B. Margon

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.

Extreme-ultraviolet observations of a flare on Proxima Centauri and implications concerning flare-star scaling theory

We report the first detection of EUV radiation from a UV Ceti-type flare star, Proxima Centauri, and derive an emission measure of 4.6 x 10/sup 52/ cm3/sup . ThisimpliesatotalthermalXhyphen/(1-250 A) of L/sub x/=7-18 x 10/sup 29/ ergs s/sup -1/. Using past X-ray flare observations of YZ CMi and UV Ceti and a recombination model, we derive limits on L/sub x//L/sub opt/ to test flare-star scaling theories. We conclude that L/sub x//L/sub opt/ in the observed flare is at variance with Mullans scaling law theory and may be highly variable among stellar flares.

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

Observations of the cosmic background radiation at 1440 angstroms

sup 0

The local interstellar helium density

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 spectroscopy of three extragalactic sources

sup 6

The atmosphere of Sirius B. II - Extreme-ultraviolet observations

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

A sensitive search for soft X-rays from Arcturus and Procyon

sup -10

Observation of X-ray emission from M31

ergs cm