6,930 research outputs found
The Ackermann Award 2012
Report on the Ackermann Award 2012
Gamma Rays from Star Formation in Clusters of Galaxies
Star formation in galaxies is observed to be associated with gamma-ray
emission. The detection of gamma rays from star-forming galaxies by the Fermi
Large Area Telescope (LAT) has allowed the determination of a functional
relationship between star formation rate and gamma-ray luminosity (Ackermann
et. al. 2012). Since star formation is known to scale with total infrared
(8-1000 micrometers) and radio (1.4 GHz) luminosity, the observed infrared and
radio emission from a star-forming galaxy can be used to quantitatively infer
the galaxy's gamma-ray luminosity. Similarly, star forming galaxies within
galaxy clusters allow us to derive lower limits on the gamma-ray emission from
clusters, which have not yet been conclusively detected in gamma rays. In this
study we apply the relationships between gamma-ray luminosity and radio and IR
luminosities derived in Ackermann et. al. 2012 to a sample of galaxy clusters
from Ackermann et. al. 2010 in order to place lower limits on the gamma-ray
emission associated with star formation in galaxy clusters. We find that
several clusters have predicted lower limits on gamma-ray emission that are
within an order of magnitude of the upper limits derived in Ackermann et. al.
2010 based on non-detection by Fermi-LAT. Given the current gamma-ray limits,
star formation likely plays a significant role in the gamma-ray emission in
some clusters, especially those with cool cores. We predict that both Fermi-LAT
over the course of its lifetime and the future Cherenkov Telescope Array will
be able to detect gamma-ray emission from star-forming galaxies in clusters.Comment: 17 pages, 2 figures, 2 tables. Minor revisions made to match version
accepted to Ap
The Distance to Nova V959 Mon from VLA Imaging
Determining reliable distances to classical novae is a challenging but
crucial step in deriving their ejected masses and explosion energetics. Here we
combine radio expansion measurements from the Karl G. Jansky Very Large Array
with velocities derived from optical spectra to estimate an expansion parallax
for nova V959 Mon, the first nova discovered through its gamma-ray emission. We
spatially resolve the nova at frequencies of 4.5-36.5 GHz in nine different
imaging epochs. The first five epochs cover the expansion of the ejecta from
2012 October to 2013 January, while the final four epochs span 2014 February to
2014 May. These observations correspond to days 126 through 199 and days 615
through 703 after the first detection of the nova. The images clearly show a
non-spherical ejecta geometry. Utilizing ejecta velocities derived from 3D
modelling of optical spectroscopy, the radio expansion implies a distance
between 0.9 +/- 0.2 and 2.2 +/- 0.4 kpc, with a most probable distance of 1.4
+/- 0.4 kpc. This distance implies a gamma-ray luminosity much less than the
prototype gamma-ray-detected nova, V407 Cyg, possibly due to the lack of a red
giant companion in the V959 Mon system. V959 Mon also has a much lower
gamma-ray luminosity than other classical novae detected in gamma-rays to date,
indicating a range of at least a factor of 10 in the gamma-ray luminosities for
these explosions.Comment: 11 pages, 8 figures, 3 tables, submitted to ApJ 2015-01-21, under
revie
Searching for Dark Matter Annihilation in the Smith High-Velocity Cloud
Recent observations suggest that some high-velocity clouds may be confined by
massive dark matter halos. In particular, the proximity and proposed dark
matter content of the Smith Cloud make it a tempting target for the indirect
detection of dark matter annihilation. We argue that the Smith Cloud may be a
better target than some Milky Way dwarf spheroidal satellite galaxies and use
gamma-ray observations from the Fermi Large Area Telescope to search for a dark
matter annihilation signal. No significant gamma-ray excess is found coincident
with the Smith Cloud, and we set strong limits on the dark matter annihilation
cross section assuming a spatially-extended dark matter profile consistent with
dynamical modeling of the Smith Cloud. Notably, these limits exclude the
canonical thermal relic cross section () for dark matter masses GeV annihilating via the or channels for certain assumptions of the dark matter
density profile; however, uncertainties in the dark matter content of the Smith
Cloud may significantly weaken these constraints.Comment: 7 pages, 5 figures. Published in Ap
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A method for comparing non-nested models with application to astrophysical searches for new physics
Searches for unknown physics and decisions between competing astrophysical
models to explain data both rely on statistical hypothesis testing. The usual
approach in searches for new physical phenomena is based on the statistical
Likelihood Ratio Test (LRT) and its asymptotic properties. In the common
situation, when neither of the two models under comparison is a special case of
the other i.e., when the hypotheses are non-nested, this test is not
applicable. In astrophysics, this problem occurs when two models that reside in
different parameter spaces are to be compared. An important example is the
recently reported excess emission in astrophysical -rays and the
question whether its origin is known astrophysics or dark matter. We develop
and study a new, simple, generally applicable, frequentist method and validate
its statistical properties using a suite of simulations studies. We exemplify
it on realistic simulated data of the Fermi-LAT -ray satellite, where
non-nested hypotheses testing appears in the search for particle dark matter.Comment: We welcome examples of non-nested models testing problem
Multiwavelength and parsec-scale properties of extragalactic jets
Extragalactic jets originating from the central supermassive black holes of
active galaxies are powerful, highly relativistic plasma outflows, emitting
light from the radio up to the gamma-ray regime. The details of their
formation, composition and emission mechanisms are still not completely clear.
The combination of high-resolution observations using very long baseline
interferometry (VLBI) and multiwavelength monitoring provides the best insight
into these objects. Here, such a combined study of sources of the TANAMI sample
is presented, investigating the parsec-scale and high-energy properties. The
TANAMI program is a multiwavelength monitoring program of a sample of the radio
and gamma-ray brightest extragalactic jets in the southern sky, below -30deg
declination. We obtain the first-ever VLBI images for most of the sources,
providing crucial information on the jet kinematics and brightness distribution
at milliarcsecond resolution. Two particular sources are discussed in detail:
PMN J1603-4904, which can be classified either as an atypical blazar or a
gamma-ray loud (young) radio galaxy, and Centaurus A, the nearest radio-loud
active galaxy. The VLBI kinematics of the innermost parsec of Centaurus A's jet
result in a consistent picture of an accelerated jet flow with a spine-sheath
like structure.Comment: Doctoral Thesis Award Lecture 2015, AN 2016, 337,
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