Browsing by Author "D'Ammando, F."
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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 Fermi LAT Detection of a New Gamma-ray Source 4C +50.11(The Astronomer's Telegram, 2014-01-31) Carpenter, B.; Ojha, R.; D'Ammando, F.; Orienti, M.; Cheung, C. C.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 Neutrinos, Cosmic Rays and the MeV Band(2019-03-13) Ojha, R.; Zhang, H.; Kadler, M.; Neilson, N. K.; Kreter, M.; McEnery, J.; Buson, S.; Caputo, R.; Coppi, P.; D'Ammando, F.; Angelis, A. De; Fang, K.; Giannios, D.; Guiriec, S.; Guo, F.; Kopp, J.; Krauss, F.; Li, H.; Meyer, M.; Moiseev, A.; Petropoulou, M.; Prescod-Weinstein, C.; Rani, B.; Shrader, C.; Venters, T.; Wadiasingh, Z.The possible association of the blazar TXS 0506+056 with a high-energy neutrino detected by IceCube holds the tantalizing potential to answer three astrophysical questions: 1. Where do high-energy neutrinos originate? 2. Where are cosmic rays produced and accelerated? 3. What radiation mechanisms produce the high-energy gamma-rays in blazars? The MeV gamma-ray band holds the key to these questions, because it is an excellent proxy for photo-hadronic processes in blazar jets, which also produce neutrino counterparts. Variability in MeV gamma-rays sheds light on the physical conditions and mechanisms that take place in the particle acceleration sites in blazar jets. In addition, hadronic blazar models also predict a high level of polarization fraction in the MeV band, which can unambiguously distinguish the radiation mechanism. Future MeV missions with a large field of view, high sensitivity, and polarization capabilities will play a central role in multi-messenger astronomy, since pointed, high-resolution telescopes will follow neutrino alerts only when triggered by an all-sky instrument.Item PKS 2123−463: a confirmed γ-ray blazar at high redshift(Oxford University Press, 2012-11-21) D'Ammando, F.; Rau, A.; Schady, P.; Finke, J.; Orienti, M.; Greiner, J.; Kann, D. A.; Ojha, R.; Foley, A. R.; Stevens, J.; Blanchard, J. M.; Edwards, P. G.; Kadler, M.; Lovell, J. E. J.The flat spectrum radio quasar (FSRQ) PKS 2123−463 was associated in the first Fermi-Large Area Telescope (LAT) source catalogue with the γ-ray source 1FGL J2126.1−4603, but when considering the full first two years of Fermi observations, no γ-ray source at a position consistent with this FSRQ was detected, and thus PKS 2123−463 was not reported in the second Fermi-LAT source catalogue. On 2011 December 14 a γ-ray source positionally consistent with PKS 2123−463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, Gamma-ray Optical/Near-Infrared Detector (GROND), Australia Telescope Compact Array (ATCA), Ceduna and Seven Dishes Karoo Array Telescope (KAT-7) observatories. Results of the localization of the γ-ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, ultraviolet (UV) and X-ray data collected soon after the γ-ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and γ-ray bands led to a firm identification of the γ-ray source with PKS 2123−463. A new photometric redshift has been estimated as z = 1.46 ± 0.05 using GROND and Swift Ultraviolet/Optical Telescope (UVOT) observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67. We fit the broad-band spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disc component is necessary to explain the optical/UV emission detected by Swift/UVOT. This disc has a luminosity of ∼1.8 × 10⁴⁶ erg s⁻¹, and a fit to the disc emission assuming a Schwarzschild (i.e. non-rotating) black hole gives a mass of ∼2 × 10⁹ M⊙. This is the first black hole mass estimate for this source.Item Radio follow-up of the γ-ray flaring gravitational lens JVAS B0218+357(Oxford University Press, 2016-02-10) Spingola, C.; Dallacasa, D.; Orienti, M.; Giroletti, M.; J. P. McKean; Cheung, C. C.; Hovatta, T.; Ciprini, S.; D'Ammando, F.; Falco, E.; Larsson, S.; Max-Moerbeck, W.; Ojha, R.; Readhead, A. C. S.; Richards, J. L.; Scargle, J.We present results on multifrequency Very Long Baseline Array (VLBA) monitoring observations of the double-image gravitationally lensed blazar JVAS B0218+357. Multi-epoch observations started less than one month after the γ-ray flare detected in 2012 by the Large Area Telescope on board Fermi, and spanned a 2-month interval. The radio light curves did not reveal any significant flux density variability, suggesting that no clear correlation between the high-energy and low-energy emission is present. This behaviour was confirmed also by the long-term Owens Valley Radio Observatory monitoring data at 15 GHz. The milliarcsecond-scale resolution provided by the VLBA observations allowed us to resolve the two images of the lensed blazar, which have a core-jet structure. No significant morphological variation is found by the analysis of the multi-epoch data, suggesting that the region responsible for the γ-ray variability is located in the core of the active galactic nuclei, which is opaque up to the highest observing frequency of 22 GHz.Item Radio–gamma-ray connection and spectral evolution in 4C +49.22 (S4 1150+49): the Fermi, Swift and Planck view(Oxford University Press, 2014-11-07) Cutini, S.; Ciprini, S.; Orienti, M.; Tramacere, A.; D'Ammando, F.; Verrecchia, F.; Polenta, G.; Carrasco, L.; D'Elia, V.; Giommi, P.; González-Nuevo, J.; Grandi, P.; Harrison, D.; Hays, E.; Larsson, S.; Lähteenmäki, A.; León-Tavares, J.; López-Caniego, M.; Natoli, P.; Ojha, R.; Partridge, B.; Porras, A.; Reyes, L.; Recillas, E.; Torresi, E.The Large Area Telescope on board the Fermi Gamma-ray Space Telescope detected a strong γ-ray flare on 2011 May 15 from a source identified as 4C +49.22, a flat spectrum radio quasar (FSRQ) also known as S4 1150+49. This blazar, characterized by a prominent radio–optical–X-ray jet, was in a low γ-ray activity state during the first years of Fermi observations. Simultaneous observations during the quiescent, outburst and post-flare γ-ray states were obtained by Swift, Planck and optical–IR–radio telescopes (Instituto Nacional de Astrofísica, Óptica y Electrónica, Catalina Sky Survey, Very Long Baseline Array [VLBA], Metsähovi). The flare is observed from microwave to X-ray bands with correlated variability and the Fermi, Swift and Planck data for this FSRQ show some features more typical of BL Lac objects, like the synchrotron peak in the optical band that outshines the thermal blue-bump emission, and the X-ray spectral softening. Multi-epoch VLBA observations show the ejection of a new component close in time with the GeV γ-ray flare. The radio-to-γ-ray spectral energy distribution is modelled and fitted successfully for the outburst and the post-flare epochs using either a single flaring blob with two emission processes (synchrotron self-Compton (SSC), and external-radiation Compton), and a two-zone model with SSC-only mechanism.Item Swift follow-up of the flaring NLSy1 PKS 2004-447(2019-10-28) D'Ammando, F.; Gokus, A.; Kadler, M.; Ojha, R.; Fermi Large Area Telescope CollaborationItem 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.