Astrophysical Neutrinos and Blazars

We review and discuss recent results on the search for correlations between astrophysical neutrinos and γ-ray-detected sources, with many extragalactic studies reporting potential associations with different types of blazars. We investigate possible dependencies on blazar sub-classes by using the largest catalogues and all the multi-frequency data available. Through the study of similarities and differences in these sources we conclude that blazars come in two distinct flavours: LBLs and IHBLs (low-energy-peaked and intermediate-high-energy-peaked objects). These are distinguished by widely different properties such as the overall spectral energy distribution shape, jet speed, cosmological evolution, broad-band spectral variability, and optical polarisation properties. Although blazars of all types have been proposed as neutrino sources, evidence is accumulating in favour of IHBLs being the counterparts of astrophysical neutrinos. If this is indeed the case, we argue that the peculiar observational properties of IHBLs may be indirectly related to proton acceleration to very high energies.

Testing two-component models on very high-energy gamma-ray-emitting BL Lac objects

Context. It has become evident that one-zone synchrotron self-Compton models are not always adequate for very high-energy (VHE) gamma-ray-emitting blazars. While two-component models perform better, they are difficult to constrain due to the large number of free parameters. Aims. In this work, we make a first attempt at taking into account the observational constraints from very long baseline interferometry (VLBI) data, long-term light curves (radio, optical, and X-rays), and optical polarisation to limit the parameter space for a two-component model and test whether or not it can still reproduce the observed spectral energy distribution (SED) of the blazars. Methods. We selected five TeV BL Lac objects based on the availability of VHE gamma-ray and optical polarisation data. We collected constraints for the jet parameters from VLBI observations. We evaluated the contributions of the two components to the optical flux by means of decomposition of long-term radio and optical light curves as well as modelling of the optical polarisation variability of the objects. We selected eight epochs for these five objects based on the variability observed at VHE gamma rays, for which we constructed the SEDs that we then modelled with a two-component model. Results. We found parameter sets which can reproduce the broadband SED of the sources in the framework of two-component models considering all available observational constraints from VLBI observations. Moreover, the constraints obtained from the long-term behaviour of the sources in the lower energy bands could be used to determine the region where the emission in each band originates. Finally, we attempt to use optical polarisation data to shed new light on the behaviour of the two components in the optical band. Our observationally constrained two-component model allows explanation of the entire SED from radio to VHE with two co-located emission regions.

Evolving optical polarisation of the black hole X-ray binary MAXI J1820+070

Aims. The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts. Whether the extra light arises from the X-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated. Optical polarisation measurements are able to distinguish the relative contributions of these components. Methods. We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018. Results. We detect small, ∼0.7%, but statistically significant polarisation, part of which is of interstellar origin. Depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between ∼0.3% and 0.7%, and the polarisation position angle is between ∼10 ° −30°. We show that the polarisation increases after MJD 58222 (2018 April 14). The change is of the order of 0.1% and is most pronounced in the R band. The change of the source Stokes parameters occurs simultaneously with the drop of the observed V-band flux and a slow softening of the X-ray spectrum. The Stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the V and R filters. Conclusions. We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense, ionised, and magnetised medium close to the black hole. The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow.

Optical polarisation variability of radio-loud narrow-line Seyfert 1 galaxies

Context. Narrow-line Seyfert 1 galaxies (NLSy1s) constitute the active galactic nuclei subclass associated with systematically lower black hole masses. A few radio-loud NLSy1s have been detected in MeV-GeV energy bands by Fermi, and evidence that blazar-like jets are operating also in radio-loud NLSy1s, has been accumulated. Aims. We wish to quantify the temporal behaviour of the optical polarisation, fraction, and angle for a selected sample of radio-loud NLSy1s. We also search for rotations of the polarisation plane similar to those commonly observed in blazars. Methods. We have conducted R-band optical linear polarisation monitoring of a sample of ten radio-loud NLSy1 galaxies; five of them have previously been detected by Fermi. The dataset obtained with our pivoting instrument, the RoboPol polarimeter of the Skinakas observatory, has been complemented with observations from the KANATA, Perkins, and Steward observatories. When evidence for long rotations of the polarisation plane was found (at least three consecutive measurements covering at least 90°), we carried out numerical simulations to assess the probability that they are caused by intrinsically evolving electric vector position angles (EVPAs) instead of observational noise. Results. Even our moderately sampled sources show clear indications of variability in both polarisation fraction and angle. For the four best-sampled objects in our sample we find multiple periods of significant polarisation angle variability. Several of these events qualify as long rotations. In the two best-sampled cases, namely J1505+0326 and J0324+3410, we find indications for three long rotations of the polarisation angle. We show that although noise can induce the observed behaviour, it is much more likely that the apparent rotation is indeed caused by intrinsic evolution of the EVPA. To our knowledge, this is the very first detection of such events in this class of sources. In the largest dataset (J0324+3410), we find that the EVPA concentrates around a direction that is at 49.3° to the 15 GHz radio jet, implying a projected magnetic field at an angle of 40.7° to that axis. Conclusions. We assess the probability that pure measurement uncertainties are the reason behind the observed long rotations of the polarisation plane. We conclude that although this is not improbable, it is much more likely that intrinsic rotations are responsible for the observed phenomenology. We conclude, however, that much better sampled and larger datasets of larger source samples are necessary to constrain the physical mechanism(s) that generate long EVPA rotations in NLSy1s.