2024: 0524 •1004 • 1213

2023: 0119 • 0519 • 1208

2021: 0416 • 0519 • 0618 • 0716 • 0917 • 1028

2020: 0630 • 0731 • 0828  • 0929 • 1030 • 1130

The HEAD Frontier Seminars are held over Zoom, and all HEAD members receive e-mail with the Zoom meeting number. If you are not a HEAD member and would like to attend, please contact the HEAD Secretary. HEAD members are encouraged to invite their non-HEAD colleagues. Videos of talks can be viewed on the AAS/HEAD youtube channel.

2024 December 13, 1pm EST (video)

NuSTAR and XMM-Newton Observations of Pulsar Wind Nebula G54.1+0.3

Investigating Effects of Metallicity Variances on Cooling in the Circumgalactic Medium Using MAIHEM Simulations

Nuclear Cross-Section Experiment Series for Astrophysical and Planetary Needs

2024 October 4, 1pm EDT (video)

Impact of Systematic Modeling Uncertainties on Kilonova Property Estimation

Daniel Brethauer (UC Berkeley)

Precision measurements of kilonova properties like ejecta mass, geometry, and velocity are critical to informing our understanding of neutron star mergers, the neutron star equation of state, and the origins of the heaviest elements. Estimates of these properties are highly model-dependent and often do not incorporate the systematic uncertainties due to unconstrained atomic, nuclear, and astro-physics. By producing and analyzing a grid of kilonova models (using the Monte-Carlo radiative transfer code Sedona) that span the expected parameter space of mass, characteristic velocity, and composition and varying the underlying physical assumptions of atomic data and thermalization efficiency prescription, I quantify the systematic uncertainties in the inferred kilonova ejecta parameters.

IXPE Observations of Stellar-Mass Black Holes through Accretion States

Nicole Rodriguez Cavero (Wash. U.)

X-ray polarimetry offers a unique insight into black hole X-ray binary systems found in soft, intermediate, and hard states of accretion. Every state is dominated by a combination of thermal disk, coronal, and reflected emission—each type containing different information about the space-time around the black hole that can be understood through polarization. To date, the Imaging X-ray Polarimetry Explorer (IXPE) has measured the polarization signature of nine stellar-mass black holes, some in multiple states of accretion. In the hard states, polarization allows us to constrain the properties of the black hole corona; in the soft states, polarization can be used to rectify the degeneracies between black hole mass, spin, and inclination. We discuss the IXPE observations of stellar-mass black holes during different accretion states and their implications on the geometry and properties of the system through state transitions.

X-ray Reflection Spectroscopy of Accreting Black Hole Binaries

Riley Connors (Villanova)

2024 May 24, 1pm EDT (video)

Type I X-ray bursts as probes of the neutron star accretion environment

Julia Speicher (Georgia Institute of Technology)

Neutron star accretion in low-mass X-ray binaries can lead to unstable nuclear burning on the neutron star surface, causing a Type I X-ray burst. The burst irradiates the accretion environment, consisting of an accretion disk and a corona. Theoretical models predict that the burst radiation will change accretion flow properties. The burst photons exert Poynting Robertson drag on the disk, which enhances the mass accretion rate in simulations, and which could be the cause of an increased persistent emission and an observed soft excess < 3 keV during the burst. The enhanced accretion rate furthermore drains the inner disk region, causing its inner edge to recede outwards. Calculations also predict a highly ionized disk during the burst. The predicted reflection spectra feature emission lines and a soft excess varying in strength as the flux irradiating the disk changes. Additionally, the burst will cool the corona, causing a drop in observed hard X-ray flux > 30 keV related to the corona geometry. In this presentation, I will review the theoretical predictions of the burst interaction with the accretion flow and how they translate into observational properties. Overall, X-ray bursts provide a unique opportunity to probe accretion disk physics with a repeatable experiment.

How do supermassive black holes grow from z=4 to z=0

Fan Zou (Penn State)

Supermassive black holes (SMBHs) grow together with their host galaxies throughout cosmic time, and two growth channels are present – accretion and merger. I will talk about our recent efforts to build a complete picture of the SMBH growth under these two channels. First, regarding the accretion channel, it has been shown that the population mean SMBH accretion rate primarily correlates with galaxy stellar mass and redshift for the general galaxy population. I will present the best measurements of this function with our compilation of an unprecedentedly large sample from nine high-quality survey fields and our development of an advanced semiparametric Bayesian method, where we use X-rays to sample the AGN accretion power and multiwavelength surveys to sample galaxies. I will then combine our observationally constrained accretion channel with modern cosmological simulations, which provide merger information, to illustrate how SMBHs evolve from z=4 to z=0. Our analyses can address multiple issues about the SMBH population, such as their mass function, their scaling relation with their host stellar mass, and the contributions of the two growth channels to the overall SMBH population growth.

First Searches for HNLs in IceCube

Julia Book Motzkin (Harvard)

Neutrino telescopes present a novel opportunity to search for a coupling between Heavy Neutral Leptons (HNLs) and tau neutrinos via mass mixing. These searches can leverage the tau neutrino flux from the oscillations of atmospheric muon neutrinos as they traverse the Earth, giving greater access to the HNL/nutau mixing than previous accelerator-based experiments. This work presents the first search for HNLs using ten years of data from IceCube’s DeepCore sub-array. These results serve as a proof-of-concept for HNL searches in IceCube, enabled by the development of tailored HNL simulation tools. This talk will present these first results, as well as the open-source event generator developed for the search. Finally, I will discuss future prospects for HNL searches in water-cherenkov experiments, particularly improvements to HNL identification made possible by new reconstruction techniques capitalizing on the HNLs unique morphology.

2023 December 8, 1pm EST (video)

Scary Barbie: An Extremely Energetic, Long-duration Tidal Disruption Event Candidate without a Detected Host Galaxy at z = 0.995

Bhagya Madimugar Subrayan (Purdue University)

We report multiwavelength observations and characterization of the ultraluminous transient AT2021lwx (ZTF20abrbeie; aka "Barbie") identified in the alert stream of the Zwicky Transient Facility (ZTF) using a Recommender Engine For Intelligent Transient Tracking filter on the ANTARES alert broker. From a spectroscopically measured redshift of 0.995, we estimate a peak-observed pseudo-bolometric luminosity of log(L_(max)) = 45.7 from slowly fading ztf-g and ztf-r light curves spanning over 1000 observer-frame days. The host galaxy is not detected in archival Pan-STARRS observations (g > 23.3 mag), implying a lower limit to the outburst amplitude of more than 5 mag relative to the quiescent host galaxy. Optical spectra exhibit strong emission lines with narrow cores from the H Balmer series and ultraviolet semi-forbidden lines of Si III] λ1892, C III] λ1909, and C II] λ2325. Typical nebular lines in Active Galactic Nucleus (AGN) spectra from ions such as [O II] and [O III] are not detected. These spectral features, along with the smooth light curve that is unlike most AGN flaring activity and the luminosity that exceeds any observed or theorized supernova, lead us to conclude that AT2021lwx is most likely an extreme tidal disruption event (TDE). Modeling of ZTF photometry with MOSFiT suggests that the TDE was between a ≈14M⊙ star and a supermassive black hole of mass MBH ∼ 10⁸M⊙. Continued monitoring of the still-evolving light curve along with deep imaging of the field once AT2021lwx has faded can test this hypothesis and potentially detect the host galaxy.

Detection of turbulence in the hot circumgalactic medium (CGM) of the Milky Way

Sanskriti Das (Stanford University)

The density of the hot ($>10^6$\,K) CGM of the Milky Way is often modeled with an underlying assumption of isotropic density distribution. However, the large ($>$order-of-magnitude) scatter in the column density of the hot CGM, N(H), contradicts that assumption. N(H) is calculated from $z=0$ absorption lines of OVII and OVIII toward extragalactic ($b>20^\circ$) sightlines using X-ray grating spectroscopy and an assumed uniform metallicity, O/H, across the sky. We calculate the structure function, $S(\theta) \propto (\delta n)^2$ from N(H) as a measure of the density fluctuation, $\delta n$. We find that $S(\theta)$ grows with $\rm\theta$, the angular separation between sightlines, at a power-law slope of $4.39\pm1.12$, deviating from the Kolmogorov-like slope of 5/3 by $>2\sigma$. The driving scale of turbulence, beyond which $S(\theta)$ saturates, is 30$^\circ$, indicating large-scale turbulence in the hot CGM.

On the evolution of high luminosity and obscured AGN in the Stripe 82X field

Alessandro Peca (University of Miami)

We present the X-ray spectral analysis of XMM and Chandra observations in the 31.3 deg2 Stripe-82X (S82X) field. Of the 6181 unique X-ray sources in this field, we analyze a sample of 2937 candidate active galactic nuclei (AGN) with solid redshifts (045.5, z>3), where the obscured AGN fraction reaches 64±12%. The total, unobscured, and obscured X-ray luminosity functions (XLFs) are determined up to z=4. We report a luminosity and density evolution of the total XLF, with obscured AGN dominating at all luminosities at z>2 and unobscured sources prevailing at log Lx>45 at lower redshifts. Our results agree with the evolutionary models in which the bulk of AGN activity is triggered by gas-rich environments and in a downsizing scenario. Also, the black hole accretion density (BHAD) is found to evolve similarly to the star formation rate density, confirming the co-evolution between AGN and host-galaxy, but suggesting different time scales in their growing history. Intriguingly, the derived BHAD evolution shows that Compton-thick AGN contribute to the accretion history of AGN as much as all other AGN populations combined, significantly exceeding previous estimates.

2023 May 19, 1pm EDT 

Unusual Hard X-Ray Flares Caught in NICER Monitoring of the Binary Supermassive Black Hole Candidate AT2019cuk/Tick Tock/SDSS J1430+2303

Megan Masterson (MIT)

The nuclear transient AT2019cuk/Tick Tock/SDSS J1430 has been suggested to harbor a supermassive black hole (SMBH) binary within a few years of coalescence (Jiang et al. 2022). As this event presented the opportunity to potentially witness the merger of a SMBH binary for the first time, we monitored this source with high-cadence NICER X-ray observations (multiple visits per day) and continued optical monitoring from January-August 2022. In this talk, I will present the results of this monitoring campaign, which shows no obvious periodic/quasi-periodic modulation on the reported timescale of ~30 days in either the X-ray or the optical bands, thus challenging the binary SMBH hypothesis. However, we do observe exotic X-ray variability in the NICER data, namely the first case of repetitive hard X-ray flares (in 2-4 keV) in an AGN. These X-ray flares last for roughly a day, are aperiodic, and exhibit anomalously hard X-ray spectra with photon indices around ~1-1.4. I will present and discuss potential models to explain these hard flares, including: (1) corona/jet variability, (2) variable obscuration, and (3) if AT2019cuk is indeed a binary, self-lensing from the secondary SMBH. Based on timescales, spectral changes, and the aperiodic behavior of the flares, we favor the variable corona model for their origin, but this type of behavior has never been seen before in a radio-quiet AGN. These findings illustrate the importance of high-cadence X-ray monitoring for our understanding of the rapid variability of the X-ray corona and necessitate further high-cadence, multiwavelength monitoring of changing-look AGN like AT2019cuk to probe the corona-jet connection.

Line Emission Diagnostics of Tidal Disruption Events

Zachary Andalman (Yale)

When a star passes sufficiently close to a supermassive black hole (BH), the star is pulled apart by the BH’s tidal field in a so-called tidal disruption event (TDE). The stellar debris falls back onto the black hole resulting in a bright flare across the electromagnetic spectrum and the formation of a “debris disk” on a timescale of weeks to months. The strong UV/optical components of most TDE spectra suggest the presence of an envelope which reprocesses the intrinsic hard X-ray emission. Previous works have shown that optical line emission from this envelope is set by wavelength-dependent optical depth effects. However, AT2018hyz’s double-peaked Hα profiles suggest that some line emission comes from continuum photons which are reprocessed by the debris disk. In this work, we show how this reflected line emission can constrain the physical properties of a TDE using a toy model. Our model combines data from the Modular Open Source Fitter for Transients and a grid of photoionization calculations in the spectral synthesis code CLOUDY. Our model predicts that the reflected H alpha emission should appear a few peak fallback times after the disruption and rapidly decrease in flux in agreement with observations. Using a GRHD simulation of a debris disk in the GPU-accelerated code H-AMR, we show that the line flux and double-peakedness of the reflected line profile can constrain the 3D viewing angle of the observer. We also show that our model can be paired with TDE spectra to constrain the geometry of the debris disk. Finally, we suggest that with high-cadence TDE spectra, we may be able to constrain the circularization efficiency of the stellar debris.

2023 January 19, Noon EST  (video)

Fermi-GBM in the era of multimessenger transients

Joshua Wood (NASA/MFSC)

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 240 gamma-ray bursts (GRBs) each year through on-board triggers alone, making it ideal for providing simultaneous gamma-ray observations of multimessenger transients. 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 multimessenger detections through on-board triggers as well as subthreshold searches for weak transients, performed both in high-time-resolution continuous data and in targeted follow-ups of gravitational wave and high-energy neutrino events. I will provide an overview of these searches and their recent results. The Tale of SN2004C Lindsay Demarchi (Northwestern) It's long been legend that stars lose their mass through steady winds (<10^-2 solar masses/year) over the course of their lifetime. However, in the last five years, there has been emerging evidence that stars may violently and unexpectedly lose large shells of material just before they die. These harbinger events can not only alert observational astronomers to when a star may explode, but may also reveal the inner mechanisms of core collapse, normally obscured by the supernova explosion. It is currently a mystery as to why and how stars lose large amounts of material, but radio observations taken during the supernova explosion reveal to us the density profile of the circumstellar material (CSM), or what was lost by the star during the final decade of its life. One such case is SN2004C, a cousin of SN2014C and SN2019yvr, a stripped-envelope supernova that encounters a shell of material at 10^16cm away from the explosion site, and continues to change the story of mass loss in stellar evolution.

The Tale of SN2004C

Lindsay Demarchi (Northwestern)

It's long been legend that stars lose their mass through steady winds (<1E-2 solar masses/year) over the course of their lifetime. However, in the last five years, there has been emerging evidence that stars may violently and unexpectedly lose large shells of material just before they die. These harbinger events can not only alert observational astronomers to when a star may explode, but may also reveal the inner mechanisms of core collapse, normally obscured by the supernova explosion. It is currently a mystery as to why and how stars lose large amounts of material, but radio observations taken during the supernova explosion reveal to us the density profile of the circumstellar material (CSM), or what was lost by the star during the final decade of its life. One such case is SN2004C, a cousin of SN2014C and SN2019yvr, a stripped-envelope supernova that encounters a shell of material at 1E^16cm away from the explosion site, and continues to change the story of mass loss in stellar evolution.

2021 October 28, 1pm EDT

Detection of TeV gamma rays from the Crab Nebula with an innovative telescope designed for the Cherenkov Telescope Array

Leslie Taylor (University of Wisconsin - Madison)

The Cherenkov Telescope Array (CTA) is the next-generation ground-based observatory for very-high-energy gamma-ray astronomy and it will detect gamma-ray sources ~100 times faster than current arrays. Its unparalleled sensitivity and angular resolution will provide unique insights into some of the most powerful sources in the non-thermal sky and enable superb multiwavelength and multimessenger observations. An innovative 9.7 m aperture, dual-mirror Schwarzschild-Couder Telescope (SCT) design is under consideration to cover the core CTA energy range. The SCT delivers higher angular resolution over a large field of view (8 degrees) and doubles the sensitivity when compared to current single-mirror telescope designs. Its high-resolution camera uses densely packed silicon photomultipliers and state-of-the-art electronics capable of imaging air showers at a rate of one billion frames per second. A prototype SCT was inaugurated in 2019 at the Fred Lawrence Whipple Observatory and began scientific operations in January of 2020. I will describe the commissioning and performance of the pSCT as well as its first detection of TeV gamma rays from the Crab Nebula. This initial result proves the viability of this novel telescope design and promises substantial gains in gamma-ray astronomy. I will also discuss plans to significantly upgrade the pSCT camera by increasing its field of view and lowering its threshold.

Energy dependent morphology of the pulsar wind nebula HESS J1825−137 with Fermi-LAT

Giacomo Principe (INAF - Istituto di Radioastronomia)

Taking advantage of more than 11.5 years of Fermi-LAT data, we perform a new and deep analysis of the pulsar wind nebula (PWN) HESS J1825-137. We present the results of the spectral analysis and of the first energy-resolved morphological study of the PWN HESS J1825-137 from 1 GeV to 1 TeV. This PWN is an archetypal system making it a perfect laboratory for studying particle transport mechanisms. Combining this analysis with recent HESS results enables a more complete picture of the nebula to emerge. Using the variation of the source extension and position, as well as the constraints on the particle transport mechanisms, we present also a scheme for the possible evolution of the system. Finally, we provide an estimate of the electron energy density and we discuss its nature in the PWN / TeV halo-like scenario.

Modeling of X-ray spectral and spectropolarimetric observations of magnetars and black holes

Henric Krawczynski (Washington University in St. Louis)

The Imaging X-ray polarimetry Explorer (IXPE) mission is scheduled for launch in Fall 2021. We have developed models for fitting the flux (Stokes I) and polarimetric (Stokes Q and U) X-ray spectra of magnetars and black holes. In this contribution, we present the models and first results from fitting archival spectroscopic XMM-Newton, Suzaku and NuSTAR data with these models. We present furthermore predictions of the expected IXPE results, and the astrophysical insights that can be inferred from these observations.

2021 September 17, 1pm EDT

The Landscape of Relativistic Stellar Explosions

Anna Ho (UC Berkeley)

Decades of observations of long-duration gamma-ray bursts (GRBs), a rare class of explosions attributed to relativistic jets launched in the collapse of massive stars, have yielded important insights on massive-star evolution, compact-object formation, and the physics of relativistic outflows. A major outstanding question is whether GRBs represent the extreme of a continuum of phenomena, ranging in mass-loading factor, viewing angle, and engine duration or jet power. Mass-loaded jets (the long-hypothesized “dirty fireballs”) and jets viewed off-axis would not produce luminous gamma-ray emission but could be detected at other wavelengths as an “orphan afterglow.” The wide field-of-view and fast cadence of the Zwicky Transient Facility (ZTF), an optical time-domain survey, makes it particularly well-suited to discovering afterglows, which are rare and fade away within a single night. Applying well-defined selection criteria to ZTF’s high-cadence optical surveys, we are for the first time routinely discovering optical afterglows without a GRB trigger. Prior to ZTF only three afterglows had been discovered without a GRB trigger; we have more than tripled the number, and have discovered several events lacking any detected GRB. Joint observations with sensitive gamma-ray monitors in the next few years will establish whether the gamma-ray dark events represent the first dirty fireballs or jets viewed off-axis.

The Hot Circumgalactic Medium of L* Galaxies

Sanskriti Das (Ohio State University)

Observations of the hot circumgalactic medium (CGM) have taken a great stride in the last two decades, and are going to be of prime importance in the next decade(s). In that context, I will talk about my work on the hot CGM of the Milky Way and a nearby L* galaxy. By probing the hot CGM of the Milky Way using absorption lines of multiple metals (e.g., C, N, Ne, Mg, Si and Fe) in addition to oxygen, I have discovered a 1e7 K gas coexisting with the well-known 1e6 K gas, with non-solar chemical composition and non-thermal line broadening. The emission analysis toward the same direction as the absorption analysis has revealed that the emitting and the absorbing gas are not probing the same temperature components, clearly ruling out the simplified picture of the single-temperature hot CGM. The observation of the nearby L* spirals complement our observation of the Milky Way. By carefully choosing the right instrument, optimum target and following a rigorous method, I have extracted the faint emission from the diffuse, extended, massive hot CGM of NGC 3221, which can account for the missing galactic baryons of NGC 3221. There is suggestive evidence of super-virial temperature, and a temperature gradient within 100 kpc of NGC 3221. This is the first external L* galaxy with such exciting findings. These results provide insight into the impact of stellar feedback on the hot CGM. Also, these will be useful information to design experiments with upcoming X-ray telescopes.

The Best Case Scenario: Towards Prompt Arcminute Localization of a GW/GRB Source

Aaron Tohuvavohu (University of Toronto)

The rich EM phenomenology in the first few hours after a compact object merger encodes the nature of the post-merger remnant, and a wide array of other compelling physics. Unfortunately, the requirement to search, find, and classify a counterpart within the large GW localization regions before targeted follow up with sensitive instruments can begin, excludes access to these first few hours, even for the most well localized GW sources. The ability to rapidly localize a GW source to within the field-of-view of a narrow field sensitive facility, would enable extraordinary science, and is uniquely enabled by GRB imagers with arcminute localization, like Swift/BAT. Such a prompt localization is the best case scenario. I will present the Swift/BAT-GUANO rapid spacecraft commanding and targeted sub-threshold GRB search pipeline, which allows significantly deeper searches for faint GRB 170817-like bursts, achieving the farthest detection range for such transients among current instruments. This pipeline has already increased the rate of arcmin localized GRBs by >15%. GW/GRB searches in the joint sub-threshold regime can also significantly extend the BNS detection horizon, and I will discuss methods and results from a joint search during LVC O3. The angular resolution of BAT allows good spatial discrimination and push to higher temporal FARs with the small spatial overlap, further increasing the sensitivity of joint searches. However, Swift/BAT's field of view (1/6 sky) decreases the expected detection yield compared to all-sky instruments, even with the increased horizon. I will discuss biased scheduling techniques that can increase the joint GRB/GW detection rate, and efforts to use GUANO-enabled rapid commanding capabilities to respond to early warning GW alerts and put the GW location within the BAT FoV at merger time. The combination of all of these will increase the chance of a best case scenario, and set the stage for next generation space telescope response.

2021 July 16, 1pm EDT

The Swift Bulge Survey: faint X-ray sources in the galactic bulge

Craig Heinke (University of Alberta)

The Swift Bulge Survey is designed to find faint (1e34Late-time radio observations of short gamma-ray bursts lend insight to the products of binary neutron star mergers Genevieve Schroeder (Northwestern University)

Short gamma-ray bursts (SGRBs), which are thought to come from the mergers of binary neutron stars (BNS), may produce massive, rapidly spinning, highly magnetized neutron stars, known as magnetars. These magnetars may deposit a fraction of their rotational energy into the surrounding kilonova ejecta, powering a synchrotron radio signal from the interaction of the ejecta with the circumburst medium. Combining new radio observations with previous studies, we uniformly analyzed 27 low-redshift(z < 0.5) SGRBs and found that >50% of SGRBs did not form stable magnetar remnants in their mergers. Assuming SGRBs are produced by BNS mergers drawn from the Galactic BNS population plus an additional component of high-mass mergers, we place constraints on the maximum mass of a non-rotating neutron star to be <2.23 solar masses. Our methods are complementary to studies in pulsars and gravitational waves which explore the masses that can be achieved for neutron stars.

Using the bispectrum to probe radio X-ray correlations in GRS 1915+105

Kavitha Arur (Georgia Institute of Technology)

Quasi-periodic oscillations (QPOs) observed from X-ray binaries, are a signature of rapid X-ray variability and a powerful diagnostic tool to study the regions closest to the central compact object. However, the physical origin of these QPOs is still under debate. One way to break degeneracies between the different models that can reproduce the observed power spectra is to use higher order time series analysis techniques, such as the bispectrum and the closely related bicoherence; which is a measure of the strength of phase coupling among triplets of frequencies. In this talk, I will focus on the black hole binary GRS 1915+105 which uniquely shows all three known bicoherence patterns that are seen from type C QPOs, and discuss how the nature of the phase coupling between the QPO and noise components is correlated to the radio properties. I will also discuss how these observations can be explained by the cooling of the jet by soft photons from the accretion disk. These results highlight the power of applying such higher order analysis to better understand the accretion around black holes.

2021 June 18, 1pm EDT

Multiwavelength Follow-up of IceCube Neutrino Alerts

Felicia Krauss (Penn State University)

Since the detection of an astrophysical flux of neutrino by IceCube, the sources of origin have remained largely mysterious. The exciting discovery of the coincidence of the neutrino event IC170922A with the AGN TXS0506+056 was the first hint at the role that AGN play as multimessenger and cosmic ray sources. We investigate several IceCube events, including IC190331A in order to identify likely counterparts. Most IceCube events are not consistent with a blazar origin. We show that IC190331A could have been produced by a radio-quiet AGN, suggesting that the cores of AGN - not only the jets - could be relevant cosmic ray emitters.

Heating the ICM via Cosmic-Ray-Driven Instabilities

Philipp Kempski (UC Berkeley)

Most of the baryonic mass in galaxy clusters resides in the hot and tenuous Intracluster Medium (ICM) that fills the space between individual galaxies. In the dense central regions, the ICM gas rapidly loses energy via X-ray emission. Observations show that despite the large radiative losses, the ICM plasma does not cool efficiently. This suggests that there is a heating source present that keeps the gas in approximate thermal balance. It is now widely accepted that central Supermassive Black Holes and their jets likely play an important role in providing energy to the ICM and thus prevent a “cooling catastrophe”. However, how this energy is transported and thermalized throughout the ICM remains an open question. In this talk, I will argue that cosmic rays may play an important role in the heating of the ICM plasma by efficiently exciting sound waves, which subsequently travel and dissipate across the ICM.

Discovery of the SGRB 181123B at z = 1.754: Implications on the Delay Time Distribution

Kerry Paterson (Northwestern University)

I will discuss the recent results of the short gamma-ray burst GRB 181123B. Thanks to optical spectroscopy with Keck, we identified this SGRB at z > 1.5, later solidified to z = 1.754 with Gemini NIR spectroscopy. As such, GRB 181123B has the 2nd highest redshift for a secure SGRB discovered with Swift to date. Rapid ToO observations on the order of hours also allowed the discovery of an optical afterglow, making it the most distant Swift SGRB with an optical afterglow detection. This discovery highlights the importance of rapid ToO observations with large telescopes, whose sensitivity can capture these faint signals, across multiple wavelengths. The discovery of the afterglow and follow-up of the host required the sensitivity of large telescopes such as Keck. With the discovery of another high-redshift SGRB, we also explore the effects of a missing high-redshift population among the current Swift sample and the implications on delay time distribution models.

2021 May 19, 1pm EDT

The Wolf-Rayet + Black Hole Binary NGC 300 X-1: What is the Mass of the Black Hole?

Breanna Binder (Cal Poly, Pomona)

We present new X-ray and UV observations of the Wolf-Rayet + black hole binary system NGC 300 X-1 with the Chandra X-ray Observatory and the Hubble Space Telescope Cosmic Origins Spectrograph. When combined with archival X-ray observations, our X-ray and UV observations sample the entire binary orbit, providing clues to the system geometry and interaction between the black hole accretion disk and the donor star wind. We measure a binary orbital period of 32.7921±0.0003 hr. We further measure radial velocity variations for several prominent FUV spectral lines, most notably He II λ1640 and C IV λ1550. We find that the He II emission lines systematically lag the expected Wolf-Rayet star orbital motion by a phase difference Δϕ∼0.3, while C IV λ1550 matches the phase of the anticipated radial velocity curve of the Wolf-Rayet donor. We assume the C IV λ1550 emission line follows a sinusoidal radial velocity curve (semi-amplitude = 250 km s−1) and infer a BH mass of 17±4 M⊙.

Probing Kilonova Ejecta Properties Using a Catalog of Short Gamma-Ray Burst Observations

Jillian Rastinejad (Northwestern University)

The discovery of GW170817 and GRB170817A in tandem with AT2017gfo cemented the connection between neutron star mergers, short gamma-ray bursts (GRBs), and kilonovae. While gravitational wave detectors are on hiatus, short GRBs present an exciting avenue for continued kilonova searches. In this talk, I will present our comprehensive catalog of optical and near-infrared observations of 85 short GRBs discovered in the last 15 years, including detections of kilonova candidates, low-luminosity afterglows, and deep upper limits. I will discuss how this catalog reveals diversity in kilonovae uniformly observed from a pole-on viewing angle, including that deep upper limits of a number of bursts probe lower luminosities than AT2017gfo. In addition, I will show that past observational follow-up of short GRBs is more constraining of lanthanide-poor rather than lanthanide-rich kilonovae. Future targeted follow-up of short GRBs may uncover further kilonovae diversity and provide insights to the progenitors and remnant of a burst.

A VLA and VLBI Proper Motion Study of Extragalactic Jets: Connecting the Parsec and Kiloparsec Scales

Agniva Roychowdhury (University of Maryland, Baltimore County)

Proper motions of extragalactic jets, primarily conducted with very long baseline interferometry (VLBI), have revealed that these jets have bulk relativistic velocities which can exceed 99.999% the speed of light (Lorentz factors up to ~80). The parsec-scale proper motions traced by VLBI observations, however, often show a flow that is still accelerating on these scales. The measurement of the full velocity profile of jets from parsec to kiloparsec scales has only been done for a handful of jets, owing to the difficulty of obtaining decades-long time baselines for comparison on the larger (kiloparsec) scales. The Very Large Array (VLA) has now been in operation for over 40 years, and the NRAO hosts a very rich archive of observations of extragalactic jets. I will present a new effort to mine the VLA archives to measure the proper motions of jet plasma on kilo-parsec scales, where I have analyzed archival VLA observations of radio galaxy 3C78 for proper motions where we detect for the first time proper motions for multiple knots with speeds of 0.1-0.4c. Although sub-luminal, we find that the maximum kiloparsec velocity (most suggestive of underlying bulk speed) is ~ 3 times higher than the maximum VLBI speed, in keeping with observations of M87 and 3C 264 which have showed that the fastest bulk speeds in these FR I jets are reached on the > 100 parsec scale. I will briefly comment on the profile and magnetic field structure of the 3C 78 jet and will conclude with a discussion of the prospects for radio and sub-mm wavelength proper-motion studies of jets and the large catalog we intend to build using new and archival data.

2021 April 16, 1pm EDT

The Detectability of Kiloparsec Scale Dual AGNs: The Impact of Galactic Structure and Black Hole Orbital Properties

Kunyang Li (Georgia Institute of Technology)

Observational searches for dual active galactic nuclei (dAGNs) at kiloparsec separations are crucial for understanding the role of galaxy mergers in the evolution of galaxies. In addition, kpc-scale dAGNs may serve as the parent population of merging massive black hole (MBH) binaries, an important source of gravitational waves. We use a semi-analytical model to describe the orbital evolution of unequal mass MBH pairs under the influence of stellar and gaseous dynamical friction in post-merger galaxies. We quantify how the detectability of approximately 40,000 kpc-scale dAGNs depends on the structure of their host galaxies and the orbital properties of the MBH pair. Our models indicate that kpc-scale dAGNs are most likely to be detected in gas-rich post-merger galaxies with smaller stellar bulges and relatively massive, rapidly rotating gas disks. The detectability is also increased in systems with MBHs of comparable masses following low eccentricity prograde orbits. In contrast, dAGNs with retrograde, low eccentricity orbits are some of the least detectable systems among our models. The dAGNs in models in which the accreting MBHs are allowed to exhibit radiative feedback are characterized by a significantly lower overall detectability. The suppression in detectability is most pronounced in gas-rich merger remnant galaxies, where radiation feedback is more likely to arise. If so, then large, relatively gas poor galaxies may be the best candidates for detecting dAGNs.

Demystifying the Prompt Emission of Gamma Ray Bursts

Tyler Parsotan (Oregon State University)

Gamma Ray Bursts (GRBs) are the most powerful explosions in the universe, emitting more energy in a few seconds than our sun will emit in its entire lifetime. As a result, these explosions are excellent laboratories for exploring the interplay between matter and radiation in extreme environments. This interplay is integral to understanding astrophysical jets and the various compact objects that are thought to power GRBs. Recent advances in simulating the initial prompt emission of GRBs attempt to simulate this interplay between the jet properties and the resulting electromagnetic signature; this has resulted in various successes in reproducing observational aspects of GRBs. Here, we present the open source Monte Carlo Radiation Transfer (MCRaT) code. MCRaT propagates and Compton-scatters individual photons that have been injected into the collimated outflow in order to produce mock observed light curves, spectra, and polarization measurements from optical to gamma rays. These light curves and spectra allow us to compare our results to GRB observational data. We find excellent agreement between our mock observed GRBs and real GRB observations in terms of spectra and polarization measurements. Furthermore, we can understand the mock observations in terms of the jet structure and what real observations of GRBs can tell us about their jet structures. There are various improvements that can be made to MCRaT, but this code paves the way to connecting observed GRB radiation to the properties of the GRB jet in a way that was not previously possible.

New Evidence for the 3.5 keV Feature in Clusters is Inconsistent with a Dark Matter Origin

Sunayana Bhargava (University of Sussex)

There have been several reports of a detection of an unexplained excess of X-ray emission at 3.5 keV in astrophysical systems. One interpretation of this excess is the decay of sterile neutrino dark matter. The most influential study to date analysed 73 clusters observed by the XMM-Newton satellite. We explore evidence for a 3.5 keV excess in the spectra of 117 redMaPPer galaxy clusters - the largest study of its kind. In our analysis of individual spectra, we identify three systems with an excess of flux at 3.5 keV (one of which might be due to a discrete emission line). We group the remaining 114 clusters into temperature bins to search for an increase in 3.5 keV flux with temperature (a reliable proxy for halo mass) and find no evidence for a positive trend. We conclude that a 3.5 keV flux excess in our sample is not a ubiquitous feature in clusters and therefore unlikely to originate from sterile neutrino dark matter decay.

2020 November 30, Noon EST

Fermi-GBM and LIGO/Virgo Analysis of Gravitational Waves from the First and Second Observing Runs

Corinne Fletcher (Universities Space Research Association)

We present results from offline searches of Fermi Gamma-ray Burst Monitor (GBM) data for transients coincident with the gravitational-wave (GW) events reported in the LIGO/Virgo catalog GWTC-1. We search for temporal coincidences between the GW signals and GBM-triggered gamma-ray bursts (GRBs). We also use the GBM subthreshold searches to find coincident gamma-rays below the onboard triggering threshold. All searches recover GRB 170817A which occurred ~1.7 s after the binary neutron star merger GW170817. Furthermore, we review results from a new search seeking GBM counterparts to LIGO single-interferometer triggers. No significant coincidences are found. Finally, we briefly discuss ongoing efforts and planned updates for future GBM follow-up of GW events.

The relation between black-hole growth and host-galaxy compactness among star-forming galaxies

Qingling Ni (Pennsylvania State University)

Recent studies show that a universal relation between black-hole (BH) growth and stellar mass (M*) or star formation rate (SFR) is an oversimplification of BH-galaxy co-evolution, and that morphological and structural properties of host galaxies must also be considered. Particularly, a possible connection between BH growth and host-galaxy compactness was identified among star-forming (SF) galaxies. Utilizing galaxies in the COSMOS field, we perform systematic partial-correlation analyses to investigate how sample-averaged BH accretion rate (BHAR) depends on host-galaxy compactness among SF galaxies, when controlling for morphology and M* (or SFR). The projected central surface-mass density within 1 kpc, Σ1, is utilized to represent host-galaxy compactness in our study. We find that the BHAR-Σ1 relation is stronger than either the BHAR-M* or BHAR-SFR relation among SF galaxies, and this BHAR-Σ1 relation applies to both bulge-dominated galaxies and galaxies that are not dominated by bulges. This BHAR-Σ1 relation among SF galaxies suggests a link between BH growth and the central gas density of host galaxies on the kpc scale, which may further imply a common origin of the gas in the vicinity of the BH and in the central ~kpc of the galaxy. This BHAR-Σ1 relation can also be interpreted as the relation between BH growth and the central velocity dispersion of host galaxies at a given gas content, indicating the role of the host-galaxy potential well in feeding BHs.

Reverberation and Relativistic reflection in black hole transients with NICER

Jingyi Wang (Massachusetts Institute of Technology)

Black hole astrophysics can be regarded as a fundamental tool for us to learn about accretion and ejection physics in the strongest gravity regime in the Universe. With only a few black holes that can be resolved spatically, time domain techniques are very powerful to study the inner region of black holes. In particular, reverberation mapping measures light echoes off the inner accretion disc near the innermost stable circular orbit. I will present some of the highest quality reverberation mapping results of a stellar mass black hole to date, from NICER observations of the 2018 outburst of MAXI J1820+070. For the first time, we measure reverberation lags during the hard-to-soft state transition, and find that during this time, the frequency of the reverberation lag decreases, suggesting a larger emitting region, with a possibility of an expanding corona. We jointly fit the lag-energy spectra with the reverberation model RELTRANS for a range of Fourier frequencies in each epoch, and find an increase in the coronal height, consistent with the qualitative expectation. I will put these results into context, comparing to reverberation in other low mass X-ray binaries, and discuss implications for unanswered questions in black hole astrophysics, including the state transition mechanism, system geometry and coupling between the disk, corona and jet.

2020 October 30, 1:00PM EDT

Imprints of cosmic history in the extragalactic gamma-ray background

Ellis Owen (National Tsing Hua University)

Star-forming galaxies would presumably be environments rich in energetic cosmic rays due to the presence of massive stars and their remnants. Stellar remnants can supply seed particles and generate the shocks (via supernova explosions and other violent events) needed to accelerate the seeds to very high energies. These can interact to deposit energy into their environment, or to drive gamma-ray emission. In this talk, I will outline how the gamma-ray emission from these galaxies contributes to the extragalactic gamma-ray background, and discuss how the spatial signatures that would emerge in the background power spectrum could be used to probe the redshift evolution of star-forming galaxy populations. I will also outline the prospects and limitations for observing these signatures with current and future facilities.

Separating accretion and mergers in the cosmic growth of black holes with X-ray and gravitational wave observations

Fabio Pacucci (Harvard University & SAO

Black holes across a broad range of masses play a key role in the evolution of galaxies. The initial seeds of black holes formed at z ~ 30 and grew over cosmic time by gas accretion and mergers. Using observational data for quasars and theoretical models for the hierarchical assembly of dark matter halos, we study the relative importance of gas accretion and mergers for black hole growth, as a function of redshift (0 < z < 10) and black hole mass (10^3 solar masses < M < 10^10 solar masses). We find that: (i) growth by accretion is dominant in a large fraction of the parameter space, especially at M > 10^8 solar masses and z > 6; (ii) growth by mergers is dominant at M < 10^5 solar masses and z > 5.5, and at M > 10^8 solar masses and z < 2. As the growth channel has direct implications for the black hole spin (with gas accretion leading to higher spin values), we test our model against ~ 20 robust spin measurements available thus far. As expected, the spin tends to decline towards the merger-dominated regime, thereby supporting our model. The next generation of X-ray and gravitational wave observatories (e.g. Lynx, AXIS, Athena and LISA) will map out populations of black holes up to very high redshift (z ~ 20), covering the parameter space investigated here in almost its entirety. Their data will be instrumental to providing a clear picture of how black holes grew across cosmic time.

The Swift/XRT Deep Galactic Plane Survey

Nick Gorgone (The George Washington University)

The Deep Galactic Plane Survey (DGPS) is an ongoing 1.83 Ms effort to establish the transient X-ray source content of the inner Scutum and Norma arms of the Milky Way (10° <|l|< 30°, |b| < 0.5°) in the 0.3-10 keV range. Our goal is to find and classify new sources, such as magnetars, X-ray binaries, and other transients. In this talk I will introduce the survey and discuss the potential nature of some of the brightest sources we have discovered thus far based on their multi-wavelength spectral and temporal characteristics. Thus far, we have identified ~190 sources above 3 sigma in any band, with approximately 40 new and unclassified sources. Our forthcoming source catalog will provide a valuable baseline for future time-domain studies and will be significant for cross matching sources in high-energy catalogs.

2020 September 29, Noon EDT

Runaway Merger Shocks in Cluster Outskirts and their Interaction with Accretion Shocks (video)

Congyao Zhang (University of Chicago)

Mergers of galaxy clusters play an important role in cluster growth. Merger shocks arise naturally in this process, and eventually propagate away from the cluster center and encounter the accretion shocks at large cluster radii. In this talk, I will present the formation and evolution of the “runaway” merger shocks. They are considered as promising candidates for powering radio relics found in many clusters. As shocks propagate in cluster peripheries, they are moving down a steep density gradient which helps them to maintain their strength over a large distance. Observations and simulations show that, beyond R_500, gas density profiles are as steep as ∼1/r^3, suggesting a “habitable zone” of the runaway shocks. Once the runaway shock eventually overtakes the accretion shock, a long-living shock is formed and constitutes a new boundary of the cluster atmosphere. It has a very high Mach number and could travel up to a few virial radii into the intercluster medium.

Jet–accretion Coupling in Luminous Accreting Neutron Stars in Galactic Globular Clusters (video)

Teresa Panurach (Michigan State University)

Magnetic reconnection and hot spot formation in black hole accretion flows (video)

Bart Ripperda (Princeton University / Flatiron Institute)