Origin of asymmetries in X-ray emission lines from the blast wave of the 2014 outburst of nova V745 Sco

The symbiotic nova V745 Sco was observed in outburst on 2014 February 6. Its observations by the Chandra X-ray Observatory at days 16 and 17 have revealed a spectrum characterized by asymmetric and blue-shifted emission lines. Here we investigate the origin of these asymmetries through three-dimensional hydrodynamic simulations describing the outburst during the first 20 days of evolution. The model takes into account thermal conduction and radiative cooling and assumes a blast wave propagates through an equatorial density enhancement. From the simulations, we synthesize the X-ray emission and derive the spectra as they would be observed with Chandra. We find that both the blast wave and the ejecta distribution are efficiently collimated in polar directions due to the presence of the equatorial density enhancement. The majority of the X-ray emission originates from the interaction of the blast with the equatorial density enhancement and is concentrated on the equatorial plane as a ring-like structure. Our "best-fit" model requires a mass of ejecta in the outburst $M_{ej} \approx 3\times 10^{-7}\,M_{\odot}$ and an explosion energy $E_b \approx 3 \times 10^{43}$ erg and reproduces the distribution of emission measure vs temperature and the evolution of shock velocity and temperature inferred from the observations. The model predicts asymmetric and blue-shifted line profiles similar to those observed and explains their origin as due to substantial X-ray absorption of red-shifted emission by ejecta material. The comparison of predicted and observed Ne and O spectral line ratios reveals no signs of strong Ne enhancement and suggests the progenitor is a CO white dwarf.Comment: 16 pages, 17 Figures; accepted for publication on MNRA

The X-ray cycle in the solar-type star HD 81809

(abridged) Our long-term XMM-Newton program of long-term monitoring of a solar-like star with a well-studied chromospheric cycle, HD 81809 aims to study whether an X-ray cycle is present, along with studying its characteristics and its relation to the chromospheric cycle. Regular observations of HD 81809 were performed with XMM-Newton, spaced by 6 months from 2001 to 2007. We studied the variations in the resulting coronal luminosity and temperature, and compared them with the chromospheric CaII variations. We also modeled the observations in terms of a mixture of active regions, using a methodology originally developed to study the solar corona. Our observations show a well-defined cycle with an amplitude exceeding 1 dex and an average luminosity approximately one order of magnitude higher than in the Sun. The behavior of the corona of HD 81809 can be modeled well in terms of varying coverage of solar-like active regions, with a larger coverage than for the Sun, showing it to be compatible with a simple extension of the solar case.Comment: In press, Astronomy & Astrophysic

Redshifted X-rays from the material accreting onto TW Hya: evidence of a low-latitude accretion spot

High resolution spectroscopy, providing constraints on plasma motions and temperatures, is a powerful means to investigate the structure of accretion streams in CTTS. In particular, the accretion shock region, where the accreting material is heated to temperatures of a few MK as it continues its inward bulk motion, can be probed by X-ray spectroscopy. To attempt to detect for the first time the motion of this X-ray-emitting post-shock material, we searched for a Doppler shift in the deep Chandra/HETGS observation of the CTTS TW Hya. This test should unveil the nature of this X-ray emitting plasma component in CTTS, and constrain the accretion stream geometry. We searched for a Doppler shift in the X-ray emission from TW Hya with two different methods, by measuring the position of a selected sample of emission lines, and by fitting the whole TW Hya X-ray spectrum, allowing the line-of-sight velocity to vary. We found that the plasma at T~2-4 MK has a line-of-sight velocity of 38.3+/-5.1 km/s with respect to the stellar photosphere. This result definitively confirms that this X-ray-emitting material originates in the post-shock region, at the base of the accretion stream, and not in coronal structures. The comparison of the observed velocity along the line of sight, 38.3+/-5.1 km/s, with the inferred intrinsic velocity of the post shock of TW Hya, v_post~110-120 km/s, indicates that the footpoints of the accretion streams on TW Hya are located at low latitudes on the stellar surface. Our results indicate that complex magnetic field geometries, such as that of TW Hya, permit low-latitude accretion spots. Moreover, since on TW Hya the redshift of the soft X-ray emission is very similar to that of the narrow component of the CIV resonance doublet at 1550 Ang, as found by Ardila et al. (2013), then the plasma at 2-4 MK and that at 0.1 MK likely originate in the same post-shock regions.Comment: Accepted for publication in Astronomy & Astrophysics; 2nd version after language editor corrections; 16 pages, 8 figures, 6 table

Turbulent transport of heat and momentum in a boundary layer subject to deceleration, suction and variable wall temperature

The relationship between the turbulent transport of heat and momentum in an adverse pressure gradient boundary layer was studied. An experimental study was conducted of turbulent boundary layers subject to strong adverse pressure gradients with suction. Near-equilibrium flows were attained, evidenced by outer-region similarity in terms of defect temperature and defect velocity profiles. The relationship between Stanton number and enthalpy thickness was shown to be the same as for a flat plate flow both for constant wall temperature boundary conditions and for steps in wall temperature. The superposition principle used with the step-wall-temperature experimental result was shown to accurately predict the Stanton number variation for two cases of arbitrarily varying wall temperature. The Reynolds stress tensor components were measured for strong adverse pressure gradient conditions and different suction rates. Two peaks of turbulence intensity were found: one in the inner and one in the outer regions. The outer peak is shown to be displaced outward by an adverse pressure gradient and suppressed by suction

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.Comment: Accepted for publication in Astronomische Nachrichten. Proceedings of the XMM-Newton Science Workshop 201

Isogrid design handbook

Handbook has been published which presents information needed for design of isogrid triangular integral-stiffened structures. It develops equations, methods, and graphs to handle wide variety of loadings, materials, and geometry. Handbook is divided into seven sections. Handbook may be used by marine and civil engineers and by students and designers without access to computers

Fifteen years in the high-energy life of the solar-type star HD 81809. XMM-Newton observations of a stellar activity cycle

Aims. The data set of the long-term XMM-Newton monitoring program of HD 81809 is analyzed to study its X-ray cycle, to investigate if the latter is related to the chromospheric one, to infer the structure of the corona of HD 81809, and to explore if the coronal activity of HD 81809 can be ascribed to phenomena similar to the solar ones and, therefore, considered an extension of the solar case. Methods. We analyze the observations of HD 81809 performed with XMM-Newton with a regular cadence of 6 months from 2001 to 2016 and representing one of the longest available observational baseline ($\sim 15$~yr) for a solar-like star with a well-studied chromospheric cycle (with a period of $\sim 8$~yr). We investigate the modulation of coronal luminosity and temperature and its relation with the chromospheric cycle. We interpret the data in terms of a mixture of solar-like coronal regions, adopting a methodology originally proposed to study the Sun as an X-ray star. Results. The observations show a well-defined regular cyclic modulation of the X-ray luminosity that reflects the activity level of HD 81809. The data covers approximately two cycles of coronal activity; the modulation has an amplitude of a factor of $\sim 5$ (excluding evident flares, as in the June 2002 observation) and a period of $7.3\pm 1.5$~yr, consistent with that of the chromospheric cycle. We demonstrate that the corona of HD 81809 can be interpreted as an extension of the solar case and it can be modeled with a mixture of solar-like coronal regions along the whole cycle. The activity level is mainly determined by a varying coverage of very bright active regions, similar to cores of active regions observed in the Sun. Evidence of unresolved significant flaring activity is present especially in proximity of cycle maxima.Comment: 11 pages, 5 Figures, A&A accepte

Three-dimensional modeling of the asymmetric blast wave from the 2006 outburst of RS Ophiuchi: Early X-ray emission

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. We investigate the origin of these asymmetries and broadening of emission lines. We perform 3-D hydrodynamic simulations of the blast wave from the 2006 outburst, propagating through the inhomogeneous CSM. The model takes into account the thermal conduction (including the effects of heat flux saturation) and the radiative cooling. From the simulations, we synthesize the X-ray emission and derive the spectra as they would be observed with Chandra/HETG. Our model reproduces the observed X-ray emission in a natural way if the CSM in which the outburst occurred is characterized by an equatorial density enhancement. Our ``best-fit'' model predicts that most of the early X-ray emission originates from a small region propagating in the direction perpendicular to the line-of-sight and localized just behind 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. These asymmetries are due to substantial X-ray absorption of red-shifted emission by ejecta material. The comparison of high quality data of Chandra/HETG with detailed hydrodynamic modeling has allowed us to unveil, for the first time, the details of the structure emitting in the X-ray band in early phases of the outburst evolution, contributing to a better understanding of the physics of interactions between nova blasts and CSM in recurrent novae. This may have implications for whether or not RS Ophiuchi is a Type Ia SN progenitor system.Comment: 12 pages, 7 Figures; paper accepted for publication in A&A; the paper with high-resolution figures can be downloaded at http://www.astropa.unipa.it/~orlando/PAPERS/rs_oph_nova.pd