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Resolving the formation channel(s) of merging binary black holes is a key goal in gravitational-wave astronomy. The orbital eccentricity is believed to be a precious tracer of the underlying formation pathway, but is largely dissipated during the usually long inspiral between black hole formation and merger. Most gravitational-wave sources are thus expected to enter the sensitivity windows of current detectors on configurations that are compatible with quasi-circular orbits. In this paper, we investigate the impact of "negligible" residual eccentricity -- lower than currently detectable by LIGO/Virgo -- on our ability to infer the formation history of binary black holes, focusing in particular on their spin orientations. We trace the evolution of both observed and synthetic gravitational-wave events backward in time, while resampling their residual eccentricities to values that are below the detectability threshold. Eccentricities in-band as low as $\sim 10^{-4}$ can lead to significant biases when reconstructing the spin directions, especially in the case of loud, highly precessing systems. Residual eccentricity thus act like a systematic uncertainty for our astrophysical inference. As a mitigation strategy, one can marginalize the posterior distribution over the residual eccentricity using astrophysical predictions., Comment: 10 pages, 4 figures, 2 tables

The growing database of gravitational-wave (GW) detections with the binary black holes (BHs) merging in the distant Universe contains subtle insights into their formation scenarios. One of the puzzling properties of detected GW sources is the possible (anti)correlation between mass ratio q of BH-BH binaries and their effective spin. We use rapid binary evolution models to demonstrate that the isolated binary evolution followed by efficient tidal spin-up of stripped helium core produces a similar pattern in Xeff vs q distributions of BH-BH mergers. In our models, the progenitors of unequal BH-BH systems in the stable mass transfer formation scenario are more likely to efficiently shrink their orbits during the second Roche-lobe overflow than the binaries that evolve into nearly equal-mass component systems. This makes it easier for unequal-mass progenitors to enter the tidal spin-up regime and later merge due to GW emission. Our results are, however, sensitive to some input assumptions, especially, the stability of mass transfer and the angular momentum loss during non-conservative mass transfer. We note that mass transfer prescriptions widely adopted in rapid codes favor the formation of BH-BH merger progenitors with unequal masses and moderate separations. We compare our results with detailed stellar model grids and find reasonable agreement after appropriate calibration of the physics models. We anticipate that future detections of unequal-mass BH-BH mergers could provide valuable constraints on the role of the stable mass transfer formation channel. A significant fraction of BH-BH detections with mass ratio q in range (0.4 - 0.7) would be consistent with the mass ratio reversal scenario during the first, relatively conservative mass transfer, and a non-enhanced angular momentum loss during the second, highly non-conservative mass transfer phase., Comment: Version accepted for publication in A&A

Properties of the to-date-observed binary black hole (BBH) merger events suggest a preference towards spin-orbit aligned mergers. Naturally, this has caused widespread interest and speculations regrading implications on various merger formation channels. Here we show that (i) not only the BBH-merger population from isolated binaries, but also (ii) BBH population formed in young massive clusters (YMC) would possess an asymmetry in favour of aligned mergers, in the distribution of the events' effective spin parameter ($\chi_{\rm eff}$). In our analysis, we utilize BBH-merger outcomes from state-of-the-art N-body evolutionary models of YMCs and isolated binary population synthesis. We incorporate, for the first time in such an analysis, misalignments due to both natal kicks and dynamical encounters. The YMC $\chi_{\rm eff}$ distribution has a mean (an anti-aligned merger fraction) of $\langle\chi_{\rm eff}\rangle\leq0.04$ ($f_X-\approx40\%$), which is smaller (larger) than but consistent with the observed asymmetry of $\langle\chi_{\rm eff}\rangle\approx0.06$ ($f_X-\approx28\%$) as obtained from the population analysis by the LIGO-Virgo-KAGRA collaboration. In contrast, isolated binaries alone tend to produce a much stronger asymmetry; for the tested physical models, $\langle\chi_{\rm eff}\rangle\approx0.25$ and $f_X-\lesssim7\%$. Although the YMC $\chi_{\rm eff}$ distribution is more similar to the observed counterpart, none of the channels correctly reproduce the observed distribution. Our results suggest that further extensive model explorations for both isolated-binary and dynamical channels as well as better observational constraints are necessary to understand the physics of 'the symmetry breaking' of the BBH-merger population., Comment: 20 pages including Appendix, 7 figures, 3 tables. Extended results and discussions; main conclusions are unchanged. Accepted for publication in ApJ

Our understanding of the convective-engine paradigm driving core-collapse supernovae has been used for 2 decades to predict the remnant mass distribution from stellar collapse. These predictions improve as our understanding of this engine increases. In this paper, we review our current understanding of convection (in particular, the growth rate of convection) in stellar collapse and study its effect on the remnant mass distribution. We show how the depth of the mass gap between neutron stars and black holes can help probe this convective growth. We include a study of the effects of stochasticity in both the stellar structure and the convective seeds caused by stellar burning. We study the role of rotation and its effect on the pair-instability mass gap. Under the paradigm limiting stellar rotation to those stars in tight binaries, we determine the effect of rotation on the remnant mass distribution., Comment: 17 pages, accepted by ApJ

Understanding astrophysical phenomena involving compact objects requires an insight about the engine behind core-collapse supernovae (SNe) and the fate of the stellar collapse of massive stars. In particular, this insight is crucial in developing an understanding of the origin and formation channels of detected population of BH-BH, BH-NS and NS-NS mergers. To gain this understanding, we must tie our current knowledge of pre-SN stars properties and their potential explosions to the final NS or BH mass distribution. The timescale of convection growth may have a large effect on the strength of SN explosion and therefore also on the mass distribution of stellar remnants. In this study we adopt the new formulas for the relation between the pre-SN star properties and its remnant from Fryer et al. 2022 into StarTrack population synthesis code and check how they impact double compact object (DCO) mergers formed via isolated binary evolution. The new formulas give one ability to test a wide spectrum of assumptions on the convection growth time. In particular, different variants allow for a smooth transition between having a deep lower mass gap and a remnant mass distribution filled by massive NSs and low mass BHs. In this paper we present distribution of masses, mass ratios and the local merger rate densities of DCO mergers for different variants of new remnant mass formulas. We test them together with different approaches to other highly uncertain processes. We find that mass distribution of DCO mergers up to m_1+m_2 < 35 Msun is sensitive to adopted assumption on SN convection growth timescale. Between the two extreme tested variants the probability of compact object formation within the lower mass gap may differ up to 2 orders of magnitude. The mass ratio distribution of DCO mergers is significantly influenced by SN model only for our standard mass transfer stability criteria., Comment: 22 pages, version accepted for publication in MNRAS

The LIGO/Virgo collaboration has reported 50 black hole-black hole (BH-BH) mergers and 8 candidates recovered from digging deeper into the detectors noise. The majority of these mergers have low effective spins pointing toward low BH spins and efficient angular momentum transport in massive stars as proposed by several models (e.g., the Tayler-Spruit dynamo). However, out of these 58 mergers, 7 are consistent with having high effective spin parameter (chi_eff>0.3). Additionally, 2 events seem to have high effective spins sourced from the spin of the primary (more massive) BH. These particular observations could be used to discriminate between the isolated binary and dynamical formation channels. It might seem that high BH spins point to a dynamical origin if angular momentum transport in stars is efficient and forms low-spinning BHs. In such a case dynamical formation is required to produce second and third generations of BH-BH mergers with typically high-spinning BHs. Here we show, however, that isolated binary BH-BH formation also naturally reproduces such highly spinning BHs. Our models start with efficient angular momentum transport in massive stars that is needed to reproduce the majority of BH-BH mergers with low effective spins. Later, some of the binaries are subject to a tidal spin-up allowing the formation of a moderate fraction (~10%) of BH-BH mergers with high effective spins (chi_eff>=0.4-0.5). In addition, isolated binary evolution can produce a small fraction of BH-BH mergers with almost maximally spinning primary BHs. Therefore, the formation scenario of these atypical BH-BH mergers remains to be found., Comment: The manuscript version after the first review in ApJL. Comments welcome

The treatment and criteria for development of unstable Roche lobe overflow (RLOF) that leads to the common envelope (CE) phase have hindered the evolutionary predictions for decades. In particular, the formation of black hole-black hole (BH-BH), black hole-neutron star (BH-NS), and neutron star-neutron star (NS-NS) merging binaries depends sensitively on the CE phase in classical isolated binary evolution model. All these mergers are now reported as LIGO/Virgo sources or source candidates. CE is even considered by some as a mandatory phase in the formation of BH-BH, BH-NS or NS-NS mergers in binary evolution. At the moment, there is no full first-principles model for development of CE. We employ the Startrack population synthesis code to test the current advancements in studies on stability of RLOF for massive donors to assess their effect on LIGO/Virgo source population. In particular, we allow for more restrictive CE development criteria for massive donors. We also test a modified condition for switching between different types of stable mass transfer. Implemented modifications significantly influence basic properties of merging double compact objects, sometimes in non-intuitive way. For one of tested models with restricted CE development criteria local merger rate density for BH-BH systems increased due to emergence of a new dominant formation scenario without any CE phase. We find that the changes in highly uncertain assumptions on RLOF physics may significantly affect (i) local merger rate density, (ii) shape of the mass and mass ratio distributions, and (iii) dominant evolutionary formation (with and without CE) scenarios of LIGO/Virgo sources. Our results demonstrate that without sufficiently strong constraints on RLOF physics, one is not able to draw fully reliable conclusions about the population of double compact object systems based on population synthesis studies., Comment: 20 pages, 11 figures, accepted for publication in A&A

We present the monitoring of the AGN continuum and MgII broad line emission for the quasar HE 0413-4031 ($z=1.38$) based on the six-year monitoring by the South African Large Telescope (SALT). We managed to estimate a time-delay of $302.6^{+28.7}_{-33.1}$ days in the rest frame of the source using seven different methods: interpolated cross-correlation function (ICCF), discrete correlation function (DCF), $z$-transformed DCF, JAVELIN, two estimators of data regularity (Von Neumann, Bartels), and $\chi^2$ method. This time-delay is below the value expected from the standard radius-luminosity relation. However, based on the monochromatic luminosity of the source and the SED modelling, we interpret this departure as the shortening of the time-delay due to the higher accretion rate of the source, with the inferred Eddington ratio of $\sim 0.4$. The MgII line luminosity of HE 0413-4031 responds to the continuum variability as $L_{\rm line}\propto L_{\rm cont}^{0.43\pm 0.10}$, which is consistent with the light-travel distance of the location of MgII emission at $R_{\rm out} \sim 10^{18}\,{\rm cm}$. Using the data of 10 other quasars, we confirm the radius-luminosity relation for broad MgII line, which was previously determined for broad H$\beta$ line for lower-redshift sources. In addition, we detect a general departure of higher-accreting quasars from this relation in analogy to H$\beta$ sample. After the accretion-rate correction of the light-travel distance, the MgII-based radius-luminosity relation has a small scatter of only $0.10$ dex., Comment: 39 pages (23 pages - Main text, 16 pages - Appendix), 21 figures, 14 tables; accepted for publication in the Astrophysical Journal