18,474 research outputs found
Searching for -ray signature in WHSP blazars: Fermi-LAT detection of 150 excess signal in the 0.3-500 GeV band
A direct search of -ray emission centered on multi-frequency selected
candidates is a valuable complementary approach to the standard search adopted
in current -ray Fermi-LAT catalogs. Our candidates are part of the
2WHSP sample, that was assembled with the aim of providing targets for Imaging
Atmospheric Cherenkov Telescopes (IACT), and is currently the largest set of
high synchrotron peaked (HSP) blazars. We perform a likelihood analysis with
the Fermi Science Tools using positions from 400 2WHSP blazars as seeds of
tentative -ray sources. This enabled us to detect 150 -ray
excess signals that have not yet been reported in previous -ray
catalogs (1FGL, 2FGL, and 3FGL). By identifying new sources, we solve a
fraction of the extragalactic isotropic -ray background (IGRB)
composition, improving the description of the -ray sky. Our analysis
considers the 0.3-500 GeV energy band, integrating over 7.2 yrs of Fermi-LAT
observation and making use of the Pass 8 data release. Within the 150 2WHSPs
that showed excess -ray signature: 85 are high-significance detections
with test statistic (TS)25, and 65 are lower-significance detections with TS
between 10 to 25. We study the -ray photon spectral index distribution,
the likelihood of detection according to the synchrotron peak brightness, and
plot the measured -ray LogN-LogS of HSP blazars, also discussing the
portion of the IGRB that has been resolved by this work. We report on four
cases where we could resolve source confusion and find counterparts for
unassociated 3FGL sources with the help of high-energy TS maps together with
multi-frequency data. The 150 new -ray sources are named with the
acronym 1BIGB for the first version of the Brazil ICRANet Gamma-ray Blazar
catalog, in reference to the cooperation agreement supporting this work.Comment: Accepted for publication in A&A: September 01, 201
Achievable Angles Between two Compressed Sparse Vectors Under Norm/Distance Constraints Imposed by the Restricted Isometry Property: A Plane Geometry Approach
The angle between two compressed sparse vectors subject to the norm/distance
constraints imposed by the restricted isometry property (RIP) of the sensing
matrix plays a crucial role in the studies of many compressive sensing (CS)
problems. Assuming that (i) u and v are two sparse vectors separated by an
angle thetha, and (ii) the sensing matrix Phi satisfies RIP, this paper is
aimed at analytically characterizing the achievable angles between Phi*u and
Phi*v. Motivated by geometric interpretations of RIP and with the aid of the
well-known law of cosines, we propose a plane geometry based formulation for
the study of the considered problem. It is shown that all the RIP-induced
norm/distance constraints on Phi*u and Phi*v can be jointly depicted via a
simple geometric diagram in the two-dimensional plane. This allows for a joint
analysis of all the considered algebraic constraints from a geometric
perspective. By conducting plane geometry analyses based on the constructed
diagram, closed-form formulae for the maximal and minimal achievable angles are
derived. Computer simulations confirm that the proposed solution is tighter
than an existing algebraic-based estimate derived using the polarization
identity. The obtained results are used to derive a tighter restricted isometry
constant of structured sensing matrices of a certain kind, to wit, those in the
form of a product of an orthogonal projection matrix and a random sensing
matrix. Follow-up applications to three CS problems, namely, compressed-domain
interference cancellation, RIP-based analysis of the orthogonal matching
pursuit algorithm, and the study of democratic nature of random sensing
matrices are investigated.Comment: submitted to IEEE Trans. Information Theor
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