Andrew G. Cantrell

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photometry of the soft X-ray transient A0620−00 spanning nearly 30 years, including previously published and previously unpublished data.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. 0 ± 0. 9, implying M = 6.6 ± 0.25 M� . Using our dynamical model and zero-disk stellar VIH magnitudes, we find d = 1.06 ± 0.12 kpc. Understanding the disk origin of nonellipsoidal 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.

The Spin of the Black Hole in the Soft X-Ray Transient A0620-00

During its year-long outburst in 1975-76, the transient source A0620-00 reached an intensity of 50 Crab, an all-time record for any X-ray binary. The source has been quiescent since then. We have recently determined accurate values for the black hole (BH) mass, orbital inclination angle, and distance. Building on these results, we have measured the radius of the inner edge of the accretion disk around the BH primary by fitting its thermal continuum spectrum to our version of the relativistic Novikov-Thorne thin-disk model. We have thereby estimated the spin of the BH. Although our spin estimate depends on a single high-quality spectrum, which was obtained in 1975 by OSO-8, we are confident of our result because of the consistent values of the inner-disk radius that we have obtained for hundreds of observations of other sources: H1743-322, XTE J1550-564, and notably LMC X-3. We have determined the dimensionless spin parameter of the BH to be a * = 0.12 ± 0.19, with a * – 0.59 at the 3σ level of confidence. This result takes into account all sources of observational and model-parameter uncertainties. Despite the low spin, the intensity and properties of the radio counterpart, both in outburst and quiescence, attest to the presence of a strong jet. If jets are driven by BH spin, then current models indicate that jet power should be a steeply increasing function of a *. Consequently, the low spin of A0620-00 suggests that its jet may be disk driven.

The Black Hole Binary V4641 Sagitarii: Activity in Quiescence and Improved Mass Determinations

We examine ∼10 years of photometric data and find that the black hole X-ray binary V4641 Sgr has two optical states, passive and active, during X-ray quiescence. The passive state is dominated by ellipsoidal variations and is stable in the shape and variability of the light curve. The active state is brighter and more variable. Emission during the active state varies over the course of the orbital period and is redder than the companion star. These optical/infrared states last for weeks or months. V4641 Sgr spends approximately 85% of X-ray quiescence in the passive state and 15% in the active. We analyze passive colors and spectroscopy of V4641 Sgr and show that they are consistent with a reddened B9III star (with E(B−V ) = 0.37 ± 0.19) with little or no contribution from the accretion disk. We use X-ray observations with an updated ephemeris to place an upper limit on the duration of an X-ray eclipse of < 8.3 ◦ in phase (∼1.6 hours). High resolution spectroscopy yields a greatly improved measurement of the rotational velocity of the companion star of Vrot sini = 100.9 ± 0.8 km s −1 . We fit ellipsoidal models to the passive state data and find an inclination angle of i = 72.3±4.1 ◦ , a mass ratio of Q = 2.2 ± 0.2, and component masses for the system of MBH = 6.4 ± 0.6 M⊙ and M2 = 2.9 ± 0.4 M⊙. Using these values we calculate an updated distance to V4641 Sgr of 6.2 ± 0.7 kpc. Subject headings: black hole physics, stars: individual: V4641 Sgr, X-rays: binaries

Jets at lowest mass accretion rates

We present results of recent observations and theoretical modeling of data from black holes accreting at very low luminosities (L/LEdd ≲ 10−8). We discuss our newly developed time-dependent model for episodic ejection of relativistic plasma within a jet framework, and a successful application of this model to describe the origin of radio flares seen in Sgr A*, the Galactic center black hole. Both the observed time lags and size-frequency relationships are reproduced well by the model. We also discuss results from new Spitzer data of the stellar black hole X-ray binary system A0620-00. Complemented by long term SMARTS monitoring, these observations indicate that once the contribution from the accretion disk and the donor star are properly included, the residual mid-IR spectral energy distribution of A0620-00 is quite flat and consistent with a non-thermal origin. The results above suggest that a significant fraction of the observed spectral energy distribution originating near black holes accreting at low luminosities could result from a mildly relativistic outflow. The fact that these outflows are seen in both stellar-mass black holes as well as in supermassive black holes at the heart of AGNs strengthens our expectation that accretion and jet physics scales with mass.

Optical State Changes in A0620‐00 and the Interpretation of Ellipsoidal Light Curves

We present optical and infrared photometry of the soft X‐ray transient A0620–00, obtained by the SMARTS and YALO consortia, spanning from 1999–2007. Although A0620–00 was X‐ray quiescent throughout this period, our data show three distinct optical states, characterized by magnitude, color, and aperiodic variability. In particular, we find that in what we call the “passive” state, A0620–00 exhibits no observable aperiodic variability on any timescale longer than our exposure length. The shape of the passive state light curve is consistent throughout our dataset. The other states are brighter than the passive state, and show enhanced aperiodic variability. These characteristics appear in NIR as well as optical data, suggesting that even NIR light curves may be contaminated if they were not obtained during the passive state. We suggest a reanalysis of historical data, using passive state data to determine the inclination and isolate additional sources of variability.