T. Hovatta
A5089910311- Astrophysics and Cosmic Phenomena
- Radio Astronomy Observations and Technology
- Gamma-ray bursts and supernovae
- Neutrino Physics Research
- Dark Matter and Cosmic Phenomena
- Astrophysical Phenomena and Observations
- Astronomical Observations and Instrumentation
- Galaxies: Formation, Evolution, Phenomena
- Solar and Space Plasma Dynamics
- Particle Accelerators and Free-Electron Lasers
- Particle Detector Development and Performance
- Pulsars and Gravitational Waves Research
- Astronomy and Astrophysical Research
- Gyrotron and Vacuum Electronics Research
- Stellar, planetary, and galactic studies
- Geophysics and Gravity Measurements
- Calibration and Measurement Techniques
- Computational Physics and Python Applications
- Cosmology and Gravitation Theories
- Particle physics theoretical and experimental studies
- GNSS positioning and interference
- Astro and Planetary Science
- Radio Wave Propagation Studies
- Precipitation Measurement and Analysis
- Radiation Detection and Scintillator Technologies
Aalto University
2015-2024
Turku Centre for Computer Science
2015-2024
University of Turku
2015-2024
CTA Observatory
2023
Sorbonne Université
2023
Institut National de Physique Nucléaire et de Physique des Particules
2023
Centre National de la Recherche Scientifique
2023
Laboratoire de Physique Nucléaire et de Hautes Énergies
2023
TU Dortmund University
2023
University of Łódź
2023
We present results from monitoring the multi-waveband flux, linear polarization, and parsec-scale structure of quasar PKS 1510-089, concentrating on eight major gamma-ray flares that occurred during interval 2009.0-2009.5. The peaks were essentially simultaneous with maxima at optical wavelengths, although flux ratio two wavebands varied by an order magnitude. polarization vector rotated 720 degrees a 5-day period encompassing six these flares. This culminated in very bright, roughly 1 day,...
<i>Aims. <i/>We have calculated variability Doppler boosting factors, Lorentz and viewing angles for a large sample of sources by using total flux density observations at 22 37 GHz VLBI data.<i>Methods. decomposed the curves into exponential flares determined brightness temperatures fastest flares. By assuming same intrinsic temperature each source, we factors 87 sources. In addition used new apparent jet speed data to calculate 67 sources.<i>Results. find that all quasars in our are...
We have investigated a frequency-dependent shift in the absolute position of optically thick apparent origin parsec-scale jets ("core shift" effect) to probe physical conditions ultra-compact relativistic outflows AGN. used multi-frequency Very Long Baseline Array (VLBA) observations 191 sources carried out 12 epochs 2006 within MOJAVE program. The were performed at 8.1, 8.4, 12.1, and 15.4 GHz. implemented method determining core vector based on (i) image registration by two-dimensional...
We investigate the dependence of gamma-ray brightness blazars on intrinsic properties their parsec-scale radio jets and implication for relativistic beaming. By combining apparent jet speeds derived from high-resolution VLBA images MOJAVE program with millimetre-wavelength flux density monitoring data Metsahovi Radio Observatory, we estimate Doppler factors, Lorentz viewing angles a sample 62 blazars. study trends in these quantities between sources which were detected gamma-rays by Fermi...
We have obtained milliarcsecond-scale spectral index distributions for a sample of 190 extragalactic radio jets through the MOJAVE project. The sources were observed in 2006 at 8.1, 8.4, 12.1, and 15.4 GHz, we study four-frequency spectrum individual jet features. performed detailed simulations to effects image alignment (u,v)-plane coverage on maps verify our results. core indices are average flat, while is general steep with mean -1.04. A simple power-law fit often inadequate regions, as...
We report observations of Faraday rotation measures for a sample 191 extragalactic radio jets observed within the MOJAVE program. Multifrequency Very Long Baseline Array were carried out over 12 epochs in 2006 at four frequencies between 8 and 15 GHz. detect parsec-scale 149 sources find quasars to have larger on average than BL Lac objects. The median core are significantly higher jet components. This is especially true where we significant negative correlation magnitude measure...
In order to determine the location of gamma-ray emission site in blazars, we investigate time-domain relationship between their radio and emission. Light curves for brightest detected blazars from first 3 yr mission Fermi Gamma-ray Space Telescope are cross-correlated with 4 15 GHz observations Owens Valley Radio Observatory 40 m monitoring programme. The large sample long light-curve duration enable us carry out a statistically robust analysis significance cross-correlations, which is...
We present a practical implementation of Monte Carlo method to estimate the significance cross-correlations in unevenly sampled time series data, whose statistical properties are modelled with simple power-law power spectral density. This builds on published methods; we introduce number improvements normalization cross-correlation function and bootstrap for estimating cross-correlations. A closely related matter is estimation model light curves, which critical estimates. graphical...
We report Fermi-LAT observations of the radio-loud AGN SBS 0846+513 (z=0.5835), optically classified as a Narrow-Line Seyfert 1 galaxy, together with new and archival radio-to-X-ray data. The source was not active at gamma-ray energies during first two years Fermi operation. A significant increase in activity observed 2010 October-2011 August. In particular strong flare 2011 June reaching an isotropic luminosity (0.1-300 GeV) 1.0x10^48 erg/s, comparable to that brightest flat spectrum radio...
We present first results on polarization swings in optical emission of blazars obtained by RoboPol, a monitoring program an unbiased sample gamma-ray bright specially designed for effective detection such events. A possible connection swing events with periods high activity gamma rays is investigated using the dataset during season operation. It was found that brightest flares tend to be located closer time rotation events, which may indication two separate mechanisms responsible rotations....
We present 10 years of R-band monitoring data 31 northern blazars which were either detected at very high energy (VHE) gamma rays or listed as potential VHE gamma-ray emitters. The comprise 11820 photometric points in the obtained 2002-2012. analyze light curves by determining their power spectral density (PSD) slopes assuming a power-law dependence with single slope $\beta$ and Gaussian probability function (PDF). use multiple fragments variance (MFVF) combined forward-casting approach...
We present average R-band optopolarimetric data, as well variability parameters, from the first and second RoboPol observing season. investigate whether gamma-ray-loud gamma-ray-quiet blazars exhibit systematic differences in their optical polarization properties. find that have a systematically higher fraction (0.092) than (0.031), with hypothesis of two samples being drawn same distribution fractions rejected at 3σ level. not found any evidence this discrepancy is related to redshift...
We present results from a parsec-scale jet kinematics study of 409 bright radio-loud AGNs based on 15 GHz VLBA data obtained between 1994 August 31 and 2016 December 26 as part the 2cm survey MOJAVE programs. tracked 1744 individual features in 382 jets over at least five epochs. A majority (59%) best-sampled showed evidence accelerated motion >3sigma level. Although most within typically have speeds ~40% characteristic median value, we identified 55 42 that had unusually slow pattern...
Identifying the most likely sources for high-energy neutrino emission has been one of main topics in astrophysics ever since first observation neutrinos by IceCube Neutrino Observatory. Active galactic nuclei with relativistic jets, also known as blazars, have considered to be candidates because their ability accelerate particles high energies. We study connection between radio and events using data from Owens Valley Radio Observatory (OVRO) Mets\"ahovi blazar monitoring programs. identify...
Abstract Most large galaxies host supermassive black holes in their nuclei and are subject to mergers, which can produce a hole binary (SMBHB), hence periodic signatures due orbital motion. We report unique radio flux density variations the blazar PKS 2131−021, strongly suggest an SMBHB with separation of ∼0.001–0.01 pc. Our 45.1 yr light curve shows two epochs strong sinusoidal variation same period phase within ≲2% ∼10%, respectively, straddling 20 when this was absent. simulated curves...
We investigate the Fermi Large Area Telescope γ-ray and 15 GHz Very Long Baseline Array radio properties of a joint radio-selected sample active galactic nuclei (AGNs) obtained during first 11 months mission (2008 August 4–2009 July 5). Our contains brightest 173 AGNs in these bands above declination −30° this period, thus probes full range loudness (γ-ray to band luminosity ratio) bright blazar population. The latter quantity spans at least 4 orders magnitude, reflecting wide spectral...
We report on 18 months of multiwavelength observations the blazar 3C 454.3 (Crazy Diamond) carried out in July 2007-January 2009. show results AGILE campaigns which took place May-June 2008, July-August and October 2008-January During May 2009 period, source average flux was highly variable, from an gamma-ray F(E>100MeV) > 200E-8 ph/cm2/s to F(E>100MeV)~80E-8 The spectrum between 100 MeV 1 GeV can be fit by a simple power law (Gamma_GRID ~ 2.0 2.2). Only 3-sigma upper limits derived 20-60...
We present the results of extensive multi-frequency monitoring radio galaxy 3C 120 between 2002 and 2007 at X-ray, optical, wave bands, as well imaging with Very Long Baseline Array (VLBA). Over 5 yr observation, significant dips in X-ray light curve are followed by ejections bright superluminal knots VLBA images. Consistent this, flux 37 GHz anti-correlated leading variations. This implies that, this galaxy, radiative state accretion disk plus corona system, where X-rays produced, has a...
We report on the detection of a very rapid TeV gamma-ray flare from BL Lacertae 2011 June 28 with Very Energetic Radiation Imaging Telescope Array System (VERITAS). The flaring activity was observed during 34.6-minute exposure, when integral flux above 200 GeV reached $(3.4\pm0.6) \times 10^{-6} \;\text{photons}\;\text{m}^{-2}\text{s}^{-1}$, roughly 125% Crab Nebula measured by VERITAS. light curve indicates that observations missed rising phase but covered significant portion decaying...
Outbursts from gamma-ray quasars provide insights on the relativistic jets of active galactic nuclei and constraints diffuse radiation fields that fill universe. The detection significant emission above 100 GeV a distant quasar would show some radiated gamma-rays escape pair-production interactions with low-energy photons, be it extragalactic background light (EBL), or near supermassive black hole lying at jet's base. VERITAS detected up to ∼200 PKS 1441+25 (z = 0.939) during 2015 April,...
We present results from four years of twice-weekly 15 GHz radio monitoring about 1500 blazars with the Owens Valley Radio Observatory 40 m telescope. Using intrinsic modulation index to measure variability amplitude, we find that, >6σ significance, radio-selected gamma-ray-loud is stronger than that gamma-ray-quiet blazars. Our extended data set also includes at least 21 months for all AGN 'clean' associations in Fermi Large Area Telescope First Catalog, 1LAC. With these additional data,...
We present measurements of rotations the optical polarization blazars during second year operation RoboPol, a monitoring programme an unbiased sample gamma-ray bright specially designed for effective detection such events, and we analyse large set rotation events discovered in two years observation. investigate patterns variability parameters total flux density compare them to behaviour non-rotating state. have searched possible correlations between average polarization-plane polarization,...
We use results of our 3 yr polarimetric monitoring programme to investigate the previously suggested connection between rotations polarization plane in optical emission blazars and their gamma-ray flares GeV band. The homogeneous set 40 rotation events 24 sources detected by RoboPol is analysed together with data provided Fermi-LAT. confirm that are indeed related closest time lags these consistent zero. Amplitudes anticorrelated amplitudes flares. This presumably caused higher relativistic...
The OVRO 40-m telescope has been monitoring the 15 GHz radio flux density of over 1200 blazars since 2008. light curve flat spectrum quasar J1359+4011 shows a strong and persistent quasi-periodic oscillation. time-scale oscillation varies between 120 150 days $\sim4$ year time span. We interpret this as active galactic nucleus mass-scaled analog low-frequency oscillations from Galactic microquasars, or evidence modulation accretion flow by thermal instabilites in "inner" disc.
Abstract We present the results of our power spectral density analysis for BL Lac object OJ 287, utilizing Fermi -LAT survey at high-energy γ -rays, Swift -XRT in X-rays, several ground-based telescopes and Kepler satellite optical, radio GHz frequencies. The light curves are modeled terms continuous-time autoregressive moving average (CARMA) processes. Owing to inclusion data, we were able construct first time optical variability spectrum a blazar without any gaps across ∼6 dex temporal Our...