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The sample of time-delay gravitational lenses appropriate for studying the geometry of the Universe continues to grow as dedicated campaigns, such as the Dark Energy Survey, the (Very Large Telescope) VLT Survey Telescope (VST) ATLAS survey, and the Large Synoptic Survey Telescope, complete their census of high-redshift sources. This catalogue now includes hundreds of strong lensing systems, at least 31 of which have reasonably accurate time-delay measurements. In this paper, we use them to compare the predictions of two competing Friedmann–Lemaître–Robertson–Walker models: flat Lambda cold dark matter (ΛCDM), characterized by two adjustable parameters (H 0 and Ωm), and the R h = ct universe (with H 0 as the single free variable). Over the past decade, the latter has accounted for the data better than the standard model, most recently the emergence of well-formed galaxies discovered by JWST at cosmic dawn. Here, we show that the current sample of time-delay lenses favours R h = ct with a likelihood of ∼ |$84{{\ \rm per\ cent}}$| versus ∼ |$16{{\ \rm per\ cent}}$| for the standard model. This level of accuracy will greatly improve as the ongoing surveys uncover many thousands of additional lens systems over the next several years. [ABSTRACT FROM AUTHOR]

ABSTRACT The activity of active galaxies may be triggered by the merging of galaxies, and present-day galaxies are probably the product of successive minor mergers. The frequent galactic mergers at high redshift imply that active galaxies harbour supermassive unequal-mass binary black holes at their centre at least once during their lifetime. The secondary black hole interacts and becomes coplanar with the accretion disc around the primary, inwardly spiraling toward their mass centre owing to the loss of orbital angular momentum to the disc mass outside the orbit of the secondary and/or to gravitational radiation. The binary black holes finally merge and form a more massive (post-merged) black hole at the centre. In this paper it is shown that the recently-discovered double-lobed FR II radio galaxies are the remnants of such supermassive binary black holes. The inwardly spiraling secondary black hole opens a gap in the accretion disc that increases with time when the loss of the orbital angular momentum via gravitational radiation becomes dominant. When the supermassive black holes merge, the inner accretion disc disappears and the gap becomes a big hole of several hundreds of Schwarzschild radii in the vicinity of the post-merged supermassive black hole, leading to an interruption of jet formation. When the outer accretion disc slowly refills the big hole on a viscous time-scale, jet formation restarts and the interaction of the recurrent jets and the inter-galactic medium forms a secondary pair of lobes. The model is applied to a particular double-lobed radio source – B1834+620 – which has an interruption time-scale ∼1 Myr . It is shown that the orbit of the secondary in B1834+620 is elliptical with a typical eccentricity e≃ 0.68 and that the ratio q of the mass of the secondary to that of the primary is 0.01 ≲q≲ 0.4 . The accretion disc is a standard α-disc with 0.01 ≲α≲ 0.04 and the ratio of disc half height H to radius r is δ≃ 0.01 . The model predicts that double-lobed radio structures form only in FR II or borderline FR I/FR II radio galaxies and that the detection rate of double-lobed radio sources among FR II radio sources is about one per cent. [ABSTRACT FROM AUTHOR]

The ESA's X-ray Multi-mirror Mission (XMM–Newton) created a new high-quality version of the XMM–Newton serendipitous source catalogue, 4XMM-DR9, which provides a wealth of information for observed sources. The 4XMM-DR9 catalogue is correlated with the Sloan Digital Sky Survey (SDSS) DR12 photometric data base and the AllWISE data base; we then get X-ray sources with information from the X-ray, optical, and/or infrared bands and obtain the XMM – WISE, XMM –SDSS, and XMM – WISE –SDSS samples. Based on the large spectroscopic surveys of SDSS and the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST), we cross-match the XMM – WISE –SDSS sample with sources of known spectral classes, and obtain known samples of stars, galaxies, and quasars. The distribution of stars, galaxies, and quasars as well as all spectral classes of stars in 2D parameter space is presented. Various machine-learning methods are applied to different samples from different bands. The better classified results are retained. For the sample from the X-ray band, a rotation-forest classifier performs the best. For the sample from the X-ray and infrared bands, a random-forest algorithm outperforms all other methods. For the samples from the X-ray, optical, and/or infrared bands, the LogitBoost classifier shows its superiority. Thus, all X-ray sources in the 4XMM-DR9 catalogue with different input patterns are classified by their respective models that are created by these best methods. Their membership of and membership probabilities for individual X-ray sources are assigned. The classified result will be of great value for the further research of X-ray sources in greater detail. [ABSTRACT FROM AUTHOR]

We investigate the possible presence of quasi-periodic oscillation (QPO) signals in 2103 blazars from the Zwicky Transient Facility (ZTF) time-domain survey. We detect a low-frequency QPO signal in five blazars observed over these 3.8-yr-long optical r -band ZTF light curves. These periods range from 144 to 196 d detected at ≳4σ significance levels in both the Lomb–Scargle periodogram and weighted wavelet Z-transform analyses. We find consistent results using the phase dispersion minimization technique. A similar peak is detected in the g -band light curves at a slightly lower significance of 3σ. Such nearly periodic signals on these time-scales in optical wavebands most likely originate from a precessing jet with high Lorentz factor, closely aligned to the observer's line of sight or the movement of plasma blobs along a helical structure in the jet. [ABSTRACT FROM AUTHOR]

The accurate estimation of photometric redshifts plays a crucial role in accomplishing science objectives of the large survey projects. Template-fitting and machine learning are the two main types of methods applied currently. Based on the training set obtained by cross-correlating the DESI Legacy Imaging Surveys DR9 galaxy catalogue and the SDSS DR16 galaxy catalogue, the two kinds of methods are used and optimized, such as eazy for template-fitting approach and catboost for machine learning. Then, the created models are tested by the cross-matched samples of the DESI Legacy Imaging Surveys DR9 galaxy catalogue with LAMOST DR7, GAMA DR3, and WiggleZ galaxy catalogues. Moreover, three machine learning methods (catboost , Multi-Layer Perceptron, and Random Forest) are compared; catboost shows its superiority for our case. By feature selection and optimization of model parameters, catboost can obtain higher accuracy with optical and infrared photometric information, the best performance (⁠|$\rm MSE=0.0032$|⁠ , σNMAD = 0.0156, and |$O=0.88{{\ \rm per\ cent}}$|⁠) with g ≤ 24.0, r ≤ 23.4, and z ≤ 22.5 is achieved. But eazy can provide more accurate photometric redshift estimation for high redshift galaxies, especially beyond the redshift range of training sample. Finally, we finish the redshift estimation of all DESI Legacy Imaging Surveys DR9 galaxies with catboost and eazy , which will contribute to the further study of galaxies and their properties. [ABSTRACT FROM AUTHOR]

We study the demographics of z ∼ 6 broad-line quasars in the black hole (BH) mass–luminosity plane using a sample of more than 100 quasars at 5.7 < z < 6.5. These quasars have well-quantified selection functions and nearly one-third of them also have virial BH masses estimated from near-IR spectroscopy. We use forward modelling of parametrized intrinsic distributions of BH masses and Eddington ratios, and account for the sample flux limits and measurement uncertainties of the BH masses and luminosities. We find significant differences between the intrinsic and observed distributions of the quantities due to measurement uncertainties and sample flux limits. There is also marginal evidence that the virial BH masses are susceptible to a positive luminosity-dependent bias (BH mass is overestimated when luminosity is above the average), and that the mean Eddington ratio increases with BH mass. Our models provide reliable constraints on the z ∼ 6 BH mass function at |$M_{\rm BH}\gt 10^{8.5}\, M_\odot$|⁠ , with a median 1σ uncertainty of ∼0.5 dex in abundance. The intrinsic Eddington ratio distribution of |$M_{\rm BH}\gt 10^{8.5}\, M_\odot$| quasars can be approximated by a mass-dependent Schechter model, with a broad peak around log (L bol/ L Edd) ∼ −0.9. We also find that, at 4.5 ≲ z ≲ 6, the number densities of more massive BHs tend to decline more rapidly with increasing redshift, contrary to the trend at 2.5 ≲ z ≲ 4.5 reported previously. [ABSTRACT FROM AUTHOR]

We extend the standard model of forward–reverse shock (FS–RS) for gamma-ray burst (GRB) afterglow to more general cases. On one hand, we derive the analytical solution to the hydrodynamics of the shocks in two limiting cases, i.e. an ultrarelativistic reverse shock case and a Newtonian reverse shock case. Based on the asymptotic solutions in these two limiting cases, we constitute a semi-analytical solution for the hydrodynamics of the shocks in the generic case, covering the mildly relativistic reverse shock case. On the other hand, we derive the evolution of the system taking into account the condition of energy conservation which is not satisfied in the standard FS–RS model. A generic solution of semi-analytical expressions is also given. In both the extended standard FS–RS model (satisfying pressure balance condition) and the model satisfying energy conservation, we find that the results in the ultrarelativistic reverse shock case and in the early stage of the Newtonian reverse shock case are different from those in the standard FS–RS model by only a factor that close to one while the same initial conditions adopted. However, the asymptotic solutions in the limiting cases are not good approximations to those in the intermediate case. Our semi-analytical results agree well with the numerical results for a large range of model parameters, and hence can be easily employed to diagnose the physical quantities of the GRB shell and circumburst environment. [ABSTRACT FROM AUTHOR]

We present the accretion disc-size estimates for a sample of 19 active galactic nuclei (AGNs) using the optical g -, r -, and i -band light curves obtained from the Zwicky Transient Facility survey. All the AGNs have reliable supermassive black hole (SMBH) mass estimates based on previous reverberation mapping measurements. The multiband light curves are cross-correlated, and the reverberation lag is estimated using the Interpolated Cross-Correlation Function method and the Bayesian method using the javelin code. As expected from the disc-reprocessing arguments, the g − r band lags are shorter than the g − i band lags for this sample. The interband lags for all, but five sources, are larger than the sizes predicted from the standard Shakura Sunyaev (SS) analytical model. We fit the light curves directly using a thin disc model implemented through the javelin code to get the accretion disc sizes. The disc sizes obtained using this model are on an average 3.9 times larger than the prediction based on the SS disc model. We find a weak correlation between the disc sizes and the known physical parameters, namely the luminosity and the SMBH mass. In the near future, a large sample of AGNs covering broader ranges of luminosity and SMBH mass from large photometric surveys would be helpful in a better understanding of the structure and physics of the accretion disc. [ABSTRACT FROM AUTHOR]

We study the luminosity function and formation rate of long gamma-ray bursts (GRBs) by using a maximum likelihood method. This is the first time this method is applied to a well-defined sample of GRBs that is complete in redshift. The sample is composed of 99 bursts detected by the Swift satellite, 81 of them with measured redshift and luminosity for a completeness level of |$82\, {\rm per\, cent}$|⁠. We confirm that a strong redshift evolution in luminosity (with an evolution index of |$\delta =2.22^{+0.32}_{-0.31}$|⁠) or in density (⁠|$\delta =1.92^{+0.20}_{-0.21}$|⁠) is needed in order to reproduce the observations well. But since the predicted redshift and luminosity distributions in the two scenarios are very similar, it is difficult to distinguish between these two kinds of evolutions only on the basis of the current sample. Furthermore, we also consider an empirical density case in which the GRB rate density is directly described as a broken power-law function and the luminosity function is taken to be non-evolving. In this case, we find that the GRB formation rate rises like |$(1+z)^{3.85^{+0.48}_{-0.45}}$| for |$z\lesssim 2$| and is proportional to |$(1+z)^{-1.07^{+0.98}_{-1.12}}$| for |$z\gtrsim 2$|⁠. The local GRB rate is |$1.49^{+0.63}_{-0.64}$|  Gpc−3 yr−1. The GRB rate may be consistent with the cosmic star formation rate (SFR) at |$z\lesssim 2$|⁠ , but shows an enhancement compared to the SFR at |$z\gtrsim 2$|⁠. [ABSTRACT FROM AUTHOR]

We aim to select quasar candidates based on the two large survey databases, Pan-STARRS and AllWISE. Exploring the distribution of quasars and stars in the colour spaces, we find that the combination of infrared and optical photometry is more conducive to select quasar candidates. Two new colour criterions (yW1W2 and iW1z W2) are constructed to distinguish quasars from stars efficiently. With iW1zW2, 98.30 per cent of star contamination is eliminated, while 99.50 per cent of quasars are retained, at least to the magnitude limit of our training set of stars. Based on the optical and infrared colour features, we put forward an efficient schema to select quasar candidates and high-redshift quasar candidates, in which two machine learning algorithms (XGBoost and SVM) are implemented. The XGBoost and SVM classifiers have proven to be very effective with accuracy of |$99.46{{\ \rm per\ cent}}$| when 8Color as input pattern and default model parameters. Applying the two optimal classifiers to the unknown Pan-STARRS and AllWISE cross-matched data set, a total of 2 006 632 intersected sources are predicted to be quasar candidates given quasar probability larger than 0.5 (i.e. P QSO > 0.5). Among them, 1 201 211 have high probability (P QSO > 0.95). For these newly predicted quasar candidates, a regressor is constructed to estimate their redshifts. Finally 7402 z > 3.5 quasars are obtained. Given the magnitude limitation and site of the LAMOST telescope, part of these candidates will be used as the input catalogue of the LAMOST telescope for follow-up observation, and the rest may be observed by other telescopes. [ABSTRACT FROM AUTHOR]

The giant X-ray bump of GRB 121027A observed by Swift is different from the typical X-ray flares in gamma-ray bursts. The observed structural variability in the rise and decay phases of the bump has four components. Of these four components, only the data in the bump from about 5300 to about 6100 s is of good enough quality to be analysed using the stepwise filter correlation method. A $86^{+5.9}_{-9.4} \rm s$ periodic oscillation is postulated, which is confirmed by the Lomb–Scargle method. A jet precession model is proposed to account for this variability. [ABSTRACT FROM AUTHOR]