ToO Proposals

1. Definition

Target of Opportunity (ToO) proposals are for observations of transient and/or rare phenomena with specific or unspecific objects based on clear observational strategy, such as nearby supernovae, Gamma-Ray Burst follow-up, etc.

2. Categories

ToO proposals are classified according to two independent criteria:

Response Timescale
Expected Trigger Rate

1. Response Timescale

Non-rapid ToO

Non-rapid ToO programs do not require immediate attention and can be executed 24 hours or more after the trigger.

Requirements
- Observation requests must be submitted by 9:00 AM (Hawaii Standard Time) on the day before the observing night.
- Suitable for predictable or less time-critical transient events.

Rapid ToO

Rapid ToO programs require immediate attention and must be executed within 24 hours of the trigger.

Requirements
- Observation requests may be submitted at any time within the 24-hour response window.
- Because proposal approval and operational processes require some time, PIs are strongly encouraged to submit trigger requests several hours before the desired observation time.
- Proposals that may require both rapid and non-rapid triggers should be submitted as Rapid ToO proposals.

2. Expected Trigger Rate

Normal ToO

ToO observations that do not qualify as rare ToO programs based on the expected trigger rate defined below are categorized as normal ToO programs.

Rare ToO

PIs may apply for a Rare ToO program if the proposed observations target phenomena with an estimated occurrence rate of:

Fewer than 10 events per sky per year at Subaru depth

This threshold is intended to correspond to:

Fewer than 0.5 expected triggers per semester

The threshold may be revised in future semesters based on operational experience and actual trigger statistics.

Eligibility

Available only for programs using Subaru instruments exclusively. Once approved, a Rare ToO proposal remains valid until the first of the following occurs.
- The approved number of observing nights has been triggered, or
- Three years have elapsed from approval,
Re-application Policy

Re-application during the three-year validity period is permitted. If the same team receives approval for a substantially similar proposal, the previous approval becomes invalid.
Justification Requirements

PIs must:
- Provide an occurrence-rate estimate in the Scientific Justification.
- Explain and justify the estimate in the Technical Justification.
The occurrence rate and its justification will be reviewed by referees. If the referees determine that the proposal does not satisfy the Rare ToO criteria, it will be reclassified as a standard ToO proposal.
Estimating the Occurrence Rate

The occurrence-rate estimate should include all relevant sample-selection effects and filtering criteria.
- Example 1: Transient A
  
  Assume a volumetric occurrence rate of \(10^{-4}\,\mathrm{yr}^{-1}\,\mathrm{Mpc}^{-3}\)
  
  Case A. [ Nearby Events ] Targeting events within 25 Mpc:
```
Estimated occurrence rate: 8 events per sky per year
```
  Case B. [ Rare Subclass ] Targeting a subclass representing 0.1% of all Transient A events within 100 Mpc:
```
Estimated occurrence rate: 0.4 events per sky per year
```
- Example 2: Transient B
  
  Assume another survey detects 15 events per sky per year.
  
  Case A. [ Well-localized Events ] Targeting events with sufficiently narrow localization regions, representing approximately 50% of detections:
```
Estimated occurrence rate: 7.5 events per sky per year
```
  Case B. [ Distant Events ] Targeting distant events representing approximately 10% of detections:
```
Estimated occurrence rate: 1.5 events per sky per year
```

3. Proposal and time allocation

ToO proposals should be submitted in the same manner as Normal or Intensive Program proposals, while clearly specifying which instruments will be used to perform the ToO observations and how much observing time will be required.

Applicants for ToO proposals are expected to conduct the observations with their own teams, including remote observing when applicable. While the participation of Observatory staff is not required, applicants are strongly encouraged to include one or more Observatory staff members as Co-Investigators to facilitate the smooth and efficient execution of the observations.

Coordination Among Accepted ToO Programs

If multiple approved ToO proposals from different groups or communities target similar events and pursue similar scientific goals, the ToO Coordinators from TAC encourage coordination among the affected teams immediately after proposal acceptance. If coordination cannot be achieved, the Observatory will determine the observing priority among the competing programs.

4. Gemini ToO Observations

Subaru users who are eligible for Subaru-Gemini Time Exchange programs may also submit ToO proposals through the time-exchange program with Gemini. Approved observations will be carried out in queue mode. Please note that Rare ToO proposals are not eligible for observations using Gemini instruments. For further information, please refer to Subaru/Gemini page.

5. Execution of ToO

When a ToO observation request is triggered, the Director will determine whether the scheduled observations should be interrupted to accommodate the ToO observations.

ToO observations are generally conducted in half-night increments. An exception is made for HSC Queue mode observations, for which ToO observations can be undertaken in hourly increments. In such cases, the minimum observation duration is 2 hours. The PI or designated observers are expected to participate in the ToO observations.

Telescope time lost by regularly scheduled observers as a result of ToO interruptions will be compensated by the observatory.

For details of the ToO procedures, please refer to the ToO procedure pages (English/Japanese).

updated on August 5, 2026