Over the course of 3-and-a-half years, the Chandra X-ray Observatory has made more than a thousand observations of cosmic objects such as comets, planets, normal stars, neutron stars, black holes, supernova remnants, galaxies, and clusters of galaxies. A frequently asked question is: who decides what Chandra will observe? To answer that question we asked Dr. Fred Seward, Assistant Director of the Observatory, to describe the process.
Q: How are the Chandra targets selected?
Fred Seward (FS): Observing time is awarded through the proposal process. Since Chandra is a national facility, built and supported with taxpayer money, anyone can propose for time. All proposals are evaluated in the same way, under the same rules.
Q: Can a proposal be made at any time, or is there a special time to submit the proposals?
FS: There is a special time. Every year a call for new proposals goes out in mid-December. Proposers are allowed 3 months to write the proposals and the deadline for receipt is in mid-March - the Ides of March!
Q: Who submits the proposals?
FS: Scientists from all over the world. In the last proposal cycle, 71 percent were from the United States, followed by Japan and the United Kingdom with 7 percent each, Germany and Italy with 5 percent each and the remaining 5 percent spread among 15 other countries.
Q: Is there some system or information available to aid proposers?
FS: Yes. A Proposers' Guide gives information about the observatory and instruments and is available as hard copy or over the Internet. A catalog of past and planned observations is available with a search engine so proposers can know if particular objects have been observed or not. A program to calculate instrument count rates from target information can be used via a web interface. Detailed instructions for preparation of proposals are on the Internet. An observing proposal consists of target information, a detailed set of instrument settings, and a 4-page science justification.
Q: Do scientists get their proposals in early, or do they procrastinate, like everyone else?
FS: Considering that most proposers are experienced professional astronomers, and that the proposals can be submitted electronically using the Remote Proposal Submission software, the submission process is somewhat bizarre. We receive a total of about 800 proposals. Four hundred are sent during the last day of the 3-month submission period and about 100 during the last hour!
Q: Who reviews the proposals?
FS: Proposals are divided among 12 panels according to topics, for example, normal stars, supernova remnants, black holes, etc. Each review panel has 8 reviewers; so about 100 peer reviewers are needed. These reviewers are from universities and research institutions, large and small, US and foreign, and are all experts in some field of astronomy. Each reviewer must read the 60-70 proposals to be evaluated by his/her panel and is responsible for a detailed understanding of 20. This is a big job and reviewers are not compensated for their time. We are fortunate that so many talented people are willing to be reviewers. Every year the panels have worked diligently to select the most interesting observations and the results have been outstanding.
Q: How long does the review process take?
FS: About 3 months are needed for the CXC to process the proposals and to arrange a mid-June review. Reading proposals and attending the review take about 2 weeks of time, which reviewers are expected to donate to the general good of the scientific community. The review itself is conducted in a Boston hotel where the reviewers are confined for 2 days during which they discuss and grade all the proposals. On the third day, a merging panel meets to blend the results of the 12 topical panels and to evaluate proposals for very long observing times.
Q: Are reviewers allowed to propose?
FS: Yes, but they are not allowed to review their own proposals or those from others at their institution. Most reviewers have submitted proposals that have been placed in other panels. Reviewers are very conscientious about being fair and often leave the room when a proposal from a good friend (or enemy) is being considered.
Q: What if two people propose to look at the same object?
FS: If several high-ranking proposals include a common target, the peer review will recommend which proposal to accept. Reports are written to return to the proposers giving results of the review and, if a proposal is not accepted, reasons why it was not ranked higher.
Q: It sounds like an exhaustive and exhausting process.
FS: It is, but it doesn't end there for the CXC staff. They must check the results, understand and fix any discrepancies, and put the new targets, ranging from the nearest planets to the most-distant quasars, in the observing program. Then it is time to solicit cost proposals, arrange the cost proposal review, and, incidentally, start preparing for the next cycle! It takes half the time of six CXC staff members to plan and implement the process.
Q: How many proposals were accepted?
FS: In the last cycle, 239 proposals to look at more than 500 different targets were selected.
Q: What was the most popular type of target?
FS: Black holes, especially the supermassive black holes that reside in the centers of active galaxies and quasars were the most popular, but all categories were included.
Q: What advice would you give to a proposer?
FS: The goal of the observing program is to maximize the science resulting from Chandra observations. Observing proposals therefore must convince the reviewer that the result will be scientifically interesting. That is, it should add to our knowledge, answer a question, or determine parameters of a model. The proposal must show that Chandra's unique capability - for example its high spatial resolution - is necessary. It must also demonstrate that the observation is feasible - for example, that the source is expected to be strong enough to detect with the proposed observation. Remember that some reviewers will not be knowledgeable about all the details of your specialty. Part of the proposal should be written for a general audience. Start early! Follow instructions. If your project is not accepted, read the report, improve the proposal, and submit it again next year.
Wallace and Karen Tucker