2,049 research outputs found
INTEGRAL observations of Sco X-1: evidence for Comptonization up to 200 keV
We have analyzed a long-term database for Sco X-1 obtained with the telescope
IBIS onboard the INTEGRAL satellite in order to study the hard X-ray behavior
of Sco X-1 from 20 up to 200 keV. Besides the data used for producing of the
INTEGRAL catalog of sources, this is the longest (412 ks) database of IBIS on
Sco X-1 up to date. The production of hard X-ray tails in low-mass X-ray
binaries is still a matter of debate. Since most of the fits to the high-energy
part of the spectra are done with powerlaw models, the physical mechanism for
the hard X-ray tail production is unclear. The purpose of this study is to
better constrain those possible mechanisms. Our main result shows a strong
correlation between the fluxes in the thermal and nonthermal part of Sco X-1
spectra. We thus suggest that Comptonization of lower energy photons is the
mechanism for producing hard X-ray tails in Sco X-1.Comment: 4 pages, 3 figures, 2 tables; officially accepted for publication (as
a Letter) by A&A in 2013 January 2
Confirming the thermal Comptonization model for black hole X-ray emission in the low-hard state
Hard X-ray spectra of black hole binaries in the low/hard state are well
modeled by thermal Comptonization of soft seed photons by a corona-type region
with \thinspace{\thinspace}keV and optical depth around 1.
Previous spectral studies of 1E{\thinspace}1740.72942, including both the
soft and the hard X-ray bands, were always limited by gaps in the spectra or by
a combination of observations with imaging and non-imaging instruments. In this
study, we have used three rare nearly-simultaneous observations of
1E{\thinspace}1740.71942 by both XMM-Newton and INTEGRAL satellites to
combine spectra from four different imaging instruments with no data gaps, and
we successfully applied the Comptonization scenario to explain the broadband
X-ray spectra of this source in the low/hard state. For two of the three
observations, our analysis also shows that, models including Compton reflection
can adequately fit the data, in agreement with previous reports. We show that
the observations can also be modeled by a more detailed Comptonization scheme.
Furthermore, we find the presence of an iron K-edge absorption feature in one
occasion, which confirms what had been previously observed by Suzaku. Our
broadband analysis of this limited sample shows a rich spectral variability in
1E{\thinspace}1740.72942 at the low/hard state, and we address the possible
causes of these variations. More simultaneous soft/hard X-ray observations of
this system and other black-hole binaries would be very helpful in constraining
the Comptonization scenario and shedding more light on the physics of these
systems.Comment: 6 pages, two figures, accepted for publication in A&
Telescope performance and image simulations of the balloon-borne coded-mask protoMIRAX experiment
In this work we present the results of imaging simulations performed with the
help of the GEANT4 package for the protoMIRAX hard X-ray balloon experiment.
The instrumental background was simulated taking into account the various
radiation components and their angular dependence, as well as a detailed mass
model of the experiment. We modeled the meridian transits of the Crab Nebula
and the Galatic Centre region during balloon flights in Brazil ( of latitude and an altitude of km) and
introduced the correspondent spectra as inputs to the imaging simulations. We
present images of the Crab and of three sources in the Galactic Centre region:
1E 1740.7-2942, GRS 1758-258 and GX 1+4. The results show that the protoMIRAX
experiment is capable of making spectral and timing observations of bright hard
X-ray sources as well as important imaging demonstrations that will contribute
to the design of the MIRAX satellite mission.Comment: 9 figure
Discovery of Two High-Magnetic-Field Radio Pulsars
We report the discovery of two young isolated radio pulsars with very high
inferred magnetic fields. PSR J1119-6127 has period P = 0.407 s, and the
largest period derivative known among radio pulsars, Pdot = 4.0e-12. Under
standard assumptions these parameters imply a characteristic spin-down age of
only tau = 1.6 kyr and a surface dipole magnetic field strength of B = 4.1e13
G. We have measured a stationary period-second-derivative for this pulsar,
resulting in a braking index of n = 2.91+-0.05. We have also observed a glitch
in the rotation of the pulsar, with fractional period change Delta_P/P =
-4.4e-9. Archival radio imaging data suggest the presence of a previously
uncataloged supernova remnant centered on the pulsar. The second pulsar, PSR
J1814-1744, has P = 3.975 s and Pdot = 7.4e-13. These parameters imply tau = 85
kyr, and B = 5.5e13 G, the largest of any known radio pulsar.
Both PSR J1119-6127 and PSR J1814-1744 show apparently normal radio emission
in a regime of magnetic field strength where some models predict that no
emission should occur. Also, PSR J1814-1744 has spin parameters similar to the
anomalous X-ray pulsar (AXP) 1E 2259+586, but shows no discernible X-ray
emission. If AXPs are isolated, high magnetic field neutron stars
(``magnetars''), these results suggest that their unusual attributes are
unlikely to be merely a consequence of their very high inferred magnetic
fields.Comment: 7 pages, 3 embedded EPS figures, to be published in Ap
Timing the Parkes Multibeam Pulsars
Measurement of accurate positions, pulse periods and period derivatives is an
essential follow-up to any pulsar survey. The procedures being used to obtain
timing parameters for the pulsars discovered in the Parkes multibeam pulsar
survey are described. Completed solutions have been obtained so far for about
80 pulsars. They show that the survey is preferentially finding pulsars with
higher than average surface dipole magnetic fields. Eight pulsars have been
shown to be members of binary systems and some of the more interesting results
relating to these are presented.Comment: 6 pages, 2 embedded EPS figures, to be published in proceedings of
"Pulsar Astronomy - 2000 and Beyond", ASP Conf. Se
Glossina fuscipes fuscipes and Glossina palpalis palpalis as joint vectors of sleeping sickness in the focus of Nola-Bilolo in the Central African Republic
Observation of spin Coulomb drag in a two-dimensional electron gas
An electron propagating through a solid carries spin angular momentum in
addition to its mass and charge. Of late there has been considerable interest
in developing electronic devices based on the transport of spin, which offer
potential advantages in dissipation, size, and speed over charge-based devices.
However, these advantages bring with them additional complexity. Because each
electron carries a single, fixed value (-e) of charge, the electrical current
carried by a gas of electrons is simply proportional to its total momentum. A
fundamental consequence is that the charge current is not affected by
interactions that conserve total momentum, notably collisions among the
electrons themselves. In contrast, the electron's spin along a given spatial
direction can take on two values, "up" and "down", so that the spin current and
momentum need not be proportional. Although the transport of spin polarization
is not protected by momentum conservation, it has been widely assumed that,
like the charge current, spin current is unaffected by electron-electron (e-e)
interactions. Here we demonstrate experimentally not only that this assumption
is invalid, but that over a broad range of temperature and electron density,
the flow of spin polarization in a two-dimensional gas of electrons is
controlled by the rate of e-e collisions
Upper Limits on the Continuum Emission from Geminga at 74 and 326 MHz
We report a search for radio continuum emission from the gamma-ray pulsar
Geminga. We have used the VLA to image the location of the optical counterpart
of Geminga at 74 and 326 MHz. We detect no radio counterpart. We derive upper
limits to the pulse-averaged flux density of Geminga, taking diffractive
scintillation into account. We find that diffractive scintillation is probably
quenched at 74 MHz and does not influence our upper limit, S < 56 mJy
(2\sigma), but that a 95% confidence level at 326 MHz is S < 5 mJy. Owing to
uncertainties on the other low-frequency detections and the possibility of
intrinsic variability or extrinsic variability (refractive interstellar
scintillation) or both, our non-detections are nominally consistent with these
previous detections.Comment: 8 pages, LaTeX2e with AASTeX 4.0, 3 figures; to be published in Ap
Les opilions dans les écosystèmes montagnards pyrénéens. I Les opilions de la haute vallée d'Ossau (Pyrénées-Atlantiques; France)
A study carried out in upper-Ossau valley (Pyrénées- Atlantiques; France) between 1987 and 1989 has revealed a high diversity of Opilionid fauna in such mountain area: eighteen species have been found. In this paper, the main features of their phenology, altitudinal distribution and ecology are presented.[fr] Une étude menée sur les Opilions (Arachnida) en haute-vallée d'Ossau (Pyrénées-Atlantiques; France) entre 1987 et 1989 a permis de souligner la richesse et la diversité de ce groupe dans les écosystèmes montagnards: dix-huit espèces ont été inventoriées. Nous présentons ici les principaux aspects de leur phénologie, de leur distribution altitudinale et de leur écologie.
[es] Un estudio sobre los Opiliones (Arachnida) en el Alto Valle de Ossau (Pirineos Atlánticos, Francia) entre 1987 y 1989 ha permitido señalarla riqueza y diversidad de este grupo en los ecosistemas de montaña: dieciocho especies se han inventariado. Presentamos aquí los principales aspectos de su fenología, distribución altitudinal y ecología
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