ABSTRACT
We report on quasi-periodic variability found in two blazars included in the Steward Observatory Blazar Monitoring data sample: the BL Lac object 3C 66A and the Flat Spectrum Radio Quasar B2 ...1633+38. We collect optical photometric and polarimetric data in V and R bands of these sources from different observatories: St. Petersburg University, Crimean Astrophysical Observatory, WEBT–GASP, Catalina Real-Time Transient Survey, Steward Observatory, STELLA Robotic Observatory, and Katzman Automatic Imaging Telescope. In addition, an analysis of the γ-ray light curves from Fermi–LAT is included. Three methods are used to search for any periodic behaviour in the data: the Z-transform Discrete Correlation Function, the Lomb–Scargle periodogram and the Weighted Wavelet Z-transform. We find pieces of evidence of possible quasi-periodic variability in the optical photometric data of both sources with periods of ∼3 yr for 3C 66A and ∼1.9 yr for B2 1633+38, with significances between 3σ and 5σ. Only B2 1633+38 shows evidence of this behaviour in the optical polarized data set at a confidence level of 2σ–4σ. This is the first reported evidence of quasi-periodic behaviour in the optical light curve of B2 1633+38. Also, a hint of quasi-periodic behaviour is found in the γ-ray light curve of B2 1633+38 with a confidence level ≥2σ, while no periodicity is observed for 3C 66A in this energy range. We propose different jet emission models that could explain the quasi-periodic variability and the differences found between these two sources.
We report the first results of a long term infrared monitoring campaign of known and candidate galactic Luminous Blue Variables (LBVs). In particular, we are able to confirm the LBV nature of ...G24.73+0.69, a luminous mid-B supergiant associated with a dusty ejection nebula. We find that prior to 2003 September G24.73+0.69 exhibited low amplitude ($\Delta JHK \sim 0.4$ mag) variability, but in the ~200 day period between 2003 September–2004 April it abruptly brightened by ~0.7 mag in the broadband J filter. Subsequently, a further ~0.4 mag increase was observed between 2004 April–October, resulting in an overall difference of ~1.1 mag between (current) photometric mimimum and maximum; similar variability also being observed in the H and K bands. In light of the numerous recent IR studies of the galactic hot star population we also compile an updated census of confirmed and candidate galactic LBVs, reporting 12 and 23 members respectively for each class. Finally, we utilise this new census to construct an H-R diagram for the galactic LBV population, resulting in a striking confirmation of the LBV-minimum light strip.
We present low-resolution spectroscopy in the near-IR (0.8-2.5 mm) of the EXor variables. These are the initial results (obtained during the period 2007-2008) from a long-term photometric and ...spectroscopic program aimed to study the variability in the accretion processes of pre-main-sequence stars, by correlating the continuum fluctuations with the spectroscopic properties. Eight sources have been observed in different epochs, for a total of 25 acquired spectra. EXor spectra show a wide variety of emission features dominated by H I recombination (Paschen and Brackett series). We have investigated whether line and continuum variability could be due to a variable extinction, but such a hypothesis is applicable only to the peculiar source PV Cep. By comparing the observed spectra with a wind model, mass loss rates in the range (2-10) X 10-8 M yr-1 are derived, along with other wind parameters. Consistent results are also obtained by assuming that H I lines are due to accretion. A CO overtone is also detected in the majority of the sources both in absorption and in emission. It appears to come from regions more compact than winds, likely the stellar photosphere (when in absorption) and the circumstellar disk (when in emission). Na I and Ca I IR lines behave as the CO does, thus they are thought to arise in the same locations. For some targets multiple spectra correspond to different activity stages of the source. Those exhibiting the largest continuum variation at 2 mm (K 1 mag) present a significant line flux fading during the continuum declining phases. In particular, CO absorption (emission) appears associated with inactive (active) stages, respectively.
We present the results of the one-year long observational campaign of the type II plateau SN 2005cs, which exploded in the nearby spiral galaxy M51 (the Whirlpool galaxy). This extensive data set ...makes SN 2005cs the best observed low-luminosity, 56Ni-poor type II plateau event so far and one of the best core-collapse supernovae ever. The optical and near-infrared spectra show narrow P-Cygni lines characteristic of this SN family, which are indicative of a very low expansion velocity (about 1000 km s−1) of the ejected material. The optical light curves cover both the plateau phase and the late-time radioactive tail, until about 380 d after core-collapse. Numerous unfiltered observations obtained by amateur astronomers give us the rare opportunity to monitor the fast rise to maximum light, lasting about 2 d. In addition to optical observations, we also present near-infrared light curves that (together with already published ultraviolet observations) allow us to construct for the first time a reliable bolometric light curve for an object of this class. Finally, comparing the observed data with those derived from a semi-analytic model, we infer for SN 2005cs a 56Ni mass of about 3 × 10−3 M⊙, a total ejected mass of 8–13 M⊙ and an explosion energy of about 3 × 1050 erg.
Context. Since 1997, BL Lacertae has undergone a phase of high optical activity, with the occurrence of several prominent outbursts. Starting in 1999, the Whole Earth Blazar Telescope (WEBT) ...consortium has organised various multifrequency campaigns on this blazar, collecting tens of thousands of data points. One of the main issues in the analysis of this huge dataset has been the study of colour variability. Aims. The immense amount of optical and near-infrared data collected during the campaigns enables us to perform a thorough analysis of multiband data, with the aim to understand the flux variability mechanisms. Methods. We use a new approach for the analysis of these data, focusing on the source spectral evolution. Results. We show that the overall behaviour of the BL Lacertae light and colour curves can be explained in terms of a changing viewing angle of a moving, discrete emitting region, which causes variable Doppler boosting of the corresponding radiation. Conclusions. A fractal helical structure is suggested to be at the origin of the different time scales of variability.
Developed and investigated are IR (850 nm) light-emitting diodes based on AlGaAs/Ga(In)As heterostractures grown by the MOCVD technique with multiple quantum wells in the active region and with a ...double optical reflector consisted of a multilayer Al
0.9
Ga
0.1
As/Al
0.1
Ga
0.9
As Bragg heterostructure and an Ag mirror layer. Light-emitting diodes with the external quantum efficiency EQE = 37.5% at current densities greater than >10 A/cm
2
have been fabricated.
Topology synthesis in integrated design of wireless communication networks is considered. An iterative method has been developed for placing base stations of a broadband network along extended ...transportation routes. The problem of base station placement is formulated as an extremal problem on a finite set in the form of a special combinatorial model. An algorithm of the branch and bound method (BBM) has been developed to solve this problem. This algorithm serves as the basis for finding a sequence of best variants for the placement of base stations. Numerical calculations confirming the efficiency of the proposed algorithm are described.
We present the results of a comparison between classical and newly identified EXor based on literature data and aimed at recognizing possible differences or similarities between the categories. ...Optical and near-IR two-color diagrams, modalities of fluctuations, and derived values of the mass accretion rates are indicative of strong similarities between the two samples. We demonstrate how the difference between the outburst and the quiescence spectral energy distribution of all EXor can be well fitted with a single blackbody, as if an additional thermal component appears during the outbursting phase. Temperatures of this additional component span between 1000 and 4500 K, while the radii of the emitting regions (assumed to be a uniform disk) span between 0.01 and 0.1 AU, sizes typical of the inner portions of the circumstellar disk. Spots persisting up to 50% of the outburst duration, not exceeding 10% of the stellar surface, and with temperatures compatible with the EXor mass accretion rates, are able to account for both the appearance of the additional thermal component and the dust sublimation in the inner structures of the disk. We also compare the EXor events with the most significant color and magnitude fluctuations of active T Tauri stars finding that (1) burst accretion phenomena should also be important for this latter class and (2) EXor events could be more frequent than those accidentally discovered. A remarkable case is that of the source V2493 Cyg, a T Tauri star recently identified as a strong outbursting object: New optical and near-IR photometric and spectroscopic data are presented in an attempt to clarify its EXor or FUor nature.
Abstract
We present the results of optical and near-IR spectroscopy and multicolor photometry of the emission-line star MWC 645, which exhibits the Be phenomenon. The presence of positionally ...variable absorption lines of a cool star detected for the first time indicates that the object is a binary system. Using a combination of the photometric and spectroscopic data as well as the Gaia EDR3 distance (
D
= 6.5 ± 0.9 kpc), we disentangled the components’ contributions and estimated their surface temperatures and luminosities (18, 000 ± 2000 K and 4250 ± 250 K, log
L
/
L
⊙
= 4.0 ± 0.5 and 3.1 ± 0.3 for the hot and cool components, respectively). Quasi-cyclic short-period (months) and long-period (∼4 yr) photometric variations were detected in both optical and near-IR regions, and are most likely due to variable circumstellar extinction. Our analysis suggests that MWC 645 is a member of the FS CMa group. The object’s properties can be described by an evolutionary model of a close binary system that currently undergoes a nonconservative mass transfer between intermediate-mass stars (e.g., 7
M
⊙
+ 2.8
M
⊙
).
Abstract
We study the optical flux and polarization variability of the binary black hole blazar OJ 287 using quasi-simultaneous observations from 2015 to 2023 carried out using telescopes in the USA, ...Japan, Russia, Crimea, and Bulgaria. This is one of the most extensive quasi-simultaneous optical flux and polarization variability studies of OJ 287. OJ 287 showed large amplitude, ∼3.0 mag flux variability, large changes of ∼37% in degree of polarization, and a large swing of ∼215° in the angle of the electric vector of polarization. During the period of observation, several flares in flux were detected. Those flares are correlated with a rapid increase in the degree of polarization and swings in electric vector of polarization angle. A peculiar behavior of anticorrelation between flux and polarization degree, accompanied by a nearly constant polarization angle, was detected from JD 2,458,156 to JD 2,458,292. We briefly discuss some explanations for the flux and polarization variations observed in OJ 287.
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