Cycle 27 Peer Review

Rodolfo Montez Jr. and the Chandra Director’s Office

This year's annual Call for Proposals, released on December 19, 2024, featured significant changes to the review procedure. Instead of holding a review of up to 100 reviewers in 13 topical panels, all proposals were subject to a distributed peer review. In Chandra's distributed peer review (DPR), all proposing teams were asked to nominate a designated reviewer (DR) who would be responsible for reviewing up to 16 proposals via the CXC's new Chandra Distributed Review Software (CDRS). TOO, Large, and Very Large proposals underwent panel review in two TOO panels and one big project panel, in addition to the DPR review. As a result, 233 DRs participated in the review and 24 panelists in the panel review. A total of 299 proposals were submitted to the Cycle 27 CfP requesting over 66 Ms of observing time and resulting in an oversubscription factor of 5x by time. One additional change to the Call for Proposals was the lack of Archive and Theory proposals in the solicitation, which typically contribute 75–100 additional proposals in any given cycle.

General Cycle 27 Proposal Statistics

Cycle 27 proposal statistics can be found in Figures 1–3 and on the CXC website.

The total amount of time allocated in Cycle 27 was 14.6 Ms. Of that, 3.4 Ms were awarded to 7 LPs and 2 Ms were awarded to 2 Very Large Proposals (VLPs), a category of proposal requiring at least 1 Ms of observing time.

The Cycle 28 Call for Proposals (CfP) will be released on December 18, 2025 and the proposal deadline is March 18, 2026.

A line plot showing seven data trends. The X-Axis shows Chandra Proposal Cycle, from 1 to 27. The Y-Axis shows Number of proposals and has a break; the bottom of the axis is linear from 0 to just over 80, while the top of the axis starts at 300 and proceeds linearly to 900. The seven data sets and their colors are GO/TOO in blue, Theory in purple, Archive in red, LP in orange, XVP in brown, VLP in green, and Total in a thicker black line. GO/TOO and Total are the only data points in the top part of the axis; they start at around 800 in Cycle 1, slowly declining to around 300 in Cycle 27. On the bottom, Archive quickly rises from Cycle 2, reaching a steady value of around 80 by Cycle 5. LP, starting in Cycle 2, and Theory, starting in Cycle 4, remain relatively consistent throughout, at roughly 60 and roughly 40 proposals per cycle, respectively. Archive and Theory are not included in Cycle 27, leading to a drop in the total count. VLPs decline from their introduction in Cycle 5 to only a few in Cycle 12; in Cycle 13, there are no VLPs, only XVPs, which remain at roughly 20, with a gradual decline over the four cycles they are present. VLPs reappear in Cycle 19, staying constant at around 10 per year.


Figure 1: The number of proposals submitted in each proposal category (e.g., GO, LP, Archive) as a function of cycle. The y-axis is broken to indicate the entire range. Since more proposal categories have become available in each cycle, the number classified as GO has decreased as others increased, while Cycle 27 did not include Archive or Theory proposals.

A line plot showing seven data trends. The X-Axis shows Chandra Proposal Cycle, from 1 to 27. The Y-Axis shows Oversubscription Ratio and stretches from 0 to just over 16. The seven data sets and their colors are GO/TOO in blue, Theory in purple, Archive in red, LP in orange, XVP in brown, VLP in green, and Total in a thicker black line. All are relatively constant, with LP and VLP showing the most extreme variability. Oversubscription rates are generally between 2 and 6, with the exception of VLPs, which are between 8 and 16.


Figure 2: The effective oversubscription ratio (in terms of observing time or money available) for each proposal category as a function of cycle. Total oversubscription ratio is shown as the black solid line. Note that some of the fluctuations are due to small number statistics (e.g., Theory proposals).

A line plot showing two data trends. The X-Axis shows Chandra Proposal Cycle, from 1 to 27. The Y-Axis is labeled Time, in units of Megaseconds, and stretches from 0 to just over 140. There are two colored data sets; these are labeled 'Requested' in blue and 'Approved' in orange. Aside from a spike to 140 Ms in Cycles 13 and 14, the requested line stays roughly between 70 and 100 Ms over the entire span. The approved line is fairly consistent over the entire Figure, other than a brief rise in the first few cycles and a slight bump in Cycles 13 and 14, hovering around 20 Ms throughout.


Figure 3: The requested and approved time as a function of cycle in Ms including allowance for the probability of triggering each TOO. The available time increased both over the first three cycles and in Cycle 5 with the introduction of Very Large Projects (VLPs). A clear spike in the requested time is seen in Cycles 13–16 in response to the calls for X-ray Visionary Projects (XVPs). Additional increases in awarded time are due to the increases in observing efficiency.