752 research outputs found
e-EVN monitoring of M87
M87 is a privileged laboratory for a detailed study of the properties of jets, owing to its proximity (D=16.7 Mpc, 1 mas = 0.080 pc), its massive black hole (~6.0 x 10^9M) and its conspicuous emission at radio wavelengths and above. We started on November 2009 a monitoring program with the e-EVN at 5 GHz, in correspondence of the season of Very High Energy (VHE) observations. Indeed, two episodes of VHE activity have been reported in February and April 2010. We present here the main results of these multi-epoch observations: the inner jet and HST-1 are both detected and resolved in our datasets. We study the apparent velocity of HST-1, which seems to be increasing since 2005, and the flux density variability in the inner jet. All in all, the radio counterpart to this year’s VHE event seems to be different from the ones in 2005 and 2008, opening new scenario for the radio-high energy connection
Global e-VLBI observations of the gamma-ray narrow line Seyfert 1 PMN J0948+0022
There is growing evidence of relativistic jets in radio-loud narrow-line
Seyfert 1 (RL-NLS1) galaxies. We constrain the observational properties of the
radio emission in the first RL-NLS1 galaxy ever detected in gamma-rays, PMN
J0948+0022, i.e., its flux density and structure in total intensity and in
polarization, its compactness, and variability. We performed three real-time
e-VLBI observations of PMN J0948+0022 at 22 GHz, using a global array including
telescopes in Europe, East Asia, and Australia. These are the first e-VLBI
science observations ever carried out with a global array, reaching a maximum
baseline length of 12458 km. The observations were part of a large
multiwavelength campaign in 2009. The source is detected at all three epochs.
The structure is dominated by a bright component, more compact than 55
microarcsec, with a fainter component at a position angle theta~ 35deg.
Relativistic beaming is required by the observed brightness temperature of
3.4x10^11 K. Polarization is detected at a level of about 1%. The parameters
derived by the VLBI observations, in addition to the broad-band properties,
confirm that PMN J0948+0022 is similar to flat spectrum radio quasars. Global
e-VLBI is a reliable and promising technique for future studies.Comment: Accepted for publication as a Letter in Astronomy and Astrophysic
Exploring the bulk of the BL Lac object population:1. parsec-scale radio structures
Context. The advent of Fermi is changing our understanding on the radio and
gamma-ray emission in Active Galactic Nuclei. Contrary to pre-Fermi ideas, BL
Lac objects are found to be the most abundant emitters in the gamma-ray band.
However, since they are relatively weak radio sources, most of their
parsec-scale structure and their multi-frequency properties are poorly
understood and/or have not been investigated in a systematically fashion. Aims.
Our main goal is to analyze the radio and gamma-ray emission properties of a
sample of 42 BL Lacs selected, for the first time in the literature, with no
constraint on their radio and gamma-ray flux densities/emission. Methods.
Thanks to new Very Long Baseline Array observations at 8 and 15 GHz for the
whole sample, we present here fundamental parameters such as radio flux
densities, spectral index information, and parsec-scale structure. Moreover, we
search for gamma-ray counterparts using data reported in the Second Catalog of
Fermi Gamma-ray sources. Results. Parsec-scale radio emission is observed in
the majority of the sources at both frequencies. Gamma-ray counterparts are
found for 14/42 sources. Conclusions. The comparison between our results in
radio and gamma-ray bands points out the presence of a large number of faint BL
Lacs showing "non classical" properties such as low source compactness, core
dominance, no gamma-ray emission and steep radio spectral indexes. A deeper
multiwavelength analysis will be needed.Comment: 19 pages, 6 figures, 6 tables, accepted for publication in A&
The 2010 M 87 VHE flare and its origin: the multi-wavelength picture
The giant radio galaxy M 87, with its proximity (16 Mpc) and its very massive
black hole ((3 - 6) \times 10^9 M_solar), provides a unique laboratory to
investigate very high energy (E>100 GeV; VHE) gamma-ray emission from active
galactic nuclei and, thereby, probe particle acceleration to relativistic
energies near supermassive black holes (SMBH) and in relativistic jets. M 87
has been established as a VHE gamma-ray emitter since 2005. The VHE gamma-ray
emission displays strong variability on timescales as short as a day. In 2008,
a rise in the 43 GHz Very Long Baseline Array (VLBA) radio emission of the
innermost region (core; extension of < 100 Rs ; Schwarzschild radii) 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. In 2010 a flare at VHE was again detected triggering further
multi-wavelength (MWL) observations with the VLBA, Chandra, and other
instruments. At the same time, M 87 was also observed with the Fermi-LAT
telescope at MeV/GeV energies, the European VLBI Network (EVN), and the
Liverpool Telescope (LT). Here, preliminary results from the 2010 campaign will
be reported.Comment: 6 pages, 2 figures; Procceedings of the workshop "High Energy
Phenomena in Relativistic Outflows III" (HEPRO III), Barcelona, June 27 -
July 1, 201
Chandra X-ray Observations of the Two Brightest Unidentified High Galactic Latitude Fermi-LAT gamma-ray Sources
We present Chandra ACIS-I X-ray observations of 0FGL J1311.9-3419 and 0FGL
J1653.4-0200, the two brightest high Galactic latitude (|b|>10 deg) gamma-ray
sources from the 3 month Fermi-LAT bright source list that are still
unidentified. Both were also detected previously by EGRET, and despite
dedicated multi-wavelength follow-up, they are still not associated with
established classes of gamma-ray emitters like pulsars or radio-loud active
galactic nuclei. X-ray sources found in the ACIS-I fields of view are
catalogued, and their basic properties are determined. These are discussed as
candidate counterparts to 0FGL J1311.9-3419 and 0FGL J1653.4-0200, with
particular emphasis on the brightest of the 9 and 13 Chandra sources detected
within their respective Fermi-LAT 95% confidence regions. Further follow-up
studies, including optical photometric and spectroscopic observations are
necessary to identify these X-ray candidate counterparts in order to ultimately
reveal the nature of these enigmatic gamma-ray objects.Comment: ApJ, accepted. 18 pages, 5 figure
Analysing the Transverse Structure of the Relativistic Jets of AGN
This paper describes a method of fitting total intensity and polarization profiles in VLBI images of astrophysical jets to profiles predicted by a theoretical model. As an example, the method is used to fit profiles of the jet in the Active Galactic Nucleus Mrk501 with profiles predicted by a model in which a cylindrical jet of synchrotron plasma is threaded by a magnetic field with helical and disordered components. This fitting yields model Stokes Q profiles that agree with the observed profiles to within the 1-2 \sigma uncertainties; the I model and observed profiles are overall not in such good agreement, with the model I profiles being generally more symmetrical than the observed profiles. Consistent fitting results are obtained for profiles derived from 6cm VLBI images at two distances from the core, and also for profiles obtained for different wavelengths at a single location in the VLBI jet. The most striking success of the model is its ability to reproduce the spine-sheath polarization structure observed across the jet. Using the derived viewing angle in the jet rest frame, \delta' approximately 83 degrees, together with a superluminal speed reported in the literature, \beta apparent = 3.3, yields a solution for the viewing angle and velocity of the jet in the observer's frame \delta degrees and \beta approximately 0.96. Although these results for Mrk501 must be considered tentative, the combined analysis of polarization profiles and apparent component speeds holds promise as a means of further elucidating the magnetic field structures and other parameters of parsec-scale AGN jets
The two sided parsec scale structure of the Low Luminosity Active Galactic Nucleus in NGC 4278
We present new Very Long Baseline Interferometry observations of the LINER
galaxy NGC 4278. The observations were taken with the Very Long Baseline Array
(VLBA) and a single antenna of the Very Large Array (VLA) at 5 GHz and 8.4 GHz
and have a linear resolution of <0.1 pc. Our radio data reveal a two sided
structure, with symmetric S-shaped jets emerging from a flat spectrum core. We
fit the jet brightness with gaussian components, which we identify from a
previous observation taken five years before. By comparing the positions of the
components in the two epochs, we measure motions between 0.45 +/- 0.14 and 3.76
+/- 0.65 mas, corresponding to apparent velocities < 0.2c, and to ages in the
range 8.3 - 65.8 years. Assuming that the radio morphology is intrinsically
symmetric and its appearance is governed by Doppler beaming effects, we find
that NGC4278 has mildly relativistic jets (beta ~ 0.75), closely aligned to the
line-of-sight (2 degrees < theta < 4 degrees). Alternatively, the source could
be oriented at a larger angle and asymmetries could be related to the jet
interaction with the surrounding medium. We also present new simultaneous VLA
observations between 1.4 and 43 GHz, and a 5 GHz light curve between 1972 and
2003. The radio spectrum can be fit by a relatively steep power-law (alpha =
0.54). We find significant variability at 5 GHz. All these arguments indicate
that the radiation from NGC 4278 is emitted via the synchrotron process by
relativistic particles accelerated by a supermassive black hole. Despite a much
lower power, this is the same process that takes place in ordinary radio loud
AGNs.Comment: 29 pages, 9 figures, ApJ accepte
The kinematic of HST-1 in the jet of M87
Aims: We aim to constrain the structural variations within the HST-1 region
downstream of the radio jet of M87, in general as well as in connection to the
episodes of activity at very high energy (VHE). Methods: We analyzed and
compared 26 VLBI observations of the M87 jet, obtained between 2006 and 2011
with the Very Long Baseline Array (VLBA) at 1.7 GHz and the European VLBI
Network (EVN) at 5 GHz. Results: HST-1 is detected at all epochs; we
model-fitted its complex structure with two or more components, the two
outermost of which display a significant proper motion with a superluminal
velocity around ~4c. The motion of a third feature that is detected upstream is
more difficult to characterize. The overall position angle of HST-1 has changed
during the time of our observations from -65deg to -90deg, while the structure
has moved by over 80 mas downstream. Our results on the component evolution
suggest that structural changes at the upstream edge of HST-1 can be related to
the VHE events.Comment: Accepted for publication in A&A Letter
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