35 research outputs found
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
The Rosette Eye: the key transition phase in the birth of a massive star
Massive protostars dramatically influence their surroundings via
accretion-induced outflows and intense radiation fields. They evolve rapidly,
the disk and infalling envelope being evaporated and dissipated in
10 years. Consequently, they are very rare and investigating this important
phase of early stellar evolution is extremely difficult. Here we present the
discovery of a key transient phase in the emergence of a massive young star, in
which ultraviolet radiation from the new-born giant has just punctured through
its natal core. The massive young stellar object AFGL 961 II is readily
resolved in the near infrared. Its morphology closely resembles a cat's eye and
is here dubbed as the Rosette Eye. Emerging ionized flows blow out an hourglass
shaped nebula, which, along with the existence of strong near-infrared excess,
suggests the existence of an accretion disk in the perpendicular direction. The
lobes of the hourglass, however, are capped with arcs of static H
emission produced by fluorescence. This study has strong implications for our
understanding of how massive stars embark on their formation.Comment: 3 figure
The Radio Structure of High-Energy Peaked BL Lacertae Objects
We present VLA and first-epoch VLBA observations that are part of a program
to study the parsec-scale radio structure of a sample of fifteen
high-energy-peaked BL Lacs (HBLs). The sample was chosen to span the range of
logarithmic X-ray to radio flux ratios observed in HBLs. As this is only the
first epoch of observations, proper motions of jet components are not yet
available; thus we consider only the structure and alignment of the parsec- and
kiloparsec-scale jets. Like most low-energy-peaked BL Lacs (LBLs), our HBL
sample shows parsec-scale, core-jet morphologies and compact, complex
kiloparsec-scale morphologies. Some objects also show evidence for bending of
the jet 10-20pc from the core, suggesting interaction of the jet with the
surrounding medium. Whereas LBLs show a wide distribution of parsec- to
kpc-scale jet misalignment angles, there is weak evidence that the jets in HBLs
are more well-aligned, suggesting that HBL jets are either intrinsically
straighter or are seen further off-axis than LBL jets.Comment: Accepted, A
An Excess of Mg II Absorbers in BL Lac Objects
Two new Mg II absorbers are presented (z=1.340 in S5 0454+844 and z=1.117 in
PKS 2029+121), bringing the total number of Mg II systems in the 1 Jy
radio-selected BL Lac sample to 10. Five of the ten absorption systems are at W
> 1A; this is a factor of four to five greater than the number expected based
upon quasar sightlines, and is 2.5 to 3 sigma greater than the expectation
value. Interpretations of this possible excess include either that some of the
Mg II absorbers might be intrinsic to the BL Lac or that there is a correlation
between the presence of absorbing gas in the foreground and the nearly
featureless spectra of these BL Lac Objects compared to quasars. Such a
correlation can be created by gravitational microlensing as suggested by
Ostriker & Vietri. The similarity between the optical spectra of BL Lacs with
Mg II absorption and the spectrum of the Gamma-ray burst source GRB 970508
suggests that models of Gamma-ray bursts as microlensed AGN should be
investigated.Comment: LaTeX, AASTeX, 3 figures: jy0138.eps, jy0454.eps, jy2029.eps ApJL,
Accepted September 2nd, 199
High-Resolution Radio Imaging of Gravitational Lensing Candidates in the 1 Jansky BL Lac Sample
While BL Lacertae objects are widely believed to be highly beamed,
low-luminosity radio galaxies, many radio-selected BL Lacs have extended radio
power levels and optical emission lines that are too luminous to be
low-luminosity radio galaxies. Also, Stocke & Rector discovered an excess of
MgII absorption systems along BL Lac sightlines compared to quasars, suggesting
that gravitational lensing may be another means of creating the BL Lac
phenomenon in some cases. We present a search for gravitationally-lensed BL
Lacs with deep, high-resolution, two-frequency VLA radio maps of seven lensing
candidates from the 1 Jansky BL Lac sample. We find that none of these objects
are resolved into an Einstein ring like B 0218+357, nor do any show multiple
images of the core. All of the lensing candidates that were resolved show a
flat-spectrum core and very unusual, steep-spectrum extended morphology that is
incompatible with a multiply lensed system. Thus, while these observations do
not rule out microlensing, no macrolensing is observed.Comment: Accepted, A