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BL Lacertae (BL Lac) is categorized as TeV blazar and considered as a possible source of astrophysical neutrinos. In 2020, the brightest X-ray flare ever detected from it. A detailed study can answer many puzzling questions related to multiband emissions and fast-flux variability often seen in this kind of source. We have performed the temporal and spectral analysis of the brightest flare. The variability is characterized by the fractional variability amplitude and the variability time. We found that the source has crossed all its previous limits of flux and reached the maximum ever seen from it in optical and X-rays. It is highly variable in X-rays with fractional variability above 100$\%$ (1.8397$\pm$0.0181) and the fastest variability time of 11.28 hours within a day. The broadband light curves correlation with X-ray suggest a time lag of one day. A broadband SED modeling is pursued to understand the possible physical mechanisms responsible for broadband emission. Modeling requires two emission regions located at two different sites to explain the low and high flux states. A significant spectral change is observed in the optical-UV and X-ray spectrum during the high state, which eventually leads to shifts in the location of the synchrotron peak towards higher energy, suggesting an emergence of a new HBL component., Comment: 13 pages, 11 figures, 3 tables, Accepted in MNRAS

This paper describes a dataset consisting of 47 hyperspectral-reflectance measurements of plastic litter samples. The plastic litter samples include virgin and real samples from the Port of Antwerp. They were measured in dry conditions in the Flemish Institute for Technological Research (VITO) calibration facility, and a selection of the samples were also measured in wet conditions and submerged in a water tank at Flanders Hydraulics. The construction on top of the tank allowed us to submerge the plastics and keep sediments in suspension. The spectral measurements were performed using an Analytical Spectral Devices (ASD) FieldSpec 4 and a Spectral Evolution (SEV) spectrometer. The datasets are available on the 4TU.ResearchData open-access repository (ASD dataset: https://doi.org/10.4121/12896312.v2, Knaeps et al., 2020; SEV dataset: https://doi.org/10.4121/uuid:9ee3be54-9132-415a-aaf2-c7fbf32d2199; Garaba et al., 2020).

Slow hot carrier cooling in halide perovskites holds the key to the development of hot carrier (HC) perovskite solar cells. For accurate modeling and pragmatic design of HC materials and devices, it is essential that HC temperatures are reliably determined. A common approach involves fitting the high-energy tail of the main photobleaching peak in a transient absorption spectrum with a Maxwell-Boltzmann distribution. However, this approach is problematic because of complications from the overlap of several photophysical phenomena and a lack of consensus in the community on the fitting procedures. Herein, we propose a simple approach that circumvents these challenges. Through tracking the broadband spectral evolution and accounting for bandgap renormalization and spectral line width broadening effects, our method extracts not only accurate and consistent carrier temperatures but also other important parameters such as the quasi-Fermi levels, bandgap renormalization constant, etc. Establishing a reliable method for the carrier temperature determination is a step forward in the study of HCs for next-generation perovskite optoelectronics. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

We demonstrate a digital twin for failure detection in optical networks. Artificial neural networks-based models for optical constellation analysis enable predicting the transmitted signal in the time domain whereas analytical models are usually used to estimate their spectral evolution. This project has received funding from the European Union Horizon 2020 MSCA-EID MENTOR (G.A. 956713), the H2020 B5G-OPEN (G.A. 101016663), the MICINN IBON (PID2020- 114135RB-I00), and the ICREA Institution.

[Aims] We estimated the spectral evolution of white dwarfs with effective temperature using the Javalambre Photometric Local Universe Survey (J-PLUS) second data release (DR2), which provides 12 photometric optical passbands over 2176 deg2. [Methods] We analyzed 5926 white dwarfs with r 19:5 mag in common between a white dwarf catalog defined from Gaia EDR3 and J-PLUS DR2. We performed a Bayesian analysis by comparing the observed J-PLUS photometry with theoretical models of hydrogen- and helium-dominated atmospheres. We estimated the probability distribution functions for effiective temperature (Teffi), surface gravity, parallax, and composition; and the probability of having a H-dominated atmosphere (pH) for each source. We applied a prior in parallax, using Gaia EDR3 measurements as a reference, and derived a self-consistent prior for the atmospheric composition as a function of Teffi. [Results] We described the fraction of white dwarfs with a He-dominated atmosphere ( fHe) with a linear function of the effiective temperature at 5000 < Teffi < 30 000 K. We find fHe = 0:24 - 0:01 at Teffi = 10 000 K, a change rate along the cooling sequence of 0:14 - 0:02 per 10 kK, and a minimum He-dominated fraction of 0:08 - 0:02 at the high-temperature end. We tested the obtained pH by comparison with spectroscopic classifications, finding that it is reliable. We estimated the mass distribution for the 351 sources with distance d < 100 pc, mass M > 0:45 M, and Teffi > 6000 K. The result for H-dominated white dwarfs agrees with previous studies, with a dominant M = 0:59 M peak and the presence of an excess at M0:8 M. This high-mass excess is absent in the He-dominated distribution, which presents a single peak. [Conclusions]The J-PLUS optical data provide a reliable statistical classification of white dwarfs into H- and He-dominated atmospheres. We find a 21 - 3% increase in the fraction of He-dominated white dwarfs from Teffi = 20 000K to Teffi = 5000 K., Funding for the J-PLUS Project has been provided by the Governments of Spain and Aragón through the Fondo de Inversiones de Teruel; the Aragonese Government through the Reseach Groups E96, E103, and E16_17R; the Spanish Ministry of Science, Innovation and Universities (MCIU/AEI/FEDER, UE) with grants PGC2018-097585-B-C21 and PGC2018-097585-B-C22; the Spanish Ministry of Economy and Competitiveness (MINECO) under AYA2015-66211-C2-1-P, AYA2015-66211-C2-2, AYA2012-30789, and ICTS-2009-14; and European FEDER funding (FCDD10-4E-867, FCDD13-4E-2685). The Brazilian agencies FINEP, FAPESP, and the National Observatory of Brazil have also contributed to this project. P.-E. T. has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme n. 677706 (white dwarf3D). J. M. C. acknowledges financial support by the Spanish Ministry of Science, Innovation and University (MICIU/FEDER, UE) through grant RTI2018-095076-B-C21, and the Institute of Cosmos Sciences University of Barcelona (ICCUB, Unidad de Excelencia ‘María de Maeztu’) through grant CEX2019-000918-M. J. V. acknowledges the technical members of the UPAD for their invaluable work: Juan Castillo, Javier Hernández, Ángel López, Alberto Moreno, and David Muniesa. B. T. G. was supported by STFC grantST/T000406/1 and by a Leverhulme Research Fellowship. F. J. E. acknowledges financial support from the Spanish MINECO/FEDER through the grant AYA2017-84089 and MDM-2017-0737 at Centro de Astrobiología (CSIC-INTA), Unidad de Excelencia María de Maeztu, and from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement no. 824064 through the ESCAPE - The European Science Cluster of Astronomy & Particle Physics ESFRI Research Infrastructures project. R. A. D. acknowledges support from the CNPq through BP grant 308105/2018-4.

We analyze the influence of cavity finesse on the spectral evolution and stability of self-phase-locking in pulsed degenerate optical parametric oscillators as a function of cavity detuning. We derive the conditions for maximum stability range.

Prominence eruption is closely related to coronal mass ejections and is an important topic in solar physics. Spectroscopic observation is an effective way to explore the plasma properties, but the spectral observations of eruptive prominences are rare. In this paper we will introduce an eruptive polar crown prominence with spectral observations from the Interface Region Imaging Spectrograph (IRIS), and try to explain some phenomena that are rarely reported in previous works. The eruptive prominence experiences a slow-rise and fast-rise phase, while the line-of-sight motions of the prominence plasma could be divided into three periods: two hours before the fast-rise phase, opposite Doppler shifts are found at the two sides of the prominence axis;then, red shifts dominate the prominence gradually; in the fast-rise phase, the prominence gets to be blue-shifted. During the second period, a faint component appears in Mg II k window with a narrow line width and a large red shift. A faint region is also found in AIA 304-angstrom images along the prominence spine, and the faint region gets darker during the expansion of the spine. We propose that the opposite Doppler shifts in the first period are a feature of the polar crown prominence that we studied. The red shifts in the second period are possibly due to mass drainage during the elevation of the prominence spine, which could accelerate the eruption in return. The blue shifts in the third period are due to that the prominence erupts toward the observer. We suggest that the faint component appears due to the decreasing of the plasma density, and the latter results from the expansion of the prominence spine., 20 pages, 9 figures. Comments are welcome to xuejc@pmo.ac.cn

Empirically characterizing the spectral evolution of cool white dwarfs is a prerequisite to understanding the physical processes that shape the evolution of these old objects. However, the high photospheric densities of cool helium-rich atmospheres seriously complicate the study of those stars. We have recently developed an updated atmosphere code that is appropriate for high densities and that can model any cool white dwarf (including DZs and DQpecs). Here, we present recent advances in our understanding of the spectral evolution of cool white dwarfs that were made possible thanks to these improved models. We discuss in particular the evolution of the hydrogen-rich to helium-rich ratio at low effective temperatures as well as the DQ→ DQpec transition.

As they evolve, white dwarfs undergo major changes in their atmospheric composition, a phenomenon known as spectral evolution. In particular, most hot He-rich (DO) stars transform into H-rich (DA) stars as they cool off, most likely as a result of the float-up of residual H. We investigate this DO-to-DA transition by taking advantage of the extensive spectroscopic dataset provided by the Sloan Digital Sky Survey (SDSS). Using our new state-of-the-art non-LTE model atmospheres, we perform a spectroscopic analysis of 1882 hot (Teff >30,000 K) white dwarfs identified in the SDSS. We find that at least 15% of all white dwarfs are born with a He-dominated atmosphere. Among these, ∼2/3 turn into H-rich stars before they reach Teff ∼40,000 K, while the remaining ∼1/3 maintain their He-rich surface throughout their entire evolution. We speculate on the origin of these two groups of objects.

The multiband photon emission and spectral evolution of G54.1+0.3 are investigated in the framework of leptonic and leptonic–hadronic models. We model the spectral energy distribution (SED) of the pulsar wind nebula (PWN) and find that both the leptonic and leptonic–hadronic models can well reproduce the multiband observations of the nebula with appropriate model parameters. Combining with dynamical evolution of the PWN, we investigate the time evolution of photon SED and radiative luminosity in the X-ray and TeV γ-ray bands of G54.1+0.3. The results indicate that the synchrotron spectrum and radiative luminosity in the X-ray band of the PWN calculated with these two models have obvious differences as the age increases to about 4 kyr, and the largest difference is present at about 40 kyr. The γ-ray luminosity calculated by the leptonic–hadronic model shows that the contribution of TeV photons arising from the decay of neutral pions produced in proton–proton interaction also changes with time and is always important for modifying the TeV γ-ray spectrum of G54.1+0.3 during the evolution of the PWN.