Cycle 25 Peer Review

Rodolfo Montez Jr. and the Chandra Director’s Office

This past June, 94 reviewers met online in 11 Topical Panels, 2 Target of Opportunity (TOO) Panels, and one Big Project Panel to discuss the 408 proposals submitted to Chandra's 25th call for proposals. The dual-anonymous Cycle 25 remote peer review was held June 20–29, 2023. The topical and TOO panels met for the first week (June 20–23) and the Big Project Panel the following week (June 26–29).

General Cycle 25 Proposal Statistics

Cycle 25 proposal statistics can be found in Figures 1–4 and on the CXC website.

The total amount of time allocated in Cycle 25 was 17.0 Ms. Of that, 3 Ms were awarded to 8 LPs and 2.1 Ms were awarded to a Very Large Proposal (VLP), a category of proposal requiring at least 1 Ms of observing time.

The Cycle 26 Call for Proposals (CfP) will be released on December 14, 2023 and the proposal deadline is March 14, 2024.

Target of Opportunity Proposals

In addition to the usual topical panels—on stars, supernovae and supernovae remnants, binaries, galaxy clusters, and AGN—this peer review hosted two target of opportunity (TOO) panels. TOO proposals request timely observations of variable phenomena (targets may be known or unknown at the time of proposing), which are triggered by criteria described in the proposal. Proposers estimate the probability of the proposal trigger and specify the response time required to secure the observations. Since TOO observations can be quite disruptive to spacecraft operations and the completion of other scheduled observations, the number of TOO triggers is limited based on the response time, with very fast triggers (requiring less than 5 day response time) limited to only 10 per cycle. In previous peer reviews, the TOO proposals were evaluated by the topical panels. From Cycle 25 onward, the TOO proposals will be evaluated together in TOO-specific panels. As a result, the TOO panels consider a broader range of scientific topics (see Figure 5), and proposers were asked to keep that in mind when writing their scientific justifications.

At the peer review, panels can award up to 10 very fast triggers (a response time from zero to less than 5 days), 20 fast triggers (from 5 to less than 20 days), 30 medium triggers (from 20 to less than 40 days), and 40 slow triggers (greater than or equal to 40 day response time). As shown in Figure 6, the number of very fast triggers had a similar demand to the previous three cycles, whereas the fast, medium, and slow triggers had a slight decline in the number of requests. The number of awarded triggers is comparable to the previous three cycles except for the slow category, which had fewer requests and therefore fewer awards.

A line plot showing seven data trends. The X-Axis shows Chandra Proposal Cycle, from 1 to 25. 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 400 (Total) and 300 (GO/TOO) in Cycle 24. 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. 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.

A line plot showing seven data trends. The X-Axis shows Chandra Proposal Cycle, from 1 to 25. 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). In Cycle 20, none of the submitted VLP proposals were selected by the peer review.

A line plot showing two data trends. The X-Axis shows Chandra Proposal Cycle, from 1 to 25. 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 ks 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.

A line plot showing three data trends. The X-Axis shows Chandra Proposal Cycle, from 1 to 25. The left Y-Axis is labeled Proposal Success Rate, and stretches from 0.0 to 0.5, and the right Y-Axis is Fraction of All Submitted Proposals, stretching from 0.0 to 1.0. There are two colored data sets, having vertical error bars of size roughly 0.1 on the left axis; these are labeled


Figure 4: The success rate of male (orange) and female (green) PIs as a function of cycle, and the overall fraction of female PIs (gray). Since Cycle 10, the success rate for female and male PIs has been statistically indistinguishable.

Two donut charts, or pie charts with the center removed so they show annular wedges instead of circular wedges. The top is in shades of black and gray and the bottom is in shades of blue. Bump-out labels note what each wedge corresponds to. In both cases, the three largest wedges are


Figure 5: The distribution of TOO proposals by topical category for Cycles 22–24 (top, shades of gray) and Cycle 25 (bottom, shades of blue). Each wedge is sized in proportion to the number of proposals received in each topical category. The top three categories are Black Holes and Neutron Stars Binaries (BH AND NS BINARIES), Supernovae, Supernovae Remnants, and Isolated Neutron Stars (SN, SNR, AND ISOLATED NS), and Active Galactic Nuclei (AGN).

Bar plot showing four pairs of bars. The X-Axis is labeled Trigger Category, with, from left to right, values of 0-5 d, 5-20 d, 20-40 d, and &gte;40 d. The Y-axis is labeled Number of Triggers, and it linearly spans from 0 to almost 100. Each pair of bars is one bar in shades of gray and one in shades of blue, identified as the average of the past 3 cycles and Cycle 25, respectively. The bottom of each bar is colored slightly lighter, denoting the fraction of proposals that were awarded. Values are mostly consistent between Cycle 25 and the previous averages.


Figure 6: The distribution of trigger requests and awards by response category for the average of the past three cycles (gray) and Cycle 25 (blue).