exciting stars

Stars and Exoplanets Meeting at Flatiron was a delight today. Lachlan Lancaster (Princeton) showed his results on a really interesting object he found in the ESA Gaia data. He was inspired by the idea that star clusters might have central black holes, which might retain a very dense, very luminous nuclear star cluster even after the cluster disrupts. But his search of the Gaia data was so simple: Look for things that are apparently bright but low in parallax (large in distance). Duh! And what he found is a very bright “star” that is variable, shows emission lines, and is above the top of the H–R diagram! The ideas from the room ranged from extremely young star to microquasar to technosignatures (who suggested that?). And the thing is incredibly variable.

But there was lots more! I won't do everything, but I will say that Thankful Cromartie (Virginia) showed data from pulsar monitoring (as part of a pulsar-timing project for gravitational waves). She showed that she can very clearly see the Shapiro time delay in the pulses when they pass by the neutron star that is in orbit around the pulsar. This lets them measure the mass of the neutron star accurately. It is very massive! i think it must be one of the most massive neutron stars known, which, in turn, will put pressure on the equations of state. Beautiful results from beautiful data.

inferring maps; detecting waves

I had a great conversation this morning with Yashar Hezaveh (Flatiron) about problems in gravitational lensing. The lensing map is in principle a linear thing (once you set the nonlinear lensing parameters) which means that it is possible to marginalize out the source plane analytically, in principle, or to apply interesting sparseness or compactness priors. We discussed computational data-analysis ideas.

Prior to that, I had an interesting conversation with Rodrigo Luger (Flatiron) and Dan Foreman-Mackey (Flatiron) about the priors we use when we do stellar-surface or exoplanet-surface modeling (map-making). Most priors that are easy to use enforce smoothness, but our maps might have sharp features (coastlines!). But we more-or-less agreed that more interesting priors are also less computationally tractable. Duh!

At mid-day, Chiara Mingarelli (Flatiron) argued in a great seminar that pulsar timing will make a detection in the next years. Her argument is conservative, so I am very optimistic about this.

CITA

I spent the day today at CITA, which is my childhood home: My first-ever scientific paper was written here (when I was an undergraduate researcher) with Scott Tremaine (now IAS) and Gerry Quinlan. At the CITA weekly grass-roots discussion of matters cosmological, Deyan Mihaylov (Cambridge) spoke about gravitational-wave detection with Gaia. He made an amazing point (which, like most amazing points, is obvious in retrospect): The GW signature in Gaia has an earth term but no “pulsar term”, in the language of pulsar timing. That is, it only depends on local metric perturbations! That is extremely good for scaling and precision.

In that same forum, I spoke for the first time ever about the correlation function estimators I have been developing with Kate Storey-Fisher (NYU). I spoke extemporaneously—it's a discussion forum—but I realized that we do have a great story to tell. It includes context from the Landy-Szalay estimator world and context from the linear-fitting world. Plus some information theory for spice! It is a great audience at CITA and they helped me sharpen my case well.

A highlight of a long day of conversations was a chat with Katie Breivik (CITA) about binary population synthesis. She is interested in predicting gravitational-wave sources. But the issues are general. We discussed what aspects of the theory are most weak, and where we might be able to patch in a data-driven replacement. That conversation is only just started, but it's something I want to bring home to NYC and think more about.

AstroFest

For many years, Columbia Astronomy has had a tradition of having everyone in the Department give a short talk in a monster, full-day event called AstroFest. This year, we extended it to three Fridays, and covering all parts of NYC Astronomy. The first of these days was today, at Columbia, and it was great! I learned many things. Here is a smattering:

There is interesting laboratory astrophysics going on at Columbia, including experiments to measure deuterium molecular formation and dissociation rates (reported by Bowen) and experiments to measure aspects of Alfven wave propagation that might be relevant to Solar Coronal heating (reported by Bose).

Spinning black holes in a magnetic field charge up, and this might lead to pulsar-like activity in the late stages of a BH-NS inspiral (reported by Levin). After that I asked if any of the electromagnetic effects might affect the gravitational-wave signal itself, and the answer is unlikely, or only at a very low level.

You can't tell the shape of a transiting object from the shape of the transit (reported by Sandford). There are strict degeneracies! That led the audience to ask about regularization. You can break these degeneracies with regularization, but the answers will depend on the form of that regularization. I was wondering if star spots or limb darkening could break the degeneracies interestingly?

If you slowed down the rotation of the Earth, it would get colder, and more uniform in temperature between equator and pole (reported by Jansen)! That was a great use of Earth climate models to inform the study of exoplanets. And it maybe violates my simplest intuitions. New cure for global warming: Slow down Earth rotation!

And I was only there for the morning.

gravitational clustering, gravitational interferometry

Today Michael Joyce (LPNHE) gave a great talk about analytic and conceptual directions towards understanding nonlinear gravitational growth of structure in the Universe. He focused on the stable-clustering approximation, which dates back to Peebles, is very predictive over a range of scales, and can be used to test simulations. At lunch afterwards, we discussed the great importance of studying gravity analytically, a point made often and well by Roman Scoccimarro (NYU).

Prior to the seminar, Ellie Schwab-Abrams (AMNH) and I discussed self-calibration for pulsar timing arrays, which we think and hope could lead to a new era of gravitational interferometry and enormously increase the sensitivity to long-term gravitational-wave signals. We decided to start by solving the radio-astronomy problem, which has yet to be solved in the literature, because no radio telescope has the problem that the relative velocities of it's elements are unknown!

group meeting, self-calibration for GR, writing

Today we had the first-ever Astronomical Data Group Meeting. The rules are: You must bring a plot, and you get a time period of (1 hr)/N where N is the number of people in the room to get feedback. It was fun: All of the plots (even Foreman-Mackey's) related to the Gaia DR2 data. I asked the crew whether the stars below the main sequence in the Gaia color–magnitude diagram are very low in metallicity? And if so, shouldn't we take spectra? Anderson thinks maybe they are just issues with crowded fields. That is, issues in the data. Problems with chasing outliers!

After that I had long sessions with Ellie Schwab-Abrams (CUNY), and Jonathan Bird (Vanderbilt). Schwab-Abrams and I are trying to convert my question about self-calibrating gravitational-wave pulsar-timing arrays into the equivalent question about self-calibrating radio telescopes. It is very similar! But we have to take into account the 6-space position not the 3-space position, and we also have to deal with light travel time issues that we can't control with delay lines! But the payoff is immense: I naively expect a factor of more than a billion increase in sensitivity of the arrays if we can do it. Yes I said billion. I hope I'm right.

Bird is finishing his paper on the age–velocity relationship in the disk. We went over discussion points. I recommended explicitly challenging the assumptions and saying what we think would happen if we relaxed them, both in terms of the results and in terms of model complexity. My problem (as it often is in projects) is that I care about the method much more than the astrophysical results.

data predicting data; bad Solar System

First thing in the morning, I met with Judy Hoffman (Berkeley) to discuss her computer-vision and machine-learning work. She suggested that machine-learning methods that are auto-encoder-like could be repurposed to make predictions from one kind of data to another kind of data on the same object. For instance, we could train an encoder to predict exoplanet RV signal, given Kepler light curve. Or etc! This appeals to me because it uses machine learning to connect data to data, without commitment to latent quantities or true labels for anything. She pointed me (relatedly) to a new kind of model called ADDA, for which she is responsible.

In the afternoon, Chiara Mingarelli (Flatiron) gave the NYU Astro Seminar about pulsar timing and gravitational radiation, expressing the hope and expectation that this method will deliver signals soon. She told a very interesting story about a false-positive detection that nearly went to press when they figured out that it was resulting from residuals in the Solar System ephemerides. The SS comes in because you have to correct Earth-bound timings to a frame that is at rest (or constant velocity with respect to) the SS barycenter.

This isn't the first time I have heard this complaint. The astronomical community really needs an open-source and probabilistic SS ephemeris, so we can use the SS model responsibly inside of inferences. Freedom-of-information act time?

quality time with the iPTA

The research highlight of the day was a couple of hours spent with the iPTA data analysis collaboration. Justin Ellis (WVU) led an overview and extremely interactive discussion of their likelihood function, which they use to detect the gravitational radiation stochastic background from pulsar timing, in the presence of systematic nuisances. These include time-variable dispersion measure, red noise, accelerations and spin-down, receiver and backend calibrations, ephemeris issues, and more! The great cleverness is that they linearize and apply Gaussian priors, so they can make use of all the beautiful linear algebra that my loyal reader hears so much about. The likelihood function is a thing of beauty, and computationally tractable. They asked me for advice, but frankly, I’m not worthy.

self-calibrating pulsar arrays, and much more

I had a great conversation with Chiara Mingarelli (Flatiron) and Ellie Schwab (AMNH) today about pulsar-timing arrays and gravitational-wave sources. We are developing some ideas about self-calibration of the arrays, such that we might be able to simultanously search for coherent sources (that is: not just stochastic backgrounds) and also precisely determine the distances to the individual pulsars to many digits of accuracy!. It is futuristic stuff, and there are lots of ways it might fail badly, but if I am right that the self-calibration of the arrays is possible, it would make the arrays a few to tens of times more sensitive to sources! We started with Mingarelli assigning us some reading homework.

In the Stars group meeting, we had a productive discussion led by Megan Bedell (Flatiron), Andrew Mann (Columbia), and John Brewer (Yale) about things learned at the recent #KnowThyStar conference. There are some new uses of machine learning and data-driven models that I might need to spend some time criticizing! And it appears that there are some serious discrepancies between asteroseismic scaling relations for stellar radii and interferometric measurements. Not bigger than those expected by the stellar experts, apparently, but much bigger than assumed by some of the exoplanet community.

Prior to that, in our weekly Gaia DR2 prep working session, we discussed the use of proper motion as a distance indicator in a post-reduced-proper-motion world. That is: The assumptions underlying reduced proper motion are not great, and will be strongly violated in the DR2 data set. So let's replace it with a much better thing!

Adrian Price-Whelan (Princeton) showed some incredible properties of (flowing from beautiful design of) the astropy coordinates package. Damn!

how to write an April Fools' paper

I had a great visit to the University of Toronto Department of Astronomy and Astrophysics (and Dunlap Institute) today. I had great conversations about scintillometry (new word?) and the future of likelihood functions and component separation in the CMB. I also discussed pairwise velocity differences in cosmology, and probabilistic supernova classification. There is lots going on. I gave my talk on The Cannon, in which I was perhaps way too pessimistic about chemical tagging!

Early in the day, I ate Toronto-style (no, not Montreal-style) bagels with Dustin Lang (Toronto) and discussed many of the things we like to discuss, like finding very faint outer-Solar-System objects in all the data Lang wrangles, like the differences between accuracy and precision, and even how to define accuracy in astrophysics, and like April Fools' papers, which have to meet four criteria:

1. conceptually interesting inference
2. extremely challenging computation
3. no long-term scientific value to the specific results found
4. non-irrelevant mention of April 1 in abstract

It is a brutal set of requirements but we have met them two times. I think this year is out (because of criterion 2), but maybe 2018?

#astrohackny, week N+1

The day started at #astrohackny with Foreman-Mackey and I arguing about convolutions of Gaussians. The question is: Consider a model (probability of the data given parameters) with two (linear) parameters of importance and 150 (linear) nuisance parameters. There is a very weak Gaussian prior on the nuisance parameters. How to write down the marginalized likelihood such that you only have to do a 2x2 least squares, not a 152x152 least squares? I had a very strong intuition about the answer but no solid argument. Very late at night I demonstrated that my intuition is correct, by the method of experimental coding. Not very satisfying, but my abilities to complete squares with high-dimensional linear operators are not strong!

Taisiya Kopytova (MPIA) is visiting NYU for a couple of months, to work on characterizing directly imaged extra-solar planets. We discussed the simultaneous fitting of photometry and spectroscopy, one of my favorite subjects! I, of course, recommended modeling the calibration (or, equivalently, continuum-normalization) issues simultaneously with the parameter estimation. We also discussed interpolation (of the model grid) and MCMC sampling and the likelihood function.

At Pizza Lunch at Columbia, Chiara Mingarelli (Caltech) talked about the Pulsar Timing Array and its project to detect the stochastic background of gravitational waves. The beautiful thing about the experiment is that it detects the motion of the Earth relative to the pulsars, not the individual motions of the pulsars, and it does so using time correlations in timing residuals as a function of angle between the pulsars. The assumption is that the ball of pulsars is far larger than the relevant wavelengths, and that different pulsars are causally unconnected in time. Interesting to think about the "multiple hypotheses" aspects of this with finite data.

Vicki Kaspi

Vicki Kaspi (McGill) gave the Physics Colloquium talk today. She compared the fastest-known millisecond pulsar (which her group discovered) to the fastest commercial blenders in spin period. The pulsar wins, but it wins far more in surface speed: The surface of a millisecond pulsar is moving a significant fraction (like 0.1) of the speed of light! She talked about the uses of pulsars for precision measurement and testing of general relativity. It is just incredible that nature delivers us these clocks! I got interested during the talk in the spin constraints on the equation of state: We often see constraints on equation of state from mass measurements, but there must be equally compelling limits from the spin: If you are spinning such that your surface is moving at or even near the sound speed in the material, I think (or I have an intuition) that everything goes to hell fast.

AAAC, day 1

Today was the first day of the Astronomy and Astrophysics Advisory Committee meeting at NSF headquarters. The Committee is established by an act of Congress to oversee the interagency cooperation and interaction and etc between NSF Astronomy and Astrophysics and NASA Astrophysics (and also DOE Cosmic Frontiers). I learned a huge amount about science at the meeting, including about a conflict between VLBI and Hipparcos parallaxes to the Pleaides. That's Huge. Of course we looked at the outrageously awesome ALMA image of HL Tau showing actual Oh-My-God rings. I learned that the black hole at the center of M82 is no longer thought to be a black hole (need to learn more about that!) and that there is a too-massive black hole found at an ultra-compact dwarf galaxy. Wow, science rocks!

We went on to learn that science rocks a lot less than I thought, for various reasons: The proposal success rates in most individual-investigator money grants are at 15 to 20 percent, with DOE being higher but with most of their (DOE's) grants going to groups already working on DOE-priority projects. These low success rates may be distorting the "game" of applying for funding; indeed it appears that proposers are writing more proposals per year than ever before.

I learned (or re-learned) that the federal budgets (primarily from the executive branch) that involve ramping down work on NASA SOFIA are also budgets that involve ramping down the whole NASA Astrophysics budget by the same amount. That is, the honesty of the community and its willingness to make hard choices about what's important leads to budget reductions. Those are some terrible incentives being set up for the community. The agencies and the powers that be above them are creating a world in which honesty and frugality is rewarded with budget cuts. I guess that's why the defense part of the US government is so (a) large and (b) dishonest. Thanks, executive branch! Okay, enough goddamn politics.

Ed Groth

I spent a great day in Princeton at the birthday and retirement celebration for Ed Groth (Princeton), who was instrumental in the HST WFPC project and is the originator of the incredibly influential Groth Strip. There were many great talks and reminiscences, a few of the highlights for me were the following:

Ed MacDonald (who worked on oceanography for the Navy and NATO) talked about moving data by paper tape from experiment to computer center, and the fact that mundane tasks are an important part of all important scientific discoveries. He noted that Bob Dicke (the leader in the Gravity Group at Princeton) was never afraid of doing mundane things in support of scientific discovery.

Bill Wickes (formerly of HP) talked about many things, not the least of which was the importance of calculators in scientific research. Indeed, calculators featured heavily in the stories and photographs from Groth's early days. Wickes is responsible for inventing and designing and improving various HP calculators. He also talked about the Gravity Group attitude of "you sit on it until it works", which is a very good principle for science!

Bruce Partridge (Haverford) discussed the precise timing of the Crab Pulsar, done at Princeton by him and Groth and others, which led to the discovery of period derivatives, second derivatives, and glitches. The timing was done very cleverly; he showed the electronics diagram. The Gravity Group was always motivated to precisely measure anything for which there was simultaneously a hope of precise measurement and a precise quantitative prediction. He showed also that the search for gravitational radiation was already in the air way back then.

Jason Rhodes (JPL) and Todd Lauer (NOAO) talked about HST imaging. Rhodes and Groth wrote one of the first papers on weak gravitational lensing. Lauer pointed out that Groth was instrumental in starting the HST Archive and our understanding of the huge legacy value of digital data sets.

Finally, Jim Peebles (Princeton) talked about correlation functions, on which he worked with Groth, and which remain the key tool of cosmology today. He showed some lovely visualizations of hand-taken data on galaxy counts from the 1960s and 70s. He highlighted the ways in which Groth's career spanned the transition from "small science" to "big science", doing important things in both modes. It was a great day!

time-series; x-ray scattering

At the final (and all-day) meeting of #NYCastroML we discussed time-series analysis, including spectral analysis, filtering, and Bayesian inference. This was followed by a hack session during which I met with Schiminovich and his group to discuss GALEX photons and Rutger van Haasteren (Caltech) and Michele Vallisneri (Caltech) to discuss application of our HODLR linear algebra tools to gravitational wave detection.

The day ended with Lia Corrales (Columbia) giving a short seminar on x-ray studies of dust, where forward scattering permits (in principle) inference of the distribution of dust in space and also grain size. The talk made me think that if you could have many x-ray point sources measured (and good knowledge of the point-spread function), you could in principle fully map the dust in three-space, and also figure out the three-dimensional positions of all the point sources. Probably not feasible, but interesting to think about.

red giants as clocks

Lars Bildsten (KITP) was in town and gave two talks today. In the first, he talked about super-luminous supernovae, and how they might be powered by the spin-down of the degenerate remnant, when spin-down times and diffusion times become comparable. In the second, he talked about making precise inferences about giant stars from Kepler and COROT photometry. The photometry shows normal modes and mode splittings, which are sensitive to the run of density in the giants; this in turn constrains what fraction of the star has burned to helium. There is a lot of interesting unexplained phenomenology related to the spin of the stellar core, which remains a puzzle. There was much more in the talk as well, but one thing that caught my interest is that some of the modes are exceedingly high in quality factor or coherence. That is, giants look like very good clocks. A discussion broke out at the end about whether or not we could use these clocks to constrain, detect, or measure gravitational radiation. Each star is much worse than a radio pulsar, but there are far, far more of them available for use. Airplane project!

pulsars, eccentricities

[This is my 2¹¹th research blog post. That's a lot of posts over the last nearly-9 years! I'll be an old man when I post my 2¹²th.]

At the brown-bag talk today, Gruzinov (NYU) talked about modeling pulsars using what he calls "Aristotelian Electrodynamics", which is an approximation valid when synchrotron radiation losses are so fast that charged particles essentially move along magnetic field lines. He claims to be able to compute realistic predictions of pulsar light-curves in the Fermi bandpass, which, if true, is a first, I think. He argued that all pulsars should live in a four-dimensional family, parameterized by two angles (viewing and dipole-misalignment), one spin period, and one magnetic dipole moment. If it all bears out, pulsars might be the new standard candles in astronomy!

In the afternoon, Foreman-Mackey and I went on the BayCEP phonecon of the exoSAMSI group, where we discussed hierarchical inference and approximations thereto. There are various projects close to doing a proper hierarchical probabilistic inference of the distribution of planets in various parameters. Eric Ford (PSU) is even implementing some of the ideas in this old paper.

KIPAC@10, day 2

Today was the second day (I missed the first) of the KIPAC@10 meeting at KIPAC at SLAC. There was a whirlwind of talks on compact objects and galaxy evolution, too many to summarize, but some highlights for me were the following:

Steiner (UW) showed neutron-star mass and radius measurements and discussed their implications for the properties of matter at extreme density. He showed some very noisy likelihood functions (ish) in mass–radius space, one per measured neutron star (and there are 8-ish with measurements) and tried to draw a curve through them. I have opinions about how to do that and he seems to be doing it right; each time we tried to discuss this over coffee something interrupted us.

Perna (Colorado) talked about magnetars; I hadn't appreciated how extremely short-lived these stars must be; their lifetimes are measured in kyr, which is not a unit you see every day. Romani (Stanford) made a pitch that Fermi-discovered gamma-ray pulsars are the bees knees. He didn't show folded light-curves but apparently there are now hundreds where you can see the periodicity in the (sparse) Fermi data. Tomsick (Berkeley) showed some outrageously awesome NuSTAR data, making me want to hear much more about that mission. It's PI is my old friend from graduate school, Fiona Harrison (Caltech), to drop a name.

Cordes (Cornell) talked about pulsar timing and gravitational radiation, a subject on which I have opinions (from a precision measurement perspective). He, like is common in that business, concentrated on the stochastic gravitational wave background; I would like to hear or think more about coherent source detection. It is usually easier! Along those lines, at one point Blandford (KIPAC) asked Aarons (Berkeley) if physical models of pulsar emission were likely to help in measurements of pulsar timing. Aarons didn't commit either way, but I think the answer has to be yes. Indeed, I have suggested previously that modeling the emission almost has to improve the measurements.

Stark (Arizona) showed very nice new data on galaxies at extremely high redshifts. He noted that almost every result at redshifts beyond six depends entirely on photometric redshifts. That's true, but is it a concern? I guess it is because there could be interloping lower-redshift objects (or stars) having a big effect on the conclusions. Kriek (Berkeley) and Lu (KIPAC), in separate talks, showed that it is difficult to explain the evolution of galaxies in sizes and stellar populations with simple models of star formation and merging. Also, Kriek called into question the star-formation-rate estimates people have been using, which is interesting; she finds a factor-of-two-ish range in the mistakes that could be being made, and this is the same order of magnitude as the amplitude of the variation in specific star-formation rate with galaxy mass. She didn't claim that there is an error there.

In the discussions at lunch, Stuart Lynn (Adler) pitched an idea from David Harris (NAS) that we start a journal of short contributions. Marshall was all over that; it might get launched tomorrow in the unconference session.

talks, Dr. Zrake, and target selection

In a talk-filled day, McWilliams (Princeton) talked about super-massive black-hole merger events and their detectability (through gravitational waves) with pulsar timing, Zrake (NYU) defended his PhD on relativistic turbulence, and Schölkopf gave the first of his Courant Lectures at NYU on causal inference and machine learning. Zrake is particularly deserving of congratulations: He has demonstrated that (warm) relativistic turbulence has very similar statistics to non-relativistic turbulence, which is very, very new. He did this by writing and operating some pretty high-end open-source code.

The day began with a discussion with Tinker (which led to a set of email trails with SDSS-IV eBOSS) about how to make a uniform sample of quasars on the sky, when quasars look very like stars (morphologically and in color) and the photometric errors and stellar density are varying significantly. The answer is: You can't make a uniform sample, for extremely deep reasons: There really is far less information when the errors get worse, and there really are different prior expectations in regions of different stellar density. However, we talked about various approaches to mitigating final, observed quasar density variance in the final observed sample. Nothing is easy here. The problem is that quasars and stars look very similar in the SDSS ugriz filter set. (The crazy thing is that LSST has now set in stone that they will use the same filters! That seems like a big mistake, given everything we now know about stars, galaxies, and quasars in the ugriz bands.)

binary pulsars as detectors

Mike Kesden (NYU) and I spent an enjoyable two hours coming to final consensus on what we think of the Hui et al paper about GW detection. We built an order-of-magnitude argument from the ground up (which confirms the Hui results but which, sadly, doesn't exist in the Hui paper). In the end, we reproduce the Hui expressions but do not confirm their final numbers. The paper is vague about how they get their final numbers, but I think the discrepancy must be in the bandwidth; I think the Hui team is mis-estimating the bandpass in which the effect is relevant.