1,274 research outputs found
Eruption of a plasma blob, associated M-class flare, and large-scale EUV wave observed by SDO
We present a multiwavelength study of the formation and ejection of a plasma
blob and associated EUV waves in AR NOAA 11176, observed by SDO/AIA and STEREO
on 25 March 2011. SDO/AIA images clearly show the formation and ejection of a
plasma blob from the lower solar atmosphere at ~9 min prior to the onset of the
M1.0 flare. This onset of the M-class flare happened at the site of the blob
formation, while the blob was rising in a parabolic path with an average speed
of ~300 km/s. The blob also showed twisting and de-twisting motion in the lower
corona, and the blob speed varied from ~10-540 km/s. The faster and slower EUV
wavefronts were observed in front of the plasma blob during its impulsive
acceleration phase. The faster EUV wave propagated with a speed of ~785 to 1020
km/s, whereas the slower wavefront speed varied in between ~245 and 465 km/s.
The timing and speed of the faster wave match the shock speed estimated from
the drift rate of the associated type II radio burst. The faster wave
experiences a reflection by the nearby AR NOAA 11177. In addition, secondary
waves were observed (only in the 171 \AA channel), when the primary fast wave
and plasma blob impacted the funnel-shaped coronal loops. The HMI magnetograms
revealed the continuous emergence of new magnetic flux along with shear flows
at the site of the blob formation. It is inferred that the emergence of twisted
magnetic fields in the form of arch-filaments/"anemone-type" loops is the
likely cause for the plasma blob formation and associated eruption along with
the triggering of M-class flare. Furthermore, the faster EUV wave formed ahead
of the blob shows the signature of fast-mode MHD wave, whereas the slower wave
seems to be generated by the field line compression by the plasma blob. The
secondary wave trains originated from the funnel-shaped loops are probably the
fast magnetoacoustic waves.Comment: A&A (in press), 22 pages, 13 figure
High Energy Emission Processes in OJ 287 during 2009 Flare
The broadband spectrum of a BL Lac object, OJ 287, from radio to
-rays obtained during a major -ray flare detected by
\emph{Fermi} in 2009 are studied to understand the high energy emission
mechanism during this episode. Using a simple one-zone leptonic model,
incorporating synchrotron and inverse Compton emission processes, we show that
the explanation of high energy emission from X-rays to -rays, by
considering a single emission mechanism, namely, synchrotron self-Compton (SSC)
or external Compton (EC) requires unlikely physical conditions. However, a
combination of both SSC and EC mechanisms can reproduce the observed high
energy spectrum satisfactorily. Using these emission mechanisms we extract the
physical parameters governing the source and its environment. Our study
suggests that the emission region of OJ 287 is surrounded by a warm infrared
(IR) emitting region of . Assuming this region as a spherical
cloud illuminated by an accretion disk, we obtain the location of the emission
region to be . This supports the claim that the -ray
emission from OJ 287 during the 2009 flare arises from a location far away from
the central engine as deduced from millimeter-gamma ray correlation study and
very long baseline array images.Comment: 22 pages, 7 figures, 1 table, accepted for publication in MNRA
Brightest Fermi-LAT Flares of PKS 1222+216: Implications on Emission and Acceleration Processes
We present a high time resolution study of the two brightest -ray
outbursts from a blazar PKS 1222+216 observed by the \textit{Fermi} Large Area
Telescope (LAT) in 2010. The -ray light-curves obtained in four
different energy bands: 0.1--3, 0.1--0.3, 0.3--1 and 1--3 GeV, with time bin of
6 hr, show asymmetric profiles with a similar rise time in all the bands but a
rapid decline during the April flare and a gradual one during the June. The
light-curves during the April flare show days long plateau in 0.1--0.3
GeV emission, erratic variations in 0.3--1 GeV emission, and a daily recurring
feature in 1--3 GeV emission until the rapid rise and decline within a day. The
June flare shows a monotonic rise until the peak, followed by a gradual decline
powered mainly by the multi-peak 0.1--0.3 GeV emission. The peak fluxes during
both the flares are similar except in the 1--3 GeV band in April which is twice
the corresponding flux during the June flare. Hardness ratios during the April
flare indicate spectral hardening in the rising phase followed by softening
during the decay. We attribute this behavior to the development of a shock
associated with an increase in acceleration efficiency followed by its decay
leading to spectral softening. The June flare suggests hardening during the
rise followed by a complicated energy dependent behavior during the decay.
Observed features during the June flare favor multiple emission regions while
the overall flaring episode can be related to jet dynamics.Comment: 17 pages, 9 figures, 4 tables, accepted for publication in Ap
Codes Detecting and Correcting Solid Burst Errors
This paper studies linear codes capable of detecting and correcting solid burst error of length b or less. The lower and upper bounds on the number of parity-check digits required for such codes are obtained. Illustrations of codes for detecting as well as correcting such errors are provided
Multi-wavelength Temporal Variability of the Blazar 3C 454.3 during 2014 Activity Phase
We present a multi-wavelength temporal analysis of the blazar 3C 454.3 during
the high -ray active period from May-December, 2014. Except for X-rays,
the period is well sampled at near-infrared (NIR)-optical by the \emph{SMARTS}
facility and the source is detected continuously on daily timescale in the
\emph{Fermi}-LAT -ray band. The source exhibits diverse levels of
variability with many flaring/active states in the continuously sampled
-ray light curve which are also reflected in the NIR-optical light
curves and the sparsely sampled X-ray light curve by the \emph{Swift}-XRT.
Multi-band correlation analysis of this continuous segment during different
activity periods shows a change of state from no lags between IR and
-ray, optical and -ray, and IR and optical to a state where
-ray lags the IR/optical by 3 days. The results are consistent
with the previous studies of the same during various -ray flaring and
active episodes of the source. This consistency, in turn, suggests an extended
localized emission region with almost similar conditions during various
-ray activity states. On the other hand, the delay of -ray with
respect to IR/optical and a trend similar to IR/optical in X-rays along with
strong broadband correlations favor magnetic field related origin with X-ray
and -ray being inverse Comptonized of IR/optical photons and external
radiation field, respectively.Comment: 15 pages, 5 figures, 1 table, MNRAS accepte
Computing Similarity between a Pair of Trajectories
With recent advances in sensing and tracking technology, trajectory data is
becoming increasingly pervasive and analysis of trajectory data is becoming
exceedingly important. A fundamental problem in analyzing trajectory data is
that of identifying common patterns between pairs or among groups of
trajectories. In this paper, we consider the problem of identifying similar
portions between a pair of trajectories, each observed as a sequence of points
sampled from it.
We present new measures of trajectory similarity --- both local and global
--- between a pair of trajectories to distinguish between similar and
dissimilar portions. Our model is robust under noise and outliers, it does not
make any assumptions on the sampling rates on either trajectory, and it works
even if they are partially observed. Additionally, the model also yields a
scalar similarity score which can be used to rank multiple pairs of
trajectories according to similarity, e.g. in clustering applications. We also
present efficient algorithms for computing the similarity under our measures;
the worst-case running time is quadratic in the number of sample points.
Finally, we present an extensive experimental study evaluating the
effectiveness of our approach on real datasets, comparing with it with earlier
approaches, and illustrating many issues that arise in trajectory data. Our
experiments show that our approach is highly accurate in distinguishing similar
and dissimilar portions as compared to earlier methods even with sparse
sampling
Crossing the Logarithmic Barrier for Dynamic Boolean Data Structure Lower Bounds
This paper proves the first super-logarithmic lower bounds on the cell probe
complexity of dynamic boolean (a.k.a. decision) data structure problems, a
long-standing milestone in data structure lower bounds.
We introduce a new method for proving dynamic cell probe lower bounds and use
it to prove a lower bound on the operational
time of a wide range of boolean data structure problems, most notably, on the
query time of dynamic range counting over ([Pat07]). Proving an
lower bound for this problem was explicitly posed as one of
five important open problems in the late Mihai P\v{a}tra\c{s}cu's obituary
[Tho13]. This result also implies the first lower bound for the
classical 2D range counting problem, one of the most fundamental data structure
problems in computational geometry and spatial databases. We derive similar
lower bounds for boolean versions of dynamic polynomial evaluation and 2D
rectangle stabbing, and for the (non-boolean) problems of range selection and
range median.
Our technical centerpiece is a new way of "weakly" simulating dynamic data
structures using efficient one-way communication protocols with small advantage
over random guessing. This simulation involves a surprising excursion to
low-degree (Chebychev) polynomials which may be of independent interest, and
offers an entirely new algorithmic angle on the "cell sampling" method of
Panigrahy et al. [PTW10]
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