X-ray Synchrotron Emission Research Articles

Constraints on relativistic jets in quiescent black hole X-ray binaries from broad-band spectral modelling

The nature of black hole jets at the lowest detectable luminosities remains an open question, largely due to a dearth of observational constraints. Here, we present a new, nearly-simultaneous broadband spectrum of the black hole X-ray binary (BHXB) XTE J1118+480 at an extremely low Eddington ratio (L_x~1e-8.5 L_Edd). Our new spectral energy distribution (SED) includes the radio, near-infrared, optical, ultraviolet, and X-ray wavebands. XTE J1118 is now the second BHXB at such a low Eddington ratio with a well-sampled SED, providing new constraints on highly sub-Eddington accretion flows and jets, and opening the door for comparison studies. We apply a multi-zone jet model to the new broadband SED, and we compare our results to previous fits to the same source using the same model at 4-5 decades higher luminosity. We find that after a BHXB transitions to the so-called quiescent spectral state, the jet base becomes more compact (by up to an order of magnitude) and slightly cooler (by at least a factor of two). Our preferred model fit indicates that jet particle acceleration is weaker after the transition into quiescence. That is, accelerated non-thermal particles no longer reach high enough Lorentz factors to contribute significant amounts of synchrotron X-ray emission. Instead, the X-ray waveband is dominated by synchrotron self-Compton emission from a population of mildly relativistic electrons with a quasi-thermal velocity distribution associated with the jet base. The corresponding (thermal) synchrotron component emits primarily in the infrared through ultraviolet wavebands. Our results on XTE J1118 are consistent with broadband modeling for A0620-00 and for Sgr A*. The above could therefore represent a canonical baseline geometry for accreting black holes in quiescence. We conclude with suggestions for future studies to further investigate the above scenario.

Synchrotron X-ray emission from old pulsars

We study the synchrotron radiation as the observed non-thermal X-ray emission from old pulsars ($\gtrsim1-10$Myr) to investigate the particle acceleration in their magnetospheres. We assume that the power-law component of the observed X-ray spectra is caused by the synchrotron radiation from electrons and positrons in the magnetosphere. We consider two pair production mechanisms of X-ray emitting particles, the magnetic and the photon-photon pair productions. High-energy photons, which ignite the pair production, are emitted via the curvature radiation of the accelerated particles. We use the analytical description for the radiative transfer and estimate the luminosity of the synchrotron radiation. We find that for pulsars with the spin-down luminosity $L_{\rm sd}\lesssim10^{33}$ erg s$^{-1}$, the locations of the particle acceleration and the non-thermal X-ray emission are within $\lesssim10^7$cm from the centre of the neutron star, where the magnetic pair production occurs. For pulsars with the spin-down luminosity $L_{\rm sd}\lesssim10^{31}$ erg s$^{-1}$ such as J0108-1431, the synchrotron radiation is difficult to explain the observed non-thermal component even if we consider the existence of the strong and small-scale surface magnetic field structures.

MAGNETIC FIELD AMPLIFICATION IN THE THIN X-RAY RIMS OF SN 1006

Several young supernova remnants (SNRs), including SN1006, emit synchrotron X-rays in narrow filaments, hereafter thin rims, along their periphery. The widths of these rims imply 50 to 100 $\mu$G fields in the region immediately behind the shock, far larger than expected for the interstellar medium compressed by unmodified shocks, assuming electron radiative losses limit rim widths. However, magnetic-field damping could also produce thin rims. Here we review the literature on rim width calculations, summarizing the case for magnetic-field amplification. We extend these calculations to include an arbitrary power-law dependence of the diffusion coefficient on energy, $D \propto E^{\mu}$. Loss-limited rim widths should shrink with increasing photon energy, while magnetic-damping models predict widths almost independent of photon energy. We use these results to analyze Chandra observations of SN 1006, in particular the southwest limb. We parameterize the full widths at half maximum (FWHM) in terms of energy as FWHM $\propto E^{m_E}_{\gamma}$. Filament widths in SN1006 decrease with energy; $m_E \sim -0.3$ to $-0.8$, implying magnetic field amplification by factors of 10 to 50, above the factor of 4 expected in strong unmodified shocks. For SN 1006, the rapid shrinkage rules out magnetic damping models. It also favors short mean free paths (small diffusion coefficients) and strong dependence of $D$ on energy ($\mu \ge 1$).

Absence of pressure-induced electron spin-state transition of iron in silicate glasses upon compression

Upon compression, silicate melts in the Earth’s interior are expected to be subject to successive structural transitions with multiple densification mechanisms that are distinct from those of their crystalline analogs. Experimental verification of this phenomenon remains a major target of glass-melt studies. Early studies of Fe spin-state transitions in silicate glasses under compression used synchrotron X-ray emission spectroscopy (XES) to develop seemingly irreconcilable interpretations that vary from complete transitions to low spin states at high pressure to a prevalence of high spin states. In an effort to reconcile the controversy, Mao et al. (February–March 2014 issue of American Mineralogist ) showed that the XES data must be properly handled with a correct reference spectrum for each spin state and suggested that the subtle effect of pressure-induced broadening in the spectra be considered. Based on the new analyses, they concluded that no pressure-induced spin-state transitions exist in iron-bearing silicate glasses at high pressure, relevant to Earth’s deep lower mantle. Thanks to several series of experimental studies, the elusive spin state in glasses under compression is being revealed, rendering the spin state of iron among the best understood for glasses at high pressure.

SIMULATIONS OF ION ACCELERATION AT NON-RELATIVISTIC SHOCKS. I. ACCELERATION EFFICIENCY

We use 2D and 3D hybrid (kinetic ions - fluid electrons) simulations to investigate particle acceleration and magnetic field amplification at non-relativistic astrophysical shocks. We show that diffusive shock acceleration operates for quasi-parallel configurations (i.e., when the background magnetic field is almost aligned with the shock normal) and, for large sonic and Alfv\'enic Mach numbers, produces universal power-law spectra proportional to p^(-4), where p is the particle momentum. The maximum energy of accelerated ions increases with time, and it is only limited by finite box size and run time. Acceleration is mainly efficient for parallel and quasi-parallel strong shocks, where 10-20% of the bulk kinetic energy can be converted to energetic particles, and becomes ineffective for quasi-perpendicular shocks. Also, the generation of magnetic turbulence correlates with efficient ion acceleration, and vanishes for quasi-perpendicular configurations. At very oblique shocks, ions can be accelerated via shock drift acceleration, but they only gain a factor of a few in momentum, and their maximum energy does not increase with time. These findings are consistent with the degree of polarization and the morphology of the radio and X-ray synchrotron emission observed, for instance, in the remnant of SN 1006. We also discuss the transition from thermal to non-thermal particles in the ion spectrum (supra-thermal region), and we identify two dynamical signatures peculiar of efficient particle acceleration, namely the formation of an upstream precursor and the alteration of standard shock jump conditions.

Spin and valence states of iron in Al-bearing silicate glass at high pressures studied by synchrotron Mossbauer and X-ray emission spectroscopy

High-pressure synchrotron Mossbauer (SMS) and X-ray emission (XES) spectroscopic measurements were conducted to investigate the spin and valence states of iron in (A1,Fe)-bearing magnesium silicate glass (Mg0.79Fe0.10Al0.10Si0.96O3) up to 126 GPa and 300 K. By analyzing the Fe K beta emission spectra using the integrated relative difference (IRD) method, which accounts for the spectral broadening effects, the derived total spin momentum (S) of the iron in the glass shows no observable changes with pressure within the experimental uncertainties. A two-doublet fitting model representing two diverse local iron atomic environments was used to satisfactorily simulate the high-pressure SMS spectra of iron in the glass. The doublet with an averaged quadrupole splitting (QS) value of 1.94(+/- 0.25) mm/s and chemical shift (CS) of 1.02(+/- 0.25) minis at ambient conditions was assigned to be high-spin Fe2+, whereas the second doublet with QS = 0.83(+/- 0.25) mm/s and CS = 0.49(+/- 0.25) mm/s was assigned to be high-spin Fe3+. Increasing pressure continuously elevates the QS of Fe2+ from similar to 2 mm/s at ambient pressure to 3.5 mm/s at 126 GPa, while Fe3+ only exhibits a slight increase in the QS to 1.34(+/- 0.25) mm/s. Comparing with previous experimental and theoretical studies on the local geometries and hyperfine parameters of silicate glasses and minerals, we conclude that the occurrence of the extremely high QS of Fe2+ in our glass above similar to 40-50 GPa can be associated with the enhanced density and diverse distortions and geometries of the local Fe2+ environments. Our combined XES and SMS results show that both Fe2+ and Fe3+ ions in Al-bearing silicate remain in the high-spin state, rather than undergoing a spin-pairing transition as proposed previously. Assuming that the silicate glass results can be used as an analog for understanding silicate melts, our results here indicate that iron ions likely experience significant changes in the local environments yet remain overall in the high-spin state in silicate melts at the extreme pressure and temperature conditions of the deep mantle.

Systematic properties of decelerating relativistic jets in low-luminosity radio galaxies

We model the kiloparsec-scale synchrotron emission from jets in 10 Fanaroff-Riley Class I radio galaxies for which we have sensitive, high-resolution imaging and polarimetry from the Very Large Array. We assume that the jets are intrinsically symmetrical, axisymmetric, decelerating, relativistic outflows and we infer their inclination angles and the spatial variations of their flow velocities, magnetic field structures and emissivities using a common set of fitting functions. The inferred inclinations agree well with independent indicators. The spreading rates increase rapidly, then decrease, in a flaring region. The jets then recollimate to form conical outer regions at distance r0 from the active galactic nucleus (AGN). The flaring regions are homologous when scaled by r0. At ~0.1r0, the jets brighten abruptly at the onset of a high-emissivity region and we find an outflow speed of ~0.8c, with a uniform transverse profile. Jet deceleration first becomes detectable at ~0.2r0 and the outflow often becomes slower at its edges than it is on-axis. Deceleration continues until ~0.6r0, after which the outflow speed is usually constant. The dominant magnetic-field component is longitudinal close to the AGN and toroidal after recollimation, but the field evolution is initially much slower than predicted by flux-freezing. In the flaring region, acceleration of ultrarelativistic particles is required to counterbalance the effects of adiabatic losses and account for observed X-ray synchrotron emission, but the brightness evolution of the outer jets is consistent with adiabatic losses alone. We interpret our results as effects of the interaction between the jets and their surroundings. (Slightly abridged).

Spin transition of Fe3+ in Al-bearing phase D: An alternative explanation for small-scale seismic scatterers in the mid-lower mantle

Among dense-hydrous magnesium silicates potentially transporting H2O into Earthʼs deep interior, phase D (MgSi2H2O6) exhibits the highest P–T stability range, extending into the lower mantle along cold slab geotherms. We have studied the compressibility and spin state of Fe in Al-bearing phase D up to 90 GPa using synchrotron X-ray diffraction and X-ray emission spectroscopy. Fe–Al-bearing phase D was synthesized at 25 GPa and 1400 °C with approximate composition MgSi1.5Fe0.15Al0.32H2.6O6, where nearly all of the Fe is ferric (Fe3+). Analysis of Fe-Kβ emission spectra reveals a gradual, pressure-induced high-spin (HS) to low-spin (LS) transition of Fe3+ extending from 40 to 65 GPa. The fitted equation of state for high-spin Fe–Al-bearing phase D results in a bulk modulus KT0=147(2) GPa with pressure derivative K′=6.3(3). An equation of state over the entire pressure range was calculated using the observed variation in low-spin fraction with pressure and a low-spin bulk modulus of KT0=253(30) GPa, derived from the data above 65 GPa. Pronounced softening in the bulk modulus occurs during the spin transition, reaching a minimum at 50 GPa (∼1500 km) where the bulk modulus of Fe–Al phase D is about 35% lower than Fe–Al-bearing silicate perovskite. Recovery of the bulk modulus at 50–65 GPa results in a structure that has a similar incompressibility as silicate perovskite above 65 GPa. Similarly, the bulk sound velocity of Fe–Al phase D reaches a minimum at ∼50 GPa, being about 10% slower than silicate perovskite. The potential association of Fe–Al phase D with subducted slabs entering the lower mantle, along with its elastic properties through the Fe3+ spin transition predicted at 1200–1800 km, suggests that phase D may provide an alternative explanation for small-scale mid-lower mantle seismic scatterers and supports the presence of deeply recycled sediments in the lower mantle.

Proper motions of Hα filaments in the supernova remnant RCW 86

We present a proper motion study of the eastern shock-region of the supernova remnant RCW 86 (MSH 14-63, G315.4-2.3), based on optical observations carried out with VLT/FORS2 in 2007 and 2010. For both the northeastern and southeastern regions, we measure an average proper motion of H-alpha filaments of 0.10 +/- 0.02 arcsec/yr, corresponding to 1200 +/- 200 km/s at 2.5kpc. There is substantial variation in the derived proper motions, indicating shock velocities ranging from just below 700 km/s to above 2200 km/s. The optical proper motion is lower than the previously measured X-ray proper motion of northeastern region. The new measurements are consistent with the previously measured proton temperature of 2.3 +/- 0.3 keV, assuming no cosmic-ray acceleration. However, within the uncertainties, moderately efficient (< 27 per cent) shock acceleration is still possible. The combination of optical proper motion and proton temperature rule out the possibility that RCW 86 has a distance less than 1.5kpc. The similarity of the proper motions in the northeast and southeast is peculiar, given the different densities and X-ray emission properties of the regions. The northeastern region has lower densities and the X-ray emission is synchrotron dominated, suggesting that the shock velocities should be higher than in the southeastern, thermal X-ray dominated, region. A possible solution is that the H-alpha emitting filaments are biased toward denser regions, with lower shock velocities. Alternatively, in the northeast the shock velocity may have decreased rapidly during the past 200yr, and the X-ray synchrotron emission is an afterglow from a period when the shock velocity was higher.

The shape of the cutoff in the synchrotron emission of SN 1006 observed withXMM-Newton

Synchrotron X-ray emission from the rims of young supernova remnants allows us to study the high-energy tail of the electrons accelerated at the shock front. The analysis of X-ray spectra can provide information on the physical mechanisms that limit the energy achieved by the electrons in the acceleration process. We aim at verifying whether the maximum electron energy in SN 1006 is limited by synchrotron losses and at obtaining information on the shape of the cutoff in the X-ray synchrotron emission. We analyzed the deep observations of the XMM-Newton SN 1006 Large Program. We performed spatially resolved spectral analysis of a set of small regions in the nonthermal limbs and studied the X-ray spectra by adopting models that assume different electron spectra. We found out that a loss-limited model provides the best fit to all the spectra and this indicates that the shape of the cutoff in the electron momentum (p) distribution has the form exp[-(p/p_cut)^2]. We also detected residual thermal emission from shocked ambient medium and confirmed the reliability of previous estimates of the post-shock density. Our results indicate that radiative losses play a fundamental role in shaping the electron spectrum in SN 1006.

Nonthermal emission properties of the northwestern rim of supernova remnant RX J0852.0-4622

The supernova remnant (SNR) RX J0852-4622 (Vela Jr., G266.6-1.2) is one of the most important SNRs for investigating the acceleration of multi-TeV particles and the origin of Galactic cosmic rays because of its strong synchrotron X-ray and TeV gamma-ray emission, which show a shell-like morphology similar to each other. Using the XMM-Newton archival data consisting of multiple pointing observations of the northwestern rim of the remnant, we investigate the spatial properties of the nonthermal X-ray emission as a function of distance from an outer shock wave. All X-ray spectra are well reproduced by an absorbed power-law model above 2 keV. It is found that the spectra show gradual softening from a photon index 2.56 in the rim region to 2.96 in the interior region. We show that this radial profile can be interpreted as a gradual decrease of the cutoff energy of the electron spectrum due to synchrotron cooling. By using a simple spectral evolution model that includes continuous synchrotron losses, the spectral softening can be reproduced with the magnetic field strength in the post-shock flow to less than several tens of uG. If this is a typical magnetic field in the SNR shell, gamma-ray emission would be accounted for by inverse Compton scattering of high-energy electrons that also produce the synchrotron X-ray emission. Future hard X-ray imaging observations with Nustar and ASTRO-H and TeV gamma-ray observations with the Cherenkov Telescope Array (CTA) will allow to us to explore other possible explanations of the systematic softening of the X-ray spectra.

DIFFUSE HARD X-RAY EMISSION IN STARBURST GALAXIES AS SYNCHROTRON FROM VERY HIGH ENERGY ELECTRONS

[Abdriged] The origin of the diffuse hard X-ray (2 - 10 keV) emission from starburst galaxies is a long-standing problem. We suggest that synchrotron emission of 10 - 100 TeV electrons and positrons (e+/-) can contribute to this emission, because starbursts have strong magnetic fields. We consider three sources of e+/- at these energies: (1) primary electrons directly accelerated by supernova remnants; (2) pionic secondary e+/- created by inelastic collisions between CR protons and gas nuclei in the dense ISMs of starbursts; (3) pair e+/- produced between the interactions between 10 - 100 TeV gamma-rays and the intense far-infrared (FIR) radiation fields of starbursts. We create one-zone steady-state models of the CR population in the Galactic Center (R <= 112 pc), NGC 253, M82, and Arp 220's nuclei, assuming a power law injection spectrum for electrons and protons. We compare these models to extant radio and GeV and TeV gamma-ray data for these starbursts, and calculate the diffuse synchrotron X-ray and Inverse Compton (IC) luminosities of these starbursts. If the primary electron spectrum extends to ~PeV energies and has a proton/electron injection ratio similar to the Galactic value, we find that synchrotron contributes 2 - 20% of their unresolved, diffuse hard X-ray emission. Inverse Compton emission is likewise a minority of the unresolved X-ray emission in these starbursts, from 0.1% in the Galactic Center to 10% in Arp 220's nuclei. We also model generic starbursts, including submillimeter galaxies, in the context of the FIR--X-ray relation, finding that up to 2% in the densest starbursts with our fiducial assumptions. Neutrino and TeV gamma-ray data can further constrain the synchrotron X-ray emission of starbursts. Our models do not constrain hard synchrotron X-ray emission from any additional hard components of primary e+/- from sources like pulsars in starbursts.

EVOLUTION OF SYNCHROTRON X-RAYS IN SUPERNOVA REMNANTS

A systematic study of the synchrotron X-ray emission from supernova remnants (SNRs) has been conducted. We selected a total of 12 SNRs whose synchrotron X-ray spectral parameters are available in the literature with reasonable accuracy, and studied how their luminosities change as a function of radius. It is found that the synchrotron X-ray luminosity tends to drop especially when the SNRs become larger than ~5 pc, despite large scatter. This may be explained by the change of spectral shape caused by the decrease of the synchrotron roll-off energy. A simple evolutionary model of the X-ray luminosity is proposed and is found to reproduce the observed data approximately, with reasonable model parameters. According to the model, the total energy of accelerated electrons is estimated to be 10^(47-48) ergs, which is well below the supernova explosion energy. The maximum energies of accelerated electrons and protons are also discussed.

Iron-rich perovskite in the Earth's lower mantle

The equations of state of perovskite with (Mg 0.75,Fe 0.25)SiO 3 and MgSiO 3 compositions have been investigated by synchrotron X-ray diffraction up to 130 GPa at 300 K in diamond anvil cells. Here we show that the addition of 25% Fe in MgSiO 3 perovskite increases its density and bulk sound velocity (V Ф) by 4–6% and 6–7%, respectively, at lower-mantle pressures. Based on concurrent synchrotron X-ray emission and Mössbauer spectroscopic studies of the samples, the increase in V Ф and density can be explained by the occurrence of the low-spin Fe 3+ and the extremely high-quadrupole component of Fe 2+. Combining these experimental results with thermodynamic modeling, our results indicate that iron-rich perovskite can produce an increase in density and a value of V Ф that is compatible with seismic observations of reduced shear-wave velocity in regions interpreted as dense, stiff piles in the lower mantle. Therefore, the existence of the Fe-rich perovskite in the lower mantle may help elucidate the cause of the lower-mantle large low-shear-velocity provinces (LLSVPs) where enhanced density and V Ф are seismically observed to anti-correlate with the reduced shear wave velocity.

Pressure induced high spin-low spin transition in FeSe superconductor studied by x-ray emission spectroscopy and ab initio calculations

FeSe is a simple binary system in the iron based superconducting family and exhibits a significant pressure induced increase in the superconducting transition temperature (Tc). In addition to pressure effect, spin fluctuations, magnetic ordering, and crystal structure all play vital roles in altering Tc. Even though various experiments and theoretical simulations explain the connection among them and superconductivity, the interplay between these important parameters is still not clearly understood. Here, we report the pressure effect on the spin state of Fe in FeSe superconductor studied using synchrotron x-ray emission spectroscopy at ambient and low temperatures down to 8 K near Tc. Pressure induced high spin to low spin transition was observed at both ambient and low temperatures with continuous suppression of Fe magnetic moments under increasing pressure. The spin transition is closely related to the pressure induced tetragonal to orthorhombic structural transition.

Modelling TeV γ-ray emission from the kiloparsec-scale jets of Centaurus A and M87

The widespread detection of synchrotron X-ray emission from the jets of low-power, nearby radio galaxies implies the presence of electrons at and above TeV energies. In this paper we explore the possibility that the TeV gamma-rays detected from the radio galaxies Cen A and M87, which both have bright, well-studied X-ray jets, are produced at least in part by inverse-Compton scattering of various photon fields by the high-energy electrons responsible for the synchrotron X-rays on kiloparsec scales. We describe a new numerical code that we have developed to carry out inverse-Compton calculations taking account of all the relevant physics and using detailed models of the jets and the photon fields in which they are embedded, and show that existing constraints on the very high-energy (VHE) gamma-ray fluxes of these two objects already place significant constraints on the magnetic field strengths in the jet in Cen A. Finally, we discuss the prospects for constraints on radio galaxy jet physics that may be obtained from observations with the Cerenkov Telescope Array (CTA).

ENERGY SPECTRUM OF NONTHERMAL ELECTRONS ACCELERATED AT A PLANE SHOCK

We calculate the energy spectra of cosmic ray (CR) protons and electrons at a plane shock with quasi-parallel magnetic fields, using time-dependent, diffusive shock acceleration (DSA) simulations, including energy losses via synchrotron emission and Inverse Compton (IC) scattering. A thermal leakage injection model and a Bohm type diffusion coeffcient are adopted. The electron spectrum at the shock becomes steady after the DSA energy gains balance the synchrotron/IC losses, and it cuts off at the equilibrium momentum peq. In the postshock region the cutoff momentum of the electron spectrum decreases with the distance from the shock due to the energy losses and the thickness of the spatial distribution of electrons scales as p?¹. Thus the slope of the downstream integrated spectrum steepens by one power of p for p br ＜ p ＜ p eq , where the break momentum decreases with the shock age as p br χ t?¹. In a CR modified shock, both the proton and electron spectrum exhibit a concave curvature and deviate from the canonical test-particle power-law, and the upstream integrated electron spectrum could dominate over the downstream integrated spectrum near the cutoff momentum. Thus the spectral shape near the cutoff of X-ray synchrotron emission could reveal a signature of nonlinear DSA.

Magnetic Fields in Supernova Remnants and Pulsar-Wind Nebulae

We review the observations of supernova remnants (SNRs) and pulsar-wind nebulae (PWNe) that give information on the strength and orientation of magnetic fields. Radio polarimetry gives the degree of order of magnetic fields, and the orientation of the ordered component. Many young shell supernova remnants show evidence for synchrotron X-ray emission. The spatial analysis of this emission suggests that magnetic fields are amplified by one to two orders of magnitude in strong shocks. Detection of several remnants in TeV gamma rays implies a lower limit on the magnetic-field strength (or a measurement, if the emission process is inverse-Compton upscattering of cosmic microwave background photons). Upper limits to GeV emission similarly provide lower limits on magnetic-field strengths. In the historical shell remnants, lower limits on B range from 25 to 1000 microGauss. Two remnants show variability of synchrotron X-ray emission with a timescale of years. If this timescale is the electron-acceleration or radiative loss timescale, magnetic fields of order 1 mG are also implied. In pulsar-wind nebulae, equipartition arguments and dynamical modeling can be used to infer magnetic-field strengths anywhere from about 5 microGauss to 1 mG. Polarized fractions are considerably higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field geometries often suggest a toroidal structure around the pulsar, but this is not universal. Viewing-angle effects undoubtedly play a role. MHD models of radio emission in shell SNRs show that different orientations of upstream magnetic field, and different assumptions about electron acceleration, predict different radio morphology. In the remnant of SN 1006, such comparisons imply a magnetic-field orientation connecting the bright limbs, with a non-negligible gradient of its strength across the remnant.

A DECLINE IN THE NONTHERMAL X-RAY EMISSION FROM CASSIOPEIA A

We present new Chandra ACIS-S3 observations of Cassiopeia A which, when combined with earlier ACIS-S3 observations, show evidence for a steady ~ 1.5-2%/yr decline in the 4.2-6.0 keV X-ray emission between the years 2000 and 2010. The computed flux from exposure corrected images over the entire remnant showed a 17% decline over the entire remnant and a slightly larger (21%) decline from regions along the remnant's western limb. Spectral fits of the 4.2-6.0 keV emission across the entire remnant, forward shock filaments, and interior filaments indicate the remnant's nonthermal spectral powerlaw index has steepened by about 10%, with interior filaments having steeper powerlaw indices. Since TeV electrons, which give rise to the observed X-ray synchrotron emission, are associated with the exponential cutoff portion of the electron distribution function, we have related our results to a change in the cutoff energy and conclude that the observed decline and steepening of the nonthermal X-ray emission is consistent with a deceleration of the remnant's ~5000 km/s forward shock of ~10--40 km/s/yr

Effects of non-uniform interstellar magnetic field on synchrotron X-ray and inverse-Comptonγ-ray morphology of supernova remnants

Observations of SNRs in X-ray and gamma-ray bands promise to contribute with important information in our understanding on the nature of galactic cosmic rays. The analysis of SNRs images collected in different energy bands requires the support of theoretical modeling of synchrotron and inverse Compton (IC) emission. We develop a numerical code (REMLIGHT) to synthesize, from MHD simulations, the synchrotron radio, X-ray and IC gamma-ray emission from SNRs expanding in non-uniform interstellar medium (ISM) and/or non-uniform interstellar magnetic field (ISMF). As a first application, the code is used to investigate the effects of non-uniform ISMF on the SNR morphology in the non-thermal X-ray and gamma-ray bands. We perform 3D MHD simulations of a spherical SNR shock expanding through a magnetized ISM with a gradient of ambient magnetic field strength. The model includes an approximate treatment of upstream magnetic field amplification and the effect of shock modification due to back reaction of accelerated cosmic rays. From the simulations, we synthesize the synchrotron radio, X-ray and IC gamma-ray emission with REMLIGHT, making different assumptions about the details of acceleration and injection of relativistic electrons. A gradient of the ambient magnetic field strength induces asymmetric morphologies in radio, X-ray and gamma-ray bands independently from the model of electron injection if the gradient has a component perpendicular to the line-of-sight. The degree of asymmetry of the remnant morphology depends on the details of the electron injection and acceleration and is different in the radio, X-ray, and gamma-ray bands. The non-thermal X-ray morphology is the most sensitive to the gradient, showing the highest degree of asymmetry. The IC gamma-ray emission is weakly sensitive to the non-uniform ISMF, the degree of asymmetry of the SNR morphology being the lowest in this band.