Predicting X-ray emission from wind-blown bubbles - Limitations of fits to ROSAT spectra

Wind-blown bubbles, from those around massive O and Wolf-Rayet stars, to superbubbles around OB associations and galactic winds in starburst galaxies, have a dominant role in determining the structure of the Interstellar Medium. X-ray observations of these bubbles are particularly important as most of their volume is taken up with hot gas, 1E5 < T (K) < 1E8. However, it is difficult to compare X-ray observations, usually analysed in terms of single or two temperature spectral model fits, with theoretical models, as real bubbles do not have such simple temperature distributions. In this introduction to a series of papers detailing the observable X-ray properties of wind-blown bubbles, we describe our method with which we aim to solve this problem, analysing a simulation of a wind-blown bubble around a massive star. We model a wind of constant mass and energy injection rate, blowing into a uniform ISM, from which we calculate X-ray spectra as would be seen by the ROSAT PSPC. We compare the properties of the bubble as would be inferred from the ROSAT data with the true properties of the bubble in the simulation. We find standard spectral models yield inferred properties that deviate significantly from the true properties, even though the spectral fits are statistically acceptable, and give no indication that they do not represent to true spectral distribution. Our results suggest that in any case where the true source spectrum does not come from a simple single or two temperature distribution the "observed" X-ray properties cannot naively be used to infer the true properties.Comment: 14 pages, LaTeX with 13 eps figures, condensed abstract. MNRAS in pres

Quick methods of analysing earthquake forces on viaduct bridges having continuous decks and tall piers

Bibliography: leaf 87.The available methods of calculating the effects of earthquakes on viaduct-type bridges with continuous decks and tall piers are examined. Particular emphasis is laid on the use of a relatively quick method of analysis using response spectra and taking account of the effects of damping. The use of this method is illustrated by applying it to a mathematical model representing a real. bridge. Manual calculations are performed and refined by the application of Rayleigh's Principle. A number of variations to the configuration of the model are analysed by the computer programme "SAP IV" under earthquake excitation in each of three mutually perpendicular directions. The computed results are compared with those obtained by manual calculation. The earthquake loading effects are compared with the effects of deadload, traffic loading and windload and there is a discussion on the design of suitable structural configurations to contend with seismic loading. The work ends with some generalisations about other types of bridges

The Energetics and Mass-loss of Mrk33

We present ROSAT HRI X-ray data and optical imaging of the important dwarf starburst Markarian 33. We find an extended, complex, shell-like morphology in the X-ray emission, with an extent of 2.3 x 1.9kpc, coincident with the bright star-forming regions at the centre of the galaxy. The physical extent of this X-ray emission from Mrk 33 is very similar to the observed Halpha emission, and suggests that the bulk of the X-ray emission is coming from an expanding superbubble. We estimate the age and mass of Mrk 33's starburst to be 5.8 Myr and 6.9 x 10^{6} Msolar respectively with the energy injection rate in the central regions of the galaxy being 10^{41} erg/s, while the associated mass-loss rate from the star-forming regions is estimated to be 0.2 Msolar/yr. We suggest that the X-ray emission is predominantly powered by starburst type activity and argue that a blowout in the form of a galactic wind is the most likely fate for Mrk 33 resulting in the loss of most of the galaxy's metal-enriched material and a small fraction (<1 per cent) of the ISM.Comment: 13 pages, 6 figures, accepted for publication in MNRA

Scalable iterative methods for sampling from massive Gaussian random vectors

Sampling from Gaussian Markov random fields (GMRFs), that is multivariate Gaussian ran- dom vectors that are parameterised by the inverse of their covariance matrix, is a fundamental problem in computational statistics. In this paper, we show how we can exploit arbitrarily accu- rate approximations to a GMRF to speed up Krylov subspace sampling methods. We also show that these methods can be used when computing the normalising constant of a large multivariate Gaussian distribution, which is needed for both any likelihood-based inference method. The method we derive is also applicable to other structured Gaussian random vectors and, in particu- lar, we show that when the precision matrix is a perturbation of a (block) circulant matrix, it is still possible to derive O(n log n) sampling schemes.Comment: 17 Pages, 4 Figure

Chandra and XMM-Newton Observations of NGC 4214: The Hot Interstellar Medium and the Luminosity Function of Dwarf Starbursts

We present results from Chandra and XMM-Newton X-ray observations of NGC 4214, a nearby dwarf starburst galaxy containing several young regions of very active star-formation. Starburst regions are known to be associated with diffuse X-ray emission, and in this case the X-ray emission from the galaxy shows an interesting morphological structure within the galaxy, clearly associated with the central regions of active star-formation. Of the two main regions of star-formation in this galaxy, X-ray emission associated with the older is identified whereas little is detected from the younger, providing an insight into the evolutionary process of the formation of superbubbles around young stellar clusters. The spectra of the diffuse emission from the galaxy can be fitted with a two temperature component thermal model with kT=0.14keV and 0.52keV, and analysis of this emission suggests that NGC 4214 will suffer a blow-out in the future. The point source population of the galaxy has an X-ray luminosity function with a slope of -0.76. This result, together with those for other dwarf starburst galaxies (NGC 4449 and NGC 5253), was added to a sample of luminosity functions for spiral and starburst galaxies. The slope of the luminosity function of dwarf starbursts is seen to be similar to that of their larger counterparts and clearly flatter than those seen in spirals. Further comparisons between the luminosity functions of starbursts and spiral galaxies are also made.Comment: 16 pages, 12 figures. Accepted for publication in MNRA

XMM-Newton Reflection Grating Spectrometer Observations of the Prototypical Starburst Galaxy M82

We present results from XMM-Newton Reflection Grating Spectrometer observations of the prototypical starburst galaxy M82. These high resolution spectra represent the best X-ray spectra to date of a starburst galaxy. A complex array of lines from species over a wide range of temperatures is seen, the most prominent being due to Lyman-alpha emission from abundant low Z elements such as N, O, Ne, Mg and Si. Emission lines from Helium-like charge states of the same elements are also seen in emission, as are strong lines from the entire Fe-L series. Further, the OVII line complex is resolved and is seen to be consistent with gas in collisional ionization equilibrium. Spectral fitting indicates emission from a large mass of gas with a differential emission measure over a range of temperatures (from 0.2 keV to 1.6 keV, peaking at 0.7 keV), and evidence for super-solar abundances of several elements is indicated. Spatial analysis of the data indicates that low energy emission is more extended to the south and east of the nucleus than to the north and west. Higher energy emission is far more centrally concentrated.Comment: 5 pages, 4 figures, MNRAS accepte

Chandra & XMM-Newton Observations of NGC5253. Analysis of the X-ray Emission from a Dwarf Starburst Galaxy

We present Chandra and XMM-Newton X-ray data of NGC5253, a local starbursting dwarf elliptical galaxy, in the early stages of a starburst episode. Contributions to the X-ray emission come from discrete point sources and extended diffuse emission, in the form of what appear to be multiple superbubbles, and smaller bubbles probably associated with individual star clusters. Chandra detects 17 sources within the optical extent of NGC5253 down to a completeness level corresponding to a luminosity of 1.5E37 erg/s.The slope of the point source X-ray luminosity function is -0.54, similar to that of other nearby dwarf starburst galaxies. Several different types of source are detected within the galaxy, including X-ray binaries and the emission associated with star-clusters. Comparison of the diffuse X-ray emission with the observed Halpha emission shows similarities in their extent. The best spectral fit to the diffuse emission is obtained with an absorbed, two temperature model giving temperatures for the two gas components of ~0.24keV and ~0.75keV.The derived parameters of the diffuse X-ray emitting gas are as follows: a total mass of \~1.4E6 f^{1/2} Msun, where f is the volume filling factor of the X-ray emitting gas, and a total thermal energy content for the hot X-ray emitting gas of \~3.4E54 f^{1/2} erg. The pressure in the diffuse gas is P/k ~ 1E6f^{-1/2}K/cm3. We find that these values are broadly commensurate with the mass and energy injection from the starburst population. Analysis of the kinematics of the starburst region suggest that the stellar ejecta contained within it can escape the gravitational potential well of the galaxy, and pollute the surrounding IGM.Comment: 19 pages, In Press in MNRA

X-ray observations of the edge-on star-forming galaxy NGC891 and its supernova SN1986J

We present XMM-Newton observations of NCG891, a nearby edge-on spiral galaxy. We analyse the extent of the diffuse emission emitted from the disk of the galaxy, and find that it has a single temperature profile with best fitting temperature of 0.26 keV, though the fit of a dual-temperature plasma with temperatures of 0.08 and 0.30 keV is also an acceptable fit. There is a considerable amount of diffuse X-ray emission protruding from the disk in the NW direction out to approximately 6 kpc. We analyse the point source population using a Chandra observation, using a maximum likelihood method to find that the slope of the cumulative luminosity function of point sources in the galaxy is -0.77^{+0.13}_{-0.1}. Using a sample of other local galaxies, we compare the X-ray and infrared properties of NGC891 with those of 'normal' and starburst spiral galaxies, and conclude that NGC891 is most likely a starburst galaxy in a quiescent state. We establish that the diffuse X-ray luminosity of spirals scales with the far infra-red luminosity as L_X \propto L_FIR ^{0.87 +- 0.07}, except for extreme starbursts, and NGC891 does not fall in the latter category. We study the supernova SN1986J in both XMM-Newton and Chandra observations, and find that the X-ray luminosity has been declining with time more steeply than expected (L_X \propto t^-3).Comment: 12 pages, 11 figures, accepted for publication in MNRA