6,295 research outputs found
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
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,
Measurement of redshift dependent cross correlation of HSC clusters and Fermi rays
The cross-correlation study of the unresolved -ray background (UGRB)
with galaxy clusters has a potential to reveal the nature of the UGRB. In this
paper, we perform a cross-correlation analysis between -ray data by the
Fermi Large Area Telescope (Fermi-LAT) and a galaxy cluster catalogue from the
Subaru Hyper Suprime-Cam (HSC) survey. The Subaru HSC cluster catalogue
provides a wide and homogeneous large-scale structure distribution out to the
high redshift at , which has not been accessible in previous
cross-correlation studies. We conduct the cross-correlation analysis not only
for clusters in the all redshift range () of the survey, but
also for subsamples of clusters divided into redshift bins, the low redshift
bin () and the high redshift bin (), to utilize
the wide redshift coverage of the cluster catalogue. We find the evidence of
the cross-correlation signals with the significance of 2.0-2.3 for all
redshift and low-redshift cluster samples. On the other hand, for high-redshift
clusters, we find the signal with weaker significance level (1.6-1.9).
We also compare the observed cross-correlation functions with predictions of a
theoretical model in which the UGRB originates from -ray emitters such
as blazars, star-forming galaxies and radio galaxies. We find that the detected
signal is consistent with the model prediction.Comment: 11 pages, 24 figures, accepted by MNRA
Linking gamma-ray spectra of supernova remnants to the cosmic ray injection properties in the aftermath of supernovae
The acceleration times of the highest-energy particles which emit gamma-rays
in young and middle-age SNRs are comparable with SNR age. If the number of
particles starting acceleration was varying during early times after the
supernova explosion then this variation should be reflected in the shape of the
gamma-ray spectrum. We use the solution of the non-stationary equation for
particle acceleration in order to analyze this effect. As a test case, we apply
our method to describe gamma-rays from IC443. As a proxy of the IC443 parent
supernova we consider SN1987A. First, we infer the time dependence of injection
efficiency from evolution of the radio spectral index in SN1987A. Then, we use
the inferred injection behavior to fit the gamma-ray spectrum of IC443. We show
that the break in the proton spectrum needed to explain the gamma-ray emission
is a natural consequence of the early variation of the cosmic ray injection,
and that the very-high energy gamma-rays originate from particles which began
acceleration during the first months after the supernova explosion. We conclude
that the shape of the gamma-ray spectrum observed today in SNRs critically
depends on the time variation of the cosmic ray injection process in the
immediate post explosion phases. With the same model, we estimate also the
possibility in the future to detect gamma-rays from SN 1987A.Comment: A&A, accepte
Gamma-ray upper limits on magnetars with 6 years of Fermi-LAT observations
We report on the search for gamma-ray emission from 20 magnetars using 6
years of Fermi, Large Area Telescope (LAT) observations. No significant
evidence for gamma-ray emission from any of the currently-known magnetars is
found. We derived the most stringent upper limits to date on the 0.1--10 GeV
emission of Galactic magnetars, which are estimated between
erg s cm. Gamma-ray pulsations were
searched for the four magnetars having reliable ephemerides over the observing
period, but none were detected. On the other hand, we also studied the
gamma-ray morphology and spectra of seven Supernova Remnants associated or
adjacent to the magnetars.Comment: 22 pages, 4 figures, 2 tables, submitted to Ap
Putting Things Back Where They Belong: Tracing Cosmic-Ray Injection with H2
At present, all physical models of diffuse Galactic gamma-ray emission assume
that the distribution of cosmic-ray sources traces the observed populations of
either OB stars, pulsars, or supernova remnants. However, since H2-rich regions
host significant star formation and numerous supernova remnants, the morphology
of observed H2 gas should also provide a physically motivated, high-resolution
tracer for cosmic-ray injection. We assess the impact of utilizing H2 as a
tracer for cosmic-ray injection on models of diffuse Galactic gamma-ray
emission. We employ state-of-the-art 3D particle diffusion and gas density
models, along with a physical model for the star-formation rate based on global
Schmidt laws. Allowing a fraction, f_H2, of cosmic-ray sources to trace the
observed H2 density, we find that a theoretically well-motivated value f_H2 ~
0.20 -- 0.25 (i) provides a significantly better global fit to the diffuse
Galactic gamma-ray sky and (ii) highly suppresses the intensity of the residual
gamma-ray emission from the Galactic center region. Specifically, in models
utilizing our best global fit values of f_H2 ~ 0.20 -- 0.25, the spectrum of
the galactic center gamma-ray excess is drastically affected, and the
morphology of the excess becomes inconsistent with predictions for dark matter
annihilation.Comment: 5 pages, 3 figures, submitted to PR
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