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HEAD 16 Special Sessions

 • First Results from the Neutron Star Interior Composition Explorer (NICER): X-ray Astrophysics from the International Space Station (Keith Gendreau, Zaven Arzoumanian)

Scheduled for launch in June 2017, NICER will be NASA's newest high-energy astrophysics mission, providing a unique high-throughput timing spectroscopy capability in the soft X-ray band. As a successor to the highly productive Rossi X-ray Timing Explorer, NICER appeals to a large community of prospective users. The Special Session will provide the first public overview of: 1) NICER's on-orbit performance following initial checkout and calibration, including early progress toward NICER's core science objectives, 2) timely plans for public data release and proposal opportunities, and 3) NICER's early contributions (together with prospects for future advances) across a broad array of X-ray astrophysics investigations -- from active stars and interacting binaries to black holes across the mass spectrum -- that will be enabled by PI Discretionary Time awards and a Guest Observer program. NICER is designed to probe the exotic interiors of neutron stars, revealing the fundamental physics of dense matter that exists nowhere else in nature, a longstanding unsolved problem. In addition to exploring neutron star structure, NICER will study dynamic phenomena powered by accretion and strong gravity, and the extreme physics of pulsar magnetospheres, perhaps the most powerful cosmic particle accelerators known. NICER's studies of neutron stars richly complement the Fermi gamma-ray telescope's remarkable successes as a pulsar discovery instrument. With a launch in June followed by a one-month checkout period leading to the start of science operations in early July, the HEAD meeting in August is perfectly timed for the mission team to make its first, highly anticipated public announcements of NICER's actual capabilities in flight, present the results of early observations of both neutron star and other targets, and discuss planning for community access to data, high-level products, and observing time. NICER's unprecedented combination of photon time-tagging precision, energy resolution, and sensitivity in the 0.2-12 keV band represents both a novel capability for studying neutron stars and an opportunity for exploring new discovery space in time-domain astrophysics.


With the first detections of gravitational waves by ground based detectors, considerable attention is now ,being directed at examining the role that gravitational wave data is going to play in dissecting the ,processes that drive and define the evolution of strongly gravitating, high energy astrophysical systems. ,In parallel, efforts to realize a space-based gravitational wave observatory sensitive to the milliHertz ,gravitational wave band have been bolstered by both the ground-based detections and the success of the ,LISA Pathfinder mission. As part of the ongoing effort to develop synergies between gravitational wave ,observations and electromagnetic observations, we propose a special session focused on the relationship ,between LISA sources and high-energy astrophysics. This special session will bring together four invited, talks that connect current theory and simulations, future gravitational wave observations, ,and current and future electromagnetic observations.

 • The first black holes in the universe (Zoltan Haiman)

While we know that supermassive black holes as massive as a billion,solar masses exist in the first Gyr of the universe, the formation,pathways of these rare objects, and the assembly history of their,more typical, less massive counterparts, remains elusive. A The,evolution of these black holes is likely to be strongly intertwined,with the formation of the first galaxies, and with the global thermal,and ionization state of the universe, and lies at the frontier that,will be probed by next-generation instruments.,This session will feature a approximately 6 talks, aiming to cover of,a mix of theoretical and observational issues (and a mix of junior and,senior speakers). A In particular: what was the typical initial seed,black hole mass, and what role did accretion vs mergers play in the,subsequent growth of the earliest black holes? What was the impact of,the growing black holes on the early IGM? A How will the next,generation of X-ray instruments, CMB experiments, 21cm, and,gravitational wave observations help us find answers to the above?

 • The Very High Energy Universe as Viewed with VERITAS and HAWC (Reshmi Mukherjee)

Gamma-ray astronomy has benefitted from the successes of,ground-based Cherenkov telescopes such as VERITAS (Very Energetic Radiation,Imaging Telescope Array System), and the commissioning of HAWC (High Altitude Water,Cherenkov), a new generation water Cherenkov observatory in Mexico. Together these,observatories complement the Fermi Gamma-ray Space Telescope. The suite of,instruments in the gamma-ray band provide exciting opportunities to explore a wide range,of energetic processes in Galactic and extragalactic sources, as well as enable insight,into cosmology and fundamental physics. In this special session we will offer a review of,Galactic and extragalactic results from the VERITAS and HAWC observatories. The,results, when combined with multi-wavelength and multi-messenger data, provide new,perspectives on familiar astrophysical systems and offer promising future prospects as we,usher in a new era in TeV astronomy with the Cherenkov Telescope Array (CTA).


 • From Stars to Accretion Disks: Why are Coronae So Hot? (Jeremy Schnittman)

Since the dawn of X-ray astronomy, hard X-rays have been observed from accreting black holes and neutron stars in almost every spectral state. The standard explanation for this hard component is due to,scattering from energetic electrons in some form of hot, diffuse,corona surrounding the accretion flow. Yet the geometry and physical,heating mechanisms for these coronae are poorly understood, despite,decades of observations. Similarly, hard X-rays are routinely seen in the,solar corona, where strong magnetic processes heat gas to millions of,degrees. Even with high-resolution imaging, timing, and spectroscopy,,the physical processes dominating heating and particle acceleration in,the solar corona remain elusive. In situ measurements with satellite,missions deployed to the outer reaches of the solar corona are,expected to break new ground.,This session will cover the latest advances in observations,,theory, and simulations of coronae around the sun and accretion,disks. We will put a particular emphasis on understanding what,underlying physical processes the different systems have in common.


• The future of spectral-timing of compact objects (Erin Kara)

Observations of the inner accretion flows around black holes are vital for our understanding of how black holes grow and effect their environments. In recent years, there has been a breakthrough in such observations due to the discovery of X-ray reverberation mapping. This technique allows us to map the gas falling on to the black hole and measure the effects of strongly curved spacetime close to the event horizon. The power of X-ray reverberation mapping is that it couples the two main observational techniques used in studies of compact objects: spectroscopy—where physical models are fit to the distribution of photons, and timing—a more phenomenological approach characterizing variability signatures—in order to understand the causal connection between the different physical components of the energy spectrum. For this HEAD Special Session, we propose to have a discussion on the future of X-ray reverberation and other spectral-techniques. We suggest a list of speakers that will review the newest observational results, the state-of-the-art techniques, the theoretical models that are being developed. This is particularly important as we look to future observatories with unprecedented timing capabilities: most immediately with NICER, and to the late 2020s with Athena.


 • Cosmic Ray Feedback: From Supernova Remnants to Galaxy Clusters (Mateusz Ruszkowski)

Recent results suggest that cosmic rays (CRs) play a vital role in astrophysical feedback pro-,cesses acting across a very large range of physical scales a from the scales of isolated supernova,remnants, progressing to the scales of individual galaxies, and continuing to the scales compara-,ble to those of galaxy clusters. Examples of phenomena where CRs play crucial roles on these,scales are: (i) CR-driven instabilities in SN remnants that strongly amplify the initial magnetic,field much beyond compression, a phenomenon not captured in MHD simulations, but necessary to,account for the role of SN in generating magnetic turbulence in the Galaxy; (ii) CR feedback from,supernovae and young stars and efficient acceleration of galactic-scale winds; (iii) self-regulation of,AGN feedback in cool cores of galaxy clusters facilitated by CR heating and CR transport processes.,These topics remain largely unexplored but can now be studied thanks to advances in numeri-,cal techniques and improvements in modern astrophysical codes. We propose to organize a special,HEAD session to discuss the role of CR in these feedback processes. We will bring together experts,in modeling of CR acceleration, propagation, and energy and momentum feedback in an attempt,to further explore connections between CR feedback processes, and to help us build a more unified,picture of the role of CR feedback operating across this vast range in the mass spectrum of astro-,physical objects (from stars to galaxy clusters). The proposed preliminary list of speakers includes,researchers with expertise in diverse techniques relevant to this problem a analytic methods, kinetic,particle-in-cell methods, adaptive mesh refinement simulations, moving mesh simulations a each of,which is suitable for a particular physical scale of interest.

 • AGN in Dwarf Galaxies (Joseph Silk, Grindlay, Reines, Volonteri )
We now know that AGNs in dwarf galaxies are more common than previously thought. The presence of ,massive black holes in nearby, low-mass galaxiesA could contribute to our understanding of black hole,formation and resolve a considerable number of problems that ariseA in discussing the properties of dwarf ,galaxies. However, observationally and theoretically,A the topic is remarkably complex. Multi-wavelength,observations are being vigorously pursued to elucidate the presence of AGN in low mass galaxies. This,special sessionA will focus on the observational evidence for AGN in dwarfs via X-ray, optical and infrared ,surveys,A and review theoretical expectationsA for the masses of black holes in dwarf galaxies, and their ,links to both the M-sigma relation and galaxy evolution.

High-Energy and Multiwavelength Polarimetry: Current Status and New Frontiers (John Tomsick, Henric Krawczynski)
Measuring the polarization of astrophysical sources has great power for constraining emission mechanisms and source geometries. In particular, making such measurements at high energies (X-rays and gamma-rays) often probes the regions where the most extreme physics is taking place. Initial pioneering X-ray observations were made in the 1970s, and there has also been some progress on Gamma-Ray Burst measurements with INTEGRAL, IKAROS/GAP, and RHESSI and also on Cyg X-1 and the Crab with INTEGRAL. Progress is now accelerating with new capabilities in the hard X-ray and soft gamma-ray on balloons (X-Calibur, COSI, GRAPE) as well as satellites (POLAR and Astrosat). In addition, in X-rays, IXPE was recently approved and other intrument concepts are being proposed. This proposed special session would include experimental results, plans for future missions, and a discussion of the scientific advances that are expected to be achieved. The proposed session is being organized as a meeting of the Physics of the Cosmos Program Analysis Group's (PhysPAG) Gamma-ray Science Interest Group (GammaSIG) and will start with an overview of the GammaSIG activities.


 • ULX Pulsars (Fiona Harrison)

Ultra-luminous X-ray sources (ULXs) are variable, off-nuclear pointsources in nearby galaxies with ,luminosities above 10e39 erg/s. Due to their high luminosity they were assumed to be accreting black ,holes; either intermediate mass black holes (100-10000 solar masses) or stellar mass black,holes (~10-40 solar masses) accreting above the Eddington limit. However, in 2014 NuSTAR observations ,revealed pulsations from the ULX M82 X-2 (Bachetti,A et al., 2014), which unmistakably identifies the compact object as a neutron star and consequently a ,mass of no more than ~2 solar masses.,In 2016 two more neutron star ULXs were discovered: NGC 5597 P13 (Fuerst et al., 2016, Isreal et al., 2017) and NGC 5907 X-1 (IsrealA  et al., ,2016). The latter is the brightest accreting neutron star known to date, accreting at up to 500 times the ,Eddington rate, assuming isotropic emission. In this special session we will present observational ,properties of these extreme objects and discuss challenges in searching for their signatures. We will also ,discuss theoretical ideas to explain these extreme accretion rates and luminosities, including possible ,accretion geometries that might also explain the observed super-orbital periods.


• Advances in Bayesian Astrostatistics: Applications to High-Energy Astrophysics (Aneta Siemiginowska)

This proposal originates from the CHASC International Center for Astrostatistics,( which has organized many well attended sessions at both the, AAS general meetings and at HEAD meetings. Our last Special Session was held in 2013 in memory of ,Alanna Connors who was a strong and early advocate for the use of modern statistical methods in ,(high-energy) astrophysics. Our last Astrostatistics topical session, held in 2011, A focused on aTime-series ,in High Energy Astrophysicsa. A Since then there has been a massive increase in the development and ,application of new statistical methods to problems in high energy astrophysics. In particular Bayesian ,methods using Markov Chain Monte Carlo have been implemented in both XSPEC and Sherpa. Such ,Bayesian methods allow researchers to fit multi-level models that, for example, account for complexities ,and uncertainties in instrumental effects, realistic representations of physical processes, selection effects, ,and/or distributions of and relationships among parameters, all in a principled statistical manner. ,Although recent tools, such as those deploying MCMC in XSPEC and Sherpa, are ,making it much easier for astronomers to access the power of modern Bayesian methods, using them ,involves a range of statistical and computation subtleties, and a variety of more sophisticated, next , generation tools are becoming available.,The goal of our session is to review advances in statistical methods, to present applications to data, and ,to discuss current issues and future perspectives. The proposed session includes talks from both ,astronomers and statisticians and a substantial time for discussion.

 • Diagnosing Astrophysics of Collisional Plasmas – A Joint HEAD/LAD Session (Randall Smith)

This session, to be jointly sponsored by HEAD and LAD will review the physics of collisional plasmas, both ,observationally, theoretically, and in the laboratory. The 1.5 hour session will consist of three 25+5, minute talks giving overviews of observational results from collisional plasmas, the theory of collisional plasmas, and the existing and future laboratory studies necessary, to improve our understanding of these systems.