Context. The iron Kα line at 6.4 keV provides a valuable spectral diagnostic in several fields of X-ray astronomy. The line often results from the reprocessing of external hard X-rays by a neutral or ...low-ionized medium, but it can also be excited by impacts of low-energy cosmic rays. Aims. This paper aims to provide signatures allowing identification of radiation from low-energy cosmic rays in X-ray spectra showing the 6.4 keV Fe Kα line. Methods. We study in detail the production of nonthermal line and continuum X-rays by interaction of accelerated electrons and ions with a neutral ambient gas. Corresponding models are then applied to XMM-Newton observations of the X-ray emission emanating from the Arches cluster region near the Galactic center. Results. Bright 6.4 keV Fe line structures are observed around the Arches cluster. This emission is very likely produced by cosmic rays. We find that it can result from the bombardment of molecular gas by energetic ions, but probably not by accelerated electrons. Using a model of X-ray production by cosmic-ray ions, we obtain a best-fit metallicity of the ambient medium of 1.7 ± 0.2 times the solar metallicity. A large flux of low-energy cosmic ray ions could be produced in the ongoing supersonic collision between the star cluster and an adjacent molecular cloud. We find that a particle acceleration efficiency in the resulting shock system of a few percent would give enough power in the cosmic rays to explain the luminosity of the nonthermal X-ray emission. Depending on the unknown shape of the kinetic energy distribution of the fast ions above ~1 GeV nucleon-1, the Arches cluster region may be a source of high-energy γ-rays detectable with the Fermi Gamma-ray Space Telescope. Conclusions. At present, the X-ray emission prominent in the 6.4 keV Fe line emanating from the Arches cluster region probably offers the best available signature for a source of low-energy hadronic cosmic rays in the Galaxy.
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Observations of the X‐ray sky after the next decade will most likely be dominated by Athena (Advanced Telescope for High ENergy Astrophysics), the second large mission of ESA's Cosmic Vision ...2015–2035 programme. Athena has been conceived to address the “Hot and Energetic Universe” science theme, which focuses on the assembly and evolution of hot baryons in cosmic structures as well as the physics and energetic output generated by accreting super‐massive black holes, along with the relationship between the two processes, dubbed cosmic feedback. Thanks to its transformational capabilities, Athena will enable bringing all of today's XMM‐Newton's hot science topics to a new stage. In this paper, we summarize the Athena science case, science requirements, as well as the expected mission performance, mission concept, and its status.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The supernova remnant SN 1006 is a powerful source of high-energy particles and evolves in a relatively tenuous and uniform environment despite interacting with an atomic cloud in its northwestern ...limb. The X-ray image of SN 1006 reveals an indentation in the southwestern part of the shock front and the H I maps show an isolated (southwestern) cloud, having the same velocity as the northwestern cloud, whose morphology fits perfectly in the indentation. We performed spatially resolved spectral analysis of a set of small regions in the southwestern nonthermal limb and studied the deep X-ray spectra obtained within the XMM-Newton SN 1006 Large Program. We also analyzed archive H I data, obtained by combining single-dish and interferometric observations. We found that the best-fit value of N sub(H) derived from the X-ray spectra significantly increases in regions corresponding to the southwestern cloud, while the cutoff energy of the synchrotron emission decreases. The N sub(H) variation corresponds perfectly with the H I column density of the southwestern cloud, as measured from the radio data. The decrease in the cutoff energy at the indentation clearly reveals that the back side of the cloud is actually interacting with the remnant. The southwestern limb therefore presents a unique combination of efficient particle acceleration and high ambient density, thus being the most promising region for gamma -ray hadronic emission in SN 1006. We estimate that such emission will be detectable with the Fermi telescope within a few years.
If a sizeable fraction of the energy of supernova remnant shocks is channeled into energetic particles (commonly identified with Galactic cosmic rays), then the morphological evolution of the ...remnants must be distinctly modified. Evidence of such modifications has been recently obtained with the Chandra and XMM-Newton X-ray satellites. To investigate these effects, we coupled a semi-analytical kinetic model of shock acceleration with a 3D hydrodynamic code (by means of an effective adiabatic index). This enables us to study the time-dependent compression of the region between the forward and reverse shocks due to the back reaction of accelerated particles, concomitantly with the development of the Rayleigh-Taylor hydrodynamic instability at the contact discontinuity. Density profiles depend critically on the injection level η of particles: for $\eta \lesssim 10^{-4}$ modifications are weak and progressive, for $\eta \sim 10^{-3}$ modifications are strong and immediate. Nevertheless, the extension of the Rayleigh-Taylor unstable region does not depend on the injection rate. A first comparison of our simulations with observations of Tycho's remnant strengthens the case for efficient acceleration of protons at the forward shock.
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Context. The Rayleigh-Taylor instabilities that are generated by the deceleration of a supernova remnant during the ejecta-dominated phase are known to produce finger-like structures in the matter ...distribution that modify the geometry of the remnant. The morphology of supernova remnants is also expected to be modified when efficient particle acceleration occurs at their shocks. Aims. The impact of the Rayleigh-Taylor instabilities from the ejecta-dominated to the Sedov-Taylor phase is investigated over one octant of the supernova remnant. We also study the effect of efficient particle acceleration at the forward shock on the growth of the Rayleigh-Taylor instabilities. Methods. We modified the Adaptive Mesh Refinement code RAMSES to study with hydrodynamic numerical simulations the evolution of supernova remnants in the framework of an expanding reference frame. The adiabatic index of a relativistic gas between the forward shock and the contact discontinuity mimics the presence of accelerated particles. Results. The great advantage of the super-comoving coordinate system adopted here is that it minimizes numerical diffusion at the contact discontinuity, since it is stationary with respect to the grid. We propose an accurate expression for the growth of the Rayleigh-Taylor structures that smoothly connects the early growth to the asymptotic self-similar behaviour. Conclusions. The development of the Rayleigh-Taylor structures is affected, although not drastically, if the blast wave is dominated by cosmic rays. The amount of ejecta that reaches the shocked interstellar medium is smaller in this case. If acceleration were to occur at both shocks, the extent of the Rayleigh-Taylor structures would be similar but the reverse shock would be strongly perturbed.
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We present a library of numerical models of cosmic ray accelerating supernova remnants (SNRs) evolving through a homogeneous ambient medium. We analyse distributions of the different energy ...components and diffusive shock acceleration time-scales for the models in various conditions. The library comprises a variety of SNR evolutionary scenarios and is used to map remnants with sufficiently known properties. This mapping constrains the respective ambient medium properties and the acceleration efficiency. Employing the library, we derive the ambient medium density, ambient magnetic field strength and the cosmic ray acceleration efficiency for models of Tycho and SN 1006 remnants and refine the ages of SNR 0509−67.5 and SNR 0519−69.0.
The center of our Galaxy harbors a four million solar mass black hole that is unusually quiet: its present X-ray luminosity is more than 10 orders of magnitude less than its Eddington luminosity. The ...observation of iron fluorescence and hard X-ray emission from some of the massive molecular clouds surrounding the Galactic center has been interpreted as an echo of a past 10(39) erg s(-1) flare. Alternatively, low-energy cosmic rays propagating inside the clouds might account for the observed emission, through inverse bremsstrahlung of low-energy ions or bremsstrahlung emission of low-energy electrons. Here, we report the observation of a clear decay of the hard X-ray emission from the molecular cloud Sgr B2 during the past seven years, thanks to more than 20 Ms of INTEGRAL exposure. This confirms the decay previously observed comparing the 6.4 keV line fluxes measured by various X-ray instruments, but without intercalibration effects. The measured decay time is 8.2 +/- 1.7 yr, compatible with the light crossing time of the molecular cloud core. Such a short timescale rules out inverse bremsstrahlung by cosmic-ray ions as the origin of the X ray emission. We also obtained 2-100 keV broadband X-ray spectra by combining INTEGRAL and XMM-Newton data and compared them with detailed models of X-ray emission due to irradiation of molecular gas by (1) low-energy cosmic-ray electrons and (2) hard X-rays. Both models can reproduce the data equally well, but the time variability constraints and the huge cosmic-ray electron luminosity required to explain the observed hard X-ray emission strongly favor the scenario in which the diffuse emission of Sgr B2 is scattered and reprocessed radiation emitted in the past by Sgr A*. The spectral index of the illuminating power-law source is found to be Gamma similar to 2 and its luminosity 1.5-5 x 10(39) erg s(-1), depending on the relative positions of Sgr B2 and Sgr A*. Using recent parallax measurements that place Sgr B2 in front of Sgr A*, we find that the period of intense activity of Sgr A* ended between 75 and 155 years ago.
Context. The supernova remnant SN 1006 is a source of high-energy particles and its southwestern limb is interacting with a dense ambient cloud, thus is a promising region for γ-ray hadronic ...emission. Aims. We aim at describing the physics and the nonthermal emission associated with the shock-cloud interaction to derive the physical parameters of the cloud (poorly constrained by the data analysis), to ascertain the origin of the observed spatial variations in the spectral properties of the X-ray synchrotron emission, and to predict spectral and morphological features of the resulting γ-ray emission. Methods. We performed 3D magnetohydrodynamic simulations modeling the evolution of SN 1006 and its interaction with the ambient cloud, and explored different model setups. By applying the REMLIGHT code on the model results, we synthesized the synchrotron X-ray emission and compared it with actual observations to constrain the parameters of the model. We also synthesized the leptonic and hadronic γ-ray emission from the models, deriving constraints on the energy content of the hadrons accelerated at the southwestern limb. Results. We found that the impact of the SN 1006 shock front with a uniform cloud with density 0.5 cm-3 can explain the observed morphology, the azimuthal variations of the cutoff frequency of the X-ray synchrotron emission, and the shock proper motion in the interaction region. Our results show that the current upper limit for the total hadronic energy in the southwestern limb is 2.5 × 1049 erg.
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Context. Recent Suzaku X-ray observations of the ejecta-dominated supernova remnant W49B have shown that there is a clear indication in the global spectrum for overionized plasma, whose physical ...origin is still under debate. Aims. In order to ascertain the physical origin of this rapidly cooling plasma, we focus on its spatial localization within the X-ray emitting ejecta. Methods. We confirm a saw-edged excess (interpreted as a strong radiative recombination continuum) in the global spectrum above 8 keV, which emerges above the ionization-equilibrium model. We produce a hardness-ratio map to determine where the plasma is overionized and perform a spectral analysis of the regions with and without strong overionization. Results. We find that the overionized plasma is localized in the center of the remnant and in its western jet, while it is not detected in the bright eastern jet, where the expansion of the ejecta is hampered by their interaction with a dense interstellar cloud. Conclusions. The location of overionized plasma suggests that the inner ejecta are rapidly cooling by expansion, unlike the outer ejecta, for which expansion is hampered by interstellar clouds seen in H2.
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We report the high S/N observation on October 3, 2002 with XMM-Newton of the brightest X-ray flare detected so far from Sgr A* with a duration shorter than one hour (~2.7 ks). The light curve is ...almost symmetrical with respect to the peak flare, and no significant difference between the soft and hard X-ray range is detected. The overall flare spectrum is well represented by an absorbed power-law with a soft photon spectral index of $\Gamma =2.5 \pm0.3$, and a peak 2–10 keV luminosity of 3.6$^{+0.3}_{-0.4}\times 10 ^{35}$ erg s-1, i.e. a factor 160 higher than the Sgr A* quiescent value. No significant spectral change during the flare is observed. This X-ray flare is very different from other bright flares reported so far: it is much brighter and softer. The present accurate determination of the flare characteristics challenge the current interpretation of the physical processes occuring inside the very close environment of Sgr A* by bringing very strong constraints for the theoretical flare models.
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