Symposium Proceedings

Populations in the High Energy Universe

The talks are in the same order as the Program Schedule.

Ultraluminous X-ray Sources in Nearby Galaxies

Douglas Swartz (USRA NASA/MSFC)

Ultraluminous X-ray sources (ULXs) are non-nuclear point-like sources in external galaxies with apparent luminosities $>10^{39}$ ergs/s. Their nature is still a mystery: If they are accreting sources at the distance of their host galaxies, then their high luminosities require either beamed emission geometries, or super-Eddington emission rates, or accretion onto compact objects more massive than predicted by stellar evolution models. I will present the evidence for and against a unique classification for ULXs, describe theoretical models for the formation of ULXs and their emission mechanisms, and demonstrate the importance of multi-wavelength campaigns to understanding the ULX phenomenon.

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Cosmic Star Formation History and Chandra Deep Field Studies

Pranab Ghosh (Tata Institute), Nicholas White (NASA/GSFC)

We discuss how recent CHANDRA deep-field surveys bear on the question of cosmic star-formation history. We show that our current understanding of the evolution of X-ray luminosities of normal/starburst and Lyman-break galaxies is qualitatively correct, while quantitative details need to be clarified. We explore the role of X-ray logN-logS plots in this context, which indicate that, while the power in the X-ray background is dominated by AGN, the number density of sources is dominated by normal/starburst galaxies at faint fluxes. We discuss various schemes currently employed to discriminate between AGN and normal/starburst galaxies, and we summarize the correlations between the emissions in the X-rays and in other wavebands -- optical, IR, submm, and radio -- which emphasize the diagnostic value of X-rays as a probe of star formation.

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The Chandra M31 Campaign: Some Surprises and M31*

Michael Garcia (SAO), Ben Williams (Penn State Univ.), Francis Primini (SAO), Lorant Sjouwerman (NRAO), Steve Murray (SAO)

Chandra has been monitoring M31 nearly monthly since A01, with a few interruptions. This nearly 6 year long dataset has allowed the X-ray variability of hundreds of sources in the bulge of this galaxy to be studied in unprecedented detail. In addition the summed exposure on the bulge is $\sim$0.5 Msec, allowing very faint sources to be studied. Here we will highlight some of the surprises that this study has revealed, which include an unexplained association between nebula and X-ray binaries. The nuclear supermassive black hole (M31*) has proven to be one of the most secure cases for a radiatively inefficient accretion flow, and bears striking differences to Sgr A*. Preliminary results from our AO6 campaign on M31* will be presented.

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Populations of Compact X-ray Sources in Galaxies

Marat Gilfanov (MPA, Garching)

We will discuss several aspects of populations of compact X-ray sources in galaxies. Among other topics we will compare luminosity distributions of low- and high- mass X-ray binaries in nearby galaxies as observed by Chandra and XMM-Newton and discuss use of compact sources as proxies for the stellar mass and star formation rate of the host galaxy. We will also consider the dependence of the HMXB population on the stellar age and constrain parameters of the binary evolution based on observations of high mass X-ray binaries in other galaxies.

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The M101 Ms


M101 is a nearby face-on Scd galaxy that is small enough in angular size to allow complete coverage with a limited number of Chandra observations. Thus, the campaign to accumulate a 1Ms exposure of M101 allows thorough study of the point source populations; the combination of depth and coverage allows a more detailed study of the luminosity function as well as a spatially resolved study of luminosity functions. I find that 1.) about a third of the sources have hardness ratios typical of thermal sources rather than X-ray binaries, 2.) the radial distribution is more centrally peaked for dimmer sources, suggesting that the brighter sources are more strongly correlated with the younger stellar populations, and 3.) the surface density of sources is correlated with the UV surface brightness.

X-Ray Emission from the Saturn System

Anil Bhardwaj (NASA Marshall Space Flight Center), Ronald F. Elsner (Presenter, NASA Marshall Space Flight Center), J. Hunter Waite Jr. (AOSS, Univ. of Michigan), G. Randall Gladstone (Southwest Research Institute), Graziella Branduardi-Raymont (MSSL, Univ. College London), Thomas E. Cravens (Dept. of Physics and Astronomy, Univ. of Kansas), Peter G. Ford (MIT)

Saturn was observed by the Advanced CCD Imaging Spectrometer on the Chandra observatory in two exposures on 20 and 26-27 January 2004 respectively; each continuous observation lasted for about one full Saturn rotation (about 10 hr). These Chandra observations have detected an X-ray flare from Saturn's disk seen in direct response to an M6-class solar X-ray flare emanating from a sunspot that was clearly visible from both Saturn and Earth. The observation showed that the Saturnian X-ray emission is highly variable: a factor of $\sim$3 variability in brightness over one week. The spectrum of Saturn disk X-rays is quite similar to that of Jupiter's equatorial emissions. In addition, there is a hint of auroral emission from Saturn's South pole. But unlike Jupiter, X-rays from Saturn's polar regions appear to have characteristics similar to those from its disk and vary in brightness inversely to the FUV aurora observed by the Hubble Space Telescope. These Chandra observations also discovered atomic oxygen $K\alpha$ X-rays from Saturn's rings. The X-ray spectrum of the rings is dominated by emission in a narrow ($\sim$130 eV wide) band centered on the atomic oxygen $K\alpha$ fluorescence line at 0.53 keV. These exciting results obtained from Chandra observations will be presented and their production mechanism will be discussed. XMM-Newton observed X-rays from Saturn during 2005 April 21-22, for about two Saturn rotations. Another observation, of similar duration, is scheduled for early November 2005. These observations are planned to take advantage of in-situ measurements being conducted simultaneously by the Cassini spacecraft. Preliminary results from the April XMM-Newton observations will also be presented and compared with the Chandra observations.

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XMM-Newton Observations of X-ray Emission from Jupiter

Graziella Branduardi-Raymont (Mullard Space Science Laboratory, Univ. College London), Anil Bhardwaj, Ronald F. Elsner (NASA Marshall Space Flight Center), G. Randall Gladstone (Southwest Research Institute), Gavin Ramsay (Mullard Space Science Laboratory, Univ. College London), Pedro Rodriguez (XMM-Newton SOC, Vilspa), Roberto Soria (Mullard Space Science Laboratory, Univ. College London), J. Hunter Waite Jr. (AOSS, Univ. of Michigan), Thomas E. Cravens (Univ. of Kansas),

Two XMM-Newton observations of Jupiter were carried out in 2003 for 100 and 250 ks (3 and 7 planet rotations) respectively.

X-ray images from the EPIC CCD cameras show bright emissions, modulated at the planet's rotation period, from Jupiter's auroral spots. Their spectra are well modelled by a combination of emission lines, including most prominently those of highly ionised oxygen (OVII and OVIII). Emission from the equatorial regions of the planet's disk is also observed: the spectrum, displaying FeXVII, Mg XI and SiXIII line emission, is consistent with that of solar X-rays scattered in the planet's upper atmosphere. Spectrally resolved EPIC images, using narrow bands centered on the brightest lines, clearly resolve the different emission areas. Remarkably, in November 2003, a large solar X-ray flare on the Sun's Jupiter-facing side is found to be associated with a corresponding feature in the Jovian X-ray lightcurve of the equatorial regions.

Jupiter's X-rays are further resolved spectrally with the RGS, which clearly separates the OVII triplet, the OVIII and FeXVII lines, the auroral emissions being mostly identified with the lower ionisation oxygen line.

Our findings suggest that the non-auroral X-ray emission from Jupiter is directly controlled by the Sun, while the auroral emissions are most likely due to capture and acceleration of energetic ions from the outer magnetosphere, or the solar wind, or both, followed by X-ray production by charge exchange.

[PDF of the talk]