Authors: Philipp M. Srivastava, Ugur Demir, Aggelos Katsaggelos, Vicky Kalogera, Elizabeth Teng, Tassos Fragos, Jeff J. Andrews, Simone S. Bavera, Max Briel, Seth Gossage, Konstantinos Kovlakas, Matthias U. Kruckow, Camille Liotine, Kyle A. Rocha, Meng Sun, Zepei Xing, Emmanouil Zapartas

Access: arXiv:2411.02586 || NASA ADS

Abstract: Modeling of large populations of binary stellar systems is an intergral part of a many areas of astrophysics, from radio pulsars and supernovae to X-ray binaries, gamma-ray bursts, and gravitational-wave mergers. Binary population synthesis codes that employ self-consistently the most advanced physics treatment available for stellar interiors and their evolution and are at the same time computationally tractable have started to emerge only recently. One element that is still missing from these codes is the ability to generate the complete time evolution of binaries with arbitrary initial conditions using pre-computed three-dimensional grids of binary sequences. Here we present a highly interpretable method, from binary evolution track interpolation. Our method implements simulation generation from irregularly sampled time series. Our results indicate that this method is appropriate for applications within binary population synthesis and computational astrophysics with time-dependent simulations in general. Furthermore we point out and offer solutions to the difficulty surrounding evaluating performance of signals exhibiting extreme morphologies akin to discontinuities.

Authors: Jeff J. Andrews, Simone S. Bavera, Max Briel, Abhishek Chattaraj, Aaron Dotter, Tassos Fragos, Monica Gallegos-Garcia, Seth Gossage, Vicky Kalogera, Eirini Kasdagli, Aggelos Katsaggelos, Chase Kimball, Konstantinos Kovlakas, Matthias U. Kruckow, Camille Liotine, Devina Misra, Kyle A. Rocha, Dimitris Souropanis, Phillip M. Srivastava, Philipp M. Srivastava, Meng Sun, Elizabeth Teng, Zepei Xing, Emmanouil Zapartas, Michael Zevin

Access: arXiv:2411.02376 || NASA ADS

Abstract: Whether considering rare astrophysical events on cosmological scales or unresolved stellar populations, accurate models must account for the integrated contribution from the entire history of star formation upon which that population is built. Here, we describe the second version of POSYDON, an open-source binary population synthesis code based on extensive grids of detailed binary evolution models computed using the \mesa code, which follows both stars' structures as a binary system evolves through its complete evolution from the zero-age main sequence, through multiple phases of mass transfer and supernovae, to their death as compact objects. To generate synthetic binary populations, POSYDON uses advanced methods to interpolate between our large, densely spaced grids of simulated binaries. In our updated version of POSYDON, we account for the evolution of stellar binaries across a cosmological range of metallicities, extending from 10^-4 Z_⊙ to 2 Z_⊙, including grids specifically focused on the Small and Large Magellanic Clouds (0.2Z_⊙ and 0.45 Z_⊙). In addition to describing our model grids and detailing our methodology, we outline several improvements to POSYDON. These include the incorporation of single stars in stellar populations, a treatment for stellar mergers, and a careful modeling of "reverse-mass transferring" binaries, in which an once-accreting star later becomes a donor star. Our simulations are focused on binaries with at least one high-mass component, such as those that host neutron stars and black holes, and we provide post-processing methods to account for the cosmological evolution of metallicity and star formation as well as rate calculations for gravitational wave events, gamma-ray bursts, and other transients.

Authors: Zepei Xing, Vicky Kalogera, Tassos Fragos, Jeff J. Andrews, Simone S. Bavera, Max Briel, Seth Gossage, Konstantinos Kovlakas, Matthias U. Kruckow, Kyle A. Rocha, Meng Sun, Philipp M. Srivastava, Emmanouil Zapartas

Access: arXiv:2410.20415 || NASA ADS

Abstract: The existence of a mass gap of 3-5 M_⊙ between the heaviest neutron stars (NSs) and the lightest black holes (BHs), inferred from the BH mass distribution in low mass X-ray binaries (LMXBs), has been suggested for decades. The recently reported gravitational-wave source GW230529 has been confidently identified as a neutron star--black hole (NSBH) merger, with the BH mass falling within this lower mass gap. This detection provides strong evidence against the existence of the latter and introduces new implications for the coalescing NSBH population, including a revised BH mass distribution and an updated local merger rate. In this study, we employ POSYDON, a binary population synthesis code that integrates detailed single- and binary-star models, to investigate coalescing NSBH binaries formed through isolated binary evolution. In particular, we focus on the BH mass distribution of the intrinsic NSBH merger population. We find that, with a high common-envelope efficiency of α_CE =2, the BH masses in NSBH mergers concentrate in the lower mass gap, aligning more closely with observations. However, after accounting for the constraints of the selection bias against mass-gap BHs in LMXBs, which suggests that the maximum NS birth mass is below ≃ 2 M_⊙, we find that introducing a high α_CE is not required to match observations. Additionally, we explore the impact of core-collapse supernova kicks on the BH mass distribution and the local merger rate density of NSBH mergers. Finally, we present the property distributions of observable NSBH mergers from our simulation and find that they match well with the observations. With a self-consistent estimate of BH spins, we find that the fraction of electromagnetic counterparts in observable populations is ≈ 4-30%, depending on different NS equations of state. Future detections of coalescing NSBH binaries would provide invaluable insights into SN mechanisms, common envelope evolution, and NS physics.

Authors: Matthias U. Kruckow, Jeff J. Andrews, Tassos Fragos, Berry Holl, Simone S. Bavera, Max Briel, Seth Gossage, Konstantinos Kovlakas, Kyle A. Rocha, Meng Sun, Philipp M. Srivastava, Zepei Xing, Emmanouil Zapartas

Access: arXiv:2410.18501 || NASA ADS

Abstract: Context. The black holes discovered using Gaia, especially Gaia BH1 and BH2, have low mass companions of solar-like metallicity in wide orbits. For standard isolated binary evolution formation channels including interactions such an extreme mass ratio is unexpected; especially in orbits of hundreds to thousands of days. Aims. Here, we investigate a non-interacting formation path for isolated binaries to explain the formation of Gaia BH1 and BH2. Methods. We use single star models computed with MESA to constrain the main characteristics of possible progenitors of long-period black hole binaries like Gaia BH1 and BH2. Then, we incorporate these model grids into the binary population synthesis code POSYDON, to explore whether the formation of the observed binaries at solar metallicity is indeed possible. Results. We find that winds of massive stars (≳80M⊙), especially during the Wolf-Rayet phase, tend to cause a plateau in the initial stellar mass to final black hole mass relation (at about 13M⊙ in our default wind prescription). However, stellar winds at earlier evolutionary phases are also important at high metallicity, as they prevent the most massive stars from expanding (<100R⊙) and filling their Roche lobe. Consequently, the strength of the applied winds affects the range of the final black hole masses in non-interacting binaries, making it possible to form systems similar to Gaia BH1 and BH2. Conclusions. We deduce that wide binaries with a black hole and a low mass companion can form at high metallicity without binary interactions. There could be hundreds of such systems in the Milky Way. The mass of the black hole in binaries evolved through the non-interacting channel can potentially provide insights into the wind strength during the progenitors evolution.

Authors: Elizabeth Teng, Ugur Demir, Zoheyr Doctor, Philipp M. Srivastava, Shamal Lalvani, Vicky Kalogera, Aggelos Katsaggelos, Jeff J. Andrews, Simone S. Bavera, Max M. Briel, Seth Gossage, Konstantinos Kovlakas, Matthias U. Kruckow, Kyle Akira Rocha, Meng Sun, Zepei Xing, Emmanouil Zapartas

Access: arXiv:2403.19743 || NASA ADS

Abstract: Knowledge about the internal physical structure of stars is crucial to understanding their evolution. The novel binary population synthesis code POSYDON includes a module for interpolating the stellar and binary properties of any system at the end of binary MESA evolution based on a pre-computed set of models. In this work, we present a new emulation method for predicting stellar profiles, i.e., the internal stellar structure along the radial axis, using machine learning techniques. We use principal component analysis for dimensionality reduction and fully-connected feed-forward neural networks for making predictions. We find accuracy to be comparable to that of nearest neighbor approximation, with a strong advantage in terms of memory and storage efficiency. By delivering more information about the evolution of stellar internal structure, these emulators will enable faster simulations of higher physical fidelity with large-scale simulations of binary star population synthesis possible with POSYDON and other population synthesis codes.

Authors: Meng Sun, Seth Gossage, Emily M. Leiner, Aaron M. Geller

Access: arXiv:2403.17279 || NASA ADS

Abstract: Motivated by measurements of the rotation speed of accretor stars in post-mass-transfer (post-MT) systems, we investigate how magnetic braking affects the spin-down of individual stars during binary evolution with the MESAbinary module. Unlike the conventional assumption of tidal synchronization coupled with magnetic braking in binaries, we first calculate whether tides are strong enough to synchronize the orbit. Subsequently, this influences the spin-down of stars and the orbital separation. In this study, we apply four magnetic braking prescriptions to reduce the spin angular momentum of the two stars throughout the entire binary evolution simulation. Our findings reveal that despite magnetic braking causing continuous spin-down of the accretor, when the donor begins to transfer material onto the accretor, the accretor can rapidly spin up to its critical rotation rate. After MT, magnetic braking becomes more important in affecting the angular momentum evolution of the stars. Post-MT accretor stars thus serve as a valuable testbed for observing how the magnetic braking prescriptions operate in spinning down stars from their critical rotation, including the saturation regimes of the magnetic braking. The rotation rate of the accretor star, combined with its mass, could provide age information since the cessation of MT. By comparing the models against observation, the magnetic braking prescription by Garraffo et al. (2018b) is found to better align with the rotation data of post-MT accretors.

Authors: Zepei Xing, Tassos Fragos, Emmanouil Zapartas, Tom M. Kwan, Lixin Dai, Ilya Mandel, Matthias U. Kruckow, Max Briel, Jeff J. Andrews, Simone S. Bavera, Seth Gossage, Konstantinos Kovlakas, Kyle A. Rocha, Meng Sun, Philipp M. Srivastava

Access: arXiv:2407.00200 || NASA ADS

Abstract: The three dynamically confirmed wind-fed black hole high-mass X-ray binaries (BH-HMXBs) are suggested to all contain a highly spinning black hole (BH). However, based on the theories of efficient angular momentum transport inside the stars, we expect that the first-born BHs in binary systems should have low spins, which is consistent with gravitational-wave observations. As a result, the origin of the high BH spins measured in wind-fed BH-HMXBs remains a mystery. In this paper, we conduct a binary population synthesis study on wind-fed BH-HMXBs at solar metallicity with the use of the newly developed code POSYDON, considering three scenarios for BH accretion: Eddington-limited, moderately super-Eddington, and fully conservative accretion. Taking into account the conditions for accretion-disk formation, we find that regardless of the accretion model, these systems are more likely to have already experienced a phase of Roche-lobe overflow after the BH formation. To account for the extreme BH spins, highly conservative accretion onto BHs is required, when assuming the accreted material carries the specific angular momentum at the innermost stable orbit. Besides, in our simulations we found that the systems with donor stars within the mass range of 10−20 M⊙ are prevalent, posing a challenge in explaining simultaneously all observed properties of the BH-HMXB in our Galaxy, Cygnus X-1, and potentially hinting that the accretion efficiency onto non-degenerate stars, before the formation of the BH, is also more conservative than assumed in our simulations.

Authors: Meng Sun, Sasha Levina, Seth Gossage, Vicky Kalogera, Emily M. Leiner, Aaron M. Geller, Zoheyr Doctor

Access: arXiv:2311.07528 || NASA ADS

Abstract: Wind Roche-Lobe Overflow (WRLOF) is a mass-transfer mechanism proposed by Mohamed and Podsiadlowski (2007) for stellar binaries wherein the wind acceleration zone of the donor star exceeds its Roche lobe radius, allowing stellar wind material to be transferred to the accretor at enhanced rates. WRLOF may explain characteristics observed in blue lurkers and blue stragglers. While WRLOF has been implemented in rapid population synthesis codes, it has yet to be explored thoroughly in detailed binary models such as MESA (a 1D stellar evolution code), and over a wide range of initial binary configurations. We incorporate WRLOF accretion in MESA to investigate wide low-mass binaries at solar metallicity. We perform a parameter study over the initial orbital period and stellar mass. In most of the models where we consider angular momentum transfer during accretion, the accretor is spun up to the critical (or break-up) rotation rate. Then we assume the star develops a boosted wind to efficiently reduce the angular momentum so that it could maintain a sub-critical rotation. Balanced by boosted wind loss, the accretor only gains ∼2% of its total mass, but can maintain a near-critical rotation rate during WRLOF. Notably, the mass-transfer efficiency is significantly smaller than in previous studies in which the rotation of the accretor is ignored. We compare our results to observational data of blue lurkers in M67 and find that the WRLOF mechanism can qualitatively explain the origin of their rapid rotation, their location on the HR diagram and their orbital periods.

Authors: Chiotellis A.; Zapartas E.; Meyer D. M.-A.

Access: arXiv:2403.19743 || NASA ADS

Abstract: Mixed-morphology supernova remnants (MMSNRs) are characterized by a shell-like morphology in the radio and centrally-peaked thermal emission in the X-ray band. The nature of this peculiar class of supernova remnants (SNRs) remains a controversial issue. In this work, by pairing the predictions of stellar evolution theory with two-dimensional hydrodynamic simulations we show that the mixed morphology properties of a SNR can arise by the interaction of the SNR with the circumstellar medium shaped by a red supergiant progenitor star, embedded in a dense environment. As a study case, we model the circumstellar medium formation and the subsequent interaction of the SNR with it of a 15 M⊙ progenitor star. The reflected shock, formed by the collision of the SNR with the density walls of the surrounding circumstellar cavity, accumulates and re-shocks the supernova ejecta at the center of the remnant, increasing its temperature so that the gas becomes X-ray bright. Such a formation mechanism may naturally explain the nature of MMSNRs resulted from Type II supernovae without the demand of additional physical mechanisms and/or ambient medium inhomogeneities. We discuss alternative evolutionary paths that potentially could be ascribed for the MMSNR formation within the framework of the reflected shock model.

Authors: de Wit, S.; Bonanos, A. Z.; Antoniadis, K.; Zapartas, E.; Ruiz, A.; Britavskiy, N.; Christodoulou, E.; De, K.; Maravelias, G.; Munoz-Sanchez, G.; Tsopela, A.

Access: arXiv:2402.12442 || NASA ADS

Abstract: Mass loss during the red supergiant (RSG) phase plays a crucial role in the evolution of an intermediate massive star, however, the underlying mechanism remains unknown. We aim to increase the sample of well-characterized RSGs at subsolar metallicity, by deriving the physical properties of 127 RSGs in nine nearby, southern galaxies presented by Bonanos et al. For each RSG, we provide spectral types and used MARCS atmospheric models to measure stellar properties from their optical spectra, such as the effective temperature, extinction, and radial velocity. By fitting the spectral energy distribution, we obtained the stellar luminosity and radius for 97 RSGs, finding ∼ 50% with log(L/L_⊙) ≥ 5.0 and 6 RSGs with R ≳ 1400 R_⊙. We also find a correlation between the stellar luminosity and mid-IR excess of 33 dusty, variable sources. Three of these dusty RSGs have luminosities exceeding the revised Humphreys-Davidson limit. We then derive a metallicity-dependent J-K_s color versus temperature relation from synthetic photometry and two new empirical J-K_s color versus temperature relations calibrated on literature TiO and J-band temperatures. To scale our derived, cool TiO temperatures to values in agreement with the evolutionary tracks, we derive two linear scaling relations calibrated on J-band and i-band temperatures. We find that the TiO temperatures are more discrepant as a function of the mass-loss rate and discuss future prospects of the TiO bands as a mass-loss probe. Finally, we speculate that 3 hot, dusty RSGs may have experienced a recent mass ejection (12% of the K-type sample) and indicate them as candidate Levesque-Massey variables.

Authors: Kyle Akira Rocha, Vicky Kalogera, Zoheyr Doctor, Jeff J. Andrews, Meng Sun, Seth Gossage, Simone S. Bavera, Tassos Fragos, Konstantinos Kovlakas, Matthias U. Kruckow, Devina Misra, Philipp M. Srivastava, Zepei Xing, Emmanouil Zapartas

Access: arXiv:2403.07172 || NASA ADS || INSPIRE-HEP

Abstract: Be X-ray binaries (Be-XRBs) are crucial in understanding high-mass X-ray binaries, featuring a rapidly rotating Be star and a neutron star companion in an eccentric orbit, intermittently accreting material from the Be star’s decretion disk. Originating from binary stellar evolution, Be-XRBs are of significant interest to binary population synthesis (BPS) studies, encapsulating the physics of supernovae, common envelope, and mass transfer (MT). Using the POSYDON BPS code, employing pre-computed grids of detailed binary stellar evolution models, we investigate the Galactic Be-XRB population. POSYDON incorporates stellar rotation self-consistently during MT phases, enabling a detailed examination of the rotational distribution of Be stars. Our fiducial BPS and Be-XRB model align well with the orbital properties of Galactic Be-XRBs, emphasizing the role of rotational constraints. Our modeling reveals a bimodal rotational distribution of Be-XRB-like systems, in excellent agreement with literature values. All Be-XRBs undergo an MT phase before the first compact object forms, with over half experiencing a second MT phase from a stripped helium companion (Case BB). Computing rotationally-limited MT efficiencies and applying them to our population, we find that the majority of Be-XRBs have undergone highly non-conservative MT (β¯rot ≃ 0.15). Our study underscores the importance of detailed angular momentum modeling during MT in interpreting Be-XRB populations, emphasizing this population as a key probe for the stability and efficiency of MT in interacting binaries.

Authors: Xing Zepei, Bavera Simone S., Fragos Tassos,Kruckow, Matthias U., Román-Garza Jaime, Andrews Jeff J., Zapartas, Emmanouil, Kovlakas Konstantinos, Dotter Aaron, Rocha Kyle Akira, Misra Devina, Srivastava Philipp M., Sun Meng

Access: A&A HTML article || NASA ADS || arXiv

Abstract:Neutron star – black hole (NSBH) merger events bring us new opportunities to constrain theories of stellar and binary evolution and understand the nature of compact objects. In this work, we investigated the formation of merging NSBH binaries at solar metallicity by performing a binary population synthesis study of merging NSBH binaries with the newly developed code POSYDON. The latter incorporates extensive grids of detailed single and binary evolution models, covering the entire evolution of a double compact object progenitor. We explored the evolution of NSBHs originating from different formation channels, which in some cases differ from earlier studies performed with rapid binary population synthesis codes. In this paper, we present the population properties of merging NSBH systems and their progenitors such as component masses, orbital features, and BH spins, and we detail our investigation of the model uncertainties in our treatment of common envelope (CE) evolution and the core-collapse process. We find that at solar metallicity, under the default model assumptions, most of the merging NSBHs have BH masses in the range of 3 − 11 M⊙ and chirp masses within 1.5 − 4 M⊙. Independently of our model variations, the BH always forms first with dimensionless spin parameter ≲0.2, which is correlated to the initial binary orbital period. Some BHs can subsequently spin up moderately (χBH ≲ 0.4) due to mass transfer, which we assume to be Eddington limited. Binaries that experience CE evolution rarely demonstrate large tilt angles. Conversely, approximately 40% of the binaries that undergo only stable mass transfer without CE evolution contain an anti-aligned BH. Finally, accounting for uncertainties in both the population modeling and the NS equation of state, we find that 0 − 18.6% of NSBH mergers may be accompanied by an electromagnetic counterpart.

Authors: Misra Devina, Kovlakas Konstantinos, Fragos Tassos, Andrews Jeff J., Bavera Simone S., Zapartas, Emmanouil, Xing, Zepei, Dotter Aaron, Rocha Kyle Akira, Srivastava Philipp M., Sun Meng

Access: A&A HTML article || NASA ADS || arXiv

Abstract: Context. Ultra-luminous X-ray sources (ULXs) are sources observed to have extreme X-ray luminosities exceeding the Eddington limit of a stellar-mass black hole (BH). A fraction of ULXs show X-ray pulsations, which are evidence for accreting neutron stars (NSs). Theoretical studies have suggested that NSs, rather than BHs, dominate the compact objects of intrinsic ULX populations, even though the majority of the observed sample is non-pulsating, implying that X-ray pulses from many NS ULXs are unobservable. Aims: We simulate populations of X-ray binaries covering a range of starburst ages spanning from 5 to 1000 Myr with the aim of comparing the properties of observed ULXs at the different ages. Additionally, we compare two models describing different assumptions for the physical processes governing binary evolution. Methods: We used the new population synthesis code POSYDON to generate multiple populations of ULXs spanning multiple burst ages. We employed a model for geometrically beamed emission from a super-Eddington accretion disk in order to estimate the luminosities of ULXs. Following theoretical predictions for the alignment of the spin axis of an NS with the accretion disk due to mass transfer, we estimated the required mass to be accreted by the NSs in the ULX populations so that the alignment suppresses observable X-ray pulses. Results: While we find that the properties of ULX populations are sensitive to model assumptions, there are certain trends that the populations follow. Generally, young and old stellar populations are dominated by BH and NS accretors, respectively. The donor stars go from being massive H-rich main-sequence stars in young populations (< 100 Myr) to low-mass post-main sequence H-rich stars in older populations (> 100 Myr), with stripped He-rich giant donors dominating the populations at around 100 Myr. In addition, we find that NS ULXs exhibit stronger geometrical beaming than BH ULXs, leading to an underrepresentation 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. Conclusions: 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.

Authors: Simone S. Bavera, Tassos Fragos, Emmanouil Zapartas, Jeff J. Andrews, Vicky Kalogera, Christopher P. L. Berry, Matthias Kruckow, Aaron Dotter, Konstantinos Kovlakas, Devina Misra, Kyle A. Rocha, Philipp M. Srivastava, Meng Sun, Zepei Xing

Access: arXiv:2212.10924 || NASA ADS || INSPIRE-HEP

Abstract: The maximum mass of black holes formed in isolated binaries is determined by stellar winds and the interactions between the binary components. We consider for the first time fully self-consistent detailed stellar structure and binary evolution calculations in population-synthesis models and a new, qualitatively different picture emerges for the formation of black-hole binaries, compared to studies employing rapid population synthesis models. We find merging binary black holes can form with a non-negligible rate (∼4×10−7M−1⊙) at solar metallicity. Their progenitor stars with initial masses ≳50M⊙ do not expand to supergiant radii, mostly avoiding significant dust-driven or luminous blue variable winds. Overall, the progenitor stars lose less mass in stellar winds, resulting in black holes as massive as ∼30M⊙, and, approximately half of them avoid a mass-transfer episode before forming the first-born black hole. Finally, binaries with initial periods of a few days, some of which may undergo episodes of Roche-lobe overflow mass transfer, result in mildly spinning first-born black holes, χBH1≲0.2, assuming efficient angular-momentum transport.

Authors: Chase Kimball, Sam Imperato, Vicky Kalogera, Kyle A. Rocha, Zoheyr Doctor, Jeff J. Andrews, Aaron Dotter, Emmanouil Zapartas, Simone S. Bavera, Konstantinos Kovlakas, Tassos Fragos, Phillip M. Srivastava, Devina Misra, Meng Sun, Zepei Xing

Access: arXiv:2211.02158 || NASA ADS || INSPIRE-HEP

Abstract: 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.

Authors: Jared C. Siegel, Ilia Kiato, Vicky Kalogera, Christopher P. L. Berry, Thomas J. Maccarone, Katelyn Breivik, Jeff J. Andrews, Simone S. Bavera, Aaron Dotter, Tassos Fragos, Konstantinos Kovlakas, Devina Misra, Kyle A. Rocha, Philipp M. Srivastava, Meng Sun, Zepei Xing, Emmanouil Zapartas

Access: arXiv:2209.06844 || NASA ADS || INSPIRE-HEP

Abstract: 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 (NS) and the least massive black holes (BH). 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 present the first quantitative study of 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 do introduce biases into the observed sample. If a gap is not inherent in BH birth masses, mass growth through LMXB accretion and selection effects can suppress mass-gap BHs in the observed sample. Our results are robust against variations of binary evolution prescriptions. We further find that a population of transient LMXBs with mass-gap BHs form through accretion induced collapse of a NS during the LMXB phase. The significance of this population is dependent on the maximum NS birth mass M_{NS,birth−max}. For M_{NS,birth−max}=3M_⊙, MESA and COSMIC models predict a similar fraction of mass gap LMXBs. However, for M_{NS,birth−max} ≾ 2M_⊙ and realistic models of the disk-instability, our MESA models produce a dearth of mass-gap LMXBs, more consistent with observations. This constraint on M_{NS,birth−max} is independent of whether low-mass BHs form at core-collapse.

Authors: Devina Misra, Konstantinos Kovlakas, Tassos Fragos, Margaret Lazzarini, Simone S. Bavera, Bret D. Lehmer, Andreas Zezas, Emmanouil Zapartas, Zepei Xing, Jeff J. Andrews, Aaron Dotter, Kyle A. Rocha, Philipp M. Srivastava, Meng Sun

Access: arXiv:2209.05505 || NASA ADS || INSPIRE-HEP

Abstract: The ever-expanding observational sample of X-ray binaries (XRBs) makes them excellent laboratories for constraining binary evolution theory. Useful insights can be obtained by studying the effects of various physical assumptions on synthetic X-ray luminosity functions (XLFs) and comparing with observed XLFs. 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 and generate 96 synthetic XRB populations corresponding to different combinations of model assumptions. The generated XLFs are feature-rich, deviating from the commonly assumed single power law. We find a break in our synthetic XLF at luminosity ~ 10³⁸ erg/s, similarly 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 orbit circularization at the onset of Roche-lobe overflow and criteria for the formation of a wind-fed X-ray emitting accretion disk around black holes can also impact our synthetic XLFs and reduce the discrepancy with observations. Due to model uncertainties, our synthetic XLFs do not always agree well with observations. However, different combinations of model parameters leave distinct imprints on the shape of the synthetic XLFs and can reduce this deviation, revealing the importance of large-scale parameter studies and highlighting the power of XRBs in constraining binary evolution theory.

Authors: Kyle Akira Rocha, Jeff J. Andrews, Christopher P. L. Berry, Zoheyr Doctor, Pablo Marchant, Vicky Kalogera, Scott Coughlin, Simone S. Bavera, Aaron Dotter, Tassos Fragos, Konstantinos Kovlakas, Devina Misra, Zepei Xing, Emmanouil Zapartas

Access: arXiv:2203.16683 || NASA ADS || INSPIRE-HEP || Journal

Abstract: 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 ∼1/4 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.

Authors: Tassos Fragos, Jeff J. Andrews, Simone S. Bavera, Christopher P. L. Berry, Scott Coughlin, Aaron Dotter, Prabin Giri, Vicky Kalogera, Aggelos Katsaggelos, Konstantinos Kovlakas, Shamal Lalvani, Devina Misra, Philipp M. Srivastava, Ying Qin, Kyle A. Rocha, Jaime Román-Garza, Juan Gabriel Serra, Petter Stahle, Meng Sun, Xu Teng, Goce Trajcevski, Nam Hai Tran, Zepei Xing, Emmanouil Zapartas, Michael Zevin

Access: arXiv:2202.05892 || NASA ADS || INSPIRE-HEP || Journal || Code release

Abstract: 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.

Authors: Xu Teng, Thomas Beckler, Bradley Gannon, Benjamin Huinker, Gabriel Huinker, Koushhik Kumar, Christina Marquez, Jacob Spooner, Goce Trajcevski, Prabin Giri, Aaron Dotter, Jeff Andrews, Scott Coughlin, Ying Qin, Juan Gabriel Serra-Pérez, Nam Tran, Jaime Román-Garja, Konstantinos Kovlakas, Emmanouil Zapartas, Simone Bavera, Devina Misra, Tassos Fragos

Access: ACM DIGITAL LIBRARY

Abstract: Traditionally, clustering of multivariate data aims at grouping objects described with multiple heterogeneous attributes based on a suitable similarity (conversely, distance) function. One of the main challenges is due to the fact that it is not straightforward to directly apply mathematical operations (e.g., sum, average) to the feature values, as they stem from heterogeneous contexts. In this work we take the challenge a step further and tackle the problem of clustering multivariate datasets based on jointly considering: (a) similarity among a subset of the attributes; and (b) distance-based diversity among another subset of the attributes. Specifically, we focus on astrophysics data, where the snapshots of the stellar evolution for different stars contain over 40 distinct attributes corresponding to various physical and categorical (e.g., 'black hole') attributes. We present CSD-CAMD -- a prototype system for Coupling Similarity and Diversity for Clustering Astrophysics Multivariate Datasets. It provides a flexibility for the users to select their preferred subsets of attributes; assign weight (to reflect their relative importance on the clustering); and select whether the impact should be in terms of proximity or distance. In addition, CSD-CAMD allows for selecting a clustring algorithm and enables visualization of the outcome of clustering.

Authors: Xu Teng, Adam Corpstein, Joel Holm, Willis Knox, Becker Mathie, Philip Payne, Ethan Vander Wiel, Prabin Giri, Goce Trajcevski, Aaron Dotter, Jeff Andrews, Scott Coughlin, Ying Qin, Juan Gabriel Serra-Pérez, Nam Tran, Jaime Román-Garja, Konstantinos Kovlakas, Emmanouil Zapartas, Simone Bavera, Devina Misra, Tassos Fragos

Access: ACM DIGITAL LIBRARY

Abstract: We present CACSE – a system for Context Aware Clustering of Stellar Evolution – for datasets corresponding to temporal evolution of stars, which are multivariate time series, usually with a large number of attributes (e.g., ≥ 40). Typically, the datasets are obtained by simulation and are relatively large in size (5 ∼ 10 GB per certain interval of values for various initial conditions). Investigating common evolutionary trends in these datasets often depends on the context – i.e., not all the attributes are always of interest, and among the subset of the context-relevant attributes, some may have more impact than others. To enable such context-aware clustering, our CACSE system provides functionalities allowing the domain experts to dynamically select attributes that matter, and assign desired weights/priorities. Our system consists of a PostgreSQL database, Python-based middleware with RESTful and Django framework, and a web-based user interface as frontend. The user interface provides multiple interactive options, including selection of datasets and preferred attributes along with the corresponding weights. Subsequently, the users can select a time instant or a time range to visualize the formed clusters. Thus, CACSE enables a detection of changes in the the set of clusters (i.e., convoys) of stellar evolution tracks. Current version provides two of the most popular clustering algorithms – k-means and DBSCAN.

Authors: Simone S. Bavera, Tassos Fragos, Emmanouil Zapartas, Enrico Ramirez-Ruiz, Pablo Marchant, Luke Z. Kelley, Michael Zevin, Jeff Andrews, Scotty Coughlin, Aaron Dotter, Konstantinos Kovlakas, Devina Misra, Juan Gabriel Serra-Pérez, Ying Qin, Kyle A. Rocha, Jaime Román-Garza, Nam Hai Tran, Zepei Xing

Access: arXiv:2106.15841 || NASA ADS || INSPIRE-HEP || Journal

Abstract: Long gamma-ray bursts are associated with the core-collapse of massive, rapidly spinning stars. However, the believed efficient angular momentum transport in stellar interiors leads to predominantly slowly-spinning stellar cores. Here, we report on binary stellar evolution and population synthesis calculations, showing that tidal interactions in close binaries not only can explain the observed sub-population of spinning, merging binary black holes, but also lead to long gamma-ray bursts at the time of black-hole formation, with rates matching the empirical ones. We find that ≈10% of the GWTC-2 reported binary black holes had a long gamma-ray burst associated with their formation, with GW190517 and GW190719 having a probability of ≈85% and ≈60%, respectively, being among them.

Authors: Emmanouil Zapartas, Mathieu Renzo, Tassos Fragos, Aaron Dotter, Jeff Andrews, Simone S. Bavera, Scotty Coughlin, Devina Misra, Konstantinos Kovlakas, Jaime Román-Garza, Juan Gabriel Serra-Pérez, Ying Qin, Kyle A. Rocha, Nam Hai Tran

Access: arXiv:2106.05228 || NASA ADS || INSPIRE-HEP || Journal

Abstract: Stripped-envelope supernovae (Type IIb, Ib, Ic) showing little or no hydrogen are one of the main classes of explosions of massive stars. Their origin and the evolution of their progenitors are not fully understood as yet. Very massive single stars stripped by their own winds (≳25 - 30M_⊙ at solar metallicity) are considered viable progenitors of these events. However, recent 1D core-collapse simulations show that some massive stars may collapse directly onto black holes after a failed explosion, with weak or no visible transient. In this letter, we estimate the effect of direct collapse onto a black hole on the rates of stripped-envelope supernovae that arise from single stars. For this, we compute single star MESA models at solar metallicity and map their final state to their core-collapse outcome following prescriptions commonly used in population synthesis. According to our models, no single stars that have lost their entire hydrogen-rich envelope are able to explode, and only a fraction of progenitors with a thin hydrogen envelope left (IIb progenitor candidates) do, unless we invoke increased wind mass-loss rates. This result increases the existing tension between the single-star scenario for stripped-envelope supernovae and their observed rates and properties. At face value, our results point towards an even higher contribution of binary progenitors for stripped-envelope supernovae. Alternatively, they may suggest inconsistencies in the common practice of mapping different stellar models to core-collapse outcomes and/or higher overall mass loss in massive stars.

Authors: Jaime Román-Garza, Simone S. Bavera, Tassos Fragos, Emmanouil Zapartas, Devina Misra, Jeff Andrews, Scotty Coughlin, Aaron Dotter, Konstantinos Kovlakas, Juan Gabriel Serra-Pérez, Ying Qin, Kyle A. Rocha, Nam Hai Tran

Access: arXiv:2012.02274 || NASA ADS || INSPIRE-HEP || Journal

Abstract: Recent detailed 1D core-collapse simulations have brought new insights on the final fate of massive stars, which are in contrast to commonly used parametric prescriptions. In this work, we explore the implications of these results to the formation of coalescing black-hole (BH) - neutron-star (NS) binaries, such as the candidate event GW190426_152155 reported in GWTC-2. Furthermore, we investigate the effects of natal kicks and the NS's radius on the synthesis of such systems and potential electromagnetic counterparts linked to them. Synthetic models based on detailed core-collapse simulations result in an increased merger detection rate of BH-NS systems (~2.3 yr^-1), 5 to 10 times larger than the predictions of "standard" parametric prescriptions. This is primarily due to the formation of low-mass BH via direct collapse, and hence no natal kicks, favored by the detailed simulations. The fraction of observed systems that will produce an electromagnetic counterpart, with the detailed supernova engine, ranges from 2-25%, depending on uncertainties in the NS equation of state. Notably, in most merging systems with electromagnetic counterparts, the NS is the first-born compact object, as long as the NS's radius is ≲12km. Furthermore, core-collapse models that predict the formation of low-mass BHs with negligible natal kicks increase the detection rate of GW190426_152155-like events to ~0.6 yr^-1; with an associated probability of electromagnetic counterpart ≤ 10% for all supernova engines. However, increasing the production of direct-collapse low-mass BHs also increases the synthesis of binary BHs, over-predicting their measured local merger density rate. In all cases, models based on detailed core-collapse simulation predict a ratio of BH-NSs to binary BHs merger rate density that is at least twice as high as other prescriptions.

Authors: Simone S. Bavera, Tassos Fragos, Michael Zevin, Christopher P. L. Berry, Pablo Marchant, Jeff J. Andrews, Scott Coughlin, Aaron Dotter, Konstantinos Kovlakas, Devina Misra, Juan G. Serra-Pérez, Ying Qin, Kyle A. Rocha, Jaime Román-Garza, Nam H. Tran, Emmanouil Zapartas

Access: arXiv:2010.16333 || NASA ADS || INSPIRE-HEP || Journal

Abstract: 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 χ_eff, Μ_chirp and q. 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 (z ~ 2), 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 χeff 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 (z ≃ 0.01) for the common envelope channel is in the range ~ 17 − 113 Gpc⁻³ yr⁻¹ considering a range of α_CE ∈ [0.2,5.0] while for the stable mass transfer channel the rate density is ~ 25 Gpc⁻³ yr>⁻¹. 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 ~ 0.2. 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.