1,746 research outputs found
Difference of optical conductivity between one- and two-dimensional doped nickelates
We study the optical conductivity in doped nickelates, and find the dramatic
difference of the spectrum in the gap (\alt4 eV) between one- (1D)
and two-dimensional (2D) nickelates. The difference is shown to be caused by
the dependence of hopping integral on dimensionality. The theoretical results
explain consistently the experimental data in 1D and
2D nickelates, YCaBaNiO and LaSrNiO,
respectively. The relation between the spectrum in the X-ray aborption
experiments and the optical conductivity in LaSrNiO is
discussed.Comment: RevTeX, 4 pages, 4 figure
Discovery of luminous pulsed hard X-ray emission from anomalous X-ray pulsars 1RXS J1708-4009, 4U 0142+61 and 1E 2259+586 by INTEGRAL and RXTE
We report on the discovery of hard spectral tails for energies above 10 keV
in the total and pulsed spectra of anomalous X-ray pulsars 1RXS J1708-4009, 4U
0142+61 and 1E 2259+586 using RXTE PCA (2-60 keV) and HEXTE (15-250 keV) data
and INTEGRAL IBIS ISGRI (20-300 keV) data. Improved spectral information on 1E
1841-045 is presented. The pulsed and total spectra measured above 10 keV have
power-law shapes and there is so far no significant evidence for spectral
breaks or bends up to ~150 keV. The pulsed spectra are exceptionally hard with
indices measured for 4 AXPs approximately in the range -1.0 -- 1.0. We also
reanalyzed archival CGRO COMPTEL (0.75-30 MeV) data to search for signatures
from our set of AXPs. No detections can be claimed, but the obtained
upper-limits in the MeV band indicate that for 1RXS J1708-4009, 4U 0142+61 and
1E 1841-045 strong breaks must occur somewhere between 150 and 750 keV.Comment: Accepted for publication in ApJ; 19 pages; 4 Tables; 15 Figures (6
color
GRB990510: on the possibility of a beamed X-ray afterglow
We discuss the prompt emission of the gamma-ray burst (GRB) 990510 and its
subsequent X-ray afterglow from 8.0 to 44.3 hrs after the prompt emission,
using observations with the Gamma-ray Burst Monitor and Narrow Field
Instruments on BeppoSAX. In the 40-700 keV band, GRB990510 had a fluence of
\~1.9x10^{-5}erg cm^{-2}, whereas it reached a peak flux of ~2.4x10^{-6}erg
cm^{-2} s^{-1}. The X-ray afterglow decay light curve can be satisfactorily
described by a single power law with index of -1.42+/-0.07. Both the X-ray and
optical behaviour of the afterglow can be explained by gamma-ray burst debris
expanding as a jet; we find that the cooling frequency is (fixed) between the
optical and X-ray wavelength bands.Comment: 16 pages, 4 figures, accepted for publication in the Astrophysical
Journa
QED can explain the non-thermal emission from SGRs and AXPs : Variability
Owing to effects arising from quantum electrodynamics (QED),
magnetohydrodynamical fast modes of sufficient strength will break down to form
electron-positron pairs while traversing the magnetospheres of strongly
magnetised neutron stars. The bulk of the energy of the fast mode fuels the
development of an electron-positron fireball. However, a small, but potentially
observable, fraction of the energy ( ergs) can generate a
non-thermal distribution of electrons and positrons far from the star. This
paper examines the cooling and radiative output of these particles. Small-scale
waves may produce only the non-thermal emission. The properties of this
non-thermal emission in the absence of a fireball match those of the quiescent,
non-thermal radiation recently observed non-thermal emission from several
anomalous X-ray pulsars and soft-gamma repeaters. Initial estimates of the
emission as a function of angle indicate that the non-thermal emission should
be beamed and therefore one would expect this emission to be pulsed as well.
According to this model the pulsation of the non-thermal emission should be
between 90 and 180 degrees out of phase from the thermal emission from the
stellar surface.Comment: 7 pages, 5 figures, to appear in the proceedings of the conference
"Isolated Neutron Stars: from the Interior to the Surface" (April 2006,
London), eds. D. Page, R. Turolla, & S. Zane, Astrophysics & Space Scienc
Penicillium verrucosum occurrence and Ochratoxin A contents in organically cultivated grain with special reference to ancient wheat types and drying practice
This study addresses the relationship between the ochratoxigenic strains of Penicillium verrucosum and ochratoxin A (OTA) contents in organically cultivated grain. It included 37 combined, non-dried grain samples from farmers with no drying facilities as well as 19 non-dried and 22 dried samples from six farms with on-farm drying facilities (Case studies 1-6). The study focused on the ancient wheat type spelt but also included samples of wheat, rye, barley, oats, triticale, emmer, and einkorn. All 78 samples were analysed for moisture content (MC) and occurrence of P. verrucosum. The latter was assessed by plating non-disinfected kernels on DYSG agar and counting those contaminated by the fungus. Fiftyfive samples were analysed for OTA. Most of the combine harvested samples (82%) were contaminated with P. verrucosum prior to drying. This was ascribed to difficult harvest conditions and many samples of spelt, which was significantly more contaminated by P. verrucosum than oats, wheat and barley. Though not statistically significant, the results also indicated that spelt was more contaminated than rye, which is usually regarded the most sensitive small grain cereal. No correlation was found between number of kernels contaminated by P. verrucosum and OTA content. Despite many non-dried samples being contaminated by P. verrucosum, only two exceeded the EU maximum limit for grain (5 ng OTA g-1), both being spring spelt with 18 and 92 ng g-1, respectively. The problems were most likely correlated to a late harvest and high MC of the grain. The case studies showed exceedings of the maximum limit in a batch of dried oats and spring wheat, respectively, probably to be explained by insufficient drying of late harvested grain with high MC. Furthermore, our results clearly indicate that OTA is not produced in significant amounts in samples with MCs below 17%. All dried samples with MCs above 18% exceeded the 5 ng OTA g-1 limit in grain. However, no correlation between MC and the amount of OTA produced was found
INTEGRAL discovery of persistent hard X-ray emission from the Soft Gamma Ray Repeater SGR 1806-20
We report the discovery of persistent hard X-ray emission extending up to 150
keV from the soft gamma-ray repeater SGR 1806-20 using data obtained with the
INTEGRAL satellite in 2003-2004. Previous observations of hard X-rays from
objects of this class were limited to short duration bursts and rare transient
episodes of strongly enhanced luminosity (``flares''). The emission observed
with the IBIS instrument above 20 keV has a power law spectrum with photon
index in the range 1.5-1.9 and a flux of 3 milliCrabs, corresponding to a
20-100 keV luminosity of ~10^36 erg s^-1 (for a distance of 15 kpc). The
spectral hardness and the luminosity correlate with the level of source
activity as measured from the number of emitted bursts.Comment: 5 pages, 3 figures, Revised version accepted for publication in
Astronomy and Astrophysics Letter
Momentum Dependence of Resonant Inelastic X-Ray Scattering Spectrum in Insulating Cuprates
The resonant inelastic x-ray scattering spectrum in insulating cuprates is
examined by using the exact diagonalization technique on small clusters in the
two-dimensional Hubbard model with second and third neighbor hopping terms.
When the incident photon energy is tuned near the Cu K absorption edges, we
find that the features of the unoccupied upper Hubbard band can be extracted
from the spectrum through an anisotropic momentum dependence. They provide an
opportunity for the understanding of the different behavior of hole- and
electron-doped superconductors.Comment: 4 pages with 4 figures, to be published in PR
The first multi-wavelength campaign of AXP 4U 0142+61 from radio to hard X-rays
For the first time a quasi-simultaneous multi-wavelength campaign has been
performed on an Anomalous X-ray Pulsar from the radio to the hard X-ray band.
4U 0142+61 was an INTEGRAL target for 1 Ms in July 2005. During these
observations it was also observed in the X-ray band with Swift and RXTE, in the
optical and NIR with Gemini North and in the radio with the WSRT. In this paper
we present the source-energy distribution. The spectral results obtained in the
individual wave bands do not connect smoothly; apparently components of
different origin contribute to the total spectrum. Remarkable is that the
INTEGRAL hard X-ray spectrum (power-law index 0.79 +/- 0.10) is now measured up
to an energy of ~230 keV with no indication of a spectral break. Extrapolation
of the INTEGRAL power-law spectrum to lower energies passes orders of magnitude
underneath the NIR and optical fluxes, as well as the low ~30 microJy (2 sigma)
upper limit in the radio band.Comment: 6 pages, 1 figure. To be published in the proceedings of the
conference "Isolated Neutron Stars: from the Interior to the Surface" (April
24-28, 2006, London, UK), eds. S. Zane, R. Turolla and D. Pag
Observational Evidence for Strong Disk Comptonization in GRO J1655-40
Analysis was made of the multiple XTE/PCA data on the promised black hole
candidate with superluminal jet, GRO J1655-40, acquired during its 1996--1997
outburst. The X-ray spectra can be adequately described by the sum of an
optically thick disk spectrum and a power-law. When the estimated 1--100 keV
power-law luminosity exceeds 1E37 erg/s (assuming a distance of 3.2 kpc), the
inner disk radius and the maximum color temperature derived from a simple
accretion disk model (a multi-color disk model) vary significantly with time.
These results reconfirm the previous report by Sobczak et al. (1999). In this
strong power-law state (once called ``very high state''), the disk luminosity
decreases with temperature, in contradiction to the prediction of the standard
Shakura-Sunyaev model. In the same state, the intensity of the power-law
component correlates negatively with that of the disk component, and positively
with the power-law photon index, suggesting that the strong power-law is simply
the missing optically thick disk emission. One possible explanation for this
behavior is inverse-Compton scattering in the disk. By re-fitting the same data
incorporating a disk Comptonization, the inner radius and temperature of the
underlying disk are found to become more constant. These results provide one of
the first observational confirmations of the scenario of disk Comptonization in
the strong power-law state. This strong power-law state seems to appear when
color temperature of the disk exceeds the certain threshold, 1.2 -- 1.3 keV.Comment: 10 pages 4 figures; submitted to ApJ
BeppoSAX measurements of the bright gamma-ray burst 010222
We analyze the BeppoSAX measurements of the prompt and afterglow emission of
the gamma-ray burst GRB010222. Among 45 GRBs detected with the Wide Field
Cameras on BeppoSAX, the 40-700 keV fluence of (9.3+/-0.3)E-5 erg cm-2 is only
surpassed by GRB990123. In terms of the isotropic 20-2000 keV energy output of
7.8E53 erg, it ranks third of all GRBs with measured distances. Since this
burst is so bright, the data provide complete and valuable coverage up to 65 hr
after the event, except for a gap between 3.5 and 8.0 hr. The 2-10 keV flux
history shows clear signs of a break which is consistent with a break seen in
the optical, and provides supporting evidence for the achromatic nature of the
break. An explanation for the break in the context of a collimated expansion is
not straightforward. Rather, a model is favored whereby the fireball is braked
to the non-relativistic regime quickly (within a fraction of day) by a dense
1E6 cm-3 circumburst medium. This implies that, after a mild beaming
correction, GRB010222 may be the most energetic burst observed thus far. The
X-ray decay index after the break is 1.33+/-0.04, the spectral index
0.97+/-0.05. The decay is, with unprecedented accuracy, identical to that
observed in the optical.Comment: Accepted on June 6 for publication in ApJ part I. Publication due in
October 2001. Accepted version has only minor modification
- âŠ