Date: Wednesday 04-Sep-2024
Speaker: Rachael Stewart (George Washington University)
Title: Phase-Resolved Broadband X-ray Analysis of the Magnetar 1RXS J170849.0-400910 with XMM-Newton, NuSTAR, and IXPE
Abstract: Magnetars are a subclass of neutron stars characterized by extreme magnetic fields reaching up to 10^15 gauss—significantly higher than those found in typical pulsars. The decay of these supercritical fields generates bright, polarized X-ray emissions. The observed spectro-polarimetric properties of these emissions reveal information about the magnetar’s magnetic field configuration, surface temperature distribution, and the characteristics of its atmosphere and surface layers. Broadband, phase-resolved spectro-polarimetry is therefore a powerful tool for probing the emission geometry of magnetars. The magnetar 1RXS J170849.0-400910 possesses bright and persistent X-ray flux, a high pulsed fraction, and significant polarization levels. These characteristics make it an ideal target for detailed phase-resolved spectroscopic and polarimetric study. In this presentation, I will discuss findings from a deep, simultaneous NuSTAR and XMM-Newton observation of 1RXS J170849.0-400910, along with a re-analysis from an IXPE observation. This analysis provides the most comprehensive broadband X-ray phase-resolved spectro-polarimetric study of this source to date. The results provide new insights into the magnetar’s source geometry and surface heat distribution, enhancing our understanding of X-ray radiative processes in high magnetic field environments.
Date: Wednesday 11-Sep-2024
Speaker: Dimitris Kantzas (LAPTh CNRS)
Title: X-ray binary and Active Galactic Nuclei jets as cosmic-ray accelerators
Abstract: Since their discovery, cosmic rays (CRs) remain among the most mysterious phenomena of modern Physics. The dominant sources, as well as the exact acceleration mechanisms, remain unknown. The CRs up to the ``knee’’ have traditionally been considered to originate entirely in the shock waves of supernova remnants (SNRs), however, due to the lack of a “smoking-gun” TeV counterpart in many cases, as well as the new population of non-SNR Galactic PeVatrons, this scenario has been recently questioned. In this talk, I will motivate how the small-scale analogues of active galactic nuclei (AGN), namely black-hole X-ray binaries (BHXBs), can potentially contribute to the Galactic CR spectrum. Based on a new multi-zone, lepto-hadronic jet model to take advantage of the entire broadband multiwavelength spectra observed by BHXBs, I will discuss the neutrino and γ-ray emissions, and what are the similarities to large scale AGN jets.
Date: Wednesday 18-Sep-2024
Speaker: Niek Bollemeijer (University of Amsterdam)
Title: Revealing disk-corona connections on short timescales
Abstract: Black hole X-ray binaries in outburst can be very bright sources of X-rays, which are emitted mainly in two spectral components. The accretion disk dominates emission at low energies, while high energy X-rays originate from a region close to the black hole known as the corona. Despite decades of research, the nature and geometry of the corona is still subject to debate. The X-ray flux of accreting black holes has been observed to be highly variable on a broad range of timescales. The study of that variability is called X-ray spectral-timing and it can constrain models of the corona. Observations by X-ray telescopes NICER and Insight-HXMT, which cover the full X-ray band together, provide an unprecedented view of the complex variability. I will show how combining data from both telescopes leads to surprising connections between the lowest and highest X-ray energies and discuss what they may imply for the nature of the corona.
Date: Wednesday 25-Sep-2024
Speaker: Jeffrey McKaig (George Mason University)
Title: What controls the strength of optical coronal lines in active galactic nuclei?
Abstract: Forbidden, collisionally excited, optical atomic transitions from high ionization potential (IP~54.8eV) ions, such as Ca4+, Ne4+, Fe6+, Fe10+, Fe13+, Ar9+, and S11+, are known as optical coronal lines (CLs). The spectral energy distribution (SED) of active galactic nuclei (AGN) typically extend to hundreds of electron volts and above, and thus causes CLs to be “smoking guns” of AGN activity. However, optical CLs are often not detected in AGN using large scale optical surveys such as the SDSS. In this talk, I will present photoionization calculations using the spectral synthesis code Cloudy which determine possible reasons for the rarity of optical CLs in AGN. I will discuss the effects that metallicity, dust content, and ionizing SED slope have on the observability of these lines and how lower IP lines (e.g., [O III] 5007A) behave in the presence of CLs. Additionally, I will show optical CLs can be produced up to hundreds of parsecs from the nucleus, without the need for another ionizing source such as shocks.
Date: Wednesday 02-Oct-2024
Speaker: Mudit Garg (University of Zurich)
Title: Decoding Astrophysics from inspiraling LISA Massive Black Hole Binaries
Abstract: Our current understanding is that an environment – mainly consisting of gas or stars – is required to bring massive black hole binaries (MBHBs) with total mass ∼ [10^4, 10^7] MSun to near-merger from parsec separation. The final inspiral is driven by radiated gravitational waves (GWs) in near milli-Hz frequencies, which the recently adopted space-based mission LISA will observe in the 2030s up to high redshifts. Despite GWs dominating the binary evolution in the LISA band, a suitable environment can still non-negligibly speed up or slow down the binary inspiral, or sustain residual orbital eccentricity. Using realistic data analysis techniques, I will show how well LISA can measure gas-induced perturbation or initial eccentricity, together or separately. I also explore if a small eccentricity can mimic a weak gas effect and vice versa to motivate synergies between LISA and electromagnetic observations. Moreover, I show how ignoring moderate environmental effects biases binary parameters and violates general relativity. Lastly, I demonstrate how spin-eccentricity correlations in the LISA band can break certain degeneracies regarding formation channels. Therefore, my results have rich implications for astrophysics, data analysis, fundamental physics, and cosmology. LISA will launch in a decade, making this study exciting and valuable in unlocking the mysteries of MBHB evolution.
Date: Wednesday 09-Oct-2024
Speaker: Sarah Moran (NASA GSFC)
Title: CANCELLED
Date: Wednesday 16-Oct-2024
Speaker: Laura Sommovigo (Flatiron Institute)
Title: A dusty dawn: the rise of metals and dust in the high redshift universe
Abstract: Over the last two years, thanks to JWST we have gathered detailed information on the rest-frame UV spectra of galaxies out to extremely high redshifts, with dust playing a crucial role in the discussion of the very blue (betaUV <-2) and unattenuated albeit massive (Mstar\sim 1e9 Msun) z>10 JWST-selected sources. Complementing the UV data with sub-mm observation probing the rest frame FIR -- thus directly constraining the amount of cold gas and dust in high-z galaxies -- is crucial. However, ALMA follow-ups of JWST bright sources at z>9 have mostly failed to detect the FIR continuum and bright lines such as [CII]158micron and [OIII]88micron (just today we have the first detection in a secure JWST-selected candidate a z=14: arXiv:2409.20549). I will discuss the implications of such findings when compared to recent results from ALMA large programs targeting FIR lines and dust continuum in large samples of z=4-7 massive and UV bright galaxies (REBELS and ALPINE). In particular, I will discuss insight from theoretical models and RT-post-processed hydrodynamical simulations from the TNG and SERRA suites.
Date: Wednesday 23-Oct-2024
Speaker: Dhayaa Anbajagane (University of Chicago)
Title: Fundamental physics on small-scales: constraining inflation using galaxy surveys
Abstract: We are in an era where widefield galaxy surveys cover more than half the sky, and have nearly perfect overlap with many other surveys. While these datasets were designed for probing the late Universe, they can also be used to study primordial physics. I will show how weak lensing measurements probe the particle physics of inflation in novel ways, providing constraints that are both competitive and complementary to those from other probes. Achieving these constraints necessitates accurate, simulation-based forward models of structure formation, including the rich astrophysics of gas and stars. I will describe our recent efforts in constructing halo-based, data-driven models of baryonic physics and propagating them to weak lensing observables. Finally, I will also highlight how these signatures of inflation can manifest on even smaller scales --- such as those of dwarf galaxies and stellar streams --- with the potential to set constraints on a new regime of primordial physics.
Date: Wednesday 30-Oct-2024
Speaker: Anirudh Chiti (University of Chicago)
Title: Uncovering the ancient Milky Way with the DECam MAGIC survey
Abstract: The ancient stellar populations of the Milky Way and its surrounding dwarf galaxies are a window on the earliest stages of galaxy formation and element production. In particular, the Milky Way’s ultra-faint dwarf galaxies (UFDs) are the products of early galaxy formation on the smallest scales, and its lowest metallicity stars are plausibly directly enriched by the first stars. Recent, metallicity-sensitive imaging techniques have dramatically increased the efficiency of identifying such stars, with potential to significantly advance this field. I will present one set of results highlighting this: the detection of an extended population of low metallicity stars around several dwarf galaxies, highlighting the Tucana II UFD, indicating that several of these relic systems may host extended stellar populations. In the case of Tucana II, in which we detect stars out to ~1 kpc (~8 r_h), I will also present their detailed chemistry and interpret these signatures in the context of formation mechanisms of its outer regions. I will use these results to motivate the DECam MAGIC survey— a new survey on the Dark Energy Camera survey— that will image a quarter of the southern sky with a new metallicity-sensitive imaging filter 2-3 magnitudes fainter than similar existing photometry in the southern hemisphere. This survey will unlock new discovery space in the low metallicity, faint regime of the Milky Way across a broad region of the sky, with potential to dramatically increase the sample of low metallicity stars and provide direct insights on the products of the first stars.
Date: Wednesday 06-Nov-2024
Speaker: Madeline Lucey (University of Pennsylvania)
Title: The Galactic Halo's Contribution to Inner Galaxy Stellar Populations
Abstract: ΛCDM cosmology predicts the hierarchical formation of galaxies, with mass building up through merger events and the accretion of smaller systems. In the innermost regions, where dynamical times are relatively short, the phase-mixed stellar populations are thought to trace the Galaxy’s cumulative accretion history. Observations indicate that most of the stellar mass in the inner Galaxy is metal-rich, participating in the Galactic bar and forming a corresponding boxy/peanut-shaped (B/P) bulge. However, about ~5% of the stellar mass consists of metal-poor stars whose dynamical structure remains a topic of debate. In this talk, I will discuss my work from the Chemodynamical Origins of Metal-Poor Bulge Stars (COMBS) survey, demonstrating that at least half of the metal-poor stars in the inner Galaxy are halo stars on orbits with apocenters > 3.5 kpc, providing no evidence for a classical bulge in the Milky Way. Additionally, I will present my work using the FIRE-2 and Auriga cosmological zoom-in simulation suites of Milky Way-mass galaxies. I find that the stellar density profiles along the minor axis are not significantly correlated with metrics of the galaxy's accretion history, but are instead strongly correlated with the dark matter profile. Across simulation suites, the galaxies have a stellar-to-dark matter mass ratio that decreases as 1/R^2 along the minor axis. Whether these findings align with or challenge Milky Way observations remains to be determined.
Date: Wednesday 13-Nov-2024
Speaker: Lizhong Zhang (Flatiron Institute)
Title: Understanding the dynamics of accretion onto neutron stars and black holes in relativistic radiation MHD simulations
Abstract: Radiation-driven dynamics plays an essential role in understanding accretion physics around neutron stars and black holes, particularly in super-Eddington regimes. In super-Eddington accretion onto a black hole, thermal instability causes the disk to expand. This expansion is constrained by magnetic tension along the jet, which shapes a conical funnel in the outflow region. For accretion onto a strongly magnetized neutron star, the magnetic field disrupts the disk geometry, channeling accretion flows along field lines toward the magnetic poles. At sufficiently high accretion rates, radiation pressure supports this magnetically confined flow, forming a column structure at the poles. To quantify these dynamics, we solve (general) relativistic MHD equations coupled with angle-dependent radiation transfer, allowing us to capture the effects introduced by radiation and magnetic fields in extreme environments. The simulation results aligns with several recent observational findings and provides the diagnostics for future observations.
Date: Wednesday 20-Nov-2024
Speaker: Thankful Cromartie (Naval Research Laboratory)
Title: Detecting Gravitational Waves With Pulsar Timing: Updates from NANOGrav and the IPTA
Abstract: The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration recently published its 15-Year Data Set, providing substantial evidence for a nHz background of gravitational waves and marking an exciting milestone for pulsar timing arrays. Since the publication of our 12.5-Year Data Set, which strongly suggested the presence of a common red noise process in NANOGrav’s millisecond pulsar (MSP) timing data, we have added 21 new MSPs and 3 years of data. This colloquium will touch on the improvements between our 12.5 and 15-year data releases, results from the 15 year gravitational wave search, and exciting synergistic science results. NANOGrav’s participation in the International Pulsar Timing Array’s Third Data Release effort, as well as what lies on the horizon for pulsar timing array experiments, will also be discussed.
Date: Wednesday 27-Nov-2024
Speaker: NO SEMINAR
Title: Thanksgiving Break
Date: Wednesday 04-Dec-2024
Speaker: Megan Bedell (Flatiron Institute)
Title: CANCELLED
