The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow ...relativistic jet or an isotropic outflow. High-spatial-resolution measurements of the source size and displacement can discriminate between these scenarios. We present very-long-baseline interferometry observations, performed 207.4 days after the merger by using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milli-arc seconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.
We discuss the relationship between a standard Shakura & Sunyaev accretion disc model and the big blue bump (BBB) observed in Type 1 active galactic nuclei. Given the similarity between the BBB and ...the predicted disc spectrum, we propose a new method to estimate black hole masses which relies on the modelling of both optical and UV data with a Shakura & Sunyaev disc spectrum. We apply this method to a sample of 23 radio-loud narrow-line Seyfert 1 (RL-NLS1) galaxies, using data from Wide-field Infrared Survey Explorer, SDSS and GALEX. Our black hole mass estimates are at least a factor of ∼6 above previous results based on single epoch virial methods, while the Eddington ratios are correspondingly lower. Hence, the black hole masses of RL-NLS1 galaxies are typically above 108 M, in agreement with the typical black hole mass of blazars.
Detections of gravitational waves (GWs) may soon uncover the signal from the coalescence of a black hole–neutron star (BHNS) binary, which is expected to be accompanied by an electromagnetic (EM) ...signal. In this paper, we present a composite semi-analytical model to predict the properties of the expected EM counterpart from BHNS mergers, focusing on the kilonova emission and on the gamma-ray burst afterglow. Four main parameters rule the properties of the EM emission: the NS mass
M
NS
, its tidal deformability
Λ
NS
, the BH mass and spin. Only for certain combinations of these parameters an EM counterpart is produced. Here we explore the parameter space, and construct light curves, analyzing the dependence of the EM emission on the NS mass and tidal deformability. Exploring the NS parameter space limiting to
M
NS
-
Λ
NS
pairs described by a physically motivated equations of state (EoS), we find that the brightest EM counterparts are produced in binaries with low-mass NSs (fixing the BH properties and the EoS). Using constraints on the NS EoS from GW170817, our modeling shows that the emission falls in a narrow range of absolute magnitudes. Within the range of explored parameters, light curves and peak times are not dissimilar to those from NSNS mergers, except in the B band. The lack of an hyper/supra-massive NS in BHNS coalescences causes a dimming of the blue kilonova emission in the absence of the neutrino interaction with the ejecta.
In this paper, we explore a possible route of black hole seed formation that appeals to a model by Davies, Miller & Bellovary who considered the case of the dynamical collapse of a dense cluster of ...stellar black holes subjected to an inflow of gas. Here, we explore this case in a broad cosmological context. The working hypotheses are that (i) nuclear star clusters form at high redshifts in pre-galactic discs hosted in dark matter haloes, providing a suitable environment for the formation of stellar black holes in their cores, (ii) major central inflows of gas occur on to these clusters due to instabilities seeded in the growing discs and/or to mergers with other gas-rich haloes and (iii) following the inflow, stellar black holes in the core avoid ejection due to the steepening to the potential well, leading to core collapse and the formation of a massive seed of ≲ 1000 M⊙. We simulate a cosmological box tracing the build-up of the dark matter haloes and their embedded baryons, and explore cluster evolution with a semi-analytical model. We show that this route is feasible, peaks at redshifts z ≲ 10 and occurs in concomitance with the formation of seeds from other channels. The channel is competitive relative to others, and is independent of the metal content of the parent cluster. This mechanism of gas-driven core collapse requires inflows with masses at least 10 times larger than the mass of the parent star cluster, occurring on time-scales shorter than the evaporation/ejection time of the stellar black holes from the core. In this respect, the results provide upper limit to the frequency of this process.
In the new era of gravitational wave (GW) and multi-messenger astrophysics, the detection of a GW signal from the coalescence of a black hole – neutron star (BHNS) binary remains a highly anticipated ...discovery. This system is expected to be within reach of the second generation of ground-based detectors. In this context, we develop a series of versatile semi-analytical models to predict the properties of all the electromagnetic (EM) counterparts of BHNS mergers. We include the nuclear-decay-powered kilonova emission, its radio remnant, the prompt emission from the jet, and the related afterglow. The properties of these counterparts depend upon those of the outflows that result from the partial disruption of the NS during the merger and from the accretion disc around the remnant, which are necessary ingredients for transient EM emission to accompany the GW signal. We therefore define ways to relate the properties of these outflows to those of the progenitor binary, establishing a link between the binary parameters and the counterpart properties. From the resulting model, we anticipate the variety of light curves that can emerge after a BHNS coalescence from the radio up to gamma-rays. These light curves feature universal traits that are the imprint of the dynamics of the emitting outflows, but at the same time, they show a clear dependence on the BH mass and spin, but with a high degree of degeneracy. The latter can be deduced by a joint GW – EM analysis. In this paper, we perform a proof-of-concept multi-messenger parameter estimation of a BHNS merger with an associated kilonova to determine how the information from the EM counterpart can complement that from the GW signal. Our results indicate that the observation and modelling of the kilonova can help to break the degeneracies in the GW parameter space, leading to better constraints on the BH spin, for example.
We study the relation between the mass accretion rate, the jet power and the black hole mass of blazars. With this aim, we make use of the Sloan Digital Sky Survey and the 11-month catalogue of ...blazars detected at energies larger than 100 MeV by the Large Area Telescope onboard the Fermi satellite. This allows us to construct a relatively large sample of blazars with information about both the luminosity (or upper limits) of their emission lines (used as a proxy for the strength of the disc luminosity) and the luminosity of the high-energy emission (used as a proxy for the jet power). We find a good correlation between the luminosity of the broad lines and the γ-ray luminosities as detected by Fermi, both using the absolute values of the luminosities and normalizing them to the Eddington value. The data we have analysed confirm that the division of blazars into BL Lacertae objects (BL Lacs) and flat spectrum radio quasars (FSRQs) is controlled by the line luminosity in Eddington units. For small values of this ratio, the object is a BL Lac, while it is a FSRQ for large values. The transition appears to be smooth, but a much larger number of objects is needed to confirm this point.
Low-metallicity (Z≲ 0.05 Z⊙) massive (≳40 M⊙) stars might end their life by directly collapsing into massive black holes (BHs, 30 ≲mBH/M⊙≲ 80). More than ∼105 massive BHs might have been generated ...via this mechanism in the metal-poor ring galaxy Cartwheel, during the last ∼107 yr. We show that such BHs might power most of the ultra-luminous X-ray sources (ULXs) observed in the Cartwheel. We also consider a sample of ULX-rich galaxies and we find a possible anticorrelation between the number of ULXs per galaxy and the metallicity in these galaxies. However, the data are not sufficient to draw any robust conclusions about this anticorrelation, and further studies are required.
Abstract
We present QSFit (Quasar Spectral Fitting package), a new software package to automatically perform the analysis of active galactic nuclei (AGNs) optical spectra. The software provides ...luminosity estimates for the AGN continuum, the Balmer continuum, both optical and ultraviolet iron blended complex, host galaxy and emission lines, as well as width, velocity offset and equivalent width of 20 emission lines. Improving on a number of previous studies on AGN spectral analysis, QSFit fits all the components simultaneously, using an AGN continuum model which extends over the entire available spectrum, and is thus a probe of the actual AGN continuum whose estimates are scarcely influenced by localized features (e.g. emission lines) in the spectrum. We used QSFit to analyse 71 251 optical spectra of Type 1 AGN at z < 2 (obtained by the Sloan Digital Sky Survey, SDSS) and to produce a publicly available catalogue of AGN spectral properties. Such catalogue
allowed us (for the first time) to estimate the AGN continuum slope and the Balmer continuum luminosity on a very large sample, and to show that there is no evident correlation between these quantities the redshift. All data in the catalogue, the plots with best-fitting model and residuals, and the idl code we used to perform the analysis, are available on a dedicated website. The whole fitting process is customizable for specific needs, and can be extended to analyse
spectra from other data sources. The ultimate purpose of QSFit is to allow astronomers to run standardized recipes to analyse the AGN data, in a simple, replicable and shareable way.
We present results of simulations aimed at tracing the formation of nuclear star clusters (NCs) and black hole (BH) seeds in the framework of the current Λcold dark matter (ΛCDM) cosmogony. These BH ...seeds are considered to be progenitors of the supermassive BHs that inhabit today's galaxies. We focus on two mechanisms for the formation of BHs at high redshifts: as end-products of (1) Population III stars in metal-free haloes, and (2) runaway stellar collisions in metal-poor NCs. Our model tracks the chemical, radiative and mechanical feedback of stars on the baryonic component of the evolving haloes. This procedure allows us to evaluate when and where the conditions for BH formation are met, and to trace the emergence of BH seeds arising from the dynamical channel, in a cosmological context. BHs start to appear already at redshift ∼30 as remnants of Population III stars. The efficiency of this mechanism begins decreasing once feedbacks become increasingly important. Around redshift z∼ 15, BHs mostly form in the centre of mildly metal-enriched haloes inside dense NCs. The seed BHs that form along the two pathways have at birth a mass of around 100-1000 M⊙. The occupation fraction of BHs is a function of both halo mass and mass growth rate: at a given redshift, heavier and faster growing haloes have a higher chance to form a native BH, or to acquire an inherited BH via merging of another system. With decreasing z, the probability of finding a BH shifts towards progressively higher mass halo intervals. This is due to the fact that, at later cosmic times, low-mass systems rarely form a seed, and already formed BHs are deposited into larger mass systems due to hierarchical mergers. Our model predicts that at z= 0, all haloes above 1011 M⊙ should host a BH (in agreement with observational results), most probably inherited during their lifetime. Haloes less massive than 109 M⊙ have a higher probability to host a native BH, but their occupation fraction decreases below 10 per cent.
We study the dynamics of supermassive black hole binaries embedded in circumbinary gaseous discs, with the smoothed particle hydrodynamics code gadget-2. The subparsec binary (of total mass M and ...mass ratio q= 1/3) has excavated a gap and transfers its angular momentum to the self-gravitating disc (M
disc= 0.2M). We explore the changes of the binary eccentricity, e, by simulating a sequence of binary models that differ in the initial eccentricity e
0 only. In initially low-eccentric binaries, the eccentricity increases with time, while in high-eccentric binaries e declines, indicating the existence of a limiting eccentricity e
crit that is found to fall in the interval 0.6, 0.8. We also present an analytical interpretation for this saturation limit. An important consequence of the existence of e
crit is the detectability of a significant residual eccentricity e
LISA by the proposed gravitational wave detector Laser Interferometer Space Antenna (LISA). It is found that at the moment of entering the LISA frequency domain e
LISA∼ 10−3-10−2, a signature of its earlier coupling with the massive circumbinary disc. We also observe large periodic inflows across the gap, occurring on the binary and disc dynamical time-scales rather than on the viscous time. These periodic changes in the accretion rate (with amplitudes up to ∼100 per cent, depending on the binary eccentricity) can be considered a fingerprint of eccentric subparsec binaries migrating inside a circumbinary disc.
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