Variability studies of a sample of IntraDay Variable sources

The discovery of IntraDay Variability (IDV) in the radio flux from Active Galactic Nuclei gave rise to a series of fundamental issues concerning the origin of the variability. The observation of large flux-density variations on time scales from hours to few days led to the question whether such extreme phenomena can physically be intrinsic to sources. From causality arguments, the variability time scales constrain the maximum size of the emitting region. For IDV sources, this frequently implies very high brightness tempera- tures, far exceeding the inverse-Compton limit. In a source-intrinsic interpretation, only very high Doppler boost factors – of the order of a few tens, or even hundreds – or partic- ular geometries of the emitting regions can explain such short time scale variability. No strong evidence in favour of the second hypothesis has been found yet, while the first one seems to contradict the results of VLBI observations. Source-extrinsic models, instead, ascribe the variability to propagation effects. Recent studies seem to indicate that inter- stellar scintillation plays an important role in the IDV shown by several sources, but it is insufficient to explain all the aspects of the phenomenon. For this thesis project, a sample of IDV sources has been monitored for more than three years with the Urumqi radio telescope, at a frequency of 4.8GHz, aiming to study the evolution of their variability characteristics and to compare it with the predictions of source-intrinsic and source-extrinsic IDV models. 22 observing sessions have been per- formed between August 2005 and April 2008 for a total of 78 days of observations. All the variability curves have been analyzed by means of classical and new meth- ods of time-series analysis. The results seem to point towards an extrinsic origin of the variability for three out of the four main targets of the monitoring campaign. For the blazar AO 025+164, instead, there is strong evidence that the evolution of its variability characteristics is correlated with changes in the structure of the source. A search for possible regular flux-density variations in IDV sources led to the discovery of a 1-day periodic contribution to their variability, which can not be ascribed to any known systematic effect. Plausibly, it can be related to variations in the total electron content of the ionosphere, or it may be caused by the magnetosphere. However, the issue is still open

The Paradox of Tik Tok Anti-Pro-Anorexia Videos: How Social Media Can Promote Non-Suicidal Self-Injury and Anorexia

The literature shows that social pressure promotes non-suicidal self-injury (NSSI) eating disorders, along with self-injury, are also favored by underregulated social media. Tik tok is one of the most used social media platforms among adolescents. It has been shown that the time young children spend on this platform doubled during the lockdown. The theme of anorexia is very common on this platform. While most "pro-ana" (pro-anorexia) videos, where users exchanged advice on how to pathologically lose weight, have been censored by the application, other "anti-pro-ana" (anti-pro-anorexia) videos, officially aimed at raising awareness of the consequences of anorexia, have become increasingly popular. However, our case shows how even these safer videos paradoxically lead the users to emulate these "guilty" behaviors

A seasonal cycle and an abrupt change in the variability characteristics of the intraday variable source S4 0954+65

The BLLac object S4 0954+65 is one of the main targets of the Urumqi monitoring program targeting IntraDay Variable (IDV) sources. Between August 2005 and December 2009, the source was included in 41 observing sessions, carried out at a frequency of 4.8 GHz. The time analysis of the collected light curves, performed by applying both a structure function analysis and a specifically developed wavelet-based algorithm, discovered an annual cycle in the variability timescales, suggesting that there is a fundamental contribution by interstellar scintillation to the IDV pattern of the source. The combined use of the two analysis methods also revealed that there was a dramatic change in the variability characteristics of the source between February and March 2008, at the starting time of a strong outburst phase. The analysis' results suggest that the flaring state of the source coincides with the appearance of multiple timescales in its light curves, indicating that changes in the structure of the relativistically moving emitting region may strongly influence the variability observed on IDV timescales.Comment: 9 pages, 8 figures and 3 tables. Accepted for publication in Astronomy and Astrophysic

Optical polarisation variability of radio loud narrow line Seyfert 1 galaxies. Search for long rotations of the polarisation plane

Narrow line Seyfert 1 galaxies (NLSy1s) constitute the AGN subclass associated with systematically smaller black hole masses. A few radio loud ones have been detected in MeV -- GeV energy bands by Fermi and evidence for the presence of blazar-like jets has been accumulated. In this study 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. We have conducted R-band optical polarisation monitoring of a sample of 10 RL NLSy1s 5 of which have been previously detected by Fermi. The dataset includes observations with the RoboPol, KANATA, Perkins and Steward polarimeters. In the cases where evidences for long rotations of the polarisation plane are found, we carry out numerical simulations to assess the probability that they are caused by intrinsically evolving EVPAs instead of observational noise. Even our moderately sampled sources show indications of variability, both in polarisation fraction and angle. For the four best sampled objects in our sample we find multiple periods of significant polarisation angle variability. In the two best sampled cases, namely J1505+0326 and J0324+3410, we find indications for three long rotations. We show that although noise can induce the observed behaviour, it is much more likely that the apparent rotation is 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 case of the largest dataset (J0324+3410) we find that the EVPA concentrates around a direction which is at 49.3\degr to the 15-GHz radio jet implying a projected magnetic field at an angle of 40.7\degr to that axis.Comment: Accepted for publication in section 2. Astrophysical processes of Astronomy and Astrophysic

Very Long Baseline Array observations of the Intraday Variable source J1128+592

Short time-scale flux density variations of flat spectrum radio sources are often explained by the scattering of radio waves in the turbulent, ionized Interstellar Matter of the Milky Way. One of the most convincing observational arguments in favor of this is the annual modulation of the variability time-scale caused by the Earth orbital motion around the Sun. J1128+592 is an IDV source with a possible annual modulation in its variability time-scale. We observed the source in 6 epochs with the VLBA at 5, 8 and 15 GHz in total intensity and polarization. The VLBA observations revealed an east-west oriented core-jet structure. Its position angle agrees with the angle of anisotropy derived from the annual modulation model. No significant long-term structural changes were observed with VLBI on mas-scales, however, compared to archival data, the VLBI core size is expanded. This expansion offers a possible explanation to the observed decrease of the strength of IDV. VLBI polarimetry revealed significant changes in the electric vector position angle and Rotation Measure of the core and jet. Part of the observed RM variability could be attributed to a scattering screen (37 pc distance), which covers the source (core and jet) and which may be responsible for the IDV. Superposition of polarized sub-components below the angular resolution limit may affect the observed RM as well.Comment: accepted for A&A (11 pages, 11 figures

How to Legislate with Wisdom and Eloquence. The art of Legislation Reconstructed From the Rhetorical Tradition

The art of legislation, that had got lost, is reborn in this book from the classic tradition, which conceives the laws like wise and eloquent civic speeches, and the rhetoric as its basic method, of a such way, that the return to the ancient will be a true progress

Memory. Oblivion

The subject of this essay is the reconstruction of some formal aspects of the relationship be-tween memory and oblivion. The reference context is that of the interpretation of the sociological theory of collective memory into cultural memory. The essay is also concerned with outlining a further shift from cultural to political theory through reference to counterfactual memory. The thesis it expresses, however, is that with respect to the political hypertrophy of memory, which operates as a guarantee of the identity of collective subjectivities, the critical work of historiography must always be guaranteed, which allows equal access to arguments based on evidences

F-GAMMA: Variability Doppler factors of blazars from multiwavelength monitoring

Recent population studies have shown that the variability Doppler factors can adequately describe blazars as a population. We use the flux density variations found within the extensive radio multi-wavelength datasets of the F-GAMMA program, a total of 10 frequencies from 2.64 up to 142.33 GHz, in order to estimate the variability Doppler factors for 58 $\gamma$-ray bright sources, for 20 of which no variability Doppler factor has been estimated before. We employ specifically designed algorithms in order to obtain a model for each flare at each frequency. We then identify each event and track its evolution through all the available frequencies for each source. This approach allows us to distinguish significant events producing flares from stochastic variability in blazar jets. It also allows us to effectively constrain the variability brightness temperature and hence the variability Doppler factor as well as provide error estimates. Our method can produce the most accurate (16\% error on average) estimates in the literature to date.Comment: 9 pages, 7 figures, accepted for publication in MNRA