520 research outputs found
Observations of the binary pulsar system PSR B1718-19 -- The Role of Tidal Circularisation
We present optical and infrared observations taken with the Very Large
Telescope of the eclipsing binary pulsar system PSR B1718-19. The candidate
companion of the pulsar, identified earlier in Hubble Space Telescope
observations, has been detected in all three bands, R, I, and J. These
detections allowed us to derive constraints on temperature, radius, and mass,
pointing to a companion that has expanded to a radius between one of a main
sequence star and one at the Roche-limit. We focus on the role of tidal
circularisation in the system, which will have transformed the initially
eccentric orbit expected from formation scenarios into the nearly circular
orbit presently observed. Based on simple energy balance arguments, we are able
to draw a picture of the companion's evolution resulting from the energy
deposition in the star due to circularisation. In this picture, our measurement
of the companion's parameters is consistent with the expected initial
eccentricity. However, with the present understanding of tidal dissipation it
remains difficult to account for the short time in which the system was
circularised.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
The (Double) White Dwarf Binary SDSS 1257+5428
SDSS 1257+5428 is a white dwarf in a close orbit with a companion that has
been suggested to be a neutron star. If so, it hosts the closest known neutron
star, and its existence implies a great abundance of similar systems and a rate
of white-dwarf neutron-star mergers similar to that of the type Ia supernova
rate. Here, we present high signal-to-noise spectra of SDSS 1257+5428, which
confirm an independent finding that the system is in fact composed of two white
dwarfs, one relatively cool and with low mass, and the other hotter and more
massive. With this, the demographics and merger rate are no longer puzzling
(various factors combine to lower the latter by more than two orders of
magnitude). We show that the spectra are fit well with a combination of two
hydrogen model atmospheres, as long as the lines of the higher-gravity
component are broadened significantly relative to what is expected from just
pressure broadening. Interpreting this additional broadening as due to
rotation, the inferred spin period is short, about 1 minute. Similarly rapid
rotation is only seen in accreting white dwarfs that are magnetic; empirically,
it appears that in non-magnetized white dwarfs, accreted angular momentum is
lost by nova explosions before it can be transferred to the white dwarf. This
suggests that the massive white dwarf in SDSS 1257+5428 is magnetic as well,
with B~10^5 G. Alternatively, the broadening seen in the spectral lines could
be due to a stronger magnetic field, of ~10^6 G. The two models could be
distinguished by further observations.Comment: 9 pages, 3 figures, submitted to Ap
A search for the optical counterpart to the magnetar CXOU J010043.1-721134
After our tentative detection of an optical counterpart to CXOU
J010043.1-721134 from archival Hubble Space Telescope (HST) imaging, we have
followed up with further images in four bands. Unfortunately, the source
originally identified is not confirmed. We provide deep photometric limits in
four bands and accurate photometry of field stars around the location of the
magnetar.Comment: 9 pages, accepted by Ap
On the masses of neutron stars
We analyze the currently available observations of X-ray binaries in a
consistent way, to re-determine the masses of the neutron stars in these
systems. In particular, our attention is focussed on a realistic and consistent
assessment of observational uncertainties and sources of systematic error.
Confidence limits for these new mass estimates are generally less optimistic
than previously assumed. The available observations, including data on six
radio pulsars, do not firmly constrain the equation of state of neutron star
matter. In particular, a firm upper mass limit cannot yet be established. An
improvement of the accuracy of optical data holds the key to further progress.Comment: 5 pages of compressed, uuencoded postscript (text+figure). Accepted
for publication in A&A. In case of problems, contact [email protected]
The IR counterpart of the black-hole candidate 4U 1630-47
We present K band photometry of the region including the radio error box of
the soft X-ray transient 4U 1630-47 during its 1998 outburst. We detect a
variable source at K=16.1 mag located inside the radio error circle which we
identify as the counterpart to the X-ray source. We discuss the properties of
the source, and conclude that it is most likely a black-hole X-ray binary
similar to 4U 1543-47, GRO J1655-40 or SAX J1819.3-2525, containing a
relatively early-type secondary.Comment: 7 pages, 3 figures, A&A, in pres
The infrared counterpart to the magnetar 1RXS J170849.0-400910
We have analyzed both archival and new infrared imaging observations of the
field of the Anomalous X-ray Pulsar 1RXS J170849.0-400910, in search of the
infrared counterpart. This field has been previously investigated, and one of
the sources consistent with the position of the AXP suggested as the
counterpart. We, however, find that this object is more likely a background
star, while another object within the positional error circle has non-stellar
colors and shows evidence for variability. These two pieces of evidence, along
with a consistency argument for the X-ray-to-infrared flux ratio, point to the
second source being the more likely infrared counterpart to the AXP.Comment: 19 pages AASTEX, 4 figure. Accepted for publication in ApJ. Full
resolution figures at: http://www.astro.utoronto.ca/~durant/1708.ps.g
Mode identification from time-resolved spectroscopy of the pulsating white dwarf G 29-38
We have used time-resolved spectroscopy to measure the colour dependence of
pulsation amplitudes in the DAV white dwarf G 29-38. Model atmospheres predict
that mode amplitudes should change with wavelength in a manner that depends on
the spherical harmonic degree l of the mode. This dependence arises from the
convolution of mode geometry with wavelength-dependent limb darkening. Our
analysis of the six largest normal modes detected in Keck observations of G
29-38 reveals one mode with a colour dependence different from the other five,
permitting us to identify the l value of all six modes and to test the model
predictions. The Keck observations also show pulsation amplitudes that are
unexpectedly asymmetric within absorption lines. We show that these asymmetries
arise from surface motions associated with the non-radial pulsations (which are
discussed in detail in a companion paper). By incorporating surface velocity
fields into line profile calculations, we are able to produce models that more
closely resemble the observations.Comment: 10 pages, 9 figures, mn.sty. Accepted for publication in MNRA
Properties of neutron stars
I review attempts made to determine the properties of neutron stars. I focus
on constraints on the maximum mass that a neutron star can have, and on
attempts to measure neutron-star radii. So far, there appears to be only one
neutron star for which there is strong evidence that its mass is above the
canonical 1.4 Msun, viz., Vela X-1, for which a mass close to 1.9 Msun is
found. Prospects for progress appear brightest for studies of systems in which
the neutron star should have accreted substantial amounts of matter. While for
individual systems the evidence that neutron stars can have high masses is
weak, the ensemble appears to show that masses around 1.6 Msun are possible.
For the radius determination, most attempts have focussed on neutron stars in
low-mass X-ray binaries in which accretion has temporarily shut down. These
neutron stars are easiest to model, since they should have pure Hydrogen
atmospheres and low magnetic fields. To obtain accurate radii, however,
requires precise distances and very high quality data.Comment: 12 pages, 2 figures, to appear in Proc. KIAS-APCTP International
Symposium in Astro-Hadron Physics "Compact Stars: Quest for New States of
Dense Matter
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