Objective Grating Spectrometer Research Articles

A ubiquitous absorption feature in the X-ray spectra of BL Lacertae objects

We present the broad-band (0.5-20 keV) X-ray spectra of five X-ray bright BL Lac objects observed with the Einstein Observatory Solid State Spectrometer (SSS) and Monitor Proportional Counter (MPC) detectors. The combination of moderate energy resolution and broad spectral coverage allows us to confirm the presence of an absorption feature at an energy of ~ 650 eV in the BL Lac object PKS 2155 - 304, originally reported by Canizares & Kruper based on higher resolution Einstein Objective Grating Spectrometer (OGS) data. Furthermore, the SSS/MPC data indicate that such a feature, presumably due to O VIII Lyα resonant absorption, is present in all BL Lac objects in our sample. This suggests that such features are ubiquitous in at least X- ray-bright BL Lac objects.

Soft X-ray emission lines from low-mass X-ray binaries. I - Collision-dominated models

The soft X-ray line emission from five low-mass X-ray binaries observed by the Einstein Observatory (HEAO 2) Objective Grating Spectrometer is examined. Observed spectra are fitted to model spectra consisting of emission from an optically thin plasma. Fits are performed using a variety of different assumptions for elemental abundances, heating and ionization mechanisms, and the distribution of gas temperature and ionization parameter. The results are discussed in the context of current models for the accretion flow in low-mass X-ray binaries.

The performance of the penning gas imager

We describe a gas proportional counter of 120 mm diameter employing two stages of charge amplification and utilising a Wedge and Strip Anode two dimensional position readout. The two stage gas amplification allows upper stage gain to be optimised for energy resolution which is further improved by the use of a Penning gas mixture. The second stage of amplification allows the overall gain to be high; a requirement for good spatial resolution. Thus optimal energy and position resolution can be obtained simultaneously. We present pre-flight performance data from the Penning Gas Imager [1] in conjunction with its flight electronics which are being prepared for inclusion in a sounding rocket experiment, the X-ray Objective Grating Spectrometer (XOGS) to be launched in mid-1988.

X-ray spectral models of Galactic bulge sources - The emission-line factor

Current difficulties in finding unique and physically meaningful models for the X-ray spectra of Galactic bulge sources are exacerbated by the presence of strong, variable emission and absorption features that are not resolved by the instruments observing them. Nine Einstein solid state spectrometer (SSS) observations of five Galactic bulge sources are presented for which relatively high resolution objective grating spectrometer (OGS) data have been published. It is found that in every case the goodness of fit of simple models to SSS data is greatly improved by adding line features identified in the OGS that cannot be resolved by the SSS but nevertheless strongly influence the spectra observed by SSS.

X-ray emission lines from three Galactic bulge sources

X-ray spectroscopic observations obtained with the Einstein objective grating spectrometer (OGS) and monitor proportional counter (MPC) instruments of three Galactic bulge sources, the globular cluster burst source 1820-30, the burster Serpens X-1, and the GX 9+9 are presented. Joint spectral fits to the OGS and MPC data are consistent for all three sources with either a thermal bremsstrahlung model with temperature ranging from 6 to 10 keV or with a two-component blackbody model, where one component may be associated with the neutron star and one with the accretion disk. The spectra of Serpens X-1 and 1820-30 harden with increases in intensity. The implications of the results for recent models of low-mass X-ray binaries are discussed.

X-ray, optical and UV observations of the AM Her system E2003+225

The AM Her type object E2003+225 was observed with EXOSAT, IUE and ground-based telescopes on 1983 Oct. 12. The brightness of the ultrasoft X-ray component allowed the Objective Grating Spectrometer (OGS) to be used, which gave a model-independent determination of the temperature of the blackbody spectrum. The star was observed again on 1984 July 24 by IUE with simultaneous optical spectrophotometry. The high resolution of this observation revealed complex line profiles, and a systematic velocity much smaller than previously reported. The composite energy distribution is presented.

High-resolution soft X-ray spectra of Scorpius X-1 - The structure of circumsource accreting material

Four observations of Scorpius X-1 with the Objective Grating Spectrometer of the Einstein Observatory have provided high-resolution spectra (lambda/Delta lambda = approximately 20-50) in the wavelength range 7-46 A. The spectra reveal the presence of absorption structure due to oxygen, nitrogen, and iron, and variable emission structure associated with ionized iron and nitrogen. The strengths of these features suggest that the N/O abundance ratio in the absorbing and line emitting gas is anomalously high, which might indicate that these spectral components are associated with processed material, probably accreting matter transferred from the surface of an evolved companion. Constraints on the inclination of the system, however, imply that this cool, dense, accreting material must be well out of the plane of the binary system. Possible models for the origin and nature of this circumsource medium are discussed. An extensive discussion of the calibration of the Objective Grating Spectrometer and of the analysis of spectra acquired by that instrument is also provided.

The High Resolution Soft X-Ray Spectrum of AM Herculis

We present a high resolution (λ/Δλ = 80) Soft X-ray spectrum (44-95 Å) of the magnetic white dwarf binary AM Her obtained with the Objective Grating Spectrometer on board the Einstein observatory. Black body fits to the continuum strongly limits the range of allowable parameters and indicate a "color" temperature in the soft X-ray range between 40 and 55 eV (90% confidence limits). After egress from the eclipse the spectrum hardens and indicates a black body temperature increase from 38 eV to 52 eV. This is interpreted as a limb darkening effect of a layer with negative outward temperature gradient. Assuming the radiative flux to be constant, both the soft X-ray spectrum and the UV spectrum are consistent with a plane parallel layer in radiative equilibrium with an effective temperature of 20 eV. The color temperature in the soft X-ray range is insensitive to the effective temperature in such layers. A variable heating source at large optical depth associated with a variable accretion rate, changes the soft X-ray intensity but does not change the soft X-ray nor the UV spectral shape.