868 research outputs found
The discovery of 12min X-ray pulsations from 1WGA J1958.2+3232
During a systematic search for periodic signals in a sample of ROSAT PSPC
(0.1-2.4 keV) light curves, we discovered 12min large amplitude X-ray
pulsations in 1WGA J1958.2+3232, an X-ray source which lies close to the
galactic plane. The energy spectrum is well fit by a power law with a photon
index of 0.8, corresponding to an X-ray flux of about 10E-12 ergs cmE-2 sE-1.
The source is probably a long period, low luminosity X-ray pulsar, similar to X
Per, or an intermediate polar.Comment: 5 pages (figures included). Accepted for publication on MNRA
V0332+53 in the outburst of 2004--2005: luminosity dependence of the cyclotron line and pulse profile
We present results of observations of the transient X-ray pulsar V0332+53
performed during a very powerful outburst in Dec, 2004 -- Feb, 2005 with the
INTEGRAL and RXTE observatories in a wide (3-100 keV) energy band. A cyclotron
resonance scattering line at an energy of ~26 keV has been detected in the
source spectrum together with its two higher harmonics at ~50 and ~73 keV,
respectively. We show that the energy of the line is not constant but linearly
changes with the source luminosity. Strong pulse profile variations, especially
near the cyclotron line, are revealed for different levels of the source
intensity. We discuss the obtained results in terms of the theoretical models
of X-ray pulsars.Comment: Accepted for publication in MNRAS. 10 pages, 9 figure
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.
The Protostellar Luminosity Function
The protostellar luminosity function (PLF) is the present-day luminosity
function of the protostars in a region of star formation. It is determined
using the protostellar mass function (PMF) in combination with a stellar
evolutionary model that provides the luminosity as a function of instantaneous
and final stellar mass. As in McKee & Offner (2010), we consider three main
accretion models: the Isothermal Sphere model, the Turbulent Core model, and an
approximation of the Competitive Accretion model. We also consider the effect
of an accretion rate that tapers off linearly in time and an accelerating star
formation rate. For each model, we characterize the luminosity distribution
using the mean, median, maximum, ratio of the median to the mean, standard
deviation of the logarithm of the luminosity, and the fraction of very low
luminosity objects. We compare the models with bolometric luminosities observed
in local star forming regions and find that models with an approximately
constant accretion time, such as the Turbulent Core and Competitive Accretion
models, appear to agree better with observation than those with a constant
accretion rate, such as the Isothermal Sphere model. We show that observations
of the mean protostellar luminosity in these nearby regions of low-mass star
formation suggest a mean star formation time of 0.30.1 Myr. Such a
timescale, together with some accretion that occurs non-radiatively and some
that occurs in high-accretion, episodic bursts, resolves the classical
"luminosity problem" in low-mass star formation, in which observed protostellar
luminosities are significantly less than predicted. An accelerating star
formation rate is one possible way of reconciling the observed star formation
time and mean luminosity.Comment: 22 pages, 9 figures, accepted to Ap
kHz Quasi Periodic Oscillations in Low Mass X-ray Binaries as Probes of General Relativity in the Strong Field Regime
We consider the interpretation of a pair of kHz Quasi Periodic Oscillations
(QPOs) in the Fourier spectra of two Low Mass X-Ray Binaries, Sco X-1 and
4U1608-52, hosting an old accreting neutron star. The observed frequency
difference of these QPOs decreaseas as their frequency increases, contrary to
simple beat frequency models, which predict a constant frequency difference. We
show that the behaviour of these QPOs is instead well matched in terms of the
fundamental frequencies (in the radial and azimuthal directions) for test
particle motion in the gravitational field of the neutron star, for reasonable
star masses, and nearly independent of the star spin. The radial frequency must
be much smaller than the azimuthal one, testifying that kHz QPOs are produced
close to the innermost stable orbit. These results are not reproduced through
the post--Newtonian (PN) approximation of General Relativity (GR). kHz QPOs
from X-ray binaries likely provide an accurate laboratory for strong field GR.Comment: to appear in Physical Review Letters, PRL Latex plus 2 figures in
standard PostScript forma
X-Ray Eclipse Time Delays in 4U2129+47
4U 2129+47 was discovered in the early 80's and classified as an accretion
disk corona source due to its broad and partial X-ray eclipses. The 5.24 hr
binary orbital period was inferred from the X-ray and optical light curve
modulation, implying a late K or M spectral type companion star.
The source entered a low state in 1983, during which the optical modulation
disappeared and an F8 IV star was revealed, suggesting that 4U 2129+47 might be
part of a triple system. The nature of 4U 2129+47 has since been investigated,
but no definitive conclusion has been reached.
Here, we present timing and spectral analyses of two XMM-Newton observations
of this source, carried out in May and June, 2005. We find evidence for a delay
between two mid-eclipse epochs measured ~22 days apart, and we show that this
delay can be naturally explained as being due to the orbital motion of the
binary 4U 2129+47 around the center of mass of a triple system. This result
thus provides further support in favor of the triple nature of 4U 2129+47.Comment: 6 pages, 5 figures, accepted by A&
A look with BeppoSAX at the low-luminosity Galactic X-ray source 4U 2206+54
A pointed observation of the low-luminosity galactic source 4U 2206+54 was
carried out in November 1998 with BeppoSAX. The light curve of 4U 2206+54 shows
erratic variability on a timescale of about 1 hour; neither hardness variations
nor time periodicities are detected throughout this 67 ks long observation.
Thanks to the wide spectral coverage capabilities of BeppoSAX we could observe
the source X-ray continuum over three energy decades, from 0.6 to 60 keV. The
spectrum could be equally well fitted either with a blackbody plus
Comptonization or with a high energy cutoff power law. No iron emission around
6.5 keV was detected, while a tentative detection of a cyclotron resonant
feature in absorption is presented. Comparison of the present BeppoSAX data
with the information available in the literature for this source suggests that
4U 2206+54 is a close binary system in which a (possibly magnetized) NS is
accreting from the companion star wind.Comment: 9 pages, 5 figures, 2 tables; accepted for publication on Astronomy &
Astrophysics, main journal. Final version of the paper including the A&A
Language Editor's comment
Second order quantum renormalisation group of XXZ chain with next nearest neighbour interactions
We have extended the application of quantum renormalisation group (QRG) to
the anisotropic Heisenberg model with next-nearest neighbour (n-n-n)
interaction. The second order correction has to be taken into account to get a
self similar renormalized Hamiltonian in the presence of n-n-n-interaction. We
have obtained the phase diagram of this model which consists of three different
phases, i.e, spin-fluid, dimerised and Ne'el types which merge at the
tri-critical point. The anisotropy of the n-n-n-term changes the phase diagram
significantly. It has a dominant role in the Ne'el-dimer phase boundary. The
staggered magnetisation as an order parameter defines the border between
fluid-Ne'el and Ne'el-dimer phases. The improvement of the second order RG
corrections on the ground state energy of the Heisenberg model is presented.
Moreover, the application of second order QRG on the spin lattice model has
been discussed generally. Our scheme shows that higher order corrections lead
to an effective Hamiltonian with infinite range of interactions.Comment: 10 pages, 4 figures and 1 tabl
An Optical Counterpart to the Anomalous X-ray Pulsar 4U 0142+61
The energy source of the anomalous X-ray pulsars is not well understood,
hence their designation as anomalous. Unlike binary X-ray pulsars, no
companions are seen, so the energy cannot be supplied by accretion of matter
from a companion star. The loss of rotational energy, which powers radio
pulsars, is insufficient to power AXPs. Two models are generally considered:
accretion from a large disk left over from the birth process, or decay of a
very strong magnetic field (10^15 G) associated with a 'magnetar'. The lack of
counterparts at other wavelengths has hampered progress in our understanding of
these objects. Here, we present deep optical observations of the field around
4U 0142+61, which is the brightest AXP in X-rays. We find an object with
peculiar optical colours at the position of the X-ray source, and argue that it
is the optical counterpart. The optical emission is too faint to admit the
presence of a large accretion disk, but may be consistent with magnetospheric
emission from a magnetar.Comment: 16 pages, 3 figures, accepted by Nature. Press embargo until 1900 hrs
London time (GMT) on 6 December 200
Discovery of 5.16s pulsations from the isolated neutron star RBS 1223
The isolated neutron star candidate RBS 1223 was observed with the Advanced
CCD Imaging Spectrometer aboard the Chandra X-ray observatory on 2000 June 24.
A timing analysis of the data yielded a periodic modulation with a period
P=5.1571696^(+1.57*10^(-4) -1.36*10^(-4)s. Using ROSAT HRI archived
observations we detected a period P=5.1561274 \pm 4.4*10^(-4)s and determined
period derivative dP/dt=(0.7 - 2.0)*10^(-11) s*s^(-1). The detection of this
period and dP/dt indicates that RBS 12223 has a ``characteristic'' age of
6000-12000 years and huge magnetic field at the surface (B(dipole)~(1.7-
3.2)*10^(+14) G) typical for anomalous X-ray pulsars (AXPs).Comment: 7 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
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