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HEAD Virtual Seminar Series

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2020: 063007310828 • 0929

Seminars are by Zoom, and all HEAD members are 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.

2020 September 29, Noon EDT

Runaway Merger Shocks in Cluster Outskirts and their Interaction with Accretion Shocks

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

Teresa Panurach (Michigan State University)

It is now established that hard-state accreting neutron stars in low-mass X-ray binaries show outflows — and sometimes jets — in the general manner of accreting black holes. However, the quantitative link between the accretion flow (traced by X-rays) and the outflow/jet (traced by radio emission) is much less well-understood for neutron stars than for black holes. Here we use the deep MAVERIC radio continuum survey of 50 Galactic globular clusters to do a systematic study of the radio and X-ray properties of all the luminous (L_X > 10^34 erg/s) persistent neutron star X-ray binaries in our survey, as well as two other transients also captured in our data. We find that these neutron star X-ray binaries show a much larger range in radio luminosity than previously observed, and some have outflows as luminous as those of black holes. These results show that neutron stars do not evince a single relation between inflow and outflow and that the accretion dynamics are more complex than for black holes.

Magnetic reconnection and hot spot formation in black hole accretion flows

Bart Ripperda (Princeton University / Flatiron Institute)

Hot spots, or plasmoids, forming due to magnetic reconnection in current sheets, are conjectured to power frequent X-ray and near-infrared flares from Sgr A*, the black hole in the center of our Galaxy. It is unclear how, where, and when current sheets form in black-hole accretion disks. We show general-relativistic resistive magnetohydrodynamics simulations to model reconnection and plasmoid formation in a range of accretion flows. Current sheets and plasmoids are ubiquitous features which form regardless of the initial magnetic field in the disk, the magnetization in the quasi-steady-state phase of accretion, and the spin of the black hole. Within 10 Schwarzschild radii from the event horizon, we observe plasmoids forming, after which they can merge, grow to macroscopic scales of the order of a few Schwarzschild radii, and are ultimately advected along the jet's sheath or into the disk. Large plasmoids are energized to relativistic temperatures via reconnection and contribute to the jet's limb-brightening. We find that only hot spots forming in magnetically arrested disks can potentially explain the energetics of Sgr A* flares. The flare period is determined by the reconnection rate, which we find to be between 0.01c and 0.03c in all cases, consistent with studies of reconnection in isolated Harris-type current sheets. We quantify magnetic dissipation and non-ideal electric fields which can efficiently inject non-thermal particles. We show that explicit resistivity allows for converged numerical solutions, such that the electromagnetic energy evolution and dissipation become independent of the grid scale for the extreme resolutions considered here.

2020 August 28, Noon EDT

Recent NuSTAR and XMM Observations of Two New Black Hole X-ray Binaries (video)

Yanjun Xu, Caltech

We present results from NuSTAR and XMM observations of two recently discovered black hole X-ray binaries, MAXI J1631-479 and MAXI J1820+070. Due to the brightness of these Galactic black hole transients, relativistic disk reflection features are clearly detected to low Eddington accretion rates during the outbursts. Multi-epoch observations reveal evolution in the strength and profile of the broad Fe K line at different accretion states. These variations encode direct information about the evolution in the accretion dynamics in the vicinity of black holes. We discuss what we have learned about disk truncation, changes in the coronal geometry, and the role of thermal disk photons in shaping the disk reflection spectrum from X-ray spectral modeling. *

A Double-Peaked Calcium-Rich Transient with Luminous X-ray Emission and Shock-Ionized Spectral Features (video)

Wynn Jacobson-Galan, Northwestern Universtiy

We present multi-wavelength observations of the first Calcium-Rich transient with a luminous X-ray detection. Our panchromatic observations of supernova (SN) 2019ehk begin 10 hours after explosion and continue for ~300 days. Additionally, we present pre-explosion HST + Spitzer + Chandra imaging of the SN explosion site, all of which constrain the parameter space of viable stellar progenitors to either (i) massive stars in the lowest mass bin (~10 Msun) in binaries that lost most of their He envelope or (ii) white dwarfs. At 1.5 days after explosion, we observed "flash-ionized" Hydrogen and Helium emission lines in the SN spectrum; this is the first confident detection of circumstellar material surrounding a Ca-rich SN progenitor. The luminous X-ray emission observed by Swift-XRT is coincident with an optical "flare" at -10 days before peak and is consistent with deep radio non-detections at >30 days post-explosion. The SN has a peak absolute magnitude of -15.1 mag and a rise-time of 13 days. We estimate total nickel and ejecta masses of ~0.03 and ~0.7 Msun, respectively. The SN has a similar spectroscopic and photometric evolution to other typical Ca-rich objects, with a rapid evolution to the optically thin regime illustrated via the presence of [Ca II] near maximum light. We will present modeling of multi-wavelength observations and discuss the preferred progenitor scenario for this object. Finally we discuss the implications of these observations on Ca-rich diversity and how this SN constrains the formation mechanism of these intriguing explosions.

The characteristics of the Galactic center excess measured with 11 years of Fermi-LAT data (video)

Mattia Di Mauro, NASA’s GSFC

The presence of an excess of gamma rays in the data measured by the Fermi Large Area Telescope from the Galactic center region is one of the most intriguing mystery in Astroparticle Physics. This excess, labeled as Galactic center excess (GCE), has been measured with different interstellar emission (IEM) models, source catalogs, data selections and techniques. Despite the claims that appeared in several papers in the last decade, the origin of the GCE is still a mystery. The main difficulty in solving this puzzle is that the uncertainties on the modeling of such a complicated region prevent to measure precisely the characteristics of the GCE. I will present precise measurements for the characteristics of the GCE: the energy spectrum, spatial morphology, position and sphericity. We use 11 years of Fermi-LAT data, state of the art IEM models and the newest 4FGL catalog of sources and new tools implemented recently to minimize the uncertainties due to the modeling of such a complicated region such as the Galactic center.

2020 July 31, Noon EDT

Ongoing Monitoring of the Tidal Disruption Event Swift J1644+57 (video)

Yvette Cendes, Center for Astrophysics | Harvard & Smithsonian

Swift J164449.3+573451 (Sw J1644+57) is a tidal disruption event (TDE) where a star became unbound after getting too close to a supermassive black hole and got torn apart by tidal forces. Sw J1644+57 was the first TDE discovered in 2011, and to date is the only TDE where the launch and subsequent turnoff of a relativistic jet has been observed in detail. In this talk, I will give an overview of almost a decade of Sw 1644+57 observations, from its initial discovery by Swift to its transition to a sub-relativistic phase. I will also provide an update of the TDE from recent Chandra and Very Large Array observations in the X-ray and radio as the shockwave continues to expand and interact with the black hole's circumnuclear environment. Finally, I will explain how Sw J1644+57 fits into the broader picture of TDE studies, and how it will continue to provide a benchmark for these transient phenomena for years to come.

Radio Shapiro Delay-Enabled Measurements of Two Millisecond Pulsar Masses (video)

Thankful Cromartie, University of Virginia

In this talk, we will present the results of two radio Shapiro delay campaigns to measure millisecond pulsar (MSP) masses, and will discuss the complementary nature of radio and X-ray observations of these sources. First, we will briefly discuss our measurement of the most massive neutron star observed to date. J0740+6620 is a ~2.14 ± 0.09 (1-sigma confidence interval) MSP for which we obtained orbital-phase-specific observations with the Green Bank Telescope in order to constrain its mass using the relativistic Shapiro delay. We will then present the results of an additional Shapiro delay-powered endeavor, this time in order to constrain the mass of the bright Gamma-ray MSP J1231—1411. This source is of particular interest to the Neutron Star Interior Composition Explorer (NICER) mission. Forthcoming modeling of the source's X-ray lightcurve promises to constrain its mass-to-radius ratio, which could further our understanding of the equation of state. An independent measurement of its mass via the radio Shapiro delay would improve the NICER team's modeling of the MSP, and in turn, the project's potential scientific payoff. We conducted a multi-wavelength analysis of timing data, including a new 22-hour campaign over orbital conjunction using the Green Bank Telescope. Both traditional chi-square minimization fitting and Markov Chain Monte Carlo (MCMC)-based techniques indicate that this source is a low-mass MSP in a highly inclined binary orbit with a low-mass white dwarf. The MCMC trials, which prove the constraining power of our measurement of the white dwarf mass and orbital inclination, are informed by priors based on white dwarf evolutionary models. We also conduct a single-photon MCMC fit to 12 years of Fermi-LAT Gamma-ray data, though the resulting constraints on MSP mass are not as stringent as the (provisional) constraints from radio data.

A Search for Detectable LISA Ultracompact Binaries (video)

Kevin Burdge, California Institute of Technology

We will give an update of ongoing work using the Zwicky Transient Facility (ZTF) to identify LISA detectable ultracompact binaries by searching for optical periodicty. We will discuss properties of fifteen systems discovered thus far, which include detached double white dwarf systems and AM CVns.

2020 June 30 (Inaugural talks)

A bright millisecond-duration radio burst from a Galactic magnetar (video)

Paul Scholz, University of Toronto 

Fast radio bursts (FRBs) are bright millisecond-duration bursts of radio waves from cosmological distances whose nature is an ongoing mystery in astrophysics. A leading model for FRBs is that they are extragalactic magnetars, young neutron stars whose emission is powered by their extremely strong magnetic fields. However, a challenge to these models has been that FRBs must have radio luminosities many orders of magnitude larger than those seen from known Galactic magnetars. On 2020 April 28, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project discovered a bright radio burst from Galactic magnetar SGR 1935+2154 during a known state of X-ray outburst. The radio burst energy of the detected burst is three orders of magnitude higher than any radio emission previously seen from a Galactic magnetar and may overlap with the faintest known extragalactic FRBs. This event thus bridges a large fraction of the radio energy gap between the population of Galactic magnetars and FRBs, strongly supporting that magnetars are the origin of at least some FRBs. In this talk I will present the CHIME/FRB discovery of the radio burst, put it in context with high-energy telescope observations of the source, and discuss the implications of this landmark result.

AGN-driven outflows and formation of dusty cold gas filaments in cool-core clusters (video)

Yu Qiu, Kavli Institute for Astronomy and Astrophysics

Galaxy clusters are the most massive collapsed structures in the Universe, with potential wells filled with hot, X-ray-emitting intracluster medium (ICM). Observations, however, show that a substantial number of clusters (the so-called cool-core clusters) also contain large amounts of cold gas in their centres, some of which is in the form of spatially extended filaments spanning scales of tens of kiloparsecs. These findings have raised questions about the origin of the cold gas, as well as its relationship with the central active galactic nucleus (AGN), whose feedback has been established as a ubiquitous feature in such galaxy clusters. Here, we report a radiation-hydrodynamic simulation of AGN feedback in a galaxy cluster, in which cold filaments form from the warm, AGN-driven outflows with temperatures between 10^4 and 10^7‚ÄâK as they rise in the cluster core. Our analysis reveals a new mechanism that, through the combination of radiative cooling and ram pressure, naturally promotes outflows whose cooling times are shorter than their rising times, giving birth to spatially extended cold gas filaments. Our results strongly suggest that the formation of cold gas and AGN feedback in galaxy clusters are inextricably linked and shed light on how AGN feedback couples to the ICM.

New Ultraluminous X-ray Sources Hosted by M87's Globular Clusters (video)

Kristen Dage, Michigan State University

Thanks to the nearly 15,000 globular clusters hosted by M87, we were able to identify 7 ultraluminous X-ray sources (ULXs) with globular cluster (GC) counterparts. This nearly doubles the sample size of these unique X-ray binaries, bringing the total known sample of GC ULXs to 17. ULXs in the old GC environment represent a new population of ULXs, and ones likely to be black holes. Two of these sources show variability in their X-ray luminosity of an order of magnitude over many years, and one of these sources shows intra-observational variability on the scale of hours. While the majority of globular cluster ULXs are predominately best fit by single component models, one of the sources studied in this paper is the second GC ULX to be best fit by a two component model.  We compare this new sample of GC ULXs to the previously studied sample, and compare the X-ray and optical properties counterparts across the samples.