S. Bowyer

Parameter estimation in X-ray astronomy

The problems of model classification and parameter estimation are examined, with the objective of establishing the statistical reliability of inferences drawn from X-ray observations. For testing the validities of classes of models, the procedure based on minimizing the chi/sup 2/ statistic is recommended; it provides a rejection criterion at any desired significance level. Once a class of models has been accepted, a related procedure based on the increase of chi/sup 2/ gives a confidence region for the values of the models adjustable parameters. The procedure allows the confidence level to be chosen exactly, even for highly nonlinear models. Numerical experiments confirm the validity of the prescribed technique.The chi/sup 2//sub min/+1 error estimation method is evaluated and found unsuitable when several parameter ranges are to be derived, because it substantially underestimates their joint errors. The ratio of variances method, while formally correct, gives parameter confidence regions which are more variable than necessary. (AIP)

Discovery of a nonsolar extreme-ultraviolet source

Results are reported for observations of EUV emission from an extrasolar object which were made with a grazing-incidence telescope during the Apollo-Soyuz mission. The total energy flux in the spectral band between 170 and 620 A is estimated to be approximately 4 by 10 to the -9th power erg/sq cm per sec. It is shown that the data support the identification of the EUV object with the ultrasoft X-ray source in Coma Berenices. The source of both emissions is suggested to be the hot white dwarf HZ 43, located at right ascension 13 hr 43 min and declination +29 deg 22 min (1950). The temperature of HZ 43 is estimated as 110,000 K, giving it the highest temperature of any known white dwarf.

An intense extreme-ultraviolet source in Cetus

We report the detection of a second extrasolar extreme ultraviolet (EUV) (100--1000 A) object with the EUV telescope aboard the Apollo-Soyuz mission. The observed intensity in the 170--620 A band is 3 x 10/sup -9/ ergs cm/sup -2/ s/sup -1/, similar to that of HZ 43, the first known EUV star. The spectrum is significantly steeper than HZ 43, implying Tapproximately-less-than90,000 K. The positional error box of 3.8 deg/sup 2/ includes Feige 24, a binary system consisting of a very blue DA white dwarf and a dMe star. A grid of LTE pure-hydrogen, high-surface-gravity model atmospheres, in combination with the EUV data and optical spectrophotometry of the DA, is used to derive the stellar parameters. We find that T/sub eff/=60,000 K, R/sub asterisk/=17,000 (d/100 pc) km, and L=7(d/100 pc)/sup 2/ L/sub sun/ provides a satisfactory fit to the lambda>170 A data. Under the assumptions of this model, pure hydrogen atmospheres with white dwarf gravities predict fluxes for lambda<170 A which are in excess of the observations. Possible resolutions of this discrepancy are considered, such as the presence of trace amounts of atmospheric helium or ionized interstellar helium. The maximum density of neutral interstellar hydrogen implied simply by the detection ofmorexa0» a finite EUV flux is n/sub H/=0.01 (100 pc/d) cm/sup -3/. (AIP)«xa0less

An ultrasoft X-ray source in Coma Berenices

We have observed an intense soft X-ray source with an extraordinary spectrum in Coma Berenices, 4

Extreme ultraviolet observation of Sirius - Evidence against a photospheric origin of the 0.25 keV flux

sup 0

Soft X-ray morphology of the Virgo, Coma, and Perseus clusters of galaxies

northeast of and unassociated with the Coma cluster of galaxies. Two spectra, obtained at different times in a sounding rocket flight, indicate that the source temperature in thermal models is less than 10

Soft X-ray profile and spectrum of the supernova remnant IC 443

sup 6

Observations of the cosmic background radiation at 1440 angstroms

K; a power-law model requires photon power-law indices steeper than n=-3. The intensity in the 44--165 A band is of the order of 5x10

The local interstellar helium density

sup -10

The Discovery of Optical Emission Knots in the Inner Jet of Centaurus A

ergs cm