Session L03: Transients and Interacting Binaries

GRB 221009A: observations 8 hours later with HAWC and Fermi-LAT

The gamma-ray burst GRB221009A has been the brightest GRB observed so far. It was even observed >10 TeV and its afterglow has been studied over a large range of the electromagnetic spectrum. After 8 hours of the initial trigger, the position of the burst started to transit the HAWC observatory. No significant detection was found in searches of timescales of hundreds of seconds to hours. However, 1.3 hours into the transit, the Fermi-LAT telescope detected a 400 GeV photon 0.1deg away from the GRB position, with a probability of being a random coincidence of 4σ. Using both HAWC and Fermi data we searched for a possible late-time flare or rebrighting of the GRB, as well as performed a spectral fit of the afterglow over the energy range of ~100 MeV to 10 TeV. We will present the results of these analyses during this talk.   

Late-Time Observations of the Ultraluminoius GRB 221009A

Gamma-ray burst GRB 221009A was the brightest-ever observed long GRB and was the focus of several observing programs, including those on HST and JWST.  Early observations showed a fading of the transient consistent with that of a GRB afterglow and no observed jet-break, leading to the conclusion that this happened at extremely early times (or is yet to occur).  A supernova component was detected in JWST NIRSPEC spectroscopy, however the supernova was found to be normal-to-low luminosity for standard GRB-SNe adding further complication to our understanding of the GRB-SN connection.  Early JWST observations also showed an extremely low metallicity of the host galaxy (low even for GRB host galaxies) and strong H₂ emission at the location of the transient, which is consistent with recent star-formation.  In this talk, I present optical and NIR imaging of the host galaxy and afterglow of this GRB from HST and JWST extending to 362 days post-burst.  The source is still detected in all epochs.  With these observations, my team and I will constrain the presence of a late jet break, the afterglow light curve and SED, and the model of the host galaxy.   

Searching for GRB Counterparts to Gravitational-waves with Fermi-GBM

The Fermi Gamma-ray Burst Monitor (GBM) is an all sky monitoring instrument sensitive to photon energies from 8 keV to 40 MeV. Its capabilities allow it to observe around 40 short gamma-ray bursts (GRBs) each year through on-board triggers alone, making it ideal for providing simultaneous gamma-ray observations of gravitational wave events. This fact was proven through the on-board detection of GRB 170817A and the associated binary neutron star merger event GW170817, which was a major milestone in multimessenger astronomy. Fermi-GBM continues to look for similar counterparts to gravitational waves from the fourth gravitational wave observation run (O4) through on-board triggers as well as subthreshold searches for weak transients. I will provide an overview of these searches and their recent results.   

Gamma-Ray Burst Classification through Machine Learning

Gamma-Ray Bursts (GRBs) are among the most energetic events in the universe. Prompt, multiwavelength, and multimessenger observations of these enigmatic transients allow us to probe physics beyond extremes that can be achieved in terrestrial laboratories. Much work remains to fully elucidate the origins of GRBs and to understand their physical processes. A holy grail in GRB studies is prompt classification of GRBs, aiding follow-up by guiding both observing profiles, prioritization of telescope time, and providing key statistical information for follow-up analysis.

We propose to build a fast GRB classification tool, based on modern unsupervised deep learning techniques and relying on a set of inputs for each event, which contains the core information relevant for GRB classification (including duration, temporal variation, pulse structure, spectral hardness and evolution, and how these parameters relate), and have not yet been explored within the machine learning framework: the GRB waterfalls.   

Kilonova Light-Curve Inference Using a Neural Network

Kilonovae are astrophysical transients which are powered by the decay of radioactive elements following a neutron-star-merger. The modeling of kilonova observables, such as light curves, is done using radiative transfer simulations. As these models encapsulate more realistic physics, their computational cost becomes prohibitive to broad parameter space sampling. Emulators, such as neural networks, are frequently employed to minimize computational cost while retaining high simulation fidelity. The use of emulators enables the generation of millions of light curves in a matter of minutes; however, it also introduces additional systematic uncertainty to an already complex problem with compounded and unknown uncertainties. In this talk, we present AT2017gfo parameter inference results using the neural network model presented in an associated poster. We also highlight important considerations with regard to our treatment of the systematic uncertainty and implications for similar future analyses.   

Time-resolved photometry of X-ray binaries with NASA's TESS spacecraft.

This work entails a time-series photometric analysis of X-ray binary stars. We have used observations from NASA’s TESS spacecraft. We have calculated the light-curves & Lomb-Scargle periodograms for both high-mass & low-mass X-ray binaries (20 objects in 4 catalogs). We have used Peranso for plotting light curves & their periodograms for all sectors of TESS observed at 2-min cadence. The periodogram peaks probably show pulsations of the compact object or the object’s orbital period. Many objects in our catalogs match with their orbital periods calculated from visible spectroscopy or radial velocity studies as mentioned in literature. Using this technique, we found multiple periodicities & the most prominent periods with their corresponding epochs have been tabulated.

We find periodic behavior in the brightness of 19 of the 20 stars. The photometric periods we measure for these stars are 6.37 days for Swift J0243.6+612, 1.08 days for HESS J0632+057, 4.4 hours for IGR J11305-6256, 3.13 days for PSR B1259-63, 3.63 days for IGR J16327-4940, 0.96 days for AX J1700.2-4220, 1.22 hr for 4U 1954+31, 6.2 days for PSR J2032+4127 and 1.32 days for HD 215227 for 9 stars in Neumann et. al (2023). From the 8 stars in Liu et. al (2007), the previously unobserved periods are 7.55 hours for 4U 1456-32, 15.96 days for 4U 1700+24, 6.42 hours for 4U 1908+005, 3.23 days for GS 2023+338 and 5.32 days for Cyg X-2. From Avakyan et. al (2023), IGR J17353-3539 is the only star we found. It has a period of 5.43 days.   

Self-consistent modeling of mass transfer and stellar evolution in eccentric binary star systems.

A majority of massive binaries will interact with a companion through Roche-lobe overflow mass transfer, impacting their component masses, orbit, and ultimate fate of the binary. Modern stellar evolution codes such as MESA can simulate mass transfer while evolving stars simultaneously where the orbit is often assumed to be circular. However observations of semi-detached binaries suggest systems can undergo mass transfer while in eccentric orbits, and remain eccentric after mass transfer episodes. We employ analytic equations for the secular orbital evolution of a binary under eccentric mass transfer into MESA, investigating binaries containing a compact object and star with a range of masses and orbital configurations. We find regions of parameter space where the orbital evolution due to eccentric mass transfer diverges significantly from the circular case, highlighting the need for more studies in this regime.   

Thorne-Żytkow objects have rejuvenated rapidly moving progenitors

When a massive star with a neutron star companion engages in a common envelope phase, there are two possible outcomes: formation of a short-period binary (a potential progenitor of a gravitational-wave source) or a merger. If the binary merges, it can form a Thorne-Żytkow Object (TŻO), an exotic star that has a neutron star core surrounded by a stellar envelope. We use population synthesis at Solar metallicity to study the demographics and rates of TŻOs. We find that most TŻO progenitors have experienced mass transfer earlier in their evolution and have therefore become rejuvenated. Recent work shows that rejuvenated stars have less bound envelopes, meaning that the envelope can be more easily ejected compared to stars that have not experienced a previous mass transfer episode. This indicates that rejuvenation could result in lower TŻO rates and higher double compact object formation and merger rates. We find that the vast majority of TŻOs are rejuvenated. Moreover, the systemic velocity distribution shows that the distribution of TŻOs with main-sequence donors has a rapidly moving extended tail not present in the distribution of TŻOs with more evolved donors. Therefore, we conclude that the most promising candidates for TŻOs are cool, luminous, and rapidly moving giants.   

On the viability for r-process nucleosynthesis in collapsars

We model a black hole – accretion disk system originating from the circularization of in-falling matter from a collapsing star – the collapsar scenario. We use the new, open source code Phoebus which includes frequency dependent general relativistic radiation magnetohydrodynamics with a dense matter equation of state. Using these improved initial conditions, we access the prospects for rapid neutron capture (r-process) nucleosynthesis occurring in the collapsar disk winds. Moreover, we access whether or not these winds may escape the in-falling stellar mantle and carry away r-process rich ejecta. We present prospects for future work on the sensitivity of these results to properties of the progenitor star.