22 research outputs found
Ways to constrain neutron star equation of state models using relativistic disc lines
Relativistic spectral lines from the accretion disc of a neutron star
low-mass X-ray binary can be modelled to infer the disc inner edge radius. A
small value of this radius tentatively implies that the disc terminates either
at the neutron star hard surface, or at the innermost stable circular orbit
(ISCO). Therefore an inferred disc inner edge radius either provides the
stellar radius, or can directly constrain stellar equation of state (EoS)
models using the theoretically computed ISCO radius for the spacetime of a
rapidly spinning neutron star. However, this procedure requires numerical
computation of stellar and ISCO radii for various EoS models and neutron star
configurations using an appropriate rapidly spinning stellar spacetime. We have
fully general relativistically calculated about 16000 stable neutron star
structures to explore and establish the above mentioned procedure, and to show
that the Kerr spacetime is inadequate for this purpose. Our work systematically
studies the methods to constrain EoS models using relativistic disc lines, and
will motivate future X-ray astronomy instruments.Comment: 6 pages, 9 figures, published in MNRA
Spectral states evolution of 4U 1728-34 observed by INTEGRAL and RXTE: non-thermal component detection
We report results of a one-year monitoring of the low mass X-ray binary
(LMXB) source (atoll type) 4U 1728-34 with INTEGRAL and RXTE. Three time
intervals were covered by INTEGRAL, during which the source showed strong
spectral evolution. We studied the broad-band X-ray spectra in detail by
fitting several models in the different sections of the hardness-intensity
diagram. The soft states are characterised by prominent blackbody emission plus
a contribution from a Comptonized emission. The hard states are characterised
by the presence of an excess flux with respect to the Comptonization model
above 50 keV while the soft component is fainter. To obtain an acceptable fit
to the data this excess is modeled either with a power law with photon index
Gamma ~ 2 or a Comptonization (CompPS) spectrum implying the presence of hybrid
thermal and non-thermal electrons in a corona. This makes 4U 1728-34 one of the
few LMXBs of atoll type showing non-thermal emission at high energy. From our
analysis, it is also apparent that the presence of the hard tail is more
prominent as the overall spectrum becames harder. We discuss also alternative
models which can discribe these hard states.Comment: Accepted for publication in MNRAS (accepted 2011 April 20. Received
2011 April 20; in original form 2010 December 07); 9 pages, 7 figure
A complete X-ray spectral coverage of the 2010 May-June outbursts of Circinus X-1
Circinus X-1 is a neutron-star-accreting X-ray binary in a wide (P = 16.6 d), eccentric orbit. After two years of relatively low X-ray
luminosity, in May 2010 Circinus X-1 went into outburst, reaching 0.4 Crab
flux. This outburst lasted for about two orbital cycles and was followed by
another shorter and fainter outburst in June. We focus here on the broadband
X-ray spectral evolution of the source as it spans about three order of
magnitudes in flux. We attempt to relate luminosity, spectral shape, local
absorption, and orbital phase. We use multiple Rossi-XTE/PCA (3.0--25 keV) and
Swift/XRT (1.0--9.0 keV) observations and a 20 ks long Chandra/HETGS
observation (1.0--9.0 keV), to comprehensively track the spectral evolution of
the source during all the outbursting phases. These observations were taken
every two/three days and cover about four orbital cycles. The PCA data mostly
cover the major outburst, the XRT data monitor the declining phase of the major
outburst and all the phases of the minor outburst, and Chandra data provide an
essential snapshot of the end of this overall outbursting phase. The X-ray
spectrum can be satisfactorily described by a thermal Comptonization model with
variable neutral local absorption in all phases of the outburst. No other
additive component is statistically required. The first outburst decays
linearly, with an ankle in the light curve as the flux decreases below
\,5 10 erg cm s. At the same time, the
source shows a clear spectral state transition from an optically thick to an
optically thin state. While the characteristics of the first, bright, outburst
can be interpreted within the disk-instability scenario, the following, minor,
outburst shows peculiarities that cannot be easily reconciled in this
framework.Comment: Accepted for publication in Astronomy and Astrophysic
ALMA observations of 4U 1728 - 34 and 4U 1820 - 30: First detection of neutron star X-ray binaries at 300 GHz
We report on the first observations of neutron star low-mass X-ray binaries with the Atacama Large Millimeter/submillimeter Array (ALMA) at ~300 GHz. Quasi-simultaneous observations of 4U 1728-34 and 4U 1820-30 were performed at radio (ATCA), infrared (VLT) and X-ray (Swift) frequencies, spanning more than eight decades in frequency coverage. Both sources are detected at high significance with ALMA. The spectral energy distribution of 4U 1728-34 is consistent with synchrotron emission from a jet with a break from optically thick to optically thin emission at 1.3-11.0 à 1013 Hz. This is the third time a jet spectral break has been reported for a neutron star X-ray binary. The radio to mm spectral energy distribution of 4U 1820-30 has significant detections at 5 and 300 GHz. This confirms the presence of radio emission during a soft state for this neutron star and represents the first detection of mm emission during such a state, unambiguously pointing to the presence of a jet. We also report on three additional unrelated sources - showing mm emission - in the ALMA fields of view of 4U 1728-34 and 4U 1820-30. © ESO, 2017
Measurement of neutron star parameters: a review of methods for low-mass X-ray binaries
Measurement of at least three independent parameters, for example, mass,
radius and spin frequency, of a neutron star is probably the only way to
understand the nature of its supranuclear core matter. Such a measurement is
extremely difficult because of various systematic uncertainties. The lack of
knowledge of several system parameter values gives rise to such systematics.
Low-mass X-ray binaries, which contain neutron stars, provide a number of
methods to constrain the stellar parameters. Joint application of these methods
has a great potential to significantly reduce the systematic uncertainties, and
hence to measure three independent neutron star parameters accurately. Here we
review the methods based on (1) thermonuclear X-ray bursts; (2)
accretion-powered millisecond-period pulsations; (3) kilohertz quasi-periodic
oscillations; (4) broad relativistic iron lines; (5) quiescent emissions; and
(6) binary orbital motions.Comment: 30 pages, 20 figures, 1 table, An Invited and Refereed Review, will
be published in "Advances in Space Research
A Preliminary Analysis of a New Chandra Observation (ObsID 6148) of Cir X-1
We present the preliminary spectral analysis of a 25 ks long Chandra observation of the peculiar source Cir X-1 near the periastron passage. We estimate more precise coordinates of the source compatible with the optical and radio counterpart coordinates. We detect emission lines associated to Mg XII, Si XIII, Si XIV, S XV, S XVI Ar XVII, Ar XVIII, Ca XIX, Ca XX, Fe XXV, Fe XXVI showing a redshift of 470 km s-1. The more intense emission features at 6.6 keV show a double-peaked shape that can be modelled with two or three Gaussian lines
A Preliminary Analysis of a New Chandra Observation (ObsID 6148) of Cir X-1
We present the preliminary spectral analysis of a 25 ks long Chandra observation of the peculiar source Cir X-1 near the periastron passage. We estimate more precise coordinates of the source compatible with the optical and radio counterpart coordinates. We detect emission lines associated to Mg XII, Si XIII, Si XIV, S XV, S XVI Ar XVII, Ar XVIII, Ca XIX, Ca XX, Fe XXV, Fe XXVI showing a redshift of 470 km s-1. The more intense emission features at 6.6 keV show a double-peaked shape that can be modelled with two or three Gaussian lines
The reflection component in NS LMXBs
Thanks to the good spectral resolution and large effective area of the EPIC/PN instrument on board of XMM-Newton, we have at hand a large number of observations of accreting low-mass X-ray binaries, that allow for the fist time a comprehensive view on the characteristics of the reflection component at different accretion regimes and to probe the effects of a magnetosphere on its formation. We focus here on a comparative analysis of the reflection component from a series of spectroscopic studies on selected sources: 4U 1705-44, observed both in the soft and hard state, the pulsating ms pulsars SAX J1808.4-3658 and IGR J17511-3057, and the intermittent pulsar HETE J1900-2455. Although the sources can present very similar accretion rates and continuum shapes, the reflection parameters do not generally result the same, moreover the effect of a magnetosphere on the formation of the reflection component appears elusive
Looking through the photoionisation wake: Vela Xâ1 at
Context. The supergiant X-ray binary Vela Xâ1 represents one of the best astrophysical sources to investigate the wind environment of an O/B star irradiated by an accreting neutron star. Previous studies and hydrodynamic simulations of the system have revealed a clumpy environment and the presence of two wakes: an accretion wake surrounding the compact object and a photoionisation wake trailing it along the orbit.
Aims. Our goal is to conduct, for the first time, high-resolution spectroscopy on Chandra/HETGS data at the orbital phase Ïorbâââ0.75, when the line of sight is crossing the photoionisation wake. We aim to conduct plasma diagnostics, inferring the structure and the geometry of the wind.
Methods. We performed a blind search employing a Bayesian block algorithm to find discrete spectral features and identify them thanks to the most recent laboratory results or through atomic databases. Plasma properties were inferred both with empirical techniques and with photoionisation models within CLOUDY and SPEX.
Results. We detect and identify five narrow radiative recombination continua (MgâŻXI-XI