258 research outputs found
X-ray variability in M87
We present the evidence for X-ray variability from the core and from knot A
in the M87 jet based on data from two observations with the Einstein
Observatory High Resolution Imager (HRI) and three observations with the ROSAT
HRI. The core intensity showed a 16% increase in 17 months ('79-'80); a 12%
increase in the 3 years '92 to '95; and a 17% drop in the last half of 1995.
The intensity of knot A appears to have decreased by 16% between 92Jun and
95Dec. Although the core variability is consistent with general expectations
for AGN nuclei, the changes in knot A provide constraints on the x-ray emission
process and geometry. Thus we predict that the x-ray morphology of knot A will
differ significantly from the radio and optical structure.Comment: 9 pages latex plus 6 ps figs. To appear in Monthly Notices of the RA
Radio polarimetry of compact steep spectrum sources at sub-arcsecond resolution
Aims - We report new Very Large Array polarimetric observations of Compact
Steep-Spectrum (CSS) sources at 8.4, 15, and 23GHz. Methods - Using
multi-frequency VLA observations we have derived sub-arcsecond resolution
images of the total intensity, polarisation, and rotation measure (RM)
distributions. Results heading - We present multi-frequency VLA polarisation
observations of CSS sources. About half of the sources are point-like even at
the resolution of about 0.1x0.1 arcseconds. The remaining sources have double
or triple structure. Low values for the percentage of polarised emission in CSS
sources is confirmed. On the average, quasars are more polarised than galaxies.
A wide range of RM values have been measured. There are clear indications of
very large RMs up to 5\,585 rad m**(-2). CSS galaxies are characterized by RM
values that are larger than CSS quasars. The majority of the objects show very
large values of RM. Conclusions - The available data on sub-arcsecond-scale
rest-frame RM estimates for CSS sources show that these have a wide range of
values extending up to about 36,000 rad m**(-2). RM estimates indicate an
overall density of the magneto-ionic medium larger than classical radio
sources.Comment: Accepted for publication by Astronomy & Astrophysic
Using TI-Nspire to Engage Preservice Mathematics Teachers in an Exploratory Geometry Module
In the mathematics classroom, most preservice mathematics teachers possess basic skills to use technology as an instructional strategy in communicating content standards. However, today’s demands for preservice teachers to engage in a variety of “best teaching practices” in their preservice teaching and edTPA requirements can oftentimes place the acquisition of technical skills and integration of new technology in content curriculum far from the forefront of their minds. Ertmer, Conklin, Lewandowski, Osika, Selo, and Wignall (2003) acknowledged preservice teachers’ desires to gain the adequate technical skills necessary to use technology in teachers’ daily tasks of facilitating and managing their classrooms. They suggested that “in order to translate these skills into practice, teachers need specific ideas about how to use these skills to achieve meaningful learning outcomes under normal classroom conditions” (p. 96). Preservice teachers need guidance and information about “how, as well as why, to use technology in meaningful ways” so they can “develop their own visions for, or ideas about, meaningful technology use” (p. 96). Thus, the instructional aid of technology integration in the mathematics classroom must look to address specific uses of technology to help preservice mathematics teachers build awareness and confidence to implement innovative teaching approaches to enhance student learning
Paper 2: Using TI-Nspire to Engage Preservice Mathematics Teachers in an Exploratory Geometry Module
In the mathematics classroom, most preservice mathematics teachers possess basic skills to use technology as an instructional strategy in communicating content standards. However, today’s demands for preservice teachers to engage in a variety of “best teaching practices” in their preservice teaching and edTPA requirements can oftentimes place the acquisition of technical skills and integration of new technology in content curriculum far from the forefront of their minds. Ertmer, Conklin, Lewandowski, Osika, Selo, and Wignall (2003) acknowledged preservice teachers’ desires to gain the adequate technical skills necessary to use technology in teachers’ daily tasks of facilitating and managing their classrooms. They suggested that “in order to translate these skills into practice, teachers need specific ideas about how to use these skills to achieve meaningful learning outcomes under normal classroom conditions” (p. 96). Preservice teachers need guidance and information about “how, as well as why, to use technology in meaningful ways” so they can “develop their own visions for, or ideas about, meaningful technology use” (p. 96). Thus, the instructional aid of technology integration in the mathematics classroom must look to address specific uses of technology to help preservice mathematics teachers build awareness and confidence to implement innovative teaching approaches to enhance student learning
VLBA polarimetric observations of the CSS quasar 3C147
Aims. We report new VLBA polarimetric observations of the compact
steep-spectrum (CSS) quasar 3C147 (B0538+498) at 5 and 8.4GHz.
Methods. By using multifrequency VLBA observations, we derived
milliarcsecond-resolution images of the total intensity, polarisation, and
rotation measure distributions, by combining our new observations with archival
data.
Results. The source shows a one-sided structure, with a compact region, and a
component extending about 200 mas to the south-west. The compact region is
resolved into two main components with polarised emission, a complex rotation
measure distribution, and a magnetic field dominated by components
perpendicular to the source axis.
Conclusions. By considering all the available data, we examine the possible
location of the core component, and discuss two possible interpretations of the
observed structure of this source: core-jet and lobe-hot spot. Further
observations to unambiguously determine the location of the core would help
distinguish between the two possibilities discussed here.Comment: 11 pages, 12 figure
High Frequency VLBI Imaging of the Jet Base of M87
VLBA and Global VLBI observations of M87 at 43 GHz, some new and some
previously published, are used to study the structural evolution of the jet
with a spatial resolution of under 100 Schwarzschild radii. The images, taken
between 1999 and 2004, have an angular resolution of 0.00043" x 0.00021". An
edge-brightened jet structure and an indication of a large opening angle at the
jet base are seen in all five epochs. In addition, a probable counter-jet is
seen in the latter three epochs. A 22 GHz VLBA image also confirms many of the
structures seen at the higher frequency, including the counter-jet. A
comparison of the counter-jet flux density at 22 and 43 GHz reveals that it is
not free-free absorbed at these frequencies.
Attempts to obtain speeds from the proper motions of jet and counter-jet
components indicate that these observations are undersampled. The closest pair
of images gives apparent speeds of 0.25 to 0.40c for the jet and 0.17c for the
counter-jet. These speeds should be treated as lower limits because of possible
errors in associating components between epochs. If they are real, they
indicate that the jet is oriented 30-45 degrees from the line-of-sight and that
the component speeds along the jet are 0.3-0.5c. Using the jet orientation
derived from proper motions, the spectral index of the the counter-jet, and a
jet-to-counter-jet brightness ratio of 14.4, the inferred bulk flow is
0.6-0.7c, which, given the considerable uncertainties in how to measure the
brightness ratio, is not significantly larger than the component speed.Comment: 16 pages, 1 table, 3 color figures, accepted to the Astrophysical
Journa
Flaring X-ray Emission from HST-1, a Knot in the M87 Jet
We present Chandra X-ray monitoring of the M87 jet in 2002, which shows that
the intensity of HST-1, an optical knot 0.8" from the core, increased by a
factor of two in 116 days and a factor of four in 2 yrs. There was also a
significant flux decrease over two months, with suggestive evidence for a
softening of the spectrum. From this variability behavior, we argue that the
bulk of the X-ray emission of HST-1 comes from synchrotron emission. None of
the other conceivable emission processes can match the range of observed
characteristics. By estimating synchrotron model parameters for various bulk
relativistic velocities, we demonstrate that a model with a Doppler factor,
delta, in the range 2 to 5 fits our preliminary estimates of light travel time
and synchrotron loss timescales.Comment: 4 pages with 3 embedded figures; 1 of which is color but prints ok in
b/w. Accepted for publication in the ApJ
M87 as a misaligned Synchrotron-Proton Blazar
The giant radio galaxy M87 is usually classified as a Fanaroff-Riley class I
source, suggesting that M87 is a mis-aligned BL Lac object. Its unresolved
nuclear region emits strong non-thermal emission from radio to X-rays which has
been interpreted as synchrotron radiation. In an earlier paper we predicted M87
as a source of detectable gamma ray emission in the context of the hadronic
Synchrotron-Proton Blazar (SPB) model. The subsequent tentative detection of
TeV energy photons by the HEGRA-telescope array would, if confirmed, make it
the first radio galaxy to be detected at TeV-energies. We discuss the emission
from the unresolved nuclear region of M87 in the context of the SPB model, and
give examples of possible model representations of its non-simultaneous
spectral energy distribution. The low-energy component can be explained as
synchrotron radiation by a primary relativistic electron population that is
injected together with energetic protons into a highly magnetized emission
region. We find that the gamma-ray power output is dominated either by
mu/pi-synchrotron or proton synchrotron radiation depending on whether the
primary electron synchrotron component peaks at low or high energies,
respectively. The predicted gamma-ray luminosity peaks at around 100 GeV at a
level comparable to that of the low-energy hump, and this makes M87 a promising
candidate source for the newly-commissioned high-sensitivity low-threshold
Cherenkov telescopes H.E.S.S., VERITAS, MAGIC and CANGAROO III. (abridged)Comment: 12 pages, 4 figures, accepted for publication in A&
Opacity in compact extragalactic radio sources and its effect on astrophysical and astrometric studies
The apparent position of the "core" in a parsec-scale radio jet (a compact,
bright emitting region at the narrow end of the jet) depends on the observing
frequency, owing to synchrotron self-absorption and external absorption. While
providing a tool probing physical conditions in the vicinity of the core, this
dependency poses problems for astrometric studies using compact radio sources.
We investigated the frequency-dependent shift in the positions of the cores
(core shift) observed with very long baseline interferometry (VLBI) in
parsec-scale jets. We discuss related physics, as well as its effect on radio
astrometry and the connection between radio and optical positions of
astrometric reference objects. We searched for the core shift in a sample of
277 radio sources imaged at 2.3 GHz (13 cm) and 8.6 GHz (4 cm) frequency bands
using VLBI observations made in 2002 and 2003. The core shift was measured by
referencing the core position to optically thin jet features whose positions
are not expected to change with frequency. We present here results for 29
selected active galactic nuclei (AGN) with bright distinct VLBI jet features
that can be used in differential measurements and that allow robust
measurements of the shift to be made. In these AGN, the magnitude of the
measured core shift between 2.3 and 8.6 GHz reaches 1.4 mas, with a median
value for the sample of 0.44 mas. Nuclear flares result in temporal variability
of the shift. An average shift between the radio (4 cm) and optical (6000
Angstrom) bands is estimated to be approximately 0.1 mas, and it should be
taken into account in order to provide the required accuracy of the
radio-optical reference frame connection. This can be accomplished with
multi-frequency VLBI measurements... (abridged)Comment: 11 pages, 8 figures, 2 tables, accepted for publication in the
Astronomy and Astrophysics; minor corrections to the manuscript are mad
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