841 research outputs found
PANIC: A Near-infrared Camera for the Magellan Telescopes
PANIC (Persson's Auxiliary Nasmyth Infrared Camera) is a near-infrared camera
designed to operate at any one of the f/11 folded ports of the 6.5m Magellan
telescopes at Las Campanas Observatory, Chile. The instrument is built around a
simple, all-refractive design that reimages the Magellan focal plane to a plate
scale of 0.125'' pixel^{-1} onto a Rockwell 1024x1024 HgCdTe detector. The
design goals for PANIC included excellent image quality to sample the superb
seeing measured with the Magellan telescopes, high throughput, a relatively
short construction time, and low cost. PANIC has now been in regular operation
for over one year and has proved to be highly reliable and produce excellent
images. The best recorded image quality has been ~0.2'' FWHM.Comment: 8 pages, 5 figures. To appear in "Astronomical Telescopes and
Instrumentation," Proc SPIE (Glasgow), June 2004. Version with higher
resolution figures is available at
http://cfa-www.harvard.edu/~pmartini/professional/publications/panic.pd
The optical depth of the Universe to ultrahigh energy cosmic ray scattering in the magnetized large scale structure
This paper provides an analytical description of the transport of ultrahigh
energy cosmic rays in an inhomogeneously magnetized intergalactic medium. This
latter is modeled as a collection of magnetized scattering centers such as
radio cocoons, magnetized galactic winds, clusters or magnetized filaments of
large scale structure, with negligible magnetic fields in between. Magnetic
deflection is no longer a continuous process, it is rather dominated by
scattering events. We study the interaction between high energy cosmic rays and
the scattering agents. We then compute the optical depth of the Universe to
cosmic ray scattering and discuss the phenomological consequences for various
source scenarios. For typical parameters of the scattering centers, the optical
depth is greater than unity at 5x10^{19}eV, but the total angular deflection is
smaller than unity. One important consequence of this scenario is the
possibility that the last scattering center encountered by a cosmic ray be
mistaken with the source of this cosmic ray. In particular, we suggest that
part of the correlation recently reported by the Pierre Auger Observatory may
be affected by such delusion: this experiment may be observing in part the last
scattering surface of ultrahigh energy cosmic rays rather than their source
population. Since the optical depth falls rapidly with increasing energy, one
should probe the arrival directions of the highest energy events beyond
10^{20}eV on an event by event basis to circumvent this effect.Comment: version to appear in PRD; substantial improvements: extended
introduction, sections added on angular images and on direction dependent
effects with sky maps of optical depth, enlarged discussion of Auger results
(conclusions unchanged); 27 pages, 9 figure
Shot Noise Enhancement in Resonant Tunneling Structures in a Magnetic Field
We have observed that the shot noise of tunnel current, I, in
GaSb-AlSb-InAs-AlSb-GaSb double-barrier structure under a magnetic field can
exceed 2qI. The measurements were done at T=4K in fields up to 5T parallel to
the current. The noise enhancement occurred at each of the several
negative-differential conductance regions induced by the tunneling of holes
through Landau levels in the InAs quantum well. The amount of the enhancement
increased with the strength of the negative conductance and reached values up
to 8qI. These results are explained qualitatively by fluctuations of the
density of states in the well, but point out the need for a detailed theory of
shot noise enhancement in resonant tunneling devices.Comment: 4 pages, RevTex, 3 figure
X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks
We study the properties of powerful X-ray flares from 161 pre-main sequence
(PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula
region. Relationships between flare properties, protoplanetary disks and
accretion are examined in detail to test models of star-disk interactions at
the inner edge of the accretion disks. Previous studies had found no
differences in flaring between diskfree and accreting systems other than a
small overall diminution of X-ray luminosity in accreting systems. The most
important finding is that X-ray coronal extents in fast-rotating diskfree stars
can significantly exceed the Keplerian corotation radius, whereas X-ray loop
sizes in disky and accreting systems do not exceed the corotation radius. This
is consistent with models of star-disk magnetic interaction where the inner
disk truncates and confines the PMS stellar magnetosphere. We also find two
differences between flares in accreting and diskfree PMS stars. First, a
subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are
preferentially present in accreting systems. Second, we tentatively find that
accreting stars produce flares with shorter durations. Both results may be
consequences of the distortion and destabilization of the stellar magnetosphere
by the interacting disk. Finally, we find no evidence that any flare types,
even slow-rise flat-top flares are produced in star-disk magnetic loops. All
are consistent with enhanced solar long-duration events with both footprints
anchored in the stellar surface.Comment: Accepted for publication in ApJ (07/17/08); 46 pages, 14 figures, 2
table
X-rays in the Orion Nebula Cluster: Constraints on the origins of magnetic activity in pre-main sequence stars
A recent Chandra/ACIS observation of the Orion Nebula Cluster detected 1075
sources (Feigelson et al. 2002), providing a uniquely large and well-defined
sample to study the dependence of magnetic activity on bulk properties for
stars descending the Hayashi tracks. The following results are obtained: (1)
X-ray luminosities L_t in the 0.5-8 keV band are strongly correlated with
bolometric luminosity with = -3.8 for stars with masses 0.7<M<2
Mo, an order of magnitude below the main sequence saturation level; (2) the
X-ray emission drops rapidly below this level in some or all stars with 2<M<3
Mo; (3) the presence or absence of infrared circumstellar disks has no apparent
relation to X-ray levels; and (4) X-ray luminosities exhibit a slight rise as
rotational periods increase from 0.4 to 20 days. This last finding stands in
dramatic contrast to the strong anticorrelation between X-rays and period seen
in main sequence stars.
The absence of a strong X-ray/rotation relationship in PMS stars, and
particularly the high X-ray values seen in some very slowly rotating stars, is
a clear indication that the mechanisms of magnetic field generation differ from
those operating in main sequence stars. The most promising possibility is a
turbulent dynamo distributed throughout the deep convection zone, but other
models such as alpha-Omega dynamo with `supersaturation' or relic core fields
are not immediately excluded. The drop in magnetic activity in
intermediate-mass stars may reflect the presence of a significant radiative
core. The evidence does not support X-ray production in large-scale star-disk
magnetic fields.Comment: 51 pages, 8 figures. To appear in the Astrophysical Journa
Rotation and X-ray emission from protostars
The ASCA satellite has recently detected variable hard X-ray emission from
two Class I protostars in the rho Oph cloud, YLW15 (IRS43) and WL6, with a
characteristic time scale ~20h. In YLW15, the X-ray emission is in the form of
quasi-periodic energetic flares, which we explain in terms of strong magnetic
shearing and reconnection between the central star and the accretion disk. In
WL6, X-ray flaring is rotationally modulated, and appears to be more like the
solar-type magnetic activity ubiquitous on T Tauri stars. We find that YLW15 is
a fast rotator (near break-up), while WL6 rotates with a significantly longer
period. We derive a mass M_\star ~ 2 M_\odot and \simlt 0.4 M_\odot for the
central stars of YLW15 and WL6 respectively. On the long term, the interactions
between the star and the disk results in magnetic braking and angular momentum
loss of the star. On time scales t_{br} ~ a few 10^5 yrs, i.e., of the same
order as the estimated duration of the Class~I protostar stage. Close to the
birthline there must be a mass-rotation relation, t_{br} \simpropto M_\star,
such that stars with M_\star \simgt 1-2 M_\odot are fast rotators, while their
lower-mass counterparts have had the time to spin down. The rapid rotation and
strong star-disk magnetic interactions of YLW15 also naturally explain the
observation of X-ray ``superflares''. In the case of YLW15, and perhaps also of
other protostars, a hot coronal wind (T~10^6 K) may be responsible for the VLA
thermal radio emission. This paper thus proposes the first clues to the
rotation status and evolution of protostars.Comment: 13 pages with 6 figures. To be published in ApJ (April 10, 2000 Part
1 issue
Multifrequency Radiation of Extragalactic Large-Scale Jets
Large-scale extragalactic jets, observed to extend from a few to a few
hundred kiloparsecs from active galactic nuclei, are now studied over many
decades in frequency of electromagnetic spectrum, from radio until (possibly)
TeV gamma rays. For hundreds of known radio jets, only about 25 are observed at
optical frequencies. Most of them are relatively short and faint, with only a
few exceptions, like 3C 273 or M 87, allowing for detailed spectroscopic and
morphological studies. Somewhat surprisingly, the large-scale jets can be very
prominent in X-rays. Up to now, about 25 jets were detected within the 1 - 10
keV energy range, although the nature of this emission is still under debate.
In general, both optical and X-ray jet observations present serious problems
for standard radiation models for the considered objects. Recent TeV
observations of M 87 suggest the possibility of generating large photon fluxes
at these high energies by its extended jet.
In this paper we summarize information about multiwavelength emission of the
large-scale jets, and we point out several modifications of the standard jet
radiation models (connected with relativistic bulk velocities, jet radial
stratification and particle energization all the way along the jet), which can
possibly explain some of the mentioned puzzling observations. We also comment
on gamma-ray emission of the discussed objects.Comment: 29 pages. Modified version, accepted for publication in Chinese
Journal of Astronomy and Astrophysic
- …