RHESSI's non-solar observations

RHESSI has some powerful capability for observing non-solar X-ray and γ-ray sources, as discussed in previous science nuggets on a fortuitously-observed magnetar. Another fortuitously-observed X-ray pulsar has not yet been described, but it (like the magnetar) chose to get bright just as it conveniently approached the Sun.

The Crab Nebula (M1 in the Messier catalog of celestial fuzzy things) is not serendipitous. Every year in June RHESSI sets out to observe it during its time of closest (apparent) approach to the Sun. Eventually these observations will result in a sort of movie of the hard X-ray structural variations of the hard X-ray Crab, at the RHESSI arc-second angular resolution. These will be the first hard X-ray imaging observations of the Crab Nebula since the pioneering imaging observations by L. E. Peterson's group from balloon-borne telescopes, plus some lunar-occultation observations. In the meanwhile this nugget explains how we do the observations with RHESSI. This is not as easy as it might seem, since the RHESSI spacecraft is designed to look at the Sun and nothing else.

Here's the quarry:

Figure 1: (left) A mosaic view of the Crab Nebula in visual light, taken by the Hubble Space Telescope. This image is about 0.1 degrees wide; the hazy blue glow in the center of the nebula results from synchrotron radiation caused by relativistic electrons accelerated in the expanding supernova envelope. (right) A Chandra Observatory soft X-ray image on a magnified scale.

How does one point a spinning spacecraft?

One essential problem is that RHESSI spins around its telescope axis. The unbalanced angular momentum of the rotation (at about 4 sec period) stabilizes the pointing direction intentionally; to change it one must apply an external torque T with a component perpendicular to the spin axis, which will cause the spacecraft to rotate around that component while conserving the total angular momentum L. As anyone familiar with gyroscopic forces knows, this is very complicated. Space flight provides wonderful opportunities to study these motions at leisurely time scales because of the weakness of the perturbing torques.

A natural source for an external torque on a spacecraft, commonly used in situations like this, is the Earth's magnetic field. A current is passed through magnetic torque bars which react to the Earth's magnetic field (since F = qv x B). This works best if the satellite is near the Earth, since the field strength falls off very rapidly. But there is an additional problem: RHESSI is spinning, so any torque derived from the field will average to zero in first order.

The solution to this problem is to provide the torque bars a modulated current in phase with the spin motion. In this fashion one can create an unbalanced torque. But one must compute the spin phase with great precision if one wants to go very far from the Sun to a specific target (e.g., the Crab). That is one of the problems that has made this difficult.

Amazingly enough, the complicated structure of hardware and software (some of the latter documented best in the manner of the oral tradition) actually works:

Figure 2: RHESSI approaches the Crab Nebula! The dots show the spin-axis location, i.e. the pointing direction of the telescope; the big plus sign is the location of the Crab. Coordinates are (RA, dec) in degrees, and the time interval is about three days. The diamonds show pointing offsets determined directly from imaging the X-ray source detected by RHESSI, and confirm the pointing as measured by RHESSI's simple quadrant-cell Sun sensor. The odd thin sigmoidal curve shows RHESSI's reaction to re-pointing torque commands, as it slews closer to the target.

The drift: a puzzle

As with previous years of offpointing RHESSI, mainly for the Crab Nebula but also for the pulsar A0535+262, we have seen some unexpected behavior. As any experimentalist knows, it is always a battle to get things to work right, and the pointing plot (Figure 2 above) displays some of our ignorance. We actually do not know the origin of the perturbing torques that causes RHESSI to change its pointing during "idle" conditions, when it is not deliberately applying magnetic torques for slewing or pointing control. There seem to be four possible candidates: first, there could be magnetic torques due to the routine operation of the spacecraft, which requires large currents to flow throughout the spacecraft. Yet these would not be phase-resolved in the spin motion, so to first order they should cancel out. There are also always also gravity-gradient torques, dependent on the mass distribution of the spacecraft. Another possibility is differential air drag. In low Earth orbit there is still quite a bit of atmosphere, mainly monatomic oxygen, and its density fluctuates strongly in space and time. Finally, there is radiation pressure, which would apply a torque if the center of gravity did not match the geometrical center (in some sense) of the spacecraft. There may be other mechanisms at work, so if an expert reads this, please feel free to straighten us out! This can be done (for example) on our blog.

Success!

Figure 3: An RHESSI snapshot image of the Crab Nebula using only the coarse (low resolution) grids. This image, with excellent SNR, shows a point source. As we learn how, we will be making images with much better angular resolution and expect thereby to see features that can be related (for the first time in hard X-rays) to the structure one sees at other wavelengths.

In the end, the 2006 Crab adventure was a success. Above is a "snapshot" image, representing only 30 minutes of accumulated data, using three of the nine RHESSI detectors. The structure of the nebula is unresolved using these detectors.