how to search for x-rays

I've mentioned that the goal of my project is to find some SNRs. But you might be wondering, how do you go from a candidate into a confirmed SNR (or other source)? The general step here is to check for emission on other wavelengths, as different objects have specific emission properties. A telltale sign of a SNR is the presence of x-rays, which traces the energetic shock of the expanding material or possibly the thermal core of a neutron star. But (as I found), thorough multiwavelenth investigation is not straightforward and unfortunately you cannot rely on one place (eg. search engine, survey) to get all the information available. In many cases, you may not even be getting the relevant information!

I thought I'd summarise here, very briefly, how you might go about a preliminary check of x-rays. This summary may end up being useful for me too, when I forget the intricacies of the search in a few months :) We will search here for a ROSAT x-ray source, but bear in mind that the ROSAT All Sky Survey (read more about it here) was not of very high angular resolution or sensitivity. Chances are, if you see a detection in RASS you will see it in other x-ray surveys, but it's not necessarily the same the other way around. Taking into account the specs of all the different telescopes out there is not easy! It also complicates searching.

So let's outline how to check for an x-ray source:

  1. Start by going here. This is the main data server of RASS and the best place to get a quick check from an image of the region your source is in.
  2. Click on RASS Field Conversion in the Tools section. This is the tool that helps you figure out which RASS field (image) your source would be found in.
  3. Enter the coordinates (longitude/latitude) of your source. You can use Galactic or Equatorial coordinates, but in the case of Equatorial you have to be very careful that you follow their structure (ie. XXhXXmXXs -XXdXXmXXs). Let's take their example position (12h35m22s 2d01m20s) as a test case.
  4. Click on RASS fields. This will load the field ID (FID) data, which tells you which image to look at. They explain the columns pretty thoroughly anyway. If there is more than one FID result, choose the one with the smallest (xoff, yoff) values. The larger these values are, the more likely your source will be at the edge of the field. Here I will choose FID "931734p".
  5. Go back to the main page. Now, click on Complete Field List under the Data heading.
  6. The far left column of the table (ROR) corresponds to your FID from above. You can either scroll down to it or do a quick search to find the FID.
  7. To see the image, click on the b.. link. This takes you to a smoothed image with known sources circled on it.
  8. Find the position of your source in the image (look at the gridlines, it's a bit confusing). In this case, we see that the test coordinate from above probably corresponds to the blurry cross-shaped source in the image. It's difficult to say for sure, but this method is not used to confirm an x-ray detection. It is useful to get a quick, rough check of what emission you might expect to find.
  9. Save the image if you want - they are very nice images! If you want to go further with an x-ray investigation, you'll need to use HEASARC (run by NASA). It's not as straightforward, but I might write it up sometime in the future. Right now, I should be writing my thesis!

And there you have it. I didn't know until today how simple it was to get a very nice ROSAT image of a fairly large field of view! Thank you to George* for going through the steps with me.

*name changed.

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License