It is well known that reverberation mapping of active galactic nuclei (AGNs) reveals a relationship between AGN luminosity and the size of the broad-line region, and that use of this relationship, combined with the Doppler width of the broad emission line, enables an estimate of the mass of the black hole at the center of the active nucleus based on a single spectrum. An unresolved key issue is the choice of parameter used to characterize the line width, either FWHM or line dispersion s line (the square root of the second moment of the line profile). We argue here that use of FWHM introduces a bias, stretching the mass scale such that high masses are overestimated and low masses are underestimated. Here we investigate estimation of black hole masses in AGNs based on individual or "singleepoch" observations, with a particular emphasis in comparing mass estimates based on line dispersion and FWHM. We confirm the recent findings that, in addition to luminosity and line width, a third parameter is required to obtain accurate masses, and that parameter seems to be Eddington ratio. We present simplified empirical formulae for estimating black hole masses from the Hβ λ4861 and C IV λ1549 emission lines. While the AGN continuum luminosity at 5100 Å is usually used to predict the Hβ reverberation lag, we show that the luminosity of the Hβ broad component can be used instead without any loss of precision, thus eliminating the difficulty of accurately accounting for the host-galaxy contribution to the observed luminosity.

Quasar broad emission lines are largely powered by photoionization from the accretion continuum. Increased central luminosity will enhance line emissivity in more distant clouds, leading to increased average distance of the broad-line-emitting clouds and decreased averaged line width, known as the broad-line region (BLR) "breathing". However, different lines breathe differently, and some highionization lines, such as C IV, can even show "anti-breathing" where the line broadens when luminosity increases. Using multi-year photometric and spectroscopic monitoring data from the Sloan Digital Sky Survey Reverberation Mapping project, we quantify the breathing effect (∆ log W = α∆ log L) of broad Hα, Hβ, Mg ii, C IV, and C III] for statistical quasar samples over z ≈ 0.1 − 2.5. We found that Hβ displays the most consistent normal breathing expected from the virial relation (α ∼ −0.25), Mg ii and Hα on average show no breathing (α ∼ 0), and C IV (and similarly C III] and Si IV) mostly shows anti-breathing (α > 0). The anti-breathing of C IV can be well understood by the presence of a nonvarying core component in addition to a reverberating broad-base component, consistent with earlier findings. The deviation from canonical breathing introduces extra scatter (a luminosity-dependent bias) in single-epoch virial BH mass estimates due to intrinsic quasar variability, which underlies the long argued caveats of C IV single-epoch masses. Using the line dispersion instead of FWHM leads to less, albeit still substantial, deviations from canonical breathing in most cases. Our results strengthen the need for reverberation mapping to provide reliable quasar BH masses, and quantify the level of variability-induced bias in single-epoch BH masses based on various lines.

We present a comprehensive X-ray study of the neutron star low-mass X-ray binary IGR J17062-6143, which has been accreting at low luminosities since its discovery in 2006. Analysing NuSTAR, XMM-Newton and Swift observations, we investigate the very faint nature of this source through three approaches: modelling the relativistic reflection spectrum to constrain the accretion geometry, performing high-resolution X-ray spectroscopy to search for an outflow, and searching for the recently reported millisecond X-ray pulsations. We find a strongly truncated accretion disk at 77 +22 −18