845 research outputs found
From Group Recommendations to Group Formation
There has been significant recent interest in the area of group
recommendations, where, given groups of users of a recommender system, one
wants to recommend top-k items to a group that maximize the satisfaction of the
group members, according to a chosen semantics of group satisfaction. Examples
semantics of satisfaction of a recommended itemset to a group include the
so-called least misery (LM) and aggregate voting (AV). We consider the
complementary problem of how to form groups such that the users in the formed
groups are most satisfied with the suggested top-k recommendations. We assume
that the recommendations will be generated according to one of the two group
recommendation semantics - LM or AV. Rather than assuming groups are given, or
rely on ad hoc group formation dynamics, our framework allows a strategic
approach for forming groups of users in order to maximize satisfaction. We show
that the problem is NP-hard to solve optimally under both semantics.
Furthermore, we develop two efficient algorithms for group formation under LM
and show that they achieve bounded absolute error. We develop efficient
heuristic algorithms for group formation under AV. We validate our results and
demonstrate the scalability and effectiveness of our group formation algorithms
on two large real data sets.Comment: 14 pages, 22 figure
A Novel Capacitive Transducer Based on a Simple Resonant Circuit
A novel room temperature capacitive transducer is described, which offers
unparalleled signal to noise ratio for detecting small relative changes in
capacitance, outperforming the best capacitance bridges. The transducer uses a
simple resonant circuit, fashioned from a discrete inductor with small
dissipation resonating with the capacitor acting as the sensor, to obtain a
relative resolution equal or better than 10-11 in a 1 Hz bandwidth. When used
as a mechanical displacement transducer, the predicted position noise is about
3 femtometre per rootHz. The circuit measures the change in capacitance by
detecting the corresponding phase modulation at the resonance frequency and is
configured to act as a close approximation of an all-pass filter to minimise
the effects of amplitude modulation in case the drive signal is imperfectly
tuned to the resonance. Some other applications of the proposed circuit are
possible such as measuring electric field, where the sensing capacitor depends
on the applied electric field. Also, the circuit can be easily adapted to
function with very small capacitance values (1 - 2 pF) as is typical in MEMS
based transducers.Comment: 16 pages, 20 figures, preprint prepared for a submission to Nature
Electronic
The enzymes LSD1 and Set1A cooperate with the viral protein HBx to establish an active hepatitis B viral chromatin state
Indexación: Web of ScienceWith about 350 million people chronically infected around the world hepatitis B is a major health problem. Template for progeny HBV synthesis is the viral genome, organized as a minichromosome (cccDNA) inside the hepatocyte nucleus. How viral cccDNA gene expression is regulated by its chromatin structure; more importantly, how the modulation of this structure impacts on viral gene expression remains elusive. Here, we found that the enzyme SetDB1 contributes to setting up a repressed cccDNA chromatin state. This repressive state is activated by the histone lysine demethylase-1 (LSD1). Consistently, inhibiting or reducing LSD1 levels led to repression of viral gene expression. This correlates with the transcriptionally repressive mark H3K9 methylation and reduction on the activating marks H3 acetylation and H3K4 methylation on viral promoters. Investigating the importance of viral proteins we found that LSD1 recruitment to viral promoters was dependent on the viral transactivator protein HBx. Moreover, the histone methyltransferase Set1A and HBx are simultaneously bound to the core promoter, and Set1A expression correlates with cccDNA H3K4 methylation. Our results shed light on the mechanisms of HBV regulation mediated by the cccDNA chromatin structure, offering new therapeutic targets to develop drugs for the treatment of chronically infected HBV patients.http://www.nature.com/articles/srep2590
Comparison of NOAA/NMC stratospheric wind analyses with UARS high resolution Doppler Imager wind measurements
The NOAA National Weather Service currently derives global stratospheric wind analyses via several procedures. The first is the operational data assimilation system that extends from the surface up to about 50 mb and is in process of being tested to about 10 mb. In addition, a balanced wind is determined from the available Climate Analysis Center stratospheric height analyses that encompass the 70-0.4 mb region. The High Resolution Doppler Imager (HRDI) recently launched as a member of the Upper Atmosphere Research Satellite (UARS) is the first satellite instrument designed to measure winds in this stratospheric region and, thus, provide a basic evaluation of the NMC derived products. The HRDI accomplishes this by utilizing a triple-etalon Fabry-Perot interferometer that allows one to measure the Doppler shift of O2 absorption and emission features of the atmosphere, from which the wind field can be determined
Afterglow rebrightenings as a signature of a long-lasting central engine activity? The emblematic case of GRB 100814A
In the past few years the number of well-sampled optical to NIR light curves
of long Gamma-Ray Bursts (GRBs) has greatly increased particularly due to
simultaneous multi-band imagers such as GROND. Combining these densely sampled
ground-based data sets with the Swift UVOT and XRT space observations unveils a
much more complex afterglow evolution than what was predicted by the most
commonly invoked theoretical models. GRB 100814A represents a remarkable
example of these interesting well-sampled events, showing a prominent late-time
rebrightening in the optical to NIR bands and a complex spectral evolution.
This represents a unique laboratory to test the different afterglow emission
models. Here we study the nature of the complex afterglow emission of GRB
100814A in the framework of different theoretical models. Moreover, we compare
the late-time chromatic rebrightening with those observed in other well-sampled
long GRBs. We analysed the optical and NIR observations obtained with the
seven-channel Gamma-Ray burst Optical and Near-infrared Detector at the 2.2 m
MPG/ESO telescope together with the X-ray and UV data detected by the
instruments onboard the Swift observatory. The broad-band afterglow evolution,
achieved by constructing multi-instrument light curves and spectral energy
distributions, will be discussed in the framework of different theoretical
models. We find that the standard models that describe the broad-band afterglow
emission within the external shock scenario fail to describe the complex
evolution of GRB 100814A, and therefore more complex scenarios must be invoked.
[abridged]Comment: 11 pages, 7 figures, 2 tables; Astronomy & Astrophysics, in pres
Multiwavelength analysis of three SNe associated with GRBs observed by GROND
After the discovery of the first connection between GRBs and SNe almost two
decades ago, tens of SN-like rebrightenings have been discovered and about
seven solid associations have been spectroscopically confirmed to date. Using
GROND optical/NIR data and Swift X-ray/UV data, we estimate the intrinsic
extinction, luminosity, and evolution of three SN rebrightenings in GRB
afterglow light curves at z~0.5. The SNe 2008hw, 2009nz, and 2010ma exhibit
0.80, 1.15, and 1.78 times the optical (r band) luminosity of SN 1998bw,
respectively. While SN 2009nz evolves similarly to SN 1998bw, SNe 2008hw and
2010ma show earlier peak times. The quasi-bolometric light curves were
corrected for the contribution of the NIR bands using data available in the
literature and blackbody fits. The large luminosity of SN 2010ma (1.4x10^43
erg/s) is confirmed, while SNe 2008hw and 2009nz reached a peak luminosity
closer to SN 1998bw. Physical parameters of the SN explosions, such as
synthesised nickel mass, ejecta mass, and kinetic energy, are estimated using
Arnett's analytic approach, which resulted in nickel masses of around 0.4-0.5
Msun. By means of the a very comprehensive data set, we found that the
luminosity and the nickel mass of SNe 2008hw, 2009nz, and 2010ma resembles
those of other known GRB-associated SNe. This findings strengthens previous
claims of GRB-SNe being brighter than type-Ic SNe unaccompanied by GRBs.Comment: 11 pages, 9 figures, accepted for publication in Astronomy &
Astrophysics, abstract abridge
The optical identifcation of events with poorly defined locations: The case of the Fermi GBM GRB140801A
We report the early discovery of the optical afterglow of gamma-ray burst
(GRB) 140801A in the 137 deg 3- error-box of the Fermi Gamma-ray
Burst Monitor (GBM). MASTER is the only observatory that automatically react to
all Fermi alerts. GRB 140801A is one of the few GRBs whose optical counterpart
was discovered solely from its GBM localization. The optical afterglow of GRB
140801A was found by MASTER Global Robotic Net 53 sec after receiving the
alert, making it the fastest optical detection of a GRB from a GBM error-box.
Spectroscopy obtained with the 10.4-m Gran Telescopio Canarias and the 6-m BTA
of SAO RAS reveals a redshift of . We performed optical and
near-infrared photometry of GRB 140801A using different telescopes with
apertures ranging from 0.4-m to 10.4-m. GRB 140801A is a typical burst in many
ways. The rest-frame bolometric isotropic energy release and peak energy of the
burst is erg and
keV, respectively, which is consistent with the
Amati relation. The absence of a jet break in the optical light curve provides
a lower limit on the half-opening angle of the jet deg. The
observed is consistent with the limit derived from the
Ghirlanda relation. The joint Fermi GBM and Konus-Wind analysis shows that GRB
140801A could belong to the class of intermediate duration. The rapid detection
of the optical counterpart of GRB 140801A is especially important regarding the
upcoming experiments with large coordinate error-box areas.Comment: in press MNRAS, 201
Super-solar metallicity at the position of the ultra-long GRB130925A
Over the last decade there has been immense progress in the follow-up of
short and long GRBs, resulting in a significant rise in the detection rate of
X-ray and optical afterglows, in the determination of GRB redshifts, and of the
identification of the underlying host galaxies. Nevertheless, our theoretical
understanding on the progenitors and central engines powering these vast
explosions is lagging behind, and a newly identified class of `ultra-long' GRBs
has fuelled speculation on the existence of a new channel of GRB formation. In
this paper we present high signal-to-noise X-shooter observations of the host
galaxy of GRB130925A, which is the fourth unambiguously identified ultra-long
GRB, with prompt gamma-ray emission detected for ~20ks. The GRB line of sight
was close to the host galaxy nucleus, and our spectroscopic observations cover
both this region along the bulge/disk of the galaxy, in addition to a bright
star-forming region within the outskirts of the galaxy. From our broad
wavelength coverage we obtain accurate metallicity and dust-extinction
measurements at both the galaxy nucleus, and an outer star-forming region, and
measure a super-solar metallicity at both locations, placing this galaxy within
the 10-20% most metal-rich GRB host galaxies. Such a high metal enrichment has
implications on the progenitor models of both long and ultra-long GRBs,
although the edge-on orientation of the host galaxy does not allow us to rule
out a large metallicity variation along our line of sight. The spatially
resolved spectroscopic data presented in this paper offer important insight
into variations in the metal and dust abundance within GRB host galaxies. They
also illustrate the need for IFU observations on a larger sample of GRB host
galaxies at varies metallicities to provide a more quantitative view on the
relation between the GRB circumburst and the galaxy-whole properties.Comment: 9 pages, 3 figures, A&A in press, matches published versio
GROND coverage of the main peak of Gamma-Ray Burst 130925A
Prompt or early optical emission in gamma-ray bursts is notoriously difficult
to measure, and observations of the dozen cases show a large variety of
properties. Yet, such early emission promises to help us achieve a better
understanding of the GRB emission process(es).
We performed dedicated observations of the ultra-long duration (T90 about
7000 s) GRB 130925A in the optical/near-infrared with the 7-channel "Gamma-Ray
Burst Optical and Near-infrared Detector" (GROND) at the 2.2m MPG/ESO
telescope. We detect an optical/NIR flare with an amplitude of nearly 2 mag
which is delayed with respect to the keV--MeV prompt emission by about 300--400
s. The decay time of this flare is shorter than the duration of the flare (500
s) or its delay.
While we cannot offer a straightforward explanation, we discuss the
implications of the flare properties and suggest ways toward understanding it.Comment: 9 pages, 9 figures, accepted for publ. in A&
The late-time afterglow of the extremely energetic short burst GRB 090510 revisited
The discovery of the short GRB 090510 has raised considerable attention
mainly because it had a bright optical afterglow and it is among the most
energetic events detected so far within the entire GRB population. The
afterglow was observed with swift/UVOT and swift/XRT and evidence of a jet
break around 1.5 ks after the burst has been reported in the literature,
implying that after this break the optical and X-ray light curve should fade
with the same decay slope. As noted by several authors, the post-break decay
slope seen in the UVOT data is much shallower than the steep decay in the X-ray
band, pointing to an excess of optical flux at late times. We reduced and
analyzed new afterglow light-curve data obtained with the multichannel imager
GROND. Based on the densely sampled data set obtained with GROND, we find that
the optical afterglow of GRB 090510 did indeed enter a steep decay phase
starting around 22 ks after the burst. During this time the GROND optical light
curve is achromatic, and its slope is identical to the slope of the X-ray data.
In combination with the UVOT data this implies that a second break must have
occurred in the optical light curve around 22 ks post burst, which, however,
has no obvious counterpart in the X-ray band, contradicting the interpretation
that this could be another jet break. The GROND data provide the missing piece
of evidence that the optical afterglow of GRB 090510 did follow a post-jet
break evolution at late times.Comment: submitted to Astronomy & Astrophysics, accepted for publication on
Dec 24, 201
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