arXiv subjects

Paul J. Green

Publications and source records attributed to Paul J. Green.

59 records · Page 4Linked to original sources

arXiv1997

Evidence Against BALS in the X-ray Bright QSO PG1416-129

Recent results from the ROSAT All Sky Survey, and from deep ROSAT pointings reveal that broad absorption line quasars (BALQSOs) are weak in the soft X-ray bandpass (with optical-to-X-ray spectral slope alpha_{ox}>1.8) in comparison to QSOs with normal OUV spectra (mean alpha_{ox}=1.4). One glaring exception appeared to be the nearby BALQSO PG1416-129, which is a bright ROSAT source showing no evidence for intrinsic soft X-ray absorption. We present here our new HST FOS spectrum of PG1416-129, in which we find no evidence for BALs. We show that the features resulting in the original BAL classification, based on IUE spectra, were probably spurious. On the basis of UV, X-ray and optical evidence, we conclude that PG1416-129, is not now, and has never been a BALQSO. Our result suggests that weak soft X-ray emission is a defining characteristic of true BALQSOs. If BALQSOs indeed harbor normal intrinsic spectral energy distributions, their observed soft X-ray weakness is most likely the result of absorption. The ubiquitous occurrence of weak soft X-ray emission with UV absorption (BALs) thus suggests absorbers in each energy regime that are physically associated, if not identical.

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arXiv1996

Understanding the Elusive Dwarf Carbon Star

Most stars in our Galaxy with photospheric C/O>1 (carbon stars) are not giants but dwarfs. The newly-recognized class of dwarf carbon (dC) stars joins the growing family of stars with peculiar abundances that are now recognized as products of mass-transfer binary (MTB) evolution. The dozen examples now known span a wide range of evolutionary histories, ages, and abundances. These stars can already provide some much-needed constraints on the formation of AGB C stars in the disk and spheroid populations, and on the parameters characterizing binary evolution there. A larger sample of dCs, with some bright members, would hasten our progress.

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arXiv1996

ROSAT Soft X-Ray Properties of the Large Bright Quasar Survey: Modeling of Stacked X-Ray Spectra

We develop and apply a novel method of analysis to study the X-ray spectral properties of 908 QSOs in the Large Bright Quasar Survey (LBQS) that were observed during the soft X-ray ROSAT All-Sky Survey (RASS). Due to the relatively short (<600sec) RASS exposure times, only 10% of the QSOs are detected in X-rays, so X-ray spectral model fits for individual QSOs are precluded by poor photon statistics. Spectral stacking provides effectively much more sensitive X-ray observations for an average QSO in bins of redshift, and for several classes of QSOs. We model the stacked X-ray spectra in a way that obviates both the distinction between detections and non-detections, and the need to stack together only those objects observed through similar Galactic column densities N_H. In application to the LBQS/RASS sample, we marginally confirm a flattening of the X-ray spectral index of QSOs toward higher redshifts. Radio-loud QSOs show flatter X-ray photon indices, in agreement with previous work. We show that the apparent flattening of the photon index with redshift is not due to an increasing fractional contribution from the radio-loud class of QSOs.

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arXiv1996

The Relationship Between the High Energy Continuum and Emission Lines in QSOs: A Low-Redshift Sample

Quasar emission lines are sensitive to both the luminosity and shape of the quasars' high energy continuum - primarily in the soft X-ray and the (obscured) extreme ultraviolet. We initiate the first large-scale, multi-line investigation of correlations between the QSO soft X-ray continuum and UV line emission, using an IUE/Einstein sample. We characterize the distributions of line equivalent and velocity widths (EW and FWHM) of 85 QSOs in the IUE atlas of Lanzetta et al. (1993), using a new error analysis for objective, automated line measurements, and contrast measured line parameters to the QSO continuum spectral energy distributions (SEDs). We find significant correlations between EW and UV luminosity (e.g., the well-studied Baldwin effect) for Lya, CIV, HeII, and CIII. EW(CIII) and EW(HeII) show previously unreported correlations with Lx which for CIII appears to be primary. The line ratios CIII/Lya and HeII/Lya both show strongest dependence on Lx. EW(Lya) correlates strongly with spectral slopes α_{uv} and α_{ox}, but NOT with Lx. We argue that one simple geometrical interpretation of the Baldwin effect (BEff) as a result of a distribution of disk inclinations is not plausible. The BEff weakens or disappears when line emission is correctly compared to the luminosity in the continuum bandpass relevant to its production, which suggests that the BEff is due to a change in SED with luminosity. Our prediction that no BEff relative to X-ray luminosity should be found for FeII or MgII emission lines, will be tested in a later paper, extended to a wider redshift/luminosity range.

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arXiv1995

Broad Absorption Line QSOs Observed by the ROSAT PSPC

Recent results from the ROSAT All-Sky Survey (RASS) have shown that broad absorption line (BAL) QSOs are either highly absorbed or underluminous in the soft X-ray bandpass. Here, we extend this work by analyzing all known bona fide BALQSOs observed within the inner 20' of the ROSAT PSPC. This sample includes both targeted and serendipitous exposures ranging from 8 to 75 ksec. Despite these deep exposures, most of the BALQSOs are undetected, and have unusually weak X-ray emission, as evidenced by large optical-to-X-ray slopes (AOX). Large values of AOX ($\gapprox$1.8) may prove to be a defining characteristic of BALQSOs. We predict that samples of QSO candidates with large AOX will yield a higher percentage of BALQSOs, particularly at low redshift. As a corollary, X-ray-selected QSO samples should yield The optical/UV emission line spectra of BAL and non-BAL QSOs are quite similar, suggesting that their intrinsic spectral energy distributions are similar as well. Absorption thus seems the likely reason for the X-ray quiet nature of BALQSOs. To constrain the total absorbing column of the BAL clouds, we compare our measured soft X-ray fluxes or upper limits to those expected from normal radio quiet QSOs of comparable optical continuum magnitude and redshift. From sensitive X-ray observations, we derive column densities of greater than about 2 times 10^22 cm-2, for intrinsic cold absorbers of solar metallicity. These new results suggest columns {\em at least} an order of magnitude larger than the columns previously estimated from optical/UV spectra alone.

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