3,928 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
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,
Fermi/LAT discovery of gamma-ray emission from a relativistic jet in the narrow-line quasar PMN J0948+0022
We report the discovery by the Large Area Telescope (LAT) onboard the Fermi
Gamma-ray Space Telescope of high-energy gamma-ray emission from the peculiar
quasar PMN J0948+0022 (z=0.5846). The optical spectrum of this object exhibits
rather narrow Hbeta (FWHM(Hbeta) ~ 1500 km s^-1), weak forbidden lines and is
therefore classified as a narrow-line type I quasar. This class of objects is
thought to have relatively small black hole mass and to accrete at high
Eddington ratio. The radio loudness and variability of the compact radio core
indicates the presence of a relativistic jet. Quasi simultaneous
radio-optical-X-ray and gamma-ray observations are presented. Both radio and
gamma-ray emission (observed over 5-months) are strongly variable. The
simultaneous optical and X-ray data from Swift show a blue continuum attributed
to the accretion disk and a hard X-ray spectrum attributed to the jet. The
resulting broad band spectral energy distribution (SED) and, in particular, the
gamma-ray spectrum measured by Fermi are similar to those of more powerful
FSRQ. A comparison of the radio and gamma-ray characteristics of PMN J0948+0022
with the other blazars detected by LAT shows that this source has a relatively
low radio and gamma-ray power, with respect to other FSRQ. The physical
parameters obtained from modelling the SED also fall at the low power end of
the FSRQ parameter region discussed in Celotti & Ghisellini (2008). We suggest
that the similarity of the SED of PMN J0948+0022 to that of more massive and
more powerful quasars can be understood in a scenario in which the SED
properties depend on the Eddington ratio rather than on the absolute power.Comment: 10 pages, 5 figures, accepted for publication on ApJ Main Journal.
Corresponding author: L. Foschin
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
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