164 research outputs found
Starbursts and black hole masses in X-shaped radio galaxies: Signatures of a merger event?
We present new spectroscopic identifications of 12 X-shaped radio galaxies
and use the spectral data to derive starburst histories and masses of the
nuclear supermassive black holes in these galaxies. The observations were done
with the 2.1-m telescope of the Observatorio Astron\'omico Nacional at San
Pedro M\'artir, M\'exico. The new spectroscopic results extend the sample of
X-shaped radio galaxies studied with optical spectroscopy. We show that the
combined sample of the X-shaped radio galaxies has statistically higher
black-hole masses and older episodes of star formation than a control sample of
canonical double-lobed radio sources with similar redshifts and luminosities.
The data reveal enhanced star-formation activity in the X-shaped sample on the
timescales expected in galactic mergers. We discuss the results obtained in the
framework of the merger scenario.Comment: 9 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
The XMM spectral catalog of SDSS optically selected Seyfert 2 galaxies
We present an X-ray spectroscopic study of optically selected (SDSS) Seyfert
2 (Sy2) galaxies. The goal is to study the obscuration of Sy2 galaxies beyond
the local universe, using good quality X-ray spectra in combination with high
S/N optical spectra for their robust classification. We analyzed all available
XMM-Newton archival observations of narrow emission line galaxies that meet the
above criteria in the redshift range 0.05<z<0.35. We initially selected narrow
line AGN using the SDSS optical spectra and the BPT classification diagram. We
further modeled and removed the stellar continuum, and we analyzed the residual
emission line spectrum to exclude any possible intermediate-type Seyferts. Our
final catalog comprises 31 Sy2 galaxies with median redshift z~0.1. X-ray
spectroscopy is performed using the available X-ray spectra from the 3XMM and
the XMMFITCAT catalogs. Implementing various indicators of obscuration, we find
seven (~23%) Compton-thick AGN. The X-ray spectroscopic Compton-thick
classification agrees with other commonly used diagnostics, such as the X-ray
to mid-IR luminosity ratio and the X-ray to [OIII] luminosity ratio. Most
importantly, we find four (~13%) unobscured Sy2 galaxies, at odds with the
simplest unification model. Their accretion rates are significantly lower than
the rest of our Sy2 sample, in agreement with previous studies that predict the
absence of the broad line region below a certain Eddington ratio threshold.Comment: 12 pages, 6 figures, accepted for publication in A&
VLBI and Single Dish Monitoring of 3C84 in the Period of 2009-2011
The radio galaxy 3C 84 is a representative of gamma-ray-bright misaligned
active galactic nuclei (AGNs) and one of the best laboratories to study the
radio properties of the sub-pc jet in connection with the gamma-ray emission.
In order to identify possible radio counterparts of the gamma-ray emissions in
3C 84, we study the change in structure within the central 1 pc and the light
curve of sub-pc-size components C1, C2, and C3. We search for any correlation
between changes in the radio components and the gamma-ray flares by making use
of VLBI and single dish data. Throughout the radio monitoring spanning over two
GeV gamma-ray flares detected by the {\it Fermi}-LAT and the MAGIC Cherenkov
Telescope in the periods of 2009 April to May and 2010 June to August, total
flux density in radio band increases on average. This flux increase mostly
originates in C3. Although the gamma-ray flares span on the timescale of days
to weeks, no clear correlation with the radio light curve on this timescale is
found. Any new prominent components and change in morphology associated with
the gamma-ray flares are not found on the VLBI images.Comment: 6 pages, 3 figures, accepted for publication in MNRAS lette
Correlation between Fermi/LAT gamma-ray and 37 GHz radio properties of northern AGN averaged over 11 months
Although the Fermi mission has increased our knowledge of gamma-ray AGN, many
questions remain, such as the site of gamma-ray production, the emission
mechanism, and the factors that govern the strength of the emission. Using data
from a high radio band, 37 GHz, uncontaminated by other radiation components
besides the jet emission, we study these questions with averaged flux densities
over the the first year of Fermi operations. We look for possible correlations
between the 100 MeV - 100 GeV band used by the Fermi satellite and 37 GHz radio
band observed at the Aalto University Metsahovi Radio Telescope, as well as for
differences between the gamma-ray emission of different AGN subsamples. We use
data averaged over the 1FGL period. Our sample includes 249 northern AGN,
including a complete sample of 68 northern AGN with a measured average flux
density exceeding 1 Jy. We find significant correlation between both the flux
densities and luminosities in gamma and radio bands. The Fermi luminosity is
inversely correlated with the peak frequency of the synchrotron component of
the AGN spectral energy distributions. We also calculate the gamma dominances,
defined as the ratio between the gamma and radio flux densities, and find an
indication that high-energy blazars are more gamma-dominated than low-energy
blazars. After studying the distributions of gamma and radio luminosities, it
is clear that BL Lacertae objects are different from quasars, with
significantly lower luminosities. It is unclear whether this is an intrinsic
difference, an effect of variable relativistic boosting across the synchrotron
peak frequency range, or the result of Fermi being more sensitive to hard
spectrum sources like BL Lacertae objects. Our results suggest that the gamma
radiation is produced co-spatially with the 37 GHz emission, i.e., in the jet.Comment: 8+5 pages, 9 figures and 2 tables. To be published in A&
The connection between gamma-ray emission and millimeter flares in Fermi/LAT blazars
We compare the gamma-ray photon flux variability of northern blazars in the
Fermi/LAT First Source Catalog with 37 GHz radio flux density curves from the
Metsahovi quasar monitoring program. We find that the relationship between
simultaneous millimeter (mm) flux density and gamma-ray photon flux is
different for different types of blazars. The flux relation between the two
bands is positively correlated for quasars and does not exist for BLLacs.
Furthermore, we find that the levels of gamma-ray emission in high states
depend on the phase of the high frequency radio flare, with the brightest
gamma-ray events coinciding with the initial stages of a mm flare. The mean
observed delay from the beginning of a mm flare to the peak of the gamma-ray
emission is about 70 days, which places the average location of the gamma-ray
production at or downstream of the radio core. We discuss alternative scenarios
for the production of gamma-rays at distances of parsecs along the length the
jet.Comment: 11 pages, 7 figures, accepted for publication in A&A, new version
contains changes suggested by refere
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