869 research outputs found
Microlensing of Lensed Supernovae
Given the number of recently discovered galaxy-galaxy lens systems, we
anticipate that a gravitationally lensed supernova will be observed within the
next few years. We explore the possibility that stars in the lens galaxy will
produce observable microlensing fluctuations in lensed supernova light curves.
For typical parameters, we predict that ~70% of lensed SNe will show
microlensing fluctuations > 0.5 mag, while ~25% will have fluctuations > 1 mag.
Thus microlensing of lensed supernova will be both ubiquitous and observable.
Additionally, we show that microlensing fluctuations will complicate
measurements of time delays from multiply imaged supernovae: time delays
accurate to better than a few days will be difficult to obtain. We also
consider prospects for extracting the lens galaxy's stellar mass fraction and
mass function from microlensing fluctuations via a new statistical measure, the
time-weighted light curve derivative.Comment: 13 pages, emulateapj format; accepted in ApJ; expanded discussion of
time delay uncertaintie
Historic Truth, Ballad Truth, and the Truth of Oral Memorate: A Study of the Floyd County, Kentucky, School Bus Disaster
A study on the recorded history, folklore, ballads and poems, of the Floyd County, Kentucky, school bus disaster of February 28, 1958 by G. Ronald Dobler in September of 1980.
Audio recordings of the ballads made by Professor Dobler can be heard at https://www.youtube.com/watch?v=36I0jwEn024
Identification of Spinning Dust in Halpha-Correlated Microwave Emission
CMB experiments commonly use maps of Halpha intensity as a spatial template
for Galactic free-free emission, assuming a power law I_nu \propto nu^-0.15 for
the spectrum. Any departure from the assumed free-free spectrum could have a
detrimental effect on determination of the primary CMB anisotropy. We show that
the Halpha-correlated emission spectrum in the diffuse warm ionized medium
(WIM) is not the expected free-free spectrum at WMAP frequencies. Instead,
there is a broad bump in the spectrum at ~50 GHz which is consistent with
emission from spinning dust grains. Spectra from both the full sky and smaller
regions of interest are well fit by a superposition of a free-free and WIM
Draine & Lazarian (1998) spinning dust model, shifted in frequency. The
spinning dust emission is ~5 times weaker than the free-free component at 50
GHz, with the null hypothesis that the Halpha-correlated spectrum is pure
free-free, ruled out at >8 sigma in all regions and >100 sigma for the full sky
fit.Comment: 8 pages, 5 figures; submitted to ApJ; LaTeX modified slightly to
reveal missing Figure
The Fermi Bubbles. I. Possible Evidence for Recent AGN Jet Activity in the Galaxy
The Fermi Gamma-ray Space Telescope reveals two large gamma-ray bubbles in
the Galaxy, which extend about 50 degrees (~ 10 kpc) above and below the
Galactic center (GC) and are symmetric about the Galactic plane. Using
axisymmetric hydrodynamic simulations with a self-consistent treatment of the
dynamical cosmic ray (CR) - gas interaction, we show that the bubbles can be
created with a recent active galactic nucleus (AGN) jet activity about 1 - 3
Myr ago, which was active for a duration of ~ 0.1 - 0.5 Myr. The bipolar jets
were ejected into the Galactic halo along the rotation axis of the Galaxy. Near
the GC, the jets must be moderately light with a typical density contrast 0.001
<~ \eta <~ 0.1 relative to the ambient hot gas. The jets are energetically
dominated by kinetic energy, and over-pressured with either CR or thermal
pressure which induces lateral jet expansion, creating fat CR bubbles as
observed. The sharp edges of the bubbles imply that CR diffusion across the
bubble surface is strongly suppressed. The jet activity induces a strong shock,
which heats and compresses the ambient gas in the Galactic halo, potentially
explaining the ROSAT X-ray shell features surrounding the bubbles. The Fermi
bubbles provide plausible evidence for a recent powerful AGN jet activity in
our Galaxy, shedding new insights into the origin of the halo CR population and
the channel through which massive black holes in disk galaxies release feedback
energy during their growth.Comment: Revised version, accepted for publication in ApJ. 17 pages, 11
figure
The Fermi Bubbles. II. The Potential Roles of Viscosity and Cosmic Ray Diffusion in Jet Models
The origin of the Fermi bubbles recently detected by the Fermi Gamma-ray
Space Telescope in the inner Galaxy is mysterious. In the companion paper Guo &
Mathews (Paper I), we use hydrodynamic simulations to show that they could be
produced by a recent powerful AGN jet event. Here we further explore this
scenario to study the potential roles of shear viscosity and cosmic ray (CR)
diffusion on the morphology and CR distribution of the bubbles. We show that
even a relatively low level of viscosity (\mu_{visc} >~ 3 g cm^{-1} s^{-1}, or
~0.1% - 1% of Braginskii viscosity in this context) could effectively suppress
the development of Kelvin-Helmholtz instabilities at the bubble surface,
resulting in smooth bubble edges as observed. Furthermore, viscosity reduces
circulating motions within the bubbles, which would otherwise mix the
CR-carrying jet backflow near bubble edges with the bubble interior. Thus
viscosity naturally produces an edge-favored CR distribution, an important
ingredient to produce the observed flat gamma-ray surface brightness
distribution. Generically, such a CR distribution often produces a
limb-brightened gamma-ray intensity distribution. However, we show that by
incorporating CR diffusion which is strongly suppressed across the bubble
surface (as inferred from sharp bubble edges) but is close to canonical values
in the bubble interior, we obtain a reasonably flat gamma-ray intensity
profile. The similarity of the resulting CR bubble with the observed Fermi
bubbles strengthens our previous result in Paper I that the Fermi bubbles were
produced by a recent AGN jet event. Studies of the nearby Fermi bubbles may
provide a unique opportunity to study the potential roles of plasma viscosity
and CR diffusion on the evolution of AGN jets and bubbles.Comment: Revised version, accepted for publication in ApJ. 14 pages, 9 figure
Evidence Of Dark Matter Annihilations In The WMAP Haze
The WMAP experiment has revealed an excess of microwave emission from the
region around the center of our Galaxy. It has been suggested that this signal,
known as the ``WMAP Haze'', could be synchrotron emission from relativistic
electrons and positrons generated in dark matter annihilations. In this letter,
we revisit this possibility. We find that the angular distribution of the WMAP
Haze matches the prediction for dark matter annihilations with a cusped density
profile, in the inner kiloparsecs. Comparing the
intensity in different WMAP frequency bands, we find that a wide range of
possible WIMP annihilation modes are consistent with the spectrum of the haze
for a WIMP with a mass in the 100 GeV to multi-TeV range. Most interestingly,
we find that to generate the observed intensity of the haze, the dark matter
annihilation cross section is required to be approximately equal to the value
needed for a thermal relic, cm/s. No
boost factors are required. If dark matter annihilations are in fact
responsible for the WMAP Haze, and the slope of the halo profile continues into
the inner Galaxy, GLAST is expected to detect gamma rays from the dark matter
annihilations in the Galactic Center if the WIMP mass is less than several
hundred GeV.Comment: 4 pages, 3 figure
Fourth moment theorems on the Poisson space in any dimension
We extend to any dimension the quantitative fourth moment theorem on the Poisson setting, recently proved by C. Döbler and G. Peccati (2017). In particular, by adapting the exchangeable pairs couplings construction introduced by I. Nourdin and G. Zheng (2017) to the Poisson framework, we prove our results under the weakest possible assumption of finite fourth moments. This yields a Peccati-Tudor type theorem, as well as an optimal improvement in the univariate case. Finally, a transfer principle “from-Poisson-to-Gaussian” is derived, which is closely related to the universality phenomenon for homogeneous multilinear sums
Microlensing of Central Images in Strong Gravitational Lens Systems
We study microlensing of the faint images that form close to the centers of
strong gravitational lens galaxies. These central images, which have finally
begun to yield to observations, naturally appear in dense stellar fields and
may be particularly sensitive to fine granularity in the mass distribution. The
microlensing magnification maps for overfocussed (i.e., demagnified) images
differ strikingly from those for magnified images. In particular, the familiar
"fold" and "cusp" features of maps for magnified images are only present for
certain values of the fraction, f, of the surface mass density contained in
stars. For central images, the dispersion in microlensing magnifications is
generally larger than for normal (minimum and saddle) images, especially when
the source is comparable to or larger than the stellar Einstein radius. The
dispersion depends in a complicated way on f; this behaviour may hold the key
to using microlensing as a probe of the relative densities of stars and dark
matter in the cores of distant galaxies. Quantitatively, we predict that the
central image C in PMN J1632-0033 has a magnification dispersion of 0.6
magnitudes for Rsrc/Rein <~ 1, or 0.3 mag for Rsrc/Rein = 10. For comparison,
the dispersions are 0.5-0.6 mag for image B and 0.05-0.1 mag for image A, if
Rsrc/Rein <~ 1; and just 0.1 mag for B and 0.008 mag for A if Rsrc/Rein = 10.
(The dispersions can be extrapolated to larger sources sizes as sigma \propto
Rsrc^{-1}.) Thus, central images are more susceptible than other lensed images
to microlensing and hence good probes for measuring source sizes.Comment: 12 pages; accepted in MNRAS; many new magnification maps and
significantly expanded analysis of magnification map structur
Indoor Navigation with MEMS sensors
AbstractAccurate positioning becomes extremely important for modern application like indoor navigation and location-based services. Standalone GPS cannot meet this accuracy. In this paper a method to couple GPS and a high resolution MEMS pressure sensor is presented to improve vertical as well as horizontal (in urban canyon environment) positioning. Further, a step counter based on an accelerometer is improved with an altimeter for stair detection and automatic step length adaptation for dead reckoning inside buildings. Finally, a stand-alone system accurately tracks floor levels inside buildings, using only a pressure sensor
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