3,171 research outputs found
Simultaneous X-ray and Radio Monitoring of the Unusual Binary LSI+61 303: Measurements of the Lightcurve and High-Energy Spectrum
The binary system, LSI+61 303, is unusual both because of the dramatic,
periodic, radio outbursts, and because of its possible association with the 100
MeV gamma-ray source, 2CG135+01. We have performed simultaneous radio and Rossi
X-ray Timing Explorer X-ray observations at eleven intervals over the 26.5 day
orbit, and in addition searched for variability on timescales ranging from
milliseconds to hours. We confirm the modulation of the X-ray emission on
orbital timescales originally reported by Taylor et al. (1996), and in addition
we find a significant offset between the peak of the X-ray and radio flux. We
argue that based on these results, the most likely X-ray emission mechanism is
inverse Compton scattering of stellar photons off of electrons accelerated at
the shock boundary between the relativistic wind of a young pulsar and the Be
star wind. In these observations we also detected 2 -- 150 keV flux from the
nearby low-redshift quasar QSO~0241+622. Comparing these measurements to
previous hard X-ray and gamma-ray observations of the region containing both
LSI+61 303 and QSO~0241+622, it is clear that emission from the QSO dominates.Comment: 23 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
Two types of shock in the hotspot of the giant quasar 4C74.26: a high-resolution comparison from Chandra, Gemini & MERLIN
New Chandra observations have resolved the structure of the X-ray luminous
southern hotspot in the giant radio quasar 4C74.26 into two distinct features.
The nearer one to the nucleus is an extremely luminous peak, extended some 5
kpc perpendicular to the orientation of the jet; 19 kpc projected further away
from the central nucleus than this is a fainter X-ray arc having similar
symmetry. This arc is co-spatial with near-IR and optical emission imaged with
Gemini, and radio emission imaged with MERLIN. The angular separation of the
double shock structure (itself ~19 kpc or 10 arcsec in size) from the active
nucleus which fuels them of ~550 kpc is a reminder of the challenge of
connecting "unidentified" hard X-ray or Fermi sources with their origins.Comment: In press at MNRA
Optical Intensity Interferometry with the Cherenkov Telescope Array
With its unprecedented light-collecting area for night-sky observations, the
Cherenkov Telescope Array (CTA) holds great potential for also optical stellar
astronomy, in particular as a multi-element intensity interferometer for
realizing imaging with sub-milliarcsecond angular resolution. Such an
order-of-magnitude increase of the spatial resolution achieved in optical
astronomy will reveal the surfaces of rotationally flattened stars with
structures in their circumstellar disks and winds, or the gas flows between
close binaries. Image reconstruction is feasible from the second-order
coherence of light, measured as the temporal correlations of arrival times
between photons recorded in different telescopes. This technique (once
pioneered by Hanbury Brown and Twiss) connects telescopes only with electronic
signals and is practically insensitive to atmospheric turbulence and to
imperfections in telescope optics. Detector and telescope requirements are very
similar to those for imaging air Cherenkov observatories, the main difference
being the signal processing (calculating cross correlations between single
camera pixels in pairs of telescopes). Observations of brighter stars are not
limited by sky brightness, permitting efficient CTA use during also bright-Moon
periods. While other concepts have been proposed to realize kilometer-scale
optical interferometers of conventional amplitude (phase-) type, both in space
and on the ground, their complexity places them much further into the future
than CTA, which thus could become the first kilometer-scale optical imager in
astronomy.Comment: Astroparticle Physics, in press; 47 pages, 10 figures, 124 reference
Tidal influence on Rutford Ice Stream, West Antarctica: observations of surface flow and basal processes from closely-spaced GPS and passive seismic stations
High-resolution surface velocity measurements and passive seismic observations from Rutford Ice Stream, West Antarctica, 40 km upstream from the grounding line are presented. These measurements indicate a complex relationship between the ocean tides and currents, basal conditions and ice-stream flow. Both the mean basal seismicity and the velocity of the ice stream are modulated by the tides. Seismic activity increases twice during each semi-diurnal tidal cycle. The tidal analysis shows the largest velocity variation is at the fortnightly period, with smaller variations superimposed at diurnal and semi-diurnal frequencies. The general pattern of the observed velocity is two velocity peaks during each semi-diurnal tidal cycle, but sometimes three peaks are observed. This pattern of two or three peaks is more regular during spring tides, when the largest-amplitude velocity variations are observed, than during neap tides. This is the first time that velocity and level of seismicity are shown to correlate and respond to tidal forcing as far as 40 km upstream from the grounding line of a large ice stream
Galactic-scale macro-engineering: Looking for signs of other intelligent species, as an exercise in hope for our own
If we consider Big History as simply 'our' example of the process of cosmic
evolution playing out, then we can seek to broaden our view of our possible
fate as a species by asking questions about what paths or trajectories other
species' own versions of Big History might take or have taken. This paper
explores the broad outlines of possible scenarios for the evolution of
long-lived intelligent engineering species---scenarios which might have been
part of another species' own Big History story, or which may yet lie ahead in
our own distant future. A sufficiently long-lived engineering-oriented species
may decide to undertake a program of macro-engineering projects that might
eventually lead to a re-engineered galaxy so altered that its artificiality may
be detectable from Earth. We consider activities that lead ultimately to a
galactic structure consisting of a central inner core surrounded by a more
distant ring of stars separated by a relatively sparser 'gap', where star
systems and stellar materials may have been removed, 'lifted' or turned into
Dyson Spheres. When one looks to the sky, one finds that such galaxies do
indeed exist---including the beautiful ringed galaxy known as 'Hoag's Object'
(PGC 54559) in the constellation Serpens. This leads us to pose the question:
Is Hoag's Object an example of galaxy-scale macro-engineering? And this
suggests a program of possible observational activities and theoretical
explorations, several of which are presented here, that could be carried out in
order to begin to investigate this beguiling question.Comment: 17 pages. Published in: Teaching and Researching Big History:
Exploring a New Scholarly Field; L. Grinin, D. Baker, E. Quaedackers and A.
Korotayev (eds). Uchitel Publ House, Volgograd, Russia, 2014, Chapter 14, pp.
283-304. ISBN: 978-5-7057-4027-
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