Peter J. Serlemitsos

A complete X-ray sample of the high-latitude /absolute value of b greater than 20 deg/ sky from HEAO 1 A-2 - Log N-log S and luminosity functions

The HEAO 2 experiment A-2 has performed a complete x-ray survey of the 8.2 sr of the sky at Vertical BargVertical Bar by 20/sup 0/ down to a limiting sensitivity of approx.3.1 x 10/sup hyphen11/ ergs cm/sup -2/ s/sup -1/ in the 2-10 keV band. Of the 85 detected sources (excluding the LMC and SMC sources), have been identified with galactic objects, 61 have been identified with extragalactic objects, and 7 remain unidentified. The log N-log S relation for the nongalactic objects is well fitted by the Euclidean relationship. We have used the x-ray spectra of these objects to construct the log N-log S relation in physical units. The complete sample of identified sources have been used to construct x-ray luminosity functions, using the absolute maximum likelihood method, for clusters of galaxies and active galactic nuclei.

The Einstein /HEAO 2/ X-ray Observatory

The Einstein (HEAO 2) X-ray Observatory, launched in 1978, includes a fully imaging focusing X-ray telescope with an angular resolution of a few arc sec, a field of view of up to one deg, and a sensitivity several hundred times greater than previously available in any X-ray astronomy experiment. A high-resolution imager, an imaging proportional counter, a focal plane crystal spectrometer, and a monitor proportional counter are among the principal instruments on board the Einstein X-ray Observatory. About 20% of the total effective observing time in the first year of the X-ray astronomy experiment has been reserved for guest observers.

The diffuse X-ray background spectrum from 3 to 50 keV

The spectrum of the extragalactic diffuse X-ray background has been measured with the GSFC Cosmic X-Ray Experiment on HEAO 1 for regions of the sky away from known point sources and more than 20 /sup 0/ from the galactic plane. A total exposure of 80 m/sup 2/-s-sr is available at present. Free-free emission from an optically thin plasma of 40 +- 5 keV provides an excellent description of the observed spectrum from 3 to 50 keV. This spectral shape is confirmed by measurements from five separate layers of three independent detectors. With an estimated absolute precision of approx.10%, the intensity of the emission at 10 keV is 3.2 keV keV/sup -1/ cm/sup -2/ s/sup -1/ sr/sup -1/, a value consistent with the average of previously reported spectra. No other spectral features, such as iron line emission, are evident. This spectrum is not typical of known extra-galactic objects. A uniform hot intergalactic medium of approximately 36% of the closure density of the universe would produce such a flux, although nonuniform models indicating less total matter are probably more realistic.

Physical Diagnostics from a Narrow Fe Kα Emission Line Detected by Chandra in the Seyfert 1 Galaxy NGC 5548

We report the detection of a narrow Fe K? emission line in the Seyfert 1 galaxy NGC 5548 with the Chandra High-Energy Transmission Grating. In the galaxy frame we measure a center energy of 6.402 keV, a FWHM of 4515 km s-1, an intensity of 3.6 ? 10-5 photons cm-2 s-1, with an equivalent width of 133 eV (errors are 90% confidence for one parameter). The line is only marginally resolved at the 90% confidence level. The line energy is consistent with an origin in cold, neutral matter, but ionization states up to ~Fe XVIII are not ruled out. We cannot constrain the detailed dynamics but, assuming Keplerian motion, the velocity width is consistent with the line being produced in the outer optical/UV broad-line region (BLR) at about a light month from the central X-ray source. We cannot rule out some contribution to the narrow Fe K? line from a putative, parsec-scale, obscuring torus that is postulated to be a key component of active galactic nuclei (AGNs) unification models. The continuum intensity during the Chandra observation was a factor ~2 less than typical historical levels. If the X-ray continuum was at least a factor of 2 higher in the recent past before the Chandra observation and the narrow Fe K? intensity had not yet responded to such a change, then the predicted line intensity and equivalent width for an origin in the BLR is within the 90% measurement errors. Anisotropic X-ray continuum illumination of the BLR and/or additional line emission from a torus structure would improve the agreement with observation. Two out of three archival ASCA data sets are consistent with the narrow line being present with the same intensity as in the Chandra observation. However, there is strong evidence that the narrow-line intensity varied and was unusually low during one of the ASCA campaigns. In general, inclusion of the narrow line to model the overall broad Fe K? line profile in terms of a rotating disk plus black-hole model can have a non-negligible effect on the disk line intensity and variability properties. Variability of the broad disk line in NGC 5548 is difficult to reconcile with the expectations of the simple disk model, even when the narrow-line component is accounted for. It will be critical to ascertain the importance of a similar nondisk Fe K? line in other Seyfert 1 galaxies. Future monitoring of the narrow Fe K? component with large collecting area and high spectral resolution will enable reverberation mapping the BLR region, complementary to similar studies using the optical/UV lines, and therefore provide independent constraints on the black-hole mass.

X-RAY CONSTRAINTS ON ACCRETION AND STARBURST PROCESSES IN GALACTIC NUCLEI. I. SPECTRAL RESULTS

The results of the analysis of 0.4-10.0 keV ASCA spectral analysis of a sample of low-luminosity active galactic nuclei (AGNs) (LLAGNs; M51, NGC 3147, and NGC 4258), low-ionization nuclear emission-line regions (LINERs; NGC 3079, NGC 3310, NGC 3998, NGC 4579, and NGC 4594), and starburst galaxies (M82, NGC 253, NGC 3628, and NGC 6946) are presented. In spite of the heterogeneous optical classifications of these galaxies, the X-ray spectra are fitted well by a canonical model consisting of an optically thin Raymond-Smith plasma soft component with T ~ 7 ? 106 K and a hard component that can be modeled by either a power law with a photon index ? ~ 1.7 or a thermal bremsstrahlung with T ~ 6 ? 107 K. The soft-component absorption is typically less than 1021 cm-2, while the hard component is typically absorbed by an additional column on the order of 1022 cm-2. The soft-component 0.4-10 keV intrinsic luminosities tend to be on the order 1039?40 ergs s-1, while the hard-component luminosities tend to be on the order of 1040?41 ergs s-1. The abundances inferred from the fits to the soft component are significantly subsolar. The Fe abundance can be measured independently of the other elemental abundances (dominated by ?-process elements) in M51, M82, NGC 253, and NGC 4258. In these galaxies the Fe abundance relative to ?-process elements is also (statistically) significantly subsolar. There is some indication (at a low statistical significance) that the abundance properties of starburst emission from starburst galaxies differs from the starburst emission from low-luminosity AGNs. However, these results on abundances are model dependent. Significant Fe K line emission is observed in M51, M82, NGC 3147, NGC 4258, and NGC 4579. An analysis of the short-term variability properties was given in Ptak et al. and detailed interpretation of these results will be given in Paper II in this series.

Spectral evolution of a long X-ray burst

Results are reported for spectroscopic observations of an X-ray burstlike event that had a duration of 100 sec and a peak count range 1.5 times that of the Crab Nebula. The best position determined is 356.4 deg galactic longitude, 2.3 deg galactic latitude. It is shown that the spectrum of the source changed as the event evolved, being very soft during the 20-sec peak of the burst, quickly hardening during the decay, and again softening as the decay proceeded. The spectra are fit best by a blackbody model with kT of 0.87 to 2.33 keV, and it is noted that a weak flux was detected some time after the burst and was well fitted by a 10-keV thermal bremsstrahlung spectrum with an indication of iron line emission. A source size is obtained which suggests a neutron star or a black hole of stellar mass.

X-radiation from clusters of galaxies - Spectral evidence for a hot evolved gas

OSO-8 observations of the X-ray flux in the range between 2 and 60 keV from the Virgo, Perseus, and Coma clusters provide strong evidence for the thermal origin of the radiation, including iron-line emission. The data are adequately described by emission from an isothermal plasma with an iron abundance in near agreement with cosmic levels. A power-law description is generally less acceptable and is ruled out in the case of Perseus. Implications of the origin of the cluster gas are discussed.

Evidence for a 16.6 day period from Circinus X-1

Analysis of All-Sky Monitor observations of Cir X-1 (3U1516-56) over the period October 1975 -- April 1976 revealed a well-defined modulation of the 3--6 keV flux at a period of 16.585 +- 0.01. The light-curve is characterized by an abrupt drop in emission occurring on a timescale of 0.07, with epoch JD 2,442,877.181 +- 0.07. No clear, correspondingly sharp increase in emission is observed during the cycle, so that a noneclipse origin for this effect cannot be ruled out. (auth)

Supermirror hard-x-ray telescope

The practical use of a grazing x-ray telescope is demonstrated for hard-x-ray imaging as hard as 40 keV by means of a depth-graded d-spacing multilayer, a so-called supermirror. Platinum-carbon multilayers of 26 layer pairs in three blocks with a different periodic length d of 3-5 nm were designed to enhance the reflectivity in the energy range from 24 to 36 keV at a grazing angle of 0.3 deg. The multilayers were deposited on thin-replica-foil mirrors by a magnetron dc sputtering system. The reflectivity was measured to be 25%-30% in this energy range; 20 mirror shells thus deposited were assembled into the tightly nested grazing-incidence telescope. The focused hard-x-ray image was observed with a newly developed position-sensitive CdZnTe solid-state detector. The angular resolution of this telescope was found to be 2.4 arc min in the half-power diameter.

X-ray spectral constraints on the broad-line cloud geometry of NGC 4151

X-ray spectral data from NGC 4151 taken with the Einstein Solid-State Spectrometer (SSS) and the HEAO 1 A-2 experiment cannot be simply reconciled with absorption from a uniform column of cold gas. The SSS data can, however, be explained in terms of a clumped absorber with approximately 10% uncovered fraction and factor-of-two overabundances in Z equal to or greater than 14 elements relative to solar oxygen. It is shown that these and previously reported spectral and variability data can be quantitatively reconciled with absorption arising in the cold clouds responsible for the broad optical line emission if the cloud dimensions are small compared to the central source size. It is suggested that the lack of significant X-ray absorption observed from much higher luminosity Seyferts and quasars is a natural consequence of the proposed picture for NGC 4151.