17 research outputs found
The origin of pulsating ultra-luminous X-ray sources: Low- and intermediate-mass X-ray binaries containing neutron star accretors
Ultra-luminous X-ray sources (ULXs) are those X-ray sources located away from
the centre of their host galaxy with luminosities exceeding the Eddington limit
of a stellar-mass black hole (). The discovery
of X-ray pulsations in some of these objects (e.g. M82~X-2) suggests that a
certain fraction of the ULX population may have a neutron star accretor. We
present systematic modelling of low- and intermediate-mass X-ray binaries
(LMXBs and IMXBs; donor-star mass range --~M and
neutron-star accretors) to explain the formation of this sub-population of
ULXs. Using MESA, we explored the allowed initial parameter space of binary
systems consisting of a neutron star and a low- or intermediate-mass donor star
that could explain the observed properties of ULXs. Our simulations take into
account beaming effects, stellar rotation, general angular momentum losses, and
a detailed and self-consistent calculation of the mass-transfer rate. We study
the conditions that lead to dynamical stability of these systems, which depends
strongly on the response of the donor star to mass loss. Using two values for
the initial neutron star mass (~M and ~M), we
present two sets of mass-transfer calculation grids. We find that LMXBs/IMXBs
can produce NS-ULXs with typical time-averaged isotropic-equivalent X-ray
luminosities of -- on a timescale up to
for the lower luminosities. We also estimate their
likelihood of detection, the types of white-dwarf remnants left behind by the
donors, and the total amount of mass accreted by the neutron stars. We also
compare our results to the observed pulsating ULXs. Our results suggest that a
large subset of the observed pulsating ULX population can be explained by
LMXBs/IMXBs undergoing a super-Eddington mass-transfer phase.Comment: 19 pages, 13 figures, Accepted by A&A. Parameter space was increased
to include low-mass XRBs and corresponding changes made to the text
(including the title) and figures 4, 6-11. Changed axes for figures 1 and 2.
Fixed typos and updated references. Added arguments about why spin period is
not an accurate reflection of mass accretion rate in the introductio
Exploring the nature of ultra-luminous X-ray sources across stellar population ages using detailed binary evolution calculations
Ultra-luminous X-ray sources (ULXs) are sources observed to exceed the
Eddington limit of a stellar-mass black hole (BH). A fraction of ULX sources
show X-ray pulses which are evidence for accreting neutron stars (NSs).
Theoretical studies have suggested that NSs dominate the compact objects of
intrinsic ULXs, even though the majority of observed sample is non-pulsating,
implying that X-ray pulses from many NS ULXs are unobservable. We use POSYDON
to generate and study X-ray binary populations spanning starburst ages 5 to
1000Myr. Following theoretical predictions for the alignment of the NS spin
axis with the accretion disc, we estimate the required accreted mass in ULXs so
that the alignment suppresses observable X-ray pulses. While the properties of
ULXs are sensitive to model assumptions, there are certain trends that the
populations follow. Young and old stellar populations are dominated by BH and
NS accretors, respectively. The donors go from massive H-rich main-sequence
(MS) stars in young populations (<100Myr) to low-mass post-MS H-rich stars in
older populations (>100Myr), with stripped He-rich giant stars dominating the
populations at around 100Myr. In addition, we find that NS ULXs exhibit
stronger geometrical beaming than BH ULXs, leading to an under-representation
of NS accretors in observed populations. Coupled with our finding that X-ray
pulses are suppressed in at least 60% of the NS ULXs, we suggest that the
observed fraction of ULXs with detectable X-ray pulses is very small, in
agreement with observations. This study investigates the effects of age on ULXs
as well as the effects of different model assumptions on ULX demographics. We
show that geometrical beaming and the mass-accretion phase are critical aspects
of understanding ULX observations. Our results suggest that even though most
ULXs have accreting NSs, those with observable X-ray pulses would be very few.Comment: 22 pages, 12 figures, Accepted by A&A. Fixed typos and updated
references. Referee's comments were addresse
X-ray luminosity function of high-mass X-ray binaries: Studying the signatures of different physical processes using detailed binary evolution calculations
The ever-expanding observational sample of X-ray binaries (XRBs) makes them
excellent laboratories for constraining binary evolution theory. Such
constraints can be obtained by studying the effects of various physical
assumptions on synthetic X-ray luminosity functions (XLFs) and comparing to
observed XLFs. In this work, we focus on high-mass XRBs (HMXBs) and study the
effects on the XLF of various, poorly-constrained assumptions regarding
physical processes such as the common-envelope phase, the core-collapse, and
wind-fed accretion. We use the new binary population synthesis code POSYDON,
which employs extensive pre-computed grids of detailed stellar structure and
binary evolution models, to simulate the evolution of binaries. We generate 96
synthetic XRB populations corresponding to different combinations of model
assumptions. The generated HMXB XLFs are feature-rich, deviating from the
commonly assumed single-power law. We find a break in our synthetic XLF at
luminosity erg s, similar to observed XLFs. However, we
find also a general overabundance of XRBs (up to a factor of 10 for
certain model parameter combinations) driven primarily by XRBs with black hole
accretors. Assumptions about the transient behavior of Be-XRBs, asymmetric
supernova kicks, and common-envelope physics can significantly affect the shape
and normalization of our synthetic XLFs. We find that less well-studied
assumptions regarding the circularization of the orbit at the onset of
Roche-lobe overflow and criteria for the formation of an X-ray emitting
accretion disk around wind-accreting black holes can also impact our synthetic
XLFs. Our study reveals the importance of large-scale parameter studies,
highlighting the power of XRBs in constraining binary evolution theory.Comment: 31 pages, 32 figures, Accepted by A&A. Fixed typos and updated
references. Referee's comments were addresse
Active Learning for Computationally Efficient Distribution of Binary Evolution Simulations
Binary stars undergo a variety of interactions and evolutionary phases,
critical for predicting and explaining observed properties. Binary population
synthesis with full stellar-structure and evolution simulations are
computationally expensive requiring a large number of mass-transfer sequences.
The recently developed binary population synthesis code POSYDON incorporates
grids of MESA binary star simulations which are then interpolated to model
large-scale populations of massive binaries. The traditional method of
computing a high-density rectilinear grid of simulations is not scalable for
higher-dimension grids, accounting for a range of metallicities, rotation, and
eccentricity. We present a new active learning algorithm, psy-cris, which uses
machine learning in the data-gathering process to adaptively and iteratively
select targeted simulations to run, resulting in a custom, high-performance
training set. We test psy-cris on a toy problem and find the resulting training
sets require fewer simulations for accurate classification and regression than
either regular or randomly sampled grids. We further apply psy-cris to the
target problem of building a dynamic grid of MESA simulations, and we
demonstrate that, even without fine tuning, a simulation set of only
the size of a rectilinear grid is sufficient to achieve the same classification
accuracy. We anticipate further gains when algorithmic parameters are optimized
for the targeted application. We find that optimizing for classification only
may lead to performance losses in regression, and vice versa. Lowering the
computational cost of producing grids will enable future versions of POSYDON to
cover more input parameters while preserving interpolation accuracies.Comment: 20 pages (16 main text), 10 figures, submitted to Ap
A Black Hole Kicked At Birth: MAXI J1305-704
When a compact object is formed in a binary, any mass lost during core
collapse will impart a kick on the binary's center of mass. Asymmetries in this
mass loss would impart an additional natal kick on the remnant black hole or
neutron star, whether it was formed in a binary or in isolation. While it is
well established that neutron stars receive natal kicks upon formation, it is
unclear whether black holes do as well. Here, we consider the low-mass X-ray
binary MAXI J1305-704, which has been reported to have a space velocity
200 km/s. In addition to integrating its trajectory to infer its
velocity upon formation of its black hole, we reconstruct its evolutionary
history, accounting for recent estimates of its period, black hole mass, mass
ratio, and donor effective temperature from photometric and spectroscopic
observations. We find that if MAXI J1305-704 formed via isolated binary
evolution in the thick Galactic disk, then its black hole received a natal kick
of at least 70 km/s with 95\% confidence.Comment: To be submitted; 9 pages, 5 figure
The impact of mass-transfer physics on the observable properties of field binary black hole populations
We study the impact of mass-transfer physics on the observable properties of
binary black hole populations formed through isolated binary evolution. We
investigate the impact of mass-accretion efficiency onto compact objects and
common-envelope efficiency on the observed distributions of ,
and . We find that low common envelope efficiency translates to
tighter orbits post common envelope and therefore more tidally spun up
second-born black holes. However, these systems have short merger timescales
and are only marginally detectable by current gravitational-waves detectors as
they form and merge at high redshifts (), outside current detector
horizons. Assuming Eddington-limited accretion efficiency and that the
first-born black hole is formed with a negligible spin, we find that all
non-zero systems in the detectable population can come only from
the common envelope channel as the stable mass-transfer channel cannot shrink
the orbits enough for efficient tidal spin-up to take place. We find the local
rate density () for the common envelope channel is in the range
considering a range of while for the stable mass transfer channel the rate density is . The latter drops by two orders of magnitude if the mass
accretion onto the black hole is not Eddington limited because conservative
mass transfer does not shrink the orbit as efficiently as non-conservative mass
transfer does. Finally, using GWTC-2 events, we constrain the lower bound of
branching fraction from other formation channels in the detected population to
be . Assuming all remaining events to be formed through either stable
mass transfer or common envelope channels, we find moderate to strong evidence
in favour of models with inefficient common envelopes.Comment: 26 pages, 13 figures, accepted for publication in A&
Investigating the Lower Mass Gap with Low Mass X-ray Binary Population Synthesis
Mass measurements from low-mass black hole X-ray binaries (LMXBs) and radio
pulsars have been used to identify a gap between the most massive neutron stars
(NSs) and the least massive black holes (BHs). BH mass measurements in LMXBs
are typically only possible for transient systems: outburst periods enable
detection via all-sky X-ray monitors, while quiescent periods enable
radial-velocity measurements of the low-mass donor. We quantitatively study
selection biases due to the requirement of transient behavior for BH mass
measurements. Using rapid population synthesis simulations (COSMIC), detailed
binary stellar-evolution models (MESA), and the disk instability model of
transient behavior, we demonstrate that transient-LMXB selection effects
introduce observational biases, and can suppress mass-gap BHs in the observed
sample. However, we find a population of transient LMXBs with mass-gap BHs form
through accretion-induced collapse of a NS during the LMXB phase, which is
inconsistent with observations. These results are robust against variations of
binary evolution prescriptions. The significance of this accretion-induced
collapse population depends upon the maximum NS birth mass . To reflect the observed dearth of low-mass BHs, COSMIC and MESA
models favor . In the absence of
further observational biases against LMXBs with mass-gap BHs, our results
indicate the need for additional physics connected to the modeling of LMXB
formation and evolution.Comment: 21 pages, accepted to Ap
POSYDON: A General-Purpose Population Synthesis Code with Detailed Binary-Evolution Simulations
Most massive stars are members of a binary or a higher-order stellar systems,
where the presence of a binary companion can decisively alter their evolution
via binary interactions. Interacting binaries are also important astrophysical
laboratories for the study of compact objects. Binary population synthesis
studies have been used extensively over the last two decades to interpret
observations of compact-object binaries and to decipher the physical processes
that lead to their formation. Here, we present POSYDON, a novel, binary
population synthesis code that incorporates full stellar-structure and
binary-evolution modeling, using the MESA code, throughout the whole evolution
of the binaries. The use of POSYDON enables the self-consistent treatment of
physical processes in stellar and binary evolution, including: realistic
mass-transfer calculations and assessment of stability, internal
angular-momentum transport and tides, stellar core sizes, mass-transfer rates
and orbital periods. This paper describes the detailed methodology and
implementation of POSYDON, including the assumed physics of stellar- and
binary-evolution, the extensive grids of detailed single- and binary-star
models, the post-processing, classification and interpolation methods we
developed for use with the grids, and the treatment of evolutionary phases that
are not based on pre-calculated grids. The first version of POSYDON targets
binaries with massive primary stars (potential progenitors of neutron stars or
black holes) at solar metallicity.Comment: 60 pages, 33 figures, 8 tables, referee's comments addressed. The
code and the accompanying documentations and data products are available at
https:\\posydon.or
Formation and evolution of X-ray binaries: Exploring Ultra-luminous X-ray Sources
X-ray binaries (XRBs) are some of the brightest X-ray sources in the Universe. The emission from XRBs is produced by the transfer of material from a star onto a compact object serving as its companion. The brightest of XRBs are ultra-luminous X-ray sources (ULXs), with X-ray luminosities greater than 1039 erg s-1 (which is about the Eddington limit of a 10Msun black hole). Observations of coherent X-ray pulsations in a subset of ULXs indicate the presence of an accreting neutron star. Considering that the Eddington limit of a neutron star is ~1038 erg s-1, it makes these pulsating ULXs highly super-Eddington. This thesis aims at studying the formation and evolution of ULXs, and by extension XRBs as a whole. We investigate ULXs in the form of XRBs undergoing super-Eddington mass-transfer phases and include super-Eddington disc models to help explain their luminosities. In doing so, we also investigate the stability of mass transfer in XRBs and formulate a method of constraining the mass-transfer stability in semi-detached binaries, using the overflow from the second Lagrange point as a constraint. We study the demographic of ULX populations at different ages of stellar populations, and the effect of different assumptions of accretion physics on ULXs. In order to carry out these studies, we used population synthesis techniques in combination with detailed stellar and binary evolutionary calculations. Given the uncertainties that currently exist in our understanding of binary interactions and assumptions that need to be made for evolutionary phases (for instance, the efficiency in the common-envelope phase), we carried out a parameter study with the aim of constraining the said physics. We base our studies of ULX populations on the results of the parameter study and draw conclusions from the inferred information, finding that observations of ULXs could be used to constrain the treatment of kicks in binaries and that NS-ULXs are almost always geometrically beamed whereas BH-ULXs are beamed in ~50% of the cases. Future studies, including a wider range of metallicities, will aid us better in our understanding of sources like ULXs that are more abundant in sub-solar metallicities.
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BMI does not accurately predict overweight in Asian Indians in northern India
Asian Indians are at high risk for the development of atherosclerosis and related complications, possibly initiated by higher body fat (BF). The present study attempted to establish appropriate cut-off levels of the BMI for defining overweight, considering percentage BF in healthy Asian Indians in northern India as the standard. A total of 123 healthy volunteers (eighty-six males aged 18–75 years and thirty-seven females aged 20–69 years) participated in the study. Clinical examination and anthropometric measurements were performed, and percentage BF was calculated. BMI for males was 21·4 (SD 3·7) KG/M2 AND FOR FEMALES WAS 23·3 (sd 5·5) kg/m2. Percentage BF was 21·3 (sd 7·6) in males and 35·4 (sd 5·0) in females. A comparison of BF data among Caucasians, Blacks, Polynesians and Asian ethnic groups (e.g. immigrant Chinese) revealed conspicuous differences. Receiver operating characteristic (ROC) curve analysis showed a low sensitivity and negative predictive value of the conventional cut-off value of the BMI (25 kg/m2) in identifying subjects with overweight as compared to the cut-off value based on percentage BF (males >25, females >30). This observation is particularly obvious in females, resulting in substantial misclassification. Based on the ROC curve, a lower cut-off value of the BMI (21·5 kg/m2 for males and 19·0 kg/m2 for females) displayed the optimal sensitivity and specificity, and less misclassification in identification of subjects with high percentage BF. Furthermore, a novel obesity variable, BF:BMI, was tested and should prove useful for interethnic comparison of body composition. In the northern Indian population, the conventional cut-off level of the BMI underestimates overweight and obesity when percentage BF is used as the standard to define overweight. These preliminary findings, if confirmed in a larger number of subjects and with the use of instruments having a higher accuracy of BF assessment, would be crucial for planning and the prevention and treatment of various obesity-related metabolic diseases in the Asian Indian population