473 research outputs found
Detecting the Elusive Blazar Counter-Jets
Detection of blazar pc scale counter-jets is difficult, but it can provide
invaluable insight into the relativistic effects, radiative processes and the
complex mechanisms of jet production, collimation and accelation in blazars. We
build on recent populations models (optimized using the MOJAVE apparent
velocity and redshift distributions) in order to derive the distribution of
jet-to-counter-jet ratios and the flux densities of the counter-jet at
different frequencies, in an effort to set minimum sensitivity limits required
for existing and future telescope arrays in order to detect these elusive
counter-jets. We find that: for the BL Lacs of their counter-jets have a
flux-density higher than 100mJy, are higher than 10 mJy, and have
higher flux-density than 1 mJy, whereas for the FSRQs have a flux-density
higher than 10mJy, are higher than 1 mJy, and are higher than 0.1
mJy (at 15 GHz). Future telescopes like the SKA and newly operating like
e-MERLIN and JVLA may detect up to of the BL Lac and of the FSRQ
counter-jets. Sources with both low apparent velocity and a low Doppler factor
make prime candidates for counter-jet detection. Combining our findings with
literature values we have identified five such counter-jet detection
candidates. Finally, we discuss possible effects beyond relativistic deboosting
that may complicate the detection of counter-jets and that need to be accounted
for in the interpretation of detections.Comment: 13 pages, 15 figures, accepted for publication in MNRA
Limb Lengthening Using the PRECICE<sup>TM</sup> Nail System: Complications and Results
Background: Three types of telescopic nails are mainly used for intramedullary limb lengthening nowadays. Despite some important advantages of this new technology (e.g. controlled distraction rate, not restricted availability, possibility to perform accordion maneuvers), few articles exist on clinical results and complications after lengthening with the PRECICETM nail (Ellipse, USA).
Objectives: The aim of the current study was to describe and analyze the complications associated with lengthening with the PRECICETM nail. Are the problems preventable when using the PRECICE, related to the distraction rate control, the lengthening goals and technique and handling?
Methods: We retrospectively reviewed the charts of 9 patients operated between 2012 and 2013 with a PRECICETM nail for a leg length discrepancy (LLD). The mean age of the patients was 32 years (range, 17 - 48 years). There were 5 femoral and 4 tibial procedures. The causes of LLD were posttraumatic (n = 5) and congenital (n = 4). The mean LLD was 36.4 ± 11.4 mm. The minimum follow-ups were 2 months (average, 5 months; range, 2 - 9 months).
Results: The mean distraction rate was 0.5 ± 0.1 mm/day. We observed in 7 patients differences in achieving the lengthening goals (average, 1.6 mm; range, -20.0 - 5.0 mm). Average lengthening was 34.7 ± 10.7 mm. All patients reached normal alignment and normal joint orientation. An unintentional loss of the achieved length during the consolidation phase was noticed in patients with delayed bone healing in two cases. In the first case (loss of 20mm distraction) the nail could be redistracted and the goal length was achieved. In the second case (loss of 10mm distraction) the nail broke shortly after the diagnosis and the nail was exchanged.
Conclusions: We report of loss of achieved length after lengthening with a telescopic nail. Weight bearing before complete consolidation of the regenerate might be a risk factor for that. Thorough examination of the limb length and careful evaluation of the radiographs are required in the follow-up period. The PRECICE nail system requires the same vigilance like the other intramedullary systems too
Scale invariant jets: from blazars to microquasars
Black holes, anywhere in the stellar-mass to supermassive range, are often
associated with relativistic jets. Models suggest that jet production may be a
universal process common in all black hole systems regardless of their mass.
Although in many cases observations support such hypotheses for microquasars
and Seyfert galaxies, little is known on whether boosted blazar jets also
comply with such universal scaling laws. We use uniquely rich multiwavelength
radio light curves from the F-GAMMA program and the most accurate Doppler
factors available to date to probe blazar jets in their emission rest frame
with unprecedented accuracy. We identify for the first time a strong
correlation between the blazar intrinsic broad-band radio luminosity and black
hole mass, which extends over 9 orders of magnitude down to microquasars
scales. Our results reveal the presence of a universal scaling law that bridges
the observing and emission rest frames in beamed sources and allows us to
effectively constrain jet models. They consequently provide an independent
method for estimating the Doppler factor, and for predicting expected radio
luminosities of boosted jets operating in systems of intermediate or
tens-of-solar mass black holes, immediately applicable to cases as those
recently observed by LIGO.Comment: 13 pages, 4 figures, accepted for publication in AP
F-GAMMA: Variability Doppler factors of blazars from multiwavelength monitoring
Recent population studies have shown that the variability Doppler factors can
adequately describe blazars as a population. We use the flux density variations
found within the extensive radio multi-wavelength datasets of the F-GAMMA
program, a total of 10 frequencies from 2.64 up to 142.33 GHz, in order to
estimate the variability Doppler factors for 58 -ray bright sources,
for 20 of which no variability Doppler factor has been estimated before. We
employ specifically designed algorithms in order to obtain a model for each
flare at each frequency. We then identify each event and track its evolution
through all the available frequencies for each source. This approach allows us
to distinguish significant events producing flares from stochastic variability
in blazar jets. It also allows us to effectively constrain the variability
brightness temperature and hence the variability Doppler factor as well as
provide error estimates. Our method can produce the most accurate (16\% error
on average) estimates in the literature to date.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Search for AGN counterparts of unidentified Fermi-LAT sources with optical polarimetry: Demonstration of the technique
The third Fermi-LAT catalog (3FGL) presented the data of the first four years
of observations from the Fermi Gamma-ray Space Telescope mission. There are
3034 sources, 1010 of which still remain unidentified. Identifying and
classifying gamma-ray emitters is of high significance with regard to studying
high-energy astrophysics. We demonstrate that optical polarimetry can be an
advantageous and practical tool in the hunt for counterparts of the
unidentified gamma-ray sources (UGSs). Using data from the RoboPol project, we
validated that a significant fraction of active galactic nuclei (AGN)
associated with 3FGL sources can be identified due to their high optical
polarization exceeding that of the field stars. We performed an optical
polarimetric survey within uncertainties of four unidentified 3FGL
sources. We discovered a previously unknown extragalactic object within the
positional uncertainty of 3FGL J0221.2+2518. We obtained its spectrum and
measured a redshift of . Using these measurements and
archival data we demonstrate that this source is a candidate counterpart for
3FGL J0221.2+2518 and most probably is a composite object: a star-forming
galaxy accompanied by AGN. We conclude that polarimetry can be a powerful asset
in the search for AGN candidate counterparts for unidentified Fermi sources.
Future extensive polarimetric surveys at high galactic latitudes (e.g.,
PASIPHAE) will allow the association of a significant fraction of currently
unidentified gamma-ray sources.Comment: accepted to A&
Kinetics of the thermal degradation of Erica arborea by DSC: Hybrid kinetic method
The scope of this work was the determination of kinetic parameters of the
thermal oxidative degradation of a Mediterranean scrub using a hybrid method
developed at the laboratory. DSC and TGA were used in this study under air
sweeping to record oxidative reactions. Two dominating and overlapped
exothermic peaks were recorded in DSC and individualized using an experimental
and numerical separation. This first stage allowed obtaining the enthalpy
variation of each exothermic phenomenon. In a second time, a model free method
was applied on each isolated curve to determine the apparent activation
energies. A reactional kinetic scheme was proposed for the global exotherm
composed of two independent and consecutive reactions. In fine mean values of
enthalpy variation and apparent activation energy previously determined were
injected in a model fitting method to obtain the reaction order and the
preexponential factor of each oxidative reaction. We plan to use these data in
a sub-model to be integrated in a wildland fire spread model
Polarimetry of the potential binary supermassive black hole system in J1430+2303
The growth of supermassive black holes (SMBHs) through merging has long been
predicted but its detection remains elusive. However, a promising target has
been discovered in the Seyfert-1 galaxy J1430+2303. If a binary system truly
lies at the center of J1430+2303, the usual symmetry expected from pole-on
views in active galactic nuclei (AGNs) responsible for the observed low (
1\%) optical linear polarization in the continuum of these objects is expected
to be broken. This should lead to higher-than-usual polarization degrees,
together with time-dependent variations of the polarization signal. We used the
specialized photopolarimeters RoboPol mounted on the 1.3m telescope at the
Skinakas Observatory and the Alhambra Faint Object Spectrograph and Camera
(ALFOSC) mounted on the 2.56m Nordic Optical Telescope (NOT) at the "Roque de
los Muchachos" Observatory to measure the B-, V-, R-, and I-band polarization
of J1430+2303. Observations were complemented using the FORS2
spectropolarimeter mounted on the VLT to acquire 3500 -- 8650 Angs polarized
spectra. We compared our set of observations to Monte Carlo radiative-transfer
predictions to look for the presence of a SMBH binary. The observed linear
continuum polarization of J1430+2303 in the V and R bands is 0.4\% with
an associated polarization angle of slightly larger than 0. We detected
no significant changes in polarization or photometry between May, June, and
July of 2022. In addition, there is no significant difference between the
polarization of H and the polarization of the continuum. A single SMBH
at the center of an AGN model is able to reproduce the observed spectrum and
polarization, while the binary hypothesis is rejected with a probability of
85\%.Comment: 10 pages, 8 figures 6 tables, accepted for publication in A&
Fermi LAT AGN classification using supervised machine learning
Classifying Active Galactic Nuclei (AGN) is a challenge, especially for BL
Lac Objects (BLLs), which are identified by their weak emission line spectra.
To address the problem of classification, we use data from the 4th Fermi
Catalog, Data Release 3. Missing data hinders the use of machine learning to
classify AGN. A previous paper found that Multiple Imputation by Chain
Equations (MICE) imputation is useful for estimating missing values. Since many
AGN have missing redshift and the highest energy, we use data imputation with
MICE and K-nearest neighbor (kNN) algorithm to fill in these missing variables.
Then, we classify AGN into the BLLs or the Flat Spectrum Radio Quasars (FSRQs)
using the SuperLearner, an ensemble method that includes several classification
algorithms like logistic regression, support vector classifiers, Random
Forests, Ranger Random Forests, multivariate adaptive regression spline (MARS),
Bayesian regression, Extreme Gradient Boosting. We find that a SuperLearner
model using MARS regression and Random Forests algorithms is 91.1% accurate for
kNN imputed data and 91.2% for MICE imputed data. Furthermore, the kNN-imputed
SuperLearner model predicts that 892 of the 1519 unclassified blazars are BLLs
and 627 are Flat Spectrum Radio Quasars (FSRQs), while the MICE-imputed
SuperLearner model predicts 890 BLLs and 629 FSRQs in the unclassified set.
Thus, we can conclude that both imputation methods work efficiently and with
high accuracy and that our methodology ushers the way for using SuperLearner as
a novel classification method in the AGN community and, in general, in the
astrophysics community.Comment: 15 pages, 8 figures, to be published in Monthly Notices of the Royal
Astronomical Societ
Reconciling inverse-Compton Doppler factors with variability Doppler factors in blazar jets
Context. Blazar population models have shown that the inverse-Compton and variability Doppler factor estimates yield consistent results at the population level for flat spectrum radio quasars (FSRQs). The two methods, however, are inconsistent when compared on a source-by-source basis.Aims. In this work, we attempt to understand the source of the discrepancy by tracing the potential sources of systematic and statistical error for the inverse-Compton Doppler factors. By eliminating these sources of error, we provide stronger constrains on the value of the Doppler factor in blazar jets.Methods. We re-estimate the inverse-Compton Doppler factor for 11 sources that meet certain criteria for their synchrotron peak frequency and the availability of Doppler factor estimates in the literature. We compare these estimates with the average of two di ff erent estimates of the variability Doppler factor obtained using various datasets and methodologies to identify any discrepancies and, in each case, trace their sources in the methodology or assumptions adopted.Results. We identify three significant sources of error for the inverse-Compton Doppler factors: a) contamination of the X-ray flux by non-synchrotron self-Compton emission; b) radio observations at frequencies other than the synchrotron turnover frequency; c) non-simultaneity between radio and X-ray observations. We discuss key aspects in the correct application of the inverse-Compton method in light of these potential errors. We are able to constrain the Doppler factor of 3C 273, 3C 345, 3C 454.3, PKS 1510-089, and PKS 1633 + 382 e ff ectively, since all available estimates from both methods converge to the same values for these five sources
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