new exoplanet, black-hole states, and more

At group meeting, Hattori showed us an exoplanet discovery, made with his search for single transits! Actually, the object was a known single transit, but Hattori showed that it is in fact a double transit and has a period very different from its recorded value. So this counts as a discovery, in my book. We are nearly ready to fully launch Hattori's code "in the data center"; we just need to run it on a bunch more cases to complete our functional testing.

Also at group meeting, Sanderson discussed the Gaia Challenge meeting from the previous week. There are lots of simulated Gaia-like data sets available for testing methods and ideas for Gaia data analysis. This is exciting. We also discussed generalizing her structure-finding code to make it also a clustering algorithm on the stars.

Also at group meeting, Daniela Huppenkothen (NYU) showed us time-series and spectral data from the famous black-hole source GRS 1915, which has about a dozen different "states". She suggested that there might be lots of low-hanging fruit in supervised and unsupervised classification of these different states, using both time features and spectral features. The data are so awesome, they could launch a thousand data-science masters-student capstone projects!

Tsvi Piran (Racah) gave a lively talk on the likely influence of gamma-ray bursts on life on earth and other habitable worlds. He argued that perhaps we live so far out in the outskirts of the Milky Way because the GRB rate is higher closer to the center of the Galaxy, and GRBs hurt. The ozone layer, that is.

cosmic origins

Short conversations today with Mei and Lang and Foreman-Mackey. Mei and I decided that we should do the continuum normalization that he is doing at the raw-data stage, before we interpolate the spectra onto the common wavelength grid. This should make the interpolation a bit safer, I think. Lang and I discussed DESI and he showed me some amazing images, plus image models from The Tractor. Foreman-Mackey and I discussed the relevance of his exoplanet research program to the NASA Origins program and the Great Observatories. Can anyone imagine why?

the sky in spectroscopy

At the end of the day, Malz came by the CDS space and we talked through first steps on a project to look at sky subtraction in spectroscopy. We had a great idea: If the sky residuals are caused by small line-shape changes, we can model the sky in each fiber with a linear combination of other sky fibers, including those same fibers shifted left and right by one pixel. This is like the auto-regression we do for variable stars—and they do on Wall St to model price changes in securities—but applied in the wavelength direction. It ought to permit the sky fitting to fit out convolution (or light deconvolution) as well as the brightness.

Group meeting included some very nice plots from Fadely showing that he can model the color-size distribution of sources in the SDSS data, potentially very strongly improving star–galaxy separation. We also talked about ABC (tm), or Bayesian inference when you can't write down a likelihood function, and also stellar centroiding.

At lunch, Goodman told the Data Science community about affine-invariant MCMC samplers. He did a good job advertising emcee and some new projects he is working on.

probabilistic cosmology

I spent time working through some inference problems in the future of cosmology, including how you use probabilistic redshift information, how you marginalize out beliefs about the density field and galaxy formation within that density field, and how you might get probabilistic information about the 2-pt function. I am confused about how much this all matters for different kinds of projects.

At lunch, Andy Haas (NYU) told us about searches for milli-charged particles at the LHC. He has a plan that would rule out a huge class of models.

p(z)?

I learned from Brant Robertson (Arizona), on twitter (tm) of all places, that the LSST level-2 data products will include probabilistic information about source redshifts. I spent some time working out how a "LSST customer" might use those data products. Most of the simple ideas one might have (weight by p(z) at each redshift, for example) are dead wrong and will wrong-ify your answers rather than make them more accurate. Now I am thinking: NSF proposal?

quasars! exoplanets! dark matter at small scales!

CampHogg group meeting was impressive today, with spontaneous appearances by Andreu Font-Ribera (LBL), Heather Knutson (Caltech), and Lucianne Walkowicz (Adler). All three told us something about their research. Font-Ribera showed a two-dimensional quasar—absorption cross-correlation, which in principle contains a huge amount of information about both large-scale structure and the illumination of the IGM. He seems to find that IGM illumination is simple or that the data are consistent with a direct relationship between IGM absorption and density.

Knutson showed us results from a study to see if stars hosting hot Jupiters on highly inclined (relative to the stellar rotation) orbits are different in their binary properties from those hosting hot Jupiters on co-planar orbits. The answer seems to be "no", although it does look like there is some difference between stars that host hot Jupiters and stars that don't. This all has implications for planet migration; it tends to push towards disk migration having a larger role.

We interviewed Walkowicz about the variability of the Sun (my loyal reader will recall that we loved her paper on the subject). She made a very interesting point for future study: The "plage" areas on the Sun (which are brighter than average) might be just as important as the sunspots (which are darker than average) in causing time variability. Also, the plage areas are very different from the sunspots in their emissivity properties, so they might really require a new kind of model. Time to call the applied mathematics team!

In the afternoon, Alyson Brooks (Rutgers) gave the astro seminar, on the various issues with CDM on small scales. Things sure have evolved since I was working in this area: She showed that the dynamical influence of baryonic physics (collapse, outflows, and so on) are either definitely or conceivably able to create the issues we see with galaxy density profiles at small scales, the numbers of visible satellites, the density distribution of satellites, and the sizes of disk-galaxy bulges. On the latter it still seems like there is a problem, but on the face of it, there is not really any strong reason to be unhappy with CDM. As my loyal reader knows, this makes me unhappy! How can CDM be the correct theory at all scales? All that said, Brooks herself is hopeful that precise tests of CDM at galaxy scales will reveal new physics and she is doing some of that work now. She also gave great shout-outs to Adi Zolotov.

single transits, redshift likelihoods

A high research day today, for the first time in what feels like months! In group meeting in the finally-gutted NYU CDS studio space, So Hattori told us about some single transits in Kepler and his putative ability to find them. We unleashed some project management on him and now he has a great to-do list. No success in CampHogg goes unpunished! Along the way, he re-discovered a ridiculously odd Kepler target that has three transits from at least two different kinds of planets, neither of which seems periodic. Or maybe it is one planet around a binary host, or maybe worse? That launched some email trail with some Kepler peeps.

Also at group meeting, Dun Wang showed some near-final tests of the hyper-parameter choices in his data-driven model of the Kepler pixels. It is getting down to details, but details matter. We came up with one final possible simplification for his hyper-parameter choices for him to test this week.

In the afternoon, Alex Malz came by to discuss Spring courses and we ended up working through a menu of possible thesis projects. One that I pitched is so sweet: It is just to write down, very carefully, what we would do if we had instead of a redshift catalog a set of low-precision redshift likelihood functions (with SED or spectral nuisance parameters). Could we then get the luminosity function and spatial clustering of galaxies? Of course we could, but we would have to go hierarchical. Is this practical at LSST scale? Not sure yet.

text, R, and politics

Today at lunch Michael Blanton organized a Data Science event in which Ken Benoit (LSE) told us about quanteda, his package for manipulating text in R. This package does lots of the data massaging and munging that used to be manual work, and gets the text data into "rectangular" form for data analysis. It also does lots of data analysis tasks too, but the munging was very interesting: Part of Benoit's motivation is to make text analyses reproducible from beginning to end. Benoit's example texts were amusing because he works on political speeches. He had examples from US and Irish politics. Some discussion in the room was about Python vs R; the key motivation for working in R is that it is by far the dominant language at the intersection of statistics and political science.

nuclear composition of UHECRs

Today Michael Unger (Karlsruhe) told us over lunch about ultra-high energy cosmic rays from Auger. There are many mysteries, but it does look like the composition moves to higher-Z nuclei as you go to higher energies, or at least that's my read. He told us also about a very intriguing extension to Auger which would make it possible to distinguish protons from iron in the ground detectors; if that became possible, it might be possible to do cosmic-ray imaging: It is thought that the cosmic magnetic fields are small enough that protons near the GZK cutoff should point back to their sources. So far this hasn't been possible, presumably because the iron (and other heavy elements) have charge-to-momentum ratios too large; they get heavily deflected by the magnetic fields they encounter.

Math-Astrophysics collaboration proposal

I spent a big chunk of the day today trying to write a draft of a collaboration proposal (really a letter of intent) for the Simons Foundation. That is only barely research.

exoplanet compositions

Today Angie Wolfgang (UCSC) gave a short morning seminar about hierarchical inference of exoplanet compositions (like are they ice or gas or rock?). She showed that the super-Earth (1 to 4 Earth-radius) planet radius distribution fairly simply translates into a composition distribution, if you are willing to make the (pretty justified, actually) assumption that the planets are rocky cores with a hydrogen/helium envelope. She inferred the distribution of gas fractions for these presumed rocky planets and got some reasonable numbers. Nice! There is much more to do, of course, since she cut to a very clean sample, and hasn't yet looked at the interdependence of composition, period, and host-star properties. There is a lot to do in exoplanet populations still!

training convolutional nets to find exoplanets

In group meeting today, a good discussion arose about training a supervised method to find exoplanet transits. Data-Science Masters student Elizabeth Lamm (NYU) is working with us to use a convolutional net (think: deep learning) to find exoplanet transits in the Kepler data. Our rough plan is to train this net using real Kepler lightcurves into which we have injected artificial planets. This will give "true positive" training examples, but we also need "true negative" examples. Since transits are rare, most of the lightcurves would make good negative training data; even if we used all of the non-injected lightcurves arbitrarily, we would only have a false-negative rate of a tiny fraction of a percent (like a hundredth of a percent).

That said, there were various intuitions (about training) represented in the discussion. One intuition is that even this low rate of false negatives might lead to some kinds of over-fitting. Another is that perhaps we should up-weight in the training data true negatives that are "threshold crossing events" or, in other words, places where simple software systems think there is a transit but close inspection says there isn't. We finished the discussion in disagreement, but realized that Lamm's project is pretty rich!

K2 pointing model

Imagine a strange "game": A crazy telescope designer put thousands of tiny pixelized detectors in the focal plane of an otherwise stable telescope and put it in space. Each detector has an arbitary position in the focal plane, orientation, and pixel scale, or even non-square (affine) pixels. But given the stability, the telescope's properties are set only by three Euler angles. How can you build a model of this? Ben Montet (Harvard CfA), Foreman-Mackey, and I worked on this problem today. Our approach is to construct a three-dimensional "latent-variable" space in which the telescope "lives" and then an affine transformation for each detector patch. It worked like crazy on the K2 data, which are the data from the two-wheel era of the NASA Kepler satellite. Montet is very optimistic about our abilities to improve both K2 and Kepler photometry.

single transits, new physics, K2

In my small amount of research time, I worked on the text for Hattori's paper on single transits in the Kepler data, including how we can search for them and what can be inferred from them. At lunch, Josh Ruderman (NYU) gave a nice talk on finding beyond-the-standard-model physics in the Atlas experiment at LHC. He made a nice argument at the beginning of his talk that there must be new physics for three reasons: baryogenesis, dark matter, and the hierarchy. The last is a naturalness argument, but the other two are pretty strong arguments! In the afternoon, while I ripped out furniture, Ben Montet (Harvard) and Foreman-Mackey worked on centroiding stars in the K2 data.

three talks

Three great talks happened today. Two by Jason Kalirai (STScI) on WFIRST and the connection between white dwarf stars and their progenitors. One by Foreman-Mackey on the new paper on M-dwarf planetary system abundances by Ballard & Johnson. Kalirai did a good job of justifying the science case for WFIRST; it will do a huge survey at good angular resolution and great depth. He distinguished it nicely from Euclid. It also has a Guest Observer program. On the white-dwarf stuff he showed some mind-blowing color-magnitude diagrams; it is incredible how well calibrated HST is and how well Kalirai and his team can do crowded-field photometry, both at the bright end and at the faint end. Foreman-Mackey's journal-club talk convinced us that there is a huge amount to do in exoplanetary system population inference going forward; papers like Ballard & Johnson only barely scratch the surface of what we might be doing.