382 research outputs found
Neutron Stars in Globular Clusters
Dynamical interactions that occur between objects in dense stellar systems
are particularly important for the question of formation of X-ray binaries. We
present results of numerical simulations of 70 globular clusters with different
dynamical properties and a total stellar mass of 2*10^7 Msun. We find that in
order to retain enough neutron stars to match observations we must assume that
NSs can be formed via electron-capture supernovae. Our simulations explain the
observed dependence of the number of LMXBs on ``collision number'' as well as
the large scatter observed between different globular clusters. For millisecond
pulsars, we obtain good agreement between our models and the numbers and
characteristics of observed pulsars in the clusters Terzan 5 and 47 TucComment: 5 pages, 3 figures, to appear in "Dynamical Evolution of Dense
Stellar Systems", IAUS 246, ed. E. Vesperin
The nature of very faint X-ray binaries; hints from light curves
Very faint X-ray binaries (VFXBs), defined as having peak luminosities Lx of
10^34-10^36 erg/s, have been uncovered in significant numbers, but remain
poorly understood. We analyse three published outburst light curves of two
transient VFXBs using the exponential and linear decay formalism of King and
Ritter (1998). The decay timescales and brink luminosities suggest orbital
periods of order 1 hour. We review various estimates of VFXB properties, and
compare these with suggested explanations of the nature of VFXBs. We suggest
that: 1) VFXB outbursts showing linear decays might be explained as partial
drainings of the disc of "normal" X-ray transients, and many VFXB outbursts may
belong to this category; 2) VFXB outbursts showing exponential decays are best
explained by old, short-period systems involving mass transfer from a low-mass
white dwarf or brown dwarf; 3) persistent (or quasi-persistent) VFXBs, which
maintain an Lx of 10^34-10^35 erg/s for years, may be explained by
magnetospheric choking of the accretion flow in a propeller effect, permitting
a small portion of the flow to accrete onto the neutron star's surface. We thus
predict that (quasi-)persistent VFXBs may also be transitional millisecond
pulsars, turning on as millisecond radio pulsars when their Lx drops below
10^32 erg/s.Comment: 12 pages, 4 figures. MNRAS, in pres
X-ray Studies of Two Neutron Stars in 47 Tucanae: Toward Constraints on the Equation of State
We report spectral and variability analysis of two quiescent low mass X-ray
binaries (X5 and X7, previously detected with the ROSAT HRI) in a Chandra
ACIS-I observation of the globular cluster 47 Tuc. X5 demonstrates sharp
eclipses with an 8.666+-0.01 hr period, as well as dips showing an increased
N_H column. The thermal spectra of X5 and X7 are well-modeled by unmagnetized
hydrogen atmospheres of hot neutron stars. No hard power law component is
required. A possible edge or absorption feature is identified near 0.64 keV,
perhaps an OV edge from a hot wind. Spectral fits imply that X7 is
significantly more massive than the canonical 1.4 \Msun neutron star mass, with
M>1.8 \Msun for a radius range of 9-14 km, while X5's spectrum is consistent
with a neutron star of mass 1.4 \Msun for the same radius range. Alternatively,
if much of the X-ray luminosity is due to continuing accretion onto the neutron
star surface, the feature may be the 0.87 keV rest-frame absorption complex (O
VIII & other metal lines) intrinsic to the neutron star atmosphere, and a mass
of 1.4 \Msun for X7 may be allowed.Comment: 16 pages, 7 figures, accepted by Ap
The Light Curve and Internal Magnetic Field of the Mode-Switching Pulsar PSR B0943+10
A number of radio pulsars exhibit intriguing mode-switching behavior. Recent
observations of PSR B0943+10 revealed correlated radio and X-ray mode switches,
providing a new avenue for understanding this class of objects. The large X-ray
pulse fraction observed during the radio quiet phase (Q mode) was previously
interpreted as a result of changing obscuration of X-rays by dense
magnetosphere plasma. We show that the large X-ray pulse fraction can be
explained by including the beaming effect of a magnetic atmosphere, while
remaining consistent with the dipole field geometry constrained by radio
observations. We also explore a more extreme magnetic field configuration,
where a magnetic dipole displaced from the center of the star produces two
magnetic polar caps of different sizes and magnetic field strengths. These
models are currently consistent with data in radio and X-rays and can be tested
or constrained by future X-ray observations.Comment: 5 pages, 5 figures, submitted to ApJ
Evolution of the disc atmosphere in the X-ray binary MXB 1659-298, during its 2015-2017 outburst
We report on the evolution of the X-ray emission of the accreting neutron
star (NS) low mass X-ray binary (LMXB), MXB 1659-298, during its most recent
outburst in 2015-2017. We detected 60 absorption lines during the soft state
(of which 21 at more than 3 ), that disappeared in the hard state
(e.g., the Fe xxv and Fe xxvi lines). The absorbing plasma is at rest, likely
part of the accretion disc atmosphere. The bulk of the absorption features can
be reproduced by a high column density () of highly
ionised () plasma. Its disappearance during the
hard state is likely the consequence of a thermal photo-ionisation instability.
MXB 1659-298's continuum emission can be described by the sum of an absorbed
disk black body and its Comptonised emission, plus a black body component. The
observed spectral evolution with state is in line with that typically observed
in atoll and stellar mass black hole LMXB. The presence of a relativistic Fe
K disk-line is required during the soft state. We also tentatively
detect the Fe xxii doublet, whose ratio suggests an electron density of the
absorber of , hence, the absorber is likely located at
from the illuminating source, well inside the Compton and
outer disc radii. MXB 1659-298 is the third well monitored atoll LMXB
showcasing intense Fe xxv and Fe xxvi absorption during the soft state that
disappears during the hard state.Comment: MNRAS in pres
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