The ROSAT HRI observations (2 pointings 3 years apart) may have already yielded a proper motion. Comparison of the positions, referenced to the postions of other sources in the field, give =0.4+/-0.2 arcsec/year to the east (a transverse velocity of 130 km s-1 at 100 pc). In this figure, the absolute positions are those of the HRI aspect solution from October 1994. The uncertainty of the 10/94 position is 1 pixel (0.5 arcsec); the errors on the 10/97 position reflect the uncertainty of aligning the other sources in the images.
The detection of a proper motion rules out accretion from ISM, because Bondi-Hoyle accretion rate decreases as the cube of the velocity.
The figure at left shows simulated AXAF
LETG spectra for a variety of surface compositions (Si-ash: purple;
This simulates a 200 Ksec exposure; noise has been added. The sharp edges are the instrument response. Note that the 4 surface compositions yield very different soft X-ray spectra; the Si-ash and Fe spectra have absorption lines. The wavelengths of the lines yield the gravitational redhift (nominally about 1.25).
For a Si-ash surface composition, we can determine also g
directly from gravity-sensitive lines.
The figure at left shows the predicted
Si-ash spectra, normalized to the black body, for
g=4 (white) and
2 (green) x1014(cgs). The lines at 19 and 35 Angstroms (rest wavelengths) are gravity-sensitive. This plot shows simulated data for a 200 Ksec observation; the planned GTO integration will likely not be sufficiently long to achieve this goal.
Note that while z is proportional to M/R, g is proportional to M/R2.
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