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The inter-band lags among the optical broad-band continua of active galactic nuclei (AGNs) have been intensively explored over the past decade. However, the nature of the lags remains under debate. Here utilizing two distinct scenarios for AGN variability, i.e., the thermal fluctuation of accretion disk and the reprocessing of both the accretion disk and clouds in the broad line region, we show that, owing to the random nature of AGN variability, the inter-band lags of an individual AGN would vary from one campaign with a finite baseline to another. Specifically, the thermal fluctuation scenario implies larger variations in the lags than the reprocessing scenario. Moreover, the former predicts a positive correlation between the lag and variation amplitude, while the latter does not result in such a correlation. For both scenarios, averaging the lags of an individual AGN measured with repeated and non-overlapping campaigns would give rise to a stable lag, which is larger for a longer baseline and gets saturation for a sufficiently long baseline. However, obtaining the stable lag for an individual AGN is very time-consuming. Alternatively, it can be equivalently inferred by averaging the lags of a sample of AGNs with similar physical properties, thus can be properly compared with predictions of AGN models. In addition, discussed are several new observational tests suggested by our simulations as well as the role of the deep high-cadence surveys of the Wide Field Survey Telescope in enriching our knowledge of the lags., Comment: 16 pages, 10 figures. Accepted for publication in Astrophysical Journal, comments are welcome!

The elemental abundances of stars reflect the complex enrichment history of the galaxy. To explore and explain the metal enrichment history of the cosmic environment near our solar system, we study the evolution of $^{56} \mathrm{Fe}$ abundance over time and [Mg/Fe] versus [Fe/H] evolution in the solar neighborhood. Core-collapse supernovae make the dominant contribution in the early stages, while Type Ia supernovae (SNe Ia) have a delayed and dominant impact in the later stages. In this work, we consider the nucleosynthesis contribution of neutrino-dominated accretion flows (NDAFs) formed at the end of the lives of massive stars. The results show that the [Fe/H] gradually increases over time and eventually reaches $\rm [Fe/H]=0$ and above, reproducing the chemical enrichment process in the solar neighborhood. Before the onset of SNe Ia, the ratio of $^{56} \mathrm{Fe}$ mass to the total gas mass increases by a factor of at most $\sim 1.44$ when NDAFs are taken into account. We find that by including NDAF in our models, the agreement with the observed metallicity distribution of metal-poor stars in the solar neighborhood ($\rm < 1~kpc$) is improved, while not significantly altering the location of the metallicity peak. This inclusion can also reproduce the observed evolutionary change of [Mg/Fe] at [Fe/H] $\sim -1.22$, bringing the ratio to match the solar abundance. Our results provide an extensive understanding of metallicity evolution in the solar environments by highlighting the nucleosynthesis contribution of NDAF outflows in the solar neighborhood., Comment: 10 pages, 4 figures, accepted for publication in ApJ

Resolving the inner structures of active galactic nuclei (AGNs) provides the "standard ruler" to measure the parallax distances of the Universe and a powerful way to weigh supermassive black holes (SMBHs). Thanks to time-domain observations, it is possible to use the reverberation mapping (RM) technique to measure time delays between different light curves that probe the structures of the SMBH accretion disks and broad line regions (BLRs), which are otherwise often too compact to be spatially resolved for most AGNs. Despite decades of RM studies, the critical physical process that controls the structures of SMBH accretion disk and BLR and their temporal evolution remains unclear. Here we report the variation of the SMBH accretion disk structure of NGC 4151 in response to changes in luminosity within 6 years. In the high-flux state, the time delays measured from our continuum RM with high-cadence (2 days) spectroscopy are 3.8 times larger than that in the low-flux state and 15 times longer than the classical standard thin disk (SSD) prediction. This result provides the first piece of direct evidence that the SMBH disk structure "breathes" in highly-variable AGN manifestations. The time-delay change severely challenges the popular X-ray reprocessing of the SSD model, with or without BLR contributions. More importantly, the continuum time delays can be comparable with the time delay between the broad Hb line and the nearby optical continuum, and the latter is commonly used to calculate the BLR sizes. Hence, the BLR sizes are significantly underestimated if the continuum time delays are not properly considered. This underestimation introduces up to 0.3 dex systematic uncertainties on RM SMBH masses and BLR parallax distances. Our findings underscore that simultaneous continuum and BLR RM studies are vital for better deciphering the SMBH mass growth and the cosmological expansion history., Comment: 18 pages, 8 figures, comments welcome

We present the timing analysis of 10 archived \XMM observations with an exposure of $>40$ ks of Markarian 421. Mrk 421 is the brightest high-frequency-peaked BL Lac object (HBL) emitting in X-rays produced by electrons accelerated in the innermost regions of a relativistic jet pointing toward us. For each observation, we construct averaged X-ray spectra in 0.5--10 keV band, as well as 100 s binned light curves (LCs) in various subbands. During these observations, the source exhibited various intensity states differing by close to an order of magnitude in flux, with the fractional variability amplitude increasing with energy through the X-ray band. Bayesian power spectral density analysis reveals that the X-ray variability can be characterized by a colored noise, with an index ranging from $\sim-1.9$ to $-3.0$. Moreover, both the standard cross-correlation function and cross-spectral methods indicate that the amount of time lags increases with the energy difference between two compared LCs. A time-dependent two-zone jet model is developed to extract physical information from the X-ray emission of Mrk 421. In the model, we assume that the jet emission mostly comprises a quasi-stationary component and a highly variable one. Our results show that the two-zone model can simultaneously provide a satisfactory description for both the X-ray spectra and time lags observed in different epochs, with the model parameters constrained in a fully acceptable interval. We suggest that shocks within the jets may be the primary energy dissipation process responsible for triggering the rapid variability, although magnetic reconnection cannot be excluded., Comment: 33 pages, 12 figures, 6 tables; Accepted for publication in ApJ supplement series

Quasars are generally divided into jetted radio-loud and non-jetted radio-quiet ones, but why only 10% quasars are radio loud has been puzzling for decades. Other than jet-induced-phenomena, black hole mass, or Eddington ratio, prominent difference between jetted and non-jetted quasars has scarcely been detected. Here we show a unique distinction between them and the mystery of jet launching could be disclosed by a prominent excess of radio emission in extremely stable quasars (ESQs, i.e., type 1 quasars with extremely weak variability in UV/optical over 10 years). Specifically, we find that $>$ 25% of the ESQs are detected by the FIRST/VLASS radio survey, while only $\sim$ 6-8% of the control sample, matched in redshift, luminosity, and Eddington ratio, are radio-detected. The excess of radio detection in ESQs has a significance of 4.4 $\sigma$ (99.9995%), and dominantly occurs at intermediate radio loudness with R $\sim$ 10 - 60. The radio detection fraction of ESQs also tends to increase in the ESQ samples selected with more stringent thresholds. Our results are in contrast to the common view that RL quasars are likely more variable in UV/optical due to jet contribution. New clues/challenge posed by our findings highlight the importance of extensive follow-up observations to probe the nature of jets in ESQs, and theoretical studies on the link between jet launching and ESQs. Moreover, our results makes ESQs, an essential population which has never been explored, unique targets in the burgeoning era of time domain astronomy, like their opposite counterparts of quasars exhibiting extreme variability or changing-look features., Comment: 11 pages, 16 figures, Accepted by ApJ

Federated recommender systems have been crucially enhanced through data sharing and continuous model updates, attributed to the pervasive connectivity and distributed computing capabilities of Internet of Things (IoT) devices. Given the sensitivity of IoT data, transparent data processing in data sharing and model updates is paramount. However, existing methods fall short in tracing the flow of shared data and the evolution of model updates. Consequently, data sharing is vulnerable to exploitation by malicious entities, raising significant data privacy concerns, while excluding data sharing will result in sub-optimal recommendations. To mitigate these concerns, we present LIBERATE, a privacy-traceable federated recommender system. We design a blockchain-based traceability mechanism, ensuring data privacy during data sharing and model updates. We further enhance privacy protection by incorporating local differential privacy in user-server communication. Extensive evaluations with the real-world dataset corroborate LIBERATE's capabilities in ensuring data privacy during data sharing and model update while maintaining efficiency and performance. Results underscore blockchain-based traceability mechanism as a promising solution for privacy-preserving in federated recommender systems.

To investigate the short-term variability and determine the size of the optical continuum emitting region of intermediate-mass black holes (IMBHs), we carried out high-cadence, multiband photometric monitoring of a Seyfert 1 galaxy J0249-0815 across two nights, together with a one-night single-band preliminary test. The presence of the broad Ha component in our target was confirmed by recent Paloma spectroscopic observations, 23 years after the Sloan Digital Sky Survey, ruling out the supernovae origin of the broad Ha line. The photometric experiment was primarily conducted utilizing four-channel imagers MuSCAT 3 and 4 mounted on 2 m telescopes within the Las Cumbres Observatory Global Telescope Network. Despite the expectation of variability, we observed no significant variation (<1.4%) on timescales of 6 to 10 hr. This nondetection is likely due to substantial host galaxy light diluting the subtle active galactic nucleus (AGN) variability. Additionally, we cannot rule out that the target was in a relatively quiescent state without intranight variability during our monitoring, owing to the stochastic nature of AGN variations. To enhance the possibility of detecting subtle variability signals and lag in future IMBH reverberation campaigns, it may be beneficial to select targets with a higher AGN-to-host flux ratio, and conduct dual-band preliminary tests and tailored simulations., Comment: 11 pages, 6 figures

Variability of active galactic nuclei (AGNs) has long been servicing as an essential avenue of exploring the accretion physics of black hole (BH). There are two commonly used methods for analyzing AGN variability. First, the AGN variability, characterized by the structure function (SF) of a single band, can be well described by a damped random walk (DRW) process on timescales longer than ~weeks, shorter than which departures have been reported. Second, the color variation (CV) between two bands behaves timescale-dependent, raising challenges to the widely accepted reprocessing scenario. However, both the departure from the DRW process and the timescale-dependent CV are hitherto limited to AGNs, mainly quasars, at the supermassive scale. Here, utilizing the high-cadence multi-wavelength monitoring on NGC 4395 harboring an intermediate-mass BH, we unveil at the intermediate-mass scale for the first time, prominent departures from the DRW process at timescales shorter than ~hours in all three nights and bands, and plausible timescale-dependent CVs in the two longest nights of observation. Furthermore, comparing SFs of NGC 4395 to four AGNs at the supermassive scale, we suggest a new scaling relation between the timescale (\uptau; across nearly three orders of magnitude) and the BH mass (M_BH): \uptau \propto M^\gamma_BH where the exponent \gamma is likely ~ 0.6 - 0.8. This exponent differs from most previous measurements, but confirms a few and is consistent with a recent theoretical prediction, suggesting a similar accretion process in AGNs across different mass scales., Comment: 22 pages, 12 figures, Accepted by ApJ

We study the critical behaviors of the ground and first excited states in the one-dimensional nonreciprocal Aubry-Andr{\'e}-Harper model using both the self-normal and biorthogonal fidelity susceptibilities. We demonstrate that fidelity susceptibilities serve as a probe for the phase transition in the nonreciprocal AAH model. For ground states, characterized by real eigenenergies across the entire regime, both fidelity susceptibilities near the critical points scale as $N^{2}$, akin to the Hermitian AAH model. However, for the first-excited states, where $\mathcal{PT}$ transitions occur, the fidelity susceptibilities exhibit distinct scaling laws, contingent upon whether the lattice consists of even or odd sites. For even lattices, both the self-normal and and biorthogonal fidelity susceptibilities near the critical points continue to scale as $N^{2}$. In contrast, for odd lattices, the biorthogonal fidelity susceptibilities diverge, while the self-normal fidelity susceptibilities exhibit linear behavior, indicating a novel scaling law., Comment: 7 pages, 4 figures

We investigate the constraints provided by the Euclid space observatory on the physical properties of dusty star forming galaxies (DSFGs) at z>~1.5 detected in wide area sub millimetre surveys with Herschel. We adopt a physical model for the high z progenitors of spheroidal galaxies, which form the bulk of the DSFGs at z>~1.5. We improve the model by combining the output of the equations of the model with a formalism for the spectral energy distribution(SED). After optimising the SED parameters to reproduce the measured infrared luminosity function and the number counts of DSFGs, we simulated a sample of DSFGs over 100 sq deg and then applied a 5 sigma detection limit of 37 mJy at 250 microns. We estimated the redshifts from the Euclid data and then fitted the Euclid and Herschel photometry with the code CIGALE to extract the physicsl parameters. We found that 100 % of the Herschel galaxies are detected in all 4 Euclid bands above 3 sigma. For 87% of the sources the accuracy on 1+z is better than 15%. The sample comprises mostly massive log(Mstar/Msun)~10.5-12.9, highly star forming, log(SFR/Msun/yr)~1.5-4, dusty, log(Mdust/Msun)~7.5-9.9, galaxies. The measured stellar mass have a dispersion of 0.19 dex around the true value, thus showing that Euclid will provide reliable stellar mass estimates for the majority of the bright DSFGs at z>~1.5 detected by Herschel. We also explored the effect of complementing the Euclid photometry with that from Vera C. Rubin Observatory/LSST., Comment: 24 pages, 24 figures

Magnetohydrodynamics (MHD) turbulence can drive significant temperature fluctuations in the accretion disk of an active galactic nucleus (AGN). As a result, the disk can be highly inhomogeneous and has a half-light radius larger than the static Shakura \& Sunyaev Disk (SSD), in agreement with quasar microlensing observations. Meanwhile, the accretion-disk sizes can also be determined using continuum reverberation mappings which measure interband cross correlations and time lags. The interband time lags are often understood in the X-ray reprocessing scenario. Here we show that the interband continuum time lags of the X-ray reprocessing of an inhomogeneous disk are similar to or even smaller than those of a static SSD. Consequently, the X-ray reprocessing of an inhomogeneous disk cannot account for the recent continuum reverberation mappings of some Seyfert 1 AGNs, whose measured time lags are larger than those of a static SSD. In contrast to the tight correlation between UV/optical variations, the cross correlation between X-ray and disk emission is rather weak in this model; this behavior is consistent with recent continuum reverberation mappings. Moreover, the time lags in this model are anti-correlated with the amplitude of disk temperature fluctuations. Our results suggest that the temperature fluctuations should be properly considered when modeling interband continuum time lags., Comment: 14 pages, 7 figures, Accepted to ApJ

The UV/optical light curves of Active Galactic Nuclei (AGNs) are commonly described by the Damped Random Walk (DRW) model. However, the physical interpretation of the damping timescale, a key parameter in the DRW model, remains unclear. Particularly, recent observations indicate a weak dependence of the damping timescale upon both wavelength and accretion rate, clearly being inconsistent with the accretion-disk theory. In this study, we investigate the damping timescale in the framework of the Corona Heated Accretion disk Reprocessing (CHAR) model, a physical model that describes AGN variability. We find that while the CHAR model can reproduce the observed power spectral densities of the 20-year light curves for 190 sources from \cite{Stone2022}, the observed damping timescale, as well as its weak dependence on wavelength, can also be well recovered through fitting the mock light curves with DRW. We further demonstrate that such weak dependence is artificial due to the effect of inadequate durations of light curves, which leads to best-fitting damping timescales lower than the intrinsic ones. After eliminating this effect, the CHAR model indeed yields a strong dependence of the intrinsic damping timescale on the bolometric luminosity and rest-frame wavelength. Our results highlight the demand for sufficiently long light curves in AGN variability studies and important applications of the CHAR model in such studies., Comment: 19 pages, 16 figures, accepted to ApJ

The variability of active galactic nuclei (AGNs) is ubiquitous but has not yet been understood. Measuring the optical variation properties of AGNs, such as variation timescale and amplitude, and then correlating them with their fundamental physical parameters, have long served as a critical way of exploring the origin of AGN variability and the associated physics of the accretion process in AGNs. Obtaining accurate variation properties of AGNs is thus essential. It has been found that the damped random walk (DRW) process can well describe the AGN optical variation, however, there is a controversy over how long a minimal monitoring baseline is required to obtain unbiased variation properties. In this work, we settle the controversy by exhaustively scrutinizing the complex combination of assumed priors, adopted best-fit values, ensemble averaging methods, and fitting methods. Then, the newly proposed is an optimized solution where unbiased variation properties of an AGN sample possessing the same variation timescale can be obtained with a minimal baseline of about 10 times their variation timescale. Finally, the new optimized solution is used to demonstrate the positive role of time domain surveys to be conducted by the Wide Field Survey Telescope in improving constraints on AGN variation properties., Comment: 14 pages, 12 figures, ApJ accepted

The well-known anti-correlation between the optical/ultraviolet (UV) emission line equivalent widths of active galactic nuclei and the continuum luminosity (the so-called Baldwin effect) is a long-standing puzzle. One common hypothesis is that more luminous sources have softer spectral energy distribution (SED) in the extreme UV (EUV), as revealed by some observational studies. In this work we revisit this issue through cross-matching SDSS quasars with GALEX far-UV/near-UV catalogs and correcting the effect of a severe observational bias of significant UV detection incompleteness, i.e., the more luminous in observed-frame optical, the more likely detected in observed-frame UV. We find that, for GALEX detected quasars at 1.8 < z < 2.2, the rest-frame mean UV SED (~ 500 -- 3000 Angstrom) bewilderingly shows no luminosity dependence at log(\nu L_\nu(2200 Angstrom)) > 45 (up to 47.3), contrary to the standard thin disc model predictions and the observed Baldwin effect in this luminosity range. Probably, the universal mean UV SED is the result of a local atomic-originated process, and in fainter quasars stronger disk turbulence launching more clouds is the main origin of the Baldwin effect. After correcting for the absorption of the intergalactic medium, a rest-frame intrinsic mean EUV SED is derived from a sub-sample of bright quasars and is found to be much redder in the EUV than all previous quasar composite spectra, highlighting the significance of properly accounting for the sample incompleteness. Interestingly, the global consistence between our extremely red mean EUV SED and the line-driven wind model again supports an origin of a local physical process., Comment: 27 pages, 15 figures, author's initial version submitted to Nature Astronomy

Recent observations of quasars show high line-flux ratios in their broad emission lines and the ratios appear to be independent of redshift up to $z \gtrsim 6$, which indicate that the broad-line regions of these early quasars are surprisingly metal-rich. Here, we revisit the chemical evolution of high-redshift quasars by adding a new ingredient, i.e., the neutrino-dominated accretion flows (NDAFs) with outflows, on top of the conventional core-collapse supernovae (CCSNe). In the presence of the chemical contribution from NDAFs with outflows, the total metal mass (i.e., the summation of the conventional CCSN and NDAFs with outflows) per CCSN depends weakly upon the mass of the progenitor star if the mass is in the range of $\sim 25-55~M_{\odot}$. We model the chemical evolution by adopting a improved open-box model with three typical initial mass functions (IMFs). We find that, with the additional chemical contribution from NDAFs with outflows, the quasar metallicity can be enriched more rapidly in the very early Universe ($z \sim 10$) and reaches higher saturation than the no-NDAF case at $z \sim 8$, after which they evolve slowly with redshift. The quasar metallicity can reach $\sim 20~Z_{\odot}$ ($Z_\odot$ denotes the metallicity of the Sun; and $\sim 20\%$ of which is produced by NDAF outflows) at $z \sim 8$ for the ``top-heavy'' IMF model in \citet{Toyouchi2022}, which readily explains the quasar observations on the super-solar metal abundance and redshift-independent evolution., Comment: 6 pages, 2 figures, accepted for publication in ApJ

The results of the recently published spectroscopically complete survey of dusty star-forming galaxies detected by the South Pole Telescope (SPT) over 2500 deg^2 proved to be challenging for galaxy formation models that generally underpredict the observed abundance of high-z galaxies. In this paper we interpret these results in the light of a physically grounded model for the evolution of spheroidal galaxies. The model accurately reproduces the measured redshift distribution of galaxies without any adjustment of the parameters. The data do not support the indications of an excess of z > 4 dusty galaxies reported by some analyses of Herschel surveys., Comment: accepted by ApJ

The analytic signal is a useful mathematical tool. It separates qualitative and quantitative information of a signal in form of the local phase and local amplitude. The Clifford Fourier transform (CFT) plays a vital role in the representation of multidimensional signals. By generalizing the CFT to the Clifford linear canonical transform (CLCT), we present a new type of Clifford biquaternionic analytic signal. Due to the advantages of more freedom, the envelop detection problems of 3D images, with the help of this new analytic signal, can get a better visual appearance. Synthesis examples are presented to demonstrate these advantages., Comment: 19 pages and 5 figures

The relation between X-ray and UV/optical variability in AGNs has been explored in many individual sources, however a large sample study is yet absent. Through matching the XMM-Newton serendipitous X-ray and UV source catalogs with SDSS quasars, we build a sample of 802 epoch-pairs of 525 quasars showing clear variability in logFx-logFuv space. After correcting for the effect of photometric noise, we find 35.6\pm2.1% of the epoch-pairs show asynchronous variability between X-ray and UV (brightening in one band but dimming in the other). This indicates only in 28.8\pm4.2% of the epoch-pairs the X-ray and UV variability are intrinsically coordinated. The variability synchronicity exhibits no dependence on physical parameters of quasars or the time lag of the epoch-pairs, except for stronger variability tends to have stronger synchronicity. Switches between synchronous and asynchronous variability are also seen in individual sources. The poor coordination clearly contradicts both the X-ray reprocessing model and the accretion rate variation model for AGN variability. The ratios of the observed X-ray variability amplitude to that in UV span a broad range and peak at ~ 2. The dominant fraction of the ratios appear too small to be attributed to X-ray reprocessing, and too large for accretion rate variation. The inhomogeneous disk model which incorporates both X-ray and UV/optical variability in AGNs is favored by the observed stochastic relation between X-ray and UV variations., Comment: 9 pages, 10 figures; accepted by MNRAS