778 research outputs found
Hard X-ray imaging from Explorer
Coded aperture X-ray detectors were applied to obtain large increases in sensitivity as well as angular resolution. A hard X-ray coded aperture detector concept is described which enables very high sensitivity studies persistent hard X-ray sources and gamma ray bursts. Coded aperture imaging is employed so that approx. 2 min source locations can be derived within a 3 deg field of view. Gamma bursts were located initially to within approx. 2 deg and X-ray/hard X-ray spectra and timing, as well as precise locations, derived for possible burst afterglow emission. It is suggested that hard X-ray imaging should be conducted from an Explorer mission where long exposure times are possible
A two-dimensional Kolmogorov-Smirnov test for crowded field source detection: ROSAT sources in NGC 6397
We present a two-dimensional version of the classical one-dimensional
Kolmogorov-Smirnov (K-S) test, extending an earlier idea due to Peacock (1983)
and an implementation proposed by Fasano & Franceschini (1987). The
two-dimensional K-S test is used to optimise the goodness of fit in an
iterative source-detection scheme for astronomical images. The method is
applied to a ROSAT/HRI x-ray image of the post core-collapse globular cluster
NGC 6397 to determine the most probable source distribution in the cluster
core. Comparisons to other widely-used source detection methods, and to a
Chandra image of the same field, show that our iteration scheme is superior in
measuring statistics-limited sources in severely crowded fields.Comment: 12 pages, 6 figures, 6 tables. Accepted by MNRA
EXITE2 Observation of the SIGMA Source GRS 1227+025
We report the EXITE2 hard X-ray imaging of the sky around 3C273. A 2h
observation on May 8, 1997, shows a 260 mCrab source detected at
in each of two bands (50-70 and 70-93 keV) and located 30'
from 3C273 and consistent in position with the SIGMA source GRS1227+025. The
EXITE2 spectrum is consistent with a power law with photon index 3 and large
low energy absorption, as indicated by the GRANAT/SIGMA results. No source was
detected in more sensitive followup EXITE2 observations in 2000 and 2001 with
3 upper limits of 190 and 65 mCrab, respectively. Comparison with the
flux detected by SIGMA shows the source to be highly variable, suggesting it
may be non-thermal and beamed and thus the first example of a ``type 2''
(absorbed) Blazar. Alternatively it might be (an unprecedented) very highly
absorbed binary system undergoing accretion disk instability outbursts,
possibly either a magnetic CV, or a black hole X-ray nova.Comment: 12 pages, 4 figures, accepted for publication in Ap
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