253 research outputs found
Digital Image Compression Using Artificial Neural Networks
The problem of storing, transmitting, and manipulating digital images is considered. Because of the file sizes involved, large amounts of digitized image information are becoming common in modern projects. Our goal is to described an image compression transform coder based on artificial neural networks techniques (NNCTC). A comparison of the compression results obtained from digital astronomical images by the NNCTC and the method used in the compression of the digitized sky survey from the Space Telescope Science Institute based on the H-transform is performed in order to assess the reliability of the NNCTC
Short-timescale Fluctuations in the Difference Light Curves of QSO 0957+561A,B: Microlensing or Noise?
From optical R band data of the double quasar QSO 0957+561A,B, we made two
new difference light curves (about 330 days of overlap between the time-shifted
light curve for the A image and the magnitude-shifted light curve for the B
image). We observed noisy behaviours around the zero line and no
short-timescale events (with a duration of months), where the term event refers
to a prominent feature that may be due to microlensing or another source of
variability. Only one event lasting two weeks and rising - 33 mmag was found.
Measured constraints on the possible microlensing variability can be used to
obtain information on the granularity of the dark matter in the main lensing
galaxy and the size of the source. In addition, one can also test the ability
of the observational noise to cause the rms averages and the local features of
the difference signals. We focused on this last issue. The combined
photometries were related to a process consisting of an intrinsic signal plus a
Gaussian observational noise. The intrinsic signal has been assumed to be
either a smooth function (polynomial) or a smooth function plus a stationary
noise process or a correlated stationary process. Using these three pictures
without microlensing, we derived some models totally consistent with the
observations. We finally discussed the sensitivity of our telescope (at Teide
Observatory) to several classes of microlensing variability.Comment: MNRAS, in press (LaTeX, 14 pages, 22 eps figures
A Large Brightness Enhancement of the QSO 0957+561 A Component
We report an increase of more than 0.2 mag in the optical brightness of the
leading image (A) of the gravitational lens Q0957+561, detected during the
09/2000 -- 06/2001 monitoring campaign (2001 observing season). The brightening
is similar to or even greater than the largest change ever detected during the
20 years of monitoring of this system. We discuss two different provisional
explanations to this event: intrinsic source variability or microlensing
(either short timescale microlensing or cessation of the historical
microlensing). An exhaustive photometric monitoring of Q0957+561 is needed
until summer of 2002 and during 2003 to discriminate between these
possibilities.Comment: 13 pages including 3 figures and 1 table. Accepted for publication in
ApJ Let
Optical studies of the X-ray transient XTE J2123-058 - II. Phase-resolved spectroscopy
We present time-resolved spectroscopy of the soft X-ray transient XTE
J2123-058 in outburst. A useful spectral coverage of 3700-6700A was achieved
spanning two orbits of the binary, with single epoch coverage extending to
\~9000A. The optical spectrum approximates a steep blue power-law, consistent
with emission on the Rayleigh-Jeans tail of a hot black body spectrum. The
strongest spectral lines are HeII 4686A and CIII/NIII 4640A (Bowen blend) in
emission. Their relative strengths suggest that XTE J2123-058 was formed in the
Galactic plane, not in the halo. Other weak emission lines of HeII and CIV are
present and Balmer lines show a complex structure, blended with HeII. HeII
4686A profiles show a complex multiple S-wave structure with the strongest
component appearing at low velocities in the lower-left quadrant of a Doppler
tomogram. H alpha shows transient absorption between phases 0.35-0.55. Both of
these effects appear to be analogous to similar behaviour in SW Sex type
cataclysmic variables. We therefore consider whether the spectral line
behaviour of XTE J2123-058 can be explained by the same models invoked for
those systems.Comment: Accepted for publication in MNRAS. 12 pages, 12 postscript figure
Observations of two super fast rotator NEAs: 2021 NY and 2022 AB
In the framework of the Visible NEAs Observations Survey (ViNOS) that uses
several telescopes at the Canary Islands observatories since 2018, we observed
two super fast rotator NEAs, 2021 NY and 2022 AB. We obtained photometry
and spectrophotometry of both targets and visible spectroscopy of 2022 AB.
Light curves of 2021 NY obtained in 4 different nights between Sept. 30 and
Oct. 16, 2021 return a rotation period minutes and a light
curve amplitude mag. We found that 2021 NY is a very elongated
super fast rotator with an axis ratio . We also report colours
, , and mag. These are compatible with an S-type asteroid. The light curves of
2022 AB obtained on Jan. 5 and Jan. 8, 2021 show a rotation period
minutes, with amplitudes and mag. 2022
AB is also an elongated object with axis ratio . The obtained
colours are , , and . These colours are similar to those of the X-types, but with
an unusually high value. Spectra obtained on Jan. 12 and Jan. 14, 2022,
are consistent with the reported colours. The spectral upturn over the 0.4 -
0.6 region of 2022 AB does not fit with any known asteroid taxonomical
class or meteorite spectrum, confirming its unusual surface properties.Comment: 9 pages, 7 figure
Recurrence of the blue wing enhancements in the high ionization lines of SDSS 1004+4112 A
We present integral field spectroscopic observations of the quadruple-lensed
QSO SDSS 1004+4112 taken with the fiber system INTEGRAL at the William Herschel
Telescope on 2004 January 19. In May 2003 a blueward enhancement in the high
ionization lines of SDSS 1004+4112A was detected and then faded. Our
observations are the first to note a second event of similar characteristics
less than one year after. Although initially attributed to microlensing, the
resemblance among the spectra of both events and the absence of
microlensing-induced changes in the continuum of component A are puzzling. The
lack of a convincing explanation under the microlensing or intrinsic
variability hypotheses makes the observed enhancements particularly relevant,
calling for close monitoring of this object.Comment: 4 pages, 5 figure
New VR magnification ratios of QSO 0957+561
We present VR magnification ratios of QSO 0957+561, which are inferred from
the GLITP light curves of Q0957+561A and new frames taken with the 2.56m Nordic
Optical Telescope about 14 months after the GLITP monitoring. From two
photometric approaches and a reasonable range for the time delay in the system
(415-430 days), we do not obtain achromatic optical continuum ratios, but
ratios depending on the wavelength. These new measurements are consistent with
differential extinction in the lens galaxy, the Lyman limit system, the damped
Ly-alpha system, or the host galaxy of the QSO. The possible values for the
differential extinction and the ratio of total to selective extinction in the V
band are reasonable. Moreover, crude probability arguments suggest that the ray
paths of the two components cross a similar dusty environment, including a
network of compact dust clouds and compact dust voids. As an alternative (in
fact, the usual interpretation of the old ratios), we also try to explain the
new ratios as caused by gravitational microlensing in the deflector. From
magnification maps for each of the gravitationally lensed images, using
different fractions of the surface mass density represented by the microlenses,
as well as different sizes and profiles of the V-band and R-band sources,
several synthetic distributions of V-band and R-band ratios are derived. In
some gravitational scenarios, there is an apparent disagreement between the
observed pair of ratios and the simulated distributions. However, several
microlensing pictures work well. To decide between either extinction, or
microlensing, or a mixed scenario (extinction + microlensing), new
observational and interpretation efforts are required.Comment: PS and PDF versions are created from the LaTeX file and 5 EPS
figures, two additional figues (Figs. 6 and 7) in JPEG format, scheduled for
the ApJ 20 January 2005 issu
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