F. Reale

The Sun as an X-Ray Star. II. Using the Yohkoh/Soft X-Ray Telescope-derived Solar Emission Measure versus Temperature to Interpret Stellar X-Ray Observations

This paper is the second of a project dedicated to using solar Yohkoh/SXT data as a guide and a template to interpret data on stellar coronae. In the light of the large diUerences in scope and approach between solar and stellar studies, we have developed a method to translate Yohkoh/SXT data of the whole solar corona into stellar-like data, i.e., to put them in the same format and context as the stellar ones. First from the Yohkoh/SXT images we derive the whole-Sun X-ray emission measure versus tem- perature (EM(T )), in the range 105.5¨108 K, during the speci—c observation. Then, we synthesize the solar X-ray spectrum; —nally, we fold the spectrum through the instrumental response of nonsolar X-ray observatories, for instance, ROSAT /PSPC and ASCA/SIS. Finally, we analyze such solar coronal data in the same band and with the same methods used for stellar observations, allowing a direct and homoge- neous comparison with them. In this paper we present in detail our method and, as an example of results, we show and discuss EM(T ) and stellar-like spectra for three phases of the solar cycle: maximum, intermediate phase, and minimum. The total amount and the distribution of the emission measure change dramatically during the cycle, in particular at temperatures above 106 K. We also show the EM(T ) of the whole solar corona during a large —are. The ROSAT /PSPC- and ASCA/SIS-like X-ray spectra of the Sun as a star that we obtain are discussed in the context of stellar coronal physics. The Suns coronal total luminosity in the ROSAT /PSPC band ranges from B2.7 ) 1026 ergs s~1 (at minimum) to B4.7 ) 1027 ergs s~1 (at maximum). We discuss future developments and possible applica- tions of our method. Subject headings: Sun: coronaSun: X-rays, gamma rays

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.

Temperature and Density Structure of Hot and Cool Loops Derived from the Analysis of TRACE Data

We study the transversal structure (in particular the filamentation) and the longitudinal plasma stratifica- tion in two sets of solar coronal loops observed with TRACE in the 171 and 195 Apassbands. The density stratification and the thermal structuring of the plasma along the fibrils that form the loops are derived using two techniques simultaneously: (1) a filter ratio diagnostic (195/171 A ˚ ) and (2) modeling intensity profiles along the fibrils with hydrostatic models; in both techniques we remove the background flux. We find consistent values of temperature and density with both methods in the coronal structures selected. We find evidence of a very warm fibril (T � 5 � 10 6 K) and of rather cold ones (T � 2 � 10 5 K). The hot fibril appears to have a thermal structure with a maximum at the apex compatible with temperature and density stratification, dictated by energy balance, typical of a nonisothermal hydrostatic loop. The cold fibrils appear to be isothermal and are probably in dynamical conditions. Subject headings: Sun: corona — Sun: UV radiation On-line material: color figures

On the origin of asymmetries in bilateral supernova remnants

Aims. We investigate whether the morphology of bilateral supernova remnants (BSNRs) observed in the radio band is determined mainly either by a non-uniform interstellar medium (ISM) or by a non-uniform ambient magnetic field. Methods. We perform 3D MHD simulations of a spherical SNR shock propagating through a magnetized ISM. Two cases of shock propagation are considered: 1) through a gradient of ambient density with a uniform ambient magnetic field; 2) through a homogeneous medium with a gradient of ambient magnetic field strength. From the simulations, we synthesize the synchrotron radio emission, making different assumptions about the details of acceleration and injection of relativistic electrons. Results. We find that asymmetric BSNRs are produced if the line-of-sight is not aligned with the gradient of ambient plasma density or with the gradient of ambient magnetic field strength. We derive useful parameters to quantify the degree of asymmetry of the remnants that may provide a powerful diagnostic of the microphysics of strong shock waves through the comparison between models and observations. Conclusions. BSNRs with two radio limbs of different brightness can be explained if a gradient of ambient density or, most likely, of ambient magnetic field strength is perpendicular to the radio limbs. BSNRs with converging similar radio arcs can be explained if the gradient runs between the two arcs.

THE SUN AS AN X-RAY STAR. III. FLARES

In previous works we have developed a method to convert solar X-ray data, collected with the Yohkoh/SXT, into templates of stellar coronal observations. Here we apply the method to several solar flares, for comparison with stellar X-ray flares. Eight flares, from weak (GOES class C5.8) to very intense ones (X9) are selected as representative of the flaring Sun. The emission measure distribution versus temperature, EM(T), of the flaring regions is derived from Yohkoh/SXT observations in the rise, peak, and decay of the flares. The EM(T) is rather peaked and centered around T ≈ 107 K for most of the time. Typically, it grows during the rise phase of the flare, and then it decreases and shifts toward lower temperatures during the decay, more slowly if there is sustained heating. The most intense flare we studied shows emission measure even at very high temperatures (T ≈ 108 K). Time-resolved X-ray spectra both unfiltered and filtered through the instrumental responses of the nonsolar instruments ASCA/SIS and ROSAT/PSPC are then derived. Synthesized ASCA/SIS and ROSAT/PSPC spectra are generally well fitted with single thermal components at temperatures close to that of the EM(T) maximum, albeit two thermal components are needed to fit some flare decays. ROSAT/PSPC spectra show that solar flares are in a 2 orders of magnitude flux range (106-108 ergs cm-2 s-1) and a narrow PSPC hardness ratio range, however, higher than that of typical nonflaring solar-like stars.

Hydrodynamic modeling of an X-ray flare on Proxima Centauri observed by the Einstein telescope

Hydrodynamic numerical calculations of a flare which occurred on Proxima Centauri and was observed by the Einstein satellite on August 20, 1980 at 12:50 UT are presented. The highlights of the hydrodynamic code are reviewed, and the physical and geometrical parameters necessary for the calculations are derived and compared with observations. The results are consistent with the stellar flare being caused by the rapid dissipation of 5.9 x 10 to the 31st ergs, within a magnetic loop structure whose semilength is 7 x 10 to the 9th cm and cross-sectional radius is 7.3 x 10 to the 8th cm. The results provide evidence that flares on late-type stars can be described by a hydrodynamic model with a relatively simple geometry, similar to solar compact flares. 39 references.

Time variability of the X-ray sources in M33

The results of a variability study of the X-ray sources detected by the Einstein Observatory in the galaxy M33 are reported. Two of the 15 known sources are variable above the 99.73 percent confidence level. The light curve of one of these sources, M33 X-7, exhibits a variability pattern of high and low states, suggesting an eclipsing binary X-ray source. Such a finding would be the first identification of a close accreting binary system with an X-ray source in an external galaxy other than the Magellanic Clouds. The data suggest a binary period of 1.7857 day and an eclipse duration of about 0.4 day. The nuclear source M33 X-8 varies only in the softest part of the spectrum. The observations suggest a rapid variability and show a rapid flare with a rise time shorter than three days together with longer timescale variability.

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

Context. 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. Aims. 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. Methods. 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. Results. 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. Conclusions. 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.

The role of radiative losses in the late evolution of pulse-heated coronal loops/strands

Context. Radiative losses from optically thin plasma are an important ingredient in modeling confined plasma in the solar corona. Spectral models are continuously updated to include the emission from more spectral lines, with significant effects on radiative losses, especially around 1 MK. Aims. We investigate the effect of changes to the radiative-loss temperature dependence caused by upgrading spectral codes on the predictions obtained when modeling confined plasma in the solar corona. Methods. We revisit the hydrodynamic simulation of a pulse-heated loop strand by comparing results obtained using an old and a recent radiative-loss function. Results. We find that significant changes occur in the plasma evolution during the late phases of plasma cooling: when the more recent radiative-loss curve is used, the plasma cooling rate indeed increases significantly when temperatures reach 1–2 MK. This more rapid cooling occurs when the plasma density is higher than a threshold value, which in impulsive heating models leads to the loop plasma becoming overdense. This rapid cooling has the effect of steepening the slope in the emission measure distribution of coronal plasmas with temperature, at temperatures lower than ∼2 MK, and of reducing the visibility of warm (1 MK) loops. Conclusions. The effects of changes to the radiative-loss curves can be important when modeling the late phases of the evolution of pulse-heated coronal loops, and, in general, of thermally unstable optically thin plasmas.

X-ray emitting hot plasma in solar active regions observed by the SphinX spectrometer

Aims. 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 the CORONAS-PHOTON mission is sensitive to X-ray emission of energies well above 1 keV and provides the opportunity to detect the hot plasma component. Methods. We analysed 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− 7k eV energy band by adopting the latest release of the APED database. Results. 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 statistical significance of the count rates 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 × 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 in both the high and low emission regimes, i.e. even excluding resolvable microflares. We also present and discuss the possibility of a non-thermal origin (which would be compatible with a weak contribution from thick-target bremsstrahlung) for this hard emission component. Conclusions. 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.