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NuSTAR observations of Mrk 766: distinguishing reflection from absorption
摘要： We present two new NuSTAR observations of the narrow-line Seyfert 1 (NLS1) galaxy Mrk 766 and give constraints on the two scenarios previously proposed to explain its spectrum and that of other NLS1s: relativistic reflection and partial covering. The NuSTAR spectra show a strong hard (>15 keV) X-ray excess, while simultaneous soft X-ray coverage of one of the observations provided by XMM–Newton constrains the ionized absorption in the source. The pure reflection model requires a black hole of high spin (a > 0.92) viewed at a moderate inclination (i = 46+1?4°). The pure partial covering model requires extreme parameters: the cut-off of the primary continuum is very low (22+7?5 keV) in one observation and the intrinsic X-ray emission must provide a large fraction (75 per cent) of the bolometric luminosity. Allowing a hybrid model with both partial covering and reflection provides more reasonable absorption parameters and relaxes the constraints on reflection parameters. The fractional variability reduces around the iron K band and at high energies including the Compton hump, suggesting that the reflected emission is less variable than the continuum.
关键词： accretion, accretion discs,galaxies: individual: Mrk 766,galaxies: Seyfert,black hole physics
更新于2025-09-23 15:22:29
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The power of infrared AGN selection in mergers: a theoretical study
摘要： The role of galaxy mergers in fuelling active galactic nuclei (AGNs) is still debated, owing partly to selection effects inherent to studies of the merger/AGN connection. In particular, luminous AGNs are often obscured in late-stage mergers. Mid-infrared (IR) colour selection of dust-enshrouded AGN with, e.g. the Wide-field Infrared Survey Explorer (WISE) has uncovered large new populations of obscured AGN. However, this method is sensitive mainly to AGN that dominate emission from the host. To understand how selection biases affect mid-IR studies of the merger/AGN connection, we simulate the evolution of AGN throughout galaxy mergers. Although mid-IR colours closely trace luminous, obscured AGN, we show that nearly half of merger-triggered AGNs are missed with common mid-IR selection criteria, even in late-stage, gas-rich major mergers. At z (cid:2) 0.5, where merger signatures and dual nuclei can most easily be detected, we find that a more lenient W1 ? W2 > 0.5 cut greatly improves completeness without significantly decreasing reliability. Extreme nuclear starbursts are briefly able to mimic this AGN signature, but this is largely irrelevant in mergers, where such starbursts are accompanied by AGN. We propose a two-colour cut that yields high completeness and reliability even in starbursting systems. Further, we show that mid-IR colour selection very effectively identifies dual AGN hosts, with the highest fraction at the smallest separations (<3 kpc). Thus, many merger hosts of mid-IR AGN should contain unresolved dual AGN; these are ideal targets for high-resolution follow-up, particularly with the James Webb Space Telescope.
关键词： galaxies: interactions,infrared: galaxies,accretion,accretion discs,galaxies: active,black hole physics
更新于2025-09-23 15:21:21
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Testing the validity of the ray-tracing code GYOTO
摘要： Context. In the next few years, the near-infrared interferometer GRAVITY will be able to observe the Galactic center. Astrometric data will be obtained with an anticipated accuracy of 10 μas. To analyze these future data, we have developed a code called GYOTO to compute orbits and ray-trace images. Aims. We want to assess the validity and accuracy of GYOTO in a variety of contexts, in particular for stellar astrometry in the Galactic center. Furthermore, we want to tackle and complete a study made on the astrometric displacements that are due to lensing effects of a star of the central parsec with GYOTO. Methods. We first validate GYOTO in the weak-deflection limit (WDL) by studying primary caustics and primary critical curves obtained for a Kerr black hole. We compare GYOTO results to available analytical approximations and estimate GYOTO errors using an intrinsic estimator. In the strong-deflection limit (SDL), we choose to compare null geodesics computed by GYOTO and the ray-tracing code named Geokerr. Finally, we use GYOTO to estimate the apparent astrometric displacements of a star for different angles from Sagittarius A* (Sgr A*). Results. In the WDL, we find a good coherence between GYOTO results and approximations. The maximal difference is around 10?5 μas. Our intrinsic estimator finds a conservative uncertainty estimate also around 10?5 μas. In the SDL, both ray-tracing codes find the same photon’s coordinates with a maximal difference of about 10?3 μas. The shift of a star located behind the plane of sky containing Sgr A* is consistent with the current study. In addition, the effect of lensing on any star in this plane of sky is a radial shift by 5 μas, independent of the distance from Sgr A* up to a very large distance. Conclusions. We have demonstrated that GYOTO is accurate to a very high level, orders of magnitude better than the GRAVITY requirements. GYOTO is also valid in weak- and strong-deflection regimes and for very long integrations. At the astrometric precision that GRAVITY is aiming for, lensing effects must always be taken into account when fitting stellar orbits in the central parsec of the Galaxy.
关键词： gravitational lensing: weak,gravitational lensing: strong,black hole physics,Galaxy: center
更新于2025-09-23 15:21:21
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A physical model of the broad-band continuum of AGN and its implications for the UV/X relation and optical variability
摘要： We develop a new spectral model for the broad-band spectral energy distribution (SED) of active galactic nuclei (AGN). This includes an outer standard disc, an inner warm Comptonizing region to produce the soft X-ray excess and a hot corona. We tie these together energetically by assuming Novikov–Thorne emissivity, and use this to de?ne a size scale for the hard X-ray corona as equal to the radius where the remaining accretion energy down to the black hole can power the observed X-ray emission. We test this on three AGN with well-de?ned SEDs as well as on larger samples to show that the average hard X-ray luminosity is always approximately a few per cent of the Eddington luminosity across a large range of Eddington ratio. As a consequence, the radial size scale required for gravity to power the X-ray corona has to decrease with increasing Eddington fraction. For the ?rst time, we hardwire this into the spectral models, and set the hard X-ray spectral index self-consistently from the ratio of the hard X-ray luminosity to intercepted seed photon luminosity from the disc. This matches the observed correlation of steeper spectral index with increasing Eddington ratio, as well as reproducing the observed tight UV/X relation of quasars. We also include the reprocessed emission produced by the hot inner ?ow illuminating the warm Comptonization and standard disc regions and show that this predicts a decreasing amount of optical variability with increasing Eddington ratio as observed, though additional processes may also be required to explain the observed optical variability.
关键词： galaxies: Seyfert,accretion,black hole physics,accretion discs
更新于2025-09-23 15:21:01
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The study of laser cooling of TeH- anion in theoretical approach
摘要： For systems with extremely low accretion rate, such as Galactic Center Sgr A* and M87 galaxy, the ion collisional mean free path can be considerably larger than its Larmor radius. In this case, the gas pressure is anisotropic to magnetic ?eld lines. In this paper, we pay attention to how the properties of out?ow change with the strength of anisotropic pressure and the magnetic ?eld. We use an anisotropic viscosity to model the anisotropic pressure. We solve the two-dimensional magnetohydrodynamic (MHD) equations in spherical coordinates and assume that the accretion ?ow is radially self-similar. We ?nd that the work done by anisotropic pressure can heat the accretion ?ow. The gas temperature is heightened when anisotropic stress is included. The out?ow velocity increases with the enhancement of strength of the anisotropic force. The Bernoulli parameter does not change much when anisotropic pressure is involved. However, we ?nd that the energy ?ux of out?ow can be increased by a factor of 20 in the presence of anisotropic stress. We ?nd strong wind (the mass out?ow is about 70% of the mass in?ow rate) is formed when a relatively strong magnetic ?eld is present. Out?ows from an active galactic nucleus can interact with gas in its host galaxies. Our result predicts that out?ow feedback effects can be enhanced signi?cantly when anisotropic pressure and a relatively powerful magnetic ?eld is considered.
关键词： accretion discs,hydrodynamics,accretion,black hole physics
更新于2025-09-11 14:15:04