A parsec-scale wobbling jet in the high-synchrotron peaked blazar PG 1553+113

Context. The detection of quasi-periodic variability in active galactic nuclei in general, and in blazars in particular, is key to our understanding of the origin and nature of these objects as well as their cosmological evolution. PG 1553+113 is the first blazar showing an approximately two-year quasi-periodic pattern in its γ-ray light curve, which is also revealed at optical frequencies. Aims: Such quasi-periodicity might have a geometrical origin, possibly related to the precessing nature of the jet, or could be intrinsic to the source and related to pulsational accretion flow instabilities. In this work we investigate and characterise the high-resolution radio emission properties of PG 1553+113 on parsec scales in order to differentiate between these different physical scenarios. Methods: We monitored the source with the very long baseline array (VLBA) at 15, 24, and 43 GHz during an entire cycle of γ-ray activity in the period 2015-2017, with a cadence of about 2 months, both in total and polarised intensity. We constrained the jet position angle across the different observing epochs by investigating the total intensity ridge lines. Results: We find a core-dominated source with a limb-brightened jet structure extending for ∼1.5 mas in the northeast direction whose position angle varies in time in the range ∼40°-60°. No clear periodic pattern can be recognized in the VLBA light curves during 2015-2017 or in the 15 GHz Owens Valley Radio Observatory light curve during the period 2008-2018. The core region polarisation percentage varies in the range ∼1-4%, and the polarisation angle varies from being roughly parallel to roughly transverse to the jet axis. We estimate a rotation measure value in the core region of ∼-1.0 ± 0.4 × 104 rad m-2. The brightness temperature (TB) is found to decrease as the frequency increases with an intrinsic value of ∼1.5 × 1010 K and the estimated Doppler factor is ∼1.4. Conclusions: Although the jet wobbling motion indicates that geometrical effects can produce an enhanced emission through the Doppler boosting modulation, additional mechanisms are required in order to account for the quasi-periodic variability patterns observed in γ rays. The intrinsic TB value indicates that the total energy in the core region is dominated by the magnetic field

Very Long Baseline polarimetry and the γ-ray connection in Markarian 421 during the broadband campaign in 2011

Context. This is the third paper in a series devoted to the analysis of the multiwavelength data from a campaign on the nearby (z = 0.03) TeV blazar Mrk 421 during 2011. Aims: We investigate the structure of the high angular resolution polarization, the magnetic topology, the total intensity light curve, the γ-ray flux, and the photon index. We describe how they evolve and how they are connected. Methods: We analyzed data in polarized intensity obtained with the Very Long Baseline Array (VLBA) at twelve epochs (one observation per month from January to December 2011) at 15, 24, and 43 GHz. For the absolute orientation of the electric vector position angles (EVPA) we used the D-terms method; we also confirm its accuracy. We also used γ-ray data from the Fermi Large Area Telescope on weekly time bins throughout 2011. Results: The source shows polarized emission, and its properties vary with time, frequency, and location along the jet. The core mean polarization fraction is generally between 1% and 2%, with a 4% peak at 43 GHz in March; the polarization angle is variable, mainly at 15 GHz, where it changes frequently, and less so at 43 GHz, where it oscillates in the range 114°-173°. The jet polarization properties are more stable, with a fractional polarization of around 16% and a polarization angle nearly perpendicular to the jet axis. The average flux and photon index at γ-ray energies are (17.4 ± 0.5) × 10-8 ph cm-2 s-1 and Γ = 1.77 ± 0.02. The γ-ray light curve shows variability, with a main peak of (38 ± 11) × 10-8 ph cm-2 s-1 at the beginning of March and two later peaks centered on September 8 and November 13. The first γ-ray peak appears to be associated with the peak in the core polarized emission at 43 GHz, as well as with the total intensity light curve. A discrete correlation function analysis yields a correlation coefficient of 0.54 at zero delay, with a significance level \u3e99.7%. Conclusions: With this multifrequency study, we accurately determine the polarization properties of Mrk 421, both in the core and in the jet region. The radio and γ-ray light curves are correlated. The observed EVPA variability at 15 GHz is partly due to opacity and partly to a variable Faraday rotation effect. To explain the residual variability of the intrinsic polarization angle and the low degree of polarization in the core region, we invoke a blend of variable cross-polarized subcomponents with different polarization properties within the beam

Fermi and Swift Observations of GRB 190114C: Tracing the Evolution of High-energy Emission from Prompt to Afterglow

We report on the observations of gamma-ray burst (GRB) 190114C by the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory. The prompt gamma-ray emission was detected by the Fermi GRB Monitor (GBM), the Fermi Large Area Telescope (LAT), and the Swift Burst Alert Telescope (BAT) and the long-lived afterglow emission was subsequently observed by the GBM, LAT, Swift X-ray Telescope (XRT), and Swift UV Optical Telescope. The early-time observations reveal multiple emission components that evolve independently, with a delayed power-law component that exhibits significant spectral attenuation above 40 MeV in the first few seconds of the burst. This power-law component transitions to a harder spectrum that is consistent with the afterglow emission observed by the XRT at later times. This afterglow component is clearly identifiable in the GBM and BAT light curves as a slowly fading emission component on which the rest of the prompt emission is superimposed. As a result, we are able to observe the transition from internal-shock- to external-shock-dominated emission. We find that the temporal and spectral evolution of the broadband afterglow emission can be well modeled as synchrotron emission from a forward shock propagating into a wind-like circumstellar environment. We estimate the initial bulk Lorentz factor using the observed high-energy spectral cutoff. Considering the onset of the afterglow component, we constrain the deceleration radius at which this forward shock begins to radiate in order to estimate the maximum synchrotron energy as a function of time. We find that even in the LAT energy range, there exist high-energy photons that are in tension with the theoretical maximum energy that can be achieved through synchrotron emission from a shock. These violations of the maximum synchrotron energy are further compounded by the detection of very high-energy (VHE) emission above 300 GeV by MAGIC concurrent with our observations. We conclude that the observations of VHE photons from GRB 190114C necessitates either an additional emission mechanism at very high energies that is hidden in the synchrotron component in the LAT energy range, an acceleration mechanism that imparts energy to the particles at a rate that is faster than the electron synchrotron energy-loss rate, or revisions of the fundamental assumptions used in estimating the maximum photon energy attainable through the synchrotron process

Unresolved Gamma-Ray Sky through its Angular Power Spectrum

The gamma-ray sky has been observed with unprecedented accuracy in the last decade by the Fermi -large area telescope (LAT), allowing us to resolve and understand the high-energy Universe. The nature of the remaining unresolved emission [unresolved gamma-ray background (UGRB)] below the LAT source detection threshold can be uncovered by characterizing the amplitude and angular scale of the UGRB fluctuation field. This Letter presents a measurement of the UGRB autocorrelation angular power spectrum based on eight years of Fermi-LAT Pass 8 data products. The analysis is designed to be robust against contamination from resolved sources and noise systematics. The sensitivity to subthreshold sources is greatly enhanced with respect to previous measurements. We find evidence (with ∼3.7σ significance) that the scenario in which two classes of sources contribute to the UGRB signal is favored over a single class. A double power law with exponential cutoff can explain the anisotropy energy spectrum well, with photon indices of the two populations being 2.55±0.23 and 1.86±0.15

The june 2008 flare of markarian 421 from optical to tev energies

We present optical, X-ray, high-energy ((sic) 30 GeV) and very high energy ((sic) 100 GeV; VHE) observations of the high-frequency peaked blazar Mrk 421 taken between 2008 May 24 and June 23. A high-energy gamma-ray signal was detected by AGILE with root TS = 4.5 between June 9 and 15, with F(E > 100 MeV) = 42(-12)(+14) x 10(-8) photons cm(-2) s(-1). This flaring state is brighter than the average flux observed by EGRET by a factor of similar to 3, but still consistent with the highest EGRET flux. In hard X-rays (20-60 keV) SuperAGILE resolved a five-day flare (June 9-15) peaking at similar to 55 mCrab. SuperAGILE, RXTE/ASM and Swift/BAT data show a correlated flaring structure between soft and hard X-rays. Hints of the same flaring behavior are also detected in the simultaneous optical data provided by the GASP-WEBT. A Swift/XRT observation near the flaring maximum revealed the highest 2-10 keV flux ever observed from this source, of 2.6 x 10(-9) erg cm(-2) s(-1) (i.e. > 100 mCrab). A peak synchrotron energy of similar to 3 keV was derived, higher than typical values of similar to 0.5-1 keV. VHE observations with MAGIC and VERITAS between June 6 and 8 showed the flux peaking in a bright state, well correlated with the X-rays. This extraordinary set of simultaneous data, covering a 12-decade spectral range, allowed for a deep analysis of the spectral energy distribution as well as of correlated light curves. The gamma-ray flare can be interpreted within the framework of the synchrotron self-Compton model in terms of a rapid acceleration of leptons in the jet

The First Fermi-LAT Solar Flare Catalog Appendix

We present the appendix of the First Fermi-Large Area Telescope (LAT) Solar flare catalog covering the 24thSolar cycle. The catalog (published in ApJS) contains 45 Fermi-LAT Solar flares (FLSFs) with emission in theγ-ray energy band (30 MeV -10 GeV) detected with a significance ≥5σ over the years 2010-2018. A subsample containing 37 of these flares exhibit delayed emission beyond the prompt-impulsive hard X-ray phase with 21 flares showing delayed emission lasting more than two hours. No prompt-impulsive emission is detected in four of these flares. We also report the first time observations of GeV emission from 3 flares originating from Active Regions located behind the limb (BTL) of the visible Solar disk. We report the light curves, spectra, best proton index and localization (when possible) for all the FLSFs and correlations with Solar multi-wavelength phenomena. The gamma-ray spectra is consistent with the decay of pions produced by \u3e300 MeV protons. The work presented in the First Fermi Solar Flare Catalog contains the largest sample of high-energy gamma-ray flares ever reported and provides the unique opportunity to perform population/correlation studies on the different phases of the flare and thus allowing to open a new window in solar physics. This work is on behalf of the Fermi Large Area Telescope Collaboration. The First Fermi LAT Solar Flare Catalog has been accepted for publication on ApJS

The Fermi LAT fourth source catalog (4FGL)

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title \u27Fermi Large Area Telescope fourth source catalog.\u27 (bibcode: 2020ApJS..247...33A

Fermi LAT third source catalog (3FGL)

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title \u27Fermi Large Area Telescope third source catalog.\u27 (bibcode: 2015ApJS..218...23A

Flaring gamma-ray sources; LAT 7.4yr (2FAV)

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title \u27The second catalog of flaring gamma-ray sources from the Fermi All-sky Variability Analysis.\u27 (bibcode: 2017ApJ...846...34A

Isotropic diffuse gamma-ray emission 0.1-820GeV

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title \u27The spectrum of isotropic diffuse gamma-ray emission between 100MeV and 820GeV.\u27 (bibcode: 2015ApJ...799...86A