arXiv subjects

M. Miceli

Publications and source records attributed to M. Miceli.

At least 73 records · Page 4Linked to original sources

arXiv2013

Evidence of non-thermal X-ray emission from HH 80

Protostellar jets appear at all stages of star formation when the accretion process is still at work. Jets travel at velocities of hundreds of km/s, creating strong shocks when interacting with interstellar medium. Several cases of jets have been detected in X-rays, typically showing soft emission. For the first time, we report evidence of hard X-ray emission possibly related to non-thermal processes not explained by previous models of the post-shock emission predicted in the jet/ambient interaction scenario. HH 80 is located at the south head of the jet associated to the massive protostar IRAS 18162-2048. It shows soft and hard X-ray emission in regions that are spatially separated, with the soft X-ray emission region situated behind the region of hard X-ray emission. We propose a scenario for HH 80 where soft X-ray emission is associated to thermal processes from the interaction of the jet with denser ambient matter and the hard X-ray emission is produced by synchrotron radiation at the front shock.

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arXiv2013

The loss-limited electron energy in SN 1006: effects of the shock velocity and of the diffusion process

The spectral shape of the synchrotron X-ray emission from SN 1006 reveals the fundamental role played by radiative losses in shaping the high-energy tail of the electron spectrum. We analyze data from the XMM-Newton SN 1006 Large Program and confirm that in both nonthermal limbs the loss-limited model correctly describes the observed spectra. We study the physical origin of the observed variations of the synchrotron cutoff energy across the shell. We investigate the role played by the shock velocity and by the electron gyrofactor. We found that the cutoff energy of the syncrotron X-ray emission reaches its maximum value in regions where the shock has experienced its highest average speed. This result is consistent with the loss-limited framework. We also find that the electron acceleration in both nonthermal limbs of SN 1006 proceeds close to the Bohm diffusion limit, the gyrofactor being in the range 1.5-4. We finally investigate possible explanations for the low values of cutoff energy measured in thermal limbs.

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arXiv2013

The shape of the cutoff in the synchrotron emission of SN 1006 observed with XMM-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.

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arXiv2013

The Hot and Energetic Universe: The astrophysics of supernova remnants and the interstellar medium

The study of both supernova remnants and the hot and cold phases of the interstellar medium are essential for understanding the final stages of stellar evolution and their feedback on the evolution of galaxies through injection of energy and heavy elements. These studies are also crucial for understanding the physics of supernovae, their cosmological implication, and the origin of galactic cosmic rays. The unique capabilities of Athena+ will allow us to explore a new parameter space. Spatially-resolved high-resolution spectroscopy using Athena+ X-IFU of young remnants will allow to characterize individual parcels of ejected material in the line of sight in terms of kinematics, ionization and composition, providing access to the three dimensional geometry of the explosion. Athena+ will also allow studying shock physics and particle acceleration in supernova remnants, as well as their interaction with their environment. Athena+ X-IFU will also characterize the ionization mechanisms competing in forming the complex structures of the hot interstellar medium, likely to keep the echo of past star formation activity, both in our Galaxy and nearby ones. For the first time the dust and gas of the densest cold medium, like in the Galactic Centre environment, will be studied. Athena+ X-IFU will observe, along with the Mg K and Si K edges, which are the main tracers of the silicates content of the ISM, the Fe K edge with unprecedented sensitivity and energy-resolution. This will allow us to study for the first time the nature of Fe-bearing dust in such regions.

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arXiv2013

Hydrodynamic modelling of ejecta shrapnel in the Vela supernova remnant

Many supernova remnants (SNRs) are characterized by a knotty ejecta structure. The Vela SNR is an excellent example of remnant in which detached clumps of ejecta are visible as X-ray emitting bullets that have been observed and studied in great detail. We aim at modelling the evolution of ejecta shrapnel in the Vela SNR, investigating the role of their initial parameters (position and density) and addressing the effects of thermal conduction and radiative losses. We performed a set of 2-D hydrodynamic simulations describing the evolution of a density inhomogeneity in the ejecta profile. We explored different initial setups. We found that the final position of the shrapnel is very sensitive to its initial position within the ejecta, while the dependence on the initial density contrast is weaker. Our model also shows that moderately overdense knots can reproduce the detached features observed in the Vela SNR. Efficient thermal conduction produces detectable effects by determining an efficient mixing of the ejecta knot with the surrounding medium and shaping a characteristic elongated morphology in the clump.

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arXiv2012

Non-thermal processes in coronae and beyond

This contribution summarizes the splinter session "Non-thermal processes in coronae and beyond" held at the Cool Stars 17 workshop in Barcelona in 2012. It covers new developments in high energy non-thermal effects in the Earth's exosphere, solar and stellar flares, the diffuse emission in star forming regions and reviews the state and the challenges of the underlying atomic databases.

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arXiv2012

AE Aurigae: first detection of non-thermal X-ray emission from a bow shock produced by a runaway star

Runaway stars produce shocks when passing through interstellar medium at supersonic velocities. Bow shocks have been detected in the mid-infrared for several high-mass runaway stars and in radio waves for one star. Theoretical models predict the production of high-energy photons by non-thermal radiative processes in a number sufficiently large to be detected in X-rays. To date, no stellar bow shock has been detected at such energies. We present the first detection of X-ray emission from a bow shock produced by a runaway star. The star is AE Aur, which was likely expelled from its birthplace by the encounter of two massive binary systems and now is passing through the dense nebula IC 405. The X-ray emission from the bow shock is detected at 30" to the northeast of the star, coinciding with an enhancement in the density of the nebula. From the analysis of the observed X-ray spectrum of the source and our theoretical emission model, we confirm that the X-ray emission is produced mainly by inverse Compton upscattering of infrared photons from dust in the shock front.

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arXiv2012

XMM-Newton evidence of shocked ISM in SN 1006: indications of hadronic acceleration

Shock fronts in young supernova remnants are the best candidates for being sites of cosmic ray acceleration up to a few PeV, though conclusive experimental evidence is still lacking. Hadron acceleration is expected to increase the shock compression ratio, providing higher postshock densities, but X-ray emission from shocked ambient medium has not firmly been detected yet in remnants where particle acceleration is at work. We exploited the deep observations of the XMM-Newton Large Program on SN 1006 to verify this prediction. We performed spatially resolved spectral analysis of a set of regions covering the southeastern rim of SN 1006. We studied the spatial distribution of the thermodynamic properties of the ambient medium and carefully verified the robustness of the result with respect to the analysis method. We detected the contribution of the shocked ambient medium. We also found that the postshock density of the interstellar medium significantly increases in regions where particle acceleration is efficient. Under the assumption of uniform preshock density, we found that the shock compression ratio reaches a value of ~6 in regions near the nonthermal limbs. Our results support the predictions of shock modification theory and indicate that effects of acceleration of cosmic ray hadrons on the postshock plasma can be observed in supernova remnants.

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arXiv2012

X-raying hot plasma in solar active regions with the SphinX spectrometer

The detection of very hot plasma in the quiescent corona is important for diagnosing heating mechanisms. The presence and the amount of such hot plasma is currently debated. The SphinX instrument on-board CORONAS-PHOTON mission is sensitive to X-ray emission well above 1 keV and provides the opportunity to detect the hot plasma component. We analyzed the X-ray spectra of the solar corona collected by the SphinX spectrometer in May 2009 (when two active regions were present). We modelled the spectrum extracted from the whole Sun over a time window of 17 days in the 1.34-7 keV energy band by adopting the latest release of the APED database. The SphinX broadband spectrum cannot be modelled by a single isothermal component of optically thin plasma and two components are necessary. In particular, the high statistics and the accurate calibration of the spectrometer allowed us to detect a very hot component at ~7 million K with an emission measure of ~2.7 x 10^44 cm^-3. The X-ray emission from the hot plasma dominates the solar X-ray spectrum above 4 keV. We checked that this hot component is invariably present both at high and low emission regimes, i.e. even excluding resolvable microflares. We also present and discuss a possible non-thermal origin (compatible with a weak contribution from thick-target bremsstrahlung) for this hard emission component. Our results support the nanoflare scenario and might confirm that a minor flaring activity is ever-present in the quiescent corona, as also inferred for the coronae of other stars.

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arXiv2012

A population of isolated hard X-ray sources near the supernova remnant Kes 69

Recent X-ray observations of the supernova remnant IC443 interacting with molecular clouds have shown the presence of a new population of hard X-ray sources related to the remnant itself, which has been interpreted in terms of fast ejecta fragment propagating inside the dense environment. Prompted by these studies, we have obtained a deep {\sl XMM-Newton} observation of the supernova remnant (SNR) Kes 69, which also shows signs of shock-cloud interaction. We report on the detection of 18 hard X-ray sources in the field of Kes 69, a significant excess of the expected galactic source population in the field, spatially correlated with CO emission from the cloud in the remnant environment. The spectra of 3 of the 18 sources can be described as hard power laws with photon index <2 plus line emission associated to K-shell transitions. We discuss the two most promising scenarios for the interpretation of the sources, namely fast ejecta fragments (as in IC443) and cataclysmic variables. While most of the observational evidences are consistent with the former interpretation, we cannot rule out the latter.

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arXiv2012

Role of ejecta clumping and back-reaction of accelerated cosmic rays in the evolution of Type Ia supernova remnants

We investigate the role played by initial clumping of ejecta and by efficient acceleration of cosmic rays (CRs) in determining the density structure of the post-shock region of a Type Ia supernova remnant (SNR) through detailed 3D MHD modeling. Our model describes the expansion of a SNR through a magnetized interstellar medium (ISM), including the initial clumping of ejecta and the effects on shock dynamics due to back-reaction of accelerated CRs. The model predictions are compared to the observations of SN 1006. We found that the back-reaction of accelerated CRs alone cannot reproduce the observed separation between the forward shock (FS) and the contact discontinuity (CD) unless the energy losses through CR acceleration and escape are very large and independent of the obliquity angle. On the contrary, the clumping of ejecta can naturally reproduce the observed small separation and the occurrence of protrusions observed in SN 1006, even without the need of accelerated CRs. We conclude that FS-CD separation is a probe of the ejecta structure at the time of explosion rather than a probe of the efficiency of CR acceleration in young SNRs.

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arXiv2012

Nanoflare Evidence from Analysis of the X-Ray Variability of an Active Region Observed with Hinode/XRT

The heating of the solar corona is one of the big questions in astrophysics. Rapid pulses called nanoflares are among the best candidate mechanisms. The analysis of the time variability of coronal X-ray emission is potentially a very useful tool to detect impulsive events. We analyze the small-scale variability of a solar active region in a high cadence Hinode/XRT observation. The dataset allows us to detect very small deviations of emission fluctuations from the distribution expected for a constant rate. We discuss the deviations in the light of the pulsed-heating scenario.

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arXiv2011

Role of ejecta clumping and back-reaction of accelerated cosmic rays in the evolution of supernova remnants

The thermal structure of the post-shock region of a young supernova remnant (SNR) is heavily affected by two main physical effects, the back-reaction of accelerated cosmic rays (CRs) and the Rayleigh-Taylor (RT) instabilities developing at the contact discontinuity between the ejecta and the shocked interstellar medium (ISM). Here, we investigate the role played by both physical mechanisms in the evolution of SNRs through detailed 3D MHD modeling. Our model describes the expansion of the remnant through a magnetized ISM, including consistently the initial ejecta clumping and the effects on shock dynamics due to back-reaction of accelerated CRs. We discuss the role of the initial ejecta clumpiness in developing strong instabilities at the contact discontinuity which may extend upstream to the main shock and beyond.

astro-ph.HE↗

arXiv2011

X-ray emission from protostellar jet HH 154: the first evidence of a diamond shock?

X-ray emission from about ten protostellar jets has been discovered and it appears as a feature common to the most energetic jets. Although X-ray emission seems to originate from shocks internal to jets, the mechanism forming these shocks remains controversial. One of the best studied X-ray jet is HH 154 that has been observed by Chandra over a time base of about 10 years. We analyze the Chandra observations of HH 154 by investigating the evolution of its X-ray source. We show that the X-ray emission consists of a bright stationary component and a faint elongated component. We interpret the observations by developing a hydrodynamic model describing a protostellar jet originating from a nozzle and compare the X-ray emission synthesized from the model with the X-ray observations. The model takes into account the thermal conduction and radiative losses and shows that the jet/nozzle leads to the formation of a diamond shock at the nozzle exit. The shock is stationary over the period covered by our simulations and generates an X-ray source with luminosity and spectral characteristics in excellent agreement with the observations. We conclude that the X-ray emission from HH 154 is consistent with a diamond shock originating from a nozzle through which the jet is launched into the ambient medium. We suggest that the physical origin of the nozzle could be related to the dense gas in which the HH 154 driving source is embedded and/or to the magnetic field at the jet launching/collimation region.

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arXiv2011

Constraints on local interstellar magnetic field from non-thermal emission of SN1006

The synchrotron radio morphology of bilateral supernova remnants depends on the mechanisms of particle acceleration and on the viewing geometry. However, unlike X-ray and $γ$-ray morphologies, the radio emission does not depend on the cut-off region of the parent electron population, making it a simpler and more straightforward tool to investigate the physics of cosmic ray production in SNRs. Our aim is to derive from the radio morphology tight constraints on the direction of the local magnetic field and its gradient, and on the obliquity dependence of the electron injection efficiency. We perform a set of 3D MHD simulations describing the expansion of a spherical SNR through a magnetized medium with a non-uniform ISMF. From the simulations, we derive non-thermal radio maps and compare them with observations of the SN1006 remnant. We find that the radio morphology of SN1006 at 1 GHz is best-fitted by a model with quasi-parallel injection efficiency, a magnetic field aspect angle of $38^\circ\pm 4^\circ$ with the line of sight, and a gradient of the field strength toward the galactic plane, higher then the expected variations of the large scale field of the Galaxy. We conclude that the radio limbs of SN1006 are polar caps, not lying in the plane of sky}. The study of the synchrotron radio emission of SNRs is of crucial importance to derive information on the galactic magnetic field in the vicinity of the remnants, and more hints on the actual injection efficiency scenario.

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arXiv2010

Observational constraints on the modeling of SN1006

Experimental spectra and images of the supernova remnant SN1006 have been reported for radio, X-ray and TeV gamma-ray bands. Several comparisons between models and observations have been discussed in the literature, showing that the broad-band spectrum from the whole remnant as well as a sharpest radial profile of the X-ray brightness can be both fitted by adopting a model of SN1006 which strongly depends on the non-linear effects of the accelerated cosmic rays; these models predict post-shock magnetic field (MF) strengths of the order of 150 micro G. Here we present a new way to compare models and observations, in order to put constraints on the physical parameters and mechanisms governing the remnant. In particular, we show that a simple model based on the classic MHD and cosmic rays acceleration theories allows us to investigate the spatially distributed characteristics of SN1006 and to put observational constraints on the kinetics and MF. Our method includes modelling and comparison of the azimuthal and radial profiles of the surface brightness in radio, hard X-rays and TeV gamma-rays as well as the azimuthal variations of the electron maximum energy. In addition, this simple model also provides good fits to the radio-to-gamma-ray spectrum of SN1006. We find that our best-fit model predicts an effective MF strength inside SN1006 of 32 micro G, in good agreement with the `leptonic' model suggested by the HESS Collaboration (2010). Finally, some difficulties in both the `classic' and the non-linear models are discussed. A number of evidences about non-uniformity of MF around SN1006 are noted.

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arXiv2010

Effects of non-uniform interstellar magnetic field on synchrotron X-ray and inverse-Compton gamma-ray morphology of SNRs

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.

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arXiv2010

Generation of radiative knots in a randomly pulsed protostellar jet II. X-ray emission

Protostellar jets are known to emit in a wide range of bands, from radio to IR to optical bands, and to date also about ten X-ray emitting jets have been detected, with a rate of discovery of about one per year. We aim at investigating the mechanism leading to the X-ray emission detected in protostellar jets and at constraining the physical parameters that describe the jet/ambient interaction by comparing our model predictions with observations. We perform 2D axisymmetric hydrodynamic simulations of the interaction between a supersonic jet and the ambient. The jet is described as a train of plasma blobs randomly ejected by the stellar source along the jet axis. We explore the parameter space by varying the ejection rate, the initial jet Mach number, and the initial density contrast between the ambient and the jet. We synthesized from the model the X-ray emission as it would be observed with the current X-ray telescopes. The mutual interactions among the ejected blobs and of the blobs with the ambient medium lead to complex X-ray emitting structures within the jet: irregular chains of knots; isolated knots with measurable proper motion; apparently stationary knots; reverse shocks. The predicted X-ray luminosity strongly depends on the ejection rate and on the initial density contrast between the ambient and the jet, with a weaker dependence on the jet Mach number. Our model represents the first attempt to describe the X-ray properties of all the X-ray emitting protostellar jets. The comparison between our model predictions and the observations can provide a useful diagnostic tool necessary for a proper interpretation of the observations. In particular, we suggest that the observable quantities derived from the spectral analysis of X-ray observations can be used to constrain the ejection rate, a parameter explored in our model that is not measurable by current observations.

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