191 research outputs found
On radiative np -> 1s + gamma transitions, induced by strong low-energy interactions,in kaonic atoms
We calculate the rates of the radiative transitions np -> 1s + gamma in
kaonic hydrogen and kaonic deuterium, induced by strong low-energy interactions
and enhanced by Coulomb interactions. The obtained results should be taken into
account for the theoretical analysis of the experimental data on the X-ray
spectra and yields in kaonic atoms.Comment: 10 pages, 1 figur
On “GSI Oscillations” as Interference of Two Closely Spaced Ground Mass Eigenstates of H-Like Mother Ions
Dissecting the long-term emission behaviour of the BL Lac object Mrk 421
We report on long-term multiwavelengthmonitoring of blazar Mrk 421 by the GLAST-AGILE
Support Program of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration and
Steward Observatory, and by the Swift and Fermi satellites. We study the source behaviour in
the period 2007–2015, characterized by several extreme flares. The ratio between the optical,
X-ray and γ -ray fluxes is very variable. The γ -ray flux variations show a fair correlation with
the optical ones starting from 2012.We analyse spectropolarimetric data and find wavelengthdependence
of the polarization degree (P), which is compatible with the presence of the
host galaxy, and no wavelength dependence of the electric vector polarization angle (EVPA).
Optical polarimetry shows a lack of simple correlation between P and flux and wide rotations of
the EVPA.We build broad-band spectral energy distributions with simultaneous near-infrared
and optical data from the GASP-WEBT and ultraviolet and X-ray data from the Swift satellite.
They show strong variability in both flux and X-ray spectral shape and suggest a shift of
the synchrotron peak up to a factor of ∼50 in frequency. The interpretation of the flux and
spectral variability is compatible with jet models including at least two emitting regions that
can change their orientation with respect to the line of sight.http://10.0.4.69/mnras/stx2185Accepted manuscrip
Exploring the connection between parsec-scale jet activity and broadband outbursts in 3C 279
We use a combination of high-resolution very long baseline interferometry
(VLBI) radio and multi-wavelength flux density and polarization observations to
constrain the physics of the dissipation mechanism powering the broadband
flares in 3C 279 during an episode of extreme flaring activity in 2013-2014.
Six bright flares superimposed on a long-term outburst are detected at
-ray energies. Four of the flares have optical and radio counterparts.
The two modes of flaring activity (faster flares sitting on top of a long term
outburst) present at radio, optical, -ray frequencies are missing in
X-rays. X-ray counterparts are only observed for two flares. The first three
flares are accompanied by ejection of a new VLBI component (NC2) suggesting the
43 GHz VLBI core as the site of energy dissipation. Another new component, NC3,
is ejected after the last three flares, which suggests that the emission is
produced upstream from the core (closer to the black hole). The study therefore
indicates multiple sites of energy dissipation in the source. An
anti-correlation is detected between the optical percentage polarization (PP)
and optical/-ray flux variations, while the PP has a positive
correlation with optical/-rays spectral indices. Given that the mean
polarization is inversely proportional to the number of cells in the emission
region, the PP vs. optical/-ray anti-correlation could be due to more
active cells during the outburst than at other times. In addition to the
turbulent component, our analysis suggests the presence of a combined turbulent
and ordered magnetic field, with the ordered component transverse to the jet
axis.Comment: ApJ (in press), 14 pages, 19 figure
The WEBT campaign on the BL Lac object PG 1553+113 in 2013. An analysis of the enigmatic synchrotron emission
A multifrequency campaign on the BL Lac object PG 1553+113 was organized by the Whole Earth Blazar Telescope (WEBT) in 2013 April-August, involving 19 optical, two near-IR, and three radio telescopes. The aim was to study the source behaviour at low energies during and around the high-energy observations by the Major Atmospheric Gamma-ray Imaging Cherenkov telescopes in April-July. We also analyse the UV and X-ray data acquired by the Swift and XMM-Newton satellites in the same period. The WEBT and satellite observations allow us to detail the synchrotron emission bump in the source spectral energy distribution (SED). In the optical, we found a general bluer-when-brighter trend. The X-ray spectrum remained stable during 2013, but a comparison with previous observations suggests that it becomes harder when the X-ray flux increases. The long XMM-Newton exposure reveals a curved X-ray spectrum. In the SED, the XMM-Newton data show a hard near-UV spectrum, while Swift data display a softer shape that is confirmed by previous Hubble Space Telescope/Cosmic Origins Spectrograph and International Ultraviolet Explorer observations. Polynomial fits to the optical-X-ray SED show that the synchrotron peak likely lies in the 4-30eV energy range, with a general shift towards higher frequencies for increasing X-ray brightness. However, the UV and X-ray spectra do not connect smoothly. Possible interpretations include: (i) orientation effects, (ii) additional absorption, (iii) multiple emission components, and (iv) a peculiar energy distribution of relativistic electrons. We discuss the first possibility in terms of an inhomogeneous helical jet mode
Multi-Wavelength Variability of BL Lacertae Measured with High Time Resolution
In an effort to locate the sites of emission at different frequencies and
physical processes causing variability in blazar jets, we have obtained high
time-resolution observations of BL Lacertae over a wide wavelength range: with
the \emph{Transiting Exoplanet Survey Satellite} (TESS) at 6,000-10,000 \AA\
with 2-minute cadence; with the Neil Gehrels \emph{Swift} satellite at optical,
UV, and X-ray bands; with the Nuclear Spectroscopic Telescope Array at hard
X-ray bands; with the \emph{Fermi} Large Area Telescope at -ray
energies; and with the Whole Earth Blazar Telescope for measurement of the
optical flux density and polarization. All light curves are correlated, with
similar structure on timescales from hours to days. The shortest timescale of
variability at optical frequencies observed with TESS is hr. The
most common timescale is ~hr, comparable with the minimum timescale of
X-ray variability, 14.5 hr. The multi-wavelength variability properties cannot
be explained by a change solely in the Doppler factor of the emitting plasma.
The polarization behavior implies that there are both ordered and turbulent
components to the magnetic field in the jet. Correlation analysis indicates
that the X-ray variations lag behind the -ray and optical light curves
by up to days. The timescales of variability, cross-frequency lags,
and polarization properties can be explained by turbulent plasma that is
energized by a shock in the jet and subsequently loses energy to synchrotron
and inverse Compton radiation in a magnetic field of strength GComment: 33 pages, 25 figures, 14 tables. Accepted to Ap
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