1,197 research outputs found
Timing Properties of Magnetars
We study the pulse morphologies and pulse amplitudes of thermally emitting
neutron stars with ultrastrong magnetic fields. The beaming of the radiation
emerging from a magnetar was recently shown to be predominantly non-radial,
with a small pencil and a broad fan component. We show that the combination of
this radiation pattern with the effects of strong lensing in the gravitational
field of the neutron star yields pulse profiles that show a qualitatively
different behavior compared to that of the radially-peaked beaming patterns
explored previously. Specifically, we find that: (i) the pulse profiles of
magnetars with a single hot emission region on their surface exhibit 1-2 peaks,
whereas those with an antipodal emission geometry have 1-4 peaks, depending on
the neutron star compactness, the observer's viewing angle, and the size of the
hot regions; (ii) the energy dependence of the beaming pattern may give rise to
weakly or strongly energy-dependent pulse profiles and may introduce phase lags
between different energy bands; (iii) the non-radial beaming pattern can give
rise to high pulsed fractions even for very relativistic neutron stars; (iv)
the pulsed fraction may not vary monotonically with neutron star compactness;
(v) the pulsed fraction does not decrease monotonically with the size of the
emitting region; (vi) the pulsed fraction from a neutron star with a single hot
pole has, in general, a very weak energy dependence, in contrast to the case of
an antipodal geometry. Comparison of these results to the observed properties
of anomalous X-ray pulsars strongly suggests that they are neutron stars with a
single hot region of ultrastrong magnetic field.Comment: 22 pages, 13 color figures, ApJ in pres
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]
A catalogue of low-mass X-ray binaries in the Galaxy, LMC, and SMC (Fourth edition)
We present a new edition of the catalogue of the low-mass X-ray binaries in
the Galaxy and the Magellanic Clouds. The catalogue contains source name(s),
coordinates, finding chart, X-ray luminosity, system parameters, and stellar
parameters of the components and other characteristic properties of 187
low-mass X-ray binaries, together with a comprehensive selection of the
relevant literature. The aim of this catalogue is to provide the reader with
some basic information on the X-ray sources and their counterparts in other
wavelength ranges (-rays, UV, optical, IR, and radio). Some sources,
however, are only tentatively identified as low-mass X-ray binaries on the
basis of their X-ray properties similar to the known low-mass X-ray binaries.
Further identification in other wavelength bands is needed to finally determine
the nature of these sources. In cases where there is some doubt about the
low-mass nature of the X-ray binary this is mentioned. Literature published
before 1 October 2006 has, as far as possible, been taken into account.Comment: 45 pages, catalogue include
Discovery of a Second Kilohertz QPO in the X-ray Binary 4U 1735-44
In recent observations with the Rossi X-Ray Timing Explorer we have detected
two simultaneous quasi-periodic oscillation (QPO) peaks in the low mass X-ray
binary and atoll source 4U 1735-44. The lower and higher frequency QPOs have
frequencies varying between 632 and 729 Hz, and 982 and 1026 Hz, respectively.
The fractional rms amplitudes are 3.7 to 8.1% and 5.0 to 5.8%. The frequency
separation between the two QPOs changes from 341+-7 Hz to 296+-12 Hz. The
inferred mass accretion rate during our observations is relatively low compared
to that during the previous observations, where only a single QPO was present.
There is weak evidence that the frequency of the QPOs correlates with the mass
accretion rate, as observed in other binaries. Five X-ray bursts were recorded
with no detectable oscillations with upper limits for the rms fraction of 4% to
13%.Comment: accepted by ApJ Letters, for more information see the QPO page at
http://www.astro.uva.nl/ecford/qpos.htm
Optical Counterpart of the Ultraluminous X-ray Source IC 342 X-1
We present Chandra and HST observations of the ultraluminous X-ray source
(ULX) IC 342 X-1. The Chandra and HST images are aligned using two X-ray
emitting foreground stars. The astrometry corrected position for X-1 is R.A. =
03h45m55.61s, Decl. = +68d04m55.3s (J2000) with an error circle of 0.2". One
extended optical source is found in the error circle, which could be the
optical counterpart of X-1. The source shows an extended feature in HST images
at long wavelengths, which is likely to be a superposition of two point
sources, although it is possible that the dimmer one could be a jet. Both
sources are much redder than typical for ULX optical counterparts. The brighter
one has an absolute magnitude M_V = -5.2 +/- 0.2 and (B-V)_0 = 0.66 +/- 0.13
and the dimmer star is not detected in B and has (B-V)_0 > 2.1. Their colors
are consistent with an F8 to G0 Ib supergiant or a carbon star, respectively.
However, it is likely that part or most of the optical emission may be due to
X-rays reprocessed by the companion star or the accretion disk. The stellar
neighborhood of IC 342 X-1 lacks O stars and has a minimum age of ~10 Myr. This
excludes the possibility that the surrounding nebula is powered by an energetic
explosion of a single massive star that formed a black hole. We suggest that
the nebula is most likely powered by an outflow from the X-ray source.Comment: 10 pages, 8 figures, accepted for publication in Ap
Simultaneous Measurements of X-Ray Luminosity and Kilohertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries
We measure simultaneously the properties of the energy spectra and the
frequencies of the kilohertz quasi-periodic oscillations (QPOs) in fifteen low
mass X-ray binaries covering a wide range of X-ray luminosities. In each source
the QPO frequencies cover the same range of approximately 300 Hz to 1300 Hz,
though the sources differ by two orders of magnitude in their X-ray
luminosities (as measured from the unabsorbed 2-50 keV flux). So the X-ray
luminosity does not uniquely determine the QPO frequency. This is difficult to
understand since the evidence from individual sources indicates that the
frequency and luminosity are very well correlated at least over short
timescales. Perhaps beaming effects or bolometric corrections change the
observed luminosities, or perhaps part of the energy in mass accretion is used
to power outflows reducing the energy emitted in X-rays. It is also possible
that the parameters of a QPO model are tuned in such a way that the same range
of frequencies appears in all sources. Different modes of accretion may be
involved for example (disk and radial) or multiple parameters may conspire to
yield the same frequencies.Comment: 14 pages, 2 figures (1 in color), accepted by ApJ, see the 'QPO
page': http://www.astro.uva.nl/~ecford/qpos.htm
The eclipsing Cataclysmic Variable GS Pavonis: Evidence for disk radius changes
We have obtained differential time series photometry of the cataclysmic
variable GS Pavonis over a timespan of 2 years. These show that this system is
deeply eclipsing (~2-3.5 mag) with an orbital period of 3.72 hr. The eclipse
depth and out-of-eclipse light levels are correlated. From this correlation we
deduce that the disk radius is changing and that the eclipses in the low state
are total. The derived distance to GS Pav is 790+/-90 pc, with a height above
the galactic plane of 420+/-60 pc. We classify GS Pav as a novalike system.Comment: 4 pages, including 2 figures, accepted for publication in A&A Letter
On the Lack of Type I X-ray Bursts in Black Hole X-ray Binaries: Evidence for the Event Horizon?
Type I X-ray bursts are very common in neutron star X-ray binaries, but no
Type I burst has been seen in the dozen or so binaries in which the accreting
compact star is too massive to be a neutron star and therefore is identified as
a black hole candidate. We have carried out a global linear stability analysis
of the accumulating fuel on the surface of a compact star to identify the
conditions under which thermonuclear bursts are triggered. Our analysis, which
improves on previous calculations, reproduces the gross observational trends of
bursts in neutron star systems. It further shows that, if black hole candidates
have surfaces, they would very likely exhibit instabilities similar to those
that lead to Type I bursts on neutron stars. The lack of bursts in black hole
candidates is thus significant, and indicates that these objects have event
horizons. We discuss possible caveats to this conclusion.Comment: 11 pages, 1 figure, to appear in 1 August 2002 edition of
Astrophysical Journal Letters, significant changes to the methods, results
unchange
The mass of X-Ray Nova Scorpii 1994 (=GRO J1655--40)]
We have obtained high and intermediate resolution optical spectra of the
black-hole candidate Nova Sco 1994 in May/June 1998, when the source was in
complete (X-ray) quiescence. We measure the radial velocity curve of the
secondary star and obtain a semi-amplitude of K_2=215.5+/-2.4 km/s, which is 6
per cent lower than the only previously determined value. This new value for
K_2 thus reduces the binary mass function to f(M) = 2.73+/-0.09 Mo. Using only
the high resolution spectra we constrain the rotational broadening of the
secondary star, vsini, to lie in the range 82.9-94.9 km/s (95 per cent
confidence) and thus constrain the binary mass ratio to lie in the range
0.337--0.436 (95 per cent confidence). We can also combine our results with
published limits for the binary inclination to constrain the mass of the
compact object and secondary star to the ranges 5.5 -- 7.9 and 1.7 -- 3.3 Mo
respectively (95 per cent confidence). Finally, we report on the detection of
the Lithium resonance line at 6707.8 A, with an equivalent width of 55+/-8 mA.Comment: 5 pages, 5 figures, accepted by MNRA
Pulse Profile Change Possibly Associated with a Glitch in an Anomalous X-Ray Pulsar 4U 0142+61
We report a glitch-like pulse frequency deviation from the simple spin-down
law in an anomalous X-ray pulsar (AXP) 4U 0142+61 detected by ASCA
observations. We also found a significant pulse profile change after the
putative glitch. The glitch parameters resemble those found in another AXP 1RXS
J170849.0400910, in the Vela pulsar, and in other radio pulsars. This
suggests that the radio pulsars and AXPs have the same internal structure and
glitch mechanism. It must be noted, however, that the pulse frequency anomaly
can also be explained by a gradual change of the spin-down rate ()
without invoking a glitch.Comment: 14 pages, 4 figures, accepted by Ap
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