In November 2023 the CXC issued the Chandra Legacy Program (CLP) call for white papers to identify science challenges for which the capabilities of Chandra are absolutely required. These science programs were to be designed to address fundamental questions about our current understanding of the components and evolution of the Universe. A committee of seven (non-CXC) members of the astrophysics community reviewed the twenty-two white papers that were submitted. After the CLP Review Committee completed its evaluation of the white papers, recommendations for major science initiatives were presented to the CXC Director. These recommendations have been used to form the themes of this Call for Legacy Proposals.

Chandra Legacy Program Major Initiatives

The CLP major initiatives identified through the white paper call are:

Initiative I: Probing the Physics of Baryon Cycles and Feedback using Deep Observations of Nearby Galaxies

Deep X-ray observations of nearby galaxies offer unique opportunities to detect and characterize the spatial distribution of elusive hot X-ray-emitting gas in and around galaxies. Chandra's superb angular resolution is essential for resolving X-ray binary populations and mapping the internal structures of the X-ray emitting circumgalactic medium (CGM) in and around galaxies. We seek an ambitious and compelling science proposal that can be uniquely enabled by high-resolution X-ray maps of a well-chosen sample of relatively nearby galaxies coupled with high resolution multi-wavelength coverage, within a CLP exposure time of ~3 Ms. Previous Chandra exposures should be considered when constructing a proposed observing program. A successful proposal must articulate the legacy values of the Chandra observations, its complementarity and synergy with multiwavelength observations, and why the proposed science is not possible as part of the General Observer (GO) program.

Probing the cycling of baryons in galaxies is critical to building a complete picture of galaxy assembly and growth. However, even at low redshift, key unknowns persist that Chandra can address via observations of the hot gas phase. Examples might include:

  1. providing an X-ray view of stellar population evolution;
  2. measuring how stellar evolution and AGN feedback circulate energy and metals;
  3. determining the structure of the multiphase ISM/CGM;
  4. establishing the influence of mergers of galaxies and of their super massive black holes (SMBHs).

The proposed Chandra legacy observations and resulting data products should leverage existing and/or proposed high spatial-resolution multi-wavelength coverage of nearby galaxies to provide a fundamental and enduring anchor point for understanding the evolution of the baryon cycle over cosmic time.

Initiative II: Deep Observation of a Galaxy Cluster to Understand Key Physical Processes

Deep X-ray observations of galaxy clusters offer a unique opportunity to study the physics of galaxy formation, plasma physics, and cosmology. Chandra's superb angular resolution is essential for providing detailed physical insights into the roles of AGN feedback in clusters, the microphysics of astrophysical plasma and the nature of dark matter using merging clusters, and the cosmological growth of structures in cluster outskirts. We seek an ambitious and compelling science proposal that can be uniquely enabled by a deep (~3 Msec) Chandra observation of an appropriately selected galaxy cluster. A successful proposal must articulate how the proposed observations and data products will open up new discovery space or significantly advance the state-of-the-art astrophysical constraints.

Potential science objectives for this theme include understanding the heating mechanisms of AGN feedback and cooling-heating balance in cluster cores, which might be achieved by:

  1. determining the equation-of-state of the intra-cluster medium (ICM);
  2. distinguishing among physical mechanisms for heating of the ICM;
  3. measuring the electron mean free path;
  4. testing the precipitation model in a multiphase cool core.

  5. Additional important details can be found in the full Call for CLP Proposals.

Important Dates

Call for CLP ProposalsFebruary 21, 2024
Chandra Cycle 26 Proposals DueMarch 14, 2024
CLP Letters of Intent DueMarch 28, 2024
CLP Proposals DueApril 15, 2024
CLP Review Results AnnouncedMay 15, 2024



The nominal Chandra time available for this initiative is 6 Ms, which will come from a combination of the Director's Discretionary Time program, the Guaranteed Time Observer program, and the General Observer program.

Nominal Exposure Time6 Ms
Maximum HRC Time500 ks
Time at ecliptic latitudes |b| > 55°1.2 Ms

Sources for CLP observing time of 6 Ms (totals over 2 Cycles)

GO4 Ms (~14%)
GTO1 Ms (~19%)
DDT1 Ms (~50%)

Joint Partner Observatories (JPOs)

For information about JPOs, please refer to the Cycle 26 Chandra Call for Proposals.

The following JPO allocations are potentially available for this CLP

HST42 orbits
JWST42 hrs
XMM280 ks
NuSTAR280 ks
Swift140 ks
NOIRLab1.4% (in nights)
AUI0.84% (in nights)

Formatting Guidelines

Letter of Intent Content

A preliminary Letter of Intent is required and should be submitted to by 6pm EDT 28 March 2024. The Letter of Intent is a PDF document that contains the following:

Proposal Formatting Rules

Page Limits

Each side of a sheet containing text or illustration will count as one page

Science Justification6 pages
References1 page

For more information, refer to the Call for Proposals

Dual-Anonymous Peer Review Guidance

Proposals should be written in anonymized format suitable for Dual Anonymous Peer Review (DAPR).

For more information, see NASA's DAPR Guidelines and Chandra Peer Review Guidance on avoiding DAPR violations


If you have any questions, please contact the CXC Helpdesk either through the web interface or by email to