Did you hear about this comet?

Dr. Scott Wolk is responsible for Monitoring & Trends Analysis of the Chandra X-Ray Observatory, working within the Development & Operations Group and Science Operations Team of the Chandra X-Ray Observatory Center. Scott discusses 17P/Holmes, a comet which was discovered November 6, 1892 by amateur astronomer Edwin Holmes. In October 2007 this comet became nearly one million times brighter, and is the largest known outburst by a comet.

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So a few days ago while I was at a meeting, a colleague met me in the lobby and said, "Did you hear about this comet which went from magnitude 17 to 3 in one day?" Of course, I hadn't so we started to talk about the possibility of observing it. Chandra has observed about 8 comets. Although comets are cold ice balls, they release X-rays through a process called charge-exchange. This fact was demonstrated by the first Chandra observation of a Comet, 1999/S4 Linear (http://chandra.harvard.edu/photo/2000/c1999s4/). The ensuing studies have taught us a lot about how the mechanism works.

The main topic of discussion at dinner that night was what would be particularly useful about observing a very distant, very bright comet. I thought about the fact that this comet was in a portion of the solar wind that we never see from Earth, about 2.5 A.U. away, beyond any of the usual space weather stations near Earth. He thought about the fact that we would be viewing this comet "face-on", usually we view them from the side and when they are close to Earth. We debated what instrument to use. The HRC-S/LETG would get the OVI line but would be noisy. ACIS-S imaging mode should be ok, unless the solar wind ionization is too low. We both worried about how long the comet would stay bright and how quickly Chandra could schedule it.

The next morning we sat together and wrote a request for "Director's Discretionary Time". This is the method of getting observing time when the thing that you want to look at was so surprising that there was no way you could have thought of it during the normal proposal cycle. A comet getting a million times brighter certainly fell into this category. But, there was still the issue that we needed to make a science case. We don't just observe comets because they get surprisingly bright. We get 1000 characters to explain our science case. We mentioned both the solar wind and geometry issues. (We also found a lot of characters we could cut out to get under the limit).

The next day my colleague was on his way back to Germany (with weather delays it took him 3 days), I was back in Cambridge from the meeting. I found a list of questions from the Director's Office in my email. They requested more details about the expected count rates and status of the comet. We now expected the comet to stay bright for three weeks based on reading up on comet Holmes' previous behavior. I sent in about a page of detailed discussion.

The Director's office sent it to an outside reader to validate the science arguments and judge its importance. They felt it was a good use of Chandra and the Director's Office approved about 8 hours of telescope time. I was then involved in the planning and spent about a day calculating how much optical light would fall on the CCDs, since CCDs see both X-ray and optical light, we use a filter to block the optical light and let the X-rays through. But, if the object is very bright, optical light can get through and ruin the X-ray observations. I ended up using some photos of the comet I took in the my backyard and some others on the web to calculate the surface brightness and calculate that about 1 optical photon would strike each pixel every ~3 seconds. This was below the limit of about 10, so we were ok. November 1 it was observed.

The data are now down. When I reduced them I didn't see a bright ball of X-rays steaming away from us at 10-20 km/s. I did see a little emission and think I see the shadow of the comet. Now the real work begins!

-Scott Wolk

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>View images at APOD: http://antwrp.gsfc.nasa.gov/apod/ap071110.html
http://antwrp.gsfc.nasa.gov/apod/ap071109.html