WATCH - a RHESSI precursor
From RHESSI Wiki
|1st Author:||Hugh Hudson|
|Published:||27 January 2007|
|Next Nugget:||A Pion Event|
|Previous Nugget:||RHESSI and submillimeter waves|
This Nugget calls attention to a remarkable instrument that in some ways anticipated RHESSI. WATCH did what its name suggests, watching the sky for bursts of X-rays and gamma-rays of all kinds, including solar. The name is an acronym for "Wide Angle Telescope for Cosmic Hard xrays" even though it was not really a telescope in the magnifying sense. Instead it was a cleverly designed rotating modulation collimator, as is RHESSI. As a wide-field monitor, its main function being the study of transients such as cosmic gamma-ray bursts, it necessarily responded to solar flares when the Sun entered its field of view. Fortunately this was often enough so that the WATCH instrument on the Russian satellite GRANAT was able to detect as many as 1551 solar events during a 30-month period around 1991-1992.
The cleverness of the WATCH designers was to use strips of different scintillation counter materials, with a pattern just matching that of an absorbing grid. Different scintillators have different pulse timing, so that the electronics could distinguish them. The shadows cast (see Figure 1) by the grids would then alternate between the two scintillating crystals, which could be viewed simultaneously by a single photomultiplier tube. The counting rate of each type would then modulate up and down, as shown. This innovation, though only permitting low angular resolution, gives twice the throughput as compared with a normal (e.g., RHESSI) two-grid collimator. If still confused about the way WATCH or RHESSI make images, please see our earlier Nugget on the RHESSI imaging technique.
Figure 1: Left, a solar flare observed by WATCH on board the GRANAT spacecraft, as an (unmodulated) time series. Right, the modulated light curve, showing the characteristic Bessel-function pattern. From this one can readily determine the position of the source (solar, in this case) if one knows the spacecraft dynamics.
WATCH solar results
There have been few publications on the solar observations from WATCH, but the 1998 paper by Norma Crosby and co-workers analyzed the 1551 events mentioned above. Other hard X-ray instruments have produced larger catalogs, but WATCH was the first in the "firm X-ray" spectral range, with a lower energy threshold typically at 10 keV, and viewing through a thin entrance window (25 microns Al). Thus it anticipated the RHESSI microflare results described in an earlier Nugget to a certain extent.
Figure 1: Two more solar flares observed by WATCH, and simultaneously by another instrument (BATSE). The good agreement here establishes that WATCH detects X-rays in the true non-thermal hard X-ray range - sometimes a question mark for this difficult observation. WATCH could therefore do the valuable service of establishing the distribution of event occurrence. Furthermore, and quite novel, the 1998 Crosby paper searched for relaxation oscillations in the event occurrence. This pattern is a strong prediction of the accepted scenario of flare build-up and release - the coronal magnetic field is supposed to wind up like a rubber band, then snap (the flare) somehow. WATCH did not see this pattern and it is still not clear exactly why.
Biographical note: Hugh Hudson is a senior RHESSI scientist at UC Berkeley.