Browsing by Author "Thompson, D. J."
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Item FERMI LARGE AREA TELESCOPE DETECTION OF GRAVITATIONAL LENS DELAYED γ-RAY FLARES FROM BLAZAR B0218+357(IOP, 2014-01-30) Cheung, C. C.; Larsson, S.; Scargle, J. D.; Amin, M. A.; Blandford, R. D.; Bulmash, D.; Chiang, J.; Ciprini, S.; Corbet, R. H. D.; Falco, E. E.; Marshall, P. J.; Wood, D. L.; Ajello, M.; Bastieri, D.; Chekhtman, A.; D'Ammando, F.; Giroletti, M.; Grove, J. E.; Lott, B.; Ojha, R.; Orienti, M.; Perkins, J. S.; Razzano, M.; Smith, A. W.; Thompson, D. J.; Wood, K. S.Using data from the Fermi Large Area Telescope (LAT), we report the first clear γ-ray measurement of a delay between flares from the gravitationally lensed images of a blazar. The delay was detected in B0218+357, a known double-image lensed system, during a period of enhanced γ-ray activity with peak fluxes consistently observed to reach >20-50 × its previous average flux. An auto-correlation function analysis identified a delay in the γ-ray data of 11.46 ± 0.16 days (1σ) that is ~1 day greater than previous radio measurements. Considering that it is beyond the capabilities of the LAT to spatially resolve the two images, we nevertheless decomposed individual sequences of superposing γ-ray flares/delayed emissions. In three such ~8-10 day-long sequences within a ~4 month span, considering confusion due to overlapping flaring emission and flux measurement uncertainties, we found flux ratios consistent with ~1, thus systematically smaller than those from radio observations. During the first, best-defined flare, the delayed emission was detailed with a Fermi pointing, and we observed flux doubling timescales of ~3-6 hr implying as well extremely compact γ-ray emitting regions.Item Gamma-Ray Blazars within the First 2 Billion Years(IOP, 2017-02-27) Ojha, R.; Ferrara, E. C.; Hays, E.; Perkins, J. S.; Rani, B.; Thompson, D. J.The detection of high-redshift (z > 3) blazars enables the study of the evolution of the most luminous relativistic jets over cosmic time. More importantly, high-redshift blazars tend to host massive black holes and can be used to constrain the space density of heavy black holes in the early universe. Here, we report the first detection with the Fermi-Large Area Telescope of five γ-ray-emitting blazars beyond z = 3.1, more distant than any blazars previously detected in γ-rays. Among these five objects, NVSS J151002+570243 is now the most distant known γ-ray-emitting blazar at z = 4.31. These objects have steeply falling γ-ray spectral energy distributions (SEDs), and those that have been observed in X-rays have a very hard X-ray spectrum, both typical of powerful blazars. Their Compton dominance (ratio of the inverse Compton to synchrotron peak luminosities) is also very large (> 20). All of these properties place these objects among the most extreme members of the blazar population. Their optical spectra and the modeling of their optical-UV SEDs confirm that these objects harbor massive black holes (MBH ~10⁸⁻¹⁰,Mͼ). We find that, at z = 4, the space density of > 10⁹ Mͼ black holes hosted in radio-loud and radio-quiet active galactic nuclei are similar, implying that radio-loudness may play a key role in rapid black hole growth in the early universe.Item High-Energy Polarimetry - a new window to probe extreme physics in AGN jets(2019-03-11) Rani, B.; Zhang, H.; Hunter, S. D.; Kislat, F.; Böttcher, M.; McEnery, J. E.; Thompson, D. J.; Giannios, D.; Guo, F.; Li, H.; Baring, M.; Agudo, I.; Buson, S.; Petropoulou, M.; Pavlidou, V.; Angelakis, E.; Myserlis, I.; Wadiasingh, Z.; Silva, R. M. Curado da; Kilian, P.; Guiriec, S.; Bozhilov, V. V.; Hodgson, J.; Antón, S.; Kazanas, D.; Coppi, P.; Venters, T.; Longo, F.; Bottacini, E.; Ojha, R.; Zhang, B.; Ciprini, S.; Moiseev, A.; Shrader, C.The constantly improving sensitivity of ground-based and space-borne observatories has made possible the detection of high-energy emission (X-rays and gamma-rays) from several thousands of extragalactic sources. Enormous progress has been made in measuring the continuum flux enabling us to perform imaging, spectral and timing studies. An important remaining challenge for high-energy astronomy is measuring polarization. The capability to measure polarization is being realized currently at X-ray energies (e.g. with IXPE), and sensitive gamma-ray telescopes capable of measuring polarization, such as AMEGO, AdEPT, e-ASTROGAM, etc., are being developed. These future gamma-ray telescopes will probe the radiation mechanisms and magnetic fields of relativistic jets from active galactic nuclei at spatial scales much smaller than the angular resolution achieved with continuum observations of the instrument. In this white paper, we discuss the scientific potentials of high-energy polarimetry, especially gamma-ray polarimetry, including the theoretical implications, and observational technology advances being made. In particular, we will explore the primary scientific opportunities and wealth of information expected from synergy of multi-wavelength polarimetry that will be brought to multi-messenger astronomy.Item HUNTING FOR TREASURES AMONG THE FERMI UNASSOCIATED SOURCES: A MULTIWAVELENGTH APPROACH(IOP, 2013-12-20) Acero, F.; Donato, D.; Ojha, R.; Stevens, J.; Edwards, P. G.; Ferrara, E.; J. Blanchard, J.; Lovell, J. E. J.; Thompson, D. J.The Fermi Gamma-Ray Space Telescope has been detecting a wealth of sources where the multiwavelength counterpart is either inconclusive or missing altogether. We present a combination of factors that can be used to identify multiwavelength counterparts to these Fermi unassociated sources. This approach was used to select and investigate seven bright, high-latitude unassociated sources with radio, UV, X-ray, and γ-ray observations. As a result, four of these sources are candidates to be active galactic nuclei, and one to be a pulsar, while two do not fit easily into these known categories of sources. The latter pair of extraordinary sources might reveal a new category subclass or a new type of γ-ray emitter. These results altogether demonstrate the power of a multiwavelength approach to illuminate the nature of unassociated Fermi sources.Item Multi-Physics of AGN Jets in the Multi-Messenger Era(2019-03-11) Rani, B.; Petropoulou, M.; Zhang, H.; D'Ammando, F.; Finke, J.; Baring, M.; Böttcher, M.; Dimitrakoudis, S.; Gan, Z.; Giannios, D.; Hartmann, D. H.; Krichbaum, T. P.; Marscher, A. P.; Mastichiadis, A.; Nalewajko, K.; Ojha, R.; Paneque, D.; Shrader, C.; Sironi, L.; Tchekhovskoy, A.; Thompson, D. J.; Vlahakis, N.; Venters, T. M.Active galactic nuclei (AGN) with relativistic jets, powered by gas accretion onto their central supermassive black hole (SMBH), are unique laboratories for studying the physics of matter and elementary particles in extreme conditions that cannot be realized on Earth. For a long time since the discovery of AGN, photons were the only way to probe the underlying physical processes. The recent discovery of a very high energy neutrino, IceCube-170922A, coincident with a flaring blazar, TXS 0506+056, provides the first evidence that AGN jets are multi-messenger sources; they are capable of accelerating hadrons to very high energies, while producing non-thermal EM radiation and high-energy neutrinos. This new era of multi-messenger astronomy, which will mature in the next decade, offers us the unprecedented opportunity to combine more than one messenger to solve some long-standing puzzles of AGN jet physics: How do jets dissipate their energy to accelerate particles? What is the jet total kinetic power? Where and how do jets produce high-energy emission and neutrinos? What physical mechanisms drive the particle acceleration?Item Radio and gamma-ray properties of extragalactic jets from the TANAMI sample(EDP sciences, 2016-05-04) Böck, M.; Kadler, M.; Müller, C.; Tosti, G.; Bastieri, D.; Burnett, T.; Carpenter, B.; Cavazzuti, E.; Dutka, M.; Blanchard, J.; Edwards, P. G.; Hase, H.; Horiuchi, S.; Jauncey, D. L.; Krauß, F.; Lister, M. L.; Lovell, J. E. J.; Lott, B.; Murphy, D. W.; Phillips, C.; Plötz, C.; Pursimo, T.; Quick, J.; Ros, E.; Taylor, G.; Thompson, D. J.; Tingay, S. J.; Tzioumis, A.; Zensus, J. A.The TANAMI program has been observing parsec-scale radio jets of southern (declination south of − 30°) γ-ray bright AGN, simultaneously with Fermi/LAT monitoring of their γ-ray emission, via high-resolution radio imaging with Very Long Baseline Interferometry techniques. We present the radio and γ-rayproperties of the TANAMI sources based on one year of contemporaneous TANAMI and Fermi/LAT data. A large fraction (72%) of the TANAMI sample can be associated with bright γ-ray sources for this time range. Association rates differ for different optical classes with all BL Lacs, 76% of quasars, and just 17% of galaxies detected by the LAT. Upper limits were established on the γ-ray flux from TANAMI sources not detected by LAT. This analysis led to the identification of three new Fermi sources whose detection was later confirmed. The γ-ray and radio luminosities are related by Lγ ∝ Lᵣ⁰·⁸⁹±⁰·⁰⁴. The brightness temperatures of the radio cores increase with the average γ-ray luminosity and the presence of brightness temperatures above the inverse Compton limit implies strong Doppler boosting in those sources. The undetected sources have lower γ/radio luminosity ratios and lower contemporaneous brightness temperatures. Unless the Fermi/LAT-undetected blazars are much γ-ray-fainter than the Fermi/LAT-detected sources, their γ-ray luminosity should not be significantly lower than the upper limits calculated here.Item TANAMI - Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry(2010-01-18) Muller, Cornelia; Kadler, Matthias; Ojha, Roopesh; Bock, M.; Booth, R.; Dutka, M. S.; Edwards, P. G.; Fey, A. L.; Fuhrmann, L.; Hase, H.; Horiuchi, S.; Hungwe, F.; Ros, E.; Taylor, G. B.; Thompson, D. J.; Tingay, S. J.; Tosti, G.; Tzioumis, A. K.; Wilms, J.; Zensus, J. A.We present a summary of the observation strategy of TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry), a monitoring program to study the parsec-scale structure and dynamics of relativistic jets in active galactic nuclei (AGN) of the Southern Hemisphere with the Australian Long Baseline Array (LBA) and the trans-oceanic antennas Hartebeesthoek, TIGO, and O'Higgins. TANAMI is focusing on extragalactic sources south of -30 degrees declination with observations at 8.4 GHz and 22 GHz every ~2 months at milliarcsecond resolution. The initial TANAMI sample of 43 sources has been defined before the launch of the Fermi Gamma Ray Space Telescope to include the most promising candidates for bright gamma-ray emission to be detected with its Large Area Telescope (LAT). Since November 2008, we have been adding new sources to the sample, which now includes all known radio- and gamma-ray bright AGN of the Southern Hemisphere. The combination of VLBI and gamma-ray observations is crucial to understand the broadband emission characteristics of AGN and the nature of relativistic jets.Item TANAMI blazars in the IceCube PeV-neutrino fields(EDP sciences, 2014-06-24) Krauß, F.; Kadler, M.; Mannheim, K.; Schulz, R.; Trüstedt, J.; Wilms, J.; Ojha, R.; Ros, E.; Anton, G.; Baumgartner, W.; Beuchert, T.; Blanchard, J.; Bürkel, C.; Carpenter, B.; Eberl, T.; Edwards, P. G.; Eisenacher, D.; Elsässer, D.; Fehn, K.; Fritsch, U.; Gehrels, N.; Gräfe, C.; Grossberger, C.; Hase, H.; Horiuchi, S.; James, C.; Kappes, A.; Katz, U.; Kreikenbohm, A.; Kreykenbohm, I.; Langejahn, M.; Leiter, K.; Litzinger, E.; Lovell, J. E. J.; Müller, C.; Phillips, C.; Plötz, C.; Quick, J.; Steinbring, T.; Stevens, J.; Thompson, D. J.; Tzioumis, A. K.The IceCube Collaboration has announced the discovery of a neutrino flux in excess of the atmospheric background. Owing to the steeply falling atmospheric background spectrum, events at PeV energies most likely have an extraterrestrial origin. We present the multiwavelength properties of the six radio-brightest blazars that are positionally coincident with these events using contemporaneous data of the TANAMI blazar sample, including high-resolution images and spectral energy distributions. Assuming the X-ray to γ-ray emission originates in the photoproduction of pions by accelerated protons, the integrated predicted neutrino luminosity of these sources is high enough to explain the two detected PeV events.Item TANAMI: Milliarcsecond Resolution Observations of Extragalactic Gamma-ray Sources(arXiv, 2009-12-31) Ojha, Roopesh; Kadler, M.; Böck, M.; Booth, R.; Dutka, M. S.; Edwards, P. G.; Fey, A. L.; Fuhrmann, L.; Gaume, R. A.; Hase, H.; Horiuchi, S.; Jauncey, D.L.; Johnston, K. J.; Katz, U.; Lister, M.; Lovell, J. E. J.; Müller, C.; Plötz, C.; Quick, J. F. H.; Ros, E.; Taylor, G. B.; Thompson, D. J.; Tingay, S. J.; Tosti, G.; Tzioumis, A. K.; Wilms, J.; Zensus, J. A.The TANAMI (Tracking AGN with Austral Milliarcsecond Interferometry) and associated programs provide comprehensive radio monitoring of extragalactic gamma-ray sources south of declination -30 degrees. Joint quasi-simultaneous observations between the Fermi Gamma-ray Space Telescope and ground based observatories allow us to discriminate between competing theoretical blazar emission models. High resolution VLBI observations are the only way to spatially resolve the sub-parsec level emission regions where the high-energy radiation originates. The gap from radio to gamma-ray energies is spanned with near simultaneous data from the Swift satellite and ground based optical observatories. We present early results from the TANAMI program in the context of this panchromatic suite of observations.Item TANAMI: tracking active galactic nuclei with austral milliarcsecond interferometry(EDP sciences, 2010-09-10) Ojha, R.; Kadler, M.; Böck, M.; Booth, R.; Dutka, M. S.; Edwards, P. G.; Fey, A. L.; Fuhrmann, L.; Gaume, R. A.; Hase, H.; Horiuchi, S.; Jauncey, D. L.; Johnston, K. J.; Katz, U.; Lister, M.; Lovell, J. E. J.; Müller, C.; Plötz, C.; Quick, J. F. H.; Ros, E.; Taylor, G. B.; Thompson, D. J.; Tingay, S. J.; Tosti, G.; Tzioumis, A. K.; Wilms, J.; Zensus, J. A.Context. A number of theoretical models vie to explain the γ-ray emission from active galactic nuclei (AGN). This was a key discovery of EGRET. With its broader energy coverage, higher resolution, wider field of view and greater sensitivity, the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope is dramatically increasing our knowledge of AGN γ-ray emission. However, discriminating between competing theoretical models requires quasi-simultaneous observations across the electromagnetic spectrum. By resolving the powerful parsec-scale relativistic outflows in extragalactic jets and thereby allowing us to measure critical physical properties, Very Long Baseline Interferometry observations are crucial to understanding the physics of extragalactic γ-ray objects. Aims. We introduce the TANAMI program (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) which is monitoring an initial sample of 43 extragalactic jets located south of -30 degrees declination at 8.4 GHz and 22 GHz since 2007. All aspects of the program are discussed. First epoch results at 8.4 GHz are presented along with physical parameters derived therefrom. Methods. These observations were made during 2007/2008 using the telescopes of the Australian Long Baseline Array in conjunction with Hartebeesthoek in South Africa. These data were correlated at the Swinburne University correlator. Results. We present first epoch images for 43 sources, some observed for the first time at milliarcsecond resolution. Parameters of these images as well as physical parameters derived from them are also presented and discussed. These and subsequent images from the TANAMI survey are available at http://pulsar.sternwarte.uni-erlangen.de/tanami/. Conclusions. We obtain reliable, high dynamic range images of the southern hemisphere AGN. All the quasars and BL Lac objects in the sample have a single-sided radio morphology. Galaxies are either double-sided, single-sided or irregular. About 28% of the TANAMI sample has been detected by LAT during its first three months of operations. Initial analysis suggests that when galaxies are excluded, sources detected by LAT have larger opening angles than those not detected by LAT. Brightness temperatures of LAT detections and non-detections seem to have similar distributions. The redshift distributions of the TANAMI sample and sub-samples are similar to those seen for the bright γ-ray AGN seen by LAT and EGRET but none of the sources with a redshift above 1.8 have been detected by LAT.Item TANAMI: Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry⋆(EDP sciences, 2018-02-07) Müller, C.; Kadler, M.; Ojha, R.; Schulz, R.; Trüstedt, J.; Edwards, P. G.; Ros, E.; Carpenter, B.; Angioni, R.; Blanchard, J.; Böck, M.; Burd, P. R.; Dörr, M.; Dutka, M. S.; Eberl, T.; Gulyaev, S.; Hase, H.; Horiuchi, S.; Katz, U.; Krauß, F.; Lovell, J. E. J.; Natusch, T.; Nesci, R.; Phillips, C.; Plötz, C.; Pursimo, T.; Quick, J. F. H.; Stevens, J.; Thompson, D. J.; Tingay, S. J.; Tzioumis, A. K.; Weston, S.; Wilms, J.; Zensus, J. A.Context. TANAMI is a multiwavelength program monitoring active galactic nuclei (AGN) south of − 30° declination including high-resolution very long baseline interferometry (VLBI) imaging, radio, optical/UV, X-ray, and γ-ray studies. We have previously published first-epoch8.4 GHz VLBI images of the parsec-scale structure of the initial sample. In this paper, we present images of 39 additional sources. The full sample comprises most of the radio- and γ-ray brightest AGN in the southern quarter of the sky, overlapping with the region from which high-energy (> 100 TeV) neutrino events have been found. Aims. We characterize the parsec-scale radio properties of the jets and compare them with the quasi-simultaneous Fermi/LAT γ-ray data. Furthermore, we study the jet properties of sources which are in positional coincidence with high-energy neutrino events compared to the full sample. We test the positional agreement of high-energy neutrino events with various AGN samples. Methods. TANAMI VLBI observations at 8.4 GHz are made with southern hemisphere radio telescopes located in Australia, Antarctica, Chile, New Zealand, and South Africa. Results. Our observations yield the first images of many jets below − 30° declination at milliarcsecond resolution. We find that γ-ray loud TANAMI sources tend to be more compact on parsec-scales and have higher core brightness temperatures than γ-ray faint jets, indicating higher Doppler factors. No significant structural difference is found between sources in positional coincidence with high-energy neutrino events and other TANAMI jets. The 22 γ-ray brightest AGN in the TANAMI sky show only a weak positional agreement with high-energy neutrinos demonstrating that the > 100 TeV IceCube signal is not simply dominated by a small number of the γ-ray brightest blazars. Instead, a larger number of sources have to contribute to the signal with each individual source having only a small Poisson probability for producing an event in multi-year integrations of current neutrino detectors.Item Unusual flaring activity in the blazar PKS 1424−418 during 2008−2011(EDP sciences, 2014-09-16) Buson, S.; Longo, F.; Larsson, S.; Cutini, S.; Finke, J.; Ciprini, S.; Ojha, R.; D'Ammando, F.; Donato, D.; Thompson, D. J.; Desiante, R.; Bastieri, D.; Wagner, S.; Hauser, M.; Fuhrmann, L.; Dutka, M.; Müller, C.; Kadler, M.; Angelakis, E.; Zensus, J. A.; Stevens, J.; Blanchard, J. M.; Edwards, P. G.; Lovell, J. E. J.; Gurwell, M. A.; Wehrle, A. E.; Zook, A.Context. Blazars are a subset of active galactic nuclei (AGN) with jets that are oriented along our line of sight. Variability and spectral energy distribution (SED) studies are crucial tools for understanding the physical processes responsible for observed AGN emission. Aims. We report peculiar behavior in the bright γ-ray blazar PKS 1424−418 and use its strong variability to reveal information about the particle acceleration and interactions in the jet. Methods. Correlation analysis of the extensive optical coverage by the ATOM telescope and nearly continuous γ-ray coverage by the Fermi Large Area Telescope is combined with broadband, time-dependent modeling of the SED incorporating supplemental information from radio and X-ray observations of this blazar. Results. We analyse in detail four bright phases at optical-GeV energies. These flares of PKS 1424−418 show high correlation between these energy ranges, with the exception of one large optical flare that coincides with relatively low γ-ray activity. Although the optical/γ-ray behavior of PKS 1424−418 shows variety, the multiwavelength modeling indicates that these differences can largely be explained by changes in the flux and energy spectrum of the electrons in the jet that are radiating. We find that for all flares the SED is adequately represented by a leptonic model that includes inverse Compton emission from external radiation fields with similar parameters. Conclusions. Detailed studies of individual blazars like PKS 1424−418 during periods of enhanced activity in different wavebands are helping us identify underlying patterns in the physical parameters in this class of AGN.