Ingrid Stairs UBC

Novel Radio Telescopes Penticton

June 13, 2011

Pulsar Science and New Telescopes

CHIME diagram

Neutron stars are an extreme state of matter with strong self-gravitation. Many are visible as radio pulsars.

Add together several hundred pulses stable “integrated profile”

Pulse-to-pulse variations

Dispersion: friend and foe… and never quite constant…

Modern observing systems use the coherent dedispersion technique over hundreds of MHz of bandwidth.

Result: sharp profile features are resolved, leading to more precise pulse Times of Arrival (TOAs). The wide bandwidths increase the signal-to-noise ratio and therefore also TOA precision.

Build up a “standard profile” over hours/days of observing.

Standard profile

Observed profile

Measure offset

Pulsar Timing in a Nutshell

Record the start time of the observation with the observatory clock (typically a maser). The offset gives us a Time of Arrival (TOA) for the observed pulse. Then count every single rotation of the pulsar across years of observing.

Two Groups of Pulsars

Young pulsars

Recycled pulsars

Nasa/Dana Berry

Timing Residuals: Actual pulse Times of Arrival (TOAs) – Predicted TOAs

Gaussian residuals imply a good ephemeris (spin, astrometry, binary parameters...).

PSR J1751-2857 – Stairs et al. (2005)

Precision measurement of pulsar timing parameters can lead to tests of gravitational theories. This is the mass-mass diagram in GR for the double pulsar J0737-3039A and B. In addition to 5 “post-Keplerian” parameters (for A), we measure the mass ratio R, which provides a unique constraint on gravitational theories.

As of July 2010; Kramer et al. in prep.

Timing of millisecond pulsars with white-dwarf companions can also lead to tests of gravity.

Here observations of multiple pulsars are combined to set a limit on violation of the Strong Equivalence Principle.

Result: |Δnet| < 0.0045 at 95% confidence (Gonzalez et al, submitted, based on method used in Stairs et al. 2005).

Gonzalez et al., submitted.

Reasons to time millisecond pulsars (MSPs):

Masses, velocities, population, binary evolution, testing gravitational theories... and potential to constrain or detect gravitational waves.

This is the goal of the International Pulsar Timing Array – European, Australian and North American collaborations.

Nanohertz GW sources:

“Monochromatic” MBH-MBH binaries of >107 solar mass.

PTA Sources

  Stochastic MBH background (Jaffe & Backer 2003, Sesana et al 2008, ...)

  Resolved MBH sources (Sesana et al 2009, Boyle & Pen 2010, ...)

  Also cosmic strings, other exotica… Slide: Paul Demorest

See Hobbs et al 2009 for correlation expectation from Hellings & Downs 1983, plus simulated data.

For a stochastic (and isotropic) background, GWs near Earth would produce correlations in timing residuals with a roughly quadrupolar dependence on angular separation. The goal is to detect this curve with good significance.

Baseline requirement: ~20 pulsars over 5—10 years with 100ns timing precision (Jenet et al 2005).

Signal sizes and detector comparisons

Demorest

NANOGrav 5-year timing results overview:

(plot: D. Nice)

NANOGrav 5-year timing results summary (analysis ongoing; P. Demorest, M. Gonzalez, D. Nice, I. Stairs, S. Ransom, R. Ferdman)

Analysis features:

2 PSRs at ~40 ns!

Two independent calibration/processing pipelines.

DM(t) and timing model in single fit.

Fit includes systematic timing vs. frequency correction (profile shape evolution).

Best timing residuals versus time:

J1713+0747

J1909-3744

Slide: Paul Demorest

Dispersion measure variation with time

J1713+0747 With DM(t): 30 ns Without: 90 ns

J1909-3744 With DM(t): 40 ns Without: 440 ns

Slide: Paul Demorest

5-year timing residual GW cross-correlation analysis

Computed using methods from Demorest PhD (2007): Assumes/optimized for hc α ω-2/3 GW spectrum.

Preliminary limit: hc(1 yr-1) = 7 x 10-15 (at 95% confidence). Current work: Inject/characterize simulated GW signals. Paper should be submitted soon!

GUPPI coherent dedispersion of J1903+0327, DM of ~297.

Note increased scattering at low frequencies – this has to be measured and accounted for in the timing. Also plain old pulse profile evolution with frequency.

Monitoring with CHIME (350-850 MHz) could provide a handle on ISM variability.

Ransom/Demorest

Scattering tails in PSR B1937+21 at 430 MHz

Scintillation is a related phenomenon; monitoring one can provide information about the other.

CHIME can monitor changes in scattering and scintillation at low frequencies; the trick will be to correct the high-frequency timing data based on this.

Another opportunity based on CHIME: monitor timing variations and associated profile changes in young/slow (and millisecond?) pulsars.

A good example is PSR B1931+24, which in “on” for about a week, then off for a few weeks, and shows associated changes in spin-down rate (Kramer et al. 2006).

Lyne et al 2010: many young/slow pulsars show quasi-periodic oscillations in their timing noise. Probably not just stochastic pulse wandering…

The spin-down rates also show ~periodic changes.

Lyne et al 2010

Lyne et al 2010

And profile shape changes are identifiable for many of the same pulsars…

Lyne et al 2010

… and the profiles and timing change together, indicating large-scale switching in the magnetospheres. This phenomenon would never have been noticed without highly regular observations – something CHIME will also provide!

What about searching for new pulsars?

To use the full field available to CHIME (or similar telescopes) is basically prohibitive with current disk/tapes/clusters.

What makes more sense is to use tied-array beams to get time series from point sources and apply a standard pulsar search algorithm – this is the same strategy that will be employed (at least at first) on ASKAP and MeerKAT.

CHIME summary from the pulsar point of view:

•  Easy to get tied-array beam for pulsar observations •  Coherent-dedispersion backend is comparatively cheap •  Wide bandwidth •  Low frequency means ISM effects are larger and potentially easier to measure accurately •  But low-frequency data not optimal for direct inclusion in timing array data sets •  Daily monitoring (minutes to hours, depending on Dec.) of Northern pulsars means access to a wide range of science including timing noise/switching, glitch monitoring, planets(?)…