The Inclination of the Soft X-ray Transient A0620--00 and the Mass of its Black Hole

We analyze photometry of the Soft X-ray Transient A0620-00 spanning nearly 30 years, including previously published and previously unpublished data. Previous attempts to determine the inclination of A0620 using subsets of these data have yielded a wide range of measured values of i. Differences in the measured value of i have been due to changes in the shape of the light curve and uncertainty regarding the contamination from the disk. We give a new technique for estimating the disk fraction and find that disk light is significant in all light curves, even in the infrared. We also find that all changes in the shape and normalization of the light curve originate in a variable disk component. After accounting for this disk component, we find that all the data, including light curves of significantly different shapes, point to a consistent value of i. Combining results from many separate data sets, we find i=51 plus or minus 0.9 degrees, implying M=6.6 plus or minus 0.25 solar masses. Using our dynamical model and zero-disk stellar VIH magnitudes, we find d=1.06 plus or minus 0.12 kpc. Understanding the disk origin of non-ellipsoidal variability may assist with making reliable determinations of i in other systems, and the fluctuations in disk light may provide a new observational tool for understanding the three-dimensional structure of the accretion disk.

💡 Research Summary

The paper presents a comprehensive re‑analysis of photometric observations of the soft X‑ray transient A0620‑00, spanning nearly three decades (1975–2005) and incorporating both previously published and unpublished data in the optical (V, I) and near‑infrared (H) bands. Earlier attempts to determine the orbital inclination (i) of the system yielded widely disparate values (≈30°–70°) because researchers used limited subsets of the data and could not accurately account for contamination from the accretion disk.

To overcome these limitations the authors introduce two key methodological advances. First, they develop a novel technique for estimating the “disk fraction” – the proportion of total light contributed by the accretion disk – by exploiting simultaneous multi‑band measurements at the same orbital phase. By comparing color differences across bands, they separate the ellipsoidal modulation of the secondary star from the variable disk component, thereby quantifying the disk’s contribution in each light curve. This analysis demonstrates that disk light is non‑negligible in every dataset, even in the infrared where one might expect stellar dominance.

Second, they test the hypothesis that all non‑ellipsoidal variability originates in a time‑variable disk. By extracting the residual (non‑ellipsoidal) component from each light curve, they find that its amplitude and shape change with time but can be fully explained by fluctuations in the disk brightness. After subtracting the disk contribution, the remaining stellar light curves exhibit a consistent shape and amplitude, indicating that the underlying ellipsoidal modulation is stable and that the inclination can be derived robustly from any epoch’s data once the disk is removed.

Using a simultaneous fit to all corrected light curves, the authors obtain a tightly constrained inclination of i = 51° ± 0.9°. Combining this inclination with the previously measured mass function, the spectral type of the secondary (K5 V), and standard mass‑luminosity relations yields a black‑hole mass of M = 6.6 ± 0.25 M⊙. Moreover, by adopting the zero‑disk stellar magnitudes in V, I, and H and applying standard extinction and distance modulus relations, they derive a distance of d = 1.06 ± 0.12 kpc.

Beyond the specific parameters of A0620‑00, the study highlights broader implications. Accurate disk‑fraction estimation dramatically reduces systematic uncertainties in inclination and mass measurements for low‑luminosity X‑ray binaries. The demonstrated link between disk variability and non‑ellipsoidal light changes provides a new diagnostic for probing the three‑dimensional structure, temperature distribution, and possible asymmetries of accretion disks. This approach can be generalized to other quiescent black‑hole binaries, improving the reliability of dynamical mass determinations across the population.

In summary, by rigorously quantifying and removing the variable disk contribution, the authors reconcile previously conflicting inclination estimates and deliver precise values for the inclination, black‑hole mass, and distance of A0620‑00. Their methodology sets a benchmark for future dynamical studies of X‑ray transients and offers a novel observational avenue for investigating accretion‑disk physics.