Optical Variability Research Articles

X-ray reverberation as an explanation for UV/optical variability in nearby Seyferts

Context. Active galactic nuclei (AGNs) are known to be variable across all wavelengths. Significant observational efforts have been invested in the last decade in studying their ultraviolet (UV) and optical variability. Long and densely sampled, multi-wavelength monitoring campaigns of numerous Seyfert galaxies have been conducted with the aim of determining the X-ray/UV/optical continuum time lags. Time-lag studies can be used to constrain theoretical models. The observed time lags can be explained by thermal reprocessing of the X-rays illuminating the accretion disc (known as the X-ray reverberation model). However, the observed light curves contain more information that can be used to further constrain physical models. Aims. Our primary objective is to investigate whether, in addition to time lags, the X-ray reverberation model can also explain the UV/optical variability amplitude of nearby Seyferts. Methods. We measured the excess variance of four sources (namely Mrk 509, NGC 4151, NGC 2617, and Mrk 142) as a function of wavelength using data from archival long, multi-wavelength campaigns with Swift, and ground-based telescopes. We also computed the model excess variance in the case of the X-ray reverberation model by determining the disc’s transfer function and assuming a bending power law for the X-ray power spectrum. We tested the validity of the model by comparing the measured and model variances for a range of accretion rates and X-ray source heights. Results. Our main result is that the X-ray thermal reverberation model can fit both the continuum, UV/optical time lags, as well as the variance (i.e. the variability amplitude) in these AGNs, for the same physical parameters. Our results suggest that the accretion disc is constant and that all the observed UV/optical variations, on timescales of days and up to a few weeks, can be fully explained by the variable X-rays as they illuminate the accretion disc.

Freezing of movements and its correspondence with MLG1 neurons response to looming stimuli in the crab Neohelice.

Upon visually detecting a moving predator animals often freeze, i.e. stop moving, to minimize being uncovered and to gather detailed information of the object's movements and properties. In certain conditions the freezing behavior can be enough to avoid a predatory menace but, when the risk is high or increases to a higher level, animals switch strategy and engage in an escape response. The neural bases underlying escape responses to visual stimuli are extensively investigated both in vertebrates and arthropods. However, those involved in freezing behaviors are much less studied. Here, we investigated the freezing behavior displayed by the crab Neohelice granulata when confronted with a variety of looming stimuli simulating objects of distinct sizes approaching on a collision course at different speeds. The experiments were performed in a treadmill-like device. Animals engaged in exploratory walks respond to the looming stimulus with freezing followed by escaping. The analysis of the stimulus optical variables shows that regardless of the looming dynamic, the freezing decision is made when the angular size of the object increases by 1.4°. In vivo intracellular recording responses of Monostratified Lobula Giant Neurons (MLG1) to the same looming stimuli show that the freezing times correlate with the times predicted by a hypothetical spike counter of this neuron.

Optical and γ-ray variability analysis of BL Lacertae object TXS 1902+556

ABSTRACT The variability data for the BL Lacertae object TXS 1902+556 in the optical and $\gamma$-ray wavebands were obtained from the 0.76-m Katzman Automatic Imaging Telescope and the Fermi Large Area Telescope (Fermi-LAT), covering periods of 14.4 and 14.7 yr, respectively. The variability properties were systematically analysed, with particular emphasis on the first comprehensive investigation of radiation variation in the optical waveband. Four well-established techniques were employed for this purpose: the Lomb–Scargle periodogram, REDFIT program, Jurkevich method, and discrete correlation function (DCF) approach. The optical waveband exhibits quasi-periodic oscillations (QPO) with a time-scale of $P_{\rm O}=276.8\pm 6.1$ d at a significance level $3.87\sigma$, while the $\gamma$-ray waveband does not exhibit any significant periodicity. However, it should be noted that the QPO time-scale is consistent with the Sun-gaps in the optical light curve within 2$\sigma$ uncertainties. The optical QPO behaviour is most likely attributed to the helical motion of the jet driven by the orbital motion in a supermassive black hole binary system. Moreover, we have provided an explanation for the absence of QPO in the $\gamma$-ray light curves. Furthermore, utilizing the DCF method, a weak correlation between the variability in the optical and $\gamma$-ray wavebands was observed, suggesting that the emission of TXS 1902+556 may be generated through a combination of synchrotron self-Compton (SSC) and external Compton (EC) processes, or a leptonic–hadronic hybrid process.

Searching for new cataclysmic variables in the Chandra Source Catalog

Aims. Cataclysmic variables (CVs) are compact binary systems in which a white dwarf accretes matter from a Roche-lobe-filling companion star. For this study we searched for new CVs in the Milky Way in the Chandra Source Catalog v2.0, cross-matched with Gaia Data Release 3 (DR3). Methods. We identified new CV candidates by combining X-ray and optical data in a color-color diagram called the X-ray main sequence. We used two different cuts in this diagram to compile pure and optically variable samples of CV candidates. We undertook optical spectroscopic follow-up observations with the Keck and Palomar Observatories to confirm the nature of these sources. Results. We assembled a sample of 25 887 Galactic X-ray sources and found 14 new CV candidates. Seven objects show X-ray and/or optical variability. All sources show X-ray luminosity in the 1029 − 1032 erg s−1 range, and their X-ray spectra can be approximated by a power-law model with photon indices in the Γ ∼ 1 − 3 range or an optically thin thermal emission model in the kT ∼ 1 − 70 keV range. We spectroscopically confirmed four CVs, discovering two new polars, one low accretion rate polar and a WZ Sge-like low accretion rate CV. X-ray and optical properties of the other nine objects suggest that they are also CVs (likely magnetic or dwarf novae), and one other object could be an eclipsing binary, but revealing their true nature requires further observations. Conclusions. These results show that a joint X-ray and optical analysis can be a powerful tool for finding new CVs in large X-ray and optical catalogs. X-ray observations such as those by Chandra are particularly efficient at discovering magnetic and low accretion rate CVs, which could be missed by purely optical surveys.

Short-term variability of the transitional pulsar candidate CXOU J110926.4−650224 from X-rays to infrared

CXOU J110926.4−650224 is a candidate transitional millisecond pulsar (tMSP) with X-ray and radio emission properties reminiscent of those observed in confirmed tMSPs in their X-ray ‘subluminous’ disc state. We present the results of observing campaigns that, for the first time, characterise the optical and near-infrared variability of this source and establish a connection with the mode-switching phenomenon observed in X-rays. The optical emission exhibited flickering activity, frequent dipping episodes where it appeared redder, and a multi-peaked flare where it was bluer. The variability pattern was strongly correlated with that of the X-ray emission. Each dip matched an X-ray low-mode episode, indicating that a significant portion of the optical emission originates from nearly the same region as the X-ray emission. The near-infrared emission also displayed remarkable variability, including a dip of 20 min in length during which it nearly vanished. Time-resolved optical spectroscopic observations reveal significant changes in the properties of emission lines from the disc and help infer the spectral type of the companion star to be between K0 and K5. We compare the properties of CXOU J110926.4−650224 with those of other tMSPs in the X-ray subluminous disc state and discuss our findings within the context of a recently proposed scenario that explains the phenomenology exhibited by the prototypical tMSP PSR J1023+0038.

Rapid optical flare in the extreme teraelectronvolt blazar 1ES 0229+200 on intraday timescales with the Transiting Exoplanet Survey Satellite

Context. The extreme teraelectronvolt (TeV) blazar 1ES 0229+200 is a high-frequency-peaked BL Lacertae object. It has not shown intraday variability in extensive optical and X-ray observations, nor has it shown any significant variability on any measurable timescale in the 1–100 GeV energy range over a 14-year span; however, variations in the source flux around its average are present in the energy range above 200 GeV. Aims. We aim to search for intraday optical variability in 1ES 0229+200 as part of an ongoing project to search for variability and quasi-periodic oscillations in the high-cadence (2 min), nearly uniformly sampled optical light curves of blazars provided by the Transiting Exoplanet Survey Satellite (TESS). Methods. 1ES 0229+200 was monitored by TESS in its Sectors 42, 43, and 44. We analysed the data of all these three sectors both with the TESS-provided lightkurve software and the eleanor reduction pipeline. We detected a strong, essentially symmetric flare that lasted about 6 h in Sector 42. We fitted the flare’s rising and declining phases to exponential functions. We also analysed the light curve of Sector 42 using the Lomb-Scargle periodogram (LSP) and continuous auto-regressive moving average (CARMA) methods. Results. The optical light curve of Sector 42 of the TESS observations displayed in the present work provides the first evidence of a strong, rapid, short-lived optical flare on the intraday timescale in the TeV blazar 1ES 0229+200. The variability timescale of the flare provides the upper limit for the size of the emission region to be within (3.3 ± 0.2–8.3 ± 0.5)×1015 cm. Away from the flare, the slope of the periodogram’s power spectrum is fairly typical of many blazars (α < 2), but the nominal slopes for the flaring regions are very steep (α ∼ 4.3), which may indicate that the electron distribution undergoes a sudden change. We discuss possible emission mechanisms that could explain this substantial and rapid flare.

Searching for the mHz variability in the TESS observations of nova-like cataclysmic variables

We investigated the fast optical variability of selected nova-like cataclysmic variables observed by the satellite. We searched for break frequencies b $) in the corresponding power density spectra (PDS). The goal is to study whether these systems in an almost permanent high optical state exhibit preferred $f_ b $ around 1\,mHz. We selected non-interrupted light curve portions with durations of 5 and 10 days. We divided these portions into ten equally long light curve subsamples and calculated mean PDS. We searched for $f_ b $ in the frequency interval from log($f$/Hz) = -3.5 to -2.4. We defined as a positive detection when the $f_ b $ was present in at least 50<!PCT!> of the light curve portions with a predefined minimum number of detections. We have measured $f_ b $ in 15 nova-like systems and confirmed that the value of this frequency is clustered around 1\,mHz with a maximum of the distribution between log($f$/Hz) = -2.95 and -2.84. The confidence that this maximum is not a random feature of a uniform distribution is at least 96<!PCT!>. This is a considerable improvement on the previous value of 69<!PCT!>. We discuss the origin of these b $ in the context of the sandwich model in which a central hot X-ray corona surrounds a central optically thick disc. This scenario could be supported by a correlation between the white dwarf mass and $f_ b $; the larger the mass, the lower the frequency. We see such a tendency in the measured data; however, the data are too scattered and based on a low number of measurements. Finally, it appears that systems with detected $f_ b $ have a lower inclination than 60-75$^ circ $. In higher-inclination binaries, the central disc is not seen and the PDS is dominated by red noise. This also supports the inner disc regions as being the source of the observed $f_ b

Optical Variability Properties of Southern TESS Blazars

We present a study of high-cadence, high-precision optical light curves from the TESS satellite of 67 blazars in the southern sky. We provide descriptive flux statistics, power spectral density (PSD) model parameters, and characteristic variability timescales. We find that only 15 BL Lacertae objects (BLLs) and 18 flat spectrum radio quasars (FSRQs) from the initial 26 and 41, respectively, exhibit statistically significant variability. We employ an adapted power spectral response method to test the goodness of fit for the PSD function to three power-law variant models. From our best-fitting description of the PSD, we extract the high-frequency power-spectral slopes, and if present, determine the significant bend or break in the model to identify characteristic timescales. We find no significant difference in the excess variance or rms scatter between blazar subpopulations. We identify a linear rms–flux relation in ∼69% of our sample, in which ∼20% show a strong correlation. We find that both subpopulations of blazars show power spectral slopes of α ∼ 2 in which a broken power-law best fits five BLLs and six FSRQs and a bending power-law best fits one BLL and five FSRQs. The shortest timescales of variability in each light-curve range widely from minutes to weeks. Additionally, these objects’ characteristic timescales range from ∼0.8 to 8 days, consistent with optical variability originating in the jet.

Aerosol climatology, variability, and trends over the Indo-Gangetic Plain in CMIP6 models

We evaluate the performance of Coupled Model Intercomparison Project Phase 6 (CMIP6) models in simulating aerosol optical depth (AOD) climatology, variability, and trends over the Indo-Gangetic Plain (IGP) considered for the period of space-borne MODIS satellite observations during 2003–2014. The multi-model ensemble (MME) mean of CMIP6 models, is considered when better correlations (≥0.5) of models against the MODIS-derived AOD observations using standard statistical skill metrics. Analysis suggests that the CMIP6 MME successfully reproduces the spatial distribution of AOD climatology and its seasonal variability, albeit with some discrepancies. In particular, CMIP6 MME underestimates AOD spatial distributions across the IGP region (∼ bais varies between 0.2 and 0.4 during different seasons). CMIP6 MME captures coarse-mode aerosol distribution similar to AERONET over the IGP, while it has limited skill in capturing the fine-mode aerosol distribution, which could be due to uncertainties pertinent to anthropogenic aerosol emissions. Furthermore, the CMIP6 MME captures the seasonal evolution of aerosols over the IGP and its sub-regions (i.e., eastern and western IGP regions). Notably, the CMIP6 MME successfully captures the dominance among various aerosol species and their contributions to total AOD over the IGP. The CMIP6 MME mimic the observed AOD trends (varies between 0.01 and 0.03 year−1) well across the study regions, except the western IGP, where the MME is unable to replicate declining AOD trends during the pre-monsoon and monsoon seasons compared to MODIS. By conducting a principal component analysis on AOD data, it is apparent that the CMIP6 MME captures AOD variances that are comparable to MODIS observations, albeit with some discrepancies. This finding of the present research will help in policymaking, and mitigation strategies in the region.

Biomass flame spectroscopy technique to identify wood species through spectral emission during combustion processes

The energy emitted by a flame and its spectra provide critical information about fuels, such as energy type, temperature, and molecular properties. While traditionally used to analyze combustion, we introduce a spectral-time-based index, optical calorific power (OCP), to calculate the total energy released per mass unit (J/Kg), relating to the calorific value of wood-based fuels. Using novel optical variables and machine learning, we identified different wood species during the combustion of the wood samples. Homogeneous samples of four wood species (sapwood and heartwood) were ignited in a temperature-controlled furnace. Spectra were measured using a calibrated spectrophotometer in the 450–900 nm range with a 100ms integration time. Continuous and discontinuous spectral patterns were observed in all samples, used to calculate spectral-time-based optical variables, and separated using the AirPLS algorithm. Discontinuous spectra correlated with Na and K emissions (589.4 nm and 766.5 nm, respectively). Continuous spectra were analyzed to determine optical variables such as flame temperature (K), total continuous radiation (TCR, μW/cm2), and total continuous energy (TCE, μJ). Five supervised classification models, XGBoost, LR, SVM, LDA, and RF, were trained using normalized physical parameters and optical variables with stratified cross-validation. The proposed optical variables yielded an identification accuracy of 93%, precision of 95%, and recall of 93% during combustion using the XGBoost method. These promising results demonstrate the potential of our method for accurately identifying wood species, while offering a cost-effective and novel alternative to NIR-SWIR reflectance spectral identification techniques.

DAVOS: Dwarf Active Galactic Nuclei from Variability for the Origins of Seeds: Properties of Variability-selected Active Galactic Nuclei in the COSMOS Field and Expectations for the Rubin Observatory

We study the black hole mass–host galaxy stellar mass relation, M BH–M *, of a sample of z < 4 optically variable active galactic nuclei (AGNs) in the COSMOS field. The parent sample of 491 COSMOS AGNs were identified by optical variability from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) program. Using publicly available catalogs and spectra, we consolidate their spectroscopic redshifts and estimate virial black hole masses using broad-line widths and luminosities. We show that variability searches with deep, high-precision photometry like the HSC-SSP can identity AGNs in low-mass galaxies up to z ∼ 1. However, their black holes are more massive given their host galaxy stellar masses than predicted by the local relation for active galaxies. We report that z ∼ 0.5–4 variability-selected AGNs are meanwhile more consistent with the M BH–M * relation for local inactive early-type galaxies. This result is in agreement with most previous studies of the M BH–M * relation at similar redshifts and indicates that AGNs selected from variability are not intrinsically different from the broad-line Type 1 AGN population at similar luminosities. Our results demonstrate the need for robust black hole and stellar mass estimates for intermediate-mass black hole candidates in low-mass galaxies at similar redshifts to anchor this scaling relation. Assuming that these results do not reflect a selection bias, they appear to be consistent with self-regulated feedback models wherein the central black hole and stars in galaxies grow in tandem.

Active Dwarf Galaxy Database. I. Overlap between Active Galactic Nuclei Selected by Different Techniques

We assemble a sample of 733 dwarf galaxies (M * ≤ 109.5 M ⊙) with signatures of active galactic nuclei (AGN) and explore the intersection between different AGN-selection techniques. Objects in our database are compiled from previous studies that identify AGN in dwarf galaxies through spectroscopy, X-ray emission, infrared colors, and optical photometric variability. We apply a uniform set of AGN diagnostic tools to the database using archival data. We find that any single selection method captures no more than half of the overall AGN population, and there is a general disagreement among the AGN-selection methods in this stellar-mass regime. The largest overlap between methods is found when both methods use optical spectroscopic data. In contrast, the populations of AGN intersect the least when comparing those methods that use photometric data at different wavelengths. These results can be used to better constrain the active fraction in dwarf galaxies, which is in turn an important constraint for black hole seed formation models. In a follow-up paper, we will explore links between the effectiveness of each selection technique and host galaxy properties.

A first systematic characterization of cataclysmic variables in SRG/eROSITA surveys

We present an account of known cataclysmic variables (CVs) that were detected as X-ray sources in eROSITA X-ray surveys and have gai DR3 counterparts. We address standard CVs with main sequence donors and white dwarfs accreting via Roche-lobe overflow (RLOF) and related objects, the double degenerates (DDs), and the symbiotic stars (SySts). We discern between nonmagnetic (dwarf novae and nova-like objects) and magnetic CVs (polars and intermediate polars (IPs)). In the publically available eROSITA catalog from the recent DR1, typically 65&lt;!PCT!&gt; of known cataloged and classified CVs are detected. This fraction rises to over 90&lt;!PCT!&gt; if the stack of all X-ray surveys (called S45 in this paper) is considered and the search volume is restricted to a radius of 500\,pc. We examine the various classes of CVs in various diagnostic diagrams relating X-ray and optical properties (luminosity, absolute magnitude, color, X-ray spectral hardness, and optical variability) and establish their average class properties. We derive spectral properties for the 22 brightest polars and confirm an increase in the ratio of soft to hard X-rays with increasing magnetic field in the accretion region. We report three new soft IPs and present a spectral analysis of all soft IPs. Their blackbody temperatures agree well with published values. The DDs represent the bluest and faintest subcategory but reach the same identification fraction as the standard CVs. The SySts are the most distant systems; only 20 (13&lt;!PCT!&gt;) were detected as X-ray sources in S45, and 7 of those are first-time detections. We investigate their mean properties using an upper limit on the flux of the nondetected CVs. Their X-ray nondetection is indeed a distance effect. We used all properties combined to select candidate CVs for all-sky optical identification programs, with the ultimate aim being to compose large CV samples in order to better constrain the impact of magnetic fields on the evolution of CVs, to derive space densities and luminosity functions, and to quantify the contribution of white-dwarf accreting systems to the Galactic Ridge X-ray emission (GRXE). The results of the optical identification program will be presented in forthcoming papers.

A study of gamma-ray emission from OJ 287 using Fermi-LAT from 2015-2023

We search for gamma-ray emission from OJ287 in the energy range from 0.1-300 GeV during 2015-2023, in coincidence with an extensive observing campaign to monitor the optical flux variability and polarization, as discussed in arXiv:2311.02372. We present results for eight segments in the aforementioned period, with each segment corresponding to an observing season. We report non-zero gamma-ray flux (with &gt;10σ significance) for all the eight segments. The photon and energy flux observed during this period is and , respectively, while the SED can be fitted with a power law with a slope of 2.12±0.02. The observed luminosity is between 1045−1046 ergs/sec.

Exploring potential of lead-free KSn0.5Ge0.5I3 based perovskite solar cell: A combined first-principle DFT and SCAPS-1D study

Perovskite solar cells present a promising alternative to conventional silicon or lead-based solar cells, offering a synergistic blend of cost-effectiveness and potential for elevated power conversion efficiency. We investigated the potential of an innovative KSn0.5Ge0.5I3-based perovskite solar cell (PSC) using density functional theory (DFT) and SCAPS-1D simulation studies. Computational analysis via DFT probed the crystal structure, elastic, mechanical, and optoelectronic properties of KSn0.5Ge0.5I3. Tolerance factor computation (0.94) and negative formation energy (-3.16 eV) affirmed the thermodynamic stability of the orthorhombic phase in KSn0.5Ge0.5I3. Optical variables including refractive index, absorption coefficient, dielectric function, optical loss, and reflectivity, were computed to explore KSn0.5Ge0.5I3 viability in PV applications. Band structure analysis, density of states (DOS), and projected density of states (PDOS) have been used to determine the band gap (∼1.87 eV), effective masses, and mobility of e-/h+. We have conducted SCAPS-1D simulation for diverse PSCs configurations using various holes transport layers (HTLs) and electron transport layers (ETLs), identified IGZO (Eg = 3.05 eV) as a proficient ETL while CuSbS2 (Eg = 1.58 eV) as an effective HTL. A comprehensive investigation of parameters such as absorber layer thickness, shunt and series resistance, donor and acceptor density, absorber defect density, back contact metal, temperature, and interface characteristics was undertaken, and the impact on PV device performances was assessed through current-voltage (IV) and quantum efficiency (Q.E) characteristics curves. These findings underscore the potential of KSn0.5Ge0.5I3 as a promising material for PSCs, offering a sustainable and eco-friendly avenue for future renewable energy.

Phenology and Plant Functional Type Link Optical Properties of Vegetation Canopies to Patterns of Vertical Vegetation Complexity

Vegetation vertical complexity influences biodiversity and ecosystem productivity. Rapid warming in the boreal region is altering patterns of vertical complexity. LiDAR sensors offer novel structural metrics for quantifying these changes, but their spatiotemporal limitations and their need for ecological context complicate their application and interpretation. Satellite variables can estimate LiDAR metrics, but retrievals of vegetation structure using optical reflectance can lack interpretability and accuracy. We compare vertical complexity from the airborne LiDAR Land Vegetation and Ice Sensor (LVIS) in boreal Canada and Alaska to plant functional type, optical, and phenological variables. We show that spring onset and green season length from satellite phenology algorithms are more strongly correlated with vegetation vertical complexity (R = 0.43–0.63) than optical reflectance (R = 0.03–0.43). Median annual temperature explained patterns of vegetation vertical complexity (R = 0.45), but only when paired with plant functional type data. Random forest models effectively learned patterns of vegetation vertical complexity using plant functional type and phenological variables, but the validation performance depended on the validation methodology (R2 = 0.50–0.80). In correlating satellite phenology, plant functional type, and vegetation vertical complexity, we propose new methods of retrieving vertical complexity with satellite data.

Simultaneous X-ray and optical variability of M dwarfs observed with eROSITA and TESS

M-dwarf stars are the most numerous stars in the Galaxy, and are highly magnetically active. They exhibit bursts of radiation and matter, called flares and coronal mass ejections which have the potential to strongly affect the habitability of their planets. We investigate variability through simultaneous optical and X-ray observations, forming the largest statistical sample of M dwarfs observed in this way so far. Such simultaneous observations at different wavelengths, which correspond to emissions from different layers of the stellar atmosphere, are required to constrain the flare frequency and energetics and to understand the physics of flares. We used light curves from the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission (SRG) and the Transiting Exoplanet Survey Satellite (TESS) for a sample of M dwarfs observed simultaneously with both instruments. Specifically, we identified 256 M dwarfs in the TESS Southern Continuous Viewing Zone (SCVZ), which corresponds to a sky area of 452.39 ($ deg^2 $), with simultaneous TESS and eROSITA detection. For this work, we selected the $25$ X-ray brightest or most X-ray variable stars. We used photometric data from Gaia and 2MASS to obtain stellar parameters such as distances, colours, masses, radii, and bolometric luminosities. X-ray fluxes and luminosities were determined from observed eROSITA count rates using appropriate rate-to-flux conversion factors. We defined and examined various variability diagnostics in both wavebands and how these parameters are related to each other. Our stars are nearby (mostly within $ 100$\,pc ), rotating fast rot &lt; 9$\,d), and display a high optical flare frequency, as expected from the selection of particularly X-ray-active objects. The optical duty cycle -- defined as the fraction of observing time in which the stars were in a high activity state -- is well correlated with the optical flare rate and was therefore used as proxy for optical variability. The X-ray and optical duty cycles are positively correlated, and there is a trend of faster rotators tending to have higher X-ray and optical variability. For stars with many X-ray flaring events, the chances of these events being found together with optical flares are high. A quantitative variability study of individual flares in the X-ray light curves is severely affected by data gaps due to the low (4\,h) cadence during the eROSITA all-sky survey. To mitigate this, we made use of the optical flares observed with TESS combined with knowledge accumulated from solar flares to put additional constraints on the peak flux and timing of X-ray events. With this method we could perform an exponential fit to X-ray light curves in the aftermath of an optical flare, and we find that the energies for these X-ray flares are well correlated with the corresponding optical flare energy. We also found two peculiar flaring events with uncharacteristically long duration and high energies observed in both their X-ray and optical light curves. Despite the substantial uncertainties associated with our analysis, which are mostly related to the poor sampling of the eROSITA light curves, our results showcase in an exemplary way the relevance of simultaneous all-sky surveys in different wavebands for obtaining unprecedented quantitative information on stellar variability.

Nonlinear Characteristics of Radio Variability in Quasar 3C 446

Quasars are characterized by violent and large amplitude variability in all the observation wavelengths, and the analysis of optical variability is useful for developing theoretical models. Long-term optical variability data of quasar 3C 446 were collected from the University of Michigan Radio Astronomy Observatory database in the 4.8, 8.0, and 14.5 GHz radio bands from 1976 to 2012. Due to the complexity of the variability data, it's hard to study by the linear time series analysis methods. For learning more about non-linear characteristics of the temporal evolution of quasar variability, the Ensemble Empirical Mode Decomposition and nonlinear analysis are used to analyse chaotic dynamics, fractal properties, and periodicity. This paper focuses on whether there is a significant difference between the periodic and non-linear properties of the quasar variability before and after the removal of the periodic or chaotic components. It turns out that the variability of quasar 3C 446 in the radio bands consists of periodic, trend, and chaotic components, and the periodic and trend components are dominant. The periods of the variability after removing the chaotic and trend components are exactly the same as the periods of the original variability data, but the chaotic and fractal characteristics of the two are significantly different. The saturated correlation dimension indicates that the reconstruction of the dynamical system requires more independent parameters than the original optical variables after the removal of the periodic and trend components. The Kolmogorov entropy indicates that the loss of information is greater for the former than the latter, and the system is more chaotic and more complex. The Hurst value indicates that the self-similarity and long-range correlation are slightly stronger for the latter than the former.

Holistic optical water type classification for ocean, coastal, and inland waters

AbstractWater constituents exhibit diverse optical properties across ocean, coastal, and inland waters, which alter their remote‐sensing reflectance obtained via satellites. Optical water type (OWT) classifications utilized in satellite data processing aim to mitigate optical complexity by identifying fitting ocean color algorithms tailored to each water type. This facilitates comprehension of biogeochemical cycles ranging from local to global scales. Previous OWT frameworks have focused narrowly on either oceanic or inland waters and have relied too heavily on specific data collections. We propose a novel holistic OWT framework applicable to all natural waters, based on state‐of‐the‐art bio‐geo‐optical modeling and radiative transfer simulations that encompass different phytoplankton groups. This framework employs a “knowledge‐driven” paradigm, combining domain knowledge and insights from previous studies to simulate the reflectance spectrum from water constituent concentrations and inherent optical properties. Our method extracts optical variables to represent the full spectrum of reflectance, consolidating both spectral shape and magnitude. We apply the framework utilizing diverse in situ, synthetic, and satellite data (Sentinel‐3 OLCI) and demonstrate its better classifiability than other frameworks. This framework lays the foundation for comprehensive global monitoring of natural waters.

The size-luminosity relation of the AGN torus determined from the comparison between optical and mid-infrared variability

The size-luminosity relation of the AGN torus determined from the comparison between optical and mid-infrared variability