81 research outputs found
The jet-disc connection in AGN
We present our latest results on the connection between accretion rate and
relativistic jet power in AGN, by using a large sample which includes mostly
blazars, but contains also some radio--galaxies. The jet power can be traced by
--ray luminosity in the case of blazars, and radio luminosity for both
classes. The accretion disc luminosity is instead traced by the broad emission
lines. Among blazars, we find a correlation between broad line emission and the
--ray or radio luminosities, suggesting a direct tight connection
between jet power and accretion rate. We confirm that the observational
differences between blazar subclasses reflect differences in the accretion
regime, but with blazars only we cannot properly access the low--accretion
regime. By introducing radio--galaxies, we succeed in observing the fingerprint
of the transition between radiatively efficient and inefficient accretion discs
in the jetted AGN family. The transition occurs at the standard critical value
and it appears smooth. Below this value, the
ionizing luminosity emitted by the accretion structure drops significantly.Comment: 11 pages, 6 figures. Accepted for publication in MNRA
The role of relativistic jets in the heaviest and most active supermassive black holes at high redshift
In powerful radio-quiet active galactic nuclei (AGN), black holes heavier
than one billion solar masses form at a redshift ~1.5-2. Supermassive black
holes in jetted radio-loud AGN seems to form earlier, at a redshift close to 4.
The ratio of active radio-loud to radio-quiet AGN hosting heavy black holes is
therefore a rather a strong function of redshift. We report on some recent
evidence supporting this conclusion, gathered from the Burst Alert Telescope
(BAT, onboard Swift) and by the Large Area Telescope (LAT, onboard Fermi). We
suggest that the more frequent occurrence of relativistic jets in the most
massive black holes at high redshifts, compared to later times, could be due to
the average black hole spin being greater in the distant past, or else to the
jet helping a fast accretion rate (or some combination of the two scenarios).
We emphasize that the large total accretion efficiency of rapidly spinning
black holes inhibits a fast growth, unless a large fraction of the available
gravitational energy of the accreted mass is not converted into radiation, but
used to form and maintain a powerful jet.Comment: 6 pages, 3 figures, accepted for publication in MNRAS, main journa
High redshift Fermi blazars observed by GROND and Swift
We observed 5 gamma-ray loud blazars at redshift greater than 2 with the
X-Ray Telescope (XRT) and the UltraViolet and Optical Telescope (UVOT) onboard
the Swift satellite, and the Gamma-Ray burst Optical Near-Infrared Detector
(GROND) instrument. These observations were quasi simultaneous, usually within
a few hours. For 4 of these blazars the near-IR to UV data show the presence of
an accretion disc, and we could reliably estimate its accretion rate and black
hole mass. One of them, PKS 1348+007, was found in an extraordinarily high
IR-optical state, almost two orders of magnitude brighter than at the epoch of
the Sloan Digital Sky Survey observations. For all the 5 quasars the physical
parameters of the jet emitting zone, derived by applying a one-zone emission
model, are similar to that found for the bulk of other gamma-ray loud quasars.
With our observations we have X-ray data for the full sample of blazars at z>2
present in the Fermi 2-yrs (2LAC) catalog. This allows to have a rather
complete view of the spectral energy distribution of all high-redshift Fermi
blazars, and to draw some conclusions about their properties, and especially
about the relation between the accretion rate and the jet power.Comment: 11 pages, 6 figures, submitted to MNRA
Blazar candidates beyond redshift 4 observed with GROND
The search for extremely massive high redshift blazars is essential to put
strong constraints on the supermassive black holes formation theories. Up to
now, the few blazars known to have a redshift larger than 4 have been
discovered serendipitously. We try a more systematic approach. Assuming
radio-loudness as a proxy for the jet orientation, we select a sample of
extremely radio-loud quasars. We measure their black hole masses with a method
based on fitting the thermal emission from the accretion disc. We achieve a
precision of a factor of two for our measures, thanks to the observations
performed with the Gamma-Ray Burst Optical Near-Infrared Detector (GROND). The
infrared to optical GROND data allow us to observe directly the peak of the
disc emission, thus constraining the overall disc luminosity. We obtain a small
range of masses, that peaks at 10^{9.3}Msun. If some of our candidates will be
confirmed as blazars, these results would introduce interesting constraints on
the mass function of extremely massive black holes at very high redshift.
Moreover, all our blazar candidates have high accretion rates. This result,
along with the high masses, opens an interesting view on the need of a fast
growth of the heaviest black holes at very high redshift.Comment: 12 pages, 10 figures, 4 tables. Accepted for publication in MNRA
EIF4B (eukaryotic translation initiation factor 4B)
Review on eIF4B, with data on DNA/RNA, on the protein encoded and where the gene is implicated
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Inhibition of Sec61-dependent translocation by mycolactone uncouples the integrated stress response from ER stress, driving cytotoxicity via translational activation of ATF4
Mycolactone is the exotoxin virulence factor of Mycobacterium ulcerans that causes the neglected tropical disease Buruli ulcer. We recently showed it to be a broad spectrum inhibitor of Sec61-dependent co-translational translocation of proteins into the endoplasmic reticulum (ER). An outstanding question is the molecular pathway linking this to its known cytotoxicity. We have now used translational profiling to better understand the reprogramming that occurs in cells exposed to mycolactone. Gene ontology identified enrichment in genes involved in cellular response to stress, and apoptosis signalling among those showing enhanced translation. Validation of these results supports a mechanism by which mycolactone activates an integrated stress response meditated by phosphorylation of eIF2α via multiple kinases (PERK, GCN, PKR) without activation of the ER stress sensors IRE1 or ATF6. The response therefore uncouples the integrated stress response from ER stress, and features translational and transcriptional modes of genes expression that feature the key regulatory transcription factor ATF4. Emphasising the importance of this uncoupled response in cytotoxicity, downstream activation of this pathway is abolished in cells expressing mycolactone-resistant Sec61α variants. Using multiple genetic and biochemical approaches, we demonstrate that eIF2α phosphorylation is responsible for mycolactone-dependent translation attenuation, which initially protects cells from cell death. However, chronic activation without stress remediation enhances autophagy and apoptosis of cells by a pathway facilitated by ATF4 and CHOP. Our findings demonstrate that priming events at the ER can result in the sensing of stress within different cellular compartmentsWellcome Trus
Study of the variability of Blazars gamma-ray emission
The gamma-ray emission of blazar jets shows a pronounced variability and this
feature provides limits to the size and to the speed of the emitting region. We
study the gamma-ray variability of bright blazars using data from the first 18
months of activity of the Large Area Telescope on the Fermi Gamma-Ray Space
Telescope. From the daily light-curves of the blazars characterized by a
remarkable activity, we firstly determine the minimum variability time-scale,
giving an upper limit for the size of the emitting region of the sources,
assumed to be spheroidal blobs in relativistic motion. These regions must be
smaller than ~10^-3 parsec. Another interesting time-scale is the duration of
the outbursts. We conclude that they cannot correspond to radiation produced by
a single blob moving relativistically along the jet, but they are either the
signature of emission from a standing shock extracting energy from a modulated
jet, or the superposition of a number of flares occurring on a shorter
time-scale. We also derive lower limits on the bulk Lorentz factor needed to
make the emitting region transparent for gamma-rays interacting through
photon-photon collisions.Comment: 11 pages, 7 figures, accepted for publication on Advances in Space
Research. Poster presented at COSPAR 2010 (Bremen), event E11 (Time
variability at high energies: a probe of AGN physics
Radio-loud Quasars above Redshift 4: VLBI Imaging of an Extended Sample
High-redshift radio sources provide plentiful opportunities for studying the
formation and evolution of early galaxies and supermassive black holes.
However, the number of known radio-loud active galactic nuclei (AGN) above
redshift 4 is rather limited. At high redshifts, it appears that blazars, with
relativistically beamed jets pointing towards the observer, are in majority
compared to radio-loud sources with jets misaligned with respect to the line of
sight. To find more of these misaligned AGN, milliarcsec-scale imaging studies
carried out with very long baseline interferometry (VLBI) are needed, as they
allow us to distinguish between compact core--jet radio sources and those with
more extended emission. Previous high-resolution VLBI studies revealed that
some of the radio sources among blazar candidates in fact show unbeamed radio
emission on milliarcsecond scales. The most accurate optical coordinates
determined with the Gaia astrometric space mission are also useful in the
classification process. Here, we report on dual-frequency imaging observations
of 13 high-redshift (4 < z < 4.5) quasars at 1.7 and 5 GHz with the European
VLBI Network. This sample increases the number of z>4 radio sources for which
VLBI observations are available by about a quarter. Using structural and
physical properties, such as radio morphology, spectral index, variability,
brightness temperature, as well as optical coordinates, we identified six
blazars and six misaligned radio AGNs, with the remaining one tentatively
identified as blazar
Radio-loud active galactic nuclei at high redshifts and the cosmic microwave background
The interaction between the emitting electrons and the cosmic microwave background (CMB) affects the observable properties of radio-loud active galactic nuclei (AGN) at early epochs. At high redshifts z, the CMB energy density [UCMB 1d (1 + z)4] can exceed the magnetic one (UB) in the lobes of radio-loud AGN. In this case, the relativistic electrons cool preferentially by scattering off CMB photons, rather than by synchrotron emission. This makes more distant sources less luminous in radio and more luminous in X-rays than their closer counterparts. In contrast, in the inner jet and the hotspots, where UB > UCMB, synchrotron radiation is unaffected by the presence of the CMB. The decrease in radio luminosity is thus more severe in misaligned (with respect to our line of sight) high-z sources, whose radio flux is dominated by the extended isotropic component. These sources can fail detection in current flux-limited radio surveys, where they are possibly underrepresented. As the cooling time is longer for lower energy electrons, the radio luminosity deficit due to the CMB is less important at low radio frequencies. Therefore, objects not detected so far at a few GHz could be picked up by low-frequency deep surveys, such as Low-Frequency Array and Square Kilometre Array. Until then, we can estimate the number of high-z radio-loud AGN through the census of their aligned proxies, i.e. blazars, since their observed radio emission arises in the inner and strongly magnetized compact core of the jet and it is not affected by inverse Compton scattering off CMB photons
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