Editor’s note: This week we’re in Sun Valley, Idaho at the 16th meeting of the AAS High Energy Astrophysics Division (HEAD). Follow along to catch some of the latest news from the field of high energy astro!

Session: Black Holes Across the Mass Spectrum

Lia Corrales (University of Wisconsin-Madison) opened the first session on Wednesday by discussing Sgr A*, the supermassive black hole at the center of our own galaxy. Compared to the supermassive black holes we discussed on Tuesday, Sgr A* is extremely dim — but it does actively accrete matter, and its flux is therefore variable, exhibiting occasional flares. The problem? Studying this variability is tricky, because our sightline to the galactic center is subject to dust scattering, which can create apparent variability that’s not due Sgr A*. Corrales tackled this problem by making measurements of X-ray transients in the galactic center to map the dust that lies along the sightline to the region. She and her collaborators found that dust scattering within 15 arcseconds of the galactic center accounts for a variation of 6–12% the flux of the source, with variability on timescales of hours — which means this is something we definitely have to account for when studying Sgr A*.

Sjoert Van Velzen (Johns Hopkins University) and Stephen Cenko (NASA GSFC) both discussed aspects of tidal disruption events (TDEs). Van Velzen addressed the question of how to tell whether the events we’re detecting are really TDEs, or if they’re imposters — accretion disk instabilities, a new kind of supernova, or collisions of stars on bound orbits around a black hole. Van Velzen pointed out that at high black-hole mass (>10⁸ solar masses), stars will be swallowed whole instead of torn apart (remember this if you plan to fall into a black hole: it’s better to choose a high-mass one!), so we expect to see a turnover in the distribution of TDEs at high black-hole masses. Though we have only 17 observations of optical/UV TDEs, Van Velzen showed that we do, indeed, see signs of this turnover — indicating that these events are TDEs rather than impostors.

Cenko explained the sleuthing we can do with the ultraviolet spectrum from TDEs: we can infer from the emission lines that TDE disks are much more radially compact and dense than typically seen in quasars. Consistent with what we learned in Tuesday’s talks, there’s evidence in the ultraviolet spectra for low-velocity, outflowing material. And we may even be able to use abundances measured in the UV spectra to learn about the type of star that was disrupted in the event.

Giorgio Matt (University Roma Tre) moved the discussion to smaller black holes, providing an overview of what we hope to learn about microquasars with the Imaging X-ray Polarimetry Explorer (IXPE), a new mission that will launch in early 2021. IXPE will simultaneously provide imaging, spectroscopy, timing, and polarimetry measurements of sources — and the polarimetry is what will make this spacecraft unique. Matt walked us through three things that X-ray polarimetry may help us to learn about microquasars, stellar-mass black holes that are actively accreting mass from a companion star, exhibiting accretion disks and jets:

1. The geometry of the corona
   Microquasars are expected to have coronae just like AGN, and we may be able to constrain their geometry based on the amount of polarization we detect from them.
2. The role of the jet
   If jets are present and play an important role, then we expect to see much higher polarization levels.
3. The spin of the black hole
   Measuring the polarization angle will provide a new way of identifying the black hole’s spin.

Session: Synergies with the Millihertz Gravitational Wave Universe

In this session, Jillian Bellovary (CUNY – Queensborough Community College) began by expanding on a topic first introduced on Monday: growing supermassive black holes in the early universe. Bellovary models this process using cosmological simulations, and she focuses on the direct-collapse method: low-metallicity, low-angular-momentum, massive clouds of gas collapse to form black holes, which then merge to grow. Bellovary and collaborators find that the massive black holes in low-mass, dwarf galaxies are often not directly in the center of the galaxy — they wander around, and ~50% end up off-nuclear. These have very low accretion rates (since they aren’t where the gas is); this will make them difficult to find, but observing them provides us with information about the original seed mass, since less than 10% of their mass is accreted gas. The mergers between massive black holes in low-mass galaxy environments will rarely be 1:1 mass ratios; it is much more common that the mass ratios are 1:10 or lower — which will influence the gravitational-wave signature we can expect to see from these. The upcoming LISA gravitational-wave mission will be critical for detecting these mergers from our early universe.

The next talk was given by Thomas Maccarone (Texas Tech University) on the topic of ultracompact binaries: binaries consisting of two compact objects, including white dwarfs, neutron stars, or black holes. Our current knowledge of double compact objects is very limited, as we have very few detections of these systems thus far. LISA will provide a look at some of these ultracompact binaries — and LISA’s frequency band is lower than Advanced LIGO’s, meaning that the binaries can be detected at earlier evolutionary stages, when they are evolving slowly enough for electromagnetic follow-up. LISA’s observations of these systems will help us to do astronomy with gravitational waves, including exploring binary evolution, common envelopes, kicks, etc., and probing mass distributions in globular clusters and galaxy clusters.

Last up, Tamara Bogdanovic (Georgia Institute of Technology) walked us through the merger of two galaxies and the supermassive black holes at their centers — mergers that LISA will be able to detect when it launches. Galaxy mergers consist of four stages:

• Stage 1: Galactic merger (separation: 100,000–1,000 pc, timescale: Gyrs)
• Stage 2: Interactions with gas and stars (separation: 1,000–10 pc, timescale: Myr–Gyr)
• Stage 3: Gravitationally bound binary (separation: 10–0.01 pc, timescale: Myrs–Gyrs)
• Stage 4: Gravitational-wave phase and coalescence (separation: <0.01 pc, timescale: short!)

How do we explore the later-stage mergers? Bogdanovic reviewed the ways that we can observe sub-parsec supermassive black-hole binaries. Direct imaging is possible, though difficult; we’ve detected one candidate using this method. Searching for quasiperiodic variability in photometry is another option, and ~150 candidates have been found in this way. These systems also have a spectroscopic signature, and another ~dozen candidates have been found by searching for this. Lastly, future detections and non-detections of gravitational-wave emission from final merger stages may result in the discovery of additional systems and provide constraints on those detected by electromagnetic means.

Dissertation Prize Talk: Stellar Death by Black Hole: How Tidal Disruption Events Unveil the High Energy Universe

This year’s HEAD Dissertation Prize winner is Eric Coughlin, who did his PhD at University of Colorado Boulder and is now an Einstein Fellow at UC Berkeley. Coughlin spoke on his theoretical work studying tidal disruption events (TDEs). He noted that in TDEs, immediately after a star is torn apart and begins to accrete onto the supermassive black hole, an initial intense phase of hyperaccretion occurs. Can accretion disks even hold themselves together under this extreme release of energy?

Coughlin and his PhD advisor, Mitch Begelman, came up with a model for how these disks survive: the disks puff up into giant spherical shapes, and then any excess accretion energy is funneled from the disk into bipolar jets. They termed the puffed-up disks “zero-Bernoulli accretion flows” — ZEBRAs for short. Coughlin showed how various simulations have backed up this model, demonstrating the formation of these puffed up, spherical disks as material falls back on a supermassive black hole after the tidal disruption of a star.

Coughlin concluded his talk by presenting simulations from his more recent work, in which he explores what TDEs look like when the star is disrupted not by an isolated black hole, but by a supermassive black hole binary. Initially, the star only experiences the effects of the black hole that is disrupting it, but within short order the tidal stream of material encounters the second black hole and forms a spectacular mess of debris in beautiful patterns. This can lead to re-brightenings in the observed light curve, creating a distinctive signature that should allow us to differentiate these events from disruption events from isolated black holes. You can check out his stunning simulations yourself in the video below (you may need to give it a minute to load, but it’s worth it to watch through the end), and you can visit his website for more movies of his work.

Session: Missions & Instruments

Wednesday afternoon’s first session provided useful introductions to a number of missions, instruments, and data analysis tools for high-energy astrophysics. These included:

• Chandra Interactive Analysis of Observations (CIAO)
  Antonella Fruscione (Smithsonian Astrophysical Observatory) discussed this modern data-analysis system for examining images produced by the Chandra X-ray Observatory.
• 360° movies of X-ray data in the galactic center
  Christopher Russell (NASA GSFC) shared his unusual 360° movies of our galactic center’s inner parsec, created from hydrodynamic simulations that model X-ray emission from hot stars in the center of the galaxy. You can check it out yourself by visiting this link on your computer, or by searching for “Christopher Russell astronomy” in the youtube app on your phone (recommended for the full 360° experience!).
• Compton Spectrometer and Imager (COSI)
  Clio Sleator (SSL, UC Berkeley) introduced this balloon-borne soft gamma-ray detector, which floated for 46 days in 2016. Data from this run included observations of the Crab pulsar and the gamma-ray burst GRB 160530A.
• 

  Artist’s illustration of LISA Pathfinder. [ESA/C.Carreau]

  LISA Pathfinder
  James Thorpe (NASA GSFC) provided us with an overview of the pathfinding mission for the Laser Interferometer Space Antenna (LISA). LISA Pathfinder was intended to test some of the technologies required for LISA, which will rely on incredibly high-precision engineering. Initial data from this pathfinder mission have shown that it’s exceeded the requirements for LISA, which is extremely promising for the future mission!
• Arcus
  Randall Smith (Smithsonian Astrophysical Observatory) gave us a cheerful overview of Arcus, a free-flying X-ray satellite that was recently selected by NASA for a concept study as a Medium-Class Explorer mission. If ARCUS makes it to the final phase of the proposal process and launches (as early as 2022), it will significantly improve on views from current missions like Chandra, providing us with new information on the formation and evolution of clusters of galaxies, black holes, and stars.
• All-sky Medium Energy Gamma-ray Observatory (AMEGO)
  Sara Buson (NASA GSFC) presented on the mission AMEGO, soon to be proposed to the 2020 Decadal Review, which would provide an all-sky survey of emission in the MeV energy band. AMEGO will provide at least a 20x improvement on sensitivity relative to its predecessor, COMPTEL, allowing us to better explore sources like MeV blazars. We hope to use AMEGO to shed light on supermassive black-hole growth, the accretion–jet connection, the MeV background, and much more.

Session: SNR/GRB/Gravitational Waves

This session was termed by the first speaker, Jeremy Schnittman (NASA GSFC), as “the miscellaneous session.” The first two talks focused on intriguing aspects of gravitational-wave astronomy. Schnittman presented his work modeling the complex radiation physics in the time-dependent spacetimes of a binary compact-object system. What would we expect to see when two black holes accreting gas are locked in a close binary? Schnittman modeled this with a radiation transport calculation of the gas accretion onto the merging binary black holes, and then used ray-tracing of photons to determine what a distant observer would see.

Cecilia Chirenti (UFABC) discussed the gravitational waves that are emitted from highly eccentric neutron-star binaries — not from the binary as a whole, but from the oscillation modes that are induced in the individual neutron stars as a result of their close passages. She demonstrated that the proposed Einstein Telescope should be able to detect up to tens of these events per year, and we may be able to use these detections to help constrain the neutron-star equation of state.

The next two speakers discussed various aspects of modeling the aftermath of supernovae. Tea Temim (Space Telescope Science Institute) presented her work simulating the interaction of a pulsar wind nebulae (which is generated by the pulsar embedded deep within a supernova remnant) with the supernova reverse shock. By matching her simulations with observations, she hopes to obtain information about the ambient medium, the supernova ejecta, the pulsar properties, and the energetic particle population injected into the interstellar medium.

Brian Williams (Space Telescope Science Institute) then discussed his three-dimensional modeling of the ejecta from Tycho’s supernova remnant, which I’ve written about in a previous AAS Nova post.

Closing out the session, Colleen Wilson-Hodge (NASA MSFC) gave an overview of the time-domain astronomy done with the Fermi Gamma-ray Burst Monitor (GBM). GBM’s large field of view — and the fact that it scans the sky once every ~90 minutes — has allowed it to detect a number of gamma-ray bursts, as well as to regularly monitor pulsars and galactic transients. In the current era of multi-messenger astronomy, GBM has also been used to follow up on gravitational-wave triggers from LIGO and neutrino detections from IceCube. We can only hope that it will prove successful in similar follow-up campaigns in the future!

Editor’s note: This week we’re in Sun Valley, Idaho at the 16th meeting of the AAS High Energy Astrophysics Division (HEAD). Follow along to catch some of the latest news from the field of high energy astro!

Session: AGN 1

Ehud Bahar (Technion) opened the meeting’s first session on active galactic nuclei (AGN) by discussing eclipses of a different kind than the one we observed on Monday. Light from AGN is often obstructed on its path to us by warm, outflowing, intervening material that absorbs some of the AGN’s light. Bahar explained the difference between what he termed “absorbers” and “obscurers”: absorbers are slow and steady outflows from the AGN that change very little over long timescales. These provide us with the opportunity to probe their detailed physics. Obscurers, on the other hand, are fast-moving and transient outflows, briefly causing dramatic drops in the X-ray flux of the AGN.

Two speakers in the session discussed the idea of particularly fast outflows from AGN: Michael Nowak (MIT Kavli Institute) presented data on ultrafast outflows moving at 5–20% of the speed of light from the AGN PG 1211+143 (that’s 15,000–60,000 km/s, as compared to more typical outflow speeds of 100–1,000 km/s), and Erin Kara (University of Maryland) discussed what we can learn from ultrafast outflows from tidal disruption events. Kara’s talk demonstrated how we can use our observations of a well-studied tidal disruption event, ASASSN-14li, to learn about how an accretion disk around a black hole can transition from a super-Eddington (especially high) accretion phase that launches winds to a sub-Eddington (lower) accretion phase in which the wind is shut off.

Andrew Fabian (University of Cambridge) wrapped up the session by providing an overview of what we know about AGN coronae — the incredibly luminous, compact regions that lie directly above the accretion disks of supermassive black holes. Coronae are the source of the majority of the hard X-ray emission from AGN, and we have used observations of this emission to constrain the size of AGN coronae to a mere 10 gravitational radii. We’ve learned that coronae are extremely hot, at 30–300 keV, and are highly magnetized and dynamic, likely containing outflowing plasma.

Session: The Very High Energy Universe as Viewed with VERITAS and HAWC

The session on very high energy observations opened with a talk by Brenda Dingus (LANL). Dingus introduced us to the High Altitude Water Cherenkov (HAWC) gamma-ray observatory, a new observatory located in Mexico that maps the northern sky in high-energy gamma rays. HAWC has a wide field of view, observing roughly 2/3 of the sky each day with long integration times. This means that the observatory is sensitive to the highest energy gamma rays. HAWC has recently released its very first catalog, 2HWC, and this is only the beginning — there is much more science expected from this observatory in the future!

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is another high-energy observatory, located in southern Arizona; Philip Kaaret (University of Iowa) provided us with an overview of this set of telescopes. VERITAS has a narrower field of view than HAWC, but its sensitivity and angular resolution are higher, allowing it to probe sources at a deeper level. It’s therefore often used for follow-up observations of known targets.

So what sources are high-energy observatories like VERITAS and HAWC observing? They hunt for photons from astrophysical sources like supernova remnants, pulsar wind nebulae, active galactic nuclei, gamma-ray bursts, and dark matter annihilation. Oleg Kargaltsev (George Washington University), Sara Buson (NASA GSFC), and Matthew Baring (Rice University) each explained some of the insights we’ve obtained about these objects from observatories like HAWC, VERITAS, Fermi, and MAGIC in conjunction with observatories exploring other wavelengths.

Mid-Career Prize Talk: X-ray Winds from Black Holes

Tuesday afternoon kicked off with the HEAD Mid-Career Prize Talk, given this year by Jon Miller (University of Michigan). Miller spoke in further depth about a topic introduced earlier in the day: winds emitted from black hole disks. He argued that these winds are worth studying because they provide information about how mass is accreted onto black holes, and therefore how the black holes grow and their spins evolve.

The dense and ionized winds from black-hole disks can potentially carry away more mass than is accreted — and this appears to hold true across the mass scale, from X-ray binaries containing stellar-mass black holes to Seyfert galaxies containing supermassive black holes. Miller discussed the different mechanisms that may launch these winds, and how observations indicate that magnetic driving is important, although other forces may also be at work.

Miller argued that many tests of disk physics are now within reach of data and simulations, such as measurements of disk magnetic fields. He also showed how extreme settings such as tidal disruption events can provide a unique and interesting regime in which to explore disks and winds, as the mass accretion rate in these events changes drastically on observable timescales.

As a final point, Miller discussed how our understanding of black hole disk winds will change with upcoming observatories. Missions like Xarm, ARCUS, ATHENA, Lynx, etc. will be transformative; ATHENA, for instance, will be able to produce observations outstripping the sensitivity and resolution of any observations obtained so far with current instrumentation, in “less than the time it took you to have lunch today,” Miller explained.

Session: ISM & Galaxies

Xian Hou (Yunnan Observatories) opened the session on the interstellar medium (ISM) and galaxies by discussing our view of M31 (the Andromeda galaxy) with the Fermi Large Area Telescope. M31 is the only other large spiral local galaxy — and it’s nearby, providing an excellent opportunity for resolved analysis of high-energy emission from a large, star-forming, spiral galaxy similar to the Milky Way. The >1 GeV emission tracked by Fermi LAT was found to be concentrated only in the inner region of the galaxy; it is not correlated with interstellar gas or star-formation sites. What could be this emission’s source, then? Hou suggests that possibilities include a population of millisecond pulsars in the galactic center, or annihilation/decay of dark matter.

Later in the session, Ann Hornschemeier (NASA GSFC) provided a complementary discussion of observations of M31 — this time in the form of NuSTAR’s deep survey of of our nearest galactic neighbor. Hornschemeier reminded us that before NuSTAR, we were unable to spatially resolve hard X-ray sources (energies over 10 keV) in other galaxies. Now, with NuSTAR, we can resolve point sources — and their hard X-ray color can help us to identify whether they are black hole X-ray binaries, neutron-star X-ray binaries, pulsars, etc. A number of neutron stars were identified in globular clusters in M31, as well as a particularly high energy source that is likely an intermediate-mass X-ray pulsar.

The work done by Francesca Fornasini (Harvard-Smithsonian CfA) and collaborators explores how low-luminosity AGN activity and star formation in its host galaxy are connected. Is there a correlation between these two types of activity? If there’s a positive correlation, we can infer that AGN feedback suppresses star formation; if there is a negative correlation, both types of activity may be fueled by a common mechanism. On the other hand, there may be no correlation at all! Because AGN are variable, and because the relation between AGN activity and star formation rate can vary with other host galaxy properties like stellar mass and redshift, we need a very large sample that covers the whole phase space to test for correlation. Fornasini and collaborators achieve this by building X-ray stacks from data from 123,000 galaxies in the Chandra COSMOS Legacy Survey. Their work is still underway, but thus far it has revealed no correlation between the black-hole accretion rate and the star formation rate of the host galaxies.

Editor’s note: This week we’re in Sun Valley, Idaho at the 16th meeting of the AAS High Energy Astrophysics Division (HEAD). Follow along to catch some of the latest news from the field of high energy astro!

A Brief Field Report from the Path of Totality

As you might have guessed, there weren’t any sessions Monday morning at the HEAD meeting. We 400 astronomers had instead distributed, with some driving to trailheads to watch the eclipse from the end of a wilderness hike, some gathered at the top of the nearby Bald Mountain in the hope of seeing the shadow of the Moon race across the valley, and many collected out on the large lawn behind the resort.

I joined the lawn crowd and enjoyed watching both amateur and professional astronomers alike share their sunspotter and telescope views throughout the morning with the gathering public setting up on the lawn. The crowd was quick to notice the point of first contact, and eclipse glasses came out all around. Over the span of the next ~75 minutes, we all excitedly watched as the Moon crept across the Sun’s face, tree shadows turned into hundreds of tiny crescents, the air started to cool, and the ambient light grew dimmer.

Totality was breathtaking. I’ll be honest: this was my first total solar eclipse, and I really didn’t expect it to live up to the hype. I was wrong. Bailey’s beads and the diamond ring were visible just before totality as promised, the corona lit up the sky in its stunning, sweeping shapes, and pink edges of the Sun’s chromosphere peeked around the side of the Moon, vivid in their color.

If you were in the path of the eclipse — either partial or total — I hope that you had a spectacular experience as well! If you missed it (or if you didn’t!) and you live in the States, now’s a great time to start making plans for 2024. And if you can’t wait that long, you can look to Argentina and Chile in 2019. I’ll see you there — this may have been my first experience in totality, but I can promise you it won’t be my last.

Session: Solar/Stellar Compact I

Scott Wolk (SAO) gave the opening talk of the first official session of the HEAD meeting, discussing what impacts we can observe of exoplanets on their stellar hosts. He demonstrated how tidal interactions between hot Jupiters and their stellar hosts can spin up the stars, and magnetic interactions between the planets and stars can induce active spots on the stars’ surfaces. This in turn can generate stellar flares, which can be energetic enough to strip the atmospheres from the planets.

Renee Ludlam (University of Michigan) spoke about what we can learn from the spectra observed from neutron-star accretion disks. By studying the reflection of X-rays off the inner edge of the accretion disk, we can infer the the location of that inner edge. Ludlam and collaborators used NuSTAR to observe neutron star low-mass X-ray binary systems (binary systems containing a neutron star accreting mass from a donor star) and measure the location of the inner disk edge for each system. They then used these measurements to determine how the inner disk radius changes with changing mass accretion rate, and to learn about the magnetic fields of the neutron stars.

Marianne Heida (Caltech) discussed the observations she and her collaborators made of the low-mass X-ray binary GX 339–4 (thought to be a black hole accreting from a donor star). The team used absorption lines from the donor star — detected in the near-infrared spectrum because the black hole’s accretion disk dominates the spectrum at optical wavelengths — to narrow down the properties of the star and the black hole. They found that the black hole is remarkably lightweight, at only 2.3–9.5 solar masses; we’ll be able to constrain this better when we have better measurements of the system’s inclination.

Rounding out the session, Jimmy Irwin (University of Alabama) outlined his observations of two X-ray flares from nearby galaxies. These flares are thought to have been emitted from within globular clusters, and they constitute the most energetic events ever observed from a globular cluster! In spite of their violence, X-ray flares result in a tiny number of photons arriving at our detectors. In one of Irwin’s two sources, the flare consisted of a whopping 10 photons measured in 51 seconds — a singularly unimpressive number until you realize that the normal output from this source is only 83 photons per 95,000 seconds! What kind of source can increase its X-ray luminosity by a factor of >100 on timescales of < 1 minute without blowing itself apart? Irwin suggests that these objects may be elusive intermediate-mass black holes with masses of hundreds of solar masses or more — or they may be smaller black holes or neutron stars that emit at higher luminosities than thought possible for short times, via some mechanism we don’t yet understand.

Session: AGN in Dwarf Galaxies

We care about active galactic nuclei (AGN) in dwarf galaxies because they provide clues that may help us answer a long-standing question: how were the first black holes formed in the early universe? We know that black holes of billions of solar masses and more were able to form within the first billion years of the universe, which means they must have grown from some type of seed. But were they seeded from the direct collapse of halos into black holes (seeds of tens of thousands to millions of solar masses), or from the deaths of massive, Pop III stars (seeds of 100s of solar masses)?

Dwarf galaxies in the local universe provide us with local analogs of the first galaxies, so observing the supermassive black holes that reside at their centers can provide clues as to how the first black holes formed. It’s for this reason that we want to search for AGN in local dwarf galaxies.

Among the presenters in this session, Amy Reines (Montana State University) discussed how multi-wavelength searches are being executed to find massive black holes in dwarf galaxies. She and her collaborators first analyzed 25,000 optical spectra of dwarf galaxies, finding over 100 galaxies that may host massive black holes. They also used radio and X-ray observations to search for additional cases: weakly accreting black holes that don’t turn up in optical surveys.

Vivienne Baldassare (Einstein Fellow at Yale University) talked about how we characterize these possible AGN, and how we can eliminate some candidates that are actually Type II supernovae masquerading as AGN. She also discussed one candidate in particular, RGG 118, which happens to be the smallest black hole ever found in the nucleus of a galaxy, weighing in at a mere 50,000 solar masses.

Brendan Miller (College of St. Scholastica) presented on how the local supermassive black hole occupation fraction — i.e., the fraction of local galaxies that contain supermassive black holes at their centers — can provide insight into how the first supermassive black holes formed. Dwarf galaxies are again key: if we can better pin down the occupation fraction for low-mass galaxies, we will be able to differentiate between the direct-collapse and death-of-a-massive-star models for early black-hole formation. Miller argues that our current sample of black holes in dwarf galaxies isn’t large enough to make this distinction, but upcoming survey results may push us over that limit.

Greetings from the 16th meeting of the High Energy Astrophysics Division (HEAD) of the American Astronomical Society in Sun Valley, Idaho! This week, I will be writing updates on just a few of the events at the meeting and posting each morning. The usual posting schedule for AAS Nova will resume next week.

Public Talk: Revealing the Hidden High-Energy Sun

The HEAD meeting unofficially kicked off this afternoon with a public talk given by Rachel Osten of the Space Telescope Science Institute. Osten’s lecture provided the public with an explanation of why 400 high-energy astronomers have invaded their small ski-resort town: because Sun Valley lies in the path of totality for August 21st’s solar eclipse, and eclipses mean exciting opportunities for high-energy science!

Osten pointed out that astronomy has a long, hallowed history of demonstrating that we aren’t that special. We’ve learned that the Sun doesn’t revolve around Earth, that we’re not located in a special center of the universe, and the matter we’re made up with isn’t even the dominant type of matter in the universe! But eclipses — this is a case where we are special. The fact that the Moon is 400 times smaller than the Sun, but also 400 times closer — such that the two bodies have the exact same angular size — is extremely lucky. No other planet in our solar system experiences eclipses like what we get to witness on Earth.

So what science can be done when the Sun’s disk is blocked by the Moon? This is an excellent time to observe the solar corona, which is normally too faint to be seen when the Sun’s disk isn’t blocked. The corona — the Sun’s outer atmosphere — is 1–3 million Kelvin. This incredibly hot, magnetized gas can take a variety of shapes depending on our timing within the 11-year solar cycle, and it can change rapidly based on the localized solar activity.

Osten herself studies the hot coronae of other stars besides our Sun. The high levels of activity in young stars in Orion, for instance — complete with X-ray flares with temperatures of 50–100 million Kelvin — can reveal information about what the Sun was like in its earlier years. And the behavior of the coronae in stars that host planets is important to know to determine the planets’ habitability: if stars emit too many X-ray flares, for instance, they can strip their planets’ atmospheres.

The lecture concluded with an overview of what we can expect when we view the solar eclipse Monday morning. A few things to look for include: a drop in temperature of up to ~10°F as totality approaches, changes in animal behavior, changes in wind speed and direction, crisper shadows as totality nears, and a diamond ring and Bailey’s beads just before totality.

As a final word, Osten urged us all to enjoy the eclipse. During the question session, an audience member asked what data she’ll be taking as a professional astronomer during the event. Her paraphrased response: “Tomorrow I won’t be a professional astronomer. I’ll be an amateur astronomer with all of you, being awed by the experience.” I look forward to the same!