796 research outputs found
Language Use Matters: Analysis of the Linguistic Structure of Question Texts Can Characterize Answerability in Quora
Quora is one of the most popular community Q&A sites of recent times.
However, many question posts on this Q&A site often do not get answered. In
this paper, we quantify various linguistic activities that discriminates an
answered question from an unanswered one. Our central finding is that the way
users use language while writing the question text can be a very effective
means to characterize answerability. This characterization helps us to predict
early if a question remaining unanswered for a specific time period t will
eventually be answered or not and achieve an accuracy of 76.26% (t = 1 month)
and 68.33% (t = 3 months). Notably, features representing the language use
patterns of the users are most discriminative and alone account for an accuracy
of 74.18%. We also compare our method with some of the similar works (Dror et
al., Yang et al.) achieving a maximum improvement of ~39% in terms of accuracy.Comment: 1 figure, 3 tables, ICWSM 2017 as poste
Chandra & HST Imaging of the Quasars PKS B0106+013 & 3C345: Inverse Compton X-rays and Magnetized Jets
We present results from deep (70 ks) Chandra ACIS observations and Hubble
Space Telescope ACS F475W observations of two highly optically polarized
quasars belonging to the MOJAVE blazar sample, viz., PKS B0106+013 and 1641+399
(3C345). These observations reveal X-ray and optical emission from the jets in
both sources. X-ray emission is detected from the entire length of the 0106+013
radio jet, which shows clear bends or wiggles - the X-ray emission is brightest
at the first prominent kpc jet bend. A picture of a helical kpc jet with the
first kpc-scale bend representing a jet segment moving close(r) to our line of
sight, and getting Doppler boosted at both radio and X-ray frequencies, is
consistent with these observations. The X-ray emission from the jet end however
peaks at about 0.4" (~3.4 kpc) upstream of the radio hot spot. Optical emission
is detected both at the X-ray jet termination peak and at the radio hot spot.
The X-ray jet termination peak is found upstream of the radio hot spot by
around 0.2" (~1.3 kpc) in the short projected jet of 3C345. HST optical
emission is seen in an arc-like structure coincident with the bright radio hot
spot, which we propose is a sharp (apparent) jet bend instead of a terminal
point, that crosses our line of sight and consequently has a higher Doppler
beaming factor. A weak radio hot spot is indeed observed less than 1"
downstream of the bright radio hot spot, but has no optical or X-ray
counterpart. By making use of the pc-scale radio and the kpc-scale radio/X-ray
data, we derive constraints on the jet Lorentz factors (Gamma_jet) and
inclination angles (theta): for a constant jet speed from pc- to kpc-scales, we
obtain a Gamma_jet of ~70 for 0106+013, and ~40 for 3C345. On relaxing this
assumption, we derive a Gamma_jet of ~2.5 for both the sources. Upper limits on
theta of ~13 degrees are obtained for the two quasars. (ABRIDGED)Comment: 46 pages, 11 figures, Accepted for publication in Ap
Magnetic Field Geometry in "Red" and "Blue" BL Lacs
We compare the systematics of the magnetic field geometry in the "red"
low-energy peaked BL Lacs (LBLs) and "blue" high-energy peaked BL Lacs (HBLs)
using VLBI polarimetric images. The LBLs are primarily "radio--selected" BL
Lacs and the HBLs are primarily "X-ray selected". In contrast to the LBLs,
which show predominantly transverse jet magnetic fields, the HBLs show
predominantly longitudinal fields. Thus, while the SED peaks of core-dominated
quasars, LBLs and HBLs form a sequence of increasing frequency, the magnetic
field geometry does not follow an analogous sequence. We briefly investigate
possible connections between the observed parsec-scale magnetic field
structures and circular polarization measurements in the literature on various
spatial scales.Comment: 12 pages, 5 figures, Proceedings of the Amsterdam workshop on
"Circular polarisation from relativistic jet sources", to be published in
Astrophysics and Space Science, eds. Rob Fender & J-P Macquar
The Powerful Jet and Gamma-Ray Flare of the Quasar PKS 0438436
PKS 0438436 at a redshift of has been previously recognized as
possessing perhaps the most luminous known synchrotron jet. Little is known
about this source since the maximum elevation above the horizon is low for the
Very Large Array (VLA). We present the first VLA radio image that detects the
radio lobes. We use both the 151 MHz luminosity, as a surrogate for the
isotropic radio lobe luminosity, and the lobe flux density from the radio image
to estimate a long term, time averaged, jet power, . We analyze two deep optical spectra with
strong broad emission lines and estimate the thermal bolometric luminosity of
the accretion flow, . The ratio, , is at
the limit of this empirical metric of jet dominance seen in radio loud quasars
and this is the most luminous accretion flow to have this limiting behavior.
Despite being a very luminous blazar, it previously had no -ray
detections (EGRET, AGILE or FERMI) until December 11 - 13 2016 (54 hours) when
FERMI detected a flare that we analyze here. The isotropic apparent luminosity
from 100 MeV - 100 GeV rivals the most luminous detected blazar flares
(averaged over 18 hours), . The
-ray luminosity varies over time by two orders of magnitude,
highlighting the extreme role of Doppler abberation and geometric alignment in
producing the inverse Compton emission.Comment: To appear in ApJ. Revision required to replace the last figure file
with the current versio
Chandra Discovery of 10 New X-Ray Jets Associated With FR II Radio Core-Selected AGNs in the MOJAVE Sample
The Chandra X-ray observatory has proven to be a vital tool for studying
high-energy emission processes in jets associated with Active Galactic Nuclei
(AGN).We have compiled a sample of 27 AGN selected from the radio flux-limited
MOJAVE (Monitoring of Jets in AGN with VLBA Experiments) sample of highly
relativistically beamed jets to look for correlations between X-ray and radio
emission on kiloparsec scales. The sample consists of all MOJAVE quasars which
have over 100 mJy of extended radio emission at 1.4 GHz and a radio structure
of at least 3" in size. Previous Chandra observations have revealed X-ray jets
in 11 of 14 members of the sample, and we have carried out new observations of
the remaining 13 sources. Of the latter, 10 have Xray jets, bringing the
overall detection rate to ~ 78%. Our selection criteria, which is based on
highly compact, relativistically beamed jet emission and large extended radio
flux, thus provides an effective method of discovering new X-ray jets
associated with AGN. The detected X-ray jet morphologies are generally well
correlated with the radio emission, except for those displaying sharp bends in
the radio band. The X-ray emission mechanism for these powerful FR II
(Fanaroff-Riley type II) jets can be interpreted as inverse Compton scattering
off of cosmic microwave background (IC/CMB) photons by the electrons in the
relativistic jets. We derive viewing angles for the jets, assuming a
non-bending, non-decelerating model, by using superluminal parsec scale speeds
along with parameters derived from the inverse Compton X-ray model. We use
these angles to calculate best fit Doppler and bulk Lorentz factors for the
jets, as well as their possible ranges, which leads to extreme values for the
bulk Lorentz factor in some cases. When both the non-bending and
non-decelerating assumptions are relaxed [abridged]Comment: 38 Pages, 4 Figures, 5 Tables, accepted for publication in Ap
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