2,179 research outputs found
GRB 090426: The Environment of a Rest-Frame 0.35-second Gamma-Ray Burst at Redshift z=2.609
We present the discovery of an absorption-line redshift of z = 2.609 for GRB
090426, establishing the first firm lower limit to a redshift for a gamma-ray
burst with an observed duration of <2 s. With a rest-frame burst duration of
T_90z = 0.35 s and a detailed examination of the peak energy of the event, we
suggest that this is likely (at >90% confidence) a member of the short/hard
phenomenological class of GRBs. From analysis of the optical-afterglow spectrum
we find that the burst originated along a very low HI column density sightline,
with N_HI < 3.2 x 10^19 cm^-2. Our GRB 090426 afterglow spectrum also appears
to have weaker low-ionisation absorption (Si II, C II) than ~95% of previous
afterglow spectra. Finally, we also report the discovery of a blue, very
luminous, star-forming putative host galaxy (~2 L*) at a small angular offset
from the location of the optical afterglow. We consider the implications of
this unique GRB in the context of burst duration classification and our
understanding of GRB progenitor scenarios.Comment: Submitted to MNRA
Active Learning to Overcome Sample Selection Bias: Application to Photometric Variable Star Classification
Despite the great promise of machine-learning algorithms to classify and
predict astrophysical parameters for the vast numbers of astrophysical sources
and transients observed in large-scale surveys, the peculiarities of the
training data often manifest as strongly biased predictions on the data of
interest. Typically, training sets are derived from historical surveys of
brighter, more nearby objects than those from more extensive, deeper surveys
(testing data). This sample selection bias can cause catastrophic errors in
predictions on the testing data because a) standard assumptions for
machine-learned model selection procedures break down and b) dense regions of
testing space might be completely devoid of training data. We explore possible
remedies to sample selection bias, including importance weighting (IW),
co-training (CT), and active learning (AL). We argue that AL---where the data
whose inclusion in the training set would most improve predictions on the
testing set are queried for manual follow-up---is an effective approach and is
appropriate for many astronomical applications. For a variable star
classification problem on a well-studied set of stars from Hipparcos and OGLE,
AL is the optimal method in terms of error rate on the testing data, beating
the off-the-shelf classifier by 3.4% and the other proposed methods by at least
3.0%. To aid with manual labeling of variable stars, we developed a web
interface which allows for easy light curve visualization and querying of
external databases. Finally, we apply active learning to classify variable
stars in the ASAS survey, finding dramatic improvement in our agreement with
the ACVS catalog, from 65.5% to 79.5%, and a significant increase in the
classifier's average confidence for the testing set, from 14.6% to 42.9%, after
a few AL iterations.Comment: 43 pages, 11 figures, submitted to Ap
Double-Blind Phase III Trial of Adjuvant Chemotherapy With and Without Bevacizumab in Patients With Lymph Node-Positive and High-Risk Lymph Node-Negative Breast Cancer (E5103)
Purpose Bevacizumab improves progression-free survival but not overall survival in patients with metastatic breast cancer. E5103 tested the effect of bevacizumab in the adjuvant setting in patients with human epidermal growth factor receptor 2-negative disease. Patients and Methods Patients were assigned 1:2:2 to receive placebo with doxorubicin and cyclophosphamide (AC) followed by weekly paclitaxel (arm A), bevacizumab only during AC and paclitaxel (arm B), or bevacizumab during AC and paclitaxel followed by bevacizumab monotherapy for 10 cycles (arm C). Random assignment was stratified and bevacizumab dose adjusted for choice of AC schedule. Radiation and hormonal therapy were administered concurrently with bevacizumab in arm C. The primary end point was invasive disease-free survival (IDFS). Results Four thousand nine hundred ninety-four patients were enrolled. Median age was 52 years; 64% of patients were estrogen receptor positive, 27% were lymph node negative, and 78% received dose-dense AC. Chemotherapy-associated adverse events including myelosuppression and neuropathy were similar across all arms. Grade â„ 3 hypertension was more common in bevacizumab-treated patients, but thrombosis, proteinuria, and hemorrhage were not. The cumulative incidence of clinical congestive heart failure at 15 months was 1.0%, 1.9%, and 3.0% in arms A, B, and C, respectively. Bevacizumab exposure was less than anticipated, with approximately 24% of patients in arm B and approximately 55% of patients in arm C discontinuing bevacizumab before completing planned therapy. Five-year IDFS was 77% (95% CI, 71% to 81%) in arm A, 76% (95% CI, 72% to 80%) in arm B, and 80% (95% CI, 77% to 83%) in arm C. Conclusion Incorporation of bevacizumab into sequential anthracycline- and taxane-containing adjuvant therapy does not improve IDFS or overall survival in patients with high-risk human epidermal growth factor receptor 2-negative breast cancer. Longer duration bevacizumab therapy is unlikely to be feasible given the high rate of early discontinuation
Constraints on the Progenitor System of the Type Ia Supernova SN 2011fe/PTF11kly
Type Ia supernovae (SNe) serve as a fundamental pillar of modern cosmology,
owing to their large luminosity and a well-defined relationship between
light-curve shape and peak brightness. The precision distance measurements
enabled by SNe Ia first revealed the accelerating expansion of the universe,
now widely believed (though hardly understood) to require the presence of a
mysterious "dark" energy. General consensus holds that Type Ia SNe result from
thermonuclear explosions of a white dwarf (WD) in a binary system; however,
little is known of the precise nature of the companion star and the physical
properties of the progenitor system. Here we make use of extensive historical
imaging obtained at the location of SN 2011fe/PTF11kly, the closest SN Ia
discovered in the digital imaging era, to constrain the visible-light
luminosity of the progenitor to be 10-100 times fainter than previous limits on
other SN Ia progenitors. This directly rules out luminous red giants and the
vast majority of helium stars as the mass-donating companion to the exploding
white dwarf. Any evolved red companion must have been born with mass less than
3.5 times the mass of the Sun. These observations favour a scenario where the
exploding WD of SN 2011fe/PTF11kly, accreted matter either from another WD, or
by Roche-lobe overflow from a subgiant or main-sequence companion star.Comment: 22 pages, 6 figures, submitte
Evidence for dust destruction from the early-time colour change of GRB 120119A
We present broad-band observations and analysis of Swift gamma-ray burst (GRB) 120119A. Our early-time afterglow detections began under 15âs after the burst in the host frame (redshift z = 1.73), and they yield constraints on the burst energetics and local environment. Late-time afterglow observations of the burst show evidence for a moderate column of dust (AV â 1.1âmag) similar to, but statistically distinct from, dust seen along Small Magellanic Cloud sightlines. Deep late-time observations reveal a dusty, rapidly star-forming host galaxy. Most notably, our early-time observations exhibit a significant red-to-blue colour change in the first âŒ200âs after the trigger at levels heretofore unseen in GRB afterglows. This colour change, which is coincident with the final phases of the prompt emission, is a hallmark prediction of the photodestruction of dust in GRB afterglows. We test whether dust-destruction signatures are significantly distinct from other sources of colour change, namely a change in the intrinsic spectral index ÎČ. We find that a time-varying power-law spectrum alone cannot adequately describe the observed colour change, and allowing for dust destruction (via a time-varying AV) significantly improves the fit. While not definitively ruling out other possibilities, this event provides the best support yet for the direct detection of dust destruction in the local environment of a GRB
SN Zwicky: uncovering a population of gravitational lens galaxies with magnified "standard candles"
We report the discovery of a very rare phenomenon, a multiply-imaged
gravitationally lensed Type Ia supernova (SNe Ia), "SN Zwicky", a.k.a. SN
2022qmx, magnified nearly twenty-five times by a foreground galaxy. The system
was identified as intrinsically bright thanks to the "standard candle" nature
of SNe Ia. Observations with high-spatial resolution instruments resolved a
system with four nearly simultaneous images, with an Einstein radius of only
, corresponding to a lens mass of solar masses
within a physical size below kiloparsecs. A smooth lens model fails to
reproduce the image flux ratios, suggesting significant additional
magnification from compact objects. Given the small image splitting and a
relatively faint deflecting galaxy, the lensing system would not have been
found through the angular separation technique generally used in large imaging
surveys
LSST Science Book, Version 2.0
A survey that can cover the sky in optical bands over wide fields to faint
magnitudes with a fast cadence will enable many of the exciting science
opportunities of the next decade. The Large Synoptic Survey Telescope (LSST)
will have an effective aperture of 6.7 meters and an imaging camera with field
of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over
20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with
fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a
total point-source depth of r~27.5. The LSST Science Book describes the basic
parameters of the LSST hardware, software, and observing plans. The book
discusses educational and outreach opportunities, then goes on to describe a
broad range of science that LSST will revolutionize: mapping the inner and
outer Solar System, stellar populations in the Milky Way and nearby galaxies,
the structure of the Milky Way disk and halo and other objects in the Local
Volume, transient and variable objects both at low and high redshift, and the
properties of normal and active galaxies at low and high redshift. It then
turns to far-field cosmological topics, exploring properties of supernovae to
z~1, strong and weak lensing, the large-scale distribution of galaxies and
baryon oscillations, and how these different probes may be combined to
constrain cosmological models and the physics of dark energy.Comment: 596 pages. Also available at full resolution at
http://www.lsst.org/lsst/sciboo
LSST: from Science Drivers to Reference Design and Anticipated Data Products
(Abridged) We describe here the most ambitious survey currently planned in
the optical, the Large Synoptic Survey Telescope (LSST). A vast array of
science will be enabled by a single wide-deep-fast sky survey, and LSST will
have unique survey capability in the faint time domain. The LSST design is
driven by four main science themes: probing dark energy and dark matter, taking
an inventory of the Solar System, exploring the transient optical sky, and
mapping the Milky Way. LSST will be a wide-field ground-based system sited at
Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m
effective) primary mirror, a 9.6 deg field of view, and a 3.2 Gigapixel
camera. The standard observing sequence will consist of pairs of 15-second
exposures in a given field, with two such visits in each pointing in a given
night. With these repeats, the LSST system is capable of imaging about 10,000
square degrees of sky in a single filter in three nights. The typical 5
point-source depth in a single visit in will be (AB). The
project is in the construction phase and will begin regular survey operations
by 2022. The survey area will be contained within 30,000 deg with
, and will be imaged multiple times in six bands, ,
covering the wavelength range 320--1050 nm. About 90\% of the observing time
will be devoted to a deep-wide-fast survey mode which will uniformly observe a
18,000 deg region about 800 times (summed over all six bands) during the
anticipated 10 years of operations, and yield a coadded map to . The
remaining 10\% of the observing time will be allocated to projects such as a
Very Deep and Fast time domain survey. The goal is to make LSST data products,
including a relational database of about 32 trillion observations of 40 billion
objects, available to the public and scientists around the world.Comment: 57 pages, 32 color figures, version with high-resolution figures
available from https://www.lsst.org/overvie
Evidence for dust destruction from the early-time colour change of GRBÂ 120119A
We present broad-band observations and analysis o
- âŠ