A limit to the X-ray luminosity of nearby normal galaxies

Emission is studied at luminosities lower than those for which individual discrete sources can be studied. It is shown that normal galaxies do not appear to provide the numerous low luminosity X-ray sources which could make up the 2-60 keV diffuse background. Indeed, upper limits suggest luminosities comparable with, or a little less than, that of the galaxy. This is consistent with the fact that the average optical luminosity of the sample galaxies within approximately 20 Mpc is slightly lower than that of the galaxy. An upper limit of approximately 1% of the diffuse background from such sources is derived

Low intensity H-beta emission from the interstellar medium

A search for diffuse galactic H beta emission not associated with any known H II regions was conducted using a 2-inch-diamenter pressure-scanned Fabry-Perot spectrometer at the Coude focus of a 36-inch telescope. Observations were made near the directions of four pulsars. Emissions with intensities from 40,000 to 400,000 photons/sq cm sec ster (corresponding to emission measures of approximately 10 - 100) were detected in three of the directions. The data indicate an average ionization rate (assuming steady state) of approximately 10 to the minus 14th power/H-atom sec for the interstellar hydrogen in these directions and temperatures between 1000 and 10,000 K for the emitting regions. Plans were made to continue the investigation of these very faint hydrogen emission sources using a 6-inch-diameter Fabry-Perot spectrometer

Rapid X-ray variability in the Seyfert galaxy NGC 6814

The HEAO-1 A-2 high time resolution X-ray observations of the X-ray emitting Seyfert I Galaxy NGC 6814 are reported. In sharp distinction with a sample of over 30 active galactic nuclei this object showed strong X-ray variability on timescales less than 3 hours. The mean flux on a timescale of 90 minutes varied by a factor of approximately 2.5 corresponding to Delta L sub x being approximately 1 x 10 to the 43rd power ergs/sec. An autocorrelation analysis shows a characteristic time for variability of 100 (+60 or -25) seconds. There is no indication of spectral variability with an upper limit on a change in the power law spectral index of the absolute value of Delta gamma .37, for a factor two change in intensity. The constraints of such rapid variability on a wide variety of X-ray source mechanisms are considered

HEAO 1 measurements of the galactic ridge

The HEAO A2 experiment data was systematically searched for unresolved galactic disc emission. Although there were suggestions of non-uniformities in the emission, the data were consistent with a disc of half-thickness 241 + 22 pc and surface emissivity (2-10 keV) at galactic radius R(kpc) of 2.2 10 to the minus 7th power exp(-R/3.5) erg/sq cm to the (-2)power/s (R 7.8 kpc). giving a luminosity of approximately 4.4 10 to the 37th power erg S to the (-1) power. If the model is extrapolated to radii less than 7.8 kpc, the unresolved disc emission is approximately 1.4 10 to the 38th power erg S to the (-1) power (2-10 keV) i.e., a few percent of the luminosity of the galaxy in resolved sources. the disc emission has a spectrum which is significantly softer than that of the high galactic latitude diffuse X-ray background and it is most probably of discrete source origin

Short-term temporal studies of the X ray emission from Cas A, Tycho and Sco X-1

No evidence for stable 2-10 keV periodic emission from Cas A or Tycho in the period range 1 msec to 10 sec is found. Upper limits to the pulsed fraction are presented as a function of the assumed light curve, with absolute 99% confidence upper limits of 0.089 and 0.195 for Cas A and Tycho, respectively. Previously reported transient 1-10 Hz oscillations from Sco X-1 are not observed

A change in the X-ray spectrum of MK 421

HEAO-1 experiment A-2 observations of the BL Lac object MK421 in May 1978 show a marked spectral change from the OSO-8 observations of May 1977. The source was not detected above 10 keV in May 1978. The 2-10 keV spectrum could be well fit by a power law of energy slope 2.2 is less than or minus 4.2; thermal bremsstrahlung models with T less than 2 X 10 to the 7th power deg K are also acceptable. There was no indication of any low energy turnover, so that the inferred column density N sub H is less than 7 X 10 to the 21st power at/sq cm. The total flux is consistent with an extrapolation of the UV data from IUE, but the slope is not consistent with the UV slope. Possible models for the origin of the spectral transition are discussed

A large scale height galactic component of the diffuse 2-60 keV background

The diffuse 2-60 keV X-ray background has a galactic component clearly detectable by its strong variation with both galactic latitude and longitude. This galactic component is typically 10 percent of the extragalactic background toward the galactic center, half that strong toward the anticenter, and extrapolated to a few percent of the extragalactic background toward the galactic poles. It is acceptably modeled by a finite radius emission disk with a scale height of several kiloparsecs. The averaged galactic spectrum is best fitted by a thermal spectrum of kT about 9 keV, a spectrum much softer than the about 40 keV spectrum of the extragalactic component. The most likely source of this emission is low luminosity stars with large scale heights such as subdwarfs. Inverse Compton emission from GeV electrons on the microwave background contributes only a fraction of the galactic component unless the local cosmic ray electron spectrum and intensity are atypical

HEAO-1 spectra of X-ray emitting Seyfert 1 galaxies

The A-2 experiment on HEAO-1 determined spectral parameters for seven Seyfert 1 galaxies: NGC3783, NGC4151, NGC5548, NGC6814, MK509, MCG8-11-11, and ES0141-G55. The X-ray spectra above 5 keV can be well fit by power laws of energy index alpha between .3 and 1.0 and, with the exception of MK509, by a high temperature (kt 15 keV) thermal bremsstrahlung spectrum. The column densities, with the exception of NGC4151, are less than 5 x 10 to the 22nd power at/sq cm with only the low luminosity objects having measurable columns. Galaxy ES0141-G55 showed a strong soft X-ray excess in March 1978 similar to that seen in the BL lac object MK421. Variability on a six month time scale was exhibited by MCG8-11-11, NGC3783, and possibly NGC6814. Various correlations between optical and X-ray properties are discussed. Using the typical Seyfert 1 spectrum their contribution to the diffuse X-ray background above five keV is computed

Cas A X-ray spectrum: Evidence for iron line emission

A sensitive measurement by rocket borne detectors of the X-ray flux from Cas A has revealed a steep continuum and a broad spectral feature in the region where line radiation from iron nuclei would be expected. The flux in this feature is .0122 plus or minus .0017 photons/sq cm/s: the total energy flux from 2 to 10 KeV is 1.02 x 10 to the minus 9th power ergs/sq cm/s. The presence of broad iron lines is consistent with a model in which approximately 13 MeV/nucleon iron nuclei charge exchange with surrounding interstellar oxygen and other heavy atoms. The model suggests that a substantial fraction of the energy from the outburst has gone into low energy cosmic rays which produce the observed HII region surrounding the remnant

The X-ray spectrum of 3C 273

An X-ray spectral measurement of the quasar 3C 273 with the HEAO-A2 experiment in June/July 1978 is reported. The best power law fit to the photon flux over the range 2-60 keV gives a slope of 1.41 + or - 0.02. However, structure is observed, indicating a slope of 1.52 between 2 keV and 9 keV and a slight flattening between 9 keV and 30 keV. Observations with the same experiment in December 1977 and OSO-8 in June 1976 allows confirmation of 40% intensity variability on the time scale of months, although within limits provided by the poorer statistical quality of the additional data no spectral change is discerned. Absorption from the source is found to be low, with the 1978 data yielding a 90% confidence upper limit to the hydrogen column density of 4.5 x 10 to the 21st power atoms/sq cm