494 research outputs found
Hubble Space Telescope Imaging of the Outburst Site of M31 RV
M31 RV is a luminous red variable star that appeared for several months in
the bulge of M31 during 1988. Unlike classical novae, M31 RV was cool
throughout its outburst. Interest in this object has revived recently because
of its strong resemblance to V838 Mon, a luminous Galactic variable star that
appeared in 2002 and is illuminating a spectacular light echo, and has evolved
to ever-cooler surface temperatures. V4332 Sgr is a third object which was also
a red supergiant throughout its eruption.
We have examined archival Hubble Space Telescope (HST) images of the site of
M31 RV, obtained fortuitously in 1999 with the WFPC2 camera in parallel mode
during spectroscopic observations of the nucleus of M31. We located the site of
M31 RV in the HST frames precisely through astrometric registration with
ground-based CCD images, including several taken during the outburst. No light
echo is seen at the M31 RV site, implying that M31 RV is not surrounded by
circumstellar (or interstellar) dust similar to that around V838 Mon, or that
its extent is less than ~1.7 pc.
The stellar population at the outburst site consists purely of old red
giants; there is no young population, such as seen around V838 Mon. There are
no stars of unusual color at the site, suggesting that M31 RV had faded below
HST detectability in the 11 years since outburst, that it is an unresolved
companion of one of the red giants in the field, or that it is one of the red
giants. We suggest future observations that may help decide among these
possibilities.Comment: 16 pages, 3 figures, 2 tables; AJ accepte
Hadronic Regge Trajectories: Problems and Approaches
We scrutinized hadronic Regge trajectories in a framework of two different
models --- string and potential. Our results are compared with broad spectrum
of existing theoretical quark models and all experimental data from PDG98. It
was recognized that Regge trajectories for mesons and baryons are not straight
and parallel lines in general in the current resonance region both
experimentally and theoretically, but very often have appreciable curvature,
which is flavor-dependent. For a set of baryon Regge trajectories this fact is
well described in the considered potential model. The standard string models
predict linear trajectories at high angular momenta J with some form of
nonlinearity at low J.Comment: 15 pages, 9 figures, LaTe
Variability of Active Galactic Nuclei from the Optical to X-ray Regions
Some progress in understanding AGN variability is reviewed. Reprocessing of
X-ray radiation to produce significant amounts of longer-wavelength continua
seems to be ruled out. In some objects where there has been correlated X-ray
and optical variability, the amplitude of the optical variability has exceeded
the amplitude of X-ray variability. We suggest that accelerated particles
striking material could be linking X-ray and optical variability (as in
activity in the solar chromosphere). Beaming effects could be significant in
all types of AGN. The diversity in optical/X-ray relationships at different
times in the same object, and between different objects, might be explained by
changes in geometry and directions of motion relative to our line of sight.
Linear shot-noise models of the variability are ruled out; instead there must
be large-scale organization of variability. Variability occurs on
light-crossing timescales rather than viscous timescales and this probably
rules out the standard Shakura-Sunyaev accretion disk. Radio-loud and
radio-quiet AGNs have similar continuum shapes and similar variability
properties. This suggests similar continuum origins and variability mechanisms.
Despite their extreme X-ray variability, narrow-line Seyfert 1s (NLS1s) do not
show extreme optical variability.Comment: Invited talk given at Euro Asian Astronomical Society meeting in
Moscow, June 2002. 20 pages, 4 figures. References update
Transient Gamma Ray Spectrometer Measurements of Gamma-Ray Lines from Novae. II. Constraining the Galactic Nova Rate from a Survey of the Southern Sky during 1995-1997
The good energy resolution (3--4 keV FWHM) of the Transient Gamma Ray
Spectrometer (TGRS) on board the WIND spacecraft makes it sensitive to
Doppler-shifted outbursts of 511 keV electron-positron annihilation radiation,
the reason being that the Doppler shift causes the cosmic line to be slightly
offset from a strong instrumental background 511 keV line at rest, which is
ubiquitous in space environments. Such a cosmic line (blueshifted) is predicted
to arise in classical novae due to the annihilation of positrons from
-decay on a timescale of a few hours in an expanding envelope. A further
advantage of TGRS - its broad field of view, containing the entire southern
ecliptic hemisphere - has enabled us to make a virtually complete and unbiased
3-year search for classical novae at distances up to ~1 kpc. We present
negative results of this search, and estimate its implications for the
highly-uncertain Galactic classical nova rate and for future space missions.Comment: 22 pp. + 3 fig
Three-body correlations in direct reactions: Example of Be populated in reaction
The Be continuum states were populated in the charge-exchange reaction
H(Li,Be) collecting very high statistics data ( events) on the three-body ++ correlations. The
Be excitation energy region below MeV is considered, where the
data are dominated by contributions from the and states. It is
demonstrated how the high-statistics few-body correlation data can be used to
extract detailed information on the reaction mechanism. Such a derivation is
based on the fact that highly spin-aligned states are typically populated in
the direct reactions.Comment: submitted to Physical Review
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