The Fundamental Plane of black hole activity is a relation between X-ray luminosity, radio luminosity and black hole mass for hard-state Galactic black holes and their supermassive analogues. The Fundamental Plane suggests that, at low-accretion rates, the physical processes regulating the conversion of an accretion flow into radiative energy could be universal across the entire black hole mass scale. However, there is still a need to further refine the Fundamental Plane in order to better discern the radiative processes and their geometry very close to the black hole, in particular the source of hard X-rays. Further refinement is necessary because error bars on the best-fitting slopes of the Fundamental Plane are generally large, and also the inferred coefficients can be sensitive to the adopted sample of black holes. In this work, we regress the Fundamental Plane with a Bayesian technique. Our approach shows that sub-Eddington black holes emit X-ray emission that is predominantly optically thin synchrotron radiation from the jet, provided that their radio spectra are flat or inverted. X-ray emission dominated by very radiatively inefficient accretion flows is excluded at the >3σ level. We also show that it is difficult to place Fanaroff-Riley type I (FR I) galaxies on to the Fundamental Plane because their X-ray jet emission is highly affected by synchrotron cooling. On the other hand, BL Lac objects (i.e. relativistically beamed sub-Eddington AGN) fit on to the Fundamental Plane. Including a uniform subset of high-energy peaked BL Lac objects from the Sloan Digital Sky Survey, we find sub-Eddington black holes with flat/inverted radio spectra follow log L X = (1.45 ± 0.04)log L R − (0.88 ± 0.06)log M BH − 6.07 ± 1.10, with σ int = 0.07 ± 0.05 dex. Finally, we discuss how the effects of synchrotron cooling of jet emission from the highest black hole masses can bias Fundamental Plane regressions, perhaps leading to incorrect inferences on X-ray radiation mechanisms.

Most transient black hole X-ray binaries (BHXBs) spend the bulk of their time in a quiescent state, where they accrete matter from their companion star at highly sub-Eddington luminosities (we define quiescence here as a normalized Eddington ratio l x = L 0.5−10 keV /L Edd < 10 −5 ). Here, we present Chandra X-ray imaging spectroscopy for three BHXB systems (H 1743-322, MAXI J1659-152, and XTE J1752-223) as they fade into quiescence following an outburst. Multiple X-ray observations were taken within one month of each other, allowing us to track each individual system's X-ray spectral evolution during its decay. We compare these three systems to other BHXB systems. We confirm that quiescent BHXBs have softer X-ray spectra than low-hard state BHXBs, and that quiescent BHXB spectral properties show no dependence on the binary system's orbital parameters. However, the observed anti-correlation between X-ray photon index (Γ) and l x in the low-hard state does not continue once a BHXB enters quiescence. Instead, Γ plateaus to an average Γ = 2.08 ± 0.07 by the time l x reaches ∼10 −5 . l x ∼ 10 −5 is thus an observationally-motivated upper limit for the beginning of the quiescent spectral state. Our results are discussed in the context of different accretion flow models, and across the black hole mass scale.

We report on deep, coordinated radio and X-ray observations of the black hole X-ray binary XTE J1118+480 in quiescence. The source was observed with the Karl G. Jansky Very Large Array for a total of 17.5 hrs at 5.3 GHz, yielding a 4.8±1.4 µJy radio source at a position consistent with the binary system. At a distance of 1.7 kpc, this corresponds to an integrated radio luminosity between 4-8 ×10 25 erg s −1 , depending on the spectral index. This is the lowest radio luminosity measured for any accreting black hole to date. Simultaneous observations with the Chandra X-ray Telescope detected XTE J1118+480 at 1.2 × 10 −14 erg s −1 cm −2 (1-10 keV), corresponding to an Eddington ratio of ∼4 × 10 −9 for a 7.5 M black hole. Combining these new measurements with data from the 2005 and 2000 outbursts available in the literature, we find evidence for a relationship of the form r =α+β X (where denotes logarithmic luminosities), with β = 0.72 ± 0.09. XTE J1118+480 is thus the third system -together with GX339-4 and V404 Cyg -for which a tight, non-linear radio/X-ray correlation has been reported over more than 5 dex in X . Confirming previous results, we find no evidence for a dependence of the correlation normalisation of an individual system on orbital parameters, relativistic boosting, reported black hole spin and/or black hole mass. We then perform a clustering and linear regression analysis on what is arguably the most up-to-date collection of coordinated radio and X-ray luminosity measurements from quiescent and hard state black hole X-ray binaries, including 24 systems. At variance with previous results, a two-cluster description is statistically preferred only for random errors < ∼ 0.3 dex in both r and X , a level which we argue can be easily reached when the known spectral shape/distance uncertainties and intrinsic variability are accounted for. A linear regression analysis performed on the whole data set returns a best-fitting slope β = 0.61 ± 0.03 and intrinsic scatter σ 0 = 0.31 ± 0.03 dex.

We present an X-ray and multiwavelength study of 33 weak emission-line quasars (WLQs) and 18 quasars that are analogs of the extreme WLQ, PHL 1811, at z ≈ 0.5-2.9. New Chandra 1.5-9.5 ks exploratory observations were obtained for 32 objects while the others have archival X-ray observations. Significant fractions of these luminous type 1 quasars are distinctly X-ray weak compared to typical quasars, including 16 (48%) of the WLQs and 17 (94%) of the PHL 1811 analogs with average X-ray weakness factors of 17 and 39, respectively. We measure a relatively hard (Γ = 1.16 +0.37 −0.32 ) effective power-law photon index for a stack of the X-ray weak subsample, suggesting X-ray absorption, and spectral analysis of one PHL 1811 analog, J1521+5202, also indicates significant intrinsic X-ray absorption. We compare composite SDSS spectra for the X-ray weak and Xray normal populations and find several optical-UV tracers of X-ray weakness; e.g., Fe II rest-frame equivalent width and relative color. We describe how orientation effects under our previously proposed "shielding-gas" scenario can likely unify the X-ray weak and X-ray normal populations. We suggest that the shielding gas may naturally be understood as a geometrically thick inner accretion disk that shields the broad line region from the ionizing continuum. If WLQs and PHL 1811 analogs have very high Eddington ratios, the inner disk could be significantly puffed up (e.g., a slim disk). Shielding of the broad emission-line region by a geometrically thick disk may have a significant role in setting the broad distributions of C IV rest-frame equivalent width and blueshift for quasars more generally.

Using the Very Long Baseline Array and the European Very Long Baseline Interferometry Network we have made a precise measurement of the radio parallax of the black hole Xray binary MAXI J1820+070, providing a model-independent distance to the source. Our parallax measurement of (0.348 ± 0.033) mas for MAXI J1820+070 translates to a distance of (2.96 ± 0.33) kpc. This distance implies that the source reached (15 ± 3) per cent of the Eddington luminosity at the peak of its outburst. Further, we use this distance to refine previous estimates of the jet inclination angle, jet velocity and the mass of the black hole in MAXI J1820+070 to be (63 ± 3) • , (0.89 ± 0.09) c and (9.2 ± 1.3) M , respectively.