Abstract

Future GW detectors like LISA and the proposed Cosmic Explorer and Einstein Telescope will herald a new era of GW science and astronomy. Together with pulsar timing arrays and inflationary probes, a frequency band spanning over 20 decades will be under observation. However, these detectors leave important gaps for GW detections, most notably the decihertz band, which would open exciting possibilities for GW cosmology, multi-messenger astronomy, and fundamental physics. The decihertz band is seen as technologically very challenging to cover. Space-borne detectors like DECIGO have been proposed, but it is not yet clear how to realize the required technologies. In this talk, we will present a new detector concept, the Lunar Gravitational-wave Antenna (LGWA). The idea is to measure vibrations of the Moon caused by GWs, which was first conceived by Joseph Weber for the Lunar Surface Gravimeter deployed on the Moon in 1972 by the crew of Apollo 17. Key of LGWA is an ultra-sensitive, cryogenic vibration sensor with sub-femtometer sensitivity in the decihertz band. An array of at least four of these sensors is to be deployed in a permanently shadowed region at the lunar north or south pole. The Moon is known to be several orders of magnitude quieter in terms of seismic perturbations than Earth, and the array serves to further reduce the extremely weak seismic background predicted from meteoroid impacts and moonquakes. In this way, LGWA becomes a technologically feasible concept fully exploiting the unique lunar environment and with the capability to achieve the first GW observations in the decihertz band.

Abstract

Globular clusters have long been known to contain ~100 times as many X-ray binaries per unit stellar mass as field star populations, due to dynamical interactions that can place compact objects into close binaries long after the compact object's formation. In the early days of Milky Way compact object studies, all the globular clusters with bright X-ray sources were found to be bursters, indicating that they had neutron star accretors. This, combined with the dynamical instability discovered by Spitzer, which was thought to efficiently eject stellar mass black holes from globular clusters, led to the conclusion that black holes were fundamentally absent from globular clusters , rather than the mere statement that they had not yet been found. Over the past decade and a half, studies have found numerous objects that meet all the criteria needed to establish the presence of black holes in globular clusters. The first such discoveries were highly variable bright X-ray sources in extragalactic globular clusters, found largely with Chandra and XMM-Newton. More recently, bright radio sources have been found that are likely to be quiescent black hole X-ray binaries. I will discuss these results, as well as their implications for gravitational wave events driven by black hole mergers in globular clusters.

Abstract

We are seeking both light and gravitational waves from binary supermassive black holes. This talk will discuss our efforts to detect multi-messenger waves from these massive pairs. Radio emission from pulsars allows us to detect low-frequency gravitational waves, while the large fields-of-view crossed with the supreme resolution allowed by radio interferometry experiments can help us map electromagnetic signatures of binary supermassive black hole candidates. When these techniques are combined, both become more sensitive, potentially allowing us to perform multi-messenger exploration of binaries in the coming 5-10 years.

Abstract

More than 20% of nearby main sequence stars are surrounded by debris disks, where planetesimals, larger bodies similar to asteroids and comets in our own Solar System, are ground down through collisions. The resulting dusty material is directly linked to any planets in the system, providing an important probe of the processes of planet formation and subsequent dynamical evolution. The Atacama Large Millimeter/submillimeter Array (ALMA) has revolutionized our ability to study planet formation, allowing us to see planets forming in disks and sculpting the surrounding material in high resolution. I will present highlights from ongoing work using ALMA and other facilities that explores how planetary systems form and evolve by (1) connecting debris disk structure to sculpting planets and (2) understanding the impact of stellar flares on planetary habitability. Together these results provide an exciting foundation to investigate the evolution of planetary systems through multi-wavelength observations.

Abstract

Fast radio bursts (FRBs), bright millisecond duration radio transients, are quickly becoming a subject of intense interest in time-domain and high energy astrophysics. FRBs have the exciting potential to be used as cosmological probes of both matter and fundamental parameters, but such studies require large populations. Advances in FRB detection using powerful new telescopes like the Canadian Hydrogen Intensity Mapping Experiment (CHIME) have enabled the growth of the population in the past few years from a few dozen to hundreds, with many surprises in the increasing FRB sample. Real-time discovery and follow-up, and new studies of the FRB population will provide us with some of the greatest insights in the coming years. In this talk I will provide a review of the FRB field from an observational and theoretical perspective. I will also discuss the most promising new developments in this field towards understanding the origins of these still-mysterious sources.

Abstract

Spectroscopic observations in the UV and X-ray bands with the newest generation of high-resolution instruments onboard current and future satellite observatories have the potential to reveal previously inaccessible details of processes in astrophysical plasmas, such as the ones in galaxy clusters and active galactic nuclei. This is essential for advancing our understanding of extreme environments and the evolution of the universe. However, what can be reconstructed from spectra is currently limited by the availability and quality of atomic data, on which plasma models are built. That is especially the case for highly charged ions (HCI), ubiquitous in hot astrophysical plasmas. For many years, electron beam ion traps (EBITs) have been valuable tools for laboratory measurements with HCI, providing a wide range of atomic data, like transtion energies and rates of ionization and recombination processes. Here, work with two generations of transportable EBITs is presented, in which radiation from ultrabrilliant UV and X-ray light sources - synchrotrons and free-electron lasers - is used to resonantly excite electronic transitions in trapped HCI. Subsequent fluorescence and changes of ion charge state are detected, which allows to gather spectroscopic data under well-controlled conditions with unprecedented resolving powers and signal-to-noise ratios. This yields atomic data valuable not only for astrophysics but also for benchmarking general atomic structure theory.

Abstract

The Compton Spectrometer and Imager (COSI) is a soft gamma-ray telescope that was recently selected to be NASA's next SMEX mission with a launch in 2025. COSI is designed to uncover the source of Galactic positrons, image diffuse emission from stellar nucleosynthesis, and perform polarization studies of gamma-ray bursts and compact objects. With unprecedented sensitivity in the MeV range, COSI is expected to provide advances in multimessenger astrophysics and reveal other sources of gamma-ray emission. COSI team members at NASA GSFC are responsible for the cryostat heat removal system, the data analysis pipelines, and science. In this presentation we will give an overview of the science goals of COSI, discuss COSI's telescope technology and the maturation path through its successful balloon program, and highlight GSFC's contribution to the future endeavors of the COSI mission.

Abstract

Small planets are ubiquitous around other stars. Excitingly, many of these worlds are rocky, with similar bulk density as the Earth. However, very little is known about their atmospheres. The composition (and even presence!) of rocky planet atmospheres is predicted to depend on a wide variety of factors, including stellar irradiation, mass loss, interior exchange, and giant impacts. In short, diversity should be expected. In this talk I will review these theoretical expectations and compare them to state-of-the-art observational constraints from Hubble, Spitzer, and ground-based telescopes. I will also discuss future prospects for atmosphere characterization with new facilities, particularly JWST. Finally, I will conclude with my outlook on the long and exciting path to biosignatures.

Abstract

A lot of exciting recent news about exoplanets has involved the discovery of "Earth-sized" planets in the "habitable zone" of "red dwarf" stars. This is partly due to the fact they are easier to find than "true Earth analogs", and partly because most stars are red dwarfs. I'll talk about these discoveries, give a background on red dwarfs, and concentrate on the current debate about whether such planets could plausibly harbor life. Strikes against them include tidal locking (so one side of the planet always faces the star) and the effects of stellar magnetic activity on their atmospheres (red dwarf habitable zones are much closer to active stars). I'll go into stellar activity in more detail, and also offer possible solutions to each of the habitability concerns.

Abstract

Measurements of the polarization of the cosmic microwave background will constrain models of inflation and can set unprecedented bounds on the sum of neutrino masses. I will discuss constraints achievable with a potential future space mission called the Probe of Inflation and Cosmic Origins (PICO). PICO's bound on models of inflation will be the strongest compared to any existing or currently foreseeable survey. However, polarized Galactic emission will overwhelm the cosmological signals in all PICO frequency bands. I will describe our work to assess PICO's efficacy in separating foregrounds for constraining or detecting the inflationary signal.

Abstract

Type Ia supernovae (SNe Ia) are thermonuclear explosions of carbon/oxygen white dwarfs (WDs). Perhaps surprisingly to researchers outside the SN subfield, fundamental questions persist regarding any further details. What is the nature of the companion(s) that trigger the explosion? What mode of burning consumes the WD? And is one model responsible for all SNe Ia, or do multiple scenarios contribute to the overall population? In this talk, I will describe our theoretical and observational work on the "Dynamically Driven Double Degenerate Double Detonation" (D6) scenario, in which the coalescence of a double WD binary leads to a converging-shock-triggered detonation in the more massive WD and a subsequent SN Ia. Our recent successes, which include the best match to the Phillips relation in the literature and the prediction and discovery of hypervelocity stars ejected from the Milky Way, provide evidence that the D6 scenario is the mechanism responsible for all non-peculiar SNe Ia.

Abstract

Brown dwarfs and directly imaged self-luminous exoplanets are interesting and complex worlds that form a critical stepping stone along the path to imaging Earth-like planets. By examining their atmospheres in detail we can better understand their thermal profiles, chemical composition, and cloud properties that are tightly coupled with their formation and evolution. In this talk, I will explain how I use atmospheric retrievals, a powerful inverse modeling technique, to examine the atmospheres of brown dwarfs. I will present results from a comparative sample of brown dwarfs showing how they have enhanced our understanding of the atmospheres of substellar objects and our retrieval approach. Lastly, I will discuss how JWST and next-generation observatories will enhance our understanding of substellar atmospheres serving as a critical bridge in our exploration of smaller and cooler worlds.

Abstract

Dynamically, cosmic rays with GeV-TeV energies may play an important role in galaxy evolution. Their pressure and pressure gradients can act at different scales to regulate the gas inflows and outflows, modifying the gas cycling between the diffuse gas reserves and the condensed phase where stars form, holding clouds against gravitational collapse to reduce star formation, helping launch galactic winds and fountains off galactic discs, modifying gas accretion onto a galaxy, and stimulating magnetic amplification. Their feedback efficiency depends on their transport properties in the different media, but the effective diffusion speeds resulting from plasma processes happening at micro-parsec scales are poorly known, both observationally and theoretically.

Gamma-ray observations with Fermi LAT reveal these cosmic rays as they interact with the gas along their interstellar journey. This is why the Milky Way stands as the most prominent source of GeV gamma rays in the sky. Fermi has also detected, if not resolved, other galaxies. Sophisticated galaxy simulations can in parallel solve the magnetohydrodynamics of the gas together with the cosmic-ray dynamics for different transport assumptions. I will review what we have recently learned from the data and from simulations and I will discuss the apparent paradox that emerges : whereas orders of magnitude changes in diffusion lengths are expected across the interstellar media, Fermi observations reveal a much more uniform behaviour, with 30% flux variations in the solar neighbourhood, smooth spatial and spectral gradients across the Milky Way, and a total gamma-ray flux per galaxy that scales primarily with the star formation rate and not so much with the interstellar medium state. A puzzle that must be solved in order to assess the true role of cosmic rays in galaxy evolution.

Abstract

We recently reported the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (T ~ 270 days), high-magnification microlensing event MOA-11-191/OGLE-11-461, in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of six years, reveals a clear relativistic astrometric deflection of the background star's apparent position. Combining the photometric, astrometric, and spectroscopic measurements, we obtain a lens mass of 7.1 +/- 1.3 solar mass and a distance of 1.58 +/- 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic-disk stars at similar distances by an amount corresponding to a transverse space velocity of ~45 km/s, suggesting that the BH received a “natal kick'' from its supernova explosion. Our mass measurement is the first ever for an isolated stellar-mass BH using any technique.