Thursday, April 26, 2012

The importance of being hardest


Here we are. The launch of the NuSTAR satellite, initially scheduled for PI day this year (03/14), will happen around June 13th. The reason for this slip was an update of the software in the Pegasus rocket that will launch the satellite.
This "cheap" (a Small Explorer mission, that cost less than 200 million dollars) telescope will be launched in space inside a Pegasus rocket and extend its 10-meter long deployable mast to provide images of the sky in the so-called hard X-rays, meaning X-rays at higher energies than it was possible with the current X-ray imaging satellites.
Is it a sort of a worthless "My satellite is harder than yours" competition stealing money from taxpayers?
'Course not.



Mother Nature gave us eyes that see more or less where we need it, i.e. using a good part of the electromagnetic spectrum that is not filtered out by the atmosphere. But as they say... there is much more to Astrophysics than meets the eye. In fact, most of the interesting things in Astrophysics happen where our eyes can't see directly (fortunately, I should add).
If we observe the sky in the visible window, we see more or less what is emitted or reprocessed at temperatures that are similar to that of the Sun, some thousand degrees Kelvin.
But, for example, things can get much hotter close to a compact object like a black hole or a neutron star. Matter can reach millions of degrees, and a good part of the emitted energy is in the X-rays. So, if we want to study the strong-gravity phenomena around a black hole, it's in the X-rays that we have to look. At these temperatures, matter is almost fully ionized and can be treated as free charged particles. If there is a magnetic field, charged particles are deviated by the field and they emit synchrotron radiation, that can be observed anywhere from radio wavelengths to very hard X-rays depending on the energy of the particles and the strength of the magnetic field. We can see if around the object there is dust, and its composition, by looking for the emission lines of typical dust particles, in the infrared. And we can study the presence of radioactive decay by looking at Gamma rays.
Here you can see an example of what the sky looks like at different wavelengths. The phenomena that can be studied in all these bands are of course many more than I briefly listed here, as with the other astro-contributors in this blog I'll try to tell you in future posts.
But we have a number of working and awesome X-ray telescopes. So, why NuSTAR? Elementary, my dear Watson. What we have now is either satellites that can look anywhere from soft to hard X-rays (up to some hundred keV) with rough angular resolution, i.e. without other bright sources closer than ~1º to our target, or satellites with very good angular resolution (down to less than 1 arcsecond!) but very limited bandpass (0.3-12 keV). NuSTAR will provide high-resolution imaging (~25") in the part of the spectrum that the other imaging instruments don't see, up to ~80keV, while being less sensitive below 10 keV.
An absolutely non-complete list of possible applications of these improved specifications is here. This year, the soft X-ray satellite XMM will devote some megaseconds to joint observations with NuSTAR, in order to exploit the unique capabilities of the two instruments together.
The bottom line is that NuSTAR is a small mission studied to cover the range of capabilities that were not technically achievable when the other current missions where studied, but are necessary to push our knowledge of Astrophysical phenomena to a higher level. So... Welcome NuSTAR!


PS: X-ray Astrophysics seeks for the next-generation large mission, that will give to the X-ray community the same leap that ALMA, SKA, E-ELT will produce in radio and visible/UV Astronomy. One of our most hoped-for missions, Athena, will be probably dropped by ESA, even if initiatives like this try to make ESA rethink their decision. At this juncture, small missions like NuSTAR are what makes X-ray Astrophysics progress.
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