745 research outputs found
Extended Emission from Cygnus X-3 Detected with Chandra
We have discovered extended X-ray emission from the microquasar Cyg X-3 in
archival Chandra X-ray Observatory observations. A 5" wide structure lies
approximately 16" to the NE from the core point source and may be extended in
that direction. This angular scale corresponds to a physical extent of roughly
0.8 lyr, at a distance of 2.5 lyr from Cyg X-3 (assuming a 10 kpc distance).
The flux varied by a factor of 2.5 during the four months separating two of the
observations, indicating significant substructure. The peak 2-10 keV luminosity
was about 5e34 ergs/s. There may also be weaker, extended emission of similar
scale oppositely directed from the core, suggesting a bipolar outflow. This
structure is not part of the dust scattering halo, nor is it caused by the
Chandra point spread function. In this Letter we describe the observations and
discuss possible origins of the extension.Comment: Submitted to ApJ Letters. 5 pages, 2 figures (1 color). Uses
emulateap
Further constraints on neutron star crustal properties in the low-mass X-ray binary 1RXS J180408.9342058
We report on two new quiescent {\it XMM-Newton} observations (in addition to
the earlier {\it Swift}/XRT and {\it XMM-Newton} coverage) of the cooling
neutron star crust in the low-mass X-ray binary 1RXS J180408.9342058. Its
crust was heated during the 4.5 month accretion outburst of the source.
From our quiescent observations, fitting the spectra with a neutron star
atmosphere model, we found that the crust had cooled from 100 eV to
73 eV from 8 days to 479 days after the end of its outburst.
However, during the most recent observation, taken 860 days after the end
of the outburst, we found that the crust appeared not to have cooled further.
This suggested that the crust had returned to thermal equilibrium with the
neutron star core. We model the quiescent thermal evolution with the
theoretical crustal cooling code NSCool and find that the source requires a
shallow heat source, in addition to the standard deep crustal heating
processes, contributing 0.9 MeV per accreted nucleon during outburst to
explain its observed temperature decay. Our high quality {\it XMM-Newton} data
required an additional hard component to adequately fit the spectra. This
slightly complicates our interpretation of the quiescent data of 1RXS
J180408.9342058. The origin of this component is not fully understood.Comment: Accepted for publication by MNRA
Dependence of the Frequency of the Kilohertz Quasi-Periodic Oscillations on X-ray Count Rate and Colors in 4U 1608-52
We present new results based on observations carried out with the Rossi X-ray
Timing Explorer during the decay of an outburst of the low-mass X-ray binary
(LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at least in
4U 1608-52, one of the long-standing issues about the phenomenology of the
kilohertz quasi-periodic oscillations (kHz QPOs), namely, the lack of a unique
relation between the frequency of the kHz QPOs and the X-ray flux. We show that
despite its complex dependence on the X-ray flux, the frequency of the kHz QPOs
is monotonically related to the position of the source in the color-color
diagram. Our findings strengthen the idea that, as in the case of Z sources, in
the atoll sources the X-ray flux is not a good indicator of , and that
the observed changes in the frequency of the kHz QPOs in LMXBs are driven by
changes in . These results raise some concern about the recently
reported detection of the orbital frequency at the innermost stable orbit in 4U
1820-30.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses
AAS LaTex v4.0 (5 pages plus 4 postscript figures
Low-frequency QPO from the 11 Hz accreting pulsar in Terzan 5: not frame dragging
We report on 6 RXTE observations taken during the 2010 outburst of the 11 Hz
accreting pulsar IGR J17480-2446 located in the globular cluster Terzan 5.
During these observations we find power spectra which resemble those seen in
Z-type high-luminosity neutron star low-mass X-ray binaries, with a
quasi-periodic oscillation (QPO) in the 35-50 Hz range simultaneous with a kHz
QPO and broad band noise. Using well known frequency-frequency correlations, we
identify the 35-50 Hz QPOs as the horizontal branch oscillations (HBO), which
were previously suggested to be due to Lense-Thirring precession. As IGR
J17480-2446 spins more than an order of magnitude more slowly than any of the
other neutron stars where these QPOs were found, this QPO can not be explained
by frame dragging. By extension, this casts doubt on the Lense-Thirring
precession model for other low-frequency QPOs in neutron-star and perhaps even
black-hole systems.Comment: 6 pages, 5 figures, Accepted for publication in ApJ
A window into the neutron star: Modelling the cooling of accretion heated neutron star crusts
In accreting neutron star X-ray transients, the neutron star crust can be
substantially heated out of thermal equilibrium with the core during an
accretion outburst. The observed subsequent cooling in quiescence (when
accretion has halted) offers a unique opportunity to study the structure and
thermal properties of the crust. Initially crust cooling modelling studies
focussed on transient X-ray binaries with prolonged accretion outbursts (> 1
year) such that the crust would be significantly heated for the cooling to be
detectable. Here we present the results of applying a theoretical model to the
observed cooling curve after a short accretion outburst of only ~10 weeks. In
our study we use the 2010 outburst of the transiently accreting 11 Hz X-ray
pulsar in the globular cluster Terzan 5. Observationally it was found that the
crust in this source was still hot more than 4 years after the end of its short
accretion outburst. From our modelling we found that such a long-lived hot
crust implies some unusual crustal properties such as a very low thermal
conductivity (> 10 times lower than determined for the other crust cooling
sources). In addition, we present our preliminary results of the modelling of
the ongoing cooling of the neutron star in MXB 1659-298. This transient X-ray
source went back into quiescence in March 2017 after an accretion phase of ~1.8
years. We compare our predictions for the cooling curve after this outburst
with the cooling curve of the same source obtained after its previous outburst
which ended in 2001.Comment: 4 pages, 1 figure, to appear in the proceedings of "IAUS 337: Pulsar
Astrophysics - The Next 50 Years" eds: P. Weltevrede, B.B.P. Perera, L. Levin
Preston & S. Sanida
Kilohertz QPOs in Neutron Star Binaries modeled as Keplerian Oscillations in a Rotating Frame of Reference
Since the discovery of kHz quasi-periodic oscillations (QPO) in neutron star
binaries, the difference between peak frequencies of two modes in the upper
part of the spectrum, i.e. Delta (omega)=omega_h-omega_K has been studied
extensively. The idea that the difference Delta(omega) is constant and (as a
beat frequency) is related to the rotational frequency of the neutron star has
been tested previously. The observed decrease of Delta(omega) when omega_h and
omega_k increase has weakened the beat frequency interpretation. We put forward
a different paradigm: a Keplerian oscillator under the influence of the
Coriolis force. For such an oscillator, omega_h and the assumed Keplerian
frequency omega_k hold an upper hybrid frequency relation:
omega^2_h-omega^2_K=4*Omega^2, where Omega is the rotational frequency of the
star's magnetosphere near the equatorial plane. For three sources (Sco X-1, 4U
1608-52 and 4U 1702-429), we demonstrate that the solid body rotation
Omega=Omega_0=const. is a good first order approximation. Within the second
order approximation, the slow variation of Omega as a function of omega_K
reveals the structure of the magnetospheric differential rotation. For Sco X-1,
the QPO have frequencies approximately 45 and 90 Hz which we interpret as the
1st and 2nd harmonics of the lower branch of the Keplerian oscillations for the
rotator with vector Omega not aligned with the normal of the disk: omega_L/2
pi=(Omega/pi)(omega_K/omega_h)sin(delta) where delta is the angle between
vector Omega and the vector normal to the disk.Comment: 13 pages, 3 figures, accepted for publications in ApJ Letter
Discovery of coherent millisecond X-ray pulsations in Aql X-1
We report the discovery of an episode of coherent millisecond X-ray pulsation
in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for
slightly more than 150 seconds, during which the pulse frequency is consistent
with being constant. No X-ray burst or other evidence of thermonuclear burning
activity is seen in correspondence with the pulsation, which can thus be
identified as occurring in the persistent emission. The pulsation frequency is
550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from
burst oscillations in this source. Hence we identify this frequency with the
neutron star spin frequency. The pulsed fraction is strongly energy dependent,
ranging from 10% (16-30 keV). We discuss possible physical
interpretations and their consequences for our understanding of the lack of
pulsation in most neutron star low-mass X-ray binaries. If interpreted as
accretion-powered pulsation, Aql X-1 might play a key role in understanding the
differences between pulsating and non-pulsating sources.Comment: 5 pages, 3 figures, accepted by ApJ Letters after minor revisions.
Slightly extended discussion. One author added. Uses emulateapj.cl
Very hard states in neutron star low-mass X-ray binaries
We report on unusually very hard spectral states in three confirmed
neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and
IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting
the Swift X-ray spectra (0.5 - 10 keV) in those states with an absorbed
power-law model, we found photon indices of \Gamma ~ 1, significantly lower
than the \Gamma = 1.5 - 2.0 typically seen when such systems are in their so
called hard state. For individual sources very hard spectra were already
previously identified but here we show for the first time that likely our
sources were in a distinct spectral state (i.e., different from the hard state)
when they exhibited such very hard spectra. It is unclear how such very hard
spectra can be formed; if the emission mechanism is similar to that operating
in their hard states (i.e., up-scattering of soft photons due to hot electrons)
then the electrons should have higher temperatures or a higher optical depth in
the very hard state compared to those observed in the hard state. By using our
obtained \Gamma as a tracer for the spectral evolution with luminosity, we have
compared our results with those obtained by Wijnands et al. (2015). We confirm
their general results in that also our sample of sources follow the same track
as the other neutron star systems, although we do not find that the accreting
millisecond pulsars are systematically harder than the non-pulsating systems.Comment: Accepted for publication in MNRA
Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1
Aql X-1 is a prolific transient neutron star low-mass X-ray binary that
exhibits an accretion outburst approximately once every year. Whether the
thermal X-rays detected in intervening quiescent episodes are the result of
cooling of the neutron star or due to continued low-level accretion remains
unclear. In this work we use Swift data obtained after the long and bright 2011
and 2013 outbursts, as well as the short and faint 2015 outburst, to
investigate the hypothesis that cooling of the accretion-heated neutron star
crust dominates the quiescent thermal emission in Aql X-1. We demonstrate that
the X-ray light curves and measured neutron star surface temperatures are
consistent with the expectations of the crust cooling paradigm. By using a
thermal evolution code, we find that ~1.2-3.2 MeV/nucleon of shallow heat
release describes the observational data well, depending on the assumed
mass-accretion rate and temperature of the stellar core. We find no evidence
for varying strengths of this shallow heating after different outbursts, but
this could be due to limitations of the data. We argue that monitoring Aql X-1
for up to ~1 year after future outbursts can be a powerful tool to break model
degeneracies and solve open questions about the magnitude, depth and origin of
shallow heating in neutron star crusts.Comment: 14 pages, 5 figures, 3 tables, accepted to MNRA
Low-level accretion in neutron-star X-ray binaries
We search the literature for reports on the spectral properties of
neutron-star low-mass X-ray binaries when they have accretion luminosities
between 1E34 and 1E36 ergs/s. We found that in this luminosity range the photon
index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV
range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the
spectrum softens). Such behaviour has been reported before for individual
sources, but here we demonstrate that very likely most (if not all)
neutron-star systems behave in a similar manner and possibly even follow a
universal relation. When comparing the neutron-star systems with black-hole
systems, it is clear that most black-hole binaries have significantly harder
spectra at luminosities of 1E34 - 1E35 erg/s. Despite a limited number of data
points, there are indications that these spectral differences also extend to
the 1E35 - 1E36 erg/s range. This observed difference between the neutron-star
binaries and black-hole ones suggests that the spectral properties (between
0.5-10 keV) at 1E34 - 1E35 erg/s can be used to tentatively determine the
nature of the accretor in unclassified X-ray binaries. We discuss our results
in the context of properties of the accretion flow at low luminosities and we
suggest that the observed spectral differences likely arise from the
neutron-star surface becoming dominantly visible in the X-ray spectra. We also
suggest that both the thermal component and the non-thermal component might be
caused by low-level accretion onto the neutron-star surface for luminosities
below a few times 1E34 erg/s.Comment: Accepted for publication in MNRA
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