Polarimetry of Millisecond Pulsars

Image: We observe different polarization properties, both the angle and the intensity, as the pulsar beam sweeps across our line of sight (Credit: Rankin 2015).

  • Precision timing of millisecond pulsars over years and decades allows us to build models of radio-pulse arrival times of incredible accuracy. These models account for the pulsar's spin period, its evolution, the dispersion of radio pulses in the interstellar medium, and many other effects. Developing precision timing models for many pulsars allows their use as tools to test fundamental physics.
  • NANOGrav requires extremely sensitive measurements of millisecond-pulsar pulse profiles in order to reach its goal of characterizing the low-frequency gravitational wave universe. We use some of the largest radio telescopes in the world to obtain our measurements.
  • Electromagnetic waves are often polarized, which means that the electric fields oscillate in one direction. Emission from pulsars is often highly polarized, and therefore the bulk of the electic fields from the waves oscillate the same way. Not only are the pulse profiles from pulsars high stable, but the polarization profiles are as well.
  • Understanding how measurements of polarization affect our timing is a focus of NANOGrav's Timing working group.

The NANOGrav 11-year Data Set

  • NANOGrav's latest data release contains radio-pulse times of arrival (TOAs) and timing-models for 45 millisecond pulsars.
  • The observations span roughly 11.4 years, from July 30, 2005, to December 31, 2015. We cover radio frequencies from 300 MHz to 2.5 GHz. The pulsar with the largest data volume is J1713+0747, with 28,000 TOAs.
  • Observations were carried out using the 100-m Robert C. Byrd Green Bank Telescope (GBT) of the Green Bank Observatory, and the 305-m William E. Gordon Telescope (Arecibo) of Arecibo Observatory. In this work, we present observations only from Arecibo.
  • In the sky map shown here, pulsar positions are marked by circles, with areas proportional to the number of TOAs in the dataset; the color scale indicates the timing baseline. The 34 pulsars with baselines greater than 3 years have solid red edges. We use only these 34 in our searches for a GW background.

Image: Sky map of NANOGrav pulsars in the 11-yr data set. (Credit: Arzoumanian et al. 2018b

Pulse Microcomponents

Image: Pulse polarization profiles for one of our best-timed pulsars, J1713+0747. The bottom shows a zoom in of the baseline, and while it looks flat in the middle panel, we see clear low-level structures.

  • To determine when a pulse arrives, we fit a template shape to each pulse profile we observe. Since our instruments are inherently noisy, we do not perfectly measure the TOA and there is uncertainty in our arrival-time estimates.
  • Proper polarization calibration is crucial since pulsar emission is highly polarized. Incorrect calibration will lead to observed changes in pulse shapes, which will induce large timing uncertainties.
  • Microcomponents, when unaccounted for, will affect our timing measurements if our template shapes differ from that of the profiles. To obtain the best possible arrival-time precision, we must account for these microcomponents. Measurements of pulsar flux densities will also be affected.
  • The observation of microcomponents implies that we are seeing low-level emission over the entirety of each pulse rotation. Therefore, we know that the emission extends beyond the geometry of a simple beam.

Calibration of the Arecibo Observatory

  • Since pulsars are highly polarized, we must ensure that we are measuring the same polarization profiles to avoid artificial change in pulse shape that might cause additional uncertainties in our arrival times.
  • As a radio telescope tracks a souce across the sky, the dipole antenna used to measure the emission will rotate, thereby changing the relative orientation of the radio waves. The change in polarization from this affect must be properly accounted for.
  • We perform different sets of calibration observations to empirically determine the variations of our system on the timescale of a single observation.
  • We measured and quantified the changes in the calibration parameters over time. These corrections describe how the emission received from the pulsar is affected by all of our instrumentation, and can be applied in the future to correct our arrival times.

Image: NANOGrav-ers taking a break from working on the 12.5-year data set to visit the Very Large Array in Soccoro, New Mexico. As the highlighted crossed dipole turns as the telescope tracks a source, it measures radio polarization differently. We correct for this effect, along with any imperfections in the cross itself (Credit: Michael Lam).

Studies of Galactic Magnetic Fields

Image: Galactic magnetic field parallel to the line of sight. The asymmetry above and below the plane of the Galaxy, representing the magnetic field towards and away from the Earth respectively, agrees with Galactic magnetic field models.

  • Magnetic fields thread through the Galaxy at very low levels, almost one-millionth the strength of that around the Earth.
  • The Faraday effect rotates the linear polarization of radio emisssion depending on the magnetic field parallel to the line of sight and the number density of electron along the path. Since we also measure the arrival-time delay from dispersion due to electrons along the propagation path, we are able to determine the parallel component of the Galaxy's magnetic field.
  • We find differences in the "Rotation Measure", which quantifies the strength of the Faraday effect, along different sightlines through the Galaxy. Interestingly, some sightlines that are very close to one another can have very different magnetic field estimates.
  • While our measurements agree with models of the Galactic magnetic field, pulsar-timing polarimetry complements other studies, such as from the dynamics of "dust" in the interstellar medium.

Future Directions

  • Future NANOGrav data sets will begin to incorporate the corrections we have measured here, thereby increasing the precision of our arrival-time estimates.
  • We are currently working on applying these studies to our observations taken with the Green Bank Telescope, so that we can correct all observations in future NANOGrav data releases.
  • Many propagation effects due to radio waves traveling through the interstellar medium are radio-frequency-dependent, as are polarization measurements. Disentangling the effects of the interstellar medium on our data is the prime focus of NANOGrav's Interstellar Medium Mitigation working group.
  • With many more pulsars in our array, we can begin to understand both the spatial and temporal variability of Galactic magnetic fields.

Image: 10,000 unique observations of 48 pulsars in the preliminary NANOGrav 12.5-year data set.

Authors

  • Members of the NANOGrav Collaboration: P. A. Gentile, M. A. McLaughlin, P. B. Demorest, I. H. Stairs, Z. Arzoumanian, K. Crowter, T. Dolch, M. E. DeCesar, J. A. Ellis, R. D. Ferdman, E. C. Ferrara, E. Fonseca, M. E. Gonzalez, G. Jones, M. L. Jones, M. T. Lam, L. Levin, D. R. Lorimer, R. S. Lynch, C. Ng, D. J. Nice, T. T. Pennucci, S. M. Ransom, P. S. Ray, R. Spiewak, K. Stovall, J. K. Swiggum, W. W. Zhu
  • Contact: Dr. Peter A. Gentile (corresponding author), Prof. Maura McLaughlin (NANOGrav chair).

The NANOGrav Collaboration at the 2017 Fall meeting in Lafayette College, PA