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One of the remarkable features of blazars is violent variability over a wide wavelength range. The variation mechanism causing the observed complex behavior is still under debate. The variability timescales range from less than a day to decades. Variation on timescales less than a day is known as “microvariability.” Such short-term variations can provide insights regarding the origin of the variability after they are distinguished from longer-term variational components. We select about 195 microvariability events from the continuous light curve of blazar W2R 1926+42 with 1-min time resolution obtained by the Kepler spacecraft, and estimate the timescale and amplitude of each event. The rise and decay timescales of the events reveal random variations over short timescales less than a day, but they indicate systematic variations on timescales longer than several days. This result implies that the events are not independent, but rather mutually correlated.

Very-long-baseline interferometry (VLBI) observations of active galactic nuclei at millimetre wavelengths have the power to reveal the launching and initial collimation region of extragalactic radio jets, down to 10–100 gravitational radii ( r g  ≡  G M / c 2 ) scales in nearby sources 1 . Centaurus A is the closest radio-loud source to Earth 2 . It bridges the gap in mass and accretion rate between the supermassive black holes (SMBHs) in Messier 87 and our Galactic Centre. A large southern declination of −43° has, however, prevented VLBI imaging of Centaurus A below a wavelength of 1 cm thus far. Here we show the millimetre VLBI image of the source, which we obtained with the Event Horizon Telescope at 228 GHz. Compared with previous observations 3 , we image the jet of Centaurus A at a tenfold higher frequency and sixteen times sharper resolution and thereby probe sub-lightday structures. We reveal a highly collimated, asymmetrically edge-brightened jet as well as the fainter counterjet. We find that the source structure of Centaurus A resembles the jet in Messier 87 on ~500  r g scales remarkably well. Furthermore, we identify the location of Centaurus A’s SMBH with respect to its resolved jet core at a wavelength of 1.3 mm and conclude that the source’s event horizon shadow 4 should be visible at terahertz frequencies. This location further supports the universal scale invariance of black holes over a wide range of masses 5 , 6 . The millimetre image of the Centaurus A nucleus by the Event Horizon Telescope reveals a highly collimated, asymmetrically edge-brightened jet. The source’s event horizon shadow should be visible at terahertz frequencies, consistent with the universal scale invariance of black holes.

We report on multi-band photopolarimetric observations of the blazars AO 0235+164 and PKS 1510–089. These two blazars became in active states in 2008 and 2009, respectively. In these active states, prominent short-term flares were detected in both objects, having amplitudes of > 1 mag within 10 days. The V – J color became bluer when the objects were brighter in these flares. On the other hand, the color of PKS 1510–089 exhibited a trend that it became redder when it was brighter, except for its prominent flare. This feature can be explained by the strong contribution of thermal emission from an accretion disk. The degree of polarization increased at the flares, and reached > 25 % at the maxima in AO 0235+164 and PKS 1510–089. We compare these flares with other short-term flares which were detected by our monitoring of 44 blazars. Those two flares had one of the largest variation amplitudes in both flux and degree of polarization. Furthermore, we found a significant positive correlation between the amplitudes of the flux and degree of polarization in the flares. It indicates that the short-term flares originate from the region where the magnetic field is aligned.

In this work, we present optical R band observations of AO 0235+164 carried out during the period of November 2006 to December 2012 using the Ap6E CCD camera attached to the primary focus of the 70 cm meniscus telescope at Abastumani Observatory, Georgia. It shows a large variation of Δ R = 4.88 mag (14.19–19.07 mag) and a short time scale of Δ T v = 73.5 min during our monitoring period. When periodicity analysis methods are applied to the R-band data from both historic and our observations, periods P 1 = 8.26 yr and P 2 = 0.54 yr are found.

The magnetic-field conditions in astrophysical relativistic jets can be probed by multiwavelength polarimetry, which has been recently extended to X-rays. For example, one can track how the magnetic field changes in the flow of the radiating particles by observing rotations of the electric vector position angle Ψ. Here we report the discovery of a ΨX rotation in the X-ray band in the blazar Markarian 421 at an average flux state. Across the 5 days of Imaging X-ray Polarimetry Explorer observations on 4–6 and 7–9 June 2022, ΨX rotated in total by ≥360°. Over the two respective date ranges, we find constant, within uncertainties, rotation rates (80 ± 9° per day and 91 ± 8° per day) and polarization degrees (ΠX = 10% ± 1%). Simulations of a random walk of the polarization vector indicate that it is unlikely that such rotation(s) are produced by a stochastic process. The X-ray-emitting site does not completely overlap the radio, infrared and optical emission sites, as no similar rotation of Ψ was observed in quasi-simultaneous data at longer wavelengths. We propose that the observed rotation was caused by a helical magnetic structure in the jet, illuminated in the X-rays by a localized shock propagating along this helix. The optically emitting region probably lies in a sheath surrounding an inner spine where the X-ray radiation is released.In June 2022, the IXPE satellite observed a shock passing through the jet of active galaxy Markarian 421. The rotation of the X-ray-polarized radiation over a 5-day period revealed that the jet contains a helical magnetic field.

Gamma-ray bursts (GRBs) are the most electromagnetically luminous cosmic explosions. They are powered by collimated streams of plasma (jets) ejected by a newborn stellar-mass black hole or neutron star at relativistic velocities. Their short-lived (typically tens of seconds) prompt γ-ray emission from within the ejecta is followed by long-lived multi-wavelength afterglow emission from the ultra-relativistic forward shock. This shock is driven into the circumburst medium by the GRB ejecta. which are in turn decelerated by a mildly relativistic reverse shock. Forward-shock emission was recently detected as teraelectronvolt-energy γ-rays. Such very-high-energy emission was also predicted from the reverse shock. Here we report the detection of optical and gigaelectronvolt-energy γ-ray emission from GRB 180720B during the first few hundred seconds, which is explained by synchrotron and inverse-Compton emission from the reverse shock propagating into the ejecta, implying a low-magnetization ejecta. Our optical measurements show a clear transition from the reverse shock to the forward shock driven into the circumburst medium, accompanied by a 90° change in the mean polarization angle and fluctuations in the polarization degree and angle. This indicates turbulence with large-scale toroidal and radially stretched magnetic-field structures in the reverse and forward shocks, respectively, which tightly couple to the physics of relativistic shocks and GRB jets, namely launching, composition, dissipation and particle acceleration. Synchrotron and inverse-Compton emission provide evidence for a reverse-shock origin of the high-energy emission from a gamma-ray burst, GRB 180720B. The polarization of the optical emission originating from the reverse shock suggests a turbulent shock that is amplified by the magnetic field in the relativistic ejecta.

We performed photo-polarimetry observations for the blazar BL Lacertae (BL Lac) in 2020 to 2021, in which BL Lac showed historical outburst. As a result, we obtained microvariability with a timescale about five minutes and wavelength dependence of polarization degree and angle. These results indicate multiple emission regions and turbulent magnetic field structure.

The detections of gravitational waves (GW) by the LIGO/Virgo collaborations provide various possibilities for both physics and astronomy. We are quite sure that GW observations will develop a lot, both in precision and in number, thanks to the continuous work on the improvement of detectors, including the expected new detector, KAGRA, and the planned detector, LIGO-India. On this occasion, we review the fundamental outcomes and prospects of gravitational wave physics and astronomy. We survey the development, focusing on representative sources of gravitational waves: binary black holes, binary neutron stars, and supernovae. We also summarize the role of gravitational wave observations as a probe of new physics.

Abstract The Laser Interferometer Gravitational-wave Observatory Scientific Collaboration and Virgo Collaboration (LVC) sent out 56 gravitational-wave (GW) notices during the third observing run (O3). The Japanese Collaboration for Gravitational wave ElectroMagnetic follow-up (J-GEM) performed optical and near-infrared observations to identify and observe an electromagnetic (EM) counterpart. We constructed a web-based system that enabled us to obtain and share information on candidate host galaxies for the counterpart, and the status of our observations. Candidate host galaxies were selected from the GLADE catalog with a weight based on the 3D GW localization map provided by LVC. We conducted galaxy-targeted and wide-field blind surveys, real-time data analysis, and visual inspection of observed galaxies. We performed galaxy-targeted follow-ups to 23 GW events during O3, and the maximum probability covered by our observations reached 9.8$\\%$. Among these, we successfully started observations for 10 GW events within 0.5 days after the detection. This result demonstrates that our follow-up observation has the potential to constrain EM radiation models for a merger of binary neutron stars at a distance of up to $\\sim$100 Mpc with a probability area of $\\leq$ 500 deg$^2$.

Long series of multi-frequency monitoring data of the active galactic nucleus (AGN) AO 0235+164 from the radio to the gamma-ray are analyzed. AO 0235+164 may be a close binary system comprised of two supermassive black holes (SMBHs). A harmonic analysis has established the presence of orbital and precessional periods of two and eight years, respectively. These values are close to the analogous periods found in other AGNs that have been proposed to be close SMBH binary systems in a stage of their evolution close to coalescence. A cross-correlation analysis is used to find the time delay between corresponding flares occurring in different wavebands. The empirical dependence found earlier for a number of other AGNS relating the delay in the flux variations with the frequency is observed for this object, and has the form of an inverse logarithmic dependence, providing evidence that the flux variability of AGNs at centimeter and shorter wavelengths is intrinsic. The standard picture of jet activity of AGNs supposes plasma formations that move from the central regions along outflows, becoming optically thin at longer and longer wavelengths as they do so. The physical and dynamical characteristics of AO 0235+164 suggest it is a close binary with SMBHs with similar masses of the order of 10 10 M ⊙ . This is one of the most massive systems of two SMBHs. The velocities of the companion and central black hole about their common center of mass are estimated to be 10 4 and 5×10 3 km/s. The results obtained suggest that observed bright AGNs form a relatiavely rare population among massive elliptical galaxies, due to their narrowly directed emission and their short lifetimes.