Article

Radio Imaging of the Very-High-Energy -Ray Emission Region in the Central Engine of a Radio Galaxy

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Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA.
(Impact Factor: 33.61). 08/2009; 325(5939):444-8. DOI: 10.1126/science.1175406
Source: PubMed

ABSTRACT

The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 10(12) electron volts and are bright sources of very-high-energy (VHE) gamma-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE gamma-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole.

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Available from: I. Oya, Jul 26, 2014
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• "Variability is also seen in objects where jets are not closely aligned with the line-of-sight to the observer, with the day-timescale TeV variability of M 87 (Aharonian et al. 2006) as the best studied case. Simultaneous VLBI and TeV observations of M 87 indicate a strong increase in flux from the nucleus during VHE flares (Acciari et al. 2009), suggesting particle acceleration is taking place very close to the central supermassive black hole. "
Article: High-energy emission from transients
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ABSTRACT: Cosmic explosions dissipate energy into their surroundings on a very wide range of time scales: producing shock waves and associated particle acceleration. The historical culprits for the acceleration of the bulk of Galactic cosmic rays are supernova remnants: explosions on approximately 10(4) year time scales. Increasingly, however, time-variable emission points to rapid and efficient particle acceleration in a range of different astrophysical systems. Gamma-ray bursts have the shortest time scales, with inferred bulk Lorentz factors of approximately 1000 and photons emitted beyond 100 GeV, but active galaxies, pulsar wind nebulae and colliding stellar winds are all now associated with time-variable emission at approximately teraelectron volt energies. Cosmic photons and neutrinos at these energies offer a powerful probe of the underlying physical mechanisms of cosmic explosions, and a tool for exploring fundamental physics with these systems. Here, we discuss the motivations for high-energy observations of transients, the current experimental situation, and the prospects for the next decade, with particular reference to the major next-generation high-energy observatory, the Cherenkov Telescope Array.
Preview · Article · Jun 2013 · Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences
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• "Theories involving particle acceleration in the jet usually place the emission region between 30 and a few thousand R Sch from the black hole, with R Sch the Schwarzschild radius (R Sch = 3×10 14 cm for a 10 9 M ¤ black hole). The observations of exceptionally strong γ-ray flares from M87 in 2008 observed by H.E.S.S., MAGIC and VERITAS in temporal coincidence with an exceptionally strong VLBA radio flare from the M87 radio core were interpreted as evidence for an origin of the γray flares within a projected distance of 50 R Sch from the central engine [31]. Unfortunately, a similar γ-flare in 2011 was not accompanied by a comparable radio flare and did not corroborate the association. "
Article: Science Drivers for AGN Observations with the Cherenkov Telescope Array
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ABSTRACT: The current generation of Imaging Atmospheric Cherenkov Telescopes (IACTs), including the H.E.S.S., MAGIC, and VERITAS telescope arrays, have made substantial contributions to our knowledge about the structure and composition of the highly relativistic jets from Active Galactic Nuclei (AGNs). In this paper, we discuss some of the outstanding scientific questions and give a qualitative overview of AGN related science topics which will be explored with the next-generation Cherenkov Telescope Array (CTA). CTA is expected to further constrain the structure and make-up of jets, and thus, to constrain models of jet formation, acceleration, and collimation. Furthermore, being the brightest well-established extragalactic sources of TeV {\gamma}- rays, AGNs can be used to probe the EBL, intergalactic magnetic fields, and the validity of the Lorentz Invariance principle at high photon energies.
Preview · Article · May 2012
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• "– 4 – M87 Cen A black hole mass m = M/M ⊙ (3.2 ± 0.9) · 10 9 (1) , (6.4 ± 0.5) · 10 9 (2) (4.5 +1.7 −1.0 ) · 10 7 , (5.5 ± 3.0) · 10 7 (3) Gravitational radius R G 4.72 · 10 14 cm= 1.53 · 10 −4 pc ( * ) , 7.375 · 10 12 cm = 2.390 · 10 −6 pc 9.44 · 10 14 cm= 3.06 · 10 −4 pc ( * * ) distance from Earth d (16.7 ± 0.2) Mpc (4) (3.8 ± 0.1) Mpc (5) inclination i ( 15 − 25) • (6) , ( 30 − 35) • (7) , ( 15 − 80) • (9) , ( 30 − 45) • (8) ( 50 − 80) • (10) (pc-scale) nuclear X-ray luminosity L X 7.0 · 10 40 erg·s −1 ((0.5 − 7)keV) (11) ∼ 5·10 41 erg·s −1 ((2−10)keV) (12) nuclear bolometric ∼ 10 42 erg s −1 (13) ∼ 10 43 erg s −1 (14) luminosity L bol ( * ) for m = 3.2 · 10 9 , ( * * ) for m = 6.4 · 10 9 , (1) Macchetto et al. (1997), (2) Gebhardt and Thomas (2009), (3) Neumayer et al. (2010), (4) Mei et al. (2007), (5) Harris et al. (2009) , (6) Acciari et al. (2009),(7) Bicknell and Begelman (1996), (8) Ly et al. (2007), (9) Aharonian et al. (2009) and references therein, (10)Tingay et al. (1998), (11) Di Matteo et al. (2003), (12)Evans et al. (2004), (13) Reynolds et al. (1996) , (14) Karovska et al. (2002) "
Article: Internal gammagamma Opacity in Active Galactic Nuclei and the Consequences for the TeV Observations of M87 and Cen A
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ABSTRACT: Low-luminosity active galactic nuclei (LLAGNs) possess the characteristic features of more luminous active galactic nuclei (AGNs) but exhibit a much lower nuclear Hα luminosity (L Hα < 1040 erg s–1) than their more luminous counterparts. M87 (NGC 4486) and Centaurus A (NGC 5128, Cen A) are well studied nearby LLAGNs. As an additional feature they show γ radiation up to TeV (1012 eV) energies, but the origin of this radiation has not been resolved. The coincident observation of a radio and TeV flare in M87 suggests that the TeV radiation is produced within around 50-100 gravitational radii of the central supermassive black hole, depending on the assumed value of the mass of the black hole. Strong radiation fields can be produced in the central region of an (LL)AGN, e.g., by the accretion flow around the black hole, the jet plasma, or stars closely orbiting the black hole. These radiation fields can lead to the absorption of emitted TeV photons, and, in fact, high optical depths of such fields can make TeV detection from inner regions impossible. In this paper, we consider the accretion flow around the black hole as the most prominent source for such a radiation field and we calculate accordingly the probability for absorption of TeV photons produced near the black holes in M87 and CenA assuming a low-luminosity Shakura-Sunyaev disk (SSD). We find that the results are very different between the two LLAGNs. While the inner region of M87 is transparent for TeV radiation up to ~20 TeV within the allowed parameter range, the optical depth in Cen A is 1, leading to an absorption of TeV photons that might be produced near the central black hole. These results imply either that the TeV γ production sites and processes are different for both sources or that LLAGN black holes do not accrete (at least only) in the form of a low-luminosity SSD.
Preview · Article · Aug 2011 · The Astrophysical Journal