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$^{42}$ erg s$^{-1}$ 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 $\sim 9$ 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 $7.1 \times 10^{-14} ergs/(cm^2 s keV)$ (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 $\mu$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