439 research outputs found
Discovery of the Orbit of the Transient X ray Pulsar SAX J2103.5+4545
Using X-ray data from the Rossi X-Ray Timing Explorer (RXTE), we carried out
pulse timing analysis of the transient X-ray pulsar SAX J2103.5+4545. An
outburst was detected by All Sky Monitor (ASM) October 25 1999 and reached a
peak X-ray brightness of 27 mCrab October 28. Between November 19 and December
27, the RXTE/PCA carried out pointed observations which provided us with pulse
arrival times. These yield an eccentric orbit (e= 0.4 \pm 0.2) with an orbital
period of 12.68 \pm 0.25 days and light travel time across the projected
semimajor axis of 72 \pm 6 sec. The pulse period was measured to be 358.62171
\pm 0.00088 s and the spin-up rate (2.50 \pm 0.15) \times 10^{-13} Hz s^{-1}.
The ASM data for the February to September 1997 outburst in which BeppoSAX
discovered SAX J2103.5+4545 (Hulleman, in't Zand and Heise 1998) are modulated
at time scales close to the orbital period. Folded light curves of the 1997 ASM
data and the 1999 PCA data are similar and show that the intensity increases at
periastron passages.Comment: To appear in The Astrophysical Journal (Letters
Emission Spectra of Fallback Disks Around Young Neutron Stars
The nature of the energy source powering anomalous X-ray pulsars is
uncertain. Proposed scenarios involve either an ultramagnetized neutron star,
or accretion onto a neutron star. We consider the accretion model proposed
recently by Chatterjee, Hernquist & Narayan, in which a disk is fed by fallback
material following a supernova. We compute the optical, infrared, and
submillimeter emission expected from such a disk, including both viscous
dissipation and reradiation of X-ray flux impinging on the disk from the
pulsar. We find that it is possible with current instruments to put serious
constraints on this and on other accretion models of AXPs. Fallback disks could
also be found around isolated radio pulsars and we compute the corresponding
spectra. We show that the excess emission in the R and I bands observed for the
pulsar PSR 0656+14 is broadly consistent with emission from a disk.Comment: 12 pages, 1 table, 4 figures, submitted to Ap
The Spin Period, Luminosity and Age Distributions of Anomalous X-Ray Pulsars
We consider the accretion model for anomalous X-ray pulsars proposed recently
by Chatterjee, Hernquist and Narayan, in which the emission is powered by
accretion from a fossil disk formed by the fallback of material from a
supernova explosion. We demonstrate that this model is able to account for the
spin period, luminosity and age distributions of the observed population of
AXPs for reasonable and broad distributions of the free parameters of the
model, namely, the surface magnetic field of the neutron star, the mass of its
accretion disk and its initial spin period. In particular, this model is able
statistically to account for the puzzlingly narrow observed spin distribution
of the AXPs. We show also that if the establishment of fallback accretion disks
around isolated neutron stars is a universal phenomenon, then a fairly large
minority () of these objects become X-ray bright AXPs or X-ray faint
systems spinning down by propeller action, while the rest become radio pulsars.Comment: 11 pages, 2 figures; accepted for publication in Ap
Consequences of Interstellar Absorption for Models of Anomalous X-ray Pulsars
We examine properties of thermal radiation emitted by strongly magnetized
neutron stars (NSs). In particular, we show that the pulsation amplitudes of
the energy-integrated flux are an increasing function of the absorption column
density to the source. This is especially important for the interpretation of
the Anomalous -ray Pulsars (AXPs) as cooling neutron stars with high
magnetic fields. We show that the high-pulsation amplitudes observed in these
objects are consistent with cooling models, if the large amount of absorption
to these sources is taken into account. We also show that cooling models imply
inferred radii of the emitting regions on the order of times smaller
than the actual NS radii, again in agreement with observations.Comment: 6 pages, 2 figures, ApJL in pres
From both sides now: Crossover effects influence navigation in patients with unilateral neglect
Unilateral neglect is a challenging disorder that pervades a range of behaviours following stroke and hampers recovery. Although a preponderance of clinical studies measure performance on a range of bedside assessments, including line bisection and cancellation tasks, there have been calls for studies to embrace more relevant functional measures. Here, for the first time, we present data from two separate tasks that characterise the performance of seven patients with unilateral neglect when navigating a power wheelchair. The tasks involved negotiating an obstacle course and steering a central path between gaps of different sizes. Results from the obstacle course confirmed the clinical observation and predicted bias of contralesional errors. However, the second task revealed a robust "crossover" effect. Patients deviated to the ipsilesional side for large gaps but deviated increasingly contralesionally when steering through small gaps in behaviour that was analogous to that previously shown on line bisection tasks. Contrary to being seen as an unintuitive finding, further analysis of these errors suggests that patients are giving disproportionate weight to the location of the ipsilesional object when plotting a midline course between two objects. Our results provide a platform for further studies to investigate the modulation and rehabilitation of this important skill
Imaging X-ray, Optical, and Infrared Observations of the Transient Anomalous X-ray Pulsar XTE J1810-197
We report X-ray imaging, timing, and spectral studies of XTE J1810-197, a
5.54s pulsar discovered by Ibrahim et al. (2003) in recent RXTE observations.
In a set of short exposures with the Chandra HRC camera we detect a strongly
modulated signal (55+/-4% pulsed fraction) with the expected period located at
(J2000) 18:09:51.08, -19:43:51.7, with a uncertainty radius of 0.6 arcsec (90%
C.L.). Spectra obtained with XMM-Newton are well fitted by a two-component
model that typically describes anomalous X-ray pulsars (AXPs), an absorbed
blackbody plus power law with parameters kT = 0.67+/-0.01 keV, Gamma=3.7+/-0.2,
N_H=(1.05+/-0.05)E22 cm^-2, and Fx(0.5-10 keV) = 3.98E-11 ergs/cm2/s.
Alternatively, a 2T blackbody fit is just as acceptable. The location of CXOU
J180951.1-194351 is consistent with a point source seen in archival Einstein,
Rosat, & ASCA images, when its flux was nearly two orders-of-magnitude fainter,
and from which no pulsations are found. The spectrum changed dramatically
between the "quiescent" and "active" states, the former can be modeled as a
softer blackbody. Using XMM timing data, we place an upper limit of 0.03 lt-s
on any orbital motion in the period range 10m-8hr. Optical and infrared images
obtained on the SMARTS 1.3m telescope at CTIO show no object in the Chandra
error circle to limits V=22.5, I=21.3, J=18.9, & K=17.5. Together, these
results argue that CXOU J180951.1-194351 is an isolated neutron star, one most
similar to the transient AXP AX J1844.8-0256. Continuing study of XTE J1810-197
in various states of luminosity is important for understanding and possibly
unifying a growing class of isolated, young neutron stars that are not powered
by rotation.Comment: 12 pages, 7 figures, AAS LaTex, uses emulateapj5.sty. Updated to
include additional archival data and a new HRC observation. To appear in The
Astrophysical Journa
Visual Search Without Selective Attention: A Cognitive Architecture Account
A key phenomenon in visual search experiments is the linear relation of reaction time (RT) to the number of objects to be searched (set size). The dominant theory of visual search claims that this is a result of covert selective attention operating sequentially to âbindâ visual features into objects, and this mechanism operates differently depending on the nature of the search task and the visual features involved, causing the slope of the RT as a function of set size to range from zero to large values. However, a cognitive architectural model presented here shows these effects on RT in three different search task conditions can be easily obtained from basic visual mechanisms, eye movements, and simple task strategies. No selective attention mechanism is needed. In addition, there are littleâexplored effects of visual crowding, which is typically confounded with set size in visual search experiments. Including a simple mechanism for crowding in the model also allows it to account for significant effects on error rate (ER). The resulting model shows the interaction between visual mechanisms and task strategy, and thus it represents a more comprehensive and fruitful approach to visual search than the dominant theory.Visual Search without Selective Attention calls into question the necessity of a covert selective attention mechanism by implementing a formal model that includes basic visual mechanisms, saccades, and simple task strategies. Across three search tasks, the model accounts for response times as well as the proportion of errors observed in human participants, including effects of item crowding in the visual stimulus.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/147754/1/tops12406.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147754/2/tops12406_am.pd
General Relativistic Constraints on Emission Models of Anomalous X-ray Pulsars
Most models of anomalous X-ray pulsars (AXPs) account for the observed X-ray
spectra and pulsations by means of radiation processes that occur on the
surfaces of neutron stars. For any such model, general relativistic deflection
of light severely suppresses the amplitude of the observed pulsations. We
calculate the expected pulsation amplitudes of AXPs according to various models
and compare the results with observations. We show that the high (<= 70%) pulse
amplitudes observed in some AXPs can be accounted for only if the surface
emission is localized (spot radius <40 degrees) and strongly beamed
(cos^n[theta'] with n>2, where theta' is the angle to the normal). These
constraints are incompatible with those cooling and magnetar models in which
the observed X-rays originate as thermal emission from the neutron-star
surface. Accretion models, on the other hand, are compatible with observations
for a wide range of parameters. Finally, definitive conclusions cannot be
reached on magnetospheric models, since their localization and beaming
properties are not well understood.Comment: 7 pages, 9 figures, submitted to The Astrophysical Journa
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