49 research outputs found
Pinpointing Cosmic Ray Propagation With The AMS-02 Experiment
The Alpha Magnetic Spectrometer (AMS-02), which is scheduled to be deployed
onboard the International Space Station later this year, will be capable of
measuring the composition and spectra of GeV-TeV cosmic rays with unprecedented
precision. In this paper, we study how the projected measurements from AMS-02
of stable secondary-to-primary and unstable ratios (such as boron-to-carbon and
beryllium-10-to-beryllium-9) can constrain the models used to describe the
propagation of cosmic rays throughout the Milky Way. We find that within the
context of fairly simple propagation models, all of the model parameters can be
determined with high precision from the projected AMS-02 data. Such
measurements are less constraining in more complex scenarios, however, which
allow for departures from a power-law form for the diffusion coefficient, for
example, or for inhomogeneity or stochasticity in the distribution and chemical
abundances of cosmic ray sources.Comment: 12 pages, 7 figures, 3 tables, matches published versio
The Orbital Period of the Ultraluminous X-ray Source in M82
The ultraluminous x-ray source (ULX) in the galaxy M82 has been identified as
a possible intermediate-mass black hole. We have found that the x-ray flux from
M82 is modulated with a peak-to-peak amplitude corresponding to an isotropic
luminosity of 2.4x10^40 erg/s in M82 and a period of 62.0 +/- 2.5 days, which
we interpret as the orbital period of the ULX binary. This orbital period
implies that the mass-donor star must be a giant or supergiant. Large
mass-transfer rates, sufficient to fuel the ULX, are expected for a giant-phase
mass donor in an x-ray binary. The giant phase has a short lifetime, indicating
that we see the ULX in M82 in a brief and unusual period of its evolution.Comment: 3 pages, appeared in Scienc
Weak lensing calibration of mass bias in the REFLEX+BCS X-ray galaxy cluster catalogue
The use of large, X-ray selected galaxy cluster catalogues for cosmological
analyses requires a thorough understanding of the X-ray mass estimates. Weak
gravitational lensing is an ideal method to shed light on such issues, due to
its insensitivity to the cluster dynamical state. We perform a weak lensing
calibration of 166 galaxy clusters from the REFLEX and BCS cluster catalogue
and compare our results to the X-ray masses based on scaled luminosities from
that catalogue. To interpret the weak lensing signal in terms of cluster
masses, we compare the lensing signal to simple theoretical Navarro-Frenk-White
models and to simulated cluster lensing profiles, including complications such
as cluster substructure, projected large-scale structure, and Eddington bias.
We find evidence of underestimation in the X-ray masses, as expected, with
stat. sys. for our best-fit model. The biases in cosmological parameters in a
typical cluster abundance measurement that ignores this mass bias will
typically exceed the statistical errors.Comment: 13 pages, 5 figures. Revised to address referee comment
Comparison of Observed Galaxy Properties with Semianalytic Model Predictions using Machine Learning
With current and upcoming experiments such as WFIRST, Euclid and LSST, we can
observe up to billions of galaxies. While such surveys cannot obtain spectra
for all observed galaxies, they produce galaxy magnitudes in color filters.
This data set behaves like a high-dimensional nonlinear surface, an excellent
target for machine learning. In this work, we use a lightcone of semianalytic
galaxies tuned to match CANDELS observations from Lu et al. (2014) to train a
set of neural networks on a set of galaxy physical properties. We add realistic
photometric noise and use trained neural networks to predict stellar masses and
average star formation rates on real CANDELS galaxies, comparing our
predictions to SED fitting results. On semianalytic galaxies, we are nearly
competitive with template-fitting methods, with biases of dex for
stellar mass, dex for star formation rate, and dex for
metallicity. For the observed CANDELS data, our results are consistent with
template fits on the same data at dex bias in and
dex bias in star formation rate. Some of the bias is driven by SED-fitting
limitations, rather than limitations on the training set, and some is intrinsic
to the neural network method. Further errors are likely caused by differences
in noise properties between the semianalytic catalogs and data. Our results
show that galaxy physical properties can in principle be measured with neural
networks at a competitive degree of accuracy and precision to template-fitting
methods.Comment: 19 pages, 10 figures, 6 tables. Accepted for publication in Ap
High Energy Positrons From Annihilating Dark Matter
Recent preliminary results from the PAMELA experiment indicate the presence
of an excess of cosmic ray positrons above 10 GeV. In this letter, we consider
possibility that this signal is the result of dark matter annihilations taking
place in the halo of the Milky Way. Rather than focusing on a specific particle
physics model, we take a phenomenological approach and consider a variety of
masses and two-body annihilation modes, including W+W-, ZZ, b bbar, tau+ tau-,
mu+ mu-, and e+e. We also consider a range of diffusion parameters consistent
with current cosmic ray data. We find that a significant upturn in the positron
fraction above 10 GeV is compatible with a wide range of dark matter
annihilation modes, although very large annihilation cross sections and/or
boost factors arising from inhomogeneities in the local dark matter
distribution are required to produce the observed intensity of the signal. We
comment on constraints from gamma rays, synchrotron emission, and cosmic ray
antiproton measurements.Comment: 4 pages, 1 figur
The Milky Way as a Kiloparsec-Scale Axionscope
Very high energy gamma-rays are expected to be absorbed by the extragalactic
background light over cosmological distances via the process of
electron-positron pair production. Recent observations of cosmologically
distant gamma-ray emitters by ground based gamma-ray telescopes have, however,
revealed a surprising degree of transparency of the universe to very high
energy photons. One possible mechanism to explain this observation is the
oscillation between photons and axion-like-particles (ALPs). Here we explore
this possibility further, focusing on photon-ALP conversion in the magnetic
fields in and around gamma-ray sources and in the magnetic field of the Milky
Way, where some fraction of the ALP flux is converted back into photons. We
show that this mechanism can be efficient in allowed regions of the ALP
parameter space, as well as in typical configurations of the Galactic Magnetic
Field. As case examples, we consider the spectrum observed from two HESS
sources: 1ES1101-232 at redshift z=0.186 and H 2356-309 at z=0.165. We also
discuss features of this scenario which could be used to distinguish it from
standard or other exotic models.Comment: 7 pages, 4 figures. Matches published versio