Einstein Probe Mission Call for Observing Proposals (Cycle-1)

Proposer's Guidance

-- 1st Cycle 


Proposal Submission Deadline:
Wednesday, December 20th 2023, 6:00 PM UTC+8 (Beijing)


      Letter of Invitation from EP Science Center for Cycle-1 (pdf)

  1. EP Mission Overview
  2. General Information
  3. Requirements of Proposal Submission
  4. Cycle-1 Timetable
  5. Supporting documents for proposal submission
  6. Supporting tools for proposal submission
  7. User support at EP Science Center


  1. EP Mission Overview

Einstein Probe (EP) is an upcoming astronomical space mission led by the Chinese Academy of Sciences (CAS) and developed in collaboration with the European Space Agency (ESA), the Max Planck Institute for extraterrestrial Physics (MPE), and the French Space Agency (CNES). The primary goals of EP are to discover high-energy transients and monitor variable objects in the soft X-ray band, and to characterize them with quick X-ray follow-up observations.

To achieve this, EP employs a large instantaneous field-of-view of 3600 deg2 with its 12 identical modules of lobster-eye Wide-field X-ray Telescopes (WXTs). WXT has a moderate angular resolution of FWHM ~ 5 arcmin and positioning accuracy of 1 arcmin (90%) in the 0.5 - 4 keV energy band. EP also has the capability of performing fast and deep follow-up observations in the 0.3 - 10 keV energy band with its two units of Follow-up X-ray Telescopes (FXTs). The total effective area is about 2 x 300 cm2 @ 1 keV. The angular resolution is HPD ≤ 30 arcsec, and the positioning accuracy is 8.6 arcsec (90%). The EP satellite can conduct quick downlink of transient alert messages, as well as quick uplink of urgent Target-of-Opportunity (ToO) observation commands. The satellite will operate in a low-Earth orbit with a period of 97 minutes. Please refer to the “EP Technical Handbook” for more details.

  1. General Information

This is an announcement for EP observing proposals during the first year of EP scientific operations (Cycle-1).

The EP launch is currently scheduled at the end of 2023. Once in orbit EP will conduct a series of tests, performance verification and instrument calibration during the first 6 months, after which scientific operations will be started.

The science operational time is composed of two parts, the Regular Operational Time (ROT) and Targets of Opportunity (ToO). The ROT observations further comprise Survey Mode observations with WXT and Follow-up observation with FXT. The WXT survey is the basic operational mode of EP designed to monitor the sky to detect transients and monitor variable sources. EP will operate in this mode unless a follow-up observation or ToO observation is triggered. During the survey mode, WXT will monitor pre-planned sky regions, whilst FXT will be normally pointed to pre-selected targets to perform pointed observations, termed FXT Survey Target Observations (FSTO). ToO observations comprise two kinds, regular ToOs requested when observing opportunities arise any time during EP operations and pre-approved anticipated observations of known targes (anticipated-ToO). FSTO and anticipated-ToO targets are based on projects proposed by the EP Science Topical Panels (STPs) and guest observers, endorsed by the EP Science Management Committee (SMC).

2.1 Who can propose:

For this first Cycle, only the following two types of prospective EP users can submit observing proposals as principal investigator (PI).

1) STP observer: EP STP members and associate members.

2) Guest observer (GO): non-STP users whose primary affiliations are Chinese institutions (including Hongkong, Macao and Taiwan).

2.2 Types of proposals available for Cycle-1:

There are two types of observing proposals available in Cycle-1.

1) FSTO observations (non-ToO observations) performed by using FXT.

2) Anticipated-ToO for known X-ray sources, including observations of known sources triggered by external facilities or EP-WXT (triggered onboard or on-ground). For WXT triggering, the proposed threshold should be at or above the detection limit for one typical survey pointing (~1 ks), which is about (2-3)×10-11 erg cm-2 s-1.

  • Concerning anticipated-ToO proposals, the total time to be allocated is less stringent and less accurate to predict as certain source characteristics, probabilities and criteria need to be fulfilled before an observation can commence. Effective observation times for anticipated-ToOs are calculated using probability factors for the occurrence of an event. Therefore, in case of an anticipated-ToO proposal, the proposers are obliged to provide an estimate of the triggering probability for their proposed target.

2.3 Properties of Cycle-1 proposals:

  1. This cycle accepts proposals using FXT to observe known targets only. All EP observations must be pointed observations.
  2. Time available in Cycle-1:
  • It is anticipated that up to approximately 5M seconds of observing time will be available for Cycle-1. Among them, 0.5M, 0.5M and 0.25M seconds will be allocated to the ESA, MPE and CNES teams, respectively, and the remaining to the CAS team.
  • In addition, the CAS team allocates 5% of its own time as GO, so the time for GO is up to 0.2M seconds.
  • In order to increase the flexibility of observation scheduling and considering the trigger probability of anticipated-ToO, the total exposure time approved may exceed the total time that can eventually be scheduled. Therefore, it is not guaranteed that all approved targets can be observed in Cycle-1.
  • Unscheduled accepted proposals will not automatically be carried over to the next observing year.
  1. Exposure constraints: the requested observing time for a single observation must be limited to between 1 ks and 100 ks.
  2. Overheads: due to the Earth obscuration, the estimated average continuous observing time per EP orbital cycle is ~3.6 ks, which also varies with the pointing direction. In addition, the SAA passage time varies from orbital cycle to another, ranging from 0 to over 10 minutes, resulting in an average loss of good observing time of ~6 min per orbit. Real-time SAA passage time can be queried on the short-term visibility-check page provided by EPSC after the launch of EP.
  3. Monitoring program: proposers may request multiple observations on a target with a specific cadence to monitor its long-term X-ray variation. The maximal number of observations per target per proposal is limited to 100. Please note that high-cadence monitoring observations of the same targets may affect the uniformity and efficiency of WXT all-sky survey, and are generally difficult to schedule. Therefore, such programs are generally discouraged unless having significantly strong scientific motivations.
  4. Targets of observing programs without time constraints are easier to be scheduled for observations than those with constraints.
  5. Tilling program: proposers may request a series of observations to cover a specific sky region with FXT (FoV~1x1 deg2). The maximal number of tills per proposal is limited to 100.
  6. Review of proposals: proposals are subject to peer review organized by each of the Parties (CAS, MPE, ESA and CNES) separately.

2.4 What are not included in Cycle-1:

  • Cycle-1 does not include scientific investigations closely related to the core science programs of the EP mission, such as the detection of transients and monitoring source variability using EP-WXT, and EP search and observation of multi-messenger sources with both WXT and FXT.
  • This cycle does not accept proposals using WXT. If proposers have special requirements for WXT observations, please directly contact EPSC to enquire the possibility.
  • This cycle does not accept proposals to observe new X-ray transients discovered by WXT or FXT.
  • This cycle does not accept proposals related to the search and follow-up observations of electromagnetic counterparts of multi-messenger events (e.g. gravitational wave events, neutrino events).
  • This cycle does not accept proposals related to observations of LHAASO UHE gamma-ray and cosmic ray sources with EP. This investigation is under the collaboration program between the LHAASO and EP CAS team.
  • Non-anticipated ToO proposals are to be submitted as regular ToO or Director's Discretionary Time (DDT) proposals during the EP science operation phase (to be open later).

2.5 Summary table of types of proposals:

Type of Proposals

Included in


Proposers in


Allowed Targets in Cycle-1

Type of Programs



STP observers,

Guest observers*

known sources

single obs, monitoring, tilling



known X-ray sources

*Primary affiliations are Chinese institutions (including Hongkong, Macao and Taiwan).

See the text above for detailed explanations and constraints.


  1. Requirements of Proposal Submission

All proposers are advised to read the following content carefully. Only complete and successfully submitted proposals will receive full consideration.

Please note that the proposal process for ESA, MPE and CNES may differ in parts from the described approach below. Additional information will be provided within each Party by the representatives.

3.1 User registration:

Every proposer should first create a user account on the EP website (https://ep.bao.ac.cn/ep/user/login). Please register with your real personal information such as name, email address and affiliation. After the approval of system administrator, you may log into the EP website and access the EP Observing Proposal System (EOPS). Your role in the EP science team will be recognized automatically. Proposers with different roles will be directed to different proposal submission interfaces.

    1. note-1: this registration process is once for all. If you have registered a user account, it can be used for future proposal submissions.
    2. note-2: by logging into the user account, users can access EOPS, where they can check the list of their previous proposals (including the status and content of each proposal), submit new proposals (including FSTO for STP members, GO for non-STP users), and download the relevant observational datasets once available.
    3. note-3: it will take some time (normally one working day) for the system administrator to approve a new registration application. So please register early before the proposal submission deadline.
    4. Access EOPS: https://ep.bao.ac.cn/ep/proposal_submit/user_proposal_create_guide

3.2 Proposal submission:

The deadline for EP Cycle-1 proposal submission is Wednesday, December 20th 2023, 6:00 PM UTC+8 (Beijing).

All EP observing proposals must be submitted via EOPS. After logging into the system, you may start creating new proposals, filling out online forms and uploading scientific justification file. Only complete proposals can be submitted (see the required items below).

There will be a countdown clock on EOPS. The submission button will be invalid after the deadline. It is possible that the system may be overloaded and react slowly close to the deadline. Thus to avoid last-minute problems, please get familiar with EOPS early, prepare and submit your proposals early.

3.3 Required items for a complete proposal:

All complete proposals submitted to Cycle-1 should consist and only consist of the following items.

    1. General forms: to be filled out online, including general proposal information such as title, abstract, category, type PI and Co-I contact information, etc.
    2. Observation forms: to be filled out online, including information related to the proposed targets and observations, such as source name, coordinates, exposure time, observing mode, instrument configuration and time constraints for every proposed exposure.
    3. Scientific justification: proposers are requested to use the EP scientific justification template released along with the other documents of Cycle-1. After filling out every section, it must be converted to a single PDF file to be uploaded to EOPS. There is a 4-page limit for the justification file, including all materials, i.e. texts, figures, tables and references. Longer proposals may be considered as invalid by the system.
  1. Cycle-1 Timetable

The current launch date of EP is the end of 2023. Once operating in orbit, EP will first perform a series of instrumental tests, calibration and performance-verification (PV) observations, which may last approximately six months. After that, the approved Cycle-1 programs will start to be scheduled and conducted. The default duration of Cycle-1 observing period is one year.

Formal Announcement of Cycle-1

October 16th 2023

Proposal Submission Due Date

December 20th 2023

Announcement of SMC-Approved Programs

Approx. March 2024

Start of Cycle-1 Observations

Approx. June 2024


  1. Supporting documents for proposal submission

The following documents can be downloaded from the Cycle-1 website.

The proposal must not exceed 4 pages and must be submitted as a PDF file. Proposers are urged to use the Latex template or MS word template provided above. A PDF file can be generated from the Latex file with the following command:

> pdflatex ep_scijust_template.tex

A font size of 12pt and a single-column format is strongly recommended. The 4-page limit includes all materials, i.e. texts, figures, tables and references. Proposers must make sure that the PDF generated is complete both when displayed with a viewer and when printed on paper.

  • EP Technical Handbook (pdf)

This document includes more detailed technical information about the EP mission, instruments and operation.

  1. Supporting tools for proposal submission

  • EP Long-term Visibility Check Tool
    Check EP long-term visibility[1] for specific source coordinates in the sky.

Link: https://ep.bao.ac.cn/ep/proposal_submit/target_visibility_tool

  • WXT Online Simulator
    Simulate point-like X-ray sources to be observed by WXT with different exposure times and sky background. The simulator will produce various simulation products, such as source count rate, detection significance, spectrum, light curve, and pseudo WXT datasets.

Link: https://ep.bao.ac.cn/ep/simulator

  • FXT Online Simulator
    Simulate point-like X-ray sources to be observed by FXT with different parameters such as exposure time, observing mode and filter. The simulator will produce various simulation products, such as source count rate, detection significance, spectrum, light curve, and pseudo-FXT datasets.

Link: http://epfxt.ihep.ac.cn/simulation/obs_sim

  • FXT Technical Evaluation Tool
    Perform technical evaluation (photon pile-up, optical loading) for the input source parameters and FXT configuration (i.e. different combinations of window modes and filters). The result will also include the recommended FXT configuration for the input source parameters.

Link: http://epfxt.ihep.ac.cn/simulation/filter_mode_eval

  • FXT Exposure Time Calculator
    For the input source parameters, calculate the exposure time required to achieve a certain signal-to-noise ratio or the signal-to-noise ratio that can be achieved with a certain exposure time.

Link: http://epfxt.ihep.ac.cn/simulation/expo_time_est

  1. User support at EP Science Center

  • Helpdesk: if you have any questions or difficulties submitting your proposal, or technical questions related to EP instruments, data and software, please submit them via EP helpdesk under appropriate categories:
    EP Helpdesk:
  • Email us: if you have more general questions, comments and suggestions related to e.g. EP’s scientific operation and performance, you may also send emails directly to EPSC via: ep@bao.ac.cn

[1] The short-tearm visibility check tool will be put online after the launch of EP.