355 research outputs found
Gamma ray and infrared emission from the M87 jet and torus
The existence of intrinsic obscuration of Fanaroff-Riley I objects is a
controversial topic. M87, the nearest such object, is puzzling in that although
it has very massive central black hole it has a relatively low luminosity,
suggesting it is in a dormant state. Despite of its proximity to us (16 Mpc) it
is not known with certainty whether or not M87 has a dusty torus.
Infrared observations indicate that if a torus exists in M87 it must have a
rather low infrared luminosity. Using arguments from unification theory of
active galactic nuclei, we have earlier suggested that the inner parsec-scale
region of M87 could still harbour a small torus sufficiently cold such that its
infrared emission is dwarfed by the jet emission. The infrared emission from
even a small cold torus could affect through photon-photon pair production
interactions the escape of 100 GeV to TeV energy gamma rays from the central
parsec of M87.
The TeV gamma-ray flux from the inner jet of M87 has recently been predicted
in the context of the Synchrotron Proton Blazar (SPB) model to extend up to at
least 100 GeV (Protheroe, Donea, Reimer, 2002). Subsequently, the detection of
gamma-rays above 730 GeV by the HEGRA Cherenkov telescopes has been reported.
We discuss the interactions of gamma-rays produced in the inner jet of M87 with
the weak infrared radiation expected from a possible dusty small-scale torus,
and show that the HEGRA detection shows that the temperature of any torus
surrounding the gamma-ray emission region must be cooler than about 250 K. We
suggest that if no gamma-rays are in future detected during extreme flaring
activity in M87 at other wavelength, this may be expected because of torus
heating.Comment: 7 pages, submitted to Prog. Theor. Phys. Suppl., ps fil
How relevant is the torus activity/geometry for the TeV gamma-rays emitted in the jet of M87 ?
Motivated by unification schemes of active galactic nuclei, we review
evidence for the existence of a small-scale dust torus in M87, a Fanaroff-Riley
Class I radio galaxy. Since there is no direct evidence of any thermal emission
from its torus we consider indirect evidence, such as BLR activity and ageing
arguments to model the cold dust structure of M87. In the context of the jet --
accretion disk -- torus symbiosis we discuss the interactions of GeV and TeV
gamma-rays produced in the jet of M87 with the infrared radiation fields
external to the jet, produced by a less active torus. A thin and cold torus
with less defined outer boundaries could still posses problems to some of the
TeV emission from the jet.Comment: 2 pages, to appear in Proceedings of the conference on Active
Galactic Nuclei: from Central Engine to Host Galaxy, meeting held in Meudon,
France, July 23-27, 2002, Eds.: S. Collin, F. Combes and I. Shlosma
Small-Angle Scattering and Diffusion: Application to Relativistic Shock Acceleration
We investigate ways of accurately simulating the propagation of energetic
charged particles over small times where the standard Monte Carlo approximation
to diffusive transport breaks down. We find that a small-angle scattering
procedure with appropriately chosen step-lengths and scattering angles gives
accurate results, and we apply this to the simulation of propagation upstream
in relativistic shock acceleration.Comment: 4 pages, 2 figures, proceedings of World Space Environment Forum
(WSEF2002) to appear in Space Science Reviews, accepte
TeV gamma rays and cosmic rays from the nucleus of M87, a mis-aligned BL Lac object
The unresolved nuclear region of M87 emits strong non-thermal emission from
radio to X-rays. Assuming this emission to originate in the pc scale jet
aligned at to the line of sight, we interpret this
emission in the context of the Synchrotron Proton Blazar (SPB) model. We find
the observed nuclear jet emission to be consistent with M87 being a mis-aligned
BL Lac Object and predict gamma-ray emission extending up to at least 100 GeV
at a level easily detectable by GLAST and MAGIC, and possibly by VERITAS
depending on whether it is high-frequency or low-frequency peaked. Predicted
neutrino emission is below the sensitivity of existing and planned neutrino
telescopes. Ultra-high energy neutrons produced in pion photoproduction
interactions decay into protons after escaping from the host galaxy. Because
energetic protons are deflected by the intergalactic magnetic field, the
protons from the decay of neutrons emitted in all directions, including along
the jet axis where the Doppler factor and hence emitted neutron energies are
higher, can contribute to the observed ultra-high energy cosmic rays. We
consider the propagation of these cosmic ray protons to Earth and conclude that
M87 could account for the observed flux if the extragalactic magnetic field
topology were favourable.Comment: 17 pages, 6 figures. 3 additional references plus minor changes,
acctepted for publication in Astroparticle Physic
Seismic Emissions from a Highly Impulsive M6.7 Solar Flare
On 10 March 2001 the active region NOAA 9368 produced an unusually impulsive
solar flare in close proximity to the solar limb. This flare has previously
been studied in great detail, with observations classifying it as a type 1
white-light flare with a very hard spectrum in hard X-rays. The flare was also
associated with a type II radio burst and coronal mass ejection. The flare
emission characteristics appeared to closely correspond with previous instances
of seismic emission from acoustically active flares. Using standard local
helioseismic methods, we identified the seismic signatures produced by the
flare that, to date, is the least energetic (in soft X-rays) of the flares
known to have generated a detectable acoustic transient. Holographic analysis
of the flare shows a compact acoustic source strongly correlated with the
impulsive hard X-ray, visible continuum, and radio emission. Time-distance
diagrams of the seismic waves emanating from the flare region also show faint
signatures, mainly in the eastern sector of the active region. The strong
spatial coincidence between the seismic source and the impulsive visible
continuum emission reinforces the theory that a substantial component of the
seismic emission seen is a result of sudden heating of the low photosphere
associated with the observed visible continuum emission. Furthermore, the
low-altitude magnetic loop structure inferred from potential--field
extrapolations in the flaring region suggests that there is a significant
inverse correlation between the seismicity of a flare and the height of the
magnetic loops that conduct the particle beams from the corona.Comment: 16 pages, 7 figures, Solar Physics Topical Issue: SOHO 19/GONG 2007
"Seismology of Magnetic Activity", Accepte
Magneto--Acoustic Energetics Study of the Seismically Active Flare of 15 February 2011
Multi--wavelength studies of energetic solar flares with seismic emissions
have revealed interesting common features between them. We studied the first
GOES X--class flare of the 24th solar cycle, as detected by the Solar Dynamics
Observatory (SDO). For context, seismic activity from this flare
(SOL2011-02-15T01:55-X2.2, in NOAA AR 11158) has been reported in the
literature (Kosovichev, 2011; Zharkov et al., 2011). Based on Dopplergram data
from the Helioseismic and Magnetic Imager (HMI), we applied standard methods of
local helioseismology in order to identify the seismic sources in this event.
RHESSI hard X-ray data are used to check the correlation between the location
of the seismic sources and the particle precipitation sites in during the
flare. Using HMI magnetogram data, the temporal profile of fluctuations in the
photospheric line-of-sight magnetic field is used to estimate the magnetic
field change in the region where the seismic signal was observed. This leads to
an estimate of the work done by the Lorentz-force transient on the photosphere
of the source region. In this instance this is found to be a significant
fraction of the acoustic energy in the attendant seismic emission, suggesting
that Lorentz forces can contribute significantly to the generation of
sunquakes. However, there are regions in which the signature of the
Lorentz-force is much stronger, but from which no significant acoustic emission
emanates.Comment: Submitted to Solar Physic
Magnetic field variations and the seismicity of solar active regions
Dynamical changes in the solar corona have proven to be very important in
inducing seismic waves into the photosphere. Different mechanisms for their
generation have been proposed. In this work, we explore the magnetic field
forces as plausible mechanisms to generate sunquakes as proposed by Hudson,
Fisher and Welsch. We present a spatial and temporal analysis of the
line-of-sight magnetic field variations induced by the seismically active 2003
October 29 and 2005 January 15 solar flares and compare these results with
other supporting observations.Comment: 4 pages, 4 figures, letter, Accepted in February by MNRA
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