John T. Stocke

The Einstein Observatory Extended Medium-Sensitivity Survey. II - The optical identifications

The optical identifications are presented of the Einstein Extended Medium-Sensitivity Survey (EMSS), including the methodology used to optically identify the EMSS sources and the uncertainties involved with that process. The optical properties of the classes of X-ray, optical, and radio data for each of the identified and, as yet, unidentified sources of the survey are described. A new class of X-ray emitters, cooling flow galaxies, is proposed. The criteria used to determine whether the proposed optical counterpart to the X-ray source is a plausible identification are described. Plausibility is based on the optical classification of the counterpart, e.g., AGN, cluster, G star, and the X-ray-to-optical flux ratios previously observed for these classes of X-ray emitters. Two independent schemes of optical classification of the counterparts are used to check the plausibility of these identifications; one is based on moderate-resolution optical spectroscopy, and the other, on inferred X-ray luminosity and the overall energy distribution. 110 refs.

The X-ray spectra of the extragalactic sources in the Einstein extended medium sensitivity survey

The X-ray energy distribution of a large sample of faint extragalactic X-ray sources is studied. Two independent methods are used to determine that the average spectral index for the sources is about 0.95. The intrinsic dispersion of the spectral index distribution is about 0.36. The sources are characterized by a range of power law slopes in the soft X-ray band with a mean of 1.03 + 0.05 or - 0.06 and an intrinsic dispersion of about 0.36. The present analysis extends knowledge of the AGN spectral properties to sources which are about three times more distant and more than 10 times fainter than those previously studied. There is no evidence in the data for any change in the mean spectral index of either extragalactic sources as a whole or AGN in particular, as a function of the X-ray flux in the flux range 10 to the -11th to 10 to the -13th ergs/sq cm/s. 22 references.

THE COSMIC ORIGINS SPECTROGRAPH

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in 2009 May, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F ? 1.0 ? 10?14?erg?cm?2?s?1 ??1, COS can achieve comparable signal to noise (when compared to Space Telescope Imaging Spectrograph echelle modes) in 1%-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (2009 September-2011 June) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is nine times than sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of 2011 June. COS has measured, for the first time with high reliability, broad Ly? absorbers and Ne VIII in the intergalactic medium, and observed the He II reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy.

The radio properties of the broad-absorption-line QSOs

A VLA radio detection survey has been conducted for a large sample of broad-absorption-line (BAL) QSOs. Together with results from a similar, previous survey and a few observations from two other surveys, 68 BALQSOs have now been well observed with no strong radio sources detected. The absence of luminous radio sources among 68 known BALQSOs reported here, together with a complementary spectroscopic study which finds no BALQSOs among a large sample of radio-loud quasars, establishes a strong anticorrelation between luminous radio sources and the BALQSO phenomenon

Multiwavelength Observations of a Dramatic High-Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (IUE) and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of 3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ~1 day. The optical-UV light curves, which are not well sampled during the high-energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron self-Compton (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a few days. This occurs naturally in the mirror model, wherein the flaring region in the jet photoionizes nearby broad emission line clouds, which, in turn, provide soft external photons that are Comptonized to γ-ray energies.

The extended medium sensitivity survey distant cluster sample - X-ray data and interpretation of the luminosity evolution

The X-ray properties of a cluster of galaxies subsample of the Einstein Extended Medium Sensitivity Survey is described. A summary of this sample and its implication has been presented previously; this paper gives the full details. The cluster subsample is 98.4 percent identified and contains 93 X-ray-selected clusters to a redshift of 0.58. The cluster X-ray luminosity function at three cosmic epochs is derived. While the present luminosity function agrees with previous determinations at the lowest redshifts, it is found that the volume density of high-luminosity clusters is greater now than it was in the past. The normalization, shape, and time dependence of the luminosity function can be described by a simple hierarchical formation model with parameters which also describe the temperature function of an independent sample of low-redshift clusters. In this model the comoving hot gas density remains constant with time at least to redshifts of order 0.35.

OBSERVATIONS OF MASS LOSS FROM THE TRANSITING EXOPLANET HD 209458b

Using the new Cosmic Origins Spectrograph on the Hubble Space Telescope, we obtained moderate-resolution, high signal/noise ultraviolet spectra of HD 209458 and its exoplanet HD 209458b during transit, both orbital quadratures, and secondary eclipse. We compare transit spectra with spectra obtained at non-transit phases to identify spectral features due to the exoplanets expanding atmosphere. We find that the mean flux decreased by 7.8% ± 1.3% for the C II 1334.5323 A and 1335.6854 A lines and by 8.2% ± 1.4% for the Si III 1206.500 A line during transit compared to non-transit times in the velocity interval –50 to +50 km s–1. Comparison of the C II and Si III line depths and transit/non-transit line ratios shows deeper absorption features near –10 and +15 km s–1 and less certain features near –40 and +30-70 km s–1, but future observations are needed to verify this first detection of velocity structure in the expanding atmosphere of an exoplanet. Our results for the C II lines and the non-detection of Si IV 1394.76 A absorption are in agreement with Vidal-Madjar et al., but we find absorption during transit in the Si III line contrary to the earlier result. The 8% ± 1% obscuration of the star during transit is far larger than the 1.5% obscuration by the exoplanets disk. Absorption during transit at velocities between –50 and +50 km s–1 in the C II and Si III lines requires high-velocity ion absorbers. Assuming hydrodynamic model values for the gas temperature and outflow velocity at the limb of the outflow as seen in the C II lines, we find mass-loss rates in the range (8-40)×1010 g s–1. These rates assume that the carbon abundance is solar, which is not the case for the giant planets in the solar system. Our mass-loss rate estimate is consistent with theoretical hydrodynamic models that include metals in the outflowing gas.

The Extended Medium Sensitivity Survey distant cluster sample : X-ray cosmological evolution

The X-ray luminosity function of clusters of galaxies is determined at different cosmic epochs using data from the Einstein Observatory Extended Medium Sensitivity Survey. The sample consists of 67 X-ray-selected clusters that have been grouped in three redshift shells. Evolution is detected in the X-ray properties of clusters. The present volume density of high-luminosity clusters is found to be greater than it was in the past. Given the still limited data set, this result should be regarded as preliminary. It can be interpreted as the consequence of either luminosity evolution or modest density evolution.

The Galaxy Environment of O VI Absorption Systems

We combine a FUSE sample of O VI absorbers (z < 0.15) with a database of 1.07 million galaxy redshifts to explore the relationship between absorbers and galaxy environments. All 37 absorbers with N ≥ 1013.2 cm-2 lie within 800 h kpc of the nearest galaxy, with no compelling evidence for O VI absorbers in voids. The O VI absorbers often appear to be associated with environments of individual galaxies. Gas with 10% ± 5% solar metallicity (O VI and C III) has a median spread in distance of 350-500 h kpc around L galaxies and 200-270 h kpc around 0.1L galaxies (ranges reflect uncertain metallicities of gas undetected in Lyα absorption). In order to match the O VI line frequency, (d/dz) ≈ 20 for N ≥ 1013.2 cm-2, galaxies with L ≤ 0.1L must contribute to the cross section. Lyα absorbers with N ≥ 1013.2 cm-2 cover ~50% of the surface area of typical galaxy filaments. Two-thirds of these show O VI and/or C III absorption, corresponding to a 33%-50% covering factor at 0.1 Z☉ and suggesting that metals are spread to a maximum distance of 800 h kpc, within typical galaxy supercluster filaments. Approximately 50% of the O VI absorbers have associated Lyα line pairs with separations (Δv)Lyα = 50-200 km s-1. These pairs could represent shocks at the speeds necessary to create copious O VI, located within 100 h kpc of the nearest galaxy and accounting for much of the two-point correlation function of low-z Lyα forest absorbers.

The Local Lyα Forest. II. Distribution of H I Absorbers,Doppler Widths, and Baryon Content

In Paper I of this series we described observations of 15 extragalactic targets taken with the Hubble Space Telescope+GHRS+G160M grating for studies of the low-z Lyα forest. We reported the detection of 111 Lyα absorbers at significance level (SL) ≥3 σ, 81 with SL ≥ 4 σ, in the redshift range 0.002 1.6. Similar to the high equivalent width ( > 240 mA) absorbers, the number density of low- absorbers at z = 0 is well above the extrapolation of d/dz from z > 2, but we observe no difference in the mean evolution of d/dz between absorbers of high ( > 240 mA) and low ( ≤ 100 mA) equivalent width. While previous work has suggested slower evolution in number density of lower absorbers, our new data do not support this conclusion. A consistent evolutionary pattern is that the slowing in the evolution of the low column density clouds occurs at lower redshift than for the higher column density clouds. A 4-5 σ signal in the two-point correlation function of Lyα absorbers for velocity separations Δv ≤ 150 km s-1 is consistent with results at high z, but with somewhat greater amplitude. Applying a photoionization correction, we find that the low-z Lyα forest may contain ~20% of the total number of baryons, with closure parameter ΩLyα = (0.008 ± 0.001)h, for a standard absorber size and ionizing radiation field. Some of these clouds appear to be primordial matter, owing to the lack of detected metals (Si III) in a composite spectrum, although current limits on composite metallicity are not strong.