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Autorzy: Abe K., Adrich P., Aihara H., Akutsu, R., Alekseev I., Ali A. , Ameli F., Anghel I., Anthony L. H. V., Antonova M. , Araya A., Asaoka Y., Ashida Y., Aushev V., Ballester F., Bandac I., Barbi M., Barker G. J., Barr G., Batkiewicz-Kwasniak M., Bellato M., Berardi V., Bergevin M., Bernard L., Bernardini E., Berns L., Bhadra S., Bian J., Blanchet A., Blaszczyk F. d. M., Blonde A., Boiano A., Bolognesi S., Bonavera L., Booth N., Borjabad S., Boschi, T., Bose D., Boyd S . B., Bozza C., Bravar A., Bravo-Berguño D., Bronner C., Brown L., Bubak Arkadiusz, Buchowicz A., Buizza Avanzini M., Cafagna F. S., Calabria N. F., Calvo-Mozota J. M., Cao S., Cartwright S.L., Carroll A., Catanesi M. G., Cebriàn, S., Chabera M., Chakraborty, S., Checchia C., Choi J.H., Choubey S., Cicerchia M., Coleman J., Collazuol G., Cook L., Cowan G., Cuen-Rochin, S., Danilov M., Díaz López G., De la Fuente E., de Perio P., De Rosa G., Dealtry T., Densham C. J., Dergacheva A., Deshmukh N., Devi M. M., Di Lodovico F., Di Meo, P., Di Palma I., Doyle T. A., Drakopoulou E., Drapier O., Dumarchez J., Dunne P., Dziewiecki M., Eklund L., El Hedri S., Ellis J., Emery S., Esmaili A., Esteve R., Evangelisti A., Feely M., Fedotov S., Feng J., Fernandez P., Fernández-Martinez E., Ferrario P., Ferrazzi,B., Feusels T., Finch A., Finley C., Fiorentini A., Fiorillo G., Fitton M., Frankiewicz K., Friend M., Fujii Y., Fukuda Y., Galinski G., Gao J., Garde C., Garfagnini A., Garode S., Gialanella L., Giganti C., Gomez-Cadenas J.J., Gonin M., González-Nuevo J., Gorin A., Gornea R., Gousy-Leblanc V. Gramegna F. Grassi M. Grella G. Guigue M. Gumplinger P. Hadley D.R. Harada M., Hartfiel B., Hartz M., Hassani S., Hastings N.C., Hayato Y., Hernando-Morata J.A., Herrero V., Hill J., Hiraide K., Hirota S., Holin A., Horiuchi S., Hoshina K., Hultqvist K., Iacob F., Ichikawa A.K., Idrissi Ibnsalih W., Iijima T., Ikeda M., Inomoto M., Inoue K., Insler J., Ioannisian A., Ishida T., Ishidoshiro K., Ishino H., Ishitsuka M., Ito H., Ito S., Itow Y., Iwamoto K., Izmaylov A., Izumi N., Izumiyama S., Jakkapu M., Jamieson B., Jang H.I., Jang J.S., Jenkins S.J., Jeon S.H., Jiang M., Jo H.S., Jonsson P., Joo K.K., Kajita T., Kakuno H., Kameda J., Kano Y., Kalaczynski P., Karlen D., Kasperek J., Kataoka Y., Kato A., Katori T., Kazarian N., Kearns E., Khabibullin M., Khotjantsev A., Kikawa T., Kekic M., Kim J.H., Kim J.Y., Kim S.B., Kim S.Y., King S., Kinoshita T., Kisiel Jan, Klekotko A., Kobayashi T., Koch L., Koga M., Koerich L., Kolev N., Konaka A., Kormos L.L., Koshio Y., Korzenev A., Kotsar Y., Kouzakov K.A., Kowalik K.L., Kravchuk L., Kryukov A.P., Kudenko Y., Kumita T., Kurjata R., Kutter T., Kuze M., Kwak K., La Commara M., Labarga L., Lagoda J., Lamers James J., Lamoureux M., Laveder M., Lavitola L., Lawe M., Learned J.G., Lee J., Leitner R., Lezaun V., Lim I.T., Lindner T., Litchfield R.P., Long K.R., Longhin A., Loverre P., Lu X., Ludovici L., Maekawa Y., Magaletti L., Magar K., Mahn K., Makida Y., Malek M., Malinský M., Marchi T., Maret L., Mariani C., Marinelli A., Martens K., Marti L., Martin J.F. Martin D., Marzec J., Matsubara T., Matsumoto R., Matsuno S., Matusiak M., Mazzucato E., McCarthy M., McCauley N., McElwee J., McGrew C., Mefodiev A., Medhi A., Mehta P., Mellet L., Menjo H., Mermod P., Metelko C., Mezzetto M., Migenda J., Migliozzi P., Mijakowski P., Miki S., Miller E.W., Minakata H., Minamino A., Mine S., Mineev O., Mitra A., Miura M., Moharana R., Mollo C.M., Mondal T., Mongelli M., Monrabal F., Moon D.H., Moon C.S., Mora F.J., Moriyama S., Mueller Th.A., Munteanu L., Murase K., Nagao Y., Nakadaira T., Nakagiri K., Nakahata M., Nakai S., Nakajima Y., Nakamura K., Nakamura KI., Nakamura H., Nakano Y., Nakaya T., Nakayama S., Nakayoshi K., Nascimento Machado L., Naseby C.E.R., Navarro-Garcia B., Needham M., Nicholls T., Niewczas K., Nishimura Y., Noah E., Nova F., Nugent J.C., Nunokawa H., Obrebski W., Ochoa-Ricoux J.P., O’Connor E., Ogawa N., Ogitsu T., Ohta K., Okamoto K., O’Keeffe H.M., Okumura K., Onishchuk Y., Orozco-Luna F., Oshlianskyi A., Ospina N., Ostrowski M., O’Sullivan E., O’Sullivan L., Ovsiannikova T., Oyama Y., Ozaki H., Pac M.Y., Paganini P., Palladino V., Paolone V., Pari M., Parsa S., Pasternak J., Pastore C., Pastuszak G., Patel D.A., Pavin M., Payne D., Peña-Garay C., Pidcott C., Pinzon Guerra E., Playfer S., Pointon B.W., Popov A., Popov B., Porwit Kamil, Posiadala-Zezula M., Poutissou J.M., Pozimski J., Pronost G., Prouse N.W., Przewlocki P., Quilain B., Quiroga A.A., Radicioni E., Radics B., Rajda P.J., Renner J., Rescigno M., Retiere F., Ricciardi G., Riccio C., Richards B., Rondio E., Rose H.J., Roskovec B., Roth S., Rott C., Rountree S.D., Rubbia A., Ruggeri A.C., Ruggles C., Russo S., Rychter A., Ryu D., Sakashita K., Samani S., Sánchez F., Sánchez M.L., Sanchez M.C., Sano S., Santos J.D., Santucci G., Sarmah P., Sashima I., Sato K., Scott M., Seiya Y., Sekiguchi T., Sekiya H., Seo J.W., Seo S.H., Sgalaberna D., Shaikhiev A., Shan Z., Shaykina A., Shimizu I., Shin C.D., Shinoki M., Shiozawa M., Sinnis G., Skrobova N., Skwarczynski K., Smy M.B., Sobczyk J., Sobel H.W., Soler F. J. P., Sonoda Y., Spina R., Spisso B., Spradlin B., Stankevich K.L., Stawarz L., Stellacci S.M., Stopa K., Studenikin A.I., Suárez Gómez S.L., Suganuma T., Suvorov S., Suwa Y., Suzuki A.T., Suzuki S.Y., Suzuki Y., Svirida D., Svoboda R., Taani M., Tada M., Takeda A., Takemoto Y., Takenaka A., Taketa A., Takeuchi Y., Takhistov V., Tanaka H., Tanaka H.A., Tanaka H.I., Tanaka M., Tashiro T., Thiesse M., Thompson L.F., Toledo J., Tomatani-Sánchez A.K., Tortone G., Tsui K.M., Tsukamoto T., Tzanov M., Uchida Y., Vagins M.R., Valder S., Valentino V., Vasseur G., Vijayvargi A., Vilela C., Vinning W. G. S., Vivolo D., Vladisavljevic T., Vogelaar R.B., Vyalkov M.M., Wachala T., Walker J., Wark D., Wascko M.O., Wendell R.A., Wilkes R.J., Wilking M.J., Wilson M.R., Wronka S., Xia J., Xie Z., Xin T., Yamaguchi Y., Yamamoto K., Yanagisawa C., Yano T., Yen S., Yershov N., Yeum D.N., Yokoyama M., Yonenaga M., Yoo J., Yu I., Yu M., Zakrzewski T., Zaldivar B., Zalipska J., Zaremba K., Zarnecki G., Ziembicki M., Zietara K., Zito M., Zsoldos S., Core-collapse supernovae are among the most magnificent events in the observable universe. They produce many of the chemical elements necessary for life to exist and their remnants-neutron stars and black holes-are interesting astrophysical objects in their own right. However, despite millennia of observations and almost a century of astrophysical study, the explosion mechanism of core-collapse supernovae is not yet well understood. Hyper-Kamiokande is a next-generation neutrino detector that will be able to observe the neutrino flux from the next galactic core-collapse supernova in unprecedented detail. We focus on the first 500 ms of the neutrino burst, corresponding to the accretion phase, and use a newly-developed, high-precision supernova event generator to simulate Hyper-Kamiokandeʼs response to five different supernova models. We show that Hyper-Kamiokande will be able to distinguish between these models with high accuracy for a supernova at a distance of up to 100 kpc. Once the next galactic supernova happens, this ability will be a powerful tool for guiding simulations toward a precise reproduction of the explosion mechanism observed in nature.

The long-term Swift monitoring of ESO 243-49 HLX-1 provides an opportunity to investigate the detailed timing and spectral behaviour of this hyper-luminous X-ray source. Swift has detected 7 outbursts since 2009 mid-August. Using different dynamical timing algorithms, we confirm an increasing trend for the time intervals between outbursts, which is manifest in the delays between the latest outbursts. The X-ray spectra of HLX-1 in quiescence can be described with a single power-law model while the thermal component dominates the X-ray emission during outburst. There is only marginal evidence for photon index (or spectral hardness) changes between quiescent states with about 1{\sigma} deviation. With the updated temporal and spectral features, we re-examine different scenarios to explain the origin of the quasi-periodic modulation of HLX-1. A significantly increasing trend without obvious stochastic fluctuations on the timescale of the detected quasi-period may not fully support an orbital period origin as might be due to mass transfer episodes from a donor star at periastron of an extremely eccentric orbit. The outburst profile seems to be consistent with the effect of tidal-induced-precession of an accretion disc or an oscillating wind scenario in the inner disc. Based on these models, we speculate that the true orbital period is much shorter than the detected quasi-periodicity., Comment: 12 pages, 4 tables and 7 figures; accepted by MNRAS

The isotopic compositions of ruthenium (Ru) are measured from presolar silicon carbide (SiC) grains. In a popular scenario, the presolar SiC grains formed in the outskirt of an asymptotic giant branch (AGB) star, left the star as a stellar wind, and joined the presolar molecular cloud from which the solar system formed. The Ru isotopes formed inside the star, moved to the stellar surface during the AGB phase, and were locked into the SiC grains. Following this scenario, we analyze the Nucleosynthesis Grid (NuGrid) data, which provide the abundances of the Ru isotopes in the stellar wind for a set of stars in a wide range of initial masses and metallicities. We apply the C > O (carbon abundance larger than the oxygen abundance) condition, which is commonly adopted for the condition of the SiC formation in the stellar wind. The NuGrid data confirm that SiC grains do not form in the winds of massive stars. The isotopic compositions of Ru in the winds of low-mass stars can explain the measurements. We find that lower-mass stars (1.65 M ☉ and 2 M ☉) with low metallicity (Z = 0.0001) can explain most of the measured isotopic compositions of Ru. We confirm that the abundance of 99 Ru inside the presolar grain includes the contribution from the in situ decay of 99 Tc. We also verify our conclusion by comparing the isotopic compositions of Ru integrated over all the pulses with those calculated at individual pulses.

Methods. We use both a Monte Carlo (MC) sampling and a Bayesian analysis to find the effects of the photospheric composition and the touchdown radius. We apply these two methods to six LMXBs that show PRE XRBs. In both methods, we solve the Eddington flux equation and the apparent angular area equation both of which include the correction terms. For the MC sampling, we have developed an iterative method in order to solve these two equations more efficiently. Results. We confirm that the effects of the photospheric composition and the touchdown radius are similar in the statistical and analytical estimation of mass and radius even when the correction terms are considered. Furthermore, in all of the six sources, we find that a H-poor photosphere and a large touchdown radius are favored statistically regardless of the statistical method. Our Bayesian analysis also hints that touchdown can occur farther from the neutron star surface when the photosphere is more H-poor. This correlation could be qualitatively understood with the Eddington flux equation. We propose a physical explanation for this correlation between the photospheric composition and the touchdown radius. Our results show that when accounting for the uncertainties of the photospheric composition and the touchdown radius, most likely radii of the neutron stars in these six LMXBs are less than 12.5 km, which is similar to the bounds for the neutron star radius placed with the tidal deformability measured from the gravitational wave signal., 19 pages, 15 figures, accepted for publication in Astronomy & Astrophysics

We propose a correction of the standard Big Bang nucleosynthesis (BBN) scenario to resolve the primordial lithium problem by considering a possibility that the primordial plasma can deviate from the ideal state. In the standard BBN, the primordial plasma is assumed to be ideal, with particles and photons satisfying the Maxwell-Boltzmann and Planck distribution, respectively. We suggest that this assumption of the primordial plasma being ideal might oversimplify the early Universe and cause the lithium problem. We find that deviation of photon distribution from the Planck distribution, which is parameterised with the help of Tsallis statistics, can resolve the primordial lithium problem when the particle distributions of the primordial plasma still follow the MaxwellBoltzmann distribution. We discuss how the primordial plasma can be weakly non-ideal in this specific fashion and its effects on the cosmic evolution., Comment: 8 pages, 6 figures, version to appear in A&A

Previous studies suggest that the estimated maximum accretion rate from approaching high velocity clouds (HVCs) on the Galactic disk can be up to ~ 0.4 solar mass per year. In this study, we point out that the hydrodynamic interaction between the HVCs and the Galactic disk is not considered in the traditional method of estimating the infall rate and therefore the true supply rate of fuel from HVCs can be different from the suggested value depending on the physical configurations of HVCs including density, velocity, and distance. We choose 11 HVC complexes and construct 4 different infall models in our simulations to give an idea of how the fuel supply rate could be different from the traditional infall rate. Our simulation results show that the fuel supply rate from HVC infall is overestimated in the traditional method and can be lowered by a factor of ~ 0.072 when the hydrodynamic interaction of the HVC complexes and the disk is considered., 8 pages, 5 figures

We present predictions of the Si iv ions in turbulent mixing layers (TMLs) between hot and cool gas and in cool high-velocity clouds (HVCs) that travel through a hot halo, complementing the C iv, N v, and O vi predictions in Kwak & Shelton, Kwak et al., and Henley et al. We find that the Si iv ions are most abundant in regions where the hot and cool gases first begin to mix or where the mixed gas has cooled significantly. The predicted column densities of high velocity Si iv and the predicted ratios of Si iv to C iv and O vi found on individual sightlines in our HVC simulations are in good agreement with observations of high velocity gas. Low velocity Si iv is also seen in the simulations, as a result of decelerated gas in the case of the HVC simulations and when looking along directions that pass perpendicular to the direction of motion in the TML simulations. The ratios of low velocity Si iv to C iv and O vi in the TML simulations are in good agreement with those recorded for Milky Way halo gas, while the ratio of Si iv to O vi from the decelerated gas in the HVC simulations is lower than that observed at normal velocity in the Milky Way halo. We attribute the shortfall of normal velocity Si iv to not having modeled the effects of photoionization and, following Henley et al., consider a composite model that includes decelerated HVC gas, supernova remnants, galactic fountain gas, and the effect of photoionization., 40 pages, 12 figures, and 6 tables

In the Galactic halo, metal-bearing Galactic halo material mixes into high velocity clouds (HVCs) as they hydrodynamically interact. This interaction begins long before the clouds completely dissipate and long before they slow to the velocity of the Galactic material. In order to make quantitative estimates of the mixing efficiency and resulting metal enrichment of HVCs, we made detailed two- and three-dimensional simulations of cloud-interstellar medium interactions. Our simulations track the hydrodynamics and time-dependent ionization levels. They assume that the cloud originally has a warm temperature and extremely low metallicity while the surrounding medium has a high temperature, low density, and substantial metallicity, but our simulations can be generalized to other choices of initial metallicities. In our simulations, mixing between cloud and halo gas noticeably raises the metallicity of the high velocity material. We present plots of the mixing efficiency and metal enrichment as a function of time.

A soft X-ray enhancement has recently been reported toward the high-velocity cloud MS30.7-81.4-118 (MS30.7), a constituent of the Magellanic Stream. In order to investigate the origin of this enhancement, we have analyzed two overlapping XMM-Newton observations of this cloud. We find that the X-ray enhancement is $\sim$6' or $\sim$100 pc across, and is concentrated to the north and west of the densest part of the cloud. We modeled the X-ray enhancement with a variety of spectral models. A single-temperature equilibrium plasma model yields a temperature of $(3.69^{+0.47}_{-0.44}) \times 10^6$ K and a 0.4-2.0 keV luminosity of $7.9 \times 10^{33}$ erg s$^{-1}$. However, this model underpredicts the on-enhancement emission around 1 keV, which may indicate the additional presence of hotter plasma ($T \gtrsim 10^7$ K), or that recombination emission is important. We examined several different physical models for the origin of the X-ray enhancement. We find that turbulent mixing of cold cloud material with hot ambient material, compression or shock heating of a hot ambient medium, and charge exchange reactions between cloud atoms and ions in a hot ambient medium all lead to emission that is too faint. In addition, shock heating in a cool or warm medium leads to emission that is too soft (for reasonable cloud speeds). We find that magnetic reconnection could plausibly power the observed X-ray emission, but resistive magnetohydrodynamical simulations are needed to test this hypothesis. If magnetic reconnection is responsible for the X-ray enhancement, the observed spectral properties could potentially constrain the magnetic field in the vicinity of the Magellanic Stream., Comment: 17 pages, 6 figures. Accepted for publication in the Astrophysical Journal

We present hydrodynamic simulations of high-velocity clouds (HVCs) traveling through the hot, tenuous medium in the Galactic halo. A suite of models was created using the FLASH hydrodynamics code, sampling various cloud sizes, densities, and velocities. In all cases, the cloud-halo interaction ablates material from the clouds. The ablated material falls behind the clouds, where it mixes with the ambient medium to produce intermediate-temperature gas, some of which radiatively cools to less than 10,000 K. Using a non-equilibrium ionization (NEI) algorithm, we track the ionization levels of carbon, nitrogen, and oxygen in the gas throughout the simulation period. We present observation-related predictions, including the expected H I and high ion (C IV, N V, and O VI) column densities on sight lines through the clouds as functions of evolutionary time and off-center distance. The predicted column densities overlap those observed for Complex C. The observations are best matched by clouds that have interacted with the Galactic environment for tens to hundreds of megayears. Given the large distances across which the clouds would travel during such time, our results are consistent with Complex C having an extragalactic origin. The destruction of HVCs is also of interest; the smallest cloud (initial mass \approx 120 Msun) lost most of its mass during the simulation period (60 Myr), while the largest cloud (initial mass \approx 4e5 Msun) remained largely intact, although deformed, during its simulation period (240 Myr)., 20 pages, 13 figures. Accepted for publication in the Astrophysical Journal

We compare the predictions of three physical models for the origin of the hot halo gas with the observed halo X-ray emission, derived from 26 high-latitude XMM-Newton observations of the soft X-ray background between $l=120\degr$ and $l=240\degr$. These observations were chosen from a much larger set of observations as they are expected to be the least contaminated by solar wind charge exchange emission. We characterize the halo emission in the XMM-Newton band with a single-temperature plasma model. We find that the observed halo temperature is fairly constant across the sky (~1.8e6-2.3e6 K), whereas the halo emission measure varies by an order of magnitude (~0.0005-0.006 cm^-6 pc). When we compare our observations with the model predictions, we find that most of the hot gas observed with XMM-Newton does not reside in isolated extraplanar supernova remnants -- this model predicts emission an order of magnitude too faint. A model of a supernova-driven interstellar medium, including the flow of hot gas from the disk into the halo in a galactic fountain, gives good agreement with the observed 0.4-2.0 keV surface brightness. This model overpredicts the halo X-ray temperature by a factor of ~2, but there are a several possible explanations for this discrepancy. We therefore conclude that a major (possibly dominant) contributor to the halo X-ray emission observed with XMM-Newton is a fountain of hot gas driven into the halo by disk supernovae. However, we cannot rule out the possibility that the extended hot halo of accreted material predicted by disk galaxy formation models also contributes to the emission., 20 pages, 14 figures. New version accepted for publication in ApJ. Changes include new section discussing systematic errors (Section 3.2), improved method for characterizing our model spectra (4.2.2), changes to discussion of other observations (5.1). Note that we can no longer rule out possibility that extended hot halo of accreted material contributes to observed halo emission (see 5.2.1)

Highly ionized species such as C IV, N V, and O VI, are commonly observed in diffuse gas in various places in the universe, such as in our Galaxy's disk and halo, high velocity clouds (HVCs), external galaxies, and the intergalactic medium. One possible mechanism for producing high ions is turbulent mixing of cool gas with hotter gas in locations where these gases slide past each other. By using hydrodynamic simulations with radiative cooling and non-equilibrium ionization (NEI) calculations, we investigate the physical properties of turbulent mixing layers and the production of high ions. We find that most of the mixing occurs on the hot side of the hot/cool interface and that the mixed region separates into a tepid zone containing radiatively cooled, C IV-rich gas and a hotter zone which is rich in C IV, N V, and O VI. Mixing occurs faster than ionization or recombination, making the mixed gas a better source of C IV, N V, and O VI in our NEI simulations than in our collisional ionization equilibrium (CIE) simulations. In addition, the gas radiatively cools faster than the ions recombine, which also allows large numbers of high ions to linger in the NEI simulations. For these reasons, our NEI calculations predict more high ions than our CIE calculations predict. We also simulate various initial configurations of turbulent mixing layers and discuss their results. We compare the results of our simulations with observations and other models, including other turbulent mixing calculations. The ratios of C IV to N V and N V to O VI are in reasonable agreement with the averages calculated from observations of the halo. There is a great deal of variation from sightline to sightline and with time in our simulations. Such spatial and temporal variation may explain some of the variation seen among observations., Comment: 19 pages, 7 figures, 8 tables, Accepted for publication by ApJ

In the Galactic fountain scenario, supernovae and/or stellar winds propel material into the Galactic halo. As the material cools, it condenses into clouds. By using FLASH three-dimensional magnetohydrodynamic simulations, we model and study the dynamical evolution of these gas clouds after they form and begin to fall toward the Galactic plane. In our simulations, we assume that the gas clouds form at a height of z=5 kpc above the Galactic midplane, then begin to fall from rest. We investigate how the cloud's evolution, dynamics, and interaction with the interstellar medium (ISM) are affected by the initial mass of the cloud. We find that clouds with sufficiently large initial densities (> 0.1 hydrogen atoms per cc) accelerate sufficiently and maintain sufficiently large column densities as to be observed and identified as high-velocity clouds (HVCs) even if the ISM is weakly magnetized (1.3 micro Gauss). We also investigate the effects of various possible magnetic field configurations. As expected, the ISM's resistance is greatest when the magnetic field is strong and perpendicular to the motion of the cloud. The trajectory of the cloud is guided by the magnetic field lines in cases where the magnetic field is oriented diagonal to the Galactic plane. The model cloud simulations show that the interactions between the cloud and the ISM can be understood via analogy to the shock tube problem which involves shock and rarefaction waves. We also discuss accelerated ambient gas, streamers of material ablated from the clouds, and the cloud's evolution from a sphere-shaped to a disk- or cigar-shaped object., 46 pages, 16 figures, 3 tables. Accepted for publication in ApJ

We test the X-ray emission predictions of galactic fountain models against XMM-Newton measurements of the emission from the Milky Way's hot halo. These measurements are from 110 sight lines, spanning the full range of Galactic longitudes. We find that a magnetohydrodynamical simulation of a supernova-driven interstellar medium, which features a flow of hot gas from the disk to the halo, reproduces the temperature but significantly underpredicts the 0.5-2.0 keV surface brightness of the halo (by two orders of magnitude, if we compare the median predicted and observed values). This is true for versions of the model with and without an interstellar magnetic field. We consider different reasons for the discrepancy between the model predictions and the observations. We find taking into account overionization in cooled halo plasma, which could in principle boost the predicted X-ray emission, is unlikely in practice to bring the predictions in line with the observations. We also find that including thermal conduction, which would tend to increase the surface brightnesses of interfaces between hot and cold gas, would not overcome the surface brightness shortfall. However, charge exchange emission from such interfaces, not included in the current model, may be significant. The faintness of the model may also be due to the lack of cosmic ray driving, meaning that the model may underestimate the amount of material transported from the disk to halo. In addition, an extended hot halo of accreted material may be important, by supplying hot electrons that could boost the emission of the material driven out from the disk. Additional model predictions are needed to test the relative importance of these processes in explaining the observed halo emission., 11 pages, 3 figures. Accepted for publication in the Astrophysical Journal

In order to determine if the material ablated from high-velocity clouds (HVCs) is a significant source of low-velocity high ions (C IV, N V, and O VI) such as those found in the Galactic halo, we simulate the hydrodynamics of the gas and the time-dependent ionization evolution of its carbon, nitrogen, and oxygen ions. Our suite of simulations examines the ablation of warm material from clouds of various sizes, densities, and velocities as they pass through the hot Galactic halo. The ablated material mixes with the environmental gas, producing an intermediate-temperature mixture that is rich in high ions and that slows to the speed of the surrounding gas. We find that the slow mixed material is a significant source of the low-velocity O VI that is observed in the halo, as it can account for at least ~1/3 of the observed O VI column density. Hence, any complete model of the high ions in the halo should include the contribution to the O VI from ablated HVC material. However, such material is unlikely to be a major source of the observed C IV, presumably because the observed C IV is affected by photoionization, which our models do not include. We discuss a composite model that includes contributions from HVCs, supernova remnants, a cooling Galactic fountain, and photoionization by an external radiation field. By design, this model matches the observed O VI column density. This model can also account for most or all of the observed C IV, but only half of the observed N V., Comment: 17 pages, 8 figures. Accepted for publication in the Astrophysical Journal

With the goal of understanding why X-rays have been reported near some high velocity clouds, we perform detailed 3 dimensional hydrodynamic and magnetohydrodynamic simulations of clouds interacting with environmental gas like that in the Galaxy's thick disk/halo or the Magellanic Stream. We examine 2 scenarios. In the first, clouds travel fast enough to shock-heat warm environmental gas. In this scenario, the X-ray productivity depends strongly on the speed of the cloud and the radiative cooling rate. In order to shock-heat environmental gas to temperatures of > or = 10^6 K, cloud speeds of > or = 300 km/s are required. If cooling is quenched, then the shock-heated ambient gas is X-ray emissive, producing bright X-rays in the 1/4 keV band and some X-rays in the 3/4 keV band due to O VII and other ions. If, in contrast, the radiative cooling rate is similar to that of collisional ionizational equilibrium plasma with solar abundances, then the shocked gas is only mildly bright and for only about 1 Myr. The predicted count rates for the non-radiative case are bright enough to explain the count rate observed with XMM-Newton toward a Magellanic Stream cloud and some enhancement in the ROSAT 1/4 keV count rate toward Complex C, while the predicted count rates for the fully radiative case are not. In the second scenario, the clouds travel through and mix with hot ambient gas. The mixed zone can contain hot gas, but the hot portion of the mixed gas is not as bright as those from the shock-heating scenario., 15 pages, 9 figures, 1 table. Accepted for publication in the Astrophysical Journal