ASTR 472 · Lecture 12

Lecture 12: Radio Transients, Pulsar Surveys, and Selection Functions

Radio Astronomy

A survey detects a population filtered by cadence, beam, propagation, sensitivity, and threshold—not an unbiased census of sources.

Objectives and route

  1. Quantify completeness and binomial uncertainty in a detection sample.
  2. Explain how dispersion smearing and scattering reshape pulsar detectability.
  3. Separate a survey detection threshold from a physical population boundary.

Route: identify what the receiver measures, derive the physical/instrument relation, calculate a scale, then test its assumptions against a diagnostic.

Terms for This Lecture

Selection function

Definition: The probability that a source with specified properties enters the analyzed sample.

Why it matters here: It is needed to answer Two sky tiles each recover 84 of 100 injections, but one tile has twice the thermal noise. Why is the same aggregate completeness not evidence of equal selection functions? and to interpret A flux-limited survey samples more volume for brighter objects, while beaming and pulse broadening affect detectability.

Relationships: Use it with Injection-recovery test and Completeness; the lecture model is \(\hat p=\frac{k}{N},\qquad\sigma_{\hat p}\simeq\sqrt{\frac{\hat p(1-\hat p)}{N}}\).

Boundary or common confusion: Binomial errors assume independent trials with one common recovery probability; a heterogeneous sky requires a stratified completeness model.

Observation before interpretation

A survey detects a population filtered by cadence, beam, propagation, sensitivity, and threshold—not an unbiased census of sources.

Keep four layers distinct in your notebook: raw correlation or spectrum, calibrated product, model parameter, and physical claim.

Physical model

For N injected sources and k recovered under a defined pipeline, completeness p_hat=k/N. A first-order binomial standard error is sqrt[p_hat(1-p_hat)/N]. This uncertainty excludes model mismatch in the injected population, spatially varying sensitivity, and correlated failures; stratify by flux, width, sky position, and DM when possible.

\(\hat p=\frac{k}{N},\qquad\sigma_{\hat p}\simeq\sqrt{\frac{\hat p(1-\hat p)}{N}}\)

Define each quantity and convention before substitution. A relation is useful only inside its stated regime.

Worked calculation

Inputs: 84 recovered of 100 injected.

completeness = 0.84

Binomial errors assume independent trials with one common recovery probability; a heterogeneous sky requires a stratified completeness model.

Independent audit: carry SI units through the derivation, then check the limiting direction and scale.

Read the geometry or data relation

injected flux densityrecovery fractioncompleteness cut
A flux-limited survey samples more volume for brighter objects, while beaming and pulse broadening affect detectability.

Identify what is directly encoded in the figure and which inference requires an instrument model or physical assumption.

Where the inference can fail

Binomial errors assume independent trials with one common recovery probability; a heterogeneous sky requires a stratified completeness model.

Reasoning prompt

Two sky tiles each recover 84 of 100 injections, but one tile has twice the thermal noise. Why is the same aggregate completeness not evidence of equal selection functions?

Write the predicted scaling first; then identify which measurement would falsify it.

Connection to the observing workflow

This lecture feeds into the paired lab: Pulsar Dispersion and Timing Residuals. Preserve the raw-to-calibrated-to-inferred chain and carry the same units and assumptions into the analysis.

For a real observation, archive the project identifier, correlator setup, antenna configuration, calibration version, flags, beam, and data-release citation.

Synthesis

Radio Transients, Pulsar Surveys, and Selection Functions is one stage in a measurement chain: celestial signal → antenna and receiver → sampled data → calibration → image/spectrum → uncertainty-aware inference.

Next: connect this result to Galactic H I: Rotation, Geometry, and Mapping.

References and scope

ATNF Pulsar Catalogue and survey papers provide real selection context; use the named survey release and injection methodology when claiming completeness.

Primary/authoritative material and provenance levels are recorded in ../reference-log.md. Numeric examples are reproducibly calculated by ../src/generate_astr472_package.py; figure geometry is original and lecture-specific.