2,920 research outputs found
Quiet Sun Magnetic Field Measurements Based on Lines with Hyperfine Structure
The Zeeman pattern of MnI lines is sensitive to hyperfine structure (HFS)
and, they respond to hG magnetic field strengths differently from the lines
used in solar magnetometry. This peculiarity has been employed to measure
magnetic field strengths in quiet Sun regions. However, the methods applied so
far assume the magnetic field to be constant in the resolution element. The
assumption is clearly insufficient to describe the complex quiet Sun magnetic
fields, biasing the results of the measurements. We present the first syntheses
of MnI lines in realistic quiet Sun model atmospheres. The syntheses show how
the MnI lines weaken with increasing field strength. In particular, kG magnetic
concentrations produce NnI 5538 circular polarization signals (Stokes V) which
can be up to two orders of magnitude smaller than the weak magnetic field
approximation prediction. Consequently, (1) the polarization emerging from an
atmosphere having weak and strong fields is biased towards the weak fields, and
(2) HFS features characteristic of weak fields show up even when the magnetic
flux and energy are dominated by kG fields. For the HFS feature of MnI 5538 to
disappear the filling factor of kG fields has to be larger than the filling
factor of sub-kG fields. Stokes V depends on magnetic field inclination
according to the simple consine law. Atmospheres with unresolved velocities
produce asymmetric line profiles, which cannot be reproduced by simple
one-component model atmospheres. The uncertainty of the HFS constants do not
limit the use of MnI lines for magnetometry.Comment: Accepted for publication in ApJ. 10 pages, 14 figure
Using X-ray catalogues to find counterparts to unassociated high-energy Fermi/LAT sources
The first Fermi Large Area Telescope (LAT) catalogue of sources (1FHL)
emitting at high energies (above 10 GeV) reports the details of 514 objects
detected in the first three years of the Fermi mission. Of these, 71 were
reported as unidentified in the 1FHL catalogue, although six are likely to be
associated with a supernova remnant (SNR), a Pulsar Wind Nebula (PWN) or a
combination of both, thereby leaving a list of 65 still unassociated objects.
Herein, we report a preliminary analysis on this sample of objects
concentrating on nine 1FHL sources, which were found to have a clear optical
extragalactic classification. They are all blazar, eight BL Lac and one flat
spectrum radio quasar, typically at redshift greater than 0.1.Comment: Proceedings of "Swift: 10 Years of Discovery", December 2-5 2014,
Rome, Italy, in Proceedings of Science (SWIFT 10
Swift/XRT counterparts to unassociated Fermi high-energy LAT sources
We report the results from our analysis of a large set of archival data
acquired with the X-ray telescope (XRT) onboard Swift, covering the sky region
surrounding objects from the first Fermi Large Area Telescope (LAT) catalogue
of high-energy sources (1FHL), which still lack an association. Of the 23
regions analysed, ten did not show any evidence of X-ray emission, but 13 were
characterised by the presence of one or more objects emitting in the 0.3-10 keV
band. Only in a couple of cases is the X-ray counterpart located outside the
Fermi positional uncertainty, while in all other cases the associations found
are compatible with the high-energy error ellipses. All counterparts we found
have been studied in detail by means of a multi-waveband approach to evaluate
their nature or class; in most cases, we have been able to propose a likely or
possible association except for one Fermi source whose nature remains doubtful
at the moment. The majority of the likely associations are extragalactic in
nature, most probably blazars of the BL Lac type.Comment: 13 pages, 5 figures, 4 tables. Accepted for publication in A&
RESIK observations of He-like Ar X-ray line emission in solar flares
The Ar XVII X-ray line group principally due to transitions 1s2 - 1s2l (l=s,
p) near 4 Anstroms was observed in numerous flares by the RESIK bent crystal
spectrometer aboard CORONAS-F between 2001 and 2003. The three line features
include the Ar XVII w (resonance line), a blend of x and y (intercombination
lines), and z (forbidden line), all of which are blended with Ar XVI
dielectronic satellites. The ratio G, equal to [I(x+y) + I(z)]/I(w), varies
with electron temperature Te mostly because of unresolved dielectronic
satellites. With temperatures estimated from GOES X-ray emission, the observed
G ratios agree fairly well with those calculated from CHIANTI and other data.
With a two-component emission measure, better agreement is achieved. Some S XV
and S XVI lines blend with the Ar lines, the effect of which occurs at
temperatures greater than 8MK, allowing the S/Ar abundance ratio to be
determined. This is found to agree with coronal values. A nonthermal
contribution is indicated for some spectra in the repeating-pulse flare of 2003
February 6.Comment: Latex file and 3 ps files. Astrophysical Journal Letters (accepted,
June 2008
Swift/XRT follow-up observations of unidentified INTEGRAL/IBIS sources
Many sources listed in the 4th IBIS/ISGRI survey are still unidentified, i.e.
lacking an X-ray counterpart or simply not studied at lower energies (< 10
keV). The cross-correlation between the list of IBIS sources in the 4th
catalogue and the Swift/XRT data archive is of key importance to search for the
X-ray counterparts; in fact, the positional accuracy of few arcseconds obtained
with XRT allows us to perform more efficient and reliable follow-up
observations at other wavelengths (optical, UV, radio). In this work, we
present the results of the XRT observations for four new gamma-ray sources: IGR
J12123-5802, IGR J1248.2-5828, IGR J13107-5626 and IGR J14080-3023. For IGR
J12123-5802 we find a likely counterpart, but further information are needed to
classified this object, IGR J1248.2-5828 is found to be a Seyfert 1.9, for IGR
J13107-5626 we suggest a possible AGN nature, while IGR J14080-3023 is
classified as a Seyfert 1.5 galaxy.Comment: 6 pages, 4 figure and 2 tables. Accepted for publication on PoS
(contribution PoS(extremesky2009)018), proceedings of "The Extreme sky:
Sampling the Universe above 10 keV", held in Otranto (Italy), 13-17 October
200
Spectral Geometry of Heterotic Compactifications
The structure of heterotic string target space compactifications is studied
using the formalism of the noncommutative geometry associated with lattice
vertex operator algebras. The spectral triples of the noncommutative spacetimes
are constructed and used to show that the intrinsic gauge field degrees of
freedom disappear in the low-energy sectors of these spacetimes. The quantum
geometry is thereby determined in much the same way as for ordinary superstring
target spaces. In this setting, non-abelian gauge theories on the classical
spacetimes arise from the K-theory of the effective target spaces.Comment: 14 pages LaTe
Flat-spectrum radio sources as likely counterparts of unidentified INTEGRAL sources (Research Note)
Many sources in the fourth INTEGRAL/IBIS catalogue are still unidentified,
since they lack an optical counterpart. An important tool that can help in
identifying/classifying these sources is the cross-correlation with radio
catalogues, which are very sensitive and positionally accurate. Moreover, the
radio properties of a source, such as the spectrum or morphology, could provide
further insight into its nature. Flat-spectrum radio sources at high Galactic
latitudes are likely to be AGN, possibly associated to a blazar or to the
compact core of a radio galaxy. Here we present a small sample of 6 sources
extracted from the fourth INTEGRAL/IBIS catalogue that are still
unidentified/unclassified, but which are very likely associated with a bright,
flat-spectrum radio object. To confirm the association and to study the source
X-ray spectral parameters, we performed X-ray follow-up observations with
Swift/XRT. We report the results obtained from this search and discuss the
nature of each source. 5 of the 6 radio associations are also detected in
X-rays; in 3 cases they are the only counterpart found. IGR J06073--0024 is a
flat-spectrum radio quasar at z=1.08, IGR J14488--4008 is a newly discovered
radio galaxy, while IGR J18129--0649 is an AGN of a still unknown type. The
nature of IGR J07225--3810 and IGR J19386--4653 is less well defined, since in
both cases we find another X-ray source in the INTEGRAL error circle;
nevertheless, the flat-spectrum radio source, likely to be a radio loud AGN,
remains a viable and more convincing association in both cases. Only for IGR
J11544--7618 could we not find any convincing counterpart since the radio
association is not an X-ray emitter.Comment: 7 pages, 7 figures, accepted for publication on Astronomy and
Astrophysic
Vortex Scattering and Intercommuting Cosmic Strings on a Noncommutative Spacetime
We study the scattering of noncommutative vortices, based on the
noncommutative field theory developed in [Phys. Rev. D 75, 045009 (2007)], as a
way to understand the interaction of cosmic strings. In the center-of-mass
frame, the effects of noncommutativity vanish, and therefore the reconnection
of cosmic strings occurs in an identical manner to the commutative case.
However, when scattering occurs in a frame other than the center-of-mass frame,
strings still reconnect but the well known 90-degree scattering no longer need
correspond to the head on collision of the strings, due to the breakdown of
Lorentz invariance in the underlying noncommutative field theory.Comment: 18 pages, 2 figure
Why Matrix theory works for oddly shaped membranes
We give a simple proof of why there is a Matrix theory approximation for a
membrane shaped like an arbitrary Riemann surface. As corollaries, we show that
noncompact membranes cannot be approximated by matrices and that the Poisson
algebra on any compact phase space is U(infinity). The matrix approximation
does not appear to work properly in theories such as IIB string theory or
bosonic membrane theory where there is no conserved 3-form charge to which the
membranes couple.Comment: 8 pages, 4 figures, revtex; references adde
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