XRT Postage Stamp Image Photo Gallery

This document is intended to provide a quick overview of XRT Postage Stamps in order to help users distinguish between X-ray sources and cosmic rays.

When the XRT arrives at a possible new GRB position following an automated slew, it takes a series of short exposures to try to find a bright X-ray source in the field of view. The first exposure is always 0.1 s. Very bright sources will produce a centroid in this Short Image exposure (see the image of GRB 050117a below). If no source is found in this exposure, a series of 2.5 s exposures will be taken, and the XRT will attempt to find a source in these Long Image exposures. If it fails (due to lack of counts, because the standard deviation of the counts is too large or too small, or because the algorithm did not converge) it reports a Centroid Error message. If it succeeds, it sends a Postage Stamp image to the ground through TDRSS, and this is distributed through the GCN.

It is important to examine these images to check for invalid centroid positions. For example, occasionally the XRT will centroid on a cosmic ray. This rarely happens for real GRBs (unless we slew to them in the South Atlantic Anomaly), but it does sometimes happen. An example is GRB 050422. The Postage Stamp image for this burst is shown below:

The compact appearance of this object is clearly not consistent with the XRT PSF (7.6 pixels HPD) or with the typical distribution of pixels for a real X-ray image (see below). Another important clue is the fact that the image gain > 1. This is because CRs deposit so much charge in some pixels that they saturate the ADC, and the software applies a gain factor to the image to compress it from 12 bits/pixel to 8 bits/pixel for the TDRSS telemetry link. This can happen for extremely bright sources, but it is unusual. (These cases are currently flagged with warnings at the bottom of the GCN Notice.)

XRT Postage Stamp Images of X-ray sources

During centroiding tests, a number of X-ray sources were observed in Image Mode and Postage Stamp images were obtained. We show here a series of typical Postage Stamp images of X-ray sources, including the Postage Stamp images of GRB 050117a and GRB 050421. Note the much broader distribution of counts compared with the cosmic ray images that follow:

Cen X-3 (2.5s)
EXO0748 (2.5s)
GRBs	GRB 050117a	GRB 050421

Rogues Gallery of Cosmic Rays

By contrast, we now present a rogues gallery of cosmic ray hits. These tend to be much more compact, although they sometimes have complex shapes, especially when two merge. Some of these are near the edge of the CCD, another clue that they are not GRBs (the accuracy of the BAT positions and S/C pointing puts most GRBs within 100 pixels of the center of the array at 300,300).

Simple Cosmic Rays
Multiple Cosmic Rays
Complex Cosmic Rays

Finally, we show some examples in which real source images have cosmic rays within the 51 x 51 pixel Postage Stamp image. In cases like this, the position is likely to be incorrect, since it is calculated as the “center of gravity” of all pixels within the centroiding window (typically 80 x 80 pixels). The Postage Stamp window is always centered on the centroid position, so the offset of the true source image in the Postage Stamp gives a direct indication of the error in the centroid position (the Postage Stamp window is 2´ x 2´).