662 research outputs found
The multi-faceted synergy between Swift and Fermi in radio-loud AGN studies
Since its launch in 2008 June, the Fermi Gamma-ray Space Telescope has opened
a new era in high-energy astrophysics. The unprecedented sensitivity, angular
resolution and effective area of the Large Area Telescope on board Fermi,
together with the nearly continuous observation of the entire gamma-ray sky
assures a formidable opportunity to study in detail gamma-ray emitting AGN of
various types. In this context the Swift satellite, thanks to its broad band
coverage and scheduling flexibility, creates a perfect synergy with Fermi.
Swift and Fermi coordinated monitoring campaigns of radio-loud AGN allowed us
to investigate correlated variability at different frequencies and to build
time-resolved spectral energy distributions from optical to gamma-rays,
constraining the emission mechanisms at work in these objects. The rapid Swift
follow-up observations of gamma-ray flaring AGN detected by Fermi-LAT were also
fundamental in firmly associating the gamma-ray sources with their low-energy
counterparts. We present some interesting results obtained from Fermi-LAT and
Swift observations of gamma-ray flaring AGN in the first six years of Fermi
operation.Comment: 7 pages, 4 figures. To appear in "Swift: 10 Years of Discovery",
Proceedings of Scienc
The first gamma-ray detection of the narrow-line Seyfert 1 FBQS J1644+2619
We report the discovery of gamma-ray emission from the narrow-line Seyfert 1
(NLSy1) galaxy FBQS J1644+2619 by the Large Area Telescope on board the Fermi
satellite. The Third Fermi LAT Source catalogue reports an unidentified
gamma-ray source, detected over the first four years of Fermi operation, 0.23
deg from the radio position of the NLSy1. Analysing 76 months of gamma-ray data
(2008 August 4 - 2014 December 31) we are able to better constrain the
localization of the gamma-ray source. The new position of the gamma-ray source
is 0.05 deg from FBQS J1644+2619, suggesting a spatial association with the
NLSy1. This is the sixth NLSy1 detected at high significance by Fermi-LAT so
far. Notably, a significant increase of activity was observed in gamma-rays
from FBQS J1644+2619 during 2012 July-October, and an increase of activity in
V-band was detected by the Catalina Real-Time Sky Survey in the same period.Comment: 6 pages, 2 figures. accepted for publication in Monthly Notices of
the Royal Astronomical Societ
The physics of the radio emission in the quiet side of the AGN population with the SKA
Despite targets of many multiwavelength campaigns, the main physical
processes at work in AGN are still under debate. In particular the origin of
the radio emission and the mechanisms involved are among the open questions in
astrophysics. In the radio-loud AGN population the radio emission is linked to
the presence of bipolar outflows of relativistic jets. However, the large
majority of the AGN population do not form powerful highly-relativistic jets on
kpc scales and are characterized by radio luminosity up to 10^23 W/Hz at 1.4
GHz, challenging our knowledge on the physical processes at the basis of the
radio emission in radio-quiet objects. The main mechanisms proposed so far are
synchrotron radiation from mildly relativistic mini-jets, thermal
cyclo-synchrotron emission by low-efficiency accretion flow (like ADAF or
ADIOS), or thermal free-free emission from the X-ray heated corona or wind. The
difficulty in understanding the main mechanism involved is related to the
weakness of these objects, which precludes the study of non-local radio-quiet
AGN. Multifrequency, high-sensitivity radio observations are crucial to
constrain the nature of the power engine, and they may help in distinguishing
between the contribution from star formation and AGN activity. The advent of
the SKA, with its sub-arcsecond resolution and unprecedented sensitivity will
allow us to investigate these processes in radio-quiet AGN, even at high
redshift for the first time. Both the broad-band radio spectrum and the
polarization information will help us in disentangling between non-thermal and
thermal origin of the radio emission. The jump in sensitivity of a few order of
magnitudes at the (sub-)uJy level will enable us to detect radio emission from
a large number of radio-quiet AGN at high redshift, providing a fundamental
step in our understanding of their cosmological evolution. (Abridged)Comment: 7 pages, to appear as part of 'Continuum Science' in Proceedings of
'Advancing Astrophysics with the SKA (AASKA14)
Young radio sources: the duty-cycle of the radio emission and prospects for gamma-ray emission
The evolutionary stage of a powerful radio source originated by an AGN is
related to its linear size. In this context, compact symmetric objects (CSOs),
which are powerful and intrinsically small objects, should represent the young
stage in the individual radio source life. However, the fraction of young radio
sources in flux density-limited samples is much higher than what expected from
the number counts of large radio sources.This indicates that a significant
fraction of young radio sources does not develop to the classical
Fanaroff-Riley radio galaxies,suggesting an intermittent jet activity. As the
radio jets are expanding within the dense and inhomogeneous interstellar
medium,the ambient may play a role in the jet growth, for example slowing down
or even disrupting its expansion when a jet-cloud interaction takes place.
Moreover, this environment may provide the thermal seed photons that scattered
by the lobes' electrons may be responsible for high energy emission, detectable
by Fermi-LAT.Comment: 4 pages, 5 figures; 2011 Fermi Symposium proceedings - eConf C11050
High-energy properties of the high-redshift flat spectrum radio quasar PKS 2149−306
We investigate the γ-ray and X-ray properties of the flat spectrum radio quasar PKS 2149−306 at redshift z = 2.345. A strong γ-ray flare from this source was detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope satellite in 2013 January, reaching on January 20 a daily peak flux of (301 ± 36) × 10^(−8) ph cm^(−2) s^(−1) in the 0.1–100 GeV energy range. This flux corresponds to an apparent isotropic luminosity of (1.5 ± 0.2) × 10^(50) erg s^(−1), comparable to the highest values observed by a blazar so far. During the flare the increase of flux was accompanied by a significant change of the spectral properties. Moreover significant flux variations on a 6-h time-scale were observed, compatible with the light crossing time of the event horizon of the central black hole. The broad-band X-ray spectra of PKS 2149−306 observed by Swift-XRT and NuSTAR are well described by a broken power-law model, with a very hard spectrum (Γ_1 ∼ 1) below the break energy, at E_(break) = 2.5–3.0 keV, and Γ_2 ∼ 1.4–1.5 above the break energy. The steepening of the spectrum below ∼3 keV may indicate that the soft X-ray emission is produced by the low-energy relativistic electrons. This is in agreement with the small variability amplitude and the lack of spectral changes in that part of the X-ray spectrum observed between the two NuSTAR and Swift joint observations. As for the other high-redshift FSRQ detected by both Fermi-LAT and Swift-BAT, the photon index of PKS 2149−306 in hard X-ray is 1.6 or lower and the average γ-ray luminosity higher than 2 × 10^(48) erg s^(−1)
On the radio and GeV-TeV gamma-ray emission connection in Fermi blazars
The Fermi-LAT revealed that the census of the gamma-ray sky is dominated by
blazars. Looking for a possible connection between radio and gamma-ray emission
is a central issue for understanding the blazar physics, and various works were
dedicated to this topic. However, while a strong and significant correlation
was found between radio and gamma-ray emission in the 0.1-100 GeV energy range,
the connection between radio and very high energy (VHE, E>0.1 TeV) emission is
still elusive. The main reason is the lack of a homogeneous VHE sky coverage,
due to the operational mode of the imaging atmospheric Cherenkov telescopes.
With the present work we aim to quantify and assess the significance of the
possible connection between high-resolution radio emission, on milliarcsecond
scale, and GeV-TeV gamma-ray emission in blazars. For achieving our goal we
extract two large and unbiased blazar samples from the 1FHL and 2FHL Fermi
catalogs, above 10 GeV and 50 GeV, respectively. To investigate how the
correlation evolves as the gamma-ray energy increases, we perform the same
analysis by using the 0.1-300 GeV 3FGL gamma-ray energy fluxes. When we
consider the 0.1-300 GeV gamma-ray energy range, we find a strong and
significant correlation for all of the blazar sub-classes. Conversely, when we
consider the gamma-ray emission above 10 GeV the correlation with the radio
emission vanishes, with the exception of the blazar sub-class of high
synchrotron peaked objects.Comment: 6 pages, 2 figures, 1 table. For the proceedings of the 7th
International Fermi Symposiu
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