Category Archives: Paper

Paper: New Tool for Pulsar Timing

Posted on by

PINT: A Modern Software Package for Pulsar Timing by Jing Luo, Scott Ransom, Paul Demorest, Paul S. Ray, Anne Archibald, Matthew Kerr, Ross J. Jennings, Matteo Bachetti, Rutger van Haasteren, Chloe A. Champagne, Jonathan Colen, Camryn Phillips, Josef Zimmerman, Kevin Stovall, Michael T. Lam, and Fredrick A. Jenet.

The defining characteristic of pulsars — that they emit regular pulses of radio waves or other radiation — also provides one of the most powerful tools for studying them and their environments: we can measure the arrival time of these pulses at our telescopes quite precisely. These arrival times carry information about the rotation of the pulsar (affected by the energy loss to a particle wind and by interaction of the crust with the internal superfluid), the orbit of the pulsar if it’s in a binary system (including tidal and relativistic effects on the orbit, and relativistic time dilation and light bending), the location of the pulsar in relation to the Earth’s orbit (including its distance), the interstellar medium (varying as our line of sight sweeps through it), and possibly even gravitational waves passing over the Earth. But to extract the interesting science from these measurements, we need a tool that models how all these physical processes affect the observed arrival times. PINT is such a tool.

Numerous data points with error bars, all lying near a horizontal line.
The difference between observed arrival times and arrival times modelled with PINT. The parameters of the model, including position on the sky, pulsar spin period, and pulsar orbital parameters, have been adjusted to match the observed data as well as possible, and the discrepancies are within about 20 microseconds, and are broadly consistent with the uncertainties on the measurements themselves.

Of the tools that exist to do this task, TEMPO has a proud history of scientific discovery but was written in FORTRAN in the 1980s and was never designed to operate with sub-microsecond accuracy. The tool TEMPO2 was meant to be its successor, written in the 2000s, but much of TEMPO2’s code is simply original TEMPO code converted to C++, and the package can be difficult to work with. PINT is intended as a largely independent development project, and indeed the process of developing PINT has revealed at least one long-standing error (now fixed) in TEMPO2’s computations. PINT is also written to be a flexible python library, well documented and well tested, so that its data structures and tools can be applied to tasks that do not fit naturally into traditional pulsar timing.

PINT is an open-source project under active development; you can find it on GitHub. It has extensive, if still in-progress, documentation as well.

Paper: When galactic foregrounds are allowed to vary

Posted on by

A minimal power-spectrum-based moment expansion for CMB B-mode searches by S. Azzoni, M. H. Abitbol, D. Alonso, A. Gough, N. Katayama, T. Matsumura

Primordial gravitational waves, thought to be sourced by inflation in the very early universe, should imprint themselves on the polarisation signal of the cosmic microwave background (CMB). Direct detection of the “B-mode” polarisation characteristic of these gravitational waves would provide strong evidence for an inflationary period in the early universe, as well as providing insight into the mechanisms driving it. The strength of the tensor perturbations which correspond to gravitational waves is parametrised by the “tensor to scalar ratio”, r. 

Detecting true B mode signal from the CMB is difficult, as the primordial B mode signal is dominated by foreground elements, such as thermal dust emission and synchrotron emission within our own galaxy. These galactic foregrounds can be dealt with by masking—cutting the portions of the sky where the galactic plane is out of the signal—though this has the disadvantage of throwing away true signal. The other approach is to model the foreground elements statistically, with some dependence on frequency.

The standard technique is to characterise the foreground elements’ spectral energy density (a function of frequency) by an amplitude and a spectral index. These spectral indices are usually considered to be constant across the sky, which, given that they are empirically fit parameters, rather than motivated by underlying physics, is not well justified. For small sky fraction surveys, the difference is thought to be negligible, but for future CMB experiments, this assumption should be relaxed. This paper is a proof of concept for an extension beyond assuming a constant spectral index.

Allowing the spectral indices of dust and synchrotron emission to vary across the sky, we propagate forward the effect on the total observed power spectrum in a very general way using a moment expansion. A highly simplified case of this model is then tested against simulations of varying degrees of realism, to see whether this kind of expansion tightens the constraints on r compared to not using the moment expansion. The result is not possible to significantly detect the effects of this variation on the foreground multi-frequency power spectra for Simon’s-Observatory-like sensitivities, at least using the strong simplifications we used to test against simulations. Nevertheless the methodology set up in this paper, as well as potential extensions, will be useful for analysis of ground based CMB experiments such as Simon’s Observatory and CMB-Stage 4.

Paper: Polarization details shed light on the origin of fast radio bursts

Posted on by

Microsecond polarimetry of the repeating FRB 20180916B by K. Nimmo, J. W. T. Hessels, A. Keimpema, A. M. Archibald, J. M. Cordes, R. Karuppusamy, F. Kirsten, D. Z. Li, B. Marcote, and Z. Paragi

Fast Radio Bursts are millisecond-long incredibly bright radio flashes from distant galaxies. They are clearly produced by some kind of coherent emission process but the details or even basic nature of this process remain mysterious. Observational clues as to what it might be are few and far between, but polarization can provide hints about the geometry and magnetic fields in or near the emitting region. For this paper we managed to find polarization structure at very short timescales inside the burst – this is a clear sign that relatively small structures are producing elements of the radio burst. We still do not know what kind of structures these might be, but the Crab pulsar produces “giant pulses”, albeit still many orders of magnitude fainter than fast radio bursts, from coherent emission regions about 30 cm across. So this hint at detailed small-scale structure in fast radio bursts sheds a little light on their still-mysterious origin.

Plot showing bursts and their polarization.
Figure 4: detailed polarization structure of different sub-bursts. Note the microsecond time-scale on the bottom panel. Black bars indicate total intensity, red indicates linearly polarized intensity, and blue circularly polarized intensity (with sign). Note that these sub-bursts are almost completely linearly polarized. The grey bars in the top panels indicate the direction of linear polarization, with fuzziness indicating uncertainty. Note that there appears to be structured variation in the direction on microsecond timescales in the sub-burst shown in the bottom panel.

Paper: How do galaxies grow? Resolving gas-phase metallicity and star-formation at cosmic noon

Posted on by

The Evolution of Gas-Phase Metallicity and Resolved Abundances in Star-forming Galaxies at z ~ 0.6 – 1.8 by S. Gillman, A. L. Tiley, A. M. Swinbank, U. Dudzeviciute, R. M. Sharples, Ian Smail, C. M. Harrison, Andrew J. Bunker, Georgios E. Magdis, J. Trevor Mendel and John P. Stott, accepted October 2020

The technique of integral field spectroscopy allows us to spatially-resolve the gas properties of individual galaxies. This is because we get a spectrum at every spatial pixel of the galaxy. By measuring the abundance of heavy elements (e.g., Nitrogen) compared to Hydrogen, the so-called “metallicity” we can learn about the star formation processes of the galaxies. Star formation inside a galaxy will enrich the gas with heavy elements (e.g., through supernovae and stellar winds) and will increase the “metallicity”. Strong star formation driven winds and supernova will distribute metals across the galaxy. In contrast, “pristine” material (mostly hydrogen) can also be accreted onto the galaxy through gas inflows. Consequently a lot can be learn by measuring the metallicity gradient inside individual galaxies we can learn about the distribution of star formation and the relevance of gas inflows.

Newcastle’s Chris Harrison is part of a team using extensive integral field spectroscopy from KMOS to study galaxies at the “cosmic noon” i.e., redshift z~1-2 when cosmic star formation was at its highest levels (KROSS, KGES and KURVS surveys). This paper uses these data to measure the galaxy-wide metallicity and metallicity gradients of ~650 star-forming galaxies at z~0.6 – 1.8. We find that for a given stellar mass, more highly star-forming, larger and irregular galaxies have lower gas-phase metallicities, which may be attributable to their lower surface mass densities and the higher gas fractions of irregular systems. Galaxies in our sample exhibit flatter metallicity gradients than local star-forming galaxies, in agreement with numerical models in which stellar feedback plays a crucial role in redistributing metals.

Metallicity gradients – i.e., the spatial gradient of the abundance of heavy elements across each galaxy – as a function of their redshifts (or equivalently, their cosmic time). There is no significant evolution between our two high redshift samples (blue and green circles); however, both samples exhibit slightly flatter gradients than observed locally (square data points). We also show theoretical predictions from two models of disc galaxies from Mott et al. (2013) with radially constant star-formation efficiency (purple dashed line) and variable star formation efficiencies (solid purple line).