9,744 research outputs found
Ulcerative colitis: let's talk about extent
Ulcerative colitis (UC) is a chronic inflammatory disease in which clinical course varies substantially between patients.
The extent of the disease is usually pointed out as one of the factors
responsible for this variation. With this study, we pretended to evaluate the differences in natural history and pharmacological therapy
prescription between left-sided and extended UC
A new class of quantum bound states: diprotons in extreme magnetic fields
This paper considers the possibility that two charged particles with an
attractive short-ranged potential between them which is not strong enough to
form bound states in free space, may bind in uniform magnetic fields. It is
shown that in the formal limit where Coulomb repulsion is negligible (q -> 0
and B_0 -> \infty with q B_0 fixed where q is the charge and B_0 the field
strength) there always exists a bound state for a system of two identical
charged particles in a constant magnetic field, provided that there is a
short-range uniformly attractive potential between them. Moreover, it is shown
that in this limit {\it any} potential with an attractive s-wave scattering
length will posses bound states provided that the range of the potential is
much smaller than the characteristic magnetic length, r_0 = (\frac{q
B_0}{4})^{-1/2}. For this case, the binding is computed numerically. We
estimate the size of the magnetic field needed to approximately reach a regime
where the formal limit considered becomes a good approximation to the dynamics.
These numerical estimates indicate that two protons in an extremely strong
magnetic field such as might be found in a magnetar will bind to form a
diproton
Very long baseline astrometry of PSR J1012+5307 and its implications on alternative theories of gravity
PSR J1012+5307, a millisecond pulsar in orbit with a helium white dwarf (WD),
has been timed with high precision for about 25 years. One of the main
objectives of this long-term timing is to use the large asymmetry in
gravitational binding energy between the neutron star and the WD to test
gravitational theories. Such tests, however, will be eventually limited by the
accuracy of the distance to the pulsar. Here, we present VLBI (very long
baseline interferometry) astrometry results spanning approximately 2.5 years
for PSR J1012+5307, obtained with the Very Long Baseline Array as part of the
MSPSRPI project. These provide the first proper motion and absolute position
for PSR J1012+5307 measured in a quasi-inertial reference frame. From the VLBI
results, we measure a distance of kpc (all the estimates
presented in the abstract are at 68% confidence) for PSR J1012+5307, which is
the most precise obtained to date. Using the new distance, we improve the
uncertainty of measurements of the unmodeled contributions to orbital period
decay, which, combined with three other pulsars, places new constraints on the
coupling constant for dipole gravitational radiation
and the fractional time derivative of
Newton's gravitational constant in the local universe. As the uncertainties of the
observed decays of orbital period for the four leading pulsar-WD systems become
negligible in years, the uncertainties for and
will be improved to and
, respectively, predominantly limited by the distance
uncertainties.Comment: published in ApJ (2020ApJ...896...85D
A Massive Neutron Star in the Globular Cluster M5
We report the results of 19 years of Arecibo timing for two pulsars in the
globular cluster NGC 5904 (M5), PSR B1516+02A (M5A) and PSR B1516+02B (M5B).
This has resulted in the measurement of the proper motions of these pulsars
and, by extension, that of the cluster itself. M5B is a 7.95-ms pulsar in a
binary system with a > 0.13 solar mass companion and an orbital period of 6.86
days. In deep HST images, no optical counterpart is detected within ~2.5 sigma
of the position of the pulsar, implying that the companion is either a white
dwarf or a low-mass main-sequence star. The eccentricity of the orbit (e =
0.14) has allowed a measurement of the rate of advance of periastron: (0.0142
+/-0.0007) degrees per year. We argue that it is very likely that this
periastron advance is due to the effects of general relativity, the total mass
of the binary system then being 2.29 +/-0.17 solar masses. The small measured
mass function implies, in a statistical sense, that a very large fraction of
this total mass is contained in the pulsar: 2.08 +/- 0.19 solar masses (1
sigma); there is a 5% probability that the mass of this object is < 1.72 solar
masses and a 0.77% probability that is is between 1.2 and 1.44 solar masses.
Confirmation of the median mass for this neutron star would exclude most
``soft'' equations of state for dense neutron matter. Millisecond pulsars
(MSPs) appear to have a much wider mass distribution than is found in double
neutron star systems; about half of these objects are significantly more
massive than 1.44 solar masses. A possible cause is the much longer episode of
mass accretion necessary to recycle a MSP, which in some cases corresponds to a
much larger mass transfer.Comment: 10 pages in ApJ emulate format, 2 tables, 6 figures. Added February
2008 data, slightly revised mass limits. Accepted for publication in Ap
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