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

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

Observations of two super fast rotator NEAs: 2021 NY1_1 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$_1$ and 2022 AB. We obtained photometry and spectrophotometry of both targets and visible spectroscopy of 2022 AB. Light curves of 2021 NY$_1$ obtained in 4 different nights between Sept. 30 and Oct. 16, 2021 return a rotation period $P=13.3449\pm0.0013$ minutes and a light curve amplitude $A = 1.00$ mag. We found that 2021 NY$_1$ is a very elongated super fast rotator with an axis ratio $a/b \ge 3.6$. We also report colours $(g-r) = 0.664 \pm 0.013$, $(r-i) = 0.186 \pm 0.013$, and $(i-z_s) = -0.117 \pm 0.012$ 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 $P=3.0304\pm0.0008$ minutes, with amplitudes $A = 0.52$ and $A =0.54$ mag. 2022 AB is also an elongated object with axis ratio $a/b \ge 1.6$. The obtained colours are $(g-r) = 0.400 \pm 0.017$, $(r-i) = 0.133 \pm 0.017$, and $(i-z_s) = 0.093 \pm 0.016$. These colours are similar to those of the X-types, but with an unusually high $(g-r)$ 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 $\mu m$ 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

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