Salvatore Orlando

Crushing of interstellar gas clouds in supernova remnants - I. The role of thermal conduction and radiative losses

We model the hydrodynamic interaction of a shock wave of an evolved supernova remnant with a small interstellar gas cloud like the ones observed in the Cygnus loop and in the Vela SNR. We investigate the interplay between radiative cooling and thermal conduction during cloud evolution and their effect on the mass and energy exchange between the cloud and the surrounding medium. Through the study of two cases characterized by different Mach numbers of the primary shock (M = 30 and 50, corresponding to a post-shock temperature T 1.7 x 10 6 K and 4.7 x 10 6 K, respectively), we explore two very different physical regimes: for M = 30. the radiative losses dominate the evolution of the shocked cloud which fragments into cold, dense, and compact filaments surrounded by a hot corona which is ablated by the thermal conduction; instead, for M = 50, the thermal conduction dominates the evolution of the shocked cloud. which evaporates in a few dynamical time-scales. In both cases we find that the thermal conduction is very effective in suppressing the hydrodynamic instabilities that would develop at the cloud boundaries.

The Sun as an X-Ray Star. I. Deriving the Emission Measure Distribution versus Temperature of the Whole Solar Corona from the Yohkoh/Soft X-Ray Telescope Data

The scope of this work is to obtain the emission measure distributions versus temperature, EM(T ), of the whole solar corona from Yohkoh Soft X-ray Telescope images. As discussed in Paper II, the EM(T ) is our starting point for studying the Sun as an X-ray star. To this purpose, we need to extract as much information as possible from the Yohkoh/SXT data covering the whole range of the Yohkoh/SXT tem- perature sensitivity, i.e., 5.5 log T (K) 8. In particular at low photon counts and temperatures below 106 K, errors on the temperature and emission measure determination are expected to be large. To this end, we have made an extensive set of simulations to explore the nominal performance of the entire system (instrument and data analysis system) in the determination of the plasma temperature and emis- sion measure at low, intermediate, and high photon counts per pixel. We have shown that low-count data with a number of photons per pixel are aUected by large errors and lead to the deriva- n phot 10 tion of an unrealistic EM(T ) characterized by a steep negative slope. As a result, we have devised an analysis method that minimizes the instrumental and statistical eUects on the determination of EM(T ) and allows us to determine the global coronal EM(T ). As a —rst application to real SXT data, we have derived the EM(T ) of the Sun close to the maximum of the solar cycle, a challenging case. The low- temperature part is in agreement with analogous studies made in the UV band, and it shows a well- de—ned maximum at T D 2 MK. Subject headings: Sun: activitySun: coronaSun: X-rays, gamma rays

The X-ray emission from Z Canis Majoris during an FUor-like outburst and the detection of its X-ray jet

Accretion shocks have been recognized as an important X-ray emission mechanism for pre-main sequence stars, and yet the X-ray properties of FUor outbursts, events that are caused by violent accretion, have been given little attention. We observed the FUor object Z CMa during optical outburst and quiescence with Chandra. No significant changes in X-ray brightness and spectral shape were found, suggesting that the X-ray emission is coronal. The binary nature of Z CMa makes the origin of the X-ray source ambiguous. However, the moderate hydrogen column density derived from our data makes it unlikely that the embedded primary star is the Xray source. The secondary star, which is the FUor object, is thus responsible for both the X-ray emission and the ongoing accretion outburst, which seem, however, to be unrelated phenomena. The secondary is also known to drive a large outflow and jet, which we detect here for the first time in X-rays. The distance of the X-ray emitting outflow source to the central star is greater than in jets of low-mass stars.

The Sun as an X-ray star: Active region evolution, rotational modulation, and implications for stellar X-ray variability

We study the contribution of an active region and its core to the luminosity and the spectrum of the Sun in the X-ray band and to the relevant solar emission measure vs. temperature distribution, EM(T ). We also study the relevant changes in the course of four solar rotations, and the solar rotational modulation due to this active region, the only one present at that time. To this end, we have used a large sample of full-disk Yohkoh/SXT observations taken between July and October 1996, covering most of the active region evolution. From the Yohkoh/SXT data we have synthesized the X-ray spectra of the whole solar corona, and the focal plane data as they would be collected with Rosat/PSPC, XMM-Newton/EPIC and Chandra/ACIS. This work is part of a project to study the Sun as an X-ray star, using the solar data as a guide and a template for stellar observations. We found that the active region contributes significantly to the X-ray spectrum of the Sun mainly during the first month of its evolution. The rotational modulation due to the active region causes a significant variability of the average X-ray flux, with only moderate spectral variation, in the pass-bands of Rosat/PSPC, XMM-Newton/EPIC and Chandra/ACIS. We investigated the characteristics of the X-ray variability due to the rotational modulation, to the solar cycle, and to flares together with the possible implications on stellar X-ray variability. We derived the diagram of X-ray surface flux vs. spectral hardness ratio in the Rosat/PSPC band; we studied the variability due to the solar cycle and to the rotational modulation and we found that both lead to the same correlation between Fpspc and HRpspc with a very steep slope. The variability due to the evolution of flares again produces a correlation between Fpspc and HRpspc but with a much flatter slope than in the other two cases. Analogous results have been found when analyzing data in the format of XMM-Newton/EPIC and Chandra/ACIS.

Observability and diagnostics in the X-ray band of shock-cloud interactions in supernova remnants

Context. X-ray emitting features originating from the interaction of supernova shock waves with small interstellar gas clouds are revealed in many X-ray observations of evolved supernova remnants (e.g., Cygnus Loop and Vela), but their interpretation is not straightforward. Aims. We develop a self-consistent method for the analysis and interpretation of shock-cloud interactions in middle-aged supernova remnants, which can provide the key parameters of the system and the role of relevant physical effects such as thermal conduction, without the need to perform ad-hoc numerical simulations and bother about morphology details. Methods. We explore all the possible values of the shock speed and cloud density contrast relevant to middle-aged SNRs with a set of hydrodynamic simulations of shock-cloud interaction including the effects of thermal conduction and radiative cooling. From the simulations, we synthesize spatially and spectrally resolved focal-plane data as they would be collected with XMM-Newton/EPIC, an X-ray instrument commonly used in these studies. Results. We develop and calibrate two diagnostic tools, the first based on the mean photon energy versus count-rate scatter plot and the second on the spectral analysis of the interaction region, that can be used to highlight the effects of thermal conduction and to derive the shock speed in case of efficient conduction at work. These tools can be used to ascertain information from X-ray observations, without the need to develop detailed and ad-hoc numerical models for the interpretation of the data.

COLLIMATION AND ASYMMETRY OF THE HOT BLAST WAVE FROM THE RECURRENT NOVA V745 Sco

J.J.D. and V.K. were funded by nNASA contract NAS8-03060 to the CXC and thank the ndirector, B. Wilkes, and the CXC science team for advice and nsupport. L.D. and M.H. acknowledge the support of the nSpanish Ministry of Economy and Competitivity (MINECO) nunder the grant ESP2014-56003-R. J.M.L. was supported by nbasic research funds of the CNR. K.L.P. acknowledges funding nfrom the UK Space Agency. S.S. acknowledges partial support nfrom NASA, NSF, and Chandra grants to ASU. R.D.G. was nsupported by NASA and the United States Air Force. C.E.W. nacknowledges support from Chandra award G04-15023A.

X-RAY EMISSION FROM STELLAR JETS BY COLLISION AGAINST HIGH-DENSITY MOLECULAR CLOUDS: AN APPLICATION TO HH 248

We investigate the plausibility of detecting X-ray emission from a stellar jet that impacts a dense molecular cloud, a scenario that may be typical for classical T Tauri stars with jets in dense star-forming complexes. We first model the impact of a jet against a dense cloud using two-dimensional axisymmetric hydrodynamic simulations, exploring different configurations of the ambient environment. Then, we compare our results with XMM-Newton observations of the Herbig–Haro object HH 248, where extended X-ray emission aligned with the optical knots is detected at the edge of the nearby IC 434 cloud. Our simulations show that a jet can produce plasma with temperatures up to 107 K, consistent with production of X-ray emission, after impacting a dense cloud. We find that jets denser than the ambient medium but less dense than the cloud produce detectable X-ray emission only at impact with the cloud. From an exploration of the model parameter space, we constrain the physical conditions (jet density and velocity and cloud density) that reproduce the intrinsic luminosity and emission measure of the X-ray source possibly associated with HH 248 well. Thus, we suggest that the extended X-ray source close to HH 248 corresponds to a jet impacting a dense cloud.

3D Modeling of the 2006 Nova Outburst of RS Ophiuchi: Collimated Outflows and Jet-Like Ejections

Chandra/HETG observations of the recurrent nova RS Ophiuchi at day 13.9 of its 2006 outburst reveal a spectrum covering a large range in plasma temperature and characterized by asymmetric and blue-shifted emission lines, suggesting a jet-like ejection, collimated by the central binary. We investigate the origin of line asymmetries by performing 3-D hydrodynamic simulations. We found that our model reproduces the observed X-ray emission in a natural way if an equatorial density enhancement is taken into account. The asymmetric nature of the circumstellar medium into which the early blast wave is driven leads to the shock collimation in the plane of the sky. Most of the early X-ray emission originates from a small region propagating in the direction perpendicular to the line-of-sight and localized at the interaction front between the blast wave and the equatorial density enhancement. The model predicts asymmetric and blue-shifted line profiles remarkably similar to those observed and explains the asymmetries as due to substantial X-ray absorption of red-shifted emission by ejecta material.

NUMERICAL SIMULATIONS AND DIAGNOSTICS IN ASTROPHYSICS: A FEW MAGNETOHYDRODYNAMICS EXAMPLES

We discuss some issues related to numerical simulations in Astrophysics and, in particular, to their use both as a theoretical tool and as a diagnostic tool, to gain insight into the physical phenomena at work. We make our point presenting some examples of Magneto-hydro-dynamic (MHD) simulations of astrophysical plasmas and illustrating their use. In particular we show the need for appropriate tools to interpret, visualize and present results in an adequate form, and the importance of spectral synthesis for a direct comparison with observations.