[en] An overview of the various Canadian tritium research and operational activities supporting the development, refurbishment and operation of CANDU^R nuclear power reactors is presented. These activities encompass tritium health and safety, tritium in the environment, tritium interaction with materials, and tritium processing, and relate to both supporting R and D advances as well as operational best practices. The collective results of these activities contribute to our goals of improving worker and public safety, and operational efficiency. (authors)

[en] The observation of energetic X-ray emission from black holes, inconsistent with thermal emission from an accretion disk, has long indicated the presence of a ''corona'' around these objects. However, our knowledge of the geometry, composition, and processes within black hole coronae is severely lacking. Basic questions regarding their size and location are still a topic of debate. In this Letter, we show that for black holes with luminosities L ∼> 10^–2 L_Edd—characteristic of many Seyferts, quasars, and stellar-mass black holes (in their brighter states)—advanced imaging and timing data strongly favor X-ray emitting regions that are highly compact, and only a few Gravitational radii above the accretion disk. The inclusion of a large number of possible systematics uncertainties does not significantly change this conclusion with our results still suggesting emission from within ∼20r_g in all cases. This result favors coronal models wherein most of the hard X-ray emission derives from magnetic reconnection in the innermost disk and/or from processes in the compact base of a central, relativistic jet.

[en] We investigate the theoretical expectations for detections of supermassive binary black holes that can be identified as sub-parsec luminous quasars. To date, only two candidates have been selected in a sample comprising 17,500 sources selected from the Sloan Digital Sky Survey (SDSS) Quasar Catalog at z < 0.70. In this Letter, we use models of assembly and growth of supermassive black holes (SMBHs) in hierarchical cosmologies to study the statistics and observability of binary quasars at sub-parsec separations. Our goal is twofold: (1) to test if such a scarce number of binaries is consistent with theoretical prediction of SMBH merger rates and (2) to provide additional predictions at higher redshifts and at lower flux levels. We determine the cumulative number of expected binaries in a complete volume-limited sample. Motivated by Boroson and Lauer, we apply the SDSS quasar luminosity cut (M_i < -22) to our theoretical sample, deriving an upper limit to the observable binary fraction. We find that sub-parsec quasar binaries are intrinsically rare. Our best models predict ∼0.01 deg^-2 sub-parsec binary quasars with separations below ∼10⁴ Schwarzschild radii (v_orb > 2000 kms) at z < 0.7, which represent a fraction ∼6 x 10^-4 of unabsorbed quasars in our theoretical sample. In a complete sample of ∼10,000 sources, we therefore predict an upper limit of ∼10 sub-parsec binary quasars. The number of binaries increases rapidly with increasing redshift. The decreasing lifetime with SMBH binary mass suggests that lowering the luminosity threshold does not lead to a significant increase in the number of detectable sub-parsec binary quasars.

[en] Swift J164449.3+573451 is an exciting transient event, likely powered by the tidal disruption of a star by a massive black hole. The distance to the source, its transient nature, and high internal column density serve to complicate several means of estimating the mass of the black hole. Utilizing newly refined relationships between black hole mass, radio luminosity, and X-ray luminosity, and de-beaming the source flux, a weak constraint on the black hole mass is obtained: log(M_BH/M_sun) = 5.5 ± 1.1 (1σ confidence). The confidence interval is determined from the current intrinsic scatter in the relation, which includes effects from X-ray variability and accretion modes. This mass range is broad, but it includes low values that are consistent with some variability arguments, and it safely excludes high-mass values where it becomes impossible for black holes to disrupt stars. Future refinements in relationships between black hole mass, radio luminosity, and X-ray luminosity will be able to reduce the uncertainty in related mass estimates by a factor of two, making this technique comparable to estimates based on the M-σ relationship. Possible difficulties in placing such events on the fundamental plane, a potential future test of their suitability, and uncertainties in mass stemming from variable X-ray emission are discussed. As near- and longer-term survey efforts such as Pan-STARRS, LSST, LOFAR, the Square Kilometer Array, and eROSITA begin to detect many tidal disruption events, black hole mass estimates from combined X-ray and radio observations may prove to be very pragmatic.

[en] X-ray reverberation mapping has emerged as a new tool to probe accretion in active galactic nuclei (AGN), providing a potentially powerful probe of accretion at the black hole scale. The lags, along with relativistic spectral signatures are often interpreted in light of the lamppost model. Focusing specifically on testing the prediction of the relativistic reverberation model, we have targeted several of the brightest Seyfert Galaxies in X-rays with different observing programs. Here, we report the results from two large campaigns with NuSTAR targeting MCG-5-23-16 and SWIFT J2127.4+5654 to test the model predictions in the 3–50 keV band. These are two of three sources that showed indications of a delayed Compton hump in early data. With triple the previously analyzed exposures, we find no evidence for relativistic reverberation in MCG-5-23-16, and the energy-dependent lags are consistent with a log-linear continuum. In SWIFT J2127.4+5654, although a continuum-only model explains the data, the relativistic reverberation model provides a significant improvement to the energy and frequency-dependent lags, but with parameters that are not consistent with the time-averaged spectrum. This adds to mounting evidence showing that the lag data is not consistent with a static lamppost model.

[en] GRS 1915+105 is a stellar-mass black hole that is well known for exhibiting at least 12 distinct classes of X-ray variability and correlated multi-wavelength behavior. Despite such extraordinary variability, GRS 1915+105 remained one of the brightest sources in the X-ray sky. However, in early 2019, the source became much fainter, apparently entering a new accretion state. Here, we report the results of an extensive, year-long monitoring campaign of GRS 1915+105 with the Neil Gehrels Swift Observatory. During this interval, the flux of GRS 1915+105 gradually diminished; the observed count rate eventually dropped by two orders of magnitude. Simple but robust spectral fits to these monitoring observations show that this new state results from the combination of a dramatic and persistent increase in internal obscuration, and a reduced mass accretion rate. The internal obscuration is the dominant effect, with a median value of N _H = 7 × 10²³ cm⁻². In a number of observations, the source appears to be Compton-thick. We suggest that this state should be identified as the “obscured state,” and discuss the implications of this new (or rarely observed) accretion mode for black holes across the mass scale.

[en] We report on a Suzaku observation of the newly discovered X-ray binary MAXI J1836-194. The source is found to be in the hard/intermediate spectral state and displays a clear and strong relativistically broadened iron emission line. We fit the spectra with a variety of phenomenological, as well as physically motivated disk reflection models, and find that the breadth and strength of the iron line are always characteristic of emission within a few gravitational radii around a black hole. This result is independent of the continuum used and strongly points toward the central object in MAXI J1836-194 being a stellar mass black hole rotating with a spin of a = 0.88 ± 0.03 (90% confidence). We discuss this result in the context of spectral state definitions, physical changes (or lack thereof) in the accretion disk, and on the potential importance of the accretion disk corona in state transitions.

[en] SAX J2224.9+5421 is an extraordinary neutron star low-mass X-ray binary. It was discovered when it was exhibiting a ≅ 10 s long thermonuclear X-ray burst, but it had faded to a 0.5-10 keV luminosity of L _X ≲ 8 × 10³²(D/7.1 kpc)² erg s^–1 only ≅ 8 hr later. It is generally assumed that neutron stars are quiescent (i.e., not accreting) at such intensity, raising questions about the trigger conditions of the X-ray burst and the origin of the faint persistent emission. We report on a ≅51 ks XMM-Newton observation aimed at finding clues explaining the unusual behavior of SAX J2224.9+5421. We identify a likely counterpart that is detected at L _X ≅ 5 × 10³¹(D/7.1 kpc)² erg s^–1 (0.5-10 keV) and has a soft X-ray spectrum that can be described by a neutron star atmosphere model with a temperature of kT ^∞ ≅ 50 eV. This would suggest that SAX J2224.9+5421 is a transient source that was in quiescence during our XMM-Newton observation and experienced a very faint (ceasing) accretion outburst at the time of the X-ray burst detection. We consider one other potential counterpart that is detected at L _X ≅ 5 × 10³²(D/7.1 kpc)² erg s^–1 and displays an X-ray spectrum that is best described by a power law with a photon index of Γ ≅ 1.7. Similarly hard X-ray spectra are seen for a few quiescent neutron stars and may be indicative of a relatively strong magnetic field or the occurrence of low-level accretion.

[en] The heating and cooling of transiently accreting neutron stars provides a powerful probe of the structure and composition of their crust. Observations of superbursts and cooling of accretion-heated neutron stars require more heat release than is accounted for in current models. Obtaining firm constraints on the depth and magnitude of this extra heat is challenging and therefore its origin remains uncertain. We report on Swift and XMM-Newton observations of the transient neutron star low-mass X-ray binary XTE J1709–267, which were made in 2012 September-October when it transitioned to quiescence after a ≅10 week long accretion outburst. The source is detected with XMM-Newton at a 0.5-10 keV luminosity of L_X ≅ 2 × 10³⁴(D/8.5 kpc)² erg s^–1. The X-ray spectrum consists of a thermal component that fits to a neutron star atmosphere model and a non-thermal emission tail, each of which contribute ≅50% to the total flux. The neutron star temperature decreases from ≅158 to ≅152 eV during the ≅8 hr long observation. This can be interpreted as cooling of a crustal layer located at a column density of y ≅ 5 × 10¹² g cm^–2 (≅50 m inside the neutron star), which is just below the ignition depth of superbursts. The required heat generation in the layers on top would be ≅0.06-0.13 MeV per accreted nucleon. The magnitude and depth rule out electron captures and nuclear fusion reactions as the heat source, but it may be accounted for by chemical separation of light and heavy nuclei. Low-level accretion offers an alternative explanation for the observed variability.

[en] It is now clear that a subset of supernovae displays evidence for jets and is observed as gamma-ray bursts (GRBs). The angular momentum distribution of massive stellar endpoints provides a rare means of constraining the nature of the central engine in core-collapse explosions. Unlike supermassive black holes, the spin of stellar-mass black holes in X-ray binary systems is little affected by accretion and accurately reflects the spin set at birth. A modest number of stellar-mass black hole angular momenta have now been measured using two independent X-ray spectroscopic techniques. In contrast, rotation-powered pulsars spin down over time, via magnetic braking, but a modest number of natal spin periods have now been estimated. For both canonical and extreme neutron star parameters, statistical tests strongly suggest that the angular momentum distributions of black holes and neutron stars are markedly different. Within the context of prevalent models for core-collapse supernovae, the angular momentum distributions are consistent with black holes typically being produced in GRB-like supernovae with jets and with neutron stars typically being produced in supernovae with too little angular momentum to produce jets via magnetohydrodynamic processes. It is possible that neutron stars are with high spin initially and rapidly spun down shortly after the supernova event, but the available mechanisms may be inconsistent with some observed pulsar properties.