878 research outputs found
Ideal velocity focusing in a reflectron time-of-flight mass spectrometer
AbstractA single-stage ion mirror in a time-of-flight (TOF) mass spectrometer (MS) can perform first order velocity focusing of ions initially located at a start focal plane while second order velocity focusing can be achieved using a double-stage reflectron. The situation is quite different when an ion source extraction field is taken into account. In this case which is common in any practical matrix-assisted laser desorption/ionization (MALDI) TOF-MS a single-stage reflectron, for example, cannot perform velocity focusing at all. In this paper an exact, analytic solution for an electric field inside a one-dimensional reflectron has been found to achieve universal temporal focusing of ions having an initial velocity distribution. The general solution is valid for arbitrary electric field distributions in the upstream (from the ion source to the reflectron) and downstream (from the reflectron to an ion detector) regions and in a decelerating part of the reflectron of a reflectron TOF mass spectrometer. The results obtained are especially useful for designing MALDI reflectron TOF mass spectrometers in which the initial velocity distribution of MALDI ions is the major limiting factor for achieving high mass resolution. Using analytical expressions obtained for an arbitrary case, convenient working formulas are derived for the case of a reflectron TOF-MS with a dual-stage extraction ion source. The special case of a MALDI reflectron TOF-MS with an ion source having a low acceleration voltage (or large extraction region) is considered. The formulas derived correct the effect of the acceleration regions in a MALDI ion source and after the reflectron before detecting ions
Stable accretion from a cold disc in highly magnetized neutron stars
The aim of this paper is to investigate the transition of a strongly
magnetized neutron star into the accretion regime with very low accretion rate.
For this purpose we monitored the Be-transient X-ray pulsar GRO J1008-57
throughout a full orbital cycle. The current observational campaign was
performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV)
between two subsequent Type I outbursts in January and September 2016. The
expected transition to the propeller regime was not observed. However, the
transitions between different regimes of accretion were detected. In
particular, after an outburst the source entered a stable accretion state
characterised by the accretion rate of ~10^14-10^15 g/s. We associate this
state with accretion from a cold (low-ionised) disc of temperature below ~6500
K. We argue that a transition to such accretion regime should be observed in
all X-ray pulsars with certain combination of the rotation frequency and
magnetic field strength. The proposed model of accretion from a cold disc is
able to explain several puzzling observational properties of X-ray pulsars.Comment: 8 pages, 3 figures, 1 table, accepted by A&
Polarimetry of the Type Ia Supernova SN 1996X
We present broad-band and spectropolarimetry of the Type Ia SN 1996X obtained
on April 14, 1996 (UT), and broad-band polarimetry of SN 1996X on May 22,1996,
when the supernova was about a week before and 4 weeks after optical maximum,
respectively. The Stokes parameters derived from the broad-band polarimetry are
consistent with zero polarization. The spectropolarimetry, however, shows broad
spectral features which are due intrinsically to an asymmetric SN atmosphere.
The spectral features in the flux spectrum and the polarization spectrum show
correlations in the wavelength range from 4900 AA up to 5500 AA. The degree of
this intrinsic component is low (<0.3 %). Theoretical polarization spectra have
been calculated. It is shown that the polarization spectra are governed by line
blending. Consequently, for similar geometrical distortions, the residual
polarization is smaller by about a factor of 2 to 3 compared to the less
blended Type II atmosphere, making it intrinsically harder to detect
asphericities in SNIa. Comparison with theoretical model polarization spectra
shows a resemblance to the observations. Taken literally, this implies an
asphericity of about 11 % in the chemical distribution in the region of partial
burning. This may not imperil the use of Type Ia supernovae as standard candles
for distance determination, but nontheless poses a source of uncertainty. SN
1996X is the first Type Ia supernova for which spectropolarimetry revealed a
polarized component intrinsic to the supernova and the first Type Ia with
spectropolarimetry well prior to optical maximum.Comment: 7 pages, 5 figures, macros 'aas2pp4.sty,psfig.tex'. LaTeX Style.
Astrophysical Journal Letters, submitted September 199
Post-Newtonian Theory for Precision Doppler Measurements of Binary Star Orbits
The determination of velocities of stars from precise Doppler measurements is
described here using relativistic theory of astronomical reference frames so as
to determine the Keplerian and post-Keplerian parameters of binary systems. We
apply successive Lorentz transformations and the relativistic equation of light
propagation to establish the exact treatment of Doppler effect in binary
systems both in special and general relativity theories. As a result, the
Doppler shift is a sum of (1) linear in terms, which include the
ordinary Doppler effect and its variation due to the secular radial
acceleration of the binary with respect to observer; (2) terms proportional to
, which include the contributions from the quadratic Doppler effect
caused by the relative motion of binary star with respect to the Solar system,
motion of the particle emitting light and diurnal rotational motion of
observer, orbital motion of the star around the binary's barycenter, and
orbital motion of the Earth; and (3) terms proportional to , which
include the contributions from redshifts due to gravitational fields of the
star, star's companion, Galaxy, Solar system, and the Earth. After
parameterization of the binary's orbit we find that the presence of
periodically changing terms in the Doppler schift enables us disentangling
different terms and measuring, along with the well known Keplerian parameters
of the binary, four additional post-Keplerian parameters, including the
inclination angle of the binary's orbit, . We briefly discuss feasibility of
practical implementation of these theoretical results, which crucially depends
on further progress in the technique of precision Doppler measurements.Comment: Minor changes, 1 Figure included, submitted to Astrophys.
Effects of Gravitational Lensing in the Double Pulsar System J0737-3039
We investigate the effects of gravitational lensing in the binary pulsar
system J0737-3039. Current measurement of the orbital inclination allows the
millisecond pulsar (A) to pass very close (at R_{min}=4000 km) in projection to
the companion pulsar (B), with R_{min} comparable to the Einstein radius (2600
km). For this separation at the conjunction, lensing causes small (about 10%)
magnification of the pulsar A signal on a timescale of several seconds, and
displaces the pulsar image on the sky plane by about 1200 km. More importantly,
lensing introduces a correction (of several microsec) to the conventional
Shapiro delay formula used in pulsar timing analysis, and gives rise to a
geometric time delay together with the delays associated with the pulsar spin
period. These lensing effects can influence the determination of the system
parameters by both timing and scintillation studies. Given the current
uncertainty in the orbital inclination, more extreme manifestations of lensing
(e.g. magnification by a factor of several) are possible. We compare our
predictions with the existing observations and discuss the possibility of
detecting gravitational lensing signatures in the system. The anomalously high
point in A's lightcurve close to superior conjunction might be caused by
gravitational lensing.Comment: Minor changes. ApJ Letters, in pres
Suppressed star formation in circumnuclear regions in Seyfert galaxies
Feedback from black hole activity is widely believed to play a key role in
regulating star formation and black hole growth. A long-standing issue is the
relation between the star formation and fueling the supermassive black holes in
active galactic nuclei (AGNs). We compile a sample of 57 Seyfert galaxies to
tackle this issue. We estimate the surface densities of gas and star formation
rates in circumnuclear regions (CNRs). Comparing with the well-known
Kennicutt-Schmidt (K-S) law, we find that the star formation rates in CNRs of
most Seyfert galaxies are suppressed in this sample. Feedback is suggested to
explain the suppressed star formation rates.Comment: 1 color figure and 1 table. ApJ Letters in pres
XRBcats: Galactic Low Mass X-ray Binary Catalogue
We present a new catalogue of low-mass X-ray binaries (LMXBs) in the Galaxy.
The catalogue contains source names, coordinates, source types, fluxes,
distances, system parameters, and other characteristic properties of 348 LMXBs,
including LMXBs that were newly discovered or re-classified since the latest
releases of the catalogues by Liu et al. (2007) and Ritter and Kolb (2003). The
aim of this catalogue is to provide a list of all currently known Galactic
objects identified as LMXBs with some basic information on each system
(including X-ray and optical/IR properties where possible). Literature
published before March 2023 has, as far as possible, been taken into account
when compiling this information. References for all reported properties as well
as object finding charts in several energy bands are provided as part of the
catalogue. We plan to update the catalogue regularly, in particular to reflect
new objects discovered in the ongoing large scale surveys such as Gaia and
eROSITA.Comment: 13 pages, 2 figures, submitted to A&A. For auxillary files, see
http://astro.uni-tuebingen.de/~xrbcat
Profile instabilities of the millisecond pulsar PSR J1022+1001
We present evidence that the integrated profiles of some millisecond pulsars
exhibit severe changes that are inconsistent with the moding phenomenon as
known from slowly rotating pulsars. We study these profile instabilities in
particular for PSR J1022+1001 and show that they occur smoothly, exhibiting
longer time constants than those associated with moding. In addition, the
profile changes of this pulsar seem to be associated with a relatively
narrow-band variation of the pulse shape. Only parts of the integrated profile
participate in this process which suggests that the origin of this phenomenon
is intrinsic to the pulsar magnetosphere and unrelated to the interstellar
medium. A polarization study rules out profile changes due to geometrical
effects produced by any sort of precession. However, changes are observed in
the circularly polarized radiation component. In total we identify four
recycled pulsars which also exhibit instabilities in the total power or
polarization profiles due to an unknown phenomenon (PSRs J1022+1001,
J1730-2304, B1821-24, J2145-0750).
The consequences for high precision pulsar timing are discussed in view of
the standard assumption that the integrated profiles of millisecond pulsars are
stable. As a result we present a new method to determine pulse times-of-arrival
that involves an adjustment of relative component amplitudes of the template
profile. Applying this method to PSR J1022+1001, we obtain an improved timing
solution with a proper motion measurement of -17 \pm 2 mas/yr in ecliptic
longitude. Assuming a distance to the pulsar as inferred from the dispersion
measure this corresponds to an one-dimensional space velocity of 50 km/s.Comment: 29 pages, 12 figures, accepted for publication in Ap
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