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Abstract The combined University of Michigan Radio Astronomy Observatory and Owens Valley Radio Observatory blazar monitoring programs at 14.5/15 GHz provide uninterrupted light curves of ∼ 46–50 yr duration for 83 blazars, selected from among the brightest and most rapidly flaring blazars north of declination −20°. In a search for supermassive black hole binary (SMBHB) candidates, we carried out tests for periodic variability using generalized Lomb–Scargle (GLS), weighted wavelet-Z, and sine-wave fitting analyses of this sample. We used simulations to test the effects of the power-law spectrum of the power spectral density (PSD) on our findings, and show that the irregular sampling in the observed light curves has very little effect on the GLS spectra. Apparent periodicities and putative harmonics appear in all 83 of the GLS spectra of the blazars in our sample. We tested the reality of these apparent periodicities and harmonics with simulations, and found that in the overwhelming majority of cases, they must be due to the steep slope of the PSD and the random nature of blazar flares, implying that, in general, apparent periodicities and harmonics in blazar light curves observed in any energy band should be treated with great caution. We find one new SMBHB candidate: PKS 1309+1154, which exhibits a 17.9 yr periodicity. The fraction of SMBHB candidates in our sample is $2.4_{-0.8}^{+3.2}\%$. 

Abstract We recently discovered a supermassive black hole binary (SMBHB) candidate, PKS J1309+1154, in the combined 46 yr University of Michigan Radio Astronomy Observatory (UMRAO) plus Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz. The light curve of PKS 1309+1154 exhibits a 17.9 yr periodicity. We also reported a hint of a first harmonic with a 9 yr periodicity in this object. Further analysis of the PKS J1309+1154 light curve provides compelling evidence that both the fundamental and the harmonic are real, confirming the existence of real periodicities in blazar light curves. This is the first case, to our knowledge, of watertight evidence for a fundamental periodicity and a harmonic periodicity in a blazar light curve. It makes PKS J1309+1154 astrongSMBHB candidate and thus the third such candidate to be revealed through long-term radio monitoring, the other two being PKS J0805–0111 and PKS 2131–021, both discovered through the OVRO 40 m Telescope monitoring program. It is argued that hundreds of SMBHB candidates will be discovered by the combination of the South Pole Telescope, the Vera Rubin Observatory, and the Simons Observatory. Coherent searches for gravitational waves from a network of SMBHB candidates, starting immediately, are strongly motivated. 

Abstract Haystack and Owens Valley Radio Observatory observations recently revealed strong, intermittent, sinusoidal total flux-density variations that maintained their coherence between 1975 and 2021 in the blazar PKS 2131−021 (z= 1.283). This was interpreted as possible evidence of a supermassive black hole binary (SMBHB). Extended observations through 2023 show a coherence over 47.9 yr, with an observed periodP_{15 GHz}= (1739.8 ± 17.4) days. We reject, withp-value = 2.09 × 10⁻⁷, the hypothesis that the variations are due to random fluctuations in the red noise tail of the power spectral density. There is clearly a physical phenomenon in PKS 2131−021 producing coherent sinusoidal flux-density variations. We find the coherent sinusoidal intensity variations extend from below 2.7 GHz to optical frequencies, from which we derive an observed periodP_optical= (1764 ± 36) days. Across this broad frequency range, there is a smoothly varying monotonic phase shift in the sinusoidal variations with frequency. Hints of periodic variations are also observed atγ-ray energies. The importance of well-vetted SMBHB candidates to searches for gravitational waves is pointed out. We estimate the fraction of blazars that are SMBHB candidates to be >1 in 100. Thus, monitoring programs covering tens of thousands of blazars could discover hundreds of SMBHB candidates. 

Abstract Owens Valley Radio Observatory observations of the supermassive black hole binary (SMBHB) candidate PKS 2131−021 revealed, for the first time, six likely characteristics of the phenomenology exhibited by SMBHBs in blazars, of which the most unexpected and critical is sinusoidal flux density variations. We have now identified a second blazar, PKS J0805−0111, showing similar variations, with a period of 1.422 ± 0.005 yr in the rest frame of thez= 1.388 object. PKS J0805−0111 displays five of the six characteristics observed in PKS 2131−021. To estimate the significance of the sinusoidal variations, we generate 10⁶simulated light curves that reproduce the radio variability characteristics of PKS J0805−0111 and show that the global probability that the periodicity we detect is due to the red-noise tail of the power spectral density isp= 6.7 × 10⁻⁵(3.82σ). This shows that PKS 2131−021 is not a unique case. The discovery of these two objects in a sample of 1158 blazars allows us to reject, at ap-value ∼0.003, the null hypothesis that the sinusoidal variations in these two blazars are all due to a red-noise process. We estimate that the number of SMBHB candidates among blazars is ∼1 in 100. 

Aims.We introduce the TELAMON program which is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics, specifically TeV blazars and candidate neutrino-associated AGN. Here, we present and characterize our main sample of TeV-detected blazars. Methods.We analyzed the data sample from the first ∼2.5 yr of observations between August 2020 and February 2023 in the range from 14 GHz to 45 GHz. During this pilot phase, we observed all 59 TeV-detected blazars in the Northern Hemisphere (i.e., Dec > 0°) known at the time of observation. We discuss the basic data reduction and calibration procedures used for all TELAMON data and introduce a sub-band averaging method used to calculate average light curves for the sources in our sample. Results.The TeV-selected sources in our sample exhibit a median flux density of 0.12 Jy at 20 mm, 0.20 Jy at 14 mm, and 0.60 Jy at 7 mm. The spectrum for most of the sources is consistent with a flat radio spectrum and we found a median spectral index (S(ν)∝ν^α) ofα = −0.11. Our results on flux density and spectral index are consistent with previous studies of TeV-selected blazars. Compared to the GeV-selected F-GAMMA sample, TELAMON sources are significantly fainter in the radio band. This is consistent with the double-humped spectrum of blazars being shifted towards higher frequencies for TeV-emitters (in particular for high-synchrotron peaked BL Lac type objects), which results in a lower radio flux density. The spectral index distribution of our TeV-selected blazar sample is not significantly different from the GeV-selected F-GAMMA sample. Moreover, we present a strategy to track the light curve evolution of sources in our sample for future variability and correlation analysis. 

Context. Optical polarimeters are typically calibrated using measurements of stars with known and stable polarization parameters. However, there is a lack of such stars available across the sky. Many of the currently available standards are not suitable for medium and large telescopes due to their high brightness. Moreover, as we find, some of the polarimetric standards used are in fact variable or have polarization parameters that differ from their cataloged values. Aims. Our goal is to establish a sample of stable standards suitable for calibrating linear optical polarimeters with an accuracy down to 10 −3 in fractional polarization. Methods. For 4 yr, we have been running a monitoring campaign of a sample of standard candidates comprised of 107 stars distributed across the northern sky. We analyzed the variability of the linear polarization of these stars, taking into account the non-Gaussian nature of fractional polarization measurements. For a subsample of nine stars, we also performed multiband polarization measurements. Results. We created a new catalog of 65 stars (see Table 2) that are stable, have small uncertainties of measured polarimetric parameters, and can be used as calibrators of polarimeters at medium and large telescopes. 

Context. In February 2017 the blazar OJ 287, one of the best super-massive binary-black-hole-system candidates, was detected for the first time at very high energies (VHEs; E  > 100 GeV) with the ground-based γ -ray observatory VERITAS. Aims. Very high energy γ rays are thought to be produced in the near vicinity of the central engine in active galactic nuclei. For this reason, and with the main goal of providing useful information for the characterization of the physical mechanisms connected with the observed teraelectronvolt flaring event, we investigate the parsec-scale source properties by means of high-resolution very long baseline interferometry observations. Methods. We use 86 GHz Global Millimeter-VLBI Array (GMVA) observations from 2015 to 2017 and combine them with additional multiwavelength radio observations at different frequencies from other monitoring programs. We investigate the source structure by modeling the brightness distribution with two-dimensional Gaussian components in the visibility plane. Results. In the GMVA epoch following the source VHE activity, we find a new jet feature (labeled K) at ∼0.2 mas from the core region and located in between two quasi-stationary components (labeled S1 and S2). Multiple periods of enhanced activity are detected at different radio frequencies before and during the VHE flaring state. Conclusions. Based on the findings of this work, we identify as a possible trigger for the VHE flaring emission during the early months of 2017 the passage of a new jet feature through a recollimation shock (represented by the model-fit component S1) in a region of the jet located at a de-projected distance of ∼10 pc from the radio core. 

In April 2019, the Event Horizon Telescope (EHT) collaboration revealed the first image of the candidate super- massive black hole (SMBH) at the centre of the giant elliptical galaxy Messier 87 (M87). This event-horizon-scale image shows a ring of glowing plasma with a dark patch at the centre, which is interpreted as the shadow of the black hole. This breakthrough result, which represents a powerful confirmation of Einstein’s theory of gravity, or general relativity, was made possible by assembling a global network of radio telescopes operating at millimetre wavelengths that for the first time included the Atacama Large Millimeter/submillimeter Array (ALMA). The addition of ALMA as an anchor station has enabled a giant leap forward by increasing the sensitivity limits of the EHT by an order of magnitude, effectively turning it into an imaging array. The published image demonstrates that it is now possible to directly study the event horizon shadows of SMBHs via electromagnetic radiation, thereby transforming this elusive frontier from a mathematical concept into an astrophysical reality. The expansion of the array over the next few years will include new stations on different continents — and eventually satellites in space. This will provide progressively sharper and higher-fidelity images of SMBH candidates, and potentially even movies of the hot plasma orbiting around SMBHs. These improvements will shed light on the processes of black hole accretion and jet formation on event-horizon scales, thereby enabling more precise tests of general relativity in the truly strong field regime.