1,371 research outputs found
Post's correspondence problem for hyperbolic and virtually nilpotent groups
Post's Correspondence Problem (the PCP) is a classical decision problem in
theoretical computer science that asks whether for pairs of free monoid
morphisms there exists any non-trivial
such that .
Post's Correspondence Problem for a group takes pairs of group
homomorphisms instead, and similarly asks
whether there exists an such that holds for non-elementary
reasons. The restrictions imposed on in order to get non-elementary
solutions lead to several interpretations of the problem; we consider the
natural restriction asking that and prove that
the resulting interpretation of the PCP is undecidable for arbitrary hyperbolic
, but decidable when is virtually nilpotent. We also study
this problem for group constructions such as subgroups, direct products and
finite extensions. This problem is equivalent to an interpretation due to
Myasnikov, Nikolev and Ushakov when one map is injective.Comment: 17 page
PROPERTIES OF THE 24 DAY MODULATION IN GX 13+1 FROM NEAR-INFRARED AND X-RAY OBSERVATIONS
A 24 day period for the low-mass X-ray binary (LMXB) GX 13+1 was previously proposed on the basis of seven years of RXTE All-Sky Monitor (ASM) observations and it was suggested that this was the orbital period of the system. This would make it one of the longest known orbital periods for a Galactic LMXB powered by Roche lobe overflow. We present here the results of (1) K-band photometry obtained with the SMARTS Consortium CTIO 1.3 m telescope on 68 nights over a 10 month interval; (2) continued monitoring with the RXTE ASM, analyzed using a semi-weighted power spectrum instead of the data filtering technique previously used; and (3) Swift Burst Alert Telescope (BAT) hard X-ray observations. Modulation near 24 days is seen in both the K band and additional statistically independent ASM X-ray observations. However, the modulation in the ASM is not strictly periodic. The periodicity is also not detected in the Swift BAT observations, but modulation at the same relative level as seen with the ASM cannot be ruled out. If the 24 day period is the orbital period of system, this implies that the X-ray modulation is caused by structure that is not fixed in location. A possible mechanism for the X-ray modulation is the dipping behavior recently reported from XMM-Newton observations
X-ray Nova XTE J1550-564: RXTE Spectral Observations
Excellent coverage of the 1998 outburst of the X-ray Nova XTE J1550-564 was
provided by the Rossi X-ray Timing Explorer. XTE J1550-564 exhibited an intense
(6.8 Crab) flare on 1998 September 19 (UT), making it the brightest new X-ray
source observed with RXTE. We present a spectral analysis utilizing 60
Proportional Counter Array spectra from 2.5-20 keV spanning 71 days, and a
nearly continuous All Sky Monitor light curve. The spectra were fit to a model
including multicolor blackbody disk and power-law components. XTE J1550-564 is
observed in the very high, high/soft, and intermediate canonical outburst
states of Black Hole X-ray Novae.Comment: 14 pages including 1 table and 4 figures, Accepted to ApJ Letter
Self Organization and a Dynamical Transition in Traffic Flow Models
A simple model that describes traffic flow in two dimensions is studied. A
sharp {\it jamming transition } is found that separates between the low density
dynamical phase in which all cars move at maximal speed and the high density
jammed phase in which they are all stuck. Self organization effects in both
phases are studied and discussed.Comment: 6 pages, 4 figure
ROSSI X-RAY TIMING EXPLORER AND BeppoSAX OBSERVATIONS OF THE TRANSIENT X-RAY PULSAR XTE J1859+083
We present observations of the 9.8 s X-ray pulsar XTE J1859+083 made with the All Sky Monitor (ASM) and Proportional Counter Array (PCA) on board the Rossi X-ray Timing Explorer, and the Wide Field Camera (WFC) on board BeppoSAX. The ASM data cover a 12 year time interval and show that an extended outburst occurred between approximately MJD 50,250 and 50,460 (1996 June 16 to 1997 January 12). The ASM data excluding this outburst interval suggest a possible modulation with a period of 60.65 ± 0.08 days. Eighteen sets of PCA observations were obtained over an approximately one month interval in 1999. The flux variability measured with the PCA appears consistent with the possible period found with the ASM. The PCA measurements of the pulse period showed it to decrease nonmonotonically and then to increase significantly. Doppler shifts due to orbital motion rather than accretion torques appear to be better able to explain the pulse period changes. Observations with the WFC during the extended outburst give a position that is consistent with a previously determined PCA error box, but which has a significantly smaller error. The transient nature of XTE J1859+083 and the length of its pulse period are consistent with it being a Be/neutron star binary. The possible 60.65 day orbital period would be of the expected length for a Be star system with a 9.8 s pulse period
An Interdisciplinary Approach to a Dental Information Technology Course
Purpose: To develop an interdisciplinary course to teach dental students about evidence-based dentistry, development of search strategies, critical appraisal of literature, and dental informatics. [See PDF for complete abstract
An Extended and More Sensitive Search for Periodicities in RXTE/ASM X-ray Light Curves
We present the results of a systematic search in approximately 14 years of
Rossi X-ray Timing Explorer All-Sky Monitor data for evidence of periodicities
not reported by Wen et al. (2006). Two variations of the commonly used Fourier
analysis search method have been employed to achieve significant improvements
in sensitivity. The use of these methods and the accumulation of additional
data have resulted in the detection of the signatures of the orbital periods of
eight low-mass X-ray binary systems and of ten high-mass X-ray binaries not
listed in the tables of Wen et al.Comment: 20 pages, 22 figures, in emulateapj format; submitted to ApJ
Complete RXTE Spectral Observations of the Black Hole X-ray Nova XTE J1550-564
We report on the X-ray spectral behavior of the exceptionally bright X-ray
nova XTE J1550-564 during its 1998-99 outburst. Our study is based on 209
pointed observations using the PCA and HEXTE instruments onboard the Rossi
X-ray Timing Explorer spanning 250 days and covering the entire double-peaked
eruption that occurred from 1998 September until 1999 May. The spectra are fit
to a model including multicolor blackbody disk and power-law components. The
source is observed in the very high and high/soft outburst states of black hole
X-ray novae. During the very high state, when the power-law component dominated
the spectrum, the inner disk radius is observed to vary by more than an order
of magnitude; the radius decreased by a factor of 16 in one day during a 6.8
Crab flare. If the larger of these observed radii is taken to be the last
stable orbit, then the smaller observed radius would imply that the inner edge
of the disk is inside the event horizon! However, we conclude that the apparent
variations of the inner disk radius observed during periods of increased
power-law emission are probably caused by the failure of the multicolor
disk/power-law model; the actual physical radius of the inner disk may remain
fairly constant. This interpretation is supported by the fact that the observed
inner disk radius remains approximately constant over 120 days in the high
state, when the power-law component is weak, even though the disk flux and
total flux vary by an order of magnitude. The mass of the black hole inferred
by equating the approximately constant inner disk radius observed in the
high/soft state with the last stable orbit for a Schwarzschild black hole is
M_BH = 7.4 M_sun (D/6 kpc) (cos i)^{-1/2}.Comment: Submitted to ApJ, 20 pages including 6 figures + 4 large table
DISCOVERY OF TWO NEW THERMALLY BLOATED LOW-MASS WHITE DWARFS AMONG THE KEPLER BINARIES
We report the discovery of two new low-mass, thermally bloated, hot white dwarfs among the Kepler sample of eclipsing binaries. These are KIC 9164561 and KIC 10727668 with orbital periods of 1.2670 and 2.3058 days, respectively. The current primary in both systems is an A star of about 2 M[subscript ʘ]. This brings the number of similar binaries among the Kepler sample to six, and the two new systems have the shortest orbital periods among them. The white dwarf (WD) in KIC 9164561 has the largest thermal bloating, compared to its cold degenerate radius, of about a factor of 14. We utilize radial velocity measurements of the A star in KIC 9164561 to determine the WD mass rather accurately: 0.197 ± 0.005 M[subscript ʘ]. The mass of the WD in KIC 10727668 is based on the Doppler boosting signal in the Kepler photometry and is less accurately determined to be 0.266 ± 0.035 M[subscript ʘ]. Based on the inferred radii and effective temperatures of these two white dwarfs, we are able to make an independent theoretical estimate of their masses to within ~0.01 M[subscript ʘ] based on evolutionary models of their cooling history after they lose their hydrogen-rich envelopes. We also present evidence that there is a third body in the KIC 9164561 system with an orbital period of 8–14 yr.Spain. Ministerio de Economia y Competitividad (Grant ESP2013-48391-C4-2 R)National Aeronautics and Space Administration (Kepler Participating Scientist Program
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