arXiv subjects

K. O. Mason

Publications and source records attributed to K. O. Mason.

77 records · Page 5Linked to original sources

arXiv1996

Double-peaked Balmer line emission in the radio-quiet AGN RX J1042+1212

We present optical and X-ray spectra of a radio-quiet X-ray selected AGN, RX J1042+1212 ($z$=0.271). The Halpha and Hbeta emission lines are very broad (with full widths at half maximum of ~10000 km s-1) and have double-peaked profiles. Such features are rarely observed in AGN in general but are even more unusual in radio-quiet objects. The analysis of the ROSAT PSPC data reveals a non-varying, unabsorbed spectrum with an energy spectral index, alphax=1.2 and little or no emission from a soft X-ray excess. The slope of the optical spectrum is similar, alphaopt=1.0, and is consistent with an extrapolation of the X-ray spectrum, suggesting that the same power-law continuum may dominate throughout and that the big blue bump component is relatively weak. We look for a link between these various properties and investigate models of double-peaked Balmer line emission in AGN. An accretion disc origin is unlikely in RX J1042+1212 as this model predicts that lines emitted by a disc should have a net gravitational redshift (both Halpha and Hbeta have a net blueshift). Emission from two broad line regions, each gravitationally bound to one component of a supermassive black hole binary, is a possibility if the two components are similar in size and nature. Alternatively, the lines (or at least the narrow peaks of the lines) may be produced by a double-sided jet or bipolar flow.

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arXiv1996

ROSAT PSPC spectra of X-ray selected Narrow Emission Line Galaxies

We analyse the ROSAT PSPC spectrum of 19 X-ray selected Narrow Emission Line Galaxies (NELGs) discovered during the optical identification of sources in the ROSAT UK Deep Survey. Their properties are compared to those of broad line Active Galactic Nuclei (AGN) in the same sample. Counts in three spectral bands have been extracted for all the sources, and have been fitted with a power-law model assuming the Galactic value for NH. The average slope of NELGs is α=0.45+-0.09, whilst for the AGN it is α=0.96+-0.03. The power-law model is a good fit for ~90% of NELGs and ~75% of AGN. Recent work shows that the fractional surface density of NELGs increases with respect to AGN at faint fluxes. Thus they are expected to be an important component of the residual soft (<2 keV) X--ray background. The slope of the X--ray background (α~0.4, 1-10 keV) is harder than that of AGN (α~1) but our results show that it is consistent with the summed spectrum of the NELGs in the deep survey (α~0.4). This may finally reconcile the spectrum of the background with the properties of the sources that constitute it.

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arXiv1996

Optical and X-ray properties of the RIXOS AGN: I - The continua

We present measurements of the optical and X-ray continua of 108 AGN (Seyfert 1s and quasars) from the Rosat International X-ray/Optical Survey (RIXOS). The sample covers a wide range in redshift (0<z<3.3), in X-ray spectral slope (-1.5 <ax<2.6) and in optical-to-X-ray ratio (0.4$<aox<1.5). A correlation is found between ax and aox; similar correlations have recently been reported in other X-ray and optical samples. We also identify previously unreported relationships between the optical slope (aopt) and ax (particularly at high redshifts) and between aopt and aox. These trends show that the overall optical-to-X-ray conti nuum changes from convex to concave as ax hardens, demonstrating a strong beha vioural link between the optical/UV big blue bump (BBB) and the soft X-ray ex cess, which is consistent with them being part of the same spectral component. By constructing models of the optical-to-X-ray continuum, we demonstrate that the observed correlations are consistent with an intrinsic spectrum which is ab sorbed through different amounts of cold gas and dust. The intrinsic spectrum is the sum of an optical-to-soft X-ray `big bump' component and an ax=1 power law; the column density of the cold gas ranges from 0 to ~4E21 cm-2 and the dust-to-gas ratio is assumed to be Galactic. The `big bump' may be represented by a T~1E6 K thermal bremsstrahlung or an accretion disk with a surrounding hot corona. The scatter in the data can accommodate a wide range in big bump tempe rature (or black hole mass) and strength. A source for the absorbing gas may be the dusty, molecular torus which lies beyond the broad line-emitting regions, although with a much lower column density than observed in Seyfert 2 galaxies. Alternatively, it may be the bulge of a spiral host galaxy or an elliptical host galaxy.

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arXiv1996

The Luminosity Function Evolution of Soft X--ray selected AGN in the RIXOS survey

A sample of 198 soft X--ray selected active galactic nuclei (AGN) from the ROSAT International X--ray Optical Survey (RIXOS), is used to investigate the X--ray luminosity function and its evolution. RIXOS, with a flux limit of 3E-14 erg s-1 cm-2 (0.5 to 2.0 keV), samples a broad range in redshift over 20 deg^2 of sky, and is almost completely identified; it is used in combination with the Einstein Extended Medium Sensitivity Survey (EMSS), to give a total sample of over 600 AGN. We find the evolution of AGN with redshift to be consistent with pure luminosity evolution (PLE) models in which the rate of evolution slows markedly or stops at high redshifts z>1.8. We find that this result is not affected by the inclusion, or exclusion, of narrow emission line galaxies at low redshift in the RIXOS and EMSS samples, and is insensitive to uncertainties in the conversion between flux values measured with ROSAT and Einstein. We confirm, using a model independent Ve/Va test, that our survey is consistent with no evolution at high redshifts.

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arXiv1995

The Deficit of Distant Galaxy Clusters in the RIXOS X-ray Survey

Clusters of galaxies are the largest gravitationally bound systems and therefore provide an important way of studying the formation and evolution of the large scale structure of the Universe. Cluster evolution can be inferred from observations of the X-ray emission of the gas in distant clusters, but interpreting these data is not straightforward. In a simplified view, clusters grow from perturbations in the matter distribution: their intracluster gas is compressed and shock-heated by the gravitational collapse$^{1}$. The resulting X-ray emission is determined by the hydrostatic equilibrium of the gas in the changing gravitational potential. However, if processes such as radiative cooling or pre-collapse heating of the gas are important, then the X-ray evolution will be strongly influenced by the thermal history of the gas. Here we present the first results from a faint flux-limited sample of X-ray selected clusters compiled as part of the ROSAT International X-ray and Optical Survey (RIXOS). Very few distant clusters have been identified. Most importantly, their redshift distribution appears to be inconsistent with simple models based on the evolution of the gravitational potential. Our results suggest that radiative cooling or non-gravitational heating of the intracluster gas must play an important role in the evolution of clusters.

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