[Paper Review] The Plane's The Thing: The Case for Wide-Fast-Deep Coverage of the Galactic Plane and Bulge

Jay Strader, E. Aydi+22 more|arXiv (Cornell University)|Nov 29, 2018

Galaxies: Formation, Evolution, Phenomena3 references4 citations

TL;DR

This paper argues for extending the LSST's wide-fast-deep (WFD) cadence to cover the Galactic Plane and Bulge, which are currently undersampled in the baseline survey. By doing so, it enables critical discoveries in transient astrophysics—such as quiescent X-ray binaries, pre-explosion nova periods, and Galactic supernovae—demonstrating that WFD-like coverage increases detection of black hole and neutron star binaries by up to fourfold and enables measurement of fractional period changes as small as 10⁻⁵ in novae.

ABSTRACT

We argue that the exclusion of the Galactic Plane and Bulge from the uniform wide-fast-deep (WFD) LSST survey cadence is fundamentally inconsistent with two of the main science drivers of LSST: Mapping the Milky Way and Exploring the Transient Optical Sky. We outline the philosophical basis for this claim and then describe a number of important science goals that can only be addressed by WFD-like coverage of the Plane and Bulge.

Motivation & Objective

• To argue that excluding the Galactic Plane and Bulge from the LSST's WFD cadence is inconsistent with core science drivers: mapping the Milky Way and exploring the transient optical sky.
• To demonstrate that WFD-like coverage of the Plane and Bulge is essential for detecting and characterizing quiescent X-ray binaries, novae, and supernovae.
• To show that current baseline cadence limits (3 visits/year per filter) are insufficient for epoch-level science like orbital period recovery and period change measurement.
• To advocate for a homogeneous WFD cadence across the Plane and Bulge, including full filter set and 2 visits/night, to ensure data consistency and maximize scientific return.
• To emphasize that the scientific value of WFD coverage in the Plane/Bulge outweighs concerns about crowding and extinction, especially given the high stellar mass and transient population density there.

Proposed method

• Proposes extending the LSST WFD cadence to include the Galactic Plane and Bulge, defined as regions within 10° of the Galactic Center and extending to l = 90° and 270°.
• Uses simulated cadences (e.g., minion_1016 and astro_sim_01_1004) to compare baseline and WFD-like coverage for period recovery and transient detection.
• Applies metrics such as median and maximum inter-night gaps, total visits per filter, and visit frequency to evaluate cadence performance for transient and variable source detection.
• Evaluates scientific impact using figures of merit: number of recovered orbital periods for X-ray binaries and measurable fractional period changes in novae.
• Relies on existing special proposals for the Plane/Bulge to avoid additional data processing burden, assuming high-density region challenges are already addressed.
• Argues for a homogeneous filter set (including u and g) across the Plane/Bulge to enable precise 3D reddening mapping and support bluer-filter-dependent science.

Experimental results

Research questions

• RQ1 Can WFD-like coverage of the Galactic Plane and Bulge enable the recovery of orbital periods for quiescent X-ray binaries down to r ~ 23?
• RQ2 Is it possible to measure fractional period changes of order 10⁻⁵ in novae using the baseline cadence, or is WFD-like cadence required?
• RQ3 What is the probability of detecting a Galactic supernova with pre-explosion variability using the current baseline cadence versus a WFD-like cadence in the Plane and Bulge?
• RQ4 How many quiescent black hole and neutron star X-ray binaries would be discovered under WFD-like coverage compared to the baseline cadence?
• RQ5 Does the inclusion of u and g filters in the Plane/Bulge improve the accuracy of 3D reddening maps and support broader astrophysical science?

Key findings

• Extending WFD coverage to the Galactic Plane and Bulge would allow recovery of orbital periods for nearly all well-behaved quiescent X-ray binaries down to r ~ 23, compared to only ~25% recovery under the baseline cadence.
• The number of quiescent black hole binaries detected would increase by a factor of four under WFD-like coverage, as 73% of known Galactic black hole candidates are located in the excluded region.
• WFD-like cadence enables measurement of fractional period changes as small as 10⁻⁵ in novae, which is critical for testing whether accreting white dwarfs can become Type Ia supernovae, but is not feasible with the baseline cadence.
• The baseline cadence provides only 3 visits per year per filter in the Plane/Bulge, insufficient for epoch-level science such as period recovery and proper motion measurements.
• The inclusion of u and g filters in the Plane/Bulge enables precise 3D reddening mapping, which benefits all Plane/Bulge science, including transient and variable source detection.
• The scientific return from WFD-like coverage of the Plane/Bulge—especially for X-ray binaries, novae, and supernovae—far outweighs concerns about crowding and extinction, justifying a homogeneous cadence and filter set.