29 research outputs found
The CLASS blazar survey: testing the blazar sequence
We discuss the properties of the sources in the CLASS Blazar survey which
aims at the selection of low radio power (P(5GHz)<10^25 W Hz^-1) blazars. We
use VLA data from available catalogues and from our own observations to
constrain the radio core-dominance of the sample which, together with the flat
radio spectral index, is a signature of the blazar activity. X-ray data from
the ROSAT All Sky Survey were also collected in order to constrain the
radio-to-X-ray luminosity ratio (alpha_RX) of the sources. The data analysis
shows that more than 30% of sources at low radio power (P(5 GHz)<10^25 W Hz^-1)
have an alpha_RX steeper than that expected in the framework of the ``blazar
sequence'' recently put forward to unify the high and low power blazars. The
possibility that this result is influenced by contaminating sources in the
current sample is discussed. The conclusion is that, even if a number of
non-blazars (typically CSO/GPS sources) are expected in the survey, it is
unlikely that this constitutes the sole reason for the observed deviation. In
particular, we show 2 examples for which the blazar nature is confirmed from
VLBI data and for which the steep alpha_RX (suggesting a synchrotron peak
frequency below 10^15.5 Hz) and the low radio power (0.6-2x10^24 W Hz^-1) put
these sources outside the ``blazar sequence''. The results presented here show
the importance of a correct and unbiased sampling of the low-power regime of
the blazar population.Comment: 20 pages, accepted for publication in MNRA
Obscuration in high redshift jetted QSO
Obscuration in high-redshift quasi-stellar objects (QSO) has a profound
impact on our understanding of the evolution of supermassive black holes across
the cosmic time. An accurate quantification of its relevance is therefore
mandatory. We present a study aimed at evaluating the importance of obscuration
in high redshift jetted QSO, i.e. those active nuclei characterized by the
presence of powerful relativistic jets. We compare the observed number of radio
detected QSO at different radio flux density limits with the value predicted by
the beaming model on the basis of the number of oriented sources (blazars). Any
significant deficit of radio-detected QSO compared to the predictions can be
caused by the presence of obscuration along large angles from the jet
direction. We apply this method to two sizable samples characterized by the
same optical limit (mag=21) but significantly different radio density limits
(30 mJy and 1 mJy respectively) and containing a total of 87 independent
radio-loud 4<z<6.8 QSO, 31 of which classified as blazars. We find a general
good agreement between the numbers predicted by the model and those actually
observed, with only a marginal discrepancy at 0.5 mJy that could be caused by
the lack of completeness of the sample. We conclude that we have no evidence of
obscuration within angles 10-20deg from the relativistic jet direction. We also
show how the ongoing deep wide-angle radio surveys will be instrumental to test
the presence of obscuration at much larger angles, up to 30-35deg. We finally
suggest that, depending on the actual fraction of obscured QSO, relativistic
jets could be much more common at high redshifts compared to what is usually
observed in the local UniverseComment: 14 pages, 5 figures. Accepted for publication on A&
SDSSJ143244.91+301435.3: a link between radio-loud narrow-line Seyfert 1 galaxies and compact steep-spectrum radio sources?
We present SDSSJ143244.91+301435.3, a new case of radio-loud narrow line
Seyfert 1 (RL NLS1) with a relatively high radio power (P1.4GHz=2.1x10^25 W
Hz^-1) and large radioloudness parameter (R1.4=600+/-100). The radio source is
compact with a linear size below ~1.4 kpc but, contrary to most of the RL NLS1
discovered so far with such a high R1.4, its radio spectrum is very steep
(alpha=0.93) and not supporting a 'blazar-like' nature. Both the small mass of
the central super-massive black-hole and the high accretion rate relative to
the Eddington limit estimated for this object (3.2x10^7 Msun and 0.27,
respectively, with a formal error of ~0.4 dex on both quantities) are typical
of the class of NLS1. Through a modeling of the spectral energy distribution of
the source we have found that the galaxy hosting SDSSJ143244.91+301435.3 is
undergoing a quite intense star-formation (SFR=50 Msun y^-1) which, however, is
expected to contribute only marginally (~1 per cent) to the observed radio
emission. The radio properties of SDSSJ143244.91+301435.3 are remarkably
similar to those of compact steep spectrum (CSS) radio sources, a class of AGN
mostly composed by young radio galaxies. This may suggest a direct link between
these two classes of AGN, with the CSS sources possibly representing the
misaligned version (the so-called parent population) of RL NLS1 showing blazar
characteristics.Comment: 14 pages, 7 figures, 4 tables, accepted for publication in MNRA
XMM-Newton spectroscopy of an X-ray selected sample of RL AGNs
This paper presents the X-ray spectroscopy of an X-ray selected sample of 25
radio-loud (RL) AGNs extracted from the XBSS sample. The main goal is to study
the origin of the X-ray spectral differences usually observed between
radio-loud and radio-quiet (RQ) AGNs. To this end, a comparison sample of 53 RQ
AGNs has been also extracted from the same XBSS sample and studied together
with the sample of RL AGNs. We have focused the analysis on the distribution of
the X-ray spectral indices of the power-law component that models the large
majority of the spectra in both samples. We find that the mean X-ray energy
spectral index is very similar in the 2 samples and close to alpha_X~1.
However, the intrinsic distribution of the spectral indices is significantly
broader in the sample of RL AGNs. In order to investigate the origin of this
difference, we have divided the RL AGNs into blazars and ``non-blazars'', on
the basis of the available optical and radio information. We find strong
evidence that the broad distribution observed in the RL AGN sample is mainly
due to the presence of the blazars. Furthermore, within the blazar class we
have found a link between the X-ray spectral index and the value of the
radio-to-X-ray spectral index suggesting that the observed X-ray emission is
directly connected to the emission of the relativistic jet. This trend is not
observed among the ``non-blazars'' RL AGNs. This favours the hypothesis that,
in these latter sources, the X-ray emission is not significantly influenced by
the jet emission and it has probably an origin similar to the RQ AGNs. Overall,
the results presented here indicate that the observed distribution of the X-ray
spectral indices in a given sample of RL AGNs is strongly dependent on the
amount of relativistic beaming present in the selected sources.Comment: 18 pages, 10 figures, accepted for publication in A&
Elusive AGN in the XMM-Newton bright serendipitous survey
AIMS: We investigate here the nature of all the sources (35 in total) in the
XBS survey (which is 86% optically identified) showing an optical spectrum
dominated by the light from the host galaxy with no evidence (or little
evidence) for the presence of an AGN. METHODS: We use the X-ray spectral
analysis to assess the presence of an AGN in these sources and to characterize
its properties. RESULTS: We detect AGN activity in 33 out of 35 sources. The
remaining 2 sources are the ones with the lowest X-ray luminosity in the sample
(L[2-10keV]<10^41 erg s^-1) and their X-ray emission could be produced within
the host galaxy. We find that the ``recognition problem'' for AGN is very
critical in the low-luminosity regime (at least 60% of the AGN with
L[2-10keV]<10^43 erg s^-1 are elusive) becoming negligible for high X-ray
luminosities (~1.5% of elusive AGN with L[2-10keV]>10^44 erg s^-1). This
problem affects mostly absorbed AGN (~40% of type~2 AGN in the survey are
elusive) but also a significant fraction of unabsorbed AGN (8%). CONCLUSIONS:
We find that the simplest explanations of why these 33 (or most of them) AGNs
are elusive are two: at low X-ray luminosities (<10^43 erg s^-1) the most
important reason is the intrinsically low AGN/galaxy contrast (optical
dilution) while at high luminosities (>10^44 erg s^-1) it is due to the optical
absorption (in the Compton-thin regime, i.e. NH<10^24 cm^-2). Alternative
hypotheses, like the presence of Compton-thick sources, BL Lac objects or
``non-standard'' AGN (e.g. with alpha_OX<1 or with weak/absorbed Narrow Line
Region) are not supported by the data although we cannot exclude the presence
in the sample of a few sources of these types.Comment: accepted for publication in A&A, 17 pages, 9 figure
A New Sample of Gamma-Ray Emitting Jetted Active Galactic Nuclei
We considered the fourth catalog of gamma-ray point sources produced by the
Fermi Large Area Telescope (LAT) and selected only jetted active galactic
nuclei (AGN) or sources with no specific classification, but with a
low-frequency counterpart. Our final list is composed of 2980 gamma-ray point
sources. We then searched for optical spectra in all the available literature
and publicly available databases, to measure redshifts and to confirm or change
the original LAT classification. Our final list of gamma-ray emitting jetted
AGN is composed of BL Lac Objects (40%), flat-spectrum radio quasars (23%),
misaligned AGN (2.8%), narrow-line Seyfert 1, Seyfert, and low-ionization
nuclear emission-line region galaxies (1.9%). We also found a significant
number of objects changing from one type to another, and vice versa
(changing-look AGN, 1.1%). About 30% of gamma-ray sources still have an
ambiguous classification or lack one altogether.Comment: 18 pages, 5 figures, 1 Table. Accepted for publication on Universe,
Special Issue "Black Holes and Relativistic Jets", edited by I. Dutan and N.
R. MacDonald. This preprint contains only the main text. The full tables A1
and A2 are available on the journal web site
(https://www.mdpi.com/2218-1997/8/11/587
Redshift Limits of BL Lacertae Objects from Optical Spectroscopy
Context: BL Lacertae objects have been the targets for numerous recent
multiwavelength campaigns, continuum spectral variability studies, and
theoretical spectral and variability modeling. A meaningful interpretation of
the results of such studies requires a reliable knowledge of the objects'
redshifts; however, the redshifts for many are still unknown or uncertain.
Aims: Therefore, we hope to determine or constrain the redshifts of six BL Lac
objects with unknown or poorly known redshifts. Methods: Observations were made
of these objects with the MDM 2.4 m Hiltner telescope. Although no spectral
features were detected, and thus no redshifts could be measured, lower redshift
limits were assigned to the objects based on the expected equivalent widths of
absorption features in their host galaxies. Redshifts were also estimated for
some objects by assuming the host galaxies are standard candles and using host
galaxy apparent magnitudes taken from the literature. Results: The commonly
used redshift of for 1219+285 is almost certainly wrong, while the
redshifts of the other objects studied remain undetermined.Comment: 4 pages, 2 figures. Accepted by A&A Research Note
The radio/gamma-ray connection in Active Galactic Nuclei in the era of the Fermi Large Area Telescope
We present a detailed statistical analysis of the correlation between radio
and gamma-ray emission of the Active Galactic Nuclei (AGN) detected by Fermi
during its first year of operation, with the largest datasets ever used for
this purpose. We use both archival interferometric 8.4 GHz data (from the VLA
and ATCA, for the full sample of 599 sources) and concurrent single-dish 15 GHz
measurements from the Owens Valley Radio Observatory (OVRO, for a sub sample of
199 objects). Our unprecedentedly large sample permits us to assess with high
accuracy the statistical significance of the correlation, using a
surrogate-data method designed to simultaneously account for common-distance
bias and the effect of a limited dynamical range in the observed quantities. We
find that the statistical significance of a positive correlation between the cm
radio and the broad band (E>100 MeV) gamma-ray energy flux is very high for the
whole AGN sample, with a probability <1e-7 for the correlation appearing by
chance. Using the OVRO data, we find that concurrent data improve the
significance of the correlation from 1.6e-6 to 9.0e-8. Our large sample size
allows us to study the dependence of correlation strength and significance on
specific source types and gamma-ray energy band. We find that the correlation
is very significant (chance probability <1e-7) for both FSRQs and BL Lacs
separately; a dependence of the correlation strength on the considered
gamma-ray energy band is also present, but additional data will be necessary to
constrain its significance.Comment: Accepted for publications by ApJ. Contact authors: M. Giroletti, V.
Pavlidou, A. Reime
The Spectral Energy Distribution of Fermi bright blazars
(Abridged) We have conducted a detailed investigation of the broad-band
spectral properties of the \gamma-ray selected blazars of the Fermi LAT Bright
AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray
spectra with Swift, radio, infra-red, optical and other hard X-ray/gamma-ray
data, collected within three months of the LBAS data taking period, we were
able to assemble high-quality and quasi-simultaneous Spectral Energy
Distributions (SED) for 48 LBAS blazars.The SED of these gamma-ray sources is
similar to that of blazars discovered at other wavelengths, clearly showing, in
the usual Log - Log F representation, the typical broad-band
spectral signatures normally attributed to a combination of low-energy
synchrotron radiation followed by inverse Compton emission of one or more
components. We have used these SEDs to characterize the peak intensity of both
the low and the high-energy components. The results have been used to derive
empirical relationships that estimate the position of the two peaks from the
broad-band colors (i.e. the radio to optical and optical to X-ray spectral
slopes) and from the gamma-ray spectral index. Our data show that the
synchrotron peak frequency is positioned between 10 and
10 Hz in broad-lined FSRQs and between and Hz in
featureless BL Lacertae objects.We find that the gamma-ray spectral slope is
strongly correlated with the synchrotron peak energy and with the X-ray
spectral index, as expected at first order in synchrotron - inverse Compton
scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton
(SSC) models cannot explain most of our SEDs, especially in the case of FSRQs
and low energy peaked (LBL) BL Lacs. (...)Comment: 85 pages, 38 figures, submitted to Ap
The mass distribution of a moderate redshift galaxy group and brightest group galaxy from gravitational lensing and kinematics
The gravitational lens system CLASS B2108+213 has two radio-loud lensed
images separated by 4.56 arcsec. The relatively large image separation implies
that the lensing is caused by a group of galaxies. In this paper, new optical
imaging and spectroscopic data for the lensing galaxies of B2108+213 and the
surrounding field galaxies are presented. These data are used to investigate
the mass and composition of the lensing structure. The redshift and stellar
velocity dispersion of the main lensing galaxy (G1) are found to be z = 0.3648
+/- 0.0002 and sigma_v = 325 +/- 25 km/s, respectively. The optical spectrum of
the lensed quasar shows no obvious emission or absorption features and is
consistent with a BL Lac type radio source. However, the tentative detection of
the G-band and Mg-b absorption lines, and a break in the spectrum of the host
galaxy of the lensed quasar gives a likely source redshift of z = 0.67.
Spectroscopy of the field around B2108+213 finds 51 galaxies at a similar
redshift to G1, thus confirming that there is a much larger structure at z ~
0.365 associated with this system. The width of the group velocity distribution
is 694 +/- 93 km/s, but is non-Gaussian, implying that the structure is not yet
viralized. The main lensing galaxy is also the brightest group member and has a
surface brightness profile consistent with a typical cD galaxy. A lensing and
dynamics analysis of the mass distribution, which also includes the newly found
group members, finds that the logarithmic slope of the mass density profile is
on average isothermal inside the Einstein radius, but steeper at the location
of the Einstein radius. This apparent change in slope can be accounted for if
an external convergence gradient, representing the underlying parent halo of
the galaxy group, is included in the mass model.Comment: 18 pages, 14 figures, 5 tables, accepted for publication in MNRA