D. Paneque

Cosmic-ray electron-positron spectrum from 7 GeV to 2 TeV with the Fermi Large Area Telescope

We present a measurement of the cosmic-ray electron+positron spectrum between 7 GeV and 2 TeV performed with almost seven years of data collected with the Fermi Large Area Telescope. We find that the spectrum is well fit by a broken power law with a break energy at about 50 GeV. Above 50 GeV, the spectrum is well described by a single power law with a spectral index of 3.07 ± 0.02 (stat+syst) ± 0.04 (energy measurement). An exponential cutoff lower than 1.8 TeV is excluded at 95% CL. PACS numbers: 98.70.Sa, 96.50.sb, 95.85.Ry, 95.55.Vj

VLBA monitoring of Mrk 421 at 15 GHz and 24 GHz during 2011

Context. High-resolution radio observations are ideal for constraining the value of physical parameters in the inner regions of activegalactic-nucleus jets and complement results on multiwavelength (MWL) observations. This study is part of a wider multifrequency campaign targeting the nearby TeV blazar Markarian 421 (z = 0.031), with observations in the sub-mm (SMA), optical/IR (GASP), UV/X-ray (Swift, RXTE, MAXI), and γ rays (Fermi-LAT, MAGIC, VERITAS). Aims. We investigate the jet’s morphology and any proper motions, and the time evolution of physical parameters such as flux densities and spectral index. The aim of our wider multifrequency campaign is to try to shed light on questions such as the nature of the radiating particles, the connection between the radio and γ-ray emission, the location of the emitting regions and the origin of the flux variability. Methods. We consider data obtained with the Very Long Baseline Array (VLBA) over twelve epochs (one observation per month from January to December 2011) at 15 GHz and 24 GHz. We investigate the inner jet structure on parsec scales through the study of model-fit components for each epoch. Results. The structure of Mrk 421 is dominated by a compact (∼0.13 mas) and bright component, with a one-sided jet detected out to ∼10 mas. We identify 5–6 components in the jet that are consistent with being stationary during the 12-month period studied here. Measurements of the spectral index agree with those of other works: they are fairly flat in the core region and steepen along the jet length. Significant flux-density variations are detected for the core component. Conclusions. From our results, we draw an overall scenario in which we estimate a viewing angle 2 ◦

The TeV blazar Markarian 421 at the highest spatial resolution

Context. High-resolution radio observations allow us to directly image the innermost region of active galactic nuclei. The Very Long Baseline Array (VLBA) data analyzed in this paper were obtained during a multiwavelength (MWL) campaign, carried out in 2011, from radio to very high energy gamma rays, on the TeV blazar Markarian 421 (Mrk 421). Aims. Our aim was to obtain information on the jet structure in Mrk 421 during the MWL campaign at the highest possible angular resolution and with high temporal frequency observations, in order to compare structural and flux density evolution with higher energy variations. Methods. We consider data obtained with the VLBA at 43 GHz through two sets of observations: one is part of a dedicated multifrequency monitoring campaign, in which we observed Mrk 421 once a month from January to December 2011 at three frequencies; the other is extracted from the Boston University monitoring program, which observes 34 blazars at 43 GHz about once per month. We model-fit the data in the visibility plane, study the proper motion of jet components, the light curve, and the spectral index of the jet features. We compare the radio data with optical light curves obtained at the Steward Observatory, considering also the optical polarization information. Results. Mrk 421 has a bright nucleus and a one-sided jet extending towards the north-west for a few parsecs. The model-fits show that brightness distribution is well described using 6–7 circular Gaussian components, four of which are reliably identified at all epochs; all components are effectively stationary except for component D, at ∼0.4 mas from the core, whose motion is, however, subluminal. Analysis of the light curve shows two different states, with the source being brighter and more variable in the first half of 2011 than in the second half. The highest flux density is reached in February. A comparison with the optical data reveals an increase of the V magnitude and of the fractional polarization simultaneous with the enhancement of the radio activity.

Fermi-LAT Observations of High-energy Behind-the-limb Solar Flares

We report on the Fermi-LAT detection of high-energy emission from the behind-the-limb (BTL) solar flares that occurred on 2013 October 11, and 2014 January 6 and September 1. The Fermi-LAT observations are associated with flares from active regions originating behind both the eastern and western limbs, as determined by STEREO. All three flares are associated with very fast coronal mass ejections (CMEs) and strong solar energetic particle events. We present updated localizations of the >100 MeV photon emission, hard X-ray (HXR) and EUV images, and broadband spectra from 10 keV to 10 GeV, as well as microwave spectra. We also provide a comparison of the BTL flares detected by Fermi-LAT with three on-disk flares and present a study of some of the significant quantities of these flares as an attempt to better understand the acceleration mechanisms at work during these occulted flares. We interpret the HXR emission to be due to electron bremsstrahlung from a coronal thin-target loop top with the accelerated electron spectra steepening at semirelativistic energies. The >100 MeV gamma-rays are best described by a pion-decay model resulting from the interaction of protons (and other ions) in a thick-target photospheric source. The protons are believed to have been accelerated (to energies >10 GeV) in the CME environment and precipitate down to the photosphere from the downstream side of the CME shock and landed on the front side of the Sun, away from the original flare site and the HXR emission.

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 >99.7%. 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.

First Nustar Observations of the Bl Lac-Type Blazar Pks 2155-304: Constraints on the Jet Content and Distribution of Radiating Particles

We report the first hard X-ray observations with NuSTAR of the BL Lac type blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and gamma-ray data from the Fermi Large Area Telescope, obtained in April 2013 when the source was in a very low flux state. A joint NuSTAR and XMM spectrum, covering the energy range 0.5 - 60 keV, is best described by a model consisting of a log-parabola component with curvature beta = 0.3(+0.2,-0.1) and a (local) photon index 3.04 +/- 0.15 at photon energy of 2 keV, and a hard power-law tail with photon index 2.2 +/- 0.4. The hard X-ray tail can be smoothly joined to the quasi-simultaneous gamma-ray spectrum by a synchrotron self-Compton component produced by an electron distribution with index p = 2.2. Assuming that the power-law electron distribution extends down to the minimum electron Lorentz factor gamma_min = 1 and that there is one proton per electron, an unrealistically high total jet power L_p of roughly 10^47 erg/s is inferred. This can be reduced by two orders of magnitude either by considering a significant presence of electron-positron pairs with lepton-to-proton ratio of at least 30, or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor gamma_br1 of roughly 100. In either case, the jet composition is expected to be strongly matter-dominated.

Multifrequency Variability and Correlations from Extensive Observing Campaigns of Mkn 421 and Mkn 501 in 2009

We are performing an unprecedentedly long and dense monitoring of the multi-frequency (radio to TeV) emission from the classical TeV blazars Mkn 421 and Mkn 501. These objects are among the brightest X-ray/TeV blazars in the sky and among the few sources whose spectral energy distributions (SED) can be completely characterised by the current instruments. This is a multi-year and multi-instrument programme which includes the participation of VLBA, Swift, RXTE, MAGIC, VERITAS, Whipple, the Fermi/LAT Gamma-ray Observatory, GASP-WEBT, among other collaborations and instruments which combined provide the most detailed temporal and energy coverage of these sources to date. In this proceedings we will focus mostly on the results obtained with the multi-frequency data from 2009, for which the SEDs of Mkn 421 and Mkn 501 are very similar and can be described by a one-zone synchrotron self-Compton scenario. We will report on the multi-frequency variability derived from these data.

Detailed Multifrequency Study of a Rapid VHE Flare of Mrk501 in May 2009

We present observations of the gamma-ray blazar Markarian 501 between April 17 and May 5, 2009, with the Whipple 10-m telescope, VERITAS, the Fermi Large Area Telescope (LAT), Swift and RXTE as part of a 4.5-month multi-wavelength campaign. The presentation will focus on the strong very high energy (VHE) gamma-ray activity detected on May 1st with Whipple and VERITAS, when the measured flux (E > 400 GeV) reached five times the flux of the Crab Nebula, coincident with an increase in the optical polarization by a factor of 5, and a rotation of the polarization angle by 15 degrees. We also show that, during this 3-week period, the largest flux and spectral variability is seen at the highest energies of the broadband spectral energy distribution.

MAGIC discovery of the BL Lac 1ES 1727+502: Multiwavelength observations, spectral behavior and variability

The MAGIC experiment is a system of two Imaging Atmospheric Cherenkov Telescopes located in the Canary Island of La Palma (Northern hemisphere). It has an energy threshold of 50 GeV, the lowest among the currently operating Cherenkov telescopes, which makes it particularly suitable for the observation of extragalactic sources at Very High Energies (VHE, E>100 GeV). MAGIC has detected numerous blazars, which are active galactic nuclei whose jet axis is pointed towards the observer. Here we present one of our latest detections, the BL Lac 1ES 1727+502, located at redshift z=0.055. The source was a promising TeV candidate based on archival data and the observation that leads this detection was not triggered by any high state alert in other wavebands. We complemented our data with multiwavelength observations: optical data from the KVA telescope, UV, optical and X-ray data taken with the instruments on board the Swift satellite and High Energy (HE, 100; MeV < E < 100; GeV) data from the Fermi-LAT (Large Area ...

Multi-instrument variability study of the classical TeV objects Mrk 421 and Mrk 501

The BL Lac objects Mrk 421 and Mrk 501 are objects of a large multi-year multiinstrument campaign including VLBA, F-GAMMA, GASP-WEBT, Swift, RXTE, Fermi-LAT, MAGIC and VERITAS, among other instruments and collaborations. This extensive radio to TeV gamma-ray dataset provides an unprecedented temporal and energy coverage, which allows for detailed studies of the evolution of their broad-band spectral energy distribution that cannot be achieved on any other BL Lac object. We report on the temporal variability and spectral correlations from the 4.5 months-long campaigns of 2009. We show that Mrk 421 and Mrk 501 have differences in their broad band variability, suggesting different mechanisms for the production of the high-energy emission. In addition, we also report on two events of high activity in very high energies (VHE) of Mrk 501 during May 2009, one of which was not accompanied by enhanced optical/X-ray activity but showed a clear correlation with an increase in the polarized optical flux and a rotatio...