An Extreme Black Hole in the Recurrent X-Ray Transient XTE J2012+381

Abstract

The black hole (BH) candidate XTE J2012+381 underwent an outburst at the end of 2022. We analyzed 105 NICER observations and two NuSTAR observations of the source during the outburst. The NuSTAR observations of the M ∼ 10 M ⊙ BH indicate clear signs of relativistic disk reflection, which we modeled to measure a BH spin of a = 0.988 − 0.030 + 0.008 and an inclination of θ = 68 − 11 + 6 deg (1σ statistical errors). In our analysis, we test an array of models and examine the effect of fitting NuSTAR spectra alone versus fitting simultaneously with NICER. We find that when the underlying continuum emission is properly accounted for, the reflected emission is similarly characterized by multiple models. We combined 52 NICER spectra to obtain a spectrum with an effective exposure of 190 ks in order to probe the presence of absorption lines that would be suggestive of disk winds, but the resulting features were not statistically significant. We discuss the implications of this measurement in relation to the overall BH spin distribution in X-ray binary systems.

Systematically Revisiting All NuSTAR Spins of Black Holes in X-Ray Binaries

“…As the parameter space is wider around the lower spin solution, the walkers converge to that solution and have trouble returning to the bestfit solution (with high spin) due to the parameter space being narrower. A similar phenomenon was described in Draghis et al (2023a). All these systematic variations are encapsulated in the uncertainties reported on our measurements.…”

“…This behavior is frequent when the quality of the data is not good enough for the fit to strongly distinguish between two similar families of solutions. This behavior is described in Draghis et al (2023a), and the measured low spin is to be interpreted as a lower limit due to the measured low inner emissivity index. Nevertheless, when combining the posterior distributions using our combining algorithm, we measure a spin of a 0.968 0.074 0.022 = -+ and an inclination 70 11…”

“…The high spin measurement for this source is in disagreement with that measured by Wang et al (2018b), who report a spin in the range −0.13 a 0.27. It is worth noting that in their analysis, Wang et al (2018b) reported that the source favors a low inner emissivity index, q 1 ∼ 3.7, which could potentially lead to an underestimation of the BH spin (see Draghis et al 2023a;Section 3.13). This source is particularly interesting, as it exhibits "heartbeat" variability (Altamirano et al 2011) similar to that observed in GRS 1915+105 (see, e.g., Belloni et al 1997;Neilsen et al 2011), for which studies have shown that the BH spin is high.…”

“…In this analysis, we fit each observation individually. Upon obtaining the best-fit combination of parameters for the models for each spectrum, we run a Markov Chain Monte Carlo (MCMC) analysis as described in Section 2.2 in Draghis et al (2023b), and combine the individual 1D posterior probability distributions using a method similar to the one described in Draghis et al (2023b), but as expanded upon in Draghis et al (2023a). When running the combining algorithm, we weight the independent measurements by the strength of reflection during the observation.…”

“…The posterior distributions produced by the MCMC analysis for the spin and inclination parameters are shown in the left and right panels of Figure 2 (online panel #5), respectively, with the width of the lines being proportional to the weighting used when combining the measurements. In the two panels of Figure 2 (online panel #5), the black dotted curves show the combined distribution obtained using the method highlighted in Draghis et al (2023b), and the black solid curves show the probability distribution of the combined measurements obtained using the method presented in Draghis et al (2023a). We measured a spin of a 0.849 0.221…”