3,087 research outputs found
Can past gamma-ray bursts explain both INTEGRAL and ATIC/PAMELA/Fermi anomalies simultaneously?
Gamma-ray bursts (GRBs) have been invoked to explain both the 511 keV
emission from the galactic bulge and the high-energy positron excess inferred
from the ATIC, PAMELA, and Fermi data. While independent explanations can be
responsible for these phenomena, we explore the possibility of their common
GRB-related origin by modeling the GRB distribution and estimating the rates.
For an expected Milky Way long GRB rate, neither of the two signals is generic;
the local excess requires a 2% coincidence, while the signal from the galactic
center requires a 20% coincidence with respect to the timing of the latest GRB.
The simultaneous explanation requires a 0.4% coincidence. Considering the large
number of statistical "trials" created by multiple searches for new physics,
the coincidences of a few per cent cannot be dismissed as unlikely.
Alternatively, both phenomena can be explained by GRBs if the galactic rate is
higher than expected. We also show that a similar result is difficult to obtain
assuming a simplified short GRB distribution.Comment: 4 pages; version accepted for publicatio
Effects of ATLAS Tile calorimeter failures on jets and missing transverse energy measurements
Failures of the ATLAS Tile calorimeter would affect the jet energy resolution and would fake tails of missing transverse energy. Significant effects are expected in processes involving high transverse momentum jets ( GeV). These effects, their consequences, as well as methods to minimize them, are studied using simulated data for various degradation topologies and for different physics processes
The Cepheid distance to the maser-host galaxy NGC 4258: Studying systematics with the Large Binocular Telescope
We identify and phase a sample of 81 Cepheids in the maser-host galaxy NGC
4258 using the Large Binocular Telescope (LBT), and obtain calibrated mean
magnitudes in up to 4 filters for a subset of 43 Cepheids using archival HST
data. We employ 3 models to study the systematic effects of extinction, the
assumed extinction law, and metallicity on the Cepheid distance to NGC 4258. We
find a correction to the Cepheid colors consistent with a grayer extinction law
in NGC 4258 compared to the Milky Way (), although we believe this is
indicative of other systematic effects. If we combine our Cepheid sample with
previously known Cepheids, we find a significant metallicity adjustment to the
distance modulus of mag/dex, for the Zaritsky et
al. (1994) metallicity scale, as well as a weak trend of Cepheid colors with
metallicity. Conclusions about the absolute effect of metallicity on Cepheid
mean magnitudes appear to be limited by the available data on the metallicity
gradient in NGC 4258, but our Cepheid data require at least some metallicity
adjustment to make the Cepheid distance consistent with independent distances
to the LMC and NGC 4258. From our ensemble of models and the geometric maser
distance of NGC 4258 ( mag), we estimate
mag ( kpc).Comment: Accepted for publication in MNRAS. 28 pages, 13 figures, 11 tables. A
brief video summarizing the key results of this paper can be found at
http://youtu.be/ICTTNyxZ89
MMT Extremely Metal Poor Galaxy Survey I. An Efficient Technique to Identify Metal Poor Galaxies
We demonstrate a successful strategy for identifying extremely metal poor
galaxies. Our preliminary survey of 24 candidates contains 10 metal poor
galaxies of which 4 have 12+log(O/H)<7.65, some of the lowest metallicity blue
compact galaxies known to date. Interestingly, our sample of metal poor
galaxies have systematically lower metallicity for their luminosity than
comparable samples of blue compact galaxies, dIrrs, and normal star-forming
galaxies. Our metal poor galaxies share very similar properties, however, with
the host galaxies of nearby long-duration gamma-ray bursts (GRBs), including
similar metallicity, stellar ages, and star formation rates. We use H\beta to
measure the number of OB stars present in our galaxies and estimate a
core-collapse supernova rate of ~10^-3 yr^-1. A larger sample of metal poor
galaxies may provide new clues into the environment where GRBs form and may
provide a list of potential GRB hosts.Comment: Accepted to AJ, 8 pages using emulateap
Nucleosynthesis of Nickel-56 from Gamma-Ray Burst Accretion Disks
We examine the prospects for producing Nickel-56 from black hole accretion
disks, by examining a range of steady state disk models. We focus on relatively
slowly accreting disks in the range of 0.05 - 1 solar masses per second, as are
thought to be appropriate for the central engines of long-duration gamma-ray
bursts. We find that significant amounts of Nickel-56 are produced over a wide
range of parameter space. We discuss the influence of entropy, outflow
timescale and initial disk position on mass fraction of Nickel-56 which is
produced. We keep careful track of the weak interactions to ensure reliable
calculations of the electron fraction, and discuss the role of the neutrinos.Comment: 10 pages, 9 figure
Pre-discovery and Follow-up Observations of the Nearby SN 2009nr: Implications for Prompt Type Ia SNe
We present photometric and spectroscopic observations of the Type Ia
supernova SN 2009nr in UGC 8255 (z=0.0122). Following the discovery
announcement at what turned out to be ten days after peak, we detected it at V
~15.7 mag in data collected by the All Sky Automated Survey (ASAS) North
telescope 2 weeks prior to the peak, and then followed it up with telescopes
ranging in aperture from 10-cm to 6.5-m. Using early photometric data available
only from ASAS, we find that the SN is similar to the over-luminous Type Ia SN
1991T, with a peak at Mv=-19.6 mag, and a slow decline rate of Dm_15(B)=0.95
mag. The early post-maximum spectra closely resemble those of SN 1991T, while
the late time spectra are more similar to those of normal Type Ia SNe.
Interestingly, SN 2009nr has a projected distance of 13.0 kpc (~4.3 disk scale
lengths) from the nucleus of the small star-forming host galaxy UGC 8255. This
indicates that the progenitor of SN 2009nr is not associated with a young
stellar population, calling into question the conventional association of
luminous SNe Ia with the "prompt" component directly correlated with current
star formation. The pre-discovery observation of SN 2009nr using ASAS
demonstrates the science utility of high cadence all sky surveys conducted
using small telescopes for the discovery of nearby (d=<50 Mpc) supernovae.Comment: 11 pages, 11 figures, 4 tables. Accepted for publication in ApJ on
11/02/201
Variability-selected quasars behind the Small Magellanic Cloud
We present followup spectroscopic observations of quasar candidates in the
Small Magellanic Cloud selected by Eyer from the OGLE database. Of twelve
observed objects identified as "QSO Candidate", five are confirmed quasars,
with the emission redshifts ranging from 0.28 to 2.16. Two of those quasars
were also recently identified independently in the MACHO database by Geha et
al. We discuss the prospects of using variability-based selection technique for
quasar searches behind other dense stellar fields. An additional criterion
utilizing the color-color diagram should reduce the number of stars in the
candidate lists.Comment: Revised version, AASTeX, 11 pages, 3 EPS figures, one table, accepted
14 Nov 2002 for publication in the Astronomical Journal, March 2003 issu
Excesses in the Cosmic Ray Spectrum and Possible Interpretations
The data collected by ATIC, PPB-BETS, FERMI-LAT and HESS all indicate that
there is an electron/positron excess in the cosmic ray energy spectrum above
100 GeV, although different instrumental teams do not agree on the
detailed spectral shape. PAMELA also reported a clear excess feature of the
positron fraction above several GeV, but no excess in anti-protons. Here we
review the observational status and theoretical models of this interesting
observational feature. We pay special attention to various physical
interpretations proposed in the literature, including modified supernova
remnant models for the background, new astrophysical sources, and new
physics (the dark matter models). We suggest that although most models can make
a case to interpret the data, with the current observational constraints the
dark matter interpretations, especially those invoking annihilation, require
much more exotic assumptions than some astrophysical interpretations. Future
observations may present some ``smoking-gun'' observational tests to
differentiate among different models and to identify the correct interpretation
to the phenomenon.Comment: 48 pages, including 10 figures and 1 tabel. Invited review to be
published in IJMP
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