519 research outputs found
Recent Results on the Anomalous X-ray Pulsars
The ''Anomalous X-ray Pulsars'' (AXPs) are a small group of X-ray pulsars
characterized by periods in the 5-10 s range and by the absence of massive
companion stars. There are now 7 possible members of this class of objects. We
review recent observational results on their X-ray spectra, spin period
evolution, and searches for orbital motion and discuss the implications for
possible models.Comment: 4 pages, 1 figures. To appear in The Active X-ray Sky: Results from
BeppoSAX and Rossi-XTE, Nuclear Physics B Proceedings Supplements, L. Scarsi,
H. Bradt, P. Giommi and F. Fiore (eds.), Elsevier Science B.
Exact renormalization-group analysis of first order phase transitions in clock models
We analyze the exact behavior of the renormalization group flow in
one-dimensional clock-models which undergo first order phase transitions by the
presence of complex interactions. The flow, defined by decimation, is shown to
be single-valued and continuous throughout its domain of definition, which
contains the transition points. This fact is in disagreement with a recently
proposed scenario for first order phase transitions claiming the existence of
discontinuities of the renormalization group. The results are in partial
agreement with the standard scenario. However in the vicinity of some fixed
points of the critical surface the renormalized measure does not correspond to
a renormalized Hamiltonian for some choices of renormalization blocks. These
pathologies although similar to Griffiths-Pearce pathologies have a different
physical origin: the complex character of the interactions. We elucidate the
dynamical reason for such a pathological behavior: entire regions of coupling
constants blow up under the renormalization group transformation. The flows
provide non-perturbative patterns for the renormalization group behavior of
electric conductivities in the quantum Hall effect.Comment: 13 pages + 3 ps figures not included, TeX, DFTUZ 91.3
Replication in situ and DNA encapsulation following induction of an excision-defective lysogen of Salmonella bacteriophage P22
The induction of an excision-defective bacteriophage P22 lysogen results in the production of particles which carry a DNA molecule of normal length within a normal capsid, but which are nonetheless defective. The DNA content of these particles was characterized physically by a restriction enzyme analysis, and genetically by two marker rescue techniques. The particles carry DNA corresponding to one side of the prophage map as well as additional DNA, apparently derived from the host chromosome to one side of the prophage insertion site. Normally, mature P22 DNA molecules are derived from a concatemer by sequential cleavage of adjacent headful lengths, beginning at a genetically unique site, the encapsulation origin (Tye et al., 1974). The defective particles appear to contain DNA matured by the same sequential mechanisms, operating on the integrated prophage and neighboring bacterial chromosome, rather than on the normal concatemeric substrate. Both the initiation and directional specificities of normal maturation are maintained during the maturation of defective particle DNA. Sequential cleavage begins within the prophage at the encapsulation origin, a site near gene 3, and proceeds into the host chromosome on the proC side of the prophage. The initiation specificity of DNA encapsulation seems to reside in the morphogenetic machinery, rather than in the mechanism of DNA replication. Replication of an induced excision-defective prophage takes place in situ on the host chromosome, apparently without disruption of the linear integrity of the prophage. Further, the entire prophage, as well as adjacent bacterial DNA, is replicated, even though only a portion of this DNA is destined to be encapsulated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22678/1/0000231.pd
Constraining compactness and magnetic field geometry of X-ray pulsars from the statistics of their pulse profiles
The light curves observed from X-ray pulsars and magnetars reflect the
radiation emission pattern, the geometry of the magnetic field, and the neutron
star compactness. We study the statistics of X-ray pulse profiles in order to
constrain the neutron star compactness and the magnetic field geometry. We
collect the data for 124 X-ray pulsars, which are mainly in high-mass X-ray
binary systems, and classify their pulse profiles according to the number of
observed peaks seen during one spin period, dividing them into two classes,
single- and double-peaked. We find that the pulsars are distributed about
equally between both groups. We also compute the probabilities predicted by the
theoretical models of two antipodal point-like spots that emit radiation
according to the pencil-like emission patterns. These are then compared to the
observed fraction of pulsars in the two classes. Assuming a blackbody emission
pattern, it is possible to constrain the neutron star compactness if the
magnetic dipole has arbitrary inclinations to the pulsar rotational axis. More
realistic pencil-beam patterns predict that 79% of the pulsars are
double-peaked independently of their compactness. The theoretical predictions
can be made consistent with the data if the magnetic dipole inclination to the
rotational axis has an upper limit of 40+/-4 deg. We also discuss the effect of
limited sensitivity of the X-ray instruments to detect weak pulses, which
lowers the number of detected double-peaked profiles and makes the theoretical
predictions to be consistent with the data even if the magnetic dipole does
have random inclinations. This shows that the statistics of pulse profiles does
not allow us to constrain the neutron star compactness. In contrast to the
previous claims by Bulik et al. (2003), the data also do not require the
magnetic inclination to be confined in a narrow interval.Comment: 14 pages, 10 figures, Astronomy and Astrophysics, in pres
The timing of erf-mediated recombination in replication, lysogenization, and the formation of recombinant progeny by Salmonella phage P22
Following infection, the development of phage P22 by either the lytic or the lysogenic pathways requires recombination, mediated either by the phage erf system or by the bacterial rec system [Botstein, D., and Matz, M. J. (1970) J. Mol. Biol. 54, 417-440]. We have investigated the timing of the essential recombinational processes with temperature-shift experiments using a temperature-sensitive erf mutant. In rec- cells, erf function appears to be required early in the infection to complete some essential step, the timing of which is the same in both the lytic and lysogenic circumstances. Once the step has taken place, subsequent development can occur without further erf function. However, the bulk of recombinant progeny arising in lytic crosses in rec- cells result from nonessential erf action late in the infection, after the time of the required early step.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23011/1/0000580.pd
Hairy Black Holes in String Theory
Solutions of bosonic string theory are constructed which correspond to
four-dimensional black holes with axionic quantum hair. The basic building
blocks are the renormalization group flows of the CP1 model with a theta term
and the SU(1,1)/U(1) WZW coset conformal field theory. However the solutions
are also found to have negative energy excitations, and are accordingly
expected to decay to the vacuum.Comment: 14 pages (References added
The doubling of the superorbital period of Cyg X-1
We study properties of the superorbital modulation of the X-ray emission of
Cyg X-1. We find that it has had a stable period of about 300 d in soft and
hard X-rays and in radio since 2005 until at least 2010, which is about double
the previously seen period. This new period, seen in the hard spectral state
only, is detected not only in the light curves but also in soft X-ray hardness
ratios and in the amplitude of the orbital modulation. On the other hand, the
spectral slope in hard X-rays, >20 keV, averaged over superorbital bins is
constant, and the soft and hard X-rays and the radio emission change in phase.
This shows that the superorbital variability consists of changing the
normalization of an intrinsic spectrum of a constant shape and of changes of
the absorbing column density with the phase. The maximum column density is
achieved at the superorbital minimum. The amplitude changes are likely to be
caused by a changing viewing angle of an anisotropic emitter, most likely a
precessing accretion disc. The constant shape of the intrinsic spectrum shows
that this modulation is not caused by a changing accretion rate. The modulated
absorbing column density shows the presence of a bulge around the disc centre,
as proposed previously. We also find the change of the superorbital period from
about 150 d to about 300 d to be associated with almost unchanged average X-ray
fluxes, making the period change difficult to explain in the framework of
disc-irradiation models. Finally, we find no correlation of the X-ray and radio
properties with the reported detections in the GeV and TeV gamma-ray range.Comment: MNRAS, in press, 8 page
The supergiant fast X-ray transient IGRJ18483-0311 in quiescence: XMM-Newton, Swift, and Chandra observations
IGR J18483-0311 was discovered with INTEGRAL in 2003 and later classified as
a supergiant fast X-ray transient. It was observed in outburst many times, but
its quiescent state is still poorly known. Here we present the results of
XMM-Newton, Swift, and Chandra observations of IGRJ18483-0311. These data
improved the X-ray position of the source, and provided new information on the
timing and spectral properties of IGR J18483-0311 in quiescence. We report the
detection of pulsations in the quiescent X-ray emission of this source, and
give for the first time a measurement of the spin-period derivative of this
source. In IGRJ18483-0311 the measured spin-period derivative of
-(1.3+-0.3)x10^(-9) s/s likely results from light travel time effects in the
binary. We compare the most recent observational results of IGRJ18483-0311 and
SAXJ1818.6-1703, the two supergiant fast X-ray transients for which a similar
orbital period has been measured.Comment: Accepted for publication in MNRA
A reaction-diffusion model for the growth of avascular tumor
A nutrient-limited model for avascular cancer growth including cell
proliferation, motility and death is presented. The model qualitatively
reproduces commonly observed morphologies for primary tumors, and the simulated
patterns are characterized by its gyration radius, total number of cancer
cells, and number of cells on tumor periphery. These very distinct
morphological patterns follow Gompertz growth curves, but exhibit different
scaling laws for their surfaces. Also, the simulated tumors incorporate a
spatial structure composed of a central necrotic core, an inner rim of
quiescent cells and a narrow outer shell of proliferating cells in agreement
with biological data. Finally, our results indicate that the competition for
nutrients among normal and cancer cells may be a determinant factor in
generating papillary tumor morphology.Comment: 9 pages, 6 figures, to appear in PR
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