What is the "Swift" spacecraft mission?

David Palmer
posted to sci.astro on Jan 17, 2000

In a message to sci.astro, James Oberg asked about high-energy astronomy missions:

> What other satellites can carry on GRO's pioneering work in high-energy
> space astrophysics?

I replied with a very little information:

> HETE-II will help -- if the launch succeeds this time :-/.  Beppo-SAX
> has been finding _and pinpointing_ gamma-ray bursts for the past few
> years, something GRO could never do.  RXTE is doing a great job at 
> X-ray energies.

David Palmer replied with this information about the Swift mission:

And in a few years we will launch Swift, which will find about as many GRBs per year as GRO/BATSE, locate each of them accurately, and make sensitive X-ray and optical observations beginning about a minute after the GRB starts.


The spacecraft will have a wide-field gamma-ray Burst Alert Telescope (BAT), covering about an eighth of the celestial sphere at any given time. BAT will be more sensitive than BATSE, so even though it only covers a quarter of the field of view, it will see fainter. The actual number of bursts it will see depends on how many faint bursts there are, which is something we will not know until Swift tells us. BAT will be able to immediately (~5 seconds) determine the location of a burst to within 4 arcminutes, and notify the spacecraft.

The spacecraft is fully autonomous, and will respond to bursts by sending the locations down to the ground (by TDRSS, for distribution by the GRB Coordinates Network to observers worldwide http://gcn.gsfc.nasa.gov/gcn/ ). . It will immediately slew around to point a pair of telescopes at the burst. We are using six of the biggest reaction wheels we can hang on the spacecraft bus to get the repointing time down to less than a minute.

The X-ray telescope and the optical telescope are both spares from other spacecraft, which means that we could get them without breaking the budget cap for Medium Explorer spacecraft. The telescopes are quite sensitive--the optical telescope, with a 30 cm aperture, can see down to 24th magnitude (point source, unfiltered, 1000 s exposure, away from zodical light) making it the equivalent of a 4 meter telescope on the ground for our purposes.

This will give us sub-arcsecond positions (2 arcseconds from the X-ray counterpart if there is no optical counterpart) and X-ray and six-color lightcurves of the afterglow. We will be able to get redshifts in most cases by a variety of techniques (X-ray lines, grism spectra, Lyman-edge spectrophotometry).

Ground-based astronomers will be able to make follow-up observations of bursts, selected based on the immediately-available information form teh spacecraft. For example, an observer at the Very Large Array may decide to look only at bursts that have optical counterparts and redshifts greater than 3, while an observer at Whipple may want to concentrate on short bursts with low redshifts. (Swift will be able to detect so many bursts that they can't all be followed up by every observer.)

This mission has been selected for funding, and we expect to meet our 2003 launch date.