What We Came For

The news at today’s weather briefing was expected, but unwelcome:  The winds at float altitude continue to be as high as 55 knots (nautical miles per hour).  Those high winds will push us east rapidly, forcing us to cut down after less than 20 hours at float to avoid crossing a mountain range.  That’s not enough time make our observations, so: we wait.  The current forecast suggests lower float winds starting next Monday, so we might have a window then.[1]

Still, now seems like an appropriate time to discuss what exactly we’re trying to observe down here in Australia.  Why isn’t twenty hours enough?  What is it we’re looking for?  To my mind, the answer is both simple and complex.  We have specific targets we expect to detect at certain significance, but this individual balloon campaign is situated in a larger context of scientific exploration.

First, the specific targets.  The center of the Milky Way Galaxy is home to two specific sources of gamma-ray emission.  The first is a cloud of antimatter positrons, produced perhaps by black hole binary systems.  When these positrons encounter electrons (their oppositely-charged matter counterpart), they annihilate and produce gamma-rays of very specific energy, 511 keV.  The shape and intensity of the emission produced by this cloud should provide clues to its origin, and satellite experiments have made progress in this direction already.

INTEGRAL 511 keV map (Weidenspointner et al. 2008)

The second major source of diffuse gamma-ray line emission near the Galactic Center and in the Galactic Plane is due to an unstable form of aluminum.  Like all heavy elements (including all those found around us on Earth!), it is produced by massive stars.  Since the aluminum decays away fairly quickly after it is produced, mapping it can tell us about the life cycles of stars and the formation of elements (known as “nucleosynthesis.”)

COMPTEL 26Al map

With its fine energy resolution and broad field of view, NCT is designed to make  maps like these.  Still, one of the great parts of high-energy astrophysics is the wide range of exotic sources, and we hope to do interesting science with some of the other sources we see along the way.  These include supernova remnants and faraway active galaxies driven by accretion disks around supermassive black holes.

One of the most interesting possibilities is to extend our New Mexico observations of the Crab Pulsar (a rapidly rotating neutron star).  As a Compton telescope, NCT is capable of measuring gamma-ray polarization.  This is a technique I am particularly interested in and which is just starting to come into its own.  Polarization measurements of pulsars should help distinguish between models of the source of their gamma-ray emission.

Of course, the sky in gamma-rays is hardly stagnant, and so there’s also room for serendipity:  transients like gamma-ray bursts (my speciality), the more rare soft gamma-repeaters, and large solar flares would all provide great discovery potential if they were to occur while we were flying.

Let’s be clear, though.  Particularly with the forecast of an abbreviated flight–well below our hopes for this campaign–it is unlikely that NCT will make “best-ever” measurements of any source we’re targeting during this flight.  Why do it, then?  First, we hope to fly NCT again in the years to come, maybe even on an around-the-world flight.  The data we obtain now can be combined with that taken later for increased exposure, and we’ll learn more about the analysis methods in the process.  Additionally, our measurements will provide important verification of results obtained by other instruments, particularly those employing different technologies and imaging techniques.  These cross-checks are crucial to the scientific process.  Finally, our efforts help lay the groundwork for future satellite missions, whose higher cost and longer timelines require proven technologies.

The gamma-ray sky is full of fun, interesting things we’d like to learn more about–now if we can just convince the winds to let us take a look!

[Back] (A note on winds:  There are three types of wind relevant to scientific balloons.  The first, surface winds, are what we feel on the ground and hear about in the weather forecast.  Above the surface to a few hundred meters are the “low-level” winds, which may have substantially different speed and direction than the surface winds.  The interplay of surface and low-level winds is crucial in determining whether a balloon can be launched.  Upon inflation, the balloon is at its most vulnerable and most dangerous–winds at different levels can push the balloon while the gondola is still attached to the crane.  A successful launch requires low-level and surface winds which are light and consistently directed.

The winds at float altitudes (around 40 km) are the largest factor in determining the length of the flight, as they tend to push the balloon towards the boundaries of the allowed flight area.  Typically, float winds have different intensities at different altitudes, so some rudimentary steering is possible by letting the balloon droop to lower altitudes at night or by dropping ballast to raise it.

CSBF has a dedicated meteorologist on staff who provides custom forecasts for the balloon flights.  He makes detailed predictions of the surface and float winds each day, simulates expected flight trajectories, and on launch days monitors the winds and determines the balloon layout direction.)


2 responses to “What We Came For

  1. I have been following your progress since you started these operations. I admire your dedication to keep working despite the complications of weather and other distractions.
    Keep up the good work and stay with it you will succeed in your final outcome. I followed the 2002 ULDB launches also with great interest.
    Regards to all the team, from James in Adelaide.

  2. Thanks for sending the progress reports.