1,780 research outputs found
Modeling Belief in Dynamic Systems, Part II: Revision and Update
The study of belief change has been an active area in philosophy and AI. In
recent years two special cases of belief change, belief revision and belief
update, have been studied in detail. In a companion paper (Friedman & Halpern,
1997), we introduce a new framework to model belief change. This framework
combines temporal and epistemic modalities with a notion of plausibility,
allowing us to examine the change of beliefs over time. In this paper, we show
how belief revision and belief update can be captured in our framework. This
allows us to compare the assumptions made by each method, and to better
understand the principles underlying them. In particular, it shows that Katsuno
and Mendelzon's notion of belief update (Katsuno & Mendelzon, 1991a) depends on
several strong assumptions that may limit its applicability in artificial
intelligence. Finally, our analysis allow us to identify a notion of minimal
change that underlies a broad range of belief change operations including
revision and update.Comment: See http://www.jair.org/ for other files accompanying this articl
Probabilistic Algorithmic Knowledge
The framework of algorithmic knowledge assumes that agents use deterministic
knowledge algorithms to compute the facts they explicitly know. We extend the
framework to allow for randomized knowledge algorithms. We then characterize
the information provided by a randomized knowledge algorithm when its answers
have some probability of being incorrect. We formalize this information in
terms of evidence; a randomized knowledge algorithm returning ``Yes'' to a
query about a fact \phi provides evidence for \phi being true. Finally, we
discuss the extent to which this evidence can be used as a basis for decisions.Comment: 26 pages. A preliminary version appeared in Proc. 9th Conference on
Theoretical Aspects of Rationality and Knowledge (TARK'03
Optical and X-ray Spectroscopy of 1E 0449.4-1823: Demise of the original type 2 QSO
New optical spectra of the original narrow-line quasar 1E 0449.4-1823 show
that it now has broad emission lines of considerable strength, eliminating it
as a "type 2 QSO" candidate. We suggest that the behavior of 1E 0449.4-1823 is
the same as that of some Seyfert 1.8 and 1.9 galaxies, in which Goodrich
attributed long-term variations of their broad Balmer lines to dynamical
motions of obscuring material located in or around the broad-line region. The
optical continuum and broad emission-line regions of 1E 0449.4-1823 may still
be partly covered in our line of sight, which would explain its large
forbidden-line equivalent widths and flat alpha_ox relative to other
low-redshift QSOs. Also present are apparent absorption features in the broad
Balmer lines and in Mg II, which may be related to the past obscuration and
current emergence of the broad-line region. However, it is difficult to
distinguish absorption from broad emission-line peaks that are displaced in
velocity; we consider the latter a plausible competing interpretation of these
peculiar line profiles. An ASCA X-ray spectrum of 1E 0449.4-1823 can be fitted
with a power-law of Gamma = 1.63, intrinsic N_H < 9 x 10^20 cm-2, and no Fe
Kalpha line emission. Its 2-10 keV luminosity is 6.7 x 10^44 ergs/s. With
regard to the still hypothetical type 2 QSOs, we argue that there is little
evidence for the existence of any among X-ray selected samples.Comment: to appear in ApJ July 1, 1998 (vol. 501). 23 pages including figures.
Uses LaTeX macro aasms4.st
New X-ray Constraints on Starburst and Seyfert Activity in the Barred Spiral Galaxy NGC 1672
The nearby barred spiral galaxy NGC 1672 shows dramatic starburst activity
and may also host a Seyfert 2 nucleus. We present new X-ray observations that
set constraints on starburst and Seyfert activity in NGC 1672. Two ROSAT HRI
exposures, taken in 1992 and 1997, are used to investigate long-term
variability of the known X-ray sources and to search for new sources of X-ray
emission. We find large-amplitude (about 69%) variability from X-3, one of the
off-nuclear sources located near an end of the galactic bar. X-3 has a peak
observed 0.2-2.0 keV luminosity of about 2.5 x 10^{39} erg/s, and it is
probably a luminous X-ray binary or young supernova remnant. We do not observe
variability of the nuclear source X-1 or the strong off-nuclear source X-2. Our
analyses also reveal two new off-nuclear sources, one of which is associated
with a bright region along a spiral arm, and we find evidence for large-scale
diffuse X-ray emission throughout part of the disk of NGC 1672. Furthermore, we
use ASCA data taken in 1995 to constrain the hard X-ray properties of NGC 1672.
While the nuclear source X-1 is the dominant soft X-ray source in NGC 1672, we
find that the bulk of the 2-10 keV and 5-10 keV emission is spatially
coincident with the off-nuclear source X-3, giving it an apparent 0.2-8 keV
luminosity of about 6 x 10^{39} erg/s. A power-law plus Raymond-Smith model
provides an acceptable fit to the full-band ASCA spectra. We do not find any
evidence for a luminous but absorbed nuclear X-ray source. If there is a
luminous Seyfert 2 nucleus in NGC 1672, it must be obscured by a
`Compton-thick' torus with a column density of > 2 x 10^{24} cm^{-2}.Comment: 19 pages, AJ, accepted, also available from
http://www.astro.psu.edu/users/niel/papers/papers.htm
Wigner's little group and Berry's phase for massless particles
The ``little group'' for massless particles (namely, the Lorentz
transformations that leave a null vector invariant) is isomorphic to
the Euclidean group E2: translations and rotations in a plane. We show how to
obtain explicitly the rotation angle of E2 as a function of and we
relate that angle to Berry's topological phase. Some particles admit both signs
of helicity, and it is then possible to define a reduced density matrix for
their polarization. However, that density matrix is physically meaningless,
because it has no transformation law under the Lorentz group, even under
ordinary rotations.Comment: 4 pages revte
Search for a Point-Source Counterpart of the Unidentified Gamma-Ray Source TeV J2032+4130 in Cygnus
We have made a multiwavelength study of the overlapping error boxes of the
unidentified gamma-ray sources TeV J2032+4130 and 3EG J2033+4118 in the
direction of the Cygnus OB2 association (d = 1.7 kpc) in order to search for a
point-source counterpart of the first unidentified TeV source. Optical
identifications and spectroscopic classifications for the brighter X-ray
sources in ROSAT PSPC and Chandra ACIS images are obtained, without finding a
compelling counterpart. The classified X-ray sources are a mix of early and
late-type stars, with one exception. The brightest source in the Chandra
observation is a new, hard absorbed source that is both transient and rapidly
variable. It lies 7' from the centroid of the TeV emission, which places it
outside of the claimed 2 sigma location (r = 4.8'). A possible eclipse or "dip"
transition is seen in its light curve. With a peak 1-10 keV luminosity of 7 x
10^(32) (d/1.7 kpc})^2 ergs s^(-1), this source could be a quiescent low-mass
X-ray binary that lies beyond the Cyg OB2 association. A coincident, reddened
optical object of R = 20.4, J = 15.4, H = 14.2, and K = 13.4 is observed, but
not yet classified due to the lack of obvious emission or absorption features
in its spectrum. Alternatively, this Chandra} and optical source might be a
considered a candidate for a proton blazar, a long hypothesized type of
radio-weak gamma-ray source. More detailed observations will be needed to
determine the nature of this variable X-ray source, and to assess the
possibility of its connection with TeV J2032+4130.Comment: 24 pages, 8 figures, accepted by Ap
The Optical Design and Characterization of the Microwave Anisotropy Probe
The primary goal of the MAP satellite, now in orbit, is to make high fidelity
polarization sensitive maps of the full sky in five frequency bands between 20
and 100 GHz. From these maps we will characterize the properties of the cosmic
microwave background (CMB) anisotropy and Galactic and extragalactic emission
on angular scales ranging from the effective beam size, <0.23 degree, to the
full sky. MAP is a differential microwave radiometer. Two back-to-back shaped
offset Gregorian telescopes feed two mirror symmetric arrays of ten corrugated
feeds. We describe the prelaunch design and characterization of the optical
system, compare the optical models to the measurements, and consider multiple
possible sources of systematic error.Comment: ApJ in press; 22 pages with 11 low resolution figures; paper is
available with higher quality figures at
http://map.gsfc.nasa.gov/m_mm/tp_links.htm
Model Checking CTL is Almost Always Inherently Sequential
The model checking problem for CTL is known to be P-complete (Clarke,
Emerson, and Sistla (1986), see Schnoebelen (2002)). We consider fragments of
CTL obtained by restricting the use of temporal modalities or the use of
negations---restrictions already studied for LTL by Sistla and Clarke (1985)
and Markey (2004). For all these fragments, except for the trivial case without
any temporal operator, we systematically prove model checking to be either
inherently sequential (P-complete) or very efficiently parallelizable
(LOGCFL-complete). For most fragments, however, model checking for CTL is
already P-complete. Hence our results indicate that, in cases where the
combined complexity is of relevance, approaching CTL model checking by
parallelism cannot be expected to result in any significant speedup. We also
completely determine the complexity of the model checking problem for all
fragments of the extensions ECTL, CTL+, and ECTL+
Multifrequency Strategies for the Identification of Gamma-Ray Sources
More than half the sources in the Third EGRET (3EG) catalog have no firmly
established counterparts at other wavelengths and are unidentified. Some of
these unidentified sources have remained a mystery since the first surveys of
the gamma-ray sky with the COS-B satellite. The unidentified sources generally
have large error circles, and finding counterparts has often been a challenging
job. A multiwavelength approach, using X-ray, optical, and radio data, is often
needed to understand the nature of these sources. This chapter reviews the
technique of identification of EGRET sources using multiwavelength studies of
the gamma-ray fields.Comment: 35 pages, 22 figures. Chapter prepared for the book "Cosmic Gamma-ray
Sources", edited by K.S. Cheng and G.E. Romero, to be published by Kluwer
Academic Press, 2004. For complete article and higher resolution figures, go
to: http://www.astro.columbia.edu/~muk/mukherjee_multiwave.pd
The ASCA X-Ray Spectrum Of The Broad-Line Radio Galaxy Pictor A: A Simple Power Law With No Fe K-alpha Line
We present the X-ray spectrum of the broad-line radio galaxy Pictor A as
observed by ASCA in 1996. The main objective of the observation was to detect
and study the profiles of the Fe~K lines. The motivation was the fact
that the Balmer lines of this object show well-separated displaced peaks,
suggesting an origin in an accretion disk. The 0.5-10 keV X-ray spectrum is
described very well by a model consisting of a power law of photon index 1.77
modified by interstellar photoelectric absorption. We find evidence for neither
a soft nor a hard (Compton reflection) excess. More importantly, we do not
detect an Fe K-alpha line, in marked contrast with the spectra of typical
Seyfert galaxies and other broad-line radio galaxies observed by ASCA. The
99%-confidence upper limit on the equivalent width of an unresolved line at a
rest energy of 6.4 keV is 100 eV, while for a broad line (FWHM of approximately
60,000 km/s) the corresponding upper limit is 135 eV. We discuss several
possible explanations for the weakness of the Fe K-alpha line in Pictor~A
paying attention to the currently available data on the properties of Fe
K-alpha lines in other broad-line radio galaxies observed by ASCA. We speculate
that the absence of a hard excess (Compton reflection) or an Fe K-alpha line is
an indication of an accretion disk structure that is different from that of
typical Seyfert galaxies, e.g., the inner disk may be an ion torus.Comment: To appear in the Astrophysical Journal (18 pages, including 8
postscript figures; uses psfig.tex
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