Purpose

tilepy concept

The growing number and improving sensitivity of gravitational-wave detectors allow us to probe deeper into the universe, increasing the observed volume by a factor proportional to the cube of the detection horizon (see Observing Capabilities). As a result, the detection rate of compact binary coalescences (BBH, NSBH, and BNS) by the international gravitational-wave network (IGWN) continues to rise.

However, gravitational-wave localization regions often remain large, ranging from several hundred to a few thousand square degrees [1], [2]. This poses a major challenge for identifying potential electromagnetic counterparts such as short gamma-ray bursts (GRBs) and kilonovae, which are fast-evolving and require rapid response.

tilepy addresses this challenge by generating optimized observation plans, starting from IGWN-provided skymaps and incorporating realistic telescope-specific constraints. It currently includes ground-based facilities such as H.E.S.S., LST, and both CTA-South and CTA-North.

It accounts for constraints and background noise like:

Current

Backgrounds:

• Milky Way dust extinction

• Galactic diffuse ultraviolet background

Constraints:

• Airmass and altitude limits

• Twilight phase or solar altitude conditions

• Sun and Moon exclusion angles

Planned

Backgrounds:

• Sky background from zodiacal light (sunlight scattered by interplanetary dust)

Constraints:

• Dynamic constraints such as telescope slew time

Telescope-specific optimization

These constraints are customized per telescope, enabling more focused and efficient tiling of the sky. This improves the chances of capturing the early light from fast transients and maximizes the scientific return of follow-up observations.

Note

For a full description of tilepy, please see [3].

How far can GW counterparts be difficult to follow-up and rare to detect?

The first phase of the fourth observing run (O4a) of the LIGO-Virgo-KAGRA Collaboration took place from May 24, 2023, to January 16, 2024. During this period, only the two LIGO detectors, the LIGO Hanford Observatory (LHO, H1) and the LIGO Livingston Observatory (LLO, L1), were operational. These detectors identified 87 gravitational wave (GW) candidate events with false alarm rates (FAR) below 1 \(\mathrm{yr}^{-1}\) for which detailed source property estimates have been provided. Based on the inferred component masses, these candidates are consistent with mergers of binary black holes (BBH) and neutron star–black hole (NSBH) systems. Combined, this brings the total number of compact binary coalescences (CBCs) observed since the first observing run (O1) to 159. Notably, no binary neutron star (BNS) mergers were detected during O4a.

Motivation

Those kinds of difficulties motivated the implementation of tilepy to optimize GRBs, early ultraviolet (UV) and optical follow-up of gravitational wave events.

References

[1]

Polina Petrov, Leo P. Singer, Michael W. Coughlin, et al. Data-driven Expectations for Electromagnetic Counterpart Searches Based on LIGO/Virgo Public Alerts. \apj , 924(2):54, January 2022. arXiv:2108.07277, doi:10.3847/1538-4357/ac366d.

[2]

R. Weizmann Kiendrebeogo, Amanda M. Farah, Emily M. Foley, et al. Updated Observing Scenarios and Multimessenger Implications for the International Gravitational-wave Networks O4 and O5. \apj , 958(2):158, December 2023. arXiv:2306.09234, doi:10.3847/1538-4357/acfcb1.

[3]

Monica Seglar-Arroyo, Halim Ashkar, Mathieu de Bony de Lavergne, et al. Cross observatory coordination with tilepy: a novel tool for observations of multimessenger transient events. The Astrophysical Journal Supplement Series, 274(1):1, aug 2024. URL: https://dx.doi.org/10.3847/1538-4365/ad5bde, doi:10.3847/1538-4365/ad5bde.