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Journal Club


"A Galactic Eclipse: The Small Magellanic Cloud is Forming Stars in Two, Superimposed Systems"

Gergely Hajdu (NCAC, Warsaw)

The structure and dynamics of the star-forming disk of the Small Magellanic Cloud (SMC) have long confounded us. The SMC is widely used as a prototype for galactic physics at low metallicity, and yet we fundamentally lack an understanding of the structure of its interstellar medium (ISM). In this work, we present a new model for the SMC by comparing the kinematics of young, massive stars with the structure of the ISM traced by high-resolution observations of neutral atomic hydrogen (HI) from the Galactic Australian Square Kilometer Array Pathfinder survey (GASKAP-HI). Specifically, we identify thousands of young, massive stars with precise radial velocity constraints from the Gaia and APOGEE surveys and match these stars to the ISM structures in which they likely formed. By comparing the average dust extinction towards these stars, we find evidence that the SMC is composed of two structures with distinct stellar and gaseous chemical compositions. We construct a simple model that successfully reproduces the observations and shows that the ISM of the SMC is arranged into two, superimposed, star-forming systems with similar gas mass separated by ~5 kpc along the line of sight.

Murray et al. (2024)


"Identifying noise transients in gravitational-wave data arising from nonlinear couplings"

Sudhagar Suyamprakasam (NCAC, Warsaw)

Noise in various interferometer systems can sometimes couple non-linearly to create excess noise in the gravitational wave (GW) strain data. Third-order statistics, such as bicoherence and biphase, can identify these couplings and help discriminate those occurrences from astrophysical GW signals. However, the conventional analysis can yield large bicoherence values even when no phase-coupling is present, thereby, resulting in false identifications. Introducing artificial phase randomization in computing the bicoherence reduces such occurrences with negligible impact on its effectiveness for detecting true phase-coupled disturbances. We demonstrate this property with simulated disturbances in this work. Statistical hypothesis testing is used for distinguishing phase-coupled disturbances from non-phase coupled ones when employing the phase-randomized bicoherence. We also obtain an expression for the bicoherence value that minimizes the sum of the probabilities of false positives and false negatives. This can be chosen as a threshold for shortlisting bicoherence triggers for further scrutiny for the presence of non-linear coupling. Finally, the utility of the phase-randomized bicoherence analysis in GW time-series data is demonstrated for the following three scenarios: (1) Finding third-order statistical similarities within categories of noise transients, such as blips and koi fish. If these non-Gaussian noise transients, or glitches, have a common source, their bicoherence maps can have similarities arising from common bifrequencies related to that source. (2) Differentiating linear or non-linear phase-coupled glitches from compact binary coalescence signals through their bicoherence maps. This is explained with a simulated signal. (3) Identifying repeated bifrequencies in the second and third observation runs (i.e., O2 and O3) of LIGO and Virgo.

Hall, Suyamprakasam, et al., (2024)


"Stellar Evolution in Real Time II: R Hydrae and an Open-Source Grid of 3000 Seismic TP-AGB Models Computed with MESA"

Oliwia Ziółkowska (NCAC, Warsaw)

We present a comprehensive characterization of the evolved thermally pulsing asymptotic giant branch (TP-AGB) star R Hydrae, building on the techniques applied in Stellar Evolution in Real Time I (Molnár et al. 2019) to T Ursae Minoris. We compute over 3000 theoretical TP-AGB pulse spectra using MESA and GYRE and combine these with classical observational constraints and nearly 400 years of measurements of R Hya's period evolution to fit R Hya's evolutionary and asteroseismic features. Two hypotheses for the mode driving R Hya's period are considered. Solutions that identify this as the fundamental mode (FM) as well as the first overtone (O1) are consistent with observations. Using a variety of statistical tests, we find that R Hya is most likely driven by the FM and currently occupies the ``power down'' phase of an intermediate pulse (TP ~ 9-16). We predict that its pulsation period will continue to shorten for millennia. Using supplementary calculations from the Monash stellar evolution code, we also find that R Hya is likely to have undergone third dredge-up in its most recent pulse. The MESA+GYRE model grid used in this analysis includes exact solutions to the adiabatic equations of stellar oscillation for the first 10 radial-order pressure modes for every time step in every evolutionary track. The grid is fully open-source and packaged with a data visualization application. This is the first publicly available grid of TP-AGB models with seismology produced with MESA.


"Observations of a black hole x-ray binary indicate formation of a magnetically arrested disk"

Piotr Życki (NCAC, Warsaw)

Accretion of material onto a black hole drags any magnetic fields present inwards, increasing their strength. Theory predicts that sufficiently strong magnetic fields can halt the accretion flow, producing a magnetically arrested disk (MAD). We analyzed archival multiwavelength observations of an outburst from the black hole x-ray binary MAXI J1820+070 in 2018. The radio and optical fluxes were delayed compared with the x-ray flux by about 8 and 17 days, respectively. We interpret this as evidence for the formation of a MAD. In this scenario, the magnetic field is amplified by an expanding corona, forming a MAD around the time of the radio peak. We propose that the optical delay is due to thermal viscous instability in the outer disk.

You et al. (2023)

 You et al. 2023 You et al. 2023


"Recent(-ish) works published by CAMK employees and students"

Gerald Hande, Amedeo Romagnolo & Alex Gormaz-Matamala, Miljenko Cemeljic (NCAC, Warsaw)

1. Gerald Hander "TIC 184743498: the first tri-axial stellar pulsator" by Zhang et al. 2024, MNRAS, 528, 3378 2. Amedeo Romagnolo & Alex Gormaz-Matamala " On the maximum black hole mass at solar metallicity" by Amedeo et al. 2024, ApJL, submitted Depending on external factors, these two talks might be followed by a special, short talk by Miljenko Cemeljic on "Art preceding science: auroras on pulsar planets".


"On the maximum black hole mass at solar metallicity"

Amedeo Romagnolo (NCAC, Warsaw)

In high metallicity environments the mass that black holes (BHs) can reach just after core-collapse widely depends on how much mass their progenitor stars lose via winds. On one hand new theoretical and observational insights suggest that early-stage winds should be weaker than what many canonical models prescribe. On the other hand the proximity to the Eddington limit should affect the formation of optically thick envelopes already during the earliest stages of stars with initial masses MZAMS≳100 M⊙, hence resulting in higher mass-loss rates during the main sequence. We use the evolutionary codes MESA and Genec to calculate a suite of tracks for massive stars at solar metallicity Z⊙=0.014 which incorporate these changes in our wind mass loss prescription. In our calculations we employ moderate rotation, high overshooting and magnetic angular momentum transport. We find a maximum BH mass MBH,max=28.3 M⊙ at Z⊙. The most massive BHs are predicted to form from stars with MZAMS≳250 M⊙, with the BH mass directly proportional to its progenitor's MZAMS. We also find in our models that at Z⊙ almost any BH progenitor naturally evolves into a Wolf-Rayet star due to the combined effect of internal mixing and wind mass loss. These results are considerably different from most recent studies regarding the final mass of stars before their collapse into BHs. While we acknowledge the inherent uncertainties in stellar evolution modelling, our study underscores the importance of employing the most up-to-date physics in BH mass predictions.


"Non-evolutionary effects on period change in Magellanic Cepheids I. New binary systems revealed from Light Travel Time Effect"

Rajeev Singh Rathour (NCAC, Warsaw)

Period change studies give a window to probe into the evolution and dynamics of Cepheids. While evolutionary period changes have been well studied both observationally and theoretically, non-evolutionary period changes lack a systematic and quantitative description. The overall objective is to have a quantitative understanding of the full picture of non-evolutionary period changes in Cepheids, to develop a formalism to disentangle it from the secular evolutionary period change. In the first part of the series of works, we aim to conduct a systematic search for non-evolutionary period changes to search for Cepheids in likely binary configuration and quantify their incidence rates in the Magellanic Clouds. We collect more than decade-long time-series photometry from the publically available survey, Optical Gravitational Lensing Experiment (OGLE), with more than 7200 Cepheids collectively from the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). Our sample contains both fundamental-mode and first overtone-mode Cepheids. Then we calculate observed minus calculated (O−C) diagrams to reveal the light-travel time effect (LTTE). In our search, out of an overall sample of more than 7200 Cepheids, we found 52 candidate Cepheid binary systems in the LMC (30 fundamental and 22 first overtone-mode) and 145 in the SMC (85 fundamental and 60 first overtone-mode). The majority of the sample is characterized by orbital periods of 2000-4000\,d and eccentricities of 0.2-0.5. Moreover, we report two candidates in each galaxy with the Cepheid likely existing with a giant companion. The incidence rate ratio for SMC to LMC calculated from our sample is in agreement with binary Cepheid population synthesis predictions.

Rathour et al. (2024)


"Similarities and differences between active galactic nuclei and black-hole X-ray binaries"

Andrzej Zdziarski (NCAC, Warsaw)