85 research outputs found
Extreme intranight variability in the BL Lacertae object AO 0235+164
We present results of two-colour photometry with high time resolution of the
violently variable BL Lac object AO 0235+164. We have found extreme intranight
variability with amplitudes of ~ 100 % over time scales of 24 hours. Changes of
0.5 magnitudes in both R and V bands were measured within a single night, and
variations up to 1.2 magnitudes occurred from night to night. A complete
outburst with an amplitude ~ 30 % was observed during one of the nights, while
the spectrum remained unchanged. This seems to support an origin based on a
thin relativistic shock propagating in such a way that it changes the viewing
angle, as recently suggested by Kraus et al. (1999) and Qian et al. (2000).Comment: 4 pages, 3 figures, to appear in Astronomy & Astrophysics (Letters
Discovery of a deep Seyfert-2 galaxy at z = 0.222 behind NGC 300
We report on the unveiling of the nature of the unidentified X-ray source
3XMM J005450.3-373849 as a Seyfert-2 galaxy located behind the spiral galaxy
NGC 300 using Hubble Space Telescope data, new spectroscopic Gemini
observations and available XMM-Newton and Chandra data. We show that the X-ray
source is positionally coincident with an extended optical source, composed by
a marginally resolved nucleus/bulge, surrounded by an elliptical disc-like
feature and two symmetrical outer rings. The optical spectrum is typical of a
Seyfert-2 galaxy redshifted to z=0.222 +/- 0.001, which confirms that the
source is not physically related to NGC 300. At this redshift the source would
be located at 909+/-4 Mpc (comoving distance in the standard model). The X-ray
spectra of the source are well-fitted by an absorbed power-law model. By tying
between the six available spectra, we found a variable index
running from ~2 in 2000-2001 years, to 1.4-1.6 in the 2005-2014
period. Alternatively, by tying , we found variable absorption columns
of N_H ~ 0.34 x cm in 2000-2001 years, and 0.54-0.75 x
cm in the 2005-2014 period. Although we cannot distinguish
between an spectral or absorption origin, from the derived unabsorbed X-ray
fluxes, we are able to assure the presence of long-term X-ray variability.
Furthermore, the unabsorbed X-ray luminosities of 0.8-2 x 10 erg
s derived in the X-ray band are in agreement with a weakly obscured
Seyfert-2 AGN at .Comment: MNRAS, accepte
Polarization and photometric observations of the gamma-ray blazar PG 1553+113
We present the results of an observational photo-polarimetry campaign of the
blazar PG 1553+113 at optical wavelengths. The blazar was recently detected at
very high energies (> 100 GeV) by the H.E.S.S and MAGIC gamma-ray Cherenkov
telescopes.
Our high-temporal resolution data show significant variations in the linear
polarization percentage and position angle at inter-night time-scales, while at
shorter (intra-night) time-scales both parameters varied less significantly, if
at all. Changes in the polarization angle seem to be common in gamma-ray
emitting blazars. Simultaneous differential photometry (through the B and R
bands) shows no significant variability in the total optical flux. We provide B
and R magnitudes, along with a finding chart, for a set of field stars suitable
for differential photometry.Comment: 4 pages, 3 figures. To be published by Astronomy and Astrophysic
Polarization microvariability of BL Lac objects
We present the results of a systematic observational campaign designed to
search for microvariability in the optical polarization of BL Lac objects. We
have observed a sample formed by 8 X-ray-selected and 10 radio-selected
sources, looking for rapid changes in both the degree of linear polarization
and the corresponding polarization angle. The whole campaign was carried out
along the last three years, and most of the objects were observed at least on
two consecutive nights. The statistical properties of both classes of BL Lac
objects are compared, and some general conclusions on the nature of the
phenomenon are drawn. In general, radio selected sources seem to display higher
duty cycles for polarimetric microvariability and, on average, they have a
stronger polarization.Comment: 12 pages, 15 figures, accepted for publication in A&
Soil salinization in marshes : study case: Samborombón Bay wetland, Argentina
En el sector sur de la BahÃa de Samborombón (Buenos Aires, Argentina) se desarrolla una extensa marisma que forma un humedal intermareal. En ella se realizaron distintas obras de ingenierÃa con el fin de evitar el ingreso del flujo mareal y lograr una mayor extensión del área de pastoreo. El objetivo del trabajo es evaluar si la regulación de los flujos mareales ocasiona una modificación en el contenido de cloruros en los sedimentos superficiales, afectando la salinidad de los suelos del humedal. Se realizaron interpretaciones de imágenes satelitales, relevamientos de campo, muestreos de sedimentos y mediciones de niveles y conductividad eléctrica del agua superficial y subterránea. Los resultados obtenidos permitieron reconocer cómo las obras de ingenierÃa (terraplenes de contención y caminos internos) alteran el comportamiento hidrológico natural y en consecuencia el contenido de cloruros en el sedimento. Problemas de salinización de suelos se reconocen en sectores donde el agua mareal se acumula y luego se evapora formando precipitados de halita. Las futuras obras deberán planificarse de forma tal de minimizar la alteración de los flujos de agua con el fin de lograr un manejo sustentable de los recursos naturales en el humedal.The southern sector of the Samborombón Bay (Buenos Aires, Argentina) is an
extensive marsh that is associated with an intertidal wetland environment. Different
engineering works were built to avoid tidal inflow and to increase the livestock grazing
areas. The objective of this work is to assess if the regulation of tidal inflow causes a
chloride content change in the surface sediments, affecting the soil salinity of the
wetland. Interpretation of satellite images, field surveys, sampling of sediments, and
electrical conductivity and levels measurements of surface and groundwater were
carried out. The results allowed us to recognize how engineering works (embankments and internal roads) alter the natural hydrological behavior, and, therefore the chloride content in the sediment. Soil salinization problems are recognized in areas where the tidal water accumulates and then evaporates forming halite precipitates. Future engineering works should be planned so as to minimize the alteration of the water flow in order to achieve a sustainable management of natural resources in the wetland.Fil: Carol, Eleonora S..
Universidad Nacional de La PlataFil: Kruse, Eduardo E..
Universidad Nacional de La PlataFil: Cellone, Francisco A..
Universidad Nacional de La Plat
The low-luminosity galaxy population in the NGC 5044 Group (Conference proceeding)
Detailed surface photometry for 79 (mostly dwarf) galaxies in the NGC 5044
Group area is analysed, revealing the existence of different morphologies among
objects originally classified as early-type dwarfs. Particularly, a significant
fraction of bright dwarf "ellipticals" show a distinct bulge+disc structure; we
thus re-classify these objects as dwarf lenticulars (dS0). Our finding points
at a possible scenario where these systems are the remnants of "harassed" disc
galaxies. This is emphasized by the discovery of a few objects with hints for
very low-surface brightness spiral-like structure. The colours, structure, and
spatial distribution of the different galaxy types suggest that our
classification may indeed be separating objects with different origins and/or
evolutionary paths.Comment: 5 pages. To appear in ESO Astrophysics Symposia: "Groups of Galaxies
in the Nearby Universe
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