77 research outputs found
Nonthermal X-Rays from the Galactic Ridge: a Tracer of Low Energy Cosmic Rays ?
A distinct low energy cosmic-ray component has been proposed to explain the
essentially constant Be/Fe ratio at low metallicities. Atomic collisions of
such low energy ions produce characteristic nonthermal X-ray emission. In this
paper, we study the possible contribution of such X-rays to the Galactic ridge
emission. We show that they would account for < 10% of the 10-60 keV luminosity
of the thin Galactic disk component detected with RXTE. They could make a more
significant contribution in the 0.5-10 keV energy range, provided that the
nonthermal ion population extends down to about 1 MeV/nucleon and delivers
about 10 erg s to the interstellar medium, comparable to the
total power suplied by the Galactic supernovae. But since the nonthermal X-rays
in this energy range are essentially produced below the thresholds of the
Be-producing cross sections, their detection does not necessarily imply a low
energy cosmic-ray origin for the spallogenic light elements. A significant
contribution of nonthermal X-rays could alleviate the problem of the origin of
the hard component observed with ASCA in the Scutum arm region.Comment: latex 9 pages, uses paspconf.sty, 4 figures. To appear in "LiBeB,
Cosmic Rays and Gamma-Ray Line Astronomy", ASP Conference Series, eds. R.
Ramaty, E. Vangioni-Flam, M. Casse and K. Oliv
RXTE Hard X-ray Observation of A754: Constraining the Hottest Temperature Component and the Intracluster Magnetic Field
Abell 754, a cluster undergoing merging, was observed in hard X-rays with the
Rossi X-ray Timing Explorer (RXTE) in order to constrain its hottest
temperature component and search for evidence of nonthermal emission.
Simultaneous modeling of RXTE data and those taken with previous missions
yields an average intracluster temperature of keV in the 1-50 keV
energy band. A multi-temperature component model derived from numerical
simulations of the evolution of a cluster undergoing a merger produces similar
quality of fit, indicating that the emission measure from the very hot gas
component is sufficiently small that it renders the two models
indistinguishable. No significant nonthermal emission was detected. However,
our observations set an upper limit of
(90% confidence limit) to the nonthermal emission flux at 20 keV. Combining
this result with the radio synchrotron emission flux we find a lower limit of
0.2 G for the intracluster magnetic field. We discuss the implications of
our results for the theories of magnetic field amplifications in cluster
mergers.Comment: Accepted for Publication in the Astrophysical Journal, 22 pages, 5
figure
On the Origin of the Iron K Line in the Spectrum of the Galactic X-ray Background
We propose a mechanism for the origin of the Galactic ridge X-ray background that naturally explains the properties of the Fe K line, specifically the detection of the centroid line energy below 6.7 keV and the apparent broadness of the line. Motivated by recent evidence of nonthermal components in the spectrum of the Galactic X-ray/gamma-ray background, we consider a model that is a mixture of thermal plasma components of perhaps supernova origin and nonthermal emission from the interaction of low energy Cosmic ray electrons (LECRe) with the interstellar medium. The LECRe may be accelerated in supernova explosions or by ambient interstellar plasma turbulence. Atomic collisions of fast electrons produce characteristic nonthermal, narrow X-ray emission lines that can explain the complex Galactic background spectrum. Using the ASCA GIS archival data from the Scutum arm region, we show that a two-temperature thermal plasma model with kT~0.6 and ~2.8 keV, plus a LECRe component models the data satisfactorily. Our analysis rules out a purely nonthermal origin for the emission. It also rules out a significant contribution from low energy Cosmic ray ions, because their nonthermal X-ray production would be accompanied by a nuclear gamma-ray line diffuse emission exceeding the upper limits obtained using OSSE, as well as by an excessive Galaxy-wide Be production rate. The proposed model naturally explains the observed complex line features and removes the difficulties associated with previous interpretations of the data which evoked a very hot thermal component (kT~7 keV)
INTEGRAL discovery of a bright highly obscured galactic X-ray binary source IGR J16318-4848
INTEGRAL regularly scans the Galactic plane to search for new objects and in
particular for absorbed sources with the bulk of their emission above 10-20
keV. The first new INTEGRAL source was discovered on 2003 January 29, 0.5
degree from the Galactic plane and was further observed in the X-rays with
XMM-Newton. This source, IGR J16318-4848, is intrinsically strongly absorbed by
cold matter and displays exceptionally strong fluorescence emission lines. The
likely infrared/optical counterpart indicates that IGR J16318-4848 is probably
a High Mass X-Ray Binary neutron star or black hole enshrouded in a Compton
thick environment. Strongly absorbed sources, not detected in previous surveys,
could contribute significantly to the Galactic hard X-ray background between 10
and 200 keV.Comment: 6 pages, 4 figures (fig 1 quality lowered), accepted for publication
in A&A letters (INTEGRAL special issue
Low-latency Science Exploration of Planetary Bodies: a Demonstration Using ISS in Support of Mars Human Exploration
We summarize a proposed experiment to use the International Space Station to formally examine the application and validation of low-latency telepresence for surface exploration from space as an alternative, precursor, or potentially as an adjunct to astronaut "boots on the ground." The approach is to develop and propose controlled experiments, which build upon previous field studies and which will assess the effects of different latencies (0 to 500 msec), task complexity, and alternate forms of feedback to the operator. These experiments serve as an example of a pathfinder for NASA's roadmap of missions to Mars with low-latency telerobotic exploration as a precursor to astronaut's landing on the surface to conduct geological tasks
Nonthermal Hard X-Ray Emission from the Galactic Ridge
We investigate the origin of the nonthermal X-ray emission from the Galactic
ridge in the range 10-200 keV. We consider bremsstrahlung of subrelativistic
cosmic ray protons and electrons as production processes. From the solution of
the kinetic equations describing the processes of particle in-situ acceleration
and spatial propagation we derive parameters of the spectra for protons and
electrons. For in-situ acceleration the flux of accelerated particles consists
mainly of protons since the ratio of the accelerated protons to electrons is
large and the flux of nuclei with charges Z>1 is strongly suppressed. Our
analysis has shown that the proton bremsstrahlung model is free from the
previously discussed problems of the high gamma-ray flux at MeV or at hundreds
of MeV energy regions. However, the model with proton bremsstrahlung requires
specific restrictions on the proton spectrum.The real problem of the proton
bremsstrahlung model is the large pressure of the accelerated protons in the
disk. For the electron origin of the ridge flux we do not have the problems of
the gamma-ray line and continuous fluxes. On the other hand the electron
bremsstrahlung model requires more effective acceleration of background
particles.Comment: A&A accepted, 16 pages, 14 figures, 2 table
Discovery and evolution of the new black hole candidate Swift J1539.2-6227 during its 2008 outburst
We report on the discovery by the Swift Gamma-Ray Burst Explorer of the black
hole candidate Swift J1539.2-6227 and the subsequent course of an outburst
beginning in November 2008 and lasting at least seven months. The source was
discovered during normal observations with the Swift Burst Alert Telescope
(BAT) on 2008 November 25. An extended observing campaign with the Rossi X-Ray
Timing Explorer (RXTE) and Swift provided near-daily coverage over 176 days,
giving us a good opportunity to track the evolution of spectral and timing
parameters with fine temporal resolution through a series of spectral states.
The source was first detected in a hard state during which strong low-frequency
quasi-periodic oscillations (QPOs) were detected. The QPOs persisted for about
35 days and a signature of the transition from the hard to soft intermediate
states was seen in the timing data. The source entered a short-lived thermal
state about 40 days after the start of the outburst. There were variations in
spectral hardness as the source flux declined and returned to a hard state at
the end of the outburst. The progression of spectral states and the nature of
the timing features provide strong evidence that Swift J1539.2-6227 is a
candidate black hole in a low-mass X-ray binary system.Comment: Accepted by the Astrophysical Journa
Gamma-ray continuum emission from the inner Galactic region as observed with INTEGRAL/SPI
The diffuse continuum emission from the Galactic plane in the energy range
18-1000 keV has been studied using 16 Ms of data from the SPI instrument on
INTEGRAL. With such an exposure we can exploit the imaging properties of SPI to
achieve a good separation of point sources from the various diffuse components.
Using a candidate-source catalogue derived with IBIS on INTEGRAL and a number
of sky distribution models we obtained spectra resolved in Galactic longitude.
We can identify spectral components of a diffuse continuum of power law shape
with index about 1.7, a positron annihilation component with a continuum from
positronium and the line at 511 keV, and a second, roughly power-law component
from detected point sources. Our analysis confirms the concentration of
positron annihilation emission in the inner region (|l|<10), the disk
(10<|l|<30) being at least a factor 7 weaker in this emission. The power-law
component in contrast drops by only a factor 2, showing a quite different
longitude distribution and spatial origin. Detectable sources constitute about
90% of the total Galactic emission between 20 and 60 keV, but have a steeper
spectrum than the diffuse emission, their contribution to the total emission
dropping rapidly to a small fraction at higher energies. In the SPI energy
range the flux is lower than found by OSSE, probably due to the more complete
accounting for sources by SPI. The power-law emission is difficult to explain
as of interstellar origin, inverse Compton giving at most 10%, and instead a
population of unresolved point sources is proposed as a possible origin, AXPs
with their spectra hardening above 100 keV being plausible candidates. We
present a broadband spectrum of the emission.Comment: accepted for publication in Astronomy and Astrophysic
RXTE observations of strongly absorbed sources IGR J16318-4848 and IGR J16358-4726
Results of analysis of RXTE observations of strongly absorbed X-ray sources
IGR J16318-4848 and IGR J16358-4726 are presented. Careful subtraction of
Galactic ridge emission contribution to the observed spectra of RXTE/PCA
allowed us to obtain the spectra of the sources in 3-25 keV energy band.
Spectra of the sources cold be well described by a power law with
photoabsorption. It is shown that the value of absorption column in the case of
RXTE observation of IGR J16318-4848 performed on March 14.1, 2003 is somewhat
higher that that obtained by XMM observatory on Feb 10.7 2003. This could imply
that the source has variable absorption, presumably connected with an orbital
phase of the binary system. It is noted, that all three X-ray sources,
discovered by INTEGRAL observatory in the sky region of (l,b)~(336,0) (IGR
J16318-4848, IGR J16320-4751 and IGR J16358-4726) have large intrinsic
photoabsorption and could be high mass X-ray binaries. This hypothesis has
indirect support from the fact that their are located close to the Norma spiral
arm tangent, i.e. in the region of enhanced concentrations of young massive
stars. If they are reside within this spiral arm some rough estimation of the
sources distance could be made -- D~6-8 kpc.Comment: 4 pages, 4 figures. Submitted to Astronomy Letter
Measurement Of the Galactic X-ray/Gamma-ray Background Radiation: Contribution of Discrete Sources
The Galactic background radiation near the Scutum Arm was observed
simultaneously with RXTE and OSSE in order to determine the spectral shape and
the origin of the emission in the hard X-ray/soft gamma-ray band. The spectrum
in the 3 keV to 1 MeV band is well modeled by 4 components: a high energy
continuum dominating above 500 keV that can be characterized by a power law of
photon index ~ 1.6 (an extrapolation from measurements above ~ 1 MeV); a
positron annihilation line at 511 keV and positronium continuum; a variable
hard X-ray/soft gamma-ray component that dominates between 10-200 keV (with a
minimum detected flux of ~ 7.7 x 10^-7 photons cm^-2 s^-1 keV^-1 deg^-2 at 100
keV averaged over the field of view of OSSE) and that is well modeled by an
exponentially cut off power law of photon index ~ 0.6 and energy cut off at ~
41 keV; and finally a thermal plasma model of solar abundances and temperature
of 2.6 keV that dominates below 10 keV. We estimate that the contribution of
bright discrete sources to the minimum flux detected by OSSE was ~ 46% at 60
keV and ~ 20% at 100 keV. The remaining unresolved emission may be interpreted
either as truly diffuse emission with a hard spectrum (such as that from
inverse Compton scattering) or the superposition of discrete sources that have
very hard spectra.Comment: Accepted for Publication in the Astrophysical Journa
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