537 research outputs found
Electronic structure and magnetic properties of pyroxenes (Li,Na)TM(Si,Ge)2O6: novel low-dimensional magnets with 90 bonds
The results of the LSDA+U calculations for pyroxenes with diverse magnetic
properties (Li,Na)TM(Si,Ge)O, where TM is the transition metal ion
(Ti,V,Cr,Mn,Fe), are presented. We show that the anisotropic orbital ordering
results in the spin-gap formation in NaTiSiO. The detailed analysis of
different contributions to the intrachain exchange interactions for pyroxenes
is performed both analytically using perturbation theory and basing on the
results of the band structure calculations. The antiferromagnetic
exchange is found to decrease gradually in going from Ti to Fe.
It turns out to be nearly compensated by ferromagnetic interaction between
half-filled and empty orbitals in Cr-based pyroxenes. The
fine-tuning of the interaction parameters by the crystal structure results in
the ferromagnetism for NaCrGeO. Further increase of the total number of
electrons and occupation of sub-shell makes the contribution
and total exchange interaction antiferromagnetic for Mn- and Fe-based
pyroxenes. Strong oxygen polarization was found in Fe-based pyroxenes. It is
shown that this effect leads to a considerable reduction of antiferromagnetic
intrachain exchange. The obtained results may serve as a basis for the analysis
of diverse magnetic properties of pyroxenes, including those with recently
discovered multiferroic behavior.Comment: 11 pages, 10 figure
In which shell-type SNRs should we look for gamma-rays and neutrinos from p-p collisions?
We present a simple analytic model for the various contributions to the
non-thermal emission from shell type SNRs, and show that this model's results
reproduce well the results of previous detailed calculations. We show that the
\geq 1 TeV gamma ray emission from the shell type SNRs RX J1713.7-3946 and RX
J0852.0-4622 is dominated by inverse-Compton scattering of CMB photons (and
possibly infra-red ambient photons) by accelerated electrons. Pion decay (due
to proton-proton collisions) is shown to account for only a small fraction,
\lesssim10^-2, of the observed flux, as assuming a larger fractional
contribution would imply nonthermal radio and X-ray synchrotron emission and
thermal X-ray Bremsstrahlung emission that far exceed the observed radio and
X-ray fluxes. Models where pion decay dominates the \geq 1 TeV flux avoid the
implied excessive synchrotron emission (but not the implied excessive thermal
X-ray Bremsstrahlung emission) by assuming an extremely low efficiency of
electron acceleration, K_ep \lesssim 10^-4 (K_ep is the ratio of the number of
accelerated electrons and the number of accelerated protons at a given energy).
We argue that observations of SNRs in nearby galaxies imply a lower limit of
K_ep \gtrsim 10^-3, and thus rule out K_ep values \lesssim 10^-4 (assuming that
SNRs share a common typical value of K_ep). It is suggested that SNRs with
strong thermal X-ray emission, rather than strong non-thermal X-ray emission,
are more suitable candidates for searches of gamma rays and neutrinos resulting
from proton-proton collisions. In particular, it is shown that the neutrino
flux from the SNRs above is probably too low to be detected by current and
planned neutrino observatories (Abridged).Comment: 13 pages, 1 figure, accepted for publication in JCAP, minor revision
Observational constraints on the modeling of SN1006
Experimental spectra and images of the supernova remnant SN1006 have been
reported for radio, X-ray and TeV gamma-ray bands. Several comparisons between
models and observations have been discussed in the literature, showing that the
broad-band spectrum from the whole remnant as well as a sharpest radial profile
of the X-ray brightness can be both fitted by adopting a model of SN1006 which
strongly depends on the non-linear effects of the accelerated cosmic rays;
these models predict post-shock magnetic field (MF) strengths of the order of
150 micro G. Here we present a new way to compare models and observations, in
order to put constraints on the physical parameters and mechanisms governing
the remnant. In particular, we show that a simple model based on the classic
MHD and cosmic rays acceleration theories allows us to investigate the
spatially distributed characteristics of SN1006 and to put observational
constraints on the kinetics and MF. Our method includes modelling and
comparison of the azimuthal and radial profiles of the surface brightness in
radio, hard X-rays and TeV gamma-rays as well as the azimuthal variations of
the electron maximum energy. In addition, this simple model also provides good
fits to the radio-to-gamma-ray spectrum of SN1006. We find that our best-fit
model predicts an effective MF strength inside SN1006 of 32 micro G, in good
agreement with the `leptonic' model suggested by the HESS Collaboration (2010).
Finally, some difficulties in both the `classic' and the non-linear models are
discussed. A number of evidences about non-uniformity of MF around SN1006 are
noted.Comment: 15 pages, 13 figures, accepted for publication on MNRA
Review: The Newsletter of the Literary Managers and Dramaturgs of the Americas, volume 13, issue 1
Contents include: Reading Review (Again), Radical from the Root, Arthur Ballet Recipient of the 2002 LMDA Loessing Award, Notes from the Lunatic Fringe, Regional Reports Info: News and Previews From All Over, Review is Inaugurating a New Section in the Next Issue: Projects-In-Process, and LMDA Prize in Drammatury 2002 The Elliott Hayes Award.
Issue editors: D.J. Hopkins, Shelley Orr, Liz Engelman, Madeline Oldham, Jacob Zimmerhttps://soundideas.pugetsound.edu/lmdareview/1026/thumbnail.jp
Hadronic gamma-ray images of Sedov supernova remnants
A number of modern experiments in high-energy astrophysics produce images of
supernova remnants (SNRs) in the TeV and GeV gamma-rays. Either relativistic
electrons (due to the inverse-Compton scattering) or protons (due to the pion
decays) may be responsible for this emission. In particular, the broad-band
spectra of SNRs may be explained in both leptonic and hadronic scenarios.
Another kind of observational data, namely, images of SNRs, is an important
part of experimental information. We present a method to model gamma-ray images
of Sedov SNRs in uniform media and magnetic field due to hadronic emission.
These gamma-rays are assumed to appear as a consequence of meson decays
produced in inelastic collisions of accelerated protons with thermal protons
downstream of the shock - a model would be relevant for SNRs without firm
confirmations of the shock-cloud interaction, as e.g. SN1006. Distribution of
surface brightness of the shell-like SNR is synthesized numerically for a
number of configurations. An approximate analytical formula for azimuthal and
radial variation of hadronic gamma-ray brightness close to the shock is
derived. The properties of images as well as the main factors determining the
surface brightness distribution are determined. Some conclusions which would be
relevant to SN1006 are discussed.Comment: 11 pages, 7 figures, to be published in MNRA
Nonthermal X-Rays from Supernova Remnant G330.2+1.0 and the Characteristics of its Central Compact Object
We present results from our X-ray data analysis of the SNR G330.2+1.0 and its
CCO, CXOU J160103.1--513353 (J1601). Using our XMM-Newton and Chandra
observations, we find that the X-ray spectrum of J1601 can be described by
neutron star atmosphere models (T ~ 2.5--3.7 MK). Assuming the distance of d ~
5 kpc for J1601 as estimated for SNR G330.2+1.0, a small emission region of R ~
1--2 km is implied. X-ray pulsations previously suggested by Chandra are not
confirmed by the XMM-Newton data, and are likely not real. However, our timing
analysis of the XMM-Newton data is limited by poor photon statistics, and thus
pulsations with a relatively low amplitude (i.e., an intrinsic pulsed-fraction
< 40%) cannot be ruled out. Our results indicate that J1601 is a CCO similar to
that in the Cassiopeia A SNR.X-ray emission from SNR G330.2+1.0 is dominated by
power law continuum (Gamma ~ 2.1--2.5) which primarily originates from thin
filaments along the boundary shell. This X-ray spectrum implies synchrotron
radiation from shock-accelerated electrons with an exponential roll-off
frequency ~ 2--3 x 10^17 Hz. For the measured widths of the X-ray filaments (D
~ 0.3 pc) and the estimated shock velocity (v_s ~ a few x 10^3 km s^-1), a
downstream magnetic field B ~ 10--50 G is derived. The estimated maximum
electron energy E_max ~ 27--38 TeV suggests that G330.2+1.0 is a candidate TeV
gamma-ray source. We detect faint thermal X-ray emission in G330.2+1.0. We
estimate a low preshock density n_0 ~ 0.1 cm^-3, which suggests a dominant
contribution from an inverse Compton mechanism (than the proton-proton
collision) to the prospective gamma-ray emission. Follow-up deep radio, X-ray,
and gamma-ray observations will be essential to reveal the details of the shock
parameters and the nature of particle accelerations in this SNR.Comment: 26 pages, 3 tables, 7 figures (4 color figures), Accepted by Ap
Fermi Large Area Telescope Observations of the Cygnus Loop Supernova Remnant
We present an analysis of the gamma-ray measurements by the Large Area
Telescope(LAT) onboard the \textit{Fermi Gamma-ray Space Telescope} in the
region of the supernova remnant(SNR) Cygnus Loop(G74.08.5). We detect
significant gamma-ray emission associated with the SNR in the energy band
0.2--100 GeV. The gamma-ray spectrum shows a break in the range 2--3 GeV. The
gamma-ray luminosity is erg s between 1--100
GeV, much lower than those of other GeV-emitting SNRs. The morphology is best
represented by a ring shape, with inner/outer radii 0.7
0.1 and 1.6 0.1. Given the association among
X-ray rims, \halpha filaments and gamma-ray emission, we argue that gamma rays
originate in interactions between particles accelerated in the SNR and
interstellar gas or radiation fields adjacent to the shock regions. The decay
of neutral pions produced in nucleon-nucleon interactions between accelerated
hadrons and interstellar gas provides a reasonable explanation for the
gamma-ray spectrum.Comment: accepted by ApJ, 34 pages, 6 figure
An X-ray view of 82 LINERs with Chandra and XMM-Newton data
We present the results of an homogeneous X-ray analysis for 82 nearby LINERs
selected from the catalogue of Carrillo et al. (1999). All sources have
available Chandra (68 sources) and/or XMM-Newton (55 sources) observations.
This is the largest sample of LINERs with X-ray spectral data (60 out of the 82
objects) and significantly improves our previous analysis based on Chandra data
for 51 LINERs (Gonzalez-Martin et al. 2006). It increases both the sample size
and adds XMM-Newton data. New models permit the inclusion of double absorbers
in the spectral fits. Nuclear X-ray morphology is inferred from the compactness
of detected nuclear sources in the hard band (4.5-8.0 keV). Sixty per cent of
the sample shows a compact nuclear source and are classified as AGN candidates.
The spectral analysis indicates that best fits involve a composite model:
absorbed primary continuum and (2) soft spectrum below 2 keV described by an
absorbed scatterer and/or a thermal component. The resulting median spectra
parameters and their standard deviations are: G=2.11, =0.54 keV,
=21.32 and =21.93. We complement our X-ray results with our
analysis of HST optical images and literature data on emission lines, radio
compactness and stellar population. Adding all these multiwavelength data, we
conclude that evidence do exist supporting the AGN nature of their nuclear
engine for 80% of the sample (66 out of 82 objects).Comment: Accepted for publications in Astronomy and Astrophysics, 49 pages, 12
figures. Catalogs only at:
http://www.star.le.ac.uk/~gmo4/O.Gonzalez-Martin-LINERs_xray.pd
Eight gamma-ray pulsars discovered in blind frequency searches of Fermi LAT data
We report the discovery of eight gamma-ray pulsars in blind frequency
searches using the LAT, onboard the Fermi Gamma-ray Space Telescope. Five of
the eight pulsars are young (tau_c10^36 erg/s), and
located within the Galactic plane (|b|<3 deg). The remaining three are older,
less energetic, and located off the plane. Five pulsars are associated with
sources included in the LAT bright gamma-ray source list, but only one, PSR
J1413-6205, is clearly associated with an EGRET source. PSR J1023-5746 has the
smallest characteristic age (tau_c=4.6 kyr) and is the most energetic
(Edot=1.1E37 erg/s) of all gamma-ray pulsars discovered so far in blind
searches. PSRs J1957+5033 and J2055+25 have the largest characteristic ages
(tau_c~1 Myr) and are the least energetic (Edot~5E33 erg/s) of the
newly-discovered pulsars. We present the timing models, light curves, and
detailed spectral parameters of the new pulsars. We used recent XMM
observations to identify the counterpart of PSR J2055+25 as XMMU
J205549.4+253959. In addition, publicly available archival Chandra X-ray data
allowed us to identify the likely counterpart of PSR J1023-5746 as a faint,
highly absorbed source, CXOU J102302.8-574606. The large X-ray absorption
indicates that this could be among the most distant gamma-ray pulsars detected
so far. PSR J1023-5746 is positionally coincident with the TeV source HESS
J1023-575, located near the young stellar cluster Westerlund 2, while PSR
J1954+2836 is coincident with a 4.3 sigma excess reported by Milagro at a
median energy of 35 TeV. Deep radio follow-up observations of the eight pulsars
resulted in no detections of pulsations and upper limits comparable to the
faintest known radio pulsars, indicating that these can be included among the
growing population of radio-quiet pulsars in our Galaxy being uncovered by the
LAT, and currently numbering more than 20.Comment: Submitted to Ap
The European Photon Imaging Camera on XMM-Newton: The MOS Cameras
The EPIC focal plane imaging spectrometers on XMM-Newton use CCDs to record
the images and spectra of celestial X-ray sources focused by the three X-ray
mirrors. There is one camera at the focus of each mirror; two of the cameras
contain seven MOS CCDs, while the third uses twelve PN CCDs, defining a
circular field of view of 30 arcmin diameter in each case. The CCDs were
specially developed for EPIC, and combine high quality imaging with spectral
resolution close to the Fano limit. A filter wheel carrying three kinds of
X-ray transparent light blocking filter, a fully closed, and a fully open
position, is fitted to each EPIC instrument. The CCDs are cooled passively and
are under full closed loop thermal control. A radio-active source is fitted for
internal calibration. Data are processed on-board to save telemetry by removing
cosmic ray tracks, and generating X-ray event files; a variety of different
instrument modes are available to increase the dynamic range of the instrument
and to enable fast timing. The instruments were calibrated using laboratory
X-ray beams, and synchrotron generated monochromatic X-ray beams before launch;
in-orbit calibration makes use of a variety of celestial X-ray targets. The
current calibration is better than 10% over the entire energy range of 0.2 to
10 keV. All three instruments survived launch and are performing nominally in
orbit. In particular full field-of-view coverage is available, all electronic
modes work, and the energy resolution is close to pre-launch values. Radiation
damage is well within pre-launch predictions and does not yet impact on the
energy resolution. The scientific results from EPIC amply fulfil pre-launch
expectations.Comment: 9 pages, 11 figures, accepted for publication in the A&A Special
Issue on XMM-Newto
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