121 research outputs found
The mass and the radius of the neutron star in the transient low mass X-ray binary SAX J1748.9−2021
We use time resolved spectroscopy of thermonuclear X-ray bursts observed from SAX J1748.9−2021 to infer the mass and the radius of the neutron star in the binary. Four X-ray bursts observed from the source with RXTE enable us to measure the angular size and the Eddington limit on the neutron star surface. Combined with a distance measurement to the globular cluster NGC 6440, in which SAX J1748.9−2021 resides, we obtain two solutions for the neutron star radius and mass, R = 8.18±1.62 km and M = 1.78±0.3 M_sun or R = 10.93±2.09 km and M = 1.33 ± 0.33 M_sun
Classification and Spectral Evolution of Outbursts of Aql X-1
We present a broad classification of all outbursts detected with the All-Sky
Monitor (ASM) on the Rossi X-Ray Timing Explorer (RXTE) and the Monitor of All
Sky X-Ray Image (MAXI) of Aql X-1. We identify three types of outbursts;
long-high, medium-low, and short-low, based on the duration and maximum flux.
We analyse the trends in the "phase-space" of flux-derivative versus flux to
demonstrate the differences in the three identified outburst types. We present
a spectral analysis of the observations of Aql X-1 performed by the
Proportional Counter Array (PCA) onboard RXTE during the 2000 and 2011
outbursts of the long-high class and the 2010 outburst of the medium-low class.
We model the source spectrum with a hybrid thermal/non-thermal hot plasma
emission model (EQPAIR in XSPEC, Coppi 2000) together with a Gaussian component
to model the Fe K_alpha emission line. We construct time histories of the
source flux, the optical depth of the corona (tau), the seed photon temperature
(kT_bb) and the hard state compactness (l_h) for these three outbursts. We show
that the physical parameters of either classes reach the same values throughout
the outbursts, the only difference being the maximum flux. We discuss our
results in the terms of modes of interaction of the star with the disc and size
of the disc kept hot by irradiation. We conclude that irradiation is the
dominant physical process leading to the different classes of outbursts.Comment: MNRAS accepted. 12 pages, 9 figures, 3 table
Mapping the Surface of the Magnetar 1E 1048.1-5937 in Outburst and Quiescence Through Phase Resolved X-ray Spectroscopy
We model the pulse profiles and the phase resolved spectra of the anomalous
X-ray pulsar 1E 1048.1-5937 obtained with XMM-Newton to map its surface
temperature distribution during an active and a quiescent epoch. We develop and
apply a model that takes into account the relevant physical and geometrical
effects on the neutron star surface, magnetosphere, and spacetime. Using this
model, we determine the observables at infinity as a function of pulse phase
for different numbers and sizes of hot spots on the surface. We show that the
pulse profiles extracted from both observations can be modeled with a single
hot spot and an antipodal cool component. The size of the hot spot changes from
in 2007, 3 months after the onset of a dramatic flux
increase, to during the quiescent observation in 2011,
when the pulsed fraction returned to the pre-outburst 65\% level. For
the 2007 observation, we also find that a model consisting of a single 0.4 keV
hot spot with a magnetic field strength of G accounts for
the spectra obtained at three different pulse phases but under predicts the
flux at the pulse minimum, where the contribution to the emission from the
cooler component is non-negligible. The inferred temperature of the spot stays
approximately constant between different pulse phases, in agreement with a
uniform temperature, single hot spot model. These results suggest that the
emitting area grows significantly during outbursts but returns to its
persistent and significantly smaller size within a few year timescale.Comment: Accepted for publication in The Astrophysical Journa
Partial accretion in the propeller stage of low mass X-ray binary Aql X--1
Aql X--1 is one of the most prolific low mass X-ray binary transients
(LMXBTs) showing outbursts almost annually. We present the results of our
spectral analyses of RXTE/PCA observations of the 2000 and the 2011 outbursts.
We investigate the spectral changes related to the changing disk-magnetosphere
interaction modes of Aql X--1. The X-ray light curves of the outbursts of
LMXBTs typically show phases of fast rise and exponential decay. The decay
phase shows a "knee" where the flux goes from the slow decay to the rapid decay
stage. We assume that the rapid decay corresponds to a weak propeller stage at
which a fraction of the inflowing matter in the disk accretes onto the star. We
introduce a novel method for inferring, from the light curve, the fraction of
the inflowing matter in the disk that accretes onto the NS depending on the
fastness parameter. We determine the fastness parameter range within which the
transition from the accretion to the partial propeller stage is realized. This
fastness parameter range is a measure of the scale-height of the disk in units
of the inner disk radius. We applied the method to a sample of outbursts of Aql
X--1 with different maximum flux and duration times. We show that different
outbursts with different maximum luminosity and duration follow a similar path
in the parameter space of accreted/inflowing mass flux fraction versus fastness
parameter.Comment: 16 pages, 5 figures, 5 tables, accepted for publication in Ap
The Distances of the Galactic Novae
Utilising the unique location of red clump giants on colour-magnitude
diagrams obtained from various near-IR surveys, we derived specific
reddening-distance relations towards 119 Galactic novae for which independent
reddening measurements are available. Using the derived distance-extinction
relation and the independent measurements of reddening we calculated the most
likely distances for each system. We present the details of our distance
measurement technique and the results of this analysis, which yielded the
distances of 73 Galactic novae and allowed us to set lower limits on the
distances of 46 systems. We also present the reddening-distance relations
derived for each nova, which may be useful to analyze the different Galactic
components present in the line of sight.Comment: Published in MNRAS (24 pages, 12 figures, 4 tables), Minor typo
correction. For the catalogue which has all reddening-distance relations, see
http://highenergyastro.istanbul.edu.tr/novae_cat/index.php,
2016MNRAS.461.1177
Population types of cataclysmic variables in the solar neighborhood
The Galactic orbital parameters of 159 cataclysmic variables in the Solar neighbourhood are calculated, for the first time, to determine their population types using published kinematical parameters. Population analysis shows that about 6 per cent of cataclysmic variables in the sample are members of the thi
ck disc component of the Galaxy. This value is consistent with the fraction obt
ained from star count analysis. The rest of the systems in the sample are found to be in the thin disc component of the Galaxy. Our analysis revealed no halo CVs in the Solar vicinity. About 60 per cent of the thick disc CVs have orbital periods be low the orbital period gap. This result is roughly consistent with the predictions of population synthesis models developed for cataclysmic variables. A kinematical age of 13 Gyr is obtained using total space velocity dispersion of the most probable thick disc CVs which is consistent with the age of thick disc component of the Galaxy
The Magnetar Nature and the Outburst Mechanism of a Transient Anomalous X-ray Pulsar
Anomalous X-ray Pulsars (AXPs) belong to a class of neutron stars believed to
harbor the strongest magnetic fields in the universe, as indicated by their
energetic bursts and their rapid spindowns. However, an unambiguous measurement
of their surface field strengths has not been made to date. It is also not
known whether AXP outbursts result from changes in the neutron star magnetic
field or crust properties. Here we report a spectroscopic measurement of the
surface magnetic field strength of an AXP, XTE J1810197, and solidify its
magnetar nature. The field strength obtained from detailed spectral analysis
and modeling, B = (2.720.03) G, is remarkably close to the
value inferred from the rate of spindown of this source and remains nearly
constant during numerous observations spanning over an order of magnitude in
source flux. The surface temperature, on the other hand, declines steadily and
dramatically following the 2003 outburst of this source. Our findings
demonstrate that heating occurs in the upper neutron star crust during an
outburst and sheds light on the transient behaviour of AXPs.Comment: Accepted by ApJ Letters with minor change
Multiwavelength observations of the black hole transient XTE J1752-223 during its 2010 outburst decay
Galactic black hole transients show many interesting phenomena during outburst decays. We present simultaneous X-ray (RXTE, Swift, and INTEGRAL), and optical/near-infrared (O/NIR) observations (SMARTS), of the X-ray transient, XTE J1752-223 during its outburst decay in 2010. The multi- wavelength observations of 150 days in 2010 cover the transition from soft to hard spectral state. The evolution of ATCA/VLBI radio observations are shown to confirm the compact jet appearance. The source shows flares in O/NIR during changes in X-ray and radio properties. One of those flares is bright and long, and starts about 20 days after the transition in timing. Other, smaller flares occur along with the transition in timing and increase in power-law flux, and also right after the detection of the core with VLBI. Furthermore, using the simultaneous broadband X-ray spectra including IN- TEGRAL, we found that a high energy cut-off is necessary with a folding energy at around 250 keV around the time that the compact jet is forming. The broad band spectrum can also be fitted equally well with a Comptonization model. In addition, using photoelectric absorption edges in the XMM– Newton RGS X-ray spectra and the extinction of red clump giants in the direction of the source, we found a lower limit on the distance of > 5 kpc
A New Catalogue of Galactic Novae: Investigation of the MMRD relation & Spatial Distribution
In this study, a new Galactic novae catalogue is introduced collecting
important parameters of these sources such as their light curve parameters,
classifications, full width half maximum (FWHM) of H line, distances
and interstellar reddening estimates. The catalogue is also published on a
website with a search option via a SQL query and an online tool to re-calculate
the distance/reddening of a nova from the derived reddening-distance relations.
Using the novae in the catalogue, the existence of a maximum magnitude-rate of
decline (MMRD) relation in the Galaxy is investigated. Although an MMRD
relation was obtained, a significant scattering in the resulting MMRD
distribution still exists. We suggest that the MMRD relation likely depends on
other parameters in addition to the decline time, as FWHM H, the light
curve shapes. Using two different samples depending on the distances in the
catalogue and from the derived MMRD relation, the spatial distributions of
Galactic novae as a function of their spectral and speed classes were studied.
The investigation on the Galactic model parameters implies that best estimates
for the local outburst density are 3.6 and 4.2 pc
yr with a scale height of 148 and 175 pc, while the space density
changes in the range of pc. The local
outburst density and scale height obtained in this study infer that the disk
nova rate in the Galaxy is in the range of to yr with
an average estimate yr.Comment: 24 pages, 9 figures, 6 tables; Accepted for publication in MNRAS on
13 Feb 201
Systematic Uncertainties in the Spectroscopic Measurements of Neutron-Star Masses and Radii from Thermonuclear X-ray Bursts. II. Eddington Limit
Time resolved X-ray spectroscopy of thermonuclear bursts observed from low
mass X-ray binaries offer a unique tool to measure neutron star masses and
radii. In this paper, we continue our systematic analysis of all the X-ray
bursts observed with RXTE from X-ray binaries. We determine the events which
show clear evidence for photospheric radius expansion and measure the Eddington
limits for these accreting neutron stars using the bolometric fluxes attained
at the touchdown moments of each X-ray burst. We employ a Bayesian technique to
investigate the degree to which the Eddington limit for each source remains
constant between bursts. We find that for sources with a large number of radius
expansion bursts, systematic uncertainties are at a 5-10% level. Moreover, in
six sources with only pairs of Eddington-limited bursts, the distribution of
fluxes is consistent with a ~10% fractional dispersion. This indicates that the
spectroscopic measurements of neutron star masses and radii using thermonuclear
X-ray bursts can reach the level of accuracy required to distinguish between
different neutron star equations of state, provided that uncertainties related
to the overall flux calibration of X-ray detectors are of comparable magnitude.Comment: accepted for publication in the February 2012 issue of the
Astrophysical Journa
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