mapping the image plane of a spectrograph

I had a phone conversation about wavelength-calibrating the multi-object APOGEE instrument with Karlo de Leon (NYU) today. He has arc images from each night, and line lists for the arc lamps. But before even using the arc lamps, I recommended that he try to find a model for the 2D images that is an outer product of 1D functions: One is the intensity as a function of wavelength from the arc lamp, and the other is the intensity as a function of slit position from the fibers on the slithead.

The thing we realized in the call is that the coordinate system is right when the image is well described as the outer product of these two functions, warped according to that coordinate system! Okay that's nice, now what will the residuals look like? One issue is that there are cosmic rays, hot pixels, and so on. Another issue is that there will be some vignetting that violates the strict outer-product model. We'll address these issues once we get close.

Dr Marco Stein Muzio

Today Marco Stein Muzio (NYU) defended his PhD dissertation on multi-messenger cosmic-ray astrophysics (and cosmic-ray physics). He gave credible arguments that the combination of hadron, neutrino, muon, and photon data imply the existence of new kinds of sources contributing at the very highest energies. He made predictions for new data. We (the audience) asked him about constraining hadronic physics, and searching for new physics. He argued that the best place to look for new physics is in the muon signals: They don't seem to fit with the rest. But overall, if I had to summarize, he was more optimistic that the data would be all explained by astrophysical sources and hadronic physics, and not require modifications to the particle model. It was an impressive and lively defense. And Dr MSM has had a huge impact on my Department, co-founding a graduate-student political group and helping us work through issues of race, representation, and culture. I can't thank him enough, and I will miss his presence.

NSF center proposal

I spent some time today discussing a possible NSF Center on real-time data analysis with Ashley Villar (Columbia) and Tyler Pritchard (NYU), based on the wide-ranging grass-roots interest we found in time-domain astrophysics we have discovered in NYC this pandemic. NSF Centers are big projects!

cosmic-ray direction anisotropy

Today I learned from Noemi Globus (NYU) That the Pierre Auger Observatory has an amazing result in cosmic-ray science: The cosmic rays at very high energy do not arrive isotropically at Earth but instead show a significant dipole in their arrival directions. As a reminder: At most energies, the magnetic field of the Galaxy randomizes their directions and they are statistically isotropic at the Earth.

The Auger result isn't new, but it is new to me. And it makes sense: At the very highest energies (above 10¹⁹ eV, apparently) the cosmic rays start to propagate more directly through the magnetic field, and preserve some of their original directional information. But the details are interesting, and Globus believes that she can explain the direction of this dipole from the local large-scale structure plus a model of the Milky Way magnetic field. That's cool! We discussed simulations of the local large-scale structure, and whether they do or can provide reasonable predictions of cosmic-ray production.

cosmic rays, alien technology

I helped Justin Alsing (Flatiron) and Maggie Lieu (ESA) search for HST data relevant to their project for training a model to find cosmic rays and asteroids. They started to decide that HST's cosmic-ray identification methods that they are already using might be good enough to just rely upon, which drops their requirements down to asteroids. That's good! But it's hard to make a good training set.

Jia Liu (Columbia) swung by to discuss the possibility of finding things at exo-L1 or exo-L2 (or the other Lagrange points). Some of the Lagrange points are unstable, so anything we find would be clear signs of alien technology. We looked at the relevant literature; we may be fully scooped, but I think there are probably things to do still. One thing we discussed is the observability; it is somehow going to depend on the relative density of the planet and star!

reading the basics

Today we decided that the newly-christened Astronomical Data Group at Flatiron will start a reading group in methods. Partially because of the words of David Blei (Columbia) a few weeks ago, we decided to start with BDA3, part 1. We will do two chapters a week, and also meet twice a week to discuss them. I haven't done this in a long time, but we realized that it will help our research to do more basic reading.

This week, Maggie Lieu (ESA) is visiting Justin Alsing (Flatiron) to work (in part) on Euclid imaging analysis. We spent some time discussing how we might build a training set for cosmic rays, asteroids, and other time-variable phenomena in imaging, in order to train some kind of model. We discussed the complications of making a ground-truth data set out of existing imaging. Next up: Look at what's in the HST Archive.

Local Group on the Local Group

Today, Kathryn Johnston (Columbia) organized a “Local Group on the Local Group” meeting at Columbia. Here are some highlights:

Lauren Anderson (Flatiron) gave an update on her data-driven model of the color–magnitude diagram of stars. This led to a conversation about which features in her deconvolved CMD are real? And are there too many red-clump stars given the total catalog size?

Steven Mohammed (Columbia) showed our GALEX Galactic-Plane survey data on the Gaia TGAS stars. The GALEX colors look very sensitive to metallicity and possibly other abundances. The audience suggested that we look at the full dependences on metallicity and temperature and surface gravity to see if we can break all degeneracies. This led to more discussion of the use of the Red Clump stars for Galactic science.

Adrian Price-Whelan (Columbia) presented a puzzle about the Galactic globular cluster system, which he has been thinking about. Are the distant clusters accreted? The in-situ formation hypothesis is unpalatable (it had to be many clusters at early times; should be many thin streams); the accreted hypothesis over-produces the smooth component of the stellar halo (unless dwarf galaxies had far more GCs per unit stellar mass in the past). These problems can be resolved, but only with strong predictions.

Yong Zheng (Columbia) spoke about the gaseous Magellanic stream and associated (or plausibly associated) high-velocity clouds. Many of the challenges in interpretation connect to the problem that we don't know where the gas is along the line of sight. She showed really nice data on something called Wright’s Cloud. For this huge structure—and for the stream as a whole—there is little to no associated stellar component.

Nicola Amorisco (Harvard) Showed theoretical simulations of the accreted part of the MW (and MW-like-galaxy) halo, with the goal of finding stellar-halo observables that strongly co-vary with the assembly history of the dark-matter halo. Both theory and observations suggest large scatter in halo properties at Milky-Way-like masses, and much less scatter at higher masses (because of central-limit-like considerations). His results are promising for understanding the MW assembly history.

Glennys Farrar (NYU) spoke about the MW magnetic field, using rotation measures and CMB to constrain the model. She showed UHECR deflections in the inferred magnetic field, and also discussed implications of her results for electron and cosmic-ray diffusion. There are also tantalizing implications for the synchrotron spectrum and CMB component separation. One interesting comment: If her results are right for the scale and amplitude of the field, there are serious questions about origin and generation; is it primordial or generated on scales much larger than the galaxy?

photon pile-up

At Blanton–Hogg group meeting, Daniela Huppenkothen brought up photon pile-up in x-ray and gamma-ray detectors. The issue is that if two photons arrive at the same time, or in the same electronics-restricted time window, they will appear as one photon, but of higher energy. It is an issue for Chandra and for Fermi, among other assets. This pile-up leads to a distortion of the spectrum (and point-spread function, and so on) of very bright sources. We discussed how one might model this, given that it is easy to simulate but hard to describe with a likelihood function. We also came up with a ridiculously simple idea for testing cosmic-ray detection in time-series imaging, which really, really needs to e done.

In the afternoon, I did text writing and problem (exercise) writing in my MCMC tutorial. Stay on target. #AcWriYear

probabilistic density inference, TESS cosmics

Boris Leistedt (UCL) showed up for the day; we discussed projects for the future when he is a Simons Postdoctoral Fellow at NYU. He even has a shared Google Doc with his plans, which is a very good idea (I should do that). In particular, we talked about small steps we can take towards fully probabilistic cosmology projects. One is performing local inference of large-scale structure to hierarchically infer (or shrink) posterior information about the redshift-space positions of objects with no redshift measurement (or imprecise ones).

Zach Berta-Thompson (MIT) reported on his efforts to optimize the hyper-parameters of my online robust statistics method for cosmic-ray mitigation in the TESS spacecraft. He found values for the two hyper-parameters such that, for some magnitude ranges, my method beats his simple and brilliant middle-eight-of-ten method. However, because my method is more complicated, and because it seems to have its success depends (possibly non-trivially) on his (somewhat naive) TESS simulation, he is inclined to stick with middle-eight-of-ten. I asked him for a full and complete search of the hyper-parameter space but agreed with his judgement in general.

online, on-board robust statistics

Zach Berta-Thompson (MIT) showed up at NYU today to discuss the on-board data analysis performed by the TESS spacecraft. His primary concern is cosmic rays: With the thick detectors in the cameras, cosmic rays will affect a large fraction of pixels in a 30-minute exposure. Fundamentally, the spacecraft takes 2-second exposures and co-adds them on-board, so there are lots of options for cosmic-ray mitigation. The catch is that the computation all has to be done on board with limited access to RAM and CPU.

Berta-Thompson showed that a "middle-eight-of-ten" strategy (every 10 sub-exposures average all but the highest and the lowest) does a pretty good job. I proposed something that looks like the standard "iteratively reweighted least squares" algorithm, but operating in an "online" mode where it can only see the last few elements of the past history. Berta-Thompson, Foreman-Mackey, and I tri-coded it in the Center for Data Science studio space. The default algorithm I wrote down didn't work great (right out of the box) but there are two hyper-parameters to tune. We put Berta-Thompson onto tuning.

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.

LSST exposure time

My very busy week ended with a seminar on Gaussian Processes, that I gave on the blackboard, supported with some projected slides showing Foreman-Mackey's demo plots. I tried to go slowly, but there is a lot to do in a one-hour lecture on something that can fill an entire semester's course. I got great questions from the crowd.

In the morning, Željko Ivezić (UW) and I discussed LSST cadence and exposure-time issues. He showed me his "conservation laws" slide, which shows that the exposure time flows down to all sorts of constraints on the survey. We talked about the plan to split the individual (probably 30-sec) exposures into two parts. What's the difference between 15+15 and 5+25 (and so on)? The expectations about the noise and the point-spread function are both related to the exposure time, and there are cosmic rays and moving objects. We tried to spec out some simple projects to analyze the relevant problems. Simplest: What are the implications for point-source position and flux measurements, in the limit that the sky is static? Even this question is not trivial.

Blandford

The highlight of a low-research day was a visit from Roger Blandford (KIPAC), who gave the Physics Colloquium on particle acceleration, especially as regards ultra high-energy particles. He pointed out that the (cosmic) accelerators are almost the opposite of thermal systems: They put all the energy very efficiently into the most energetic particles, with a steep power-law distribution. He made the argument that the highest energy particles are probably accelerated by shocks in the intergalactic media of the largest galaxy clusters and groups. This model makes predicitions, one of which is that the cosmic rays pretty-much must be iron nuclei. In conversations over coffee and dinner we touched on many other subjects, including gravitational lensing and (separately) stellar spectroscopy.

Kepler day

Today was Kepler Day at Camp Hogg, kicking off Kepler Week. Tom Barclay (Ames) came into town for a week to help Foreman-Mackey and me understand the Kepler data in more detail. We spent a lot of the day discussing the various physical effects coming in to the instrument-induced variations we see in the Kepler lightcurves. There are some crazy things, including stellar aberration variations, temperature and point-spread function variations, CCD electronics cross-talk, cosmic-rays and bad cosmic-ray removal, and thruster firings. For many of these things we might be able to build a model or help with modeling. The goal for tomorrow is to decide on week-scale goals and execute. At lunch-time, Foreman-Mackey gave a very nice blackboard talk on Kepler systematics and population modeling, which was pretty relevant to everything we did today.

measuring the shear map

In separate conversations, I spoke with Yike Tang and MJ Vakili (both NYU graduate students) about doing hierarchical inference to infer the weak-lensing cosmological shear map at higher resolution than is possible by any shape-averaging method. Tang is working on building priors over shapes, and Vakili is looking at whether we could build more general priors over galaxy images. That, as my loyal reader knows, is one of my long-term goals; it could have many applications beyond lensing. At lunch, Glennys Farrar (NYU) gave a provocative talk about the possible effect of pion–matter effects (like an index of refraction) to resolve issues in understanding ultra-high energy cosmic ray showers in the atmosphere, issues that have been pointing to possible new physics.

large galaxies, cosmic rays

It comes as a surprise to many that it is much harder to precisely measure the properties of very bright, nearby galaxies than it is to measure the properties of much more distant but similar objects! (Same for very bright stars too, in modern digital imaging.) Part of this is because at high signal-to-noise you see the (badly modeled) details of your point-spread function better. But the bigger issues are that nearby galaxies span field boundaries (in any blind survey, like SDSS), span flat-field and sky variations (because of their large angular sizes), and tend to be blended with background galaxies and foreground stars. Mykytyn, Foreman-Mackey, and I discussed all these issues over a long post-lunch meeting.

In the early morning, Andrew Flockhart (NYU), Fergus, and I discussed our project to use supervised classification machine-learning techniques to identify the cosmic rays robustly in single-epoch, single-exposure HST imaging. We decided to start with nearest-neighbor techniques and move to support vector machines, before going to any heavy machinery. We built our training set with multi-exposure imaging from the HST Archive.