52 research outputs found
BL Lac evolution revisited
BL Lac objects are an elusive and rare class of active galactic nuclei. For
years their evolutionary behavior has appeared inconsistent with the trend
observed in the population of AGN at large. The so-called ``negative''
evolution implies that BL Lacs were either less or fainter in the past. This
effect is stronger for BL Lacs selected in X-ray surveys. We have investigated
if one of the selection criteria, namely the flat-radio spectrum (imposed on
the Radio-selected but not on the X-ray-selected samples), might explain the
different evolutionary trend.Comment: Proceedings of "Multiwavelength AGN Surveys", Cozumel, Dec 200
Radio spectra of a sample of X-ray selected BL Lacs
We present simultaneous multifrequency radio observations for a complete
subsample of 26 XBLs from the Einstein Extended Medium-Sensitivity Survey,
obtained with the Very Large Array (VLA). Spectra are computed using fluxes at
20, 6 and 3.6 cm. Unlike many radio selected samples, the EMSS did not impose
any criterion on the radio spectrum to identify BL Lac objects. It is therefore
possible to investigate the intrinsic radio spectral slope distribution and to
determine the effect produced by this selection criterion. We find that 15% of
the observed objects do not meet the flat-spectrum criterion imposed on some
other BL Lac samples. A dataset that includes non-simultaneous data (that are
also taken with different VLA configurations) shows an even higher percentage
of steep spectrum sources. This effect can be ascribed to a larger fraction of
extended flux detected with the more compact VLA configuration.Possible biases
introduced by the flat--radio-spectrum criterion in the radio-selected BL Lac
samples cannot explain the discrepancies observed in the evolutionary
properties of Radio and X-ray selected samples of BL Lacs.Comment: 9 pages, 5 figures, to be published in Astronomy and Astrophysic
Processing Color in Astronomical Imagery
Every year, hundreds of images from telescopes on the ground and in space are
released to the public, making their way into popular culture through
everything from computer screens to postage stamps. These images span the
entire electromagnetic spectrum from radio waves to infrared light to X-rays
and gamma rays, a majority of which is undetectable to the human eye without
technology. Once these data are collected, one or more specialists must process
the data to create an image. Therefore, the creation of astronomical imagery
involves a series of choices. How do these choices affect the comprehension of
the science behind the images? What is the best way to represent data to a
non-expert? Should these choices be based on aesthetics, scientific veracity,
or is it possible to satisfy both? This paper reviews just one choice out of
the many made by astronomical image processors: color. The choice of color is
one of the most fundamental when creating an image taken with modern
telescopes. We briefly explore the concept of the image as translation,
particularly in the case of astronomical images from invisible portions of the
electromagnetic spectrum. After placing modern astronomical imagery and
photography in general in the context of its historical beginnings, we review
the standards (or lack thereof) in making the basic choice of color. We discuss
the possible implications for selecting one color palette over another in the
context of the appropriateness of using these images as science communication
products with a specific focus on how the non-expert perceives these images and
how that affects their trust in science. Finally, we share new data sets that
begin to look at these issues in scholarly research and discuss the need for a
more robust examination of this and other related topics in the future to
better understand the implications for science communications.Comment: 10 pages, 6 figures, published in Studies in Media and Communicatio
Image-Processing Techniques for the Creation of Presentation-Quality Astronomical Images
The quality of modern astronomical data, the power of modern computers and
the agility of current image-processing software enable the creation of
high-quality images in a purely digital form. The combination of these
technological advancements has created a new ability to make color astronomical
images. And in many ways it has led to a new philosophy towards how to create
them. A practical guide is presented on how to generate astronomical images
from research data with powerful image-processing programs. These programs use
a layering metaphor that allows for an unlimited number of astronomical
datasets to be combined in any desired color scheme, creating an immense
parameter space to be explored using an iterative approach. Several examples of
image creation are presented.
A philosophy is also presented on how to use color and composition to create
images that simultaneously highlight scientific detail and are aesthetically
appealing. This philosophy is necessary because most datasets do not correspond
to the wavelength range of sensitivity of the human eye. The use of visual
grammar, defined as the elements which affect the interpretation of an image,
can maximize the richness and detail in an image while maintaining scientific
accuracy. By properly using visual grammar, one can imply qualities that a
two-dimensional image intrinsically cannot show, such as depth, motion and
energy. In addition, composition can be used to engage viewers and keep them
interested for a longer period of time. The use of these techniques can result
in a striking image that will effectively convey the science within the image,
to scientists and to the public.Comment: 104 pages, 38 figures, submitted to A
X-Ray Spectral Variability of PKS 2005-489 During the Spectacular November 1998 Flare
We report on monitoring of the BL Lac object PKS 2005-489 by the Rossi X-ray
Timing Explorer (RXTE) in October-December 1998. During these months, the
source underwent a spectacular flare; at its peak on November 10, its 2-10 keV
flux was , over 30 times
brighter than in quiescence. During the rising phase, the X-ray spectrum of PKS
2005-489 hardened considerably, reaching near maximum. During the declining phase, the X-ray spectrum
steepened rapidly, reaching , then became somewhat harder
towards the end of December (). While such behavior has been
seen before, the simplicity, magnitude and duration of this flare allowed us to
study it in great detail. We argue that this flare was caused by either the
injection of particles into the jet or {\it in situ} particle acceleration, and
that the spectral steepening which followed the flare maximum was the result of
synchrotron cooling. Contrary to other recently observed blazar flares (e.g.,
Mkn 501, 3C 279, PKS 2155-304), our results do not imply a major shift in the
location of the synchrotron peak during this flare.Comment: ApJ Letters in press, 6 pages, 2 figures Corrected reference
BL Lac X-ray Spectra: simpler than we thought
We report results from {\it XMM-Newton} observations of thirteen X-ray bright
BL Lacertae objects, selected from the {\it Einstein} Slew Survey sample. The
spectra are generally well fit by power-law models, with four objects having
hard () spectra that indicates
synchrotron peaks at keV. None of our spectra show line features,
indicating that soft X-ray absorption ``notches'' must be rare amongst BL Lacs,
rather than common or ubiquitous as had previously been asserted. We find
significant curvature in most of the spectra. This curvature is almost
certainly intrinsic, as it appears nearly constant from 0.5 to 6 keV, an
observation which is inconsistent with the small columns seen in these sources.Comment: 4 pages, 1 figure; to be published in proceedings of the Cozumel
meeting on "Multiwavelength Surveys for AGN", Cozumel 200
CCD detector performance for NOAO's wide-field MOSAIC cameras
In July of 1998 the National Optical Astronomy Observatories (NOAO) successfully upgraded MOSAIC 1, an 8192 by 8192 pixel array using eight Scientific Imaging Technologies, Inc. (SITe) St-002A thinned backside 2k by 4k charge coupled devices (CCDs). In July of 1999 MOSAIC II, a clone of MOSAIC I was commissioned also using eight SITe ST-002A CCDs. Additionally in December of 1998 NOAO implemented Mini- MOSAIC a 4096 by 4096 pixel array using two SITe ST-002A thinned CCDs. This report will discuss the performance, characterization and capabilities of the three wide field imagers now in operation at NOAO's Kitt Peak Observatory, Cerro Tololo Inter-American Observatory and at the WIYN Consortium 3.5-Meter telescope on Kitt Peak
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