HD 5501, a hitherto little-studied eclipsing binary with an early A-type primary, has been caught in a short-lived, astrophysically interesting phase of its binary evolution. Recent photometric and spectroscopic observations, including photometric data from TESS, show it has a highly variable light curve as well as complex spectral variability, particularly in both the absorption and emission components at H alpha. Our current campaign, including both professional and amateur observers, has determined that the primary is evolving rapidly across the Hertzsprung gap and that, unusually in the case of mass transfer, the orbital period is declining with a characteristic time-scale P/P-center dot approximate to 170 000 yr. Significantly, the orbit is eccentric and it appears that mass transfer from the primary to the secondary occurs only near periastron. Modelling indicates the presumed B7 V secondary to be surrounded by an accretion torus, which likely has dynamically chaotic variations in size and shape. Our analysis further implies the presence of a circumbinary disc or shell supplied by mass loss through the Lagrange L-3 point. That mass loss appears to account for most of the emission at H alpha. We describe how this astrophysically interesting system may yield valuable information about binary star evolution at the onset of Roche lobe overflow, as well as insights into eccentricity-modifying mechanisms such as the Soker mechanism.

We present a detailed study of the MASTER OT J030227.28+191754.5 outburst in 2021-2022, which reached an amplitude of 10.2mag and a duration of 60d. The detections of (1) the double-peaked optical emission lines, and (2) the early and ordinary superhumps, established that MASTER OT J030227.28+191754.5 is an extremely energetic WZ Sge-type dwarf nova (DN). Based on the superhump observations, we obtained its orbital period and mass ratio as 0.05986(1)d and 0.063(1), respectively. These values are within a typical range for low-mass-ratio DNe. According to the binary parameters derived based on the thermal-tidal instability model, our analyses showed that (1) the standard disk model requires an accretion rate similar or equal to 1020gs(-1) to explain its peak optical luminosity, and (2) large mass was stored in the disk at the outburst onset. These factors cannot be explained solely by the impact of its massive (greater than or similar to 1.15M circle dot) primary white dwarf implied by Kimura et al. (2023, ApJ, 951, 124). Instead, we propose that the probable origin of this enormously energetic DN outburst is the even lower quiescence viscosity than other WZ Sge-type DNe. This discussion is qualitatively valid for most possible binary parameter spaces unless the inclination is low enough (less than or similar to 40 degrees) for the disk to be bright, explaining the outburst amplitude. Such low inclinations, however, would not allow detectable amplitude of early superhumps in the current thermal-tidal instability model. The optical spectra at outburst maximum showed strong emission lines of the Balmer, He i, and He ii series, the core of which is narrower than similar to 800kms(-1). Considering its binary parameters, a Keplerian disk cannot explain this narrow component, but the presumable origin is disk winds.

We present an analysis of the BVRI photometry of the blazar BL Lacertae on diverse timescales from 2020 mid-July to mid-September. We have used 11 different optical telescopes around the world and have collected data over 84 observational nights. The observations cover the onset of a new activity phase of BL Lacertae that started in 2020 August (termed as the 2020 August flare by us), and the analysis is focused on the intranight variability. On short-term timescales, (i) flux varied with ∼2.2 mag in the R band, (ii) the spectral index was found to be weakly dependent on the flux (i.e., the variations could be considered mildly chromatic), and (iii) no periodicity was detected. On intranight timescales, BL Lacertae was found to predominantly show bluer-when-brighter chromatism. We also found two cases of significant interband time lags of the order of a few minutes. The duty cycle of the blazar during the 2020 August flare was estimated to be quite high (∼90% or higher). We decomposed the intranight light curves into individual flares and determined their characteristics. On the basis of our analysis and assuming the turbulent jet model, we determined some characteristics of the emitting regions: Doppler factor, magnetic field strength, electron Lorentz factor, and radius. The radii determined were discussed in the framework of the Kolmogorov theory of turbulence. We also estimated the weighted mean structure function slope on intranight timescales, related it to the slope of the power spectral density, and discussed it with regard to the origin of intranight variability.

Context. The close approach of the near-Earth asteroid (99942) Apophis to Earth in 2029 will provide a unique opportunity to examine how the physical properties of the asteroid could be changed due to the Earth’s gravitational perturbation. As a result, the Republic of Korea is planning a rendezvous mission to Apophis. Aims. Our aim was to use photometric data from the apparitions in 2020−2021 to refine the shape model and spin state of Apophis. Methods. Using thirty-six 1- to 2-meter-class ground-based telescopes and the Transiting Exoplanet Survey Satellite, we carried out a photometric observation campaign throughout the 2020−2021 apparition. The convex shape model and spin state were refined using the light-curve inversion method. Results. According to our best-fit model, Apophis is rotating in a short-axis mode with rotation and precession periods of 264.178 h and 27.38547 h, respectively. The angular momentum vector orientation of Apophis was found to be (275°, −85°) in the ecliptic coordinate system. The ratio of the dynamic moments of inertia of this asteroid was fitted to Ia : Ib : Ic = 0.64 : 0.97 : 1, which corresponds to an elongated prolate ellipsoid. These findings regarding the spin state and shape model can be used to both design the space mission scenario and investigate the impact of the Earth’s tidal force during close encounters.

ABSTRACTOJ 287 is a BL Lacertae type quasar in which the active galactic nucleus (AGN) outshines the host galaxy by an order of magnitude. The only exception to this may be at minimum light when the AGN activity is so low that the host galaxy may make quite a considerable contribution to the photometric intensity of the source. Such a dip or a fade in the intensity of OJ 287 occurred in 2017 November, when its brightness was about 1.75 mag lower than the recent mean level. We compare the observations of this fade with similar fades in OJ 287 observed earlier in 1989, 1999, and 2010. It appears that there is a relatively strong reddening of the B− V colours of OJ 287 when its V-band brightness drops below magnitude 17. Similar changes are also seen in V− R, V− I, and R− I colours during these deep fades. These data support the conclusion that the total magnitude of the host galaxy is V = 18.0 ± 0.3, corresponding to MK = −26.5 ± 0.3 in the K-band. This is in agreement with the results, obtained using the integrated surface brightness method, from recent surface photometry of the host. These results should encourage us to use the colour separation method also in other host galaxies with strongly variable AGN. In the case of OJ 287, both the host galaxy and its central black hole are among the biggest known, and its position in the black hole mass–galaxy mass diagram lies close to the mean correlation.

We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV γ -ray light curves covering timescales of decades/years to days/minutes for the blazars 3C 279 and PKS 1510−089. The PSDs are modeled as single power laws, and the best-fit spectral shape is derived using the “power spectral response” method. With more than 10 yr of data obtained with weekly/daily sampling intervals, most of the PSDs cover ∼2–4 decades in temporal frequency; moreover, in the optical band, the PSDs cover ∼6 decades for 3C 279 due to the availability of intranight light curves. Our main results are the following: (1) on timescales ranging from decades to days, the synchrotron and the inverse-Compton spectral components, in general, exhibit red-noise (slope ∼2) and flicker-noise (slope ∼1) type variability, respectively; (2) the slopes of γ -ray variability PSDs obtained using a 3 hr integration bin and 3 weeks total duration exhibit a range between ∼1.4 and ∼2.0 (mean slope = 1.60 ± 0.70), consistent within errors with the slope on longer timescales; (3) comparisons of fractional variability indicate more power on timescales ≤100 days at γ -ray frequencies compared to longer wavelengths, in general (except between the γ -ray and optical wavelengths for PKS 1510−089); (4) the normalization of intranight optical PSDs for 3C 279 appears to be a simple extrapolation from longer timescales, indicating a continuous (single) process driving the variability at optical wavelengths; and (5) the emission at optical/infrared wavelengths may involve a combination of disk and jet processes for PKS 1510−089.

We characterize the optical variability properties of eight lobe-dominated radio quasars (QSOs): B2 0709+37, FBQS J095206.3+235245, PG 1004+130, [HB89] 1156+631, [HB89] 1425+267, [HB89] 1503+691, [HB89] 1721+343, and 4C +74.26, systematically monitored for a duration of 13 yr since 2009. The quasars are radio-loud objects with extended radio lobes that indicate their orientation close to the sky plane. Five of the eight QSOs are classified as giant radio quasars. All quasars showed variability during our monitoring, with magnitude variations between 0.3 and 1 mag for the least variable and the most variable QSOs, respectively. We performed both structure function (SF) analysis and power spectral density (PSD) analysis for the variability characterization and search for characteristic timescales and periodicities. As a result of our analysis, we obtained relatively steep SF slopes ( α ranging from 0.49 to 0.75) that are consistent with the derived PSD slopes (∼2–3). All the PSDs show a good fit to single power-law forms, indicating a red-noise character of variability between timescales of ∼13 yr and weeks. We did not measure reliable characteristic timescales of variability from the SF analysis, which indicates that the duration of the gathered data is too short to reveal them. The absence of bends in the PSDs (change of slope from ≥1 to ∼0) on longer timescales indicates that optical variations are most likely caused by thermal instabilities in the accretion disk.

Context. A stellar occultation by Neptune’s main satellite, Triton, was observed on 5 October 2017 from Europe, North Africa, and the USA. We derived 90 light curves from this event, 42 of which yielded a central flash detection. Aims. We aimed at constraining Triton’s atmospheric structure and the seasonal variations of its atmospheric pressure since the Voyager 2 epoch (1989). We also derived the shape of the lower atmosphere from central flash analysis. Methods. We used Abel inversions and direct ray-tracing code to provide the density, pressure, and temperature profiles in the altitude range ~8 km to ~190 km, corresponding to pressure levels from 9 µbar down to a few nanobars. Results. (i) A pressure of 1.18 ± 0.03 µbar is found at a reference radius of 1400 km (47 km altitude). (ii) A new analysis of the Voyager 2 radio science occultation shows that this is consistent with an extrapolation of pressure down to the surface pressure obtained in 1989. (iii) A survey of occultations obtained between 1989 and 2017 suggests that an enhancement in surface pressure as reported during the 1990s might be real, but debatable, due to very few high S/N light curves and data accessible for reanalysis. The volatile transport model analysed supports a moderate increase in surface pressure, with a maximum value around 2005-2015 no higher than 23 µbar. The pressures observed in 1995-1997 and 2017 appear mutually inconsistent with the volatile transport model presented here. (iv) The central flash structure does not show evidence of an atmospheric distortion. We find an upper limit of 0.0011 for the apparent oblateness of the atmosphere near the 8 km altitude.

In this paper, we describe the Skynet Robotic Telescope Network, a network of about 2 dozen small optical telescopes spanning four continents. For a prospective user of the network, we provide details on its operation and mention certain caveats potentially affecting observations taken under the control of Skynet software. The actual performance of the network is demonstrated by long-term observations of two blazars: OJ 49 and 3C 279. We show their photometric light curves acquired with a subset of Skynet telescopes in the 2018/19 observing seasons. Preliminary results of the analysis of the variability of these two targets are given, including the quantitative properties of light variations and the periods of possible Quasi Periodic Oscillations (QPOs) found in the seasonal light curves.

V642 Vir is a polar spotted, well-detached, UV Leo-type, low-mass, pre-WUMa (T1 ∼ 4250K, ∼K6V) eclipsing binary. It was observed in 2020 April, May, and June at the Dark Sky Observatory in North Carolina, USA with the 0.81 m reflector of Appalachian State University. A total of 88 timings were used in our 22-year period study which included 12 Transiting Exoplanet Survey Satellite (TESS) timings. The O − C plots show a low-amplitude oscillation of residuals that points to the existence of an orbiting third body, a dwarf of minimum mass, 0.15 M ⊙ in an eccentric orbit (e = 0.41), with an orbital period of 20.07 yr. The odd light curves of V642 Virginis indicate that it has polar spots similar to UV Leo and the recently published V1023 Per. Its present large polar spot region indicates that it must have a strong magnetic field and that it is synchronously rotating. The BVRcIc simultaneous Wilson–Devinney Program solution gives a detached binary (primary and secondary components are underfilling their respective Roche Lobes, with 76% and 78% fill outs respectively). The cool spot region models near the pole of the primary component (centered at 10° colatitude) and is angled toward the secondary component. Its large radius (68°) and T-fact (Tspot/Tsurface = 0.69) also attest to the conclusion of the strength of the magnetic field. The small ΔT in the components (∼318 K) and mass ratio near unity (0.9542 ± 0.0005) show that the stars are similar in spectral type (secondary ∼K9V). The inclination is high, ∼86.87 ± 0.04°, yet there is no time of constant light due to the two stars’ essentially equal radii.

Gaia16aye was a binary microlensing event discovered in the direction towards the northern Galactic disc and was one of the first microlensing events detected and alerted to by the Gaia space mission. Its light curve exhibited five distinct brightening episodes, reaching up to I=12 mag, and it was covered in great detail with almost 25,000 data points gathered by a network of telescopes. We present the photometric and spectroscopic follow-up covering 500 days of the event evolution. We employed a full Keplerian binary orbit microlensing model combined with the motion of Earth and Gaia around the Sun to reproduce the complex light curve. The photometric data allowed us to solve the microlensing event entirely and to derive the complete and unique set of orbital parameters of the binary lensing system. We also report on the detection of the first-ever microlensing space-parallax between the Earth and Gaia located at L2. The properties of the binary system were derived from microlensing parameters, and we found that the system is composed of two main-sequence stars with masses 0.57±0.05 M_⊙ and 0.36±0.03 M_⊙ at 780 pc, with an orbital period of 2.88 years and an eccentricity of 0.30. We also predict the astrometric microlensing signal for this binary lens as it will be seen by Gaia as well as the radial velocity curve for the binary system. Events such as Gaia16aye indicate the potential for the microlensing method of probing the mass function of dark objects, including black holes, in directions other than that of the Galactic bulge. This case also emphasises the importance of long-term time-domain coordinated observations that can be made with a network of heterogeneous telescopes.

Binary black hole (BH) central engine description for the unique blazar OJ 287 predicted that the next secondary BH impact-induced bremsstrahlung flare should peak on 2019 July 31. This prediction was based on detailed general relativistic modeling of the secondary BH trajectory around the primary BH and its accretion disk. The expected flare was termed the Eddington flare to commemorate the centennial celebrations of now-famous solar eclipse observations to test general relativity by Sir Arthur Eddington. We analyze the multi-epoch Spitzer observations of the expected flare between 2019 July 31 and 2019 September 6, as well as baseline observations during 2019 February-March. Observed Spitzer flux density variations during the predicted outburst time display a strong similarity with the observed optical pericenter flare from OJ 287 during 2007 September. The predicted flare appears comparable to the 2007 flare after subtracting the expected higher base-level Spitzer flux densities at 3.55 and 4.49 $\mu$m compared to the optical R-band. Comparing the 2019 and 2007 outburst lightcurves and the previously calculated predictions, we find that the Eddington flare arrived within 4 hours of the predicted time. Our Spitzer observations are well consistent with the presence of a nano-Hertz gravitational wave emitting spinning massive binary BH that inspirals along a general relativistic eccentric orbit in OJ 287. These multi-epoch Spitzer observations provide a parametric constraint on the celebrated BH no-hair theorem.

We report on the results of a multiwavelength monitoring campaign of the bright, nearby Seyfert galaxy, Ark 120 using a ~50-day observing programme with Swift and a ~4-month co-ordinated ground-based observing campaign, predominantly using the Skynet Robotic Telescope Network. We find Ark 120 to be variable at all optical, UV, and X-ray wavelengths, with the variability observed to be well-correlated between wavelength bands on short timescales. We perform cross-correlation analysis across all available wavelength bands, detecting time delays between emission in the X-ray band and the Swift V, B and UVW1 bands. In each case, we find that the longer-wavelength emission is delayed with respect to the shorter-wavelength emission. Within our measurement uncertainties, the time delays are consistent with the \tau ~ \lambda^{4/3} relation, as predicted by a disc reprocessing scenario. The measured lag centroids are \tau_{cent} = 11.90 +/- 7.33, 10.80 +/- 4.08, and 10.60 +/- 2.87 days between the X-ray and V, B, and UVW1 bands, respectively. These time delays are longer than those expected from standard accretion theory and, as such, Ark 120 may be another example of an active galaxy whose accretion disc appears to exist on a larger scale than predicted by the standard thin-disc model. Additionally, we detect further inter-band time delays: most notably between the ground-based I and B bands (\tau_{cent} = 3.46 +/- 0.86 days), and between both the Swift XRT and UVW1 bands and the I band (\tau_{cent} = 12.34 +/- 4.83 and 2.69 +/- 2.05 days, respectively), highlighting the importance of co-ordinated ground-based optical observations.

V1023 Per is a polar spotted, but well-detached dwarf binary, very likely a Pre-WUMa eclipsing binary. It was observed on 11 nights in 2015 October, November, and December at Dark Sky Observatory in North Carolina with the 0.81 m reflector of Appalachian State University. The period behavior is complex and may be increasing with a cubic or quadratic ephemeris. Its odd light curves are of high amplitude but have no totality and reach maximum light just before and following the secondary eclipse, indicating that it has polar spots similar to UV Leo. The 28 yr orbital period study calls the attention of observers to further monitor this binary to understand the complex nature of the period evolution. Its presently fixed polar spot does indicate that it must have a strong magnetic field and that it is synchronously rotating. TheBVR(c)I(c) simultaneous 2016 Wilson-Devinney program (W-D) solution gives fillouts of 68% and 75% for the primary and secondary components, respectively. The polar position of the spot, its radius (24 degrees), and T-fact (0.75) indicate that a strong magnetic field is present. The small Delta Tin the components (similar to 289 K) show that the stars are similar in spectral type despite them being well detached. The inclination is high, similar to 8519 004, but there is not a total eclipse due to the high mass ratio (similar to 0.739 0.001). Due to the low galactic position (longitude = 150390, latitude = -1023), its reddening is addressed. The primary components temperature is estimated to be 5250 250 K.