Abstract Deep surveys with the James Webb Space Telescope (JWST) have revealed an emergent population of moderate-luminosity, broad-line active galactic nuclei (AGNs) at 4 ≲ z ≲ 13 powered by accretion onto early massive black holes. The high number densities reported, together with the large Lyman-continuum (LyC) production efficiency and leakiness into the intergalactic medium that are typical of UV-selected AGNs, lead us to reassess a scenario where AGNs are the sole drivers of the cosmic hydrogen/helium reionization process. Our approach is based on the assumptions, grounded in recent observations, that (a) the fraction of broad-line AGNs among galaxies is around 10%–15%; (b) the mean escape fraction of hydrogen LyC radiation is high, at ≳80%, in AGN hosts and is negligible otherwise; and (c) internal absorption at 4 ryd or a steep ionizing EUV spectrum delay full reionization of He ii until z ≃ 2.8–3.0, in agreement with observations of the He ii Ly α forest. In our fiducial models, (1) hydrogen reionization is 99% completed by redshift z ≃ 5.3–5.5 and reaches its midpoint at z ≃ 6.5–6.7; (2) the integrated Thomson scattering optical depth to reionization is ≃0.05, consistent with constraints from cosmic microwave background anisotropy data; and (3) the abundant AGN population detected by JWST does not violate constraints on the unresolved X-ray background.
We investigate a hierarchical structure formation scenario in which galaxy stellar cores are created from the binding energy liberated by shrinking supermassive black hole (SMBH) binaries. The binary orbital decay heats the surrounding stars, eroding a preexisting stellar cusp ∝r-2. We follow the merger history of dark matter halos and associated SMBHs via cosmological Monte Carlo realizations of the merger hierarchy from early times to the present in a ΛCDM cosmology. Massive black holes get incorporated through a series of mergers into larger and larger halos, sink to the center through dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, and form a binary system. Stellar dynamical processes drive the binary to harden and eventually coalesce. A simple scheme is applied in which the loss cone is constantly refilled and a constant density core forms because of the ejection of stellar mass. We find that a model in which the effect of the hierarchy of SMBH interactions is cumulative and cores are preserved during galaxy mergers produces at the present epoch a correlation between the "mass deficit" (the mass needed to bring a flat inner density profile to a r-2 cusp) and the mass of the nuclear SMBH, with a normalization and slope comparable to the observed relation. Models in which the mass displaced by the SMBH binary is replenished after every major galaxy merger appear instead to underestimate the mass deficit observed in "core" galaxies.
Context. Molecular clouds in the Galactic center (GC) reprocess radiation from past outbursts of nearby high-energy sources, generating a bright Fe K α fluorescence at 6.4 keV. The closest clouds to the GC are only ≃1.5 pc from Sgr A ⋆ , forming a torus-like structure known as the circumnuclear disk (CND). The study of fluorescence emission can lead to a characterization of the illuminating source(s), the reflecting clouds, and the global geometry of such a system lying in the GC. Aims. The primary purpose of our study is to analyze possible fluorescence signals arising in the CND. This signal would allow us to constrain the CND’s physical properties and the source-reflector system’s geometry. Methods. By exploiting the last ≃20 yr of XMM-Newton observations of the GC, we studied the variability of the Fe K α line in the region around Sgr A ⋆ . We identified regions with a flux excess and computed the spectrum therein. We then derived the hydrogen column density of the CND after relating the intensity of the 6.4 keV line to the total energy emitted by known transient sources in the region. Results. Starting from data collected in 2019, we find significant line excesses in a region compatible with the eastern portion of the CND. The echo radiation can be linked to the 2013 outburst of the magnetar SGR J1745-2900. We derive a mean effective hydrogen column density of the CND in the eastern region of ≃10 23 cm −2 . Conclusions. The scenario depicted is physically plausible, given the luminosity, the position of the illuminating source, and the expected density of the CND. Further observations could link the variability of the echo signal to the light curve of the illuminating source. In this way, it would be possible to characterize the cloud response to the radiation front, achieving a more accurate estimate of the cloud parameters.
We report on results of a Target-of-Opportunity observation of the X-ray transient XTE J1118+480 performed on 2000 April 14-15 with the Narrow Field Instruments (0.1-200 keV) of the BeppoSAX satellite. The measured spectrum is a power law with a photon index of ~1.7 modified by an ultrasoft X-ray excess and a high-energy cutoff above ~100 keV. The soft excess is consistent with a blackbody with a temperature of ~40 eV and a low flux, while the cutoff power law is well fitted by thermal Comptonization in a plasma with an electron temperature of ~102 keV and an optical depth of order unity. Consistent with the weakness of the blackbody, Compton reflection is weak. Although the data are consistent with various geometries of the hot and cold phases of the accreting gas, we conclude that a hot accretion disk is the most plausible model. The Eddington ratio implied by recent estimates of the mass and the distance is ~10-3, which may indicate that advection is probably not the dominant cooling mechanism. We finally suggest that the reflecting medium has a low metallicity, consistent with the location of the system in the halo.
Models for gamma–ray burst afterglows envisage an hyper–relativistic fireball that is decelerated in the ambient medium around the explosion site. This interaction produces a shock wave which amplifies the magnetic field and accelerates electrons to relativistic energies, setting the conditions for an efficient production of synchrotron photons. If produced in a region of large– scale ordered magnetic field, synchrotron radiation can be highly polarized. The optical transient associated with GRB 990510 was observed ∼ 18.5 hr after the event and linear polarization in the R band was measured at a level of 1.7 ± 0.2%. This is the first detection of linear polarization in the optical afterglow of a gamma–ray burst. We exclude that this polarization is due to dust in the interstellar material, either in our Galaxy or in the host galaxy of the gamma–ray burst. These results provide important new evidence in favor of the synchrotron origin of the afterglow emission, and constrains the geometry of the fireball and/or magnetic field lines.
Abstract We study the pairing of massive black holes embedded in a massive circum–nuclear, rotationally supported disc, until they form a close binary. Using high resolution SPH simulations, we follow the black hole dynamics, and in particular the eccentricity evolution, as a function of the composition in stars and gas of the disc. Binary–disc interaction always leads to orbital decay and, in case of co–rotating black holes, to orbit circularization. We present also a higher resolution simulation performed using the particle–splitting technique showing that the binary orbital decay is efficient down to a separation of ~ 0.1 pc, comparable to our new resolution limit. We detail the gaseous mass profile bound to each black hole. Double nuclear activity is expected to occur on an estimated timescale of ≲ 10 Myrs.
We highlight some subtleties that affect naive implementations of quadrupolar and octupolar gravitational waveforms from numerically-integrated trajectories of three-body systems. Some of those subtleties arise from the requirement that the source be contained in its "coordinate near zone" when applying the standard PN formulae for gravitational-wave emission, and from the need to use the non-linear Einstein equations to correctly derive the quadrupole emission formula. We show that some of these subtleties were occasionally overlooked in the literature, with consequences for published results. We also provide prescriptions that lead to correct and robust predictions for the waveforms computed from numerically-integrated orbits.
The putative ubiquity of massive black holes (MBH) at the center of galaxies, and the hierarchical progress of structure formation along the cosmic history, together necessarily imply the existence of a large population of cosmic MBH binaries. Such systems are understood to be the loudest sources of gravitational waves at mHz frequencies, the regime that will be probed by the next Laser Interferometer Space Antenna (LISA). It has been proposed that the rate at which MBHs pair and then bind to form binaries is critically dependent upon the feedback exerted by the MBHs on the surrounding gaseous environment. Using the publicly available code GIZMO, we perform a suite of simulations aimed at studying the dynamics of a MBH pair embedded in a gaseous disk on 100 pc scale. By means of dedicated modules, we follow the dynamics of MBHs in the presence of different spin-dependent radiative feedback models, and compare the results to a benchmark case with no feedback at all. Our main finding is that feedback causes the secondary MBH to shrink its orbit at a reduced pace, when compared to models where feedback is absent. Moreover, such slower inspiral occurs on eccentric orbits, as feedback has the net effect of hampering the circularization process. Though idealized in many aspects, our study highlights and quantifies the importance of including spin-dependent feedback recipes in hydrodynamic simulations of MBH pairs, and ultimately in assessing the cosmological coalescence rate of such systems in view of their detection through gravitational waves.
