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HEAD Virtual Seminar Series

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202006300731
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.


2020 July 31, Noon EDT


Ongoing Monitoring of the Tidal Disruption Event Swift J1644+57

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

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

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 

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 

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 

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.