can be explained with an intermediate-mass black hole secondary on a mildly eccentric orbit at a mean distance of about 100 gravitational radii from the primary. Our work suggests that QPOuts could be a new way to identify intermediate/extreme-mass ratio binary candidates.

Fig. 1.. ASASSN-20qc’s long-term evolution and a sample x-ray spectrum highlighting the outflow.

(A) ASASSN-20qc’s observed x-ray and optical evolution. Orange data represents x-ray (0.3 to 1.1 keV) data acquired by various instruments. The blue data show the Gaia magnitude. The horizontal (dashed) line represents NICER’s sensitivity limit of 3 × 10⁴² erg s⁻¹ for a source at redshift, z, = 0.056. (B) Combined x-ray spectrum using all NICER data acquired over epochs of high absorption (yellow) and the best-fit emission model (black histogram). (C) Zoom-in of the outburst near the x-ray peak. (D) Ratio of the average energy spectrum using all NICER data acquired over epochs of minima in ODR and the best-fit thermal model. The outflow band is defined as the 0.75- to 1.00-keV band, while the inflow/accretion band is defined as the bandpass where the ratio is near 1, i.e., 0.30- to 0.55-keV band.

Fig. 2.. Summary of ASASSN-20qc’s timing analysis.

(A) ASASSN-20qc’s ODR versus time. ODR is defined as the ratio of background-subtracted count rates in 0.75- to 1.00-keV (outflow) and 0.3- to 0.55-keV (continuum) bands. A lower ODR value implies a stronger outflow and vice versa. The dashed vertical red lines are uniformly separated by 8.5 days. (B) Lomb-Scargle periodogram (LSP) of the ODR. The strongest signal is near 8.5 days. The horizontal dashed red lines show the 3 and 4σ global false alarm probabilities as per (6). The noise in the periodogram is consistent with white with a mean LSP power value of 1 (see Materials and Methods, “Values in the LSP are consistent with white noise” section). (C) Global (trials-accounted) false alarm probability. This curve was generated using extensive Monte Carlo simulations (see Materials and Methods, “ODR timing analysis” section). The global statistical significance of the 8.5-day quasi-periodicity is >4.2σ.

Fig. 3.. A sample snapshot from our GRMHD simulation (2D HARM, run 14 from table S7).

For this case, the SMBH mass was set to 10^7.4M_⊙ and the perturbing companion is in an elliptical orbit (eccentricity, e = 0.5) with an observed orbital period of 8.5 days and has an influence radius of three gravitational radii [1M = GM_•/c² = 0.25(M_•/10^7.4M_⊙)AU]. (A) Spatial distribution of the logarithm of mass density expressed in arbitrary units. The horizontal and the vertical axes are spatial coordinates expressed in gravitational radii (units of M). The white contours indicate the magnetic field configuration. The position and size of the perturber are shown by the black circle, while the gray line displays its trajectory in the 2D slice. (B) Spatial distribution of the Lorentz factor of the gas bulk motion. (C) Spatial distribution of the mass outflow rate with v > 0.2c. The outflow rate is color-coded using arbitrary units according to the color bar to the right. (D) Temporal profiles of the inflow rate (blue), the outflow rate through the upper funnel (purple), and the outflow rate through the lower funnel (green). The inflow and outflow rates are expressed in arbitrary units. The time is expressed in days in the observed frame. The colored points/dots indicate the time of the snapshot. Vertical lines are uniformly separated by 8.5 days. (E and F) LSP of the ratio of the outflow to the inflow rates (E) and the accretion rate (F) from run 14 sampled exactly as the real data. The peak signal in (E) is broad with a value of ${8.5}_{-1.1}^{+0.7}$ days and is consistent with the observed value of 8.3 ± 0.3 days (shaded blue band), while no such signal is present in the accretion rate periodogram (F), i.e., an elliptical binary can reproduce the observed quasi-periodicity in the outflow strength without similar variations in the continuum.

Fig. 4.. Schematic of a potential model for ASASSN-20qc.

A gravitationally bound (preexisting) IMBH located at roughly 100 R_g from the central SMBH can explain the repeated outflows seen here. The overall outburst could have been triggered by a tidal disruption of a passing star by the SMBH, which creates a compact accretion disk that naturally enhances the x-ray emission and consequently illuminates the surrounding environment and the presence of the IMBH secondary. Secondary plunges through the preexisting (non-TDE) accretion flow, modulating the outflow on the orbital period. Relative sizes are not to scale.