2017 HEAD: Day 1

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.