The feasibility and flexibility of selecting quasars by variability using ensemble machine learning algorithms

In this work,we train three decision-tree based ensemble machine learning algorithms(Random Forest Classifier,Adaptive Boosting and Gradient Boosting Decision Tree respectively)to study quasar selection in the variable source catalog in SDSS Stripe 82.We build training and test samples(both containing 1:1 of quasars and stars)using the spectroscopic confirmed sources in SDSS DR14(including8330 quasars and 3966 stars).We find that when trained with variation parameters alone,all three models can select quasars with similarly and remarkably high precision and completeness(~98.5%and 97.5%),even better than trained with SDSS colors alone(~97.2%and 96.5%),consistent with previous studies.By applying the trained models on the variable sources without spectroscopic identifications,we estimate the spectroscopically confirmed quasar sample in Stripe 82 variable source catalog is~93%complete(95%for mi<19.0).Using the Random Forest Classifier we derive the relative importance of the observational features utilized for classifications.We further show that even using one-or two-year time domain observations,variability-based quasar selection could still be highly efficient.

Synergy between CSST galaxy survey and gravitational-wave observation: Inferring the Hubble constant from dark standard sirens

Gravitational waves(GWs) from compact binary coalescences encode the absolute luminosity distances of GW sources. Once the redshifts of GW sources are known, one can use the distance-redshift relation to constrain cosmological parameters. One way to obtain the redshifts is to localize GW sources by GW observations and then use galaxy catalogs to determine redshifts from a statistical analysis of redshift information of the potential host galaxies, commonly referred to as the dark siren method. The third-generation(3G) GW detectors are planned to work in the 2030s and will observe numerous compact binary coalescences.Using these GW events as dark sirens requires high-quality galaxy catalogs from future sky survey projects. The China Space Station Telescope(CSST) will be launched in 2024 and will observe billions of galaxies within a 17500 deg^(2) survey area with redshift up to z ~ 4, providing photometric and spectroscopic galaxy catalogs. In this work, we simulate the CSST galaxy catalogs and the 5-year GW data from the 3G GW detectors and combine them to infer the Hubble constant(H_(0)). Our results show that the measurement precision of H0could reach the sub-percent level, meeting the standard of precision cosmology. We conclude that the synergy between CSST and the 3G GW detectors is of great significance in measuring the Hubble constant.

Erratum to:Forecasts on CMB lensing observations with AliCPT-1

One of the authors regrets that there was an omission of affiliation in this published article[1].The affiliation information of author Yi-WenWu only included the institution name for the author,and it is advised to add the university as the author’s second affiliation,which is the 16th affiliation of this article.

A Possible 250s X-Ray Quasi-periodicity in the Fast Blue Optical Transient AT2018cow

The fast blue optical transients(FBOTs)are a new population of extragalactic transients of unclear physical origin.A variety of mechanisms has been proposed including failed supernova explosion,shock interaction with a dense medium,young magnetar,accretion onto a compact object and stellar tidal disruption event,but none is conclusive.Here we report the discovery of a possible X-ray quasi-periodicity signal with a period of~250 s(at a significance level of 99.76%)in the brightest FBOT AT2018cow through the analysis of XMM-Newton/PN data.The signal is independently detected at the same frequency in the average power density spectrum from data taken from the Swift telescope,with observations covering from 6 to 37 days after the optical discovery,though the significance level is lower(94.26%).This suggests that the quasi-periodic oscillation(QPO)frequency may be stable over at least 1.1×10^(4)cycles.Assuming the~250 s QPO to be a scaled-down analog of that typically seen in stellar mass black holes,a black hole mass of~103–10^(5)solar masses could be inferred.The overall X-ray luminosity evolution could be modeled with a stellar tidal disruption by a black hole of~10^(4)solar masses,providing a viable mechanism to produce AT2018cow.Our findings suggest that other bright FBOTs may also harbor intermediate-mass black holes.

Wave effect of gravitational waves intersected with a microlens field:A new algorithm and supplementary study

The increase in gravitational wave(GW) events has allowed receiving strong lensing image pairs of GWs. However, the wave effect(diffraction and interference) due to the microlens field contaminates the parameter estimation of the image pair, which may lead to a misjudgment of strong lensing signals. To quantify the influence of the microlens field, researchers need a large sample of statistical research. Nevertheless, due to the oscillation characteristic, the Fresnel-Kirchhoff diffraction integral’s computational time hinders this aspect’s study. Although many algorithms are available, most cannot be well applied to the case where the microlens field is embedded in galaxy/galaxy clusters. This work proposes a faster and more accurate algorithm for studying the wave optics effect of microlenses embedded in different types of strong lensing images. Additionally, we provide a quantitative estimation criterion for the lens plane boundary for the Fresnel-Kirchhoff diffraction integral. This algorithm can significantly facilitate the study of wave optics, particularly in the case of microlens fields embedded in galaxy/galaxy clusters.

The cosmological significance of the Taiji-TianQin-LISA network

Gravitational waves(GWs)can provide important new probes for exploring the evolution of the universe.One significant advantage of GWs is their ability to measure absolute distances on cosmological scales,rather than relative distances,which is crucial for cosmological research,especially in determining the Hubble constant.Analogous to the“standard candles”and“standard ruler”in cosmology,distance measurements based on GWs are referred to as“standard sirens”.

Primordial black hole mass functions as a probe of cosmic origin

We discuss a novel window to probe the origin of our universe via the mass functions of primordial black holes(PBHs).The mass functions of PBHs are simply estimated using the conventional Press-Schechter formalism for two paradigms of cosmic origin,including inflationaryΛCDM and bounce cosmology.The standard inflationaryΛCDM model cannot generate an appreciable number of massive PBHs;however,non-trivial inflation models with blue-tilted power spectra at small scales and matter bounce cosmology provide formation mechanisms for heavy PBHs,which in turn,may seed the observed supermassive black holes(SMBHs).By fitting the SMBH mass functions at high redshift(z~6)derived from Sloan Digital Sky Survey(SDSS)and Canada-France High-z Quasar Survey(CFHQS)quasars,for two paradigms of cosmic origin,we derive constraints on the PBH density fraction fPBHat z~6 and the characteristic mass M_(★),with the prior assumption that all SMBHs stem from PBHs.We demonstrate that this newly proposed procedure,relying on astronomical measurements that utilize deep-field surveys of SMBHs at high redshift,can be used to constrain models of cosmic origin.Additionally,although not the main focus of this paper,we evolve the mass function from z~6 to z~0 through an assumption of 3×10^(8)-year Eddington’s accretion,and give a rough estimation of fPBHat z~0.

Forecasts on CMB lensing observations with AliCPT-1

Ali CPT-1 is the first Chinese cosmic microwave background(CMB) experiment aiming for the high-precision measurement of CMB B-mode polarization. The telescope, currently under deployment in Tibet, will observe in two frequency bands centered at 90 and 150 GHz. We forecast the CMB lensing reconstruction, lensing-galaxy, and lensing-cosmic infrared background(CIB) cross-correlation signal-to-noise ratio(SNR) for Ali CPT-1. We consider two stages with different integrated observation times, namely “4 module*yr”(first stage) and “48 module*yr”(final stage). For lensing reconstruction, we use three different quadratic estimators, namely temperature-only, polarization-only and minimum-variance(MV) estimators, using curved sky geometry. We take into account the impacts of inhomogeneous hit counts and mean-field bias due to incomplete sky coverage.In the first stage, our results show that the 150 GHz channel can measure the lensing signals at 15σ significance with the MV estimator. In the final stage, the measurement significance will increase to 31σ. We also combine the two frequency data in the harmonic domain to optimize the SNR. Our results show that the coadding procedure can significantly reduce the reconstruction bias in the high multiple range. Owning to the high quality of the polarization data in the final stage of Ali CPT-1, the EB estimator will dominate the lensing reconstruction in this stage. We also estimate the SNR of cross-correlations between Ali CPT-1 CMB lensing and other tracers of the large scale structure of the universe. For its cross-correlation with Dark Energy Spectroscopic Instrument(DESI) galaxies/quasars, we report the cross-correlation SNR = 10-20 for the four redshift bins at 0.05 < z < 2.1. In the first stage, the total SNR is approximately 32. In the final stage, the lensing-galaxy cross-correlation can reach SNR = 52. For lensing-CIB cross-correlation, in the first stage, the cross-correlations between Ali CPT-1 lensing and Planck CIB 353, 545 and857 GHz channels are approximately SNR = 18, 19, and 23, respectively. In the final stage, the cross-correlations can reach SNR = 25, 33 and 42. Due to the strong correlations between frequency bands, the total lensing-CIB cross-correlations by combining the three frequencies in Planck CIBs are SNR = 23 and 43 for the Ali CPT-1 first and final stages, respectively.

Primordial magnetic fields from gravitationally coupled electrodynamics in nonsingular bounce cosmology

We in this paper study a class of mechanism of the production of the primordial magnetic field(PMF) in the non-singular bouncing cosmology, through the coupling of the electromagnetic field to gravity. We adopt an electrodynamic model with a coupling coefficient as a function of the scale factor a, i.e., f = 1 +(a/a?)^(-n), with a? and n > 0 being constants. With analytical calculations, we find that this model can yield a blue tilted power spectrum of PMF on large scales from 1 Mpc to the Hubble length if the bounce scenario has experienced a contracting phase with an equation-of-state parameter larger than-1/3. Furthermore, in order to satisfy the constraints of observational data, the present mechanism favors the so-called ekpyrotic-bounce paradigm. The back-reaction of the energy density of PMF at the bouncing point can lead to additional theoretical constraints on the underlying bouncing paradigm.

A massless scalar field in Robertson-Walker spacetimes:Adiabatic regularization and Green’s function

We study adiabatic regularization of a coupling massless scalar field in general spatially flat Robertson-Walker(RW)spacetimes.For the conformal coupling,the 2nd-order regularized power spectrum and 4th-order regularized stress tensor are zero,and no trace anomaly exists in general RW spacetimes.This is a new result that exceeds those found in de Sitter space.For the minimal coupling,the regularized spectra are also zero in the radiationdominant and matter-dominant stages,as well as in de Sitter space.The vanishing of these adiabatically regularized spectra is further confirmed by direct regularization of the Green's function.For a general coupling and general RW spacetimes,the regularized spectra can be negative under the conventional prescription.At a higher order of regularization,the spectra will generally become positive,but will also acquire IR divergence,which is inevitable for a massless field.To avoid the IR divergence,the inside-horizon regularization is applied.Through these procedures,nonnegative UV-and IR-convergent power spectrum and spectral energy density will eventually be achieved.

Probing the origin of our universe through primordial gravitational waves by Ali CMB project

Gravitational waves(GW),which were predicted by Einstein in 1916 based on the classical theory of General Relativity(GR),were recently detected by LIGO[1].This breakthrough is expected to initiate a novel probe of cosmology,the nature of gravity as well as fundamental physics.In general,signals of GWs can be classified into two categories。

Constraining the non-Einsteinian polarizations of gravitational waves by pulsar timing array

Pulsar timing array(PTA) provides an excellent opportunity to detect the gravitational waves(GWs) in nanoHertz frequency band.In particular, due to the larger number of "arms" in PTA, it can be used to test gravity by probing the non-Einsteinian polarization modes of GWs, including two spin-1 shear modes labeled by "sn" and "se", the spin-0 transverse mode labeled by "b" and the longitudinal mode labeled by "l". In this paper, we investigate the capabilities of the current and potential future PTAs, which are quantified by the constraints on the amplitudes parameters(c_b, c_(sn), c_(se), c_l), by observing an individual supermassive black hole binary in Virgo cluster. We find that for binary with total mass M_c= 8.77 × 10~8 M_⊙ and GW frequency f = 10^(-9) Hz, the PTA at current level can detect these GW modes if c_b> 0.00106, c_l> 0.00217, c_(se)> 0.00271, c_(sn)> 0.00141, which will be improved by about two orders if considering the potential PTA in SKA era. Interesting enough, due to effects of the geometrical factors, we find that in SKA era, the constraints on the l, sn, se modes of GWs are purely dominated by several pulsars, instead of the full pulsars in PTA.

A possible interpretation of the Higgs mass by the cosmological attractive relaxion

Recently, a novel idea [1] has been proposed to relax the electroweak hierarchy problem through the cosmological inflation and the axion periotic potential. Here, we further assume that only the attractive inflation is needed to explain the light mass of the Higgs boson, where we do not need a specified periodic potential of the axion field. Attractive inflation during the early universe drives the Higgs boson mass from the large value in the early universe to the small value at present, where the Higgs mass is an evolving parameter of the Universe. Thus, the small Higgs mass can technically originate from the cosmological evolution rather than dynamical symmetry or anthropics. Further, we study the possible collider signals or constraints at a future lepton collier and the possible constraints from the muon anomalous magnetic moment. A concrete attractive relaxion model is also discussed, which is consistent with the data of Planck 2015.

Is the W-boson mass enhanced by the axion-like particle,dark photon,or chameleon dark energy?

The W-boson mass(m W=(80.4335±0.0094)Ge V)measured by the Collider Detector at Fermilab Collaboration is greater than the standard model(SM)prediction at a confidence level of 7σ,strongly suggesting the presence of new particles or fields.In the literature,various new particles and/or fields have been introduced to explain the astrophysical and experimental data,and their presence,in principle,may also enhance the W-boson mass.In this study,we investigate axion-like particle(ALP),dark photon(DP),and chameleon dark energy(DE)models for a solution to the W-boson mass excess.We find that the ALP and DP interpretations have been significantly narrowed down by global electroweak fits.The possibility of attributing the W-boson mass anomaly to the chameleon DE is ruled out by other experiments.

Remarkable effects of multiple photon spheres on black hole observations

The recent release of the first image of the super-massive black hole M87^(*),photographed by the Event Horizon Telescope Collaboration,has attracted a lot of attention in black hole appearances.In a spherically symmetric spacetime,the photon sphere plays a vital role in the black hole image.