What if astronomers had Superman's X-ray vision?

Today’s guest blogger is Shanil Virani, a grad student at Yale who has been working on the X-ray follow-up of IC2497 & Hanny’s Voorwerp:

Most of you, if not all, are familiar with the story of Hanny’s Voorwerp — an unusual object found by the GZ team near the disk galaxy IC 2497. The optical spectrum of the object (taking the light and splitting it up into its “rainbow” of colors) suggests it was “lit up” by an active galactic nucleus in the nearby galaxy IC 2497. Active galactic nuclei, or AGN, are believed to be powered by supermassive black holes that reside in the centers of galaxies devouring the gas and dust that come within their reach. Every galaxy has one, including our own Galaxy! But how do you find something you can’t see? The best evidence for the existence of supermassive black holes actually comes from near-infrared observations of the orbits of stars right near the central black hole in our own Galaxy (see this short animated movie if you have never seen it before!). Simple application of Newtonian mechanics demonstrates that these stars orbit a central object whose mass is approximately 4 million times that of the Sun and is confined to a region roughly the size of our Solar System! The only physical object we know of with such properties is a supermassive black hole. However, with the launch of a new generation of X-ray observatories we now know that observations at X-ray wavelengths also provide definitive evidence of whether an object is a black hole as X-ray observations probe the extreme physical conditions in the immediate vicinity of a black hole. Recently, we have been awarded significant observing time with two space-based X-ray observatories — the primarily European XMM-Newton telescope and the Japanese/American Suzaku Observatory — to determine whether IC 2497 hosts an active supermassive black hole that can explain the mystery of what’s lighting up Hanny’s Voorwerp.

Since my Ph.D. dissertation involves X-ray astronomy, principally carrying out one of the deepest surveys of the X-ray sky performed to date (see my web site), Kevin and I thought it would be cool to blog about what X-ray astronomy is and how we do it since it is completely different than optical astronomy. In a second blog post later this summer, I will provide a more physical picture of what we think is going on in IC 2497 and how we are testing this hypothesis with data from these X-ray observations.

This year we are celebrating the International Year of Astronomy which in part commemorates the 400th anniversary of Galileo’s first astronomical use of the telescope. From 1609 to the present, we’ve seen a remarkable revolution in technology in optical astronomy that has allowed us to see deeper into the cosmos. X-ray astronomy, on the other hand, is a relative newcomer having only begun in the 1950s. X-ray photons are energetic enough to go through objects compared to optical photons but they are also easily absorbed. Only a few millimeters of bone or a few meters of air will stop them. The latter is critical for life to occur on the surface but it also means we need to get above the atmosphere if we wish to do X-ray astronomy. The former tendency, for X-ray photons to be absorbed, is what is exploited when we go to the doctor’s office and get an X-ray of our teeth or bones. In this case, a film is placed behind the object we are interested in (teeth in the case of a dentist) and X-rays are then shone on the patient. The teeth and bones easily absorb these X-rays while the tissue does not. The dentist then uses this image to identify cavities, etc. In the case of X-ray astronomy, however, we collect X-ray photons from celestial objects rather than exposing objects to human-made X-rays and taking a picture (like in the dentist example). This also means that the way in which X-rays are collected are fundamentally different than optical telescopes. In optical astronomy, we all have this picture in our mind of a large telescope tube at the back of which sits a large mirror to collect the light which is then focused onto an instrument. This prescription does not work in the X-rays because they are too energetic to be collected in this way — they would just be absorbed by the mirror. Instead, X-ray photons are softly deflected several times so that they can be focused directly onto the instrument. Think of skipping pebbles off the surface of the water at a beach. The image below (courtesy of the Chandra X-ray Observatory’s education web site) provides a good schematic of how modern X-ray satellites work. The more mirrors you have nestled together, the more collecting area you have, and therefore the more fainter you can go.

The data that come back from these observatories are also different then the kind of data that you are used to seeing from optical telescopes. For example, we are all amazed by the beautiful images produced the Hubble Space Telescope. It has produced some of the prettiest pictures such that even rock bands have used its images for album covers (see Pearl Jam’s Binaural for a great example!). This is the basic product, an image, that the HST produces and its observers analyze. Now, they also have more advanced facilities so that you can take a spectrum of a source but you generally can’t do both simultaneously. With X-ray instruments, you get imaging and spectroscopic data and timing data all in one go if your source is bright enough or if you stare at it long enough! It is exactly these attributes we intend to exploit in the data returned by our Suzaku and XMM-Newton observations of Hanny’s Voorwerp to determine if IC 2497 does indeed host a supermassive black hole! The imaging, spectroscopic and perhaps even the timing data will allow us to conclusively demonstrate if the black hole in IC 2497 is currently active enough to explain Hanny’s Voorwerp. If it is not, then that may even be more interesting as it would be providing us with a rough diary of what are the eating habits of the supermassive black hole! Either way, a great story is about to unfold and will reveal another level of detail about this exciting object.

Stay tuned for more details as the data begin to come in and are analyzed. In the mean time, if you are interested in learning more about the history of X-ray astronomy, check out this excellent page at the University of Cambridge.

Shanil Virani