818 research outputs found
Gamma-ray Spectral Evolution of NGC1275 Observed with Fermi-LAT
We report on a detailed investigation of the high-energy gamma-ray emission
from NGC\,1275, a well-known radio galaxy hosted by a giant elliptical located
at the center of the nearby Perseus cluster. With the increased photon
statistics, the center of the gamma-ray emitting region is now measured to be
separated by only 0.46' from the nucleus of NGC1275, well within the 95%
confidence error circle with radius ~1.5'. Early Fermi-LAT observations
revealed a significant decade-timescale brightening of NGC1275 at GeV photon
energies, with a flux about seven times higher than the one implied by the
upper limit from previous EGRET observations. With the accumulation of one-year
of Fermi-LAT all-sky-survey exposure, we now detect flux and spectral
variations of this source on month timescales, as reported in this paper. The
average >100 MeV gamma-ray spectrum of NGC1275 shows a possible deviation from
a simple power-law shape, indicating a spectral cut-off around an observed
photon energy of E = 42.2+-19.6 GeV, with an average flux of F = (2.31+-0.13) X
10^{-7} ph/cm^2/s and a power-law photon index, Gamma = 2.13+-0.02. The largest
gamma-ray flaring event was observed in April--May 2009 and was accompanied by
significant spectral variability above E > 1-2 GeV. The gamma-ray activity of
NGC1275 during this flare can be described by a hysteresis behavior in the flux
versus photon index plane. The highest energy photon associated with the
gamma-ray source was detected at the very end of the observation, with the
observed energy of E = 67.4GeV and an angular separation of about 2.4' from the
nucleus. In this paper we present the details of the Fermi-LAT data analysis,
and briefly discuss the implications of the observed gamma-ray spectral
evolution of NGC1275 in the context of gamma-ray blazar sources in general.Comment: 20 pages, 6 figures, accepted for publication in the Ap
A Modified Synchrotron Model for Knots in the M87 Jet
For explaining the broadband spectral shape of knots in the M87 jet from
radio through optical to X-ray, we propose a modified synchrotron model that
considers the integrated effect of particle injection from different
acceleration sources in the thin acceleration region. This results in two break
frequencies at two sides of which the spectral index of knots in the M87 jet
changes. We discuss the possible implications of these results for the physical
properties in the M87 jet. The observed flux of the knots in the M87 jet from
radio to X-ray can be satisfactorily explained by the model, and the predicted
spectra from ultraviolet to X-ray could be further tested by future
observations. The model implies that the knots D, E, F, A, B, and C1 are
unlikely to be the candidate for the TeV emission recently detected in M87.Comment: 12 pages, 1 figure, 2 tables, Accepted for publication in ApJ Letter
The Highest Redshift Relativistic Jets
We describe our efforts to understand large-scale (10's-100's kpc)
relativistic jet systems through observations of the highest-redshift quasars.
Results from a VLA survey search for radio jets in ~30 z>3.4 quasars are
described along with new Chandra observations of 4 selected targets.Comment: 5 pages, 2 figures, to appear in Extragalactic Jets: Theory and
Observation from Radio to Gamma Ray, Eds. T.A. Rector and D.S. De Youn
Probing the origin of VHE emission from M 87 with MWL observations in 2010
The large majority of extragalactic very high energy (VHE; E>100 GeV) sources
belongs to the class of active galactic nuclei (AGN), in particular the BL Lac
sub-class. AGNs are characterized by an extremely bright and compact emission
region, powered by a super-massive black hole (SMBH) and an accretion disk, and
relativistic outflows (jets) detected all across the electro-magnetic spectrum.
In BL Lac sources the jet axis is oriented close to the line of sight, giving
rise to a relativistic boosting of the emission. In radio galaxies, on the
other hand, the jet makes a larger angle to the line of sight allowing to
resolve the central core and the jet in great details. The giant radio galaxy M
87 with its proximity (1 6Mpc) and its very massive black hole ((3-6) x 10^9
M_solar) provides a unique laboratory to investigate VHE emission in such
objects and thereby probe particle acceleration to relativistic energies near
SMBH and in jets. M 87 has been established as a VHE emitter since 2005. The
VHE emission displays strong variability on time-scales as short as a day. It
has been subject of a large joint VHE and multi-wavelength (MWL) monitoring
campaign in 2008, where a rise in the 43 GHz VLBA radio emission of the
innermost region (core) was found to coincide with a flaring activity at VHE.
This had been interpreted as a strong indication that the VHE emission is
produced in the direct vicinity of the SMBH black hole. In 2010 again a flare
at VHE was detected triggering further MWL observations with the VLBA, Chandra,
and other instruments. At the same time M 87 was also observed with the
Fermi-LAT telescope at GeV energies and the European VLBI Network (EVN). In
this contribution preliminary results from the campaign will be presented.Comment: 5 pages, 2 figures, in the proceedings of the "International Workshop
on Beamed and Unbeamed Gamma-Rays from Galaxies" 11-15 April 2011, Lapland
Hotel Olos, Muonio, Finland, Journal of Physics: Conference Series Volume
355, 201
Kiloparsec-Scale Jets in FR I Radio Galaxies and the Gamma-Ray Background
We discuss the contribution of kiloparsec-scale jets in FR I radio galaxies
to the diffuse gamma-ray background radiation. The analyzed gamma-ray emission
comes from inverse-Compton scattering of starlight photon fields by the
ultrarelativistic electrons whose synchrotron radiation is detected from such
sources at radio, optical and X-ray energies. We find that these objects, under
the minimum-power hypothesis (corresponding to a magnetic field of 300 muG in
the brightest knots of these jets), can contribute about one percent to the
extragalactic gamma-ray background measured by EGRET. We point out that this
result already indicates that the magnetic fields in kpc-scale jets of
low-power radio galaxies are not likely to be smaller than 10 muG on average,
as otherwise the extragalactic gamma-ray background would be overproduced.Comment: 18 pages, 3 figures included. ApJ accepte
A Flare in the Jet of Pictor A
A Chandra X-ray imaging observation of the jet in Pictor A showed a feature
that appears to be a flare that faded between 2000 and 2002. The feature was
not detected in a follow-up observation in 2009. The jet itself is over 150 kpc
long and a kpc wide, so finding year-long variability is surprising. Assuming a
synchrotron origin of the observed high-energy photons and a minimum energy
condition for the outflow, the synchrotron loss time of the X-ray emitting
electrons is of order 1200 yr, which is much longer than the observed
variability timescale. This leads to the possibility that the variable X-ray
emission arises from a very small sub-volume of the jet, characterized by
magnetic field that is substantially larger than the average over the jet.Comment: 12 pages, 3 figures, to appear in Ap. J. Letter
On the Magnetic Field in the Kiloparsec-Scale Jet of Radio Galaxy M 87
Several low-power kiloparsec-scale jets in nearby radio galaxies are known
for their synchrotron radiation extending up to optical and X-ray photon
energies. Here we comment on high-energy gamma-ray emission of one particular
object of this kind, i.e. the kiloparsec-scale jet of M 87 radio galaxy,
resulting from comptonization of the starlight photon field of the host galaxy
by the synchrotron-emitting jet electrons. In the analysis, we include
relativistic bulk velocity of the jet, as well as the Klein-Nishina effects. We
show, that upper limits to the kiloparsec-scale jet inverse-Compton radiation
imposed by HESS and HEGRA Cherenkov Telescopes - which detected a variable
source of VHE gamma-ray emission within 0.1 deg (~30 kpc) of the M 87 central
region - give us an important constraint on the magnetic field strength in this
object, namely that the magnetic field cannot be smaller than the equipartition
value (referring solely to the radiating electrons) in the brightest knot of
the jet, and most likely, is even stronger. In this context, we point out a
need for the amplification of the magnetic energy flux along the M 87 jet from
the sub-parsec to kiloparsec scales, suggesting the turbulent dynamo as a
plausible process responsible for the aforementioned amplification.Comment: 25 pages, 8 figures included. Accepted for publication in The
Astrophysical Journa
The 300 kpc Long X-ray Jet in PKS 1127-145, z=1.18 Quasar: Constraining X-ray Emission Models
We present a ~100 ksec Chandra X-ray observation and new VLA radio data of the large scale, 300 kpc long X-ray jet in PKS1127-145, a radio loud quasar at redshift z=1.18. With this deep X-ray observation we now clearly discern the complex X-ray jet morphology and see substructure within the knots. The X-ray and radio jet intensity profiles are seen to be strikingly different with the radio emission peaking strongly at the two outer knots while the X-ray emission is strongest in the inner jet region. The jet X-ray surface brightness gradually decreases by an order of magnitude going out from the core. The new X-ray data contain sufficient counts to do spectral analysis of the key jet features. The X-ray energy index of the inner jet is relatively flat with alpha_X = 0.66 +/-0.15 and steep in the outer jet with alpha_X = 1.0 +/-0.2. We discuss the constraints implied by the new data on the X-ray emission models and conclude that ``one-zone'' models fail and at least a two component model is needed to explain the jet's broad-band emission. We propose that the X-ray emission originates in the jet proper while the bulk of the radio emission comes from a surrounding jet sheath. We also consider intermittent jet activity as a possible cause of the observed jet morphology
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