The optical "afterglow" of a Gamma Ray Burst, or GRB, is usually difficult to detect. However, on March 29, 2003 (at 11:37:15 UT), a GRB was discovered that produced an amazing afterglow so bright that any amateur with a telescope and CCD would be able to image it.
GRBs are so far away that only a handfull have been associated with a galaxy. The total amount of energy in throughout the gamma ray, x-ray and optical spectrum is immense, considering the great distances to these objects. One theory for their origin involves a supernova whose core collapses to a neutron star, and later to a black hole. The creation of the black hole produces a pair of oppositely aligned jets of material that moves out at 10% the speed of light and interacts with the supernova shell created a month earlier. This interaction then produces the immense flux of photons (gamma rays, etc).
Amateurs who monitor GRB announcements have as their first goal the mere "detection" of the GRB optical afterglow. This usually reguires that they learn of the GRB detection within hours, since the optical afterglow fades rapidly. Only after a "detection" will the typical amateur try for the more scientificcally useful filtered observation. It's the V- and R-filter magnitudes that the professionals can make use of in testing their models. The main observational goal is to acquire a set of V-magnitude and R-magnitude brightnesses versus time for the creation of a "light curve."
GRB030329 is unusual in two respects, so far: 1) it's very bright, and 2) it is fading very slowly. It may be unusual in a third respect: its brightness may exhibit oscillations. Such oscillations are more easily detected when the fading rate is slow.
Here's an image I took 20 hours after the GRB detection.
Figure 1. Red filter image showing the GRB optical afterglow approximately 20 hours after discovery. The GRB afterglow has a red magnitude, Mr, of 15.74 +/- 0.04. The faintest stars visible in this image have a red magnitude of ~19 (19.4 in the original 16-bit image). Field of view is 23.3 x 14.1 'arc, north up and east left. [10-inch Meade LX200 telescope, SBIG ST-8XE CCD, Schuler Rs filter, 10.5-minute total exposure, 2003 March 30, Hereford, AZ]
Figure 2. Seven days later the GRB afterglow had faded considerably, as shown by this much longer exposure. The faintest stars visible have a red magnitude of ~21.0 (the "limiting magnitude" is 21.8, 3-sigma). All stars appear brighter, yet the GRB afterglow is barely visible. For this average image the GRB afterglow had a red magnitude of 19.0. The GRB afterglow faded 3.1 magnitudes during an 8-day interval. [176-minute total exposure, April 5, 6 and 7 UT]
Figure 3. Light curve for RED, VIS and BLU filters (red, green and blue symbols) for all AAVSO member submissions (provisional data). The large cicles are my data.
I've imaged this GRB afterglow successfully on 14 occasions, 6 with a red filter, 6 with a visible (green) filter and once unfiltered. All of these measurements appear to be good quality, as they agree with the fitted line in the graph.
Just for the heck of it, I tried to image this GRB April 21, 23 days after it's burst, and here's what I got:
Figure 4. 104-minute total exposure, unfiltered, taken April 21.4. The GRB is at the cross-hairs.
Figure 5. Zoom factor of 2.0, centered on the GRB. The GRB has a unfiltered magnitude of ~19.7 +/- 0.1. The faintest stars have magnitudes of ~20.5 (limiting magnitude is 21.3, SNR = 3).
Normally, a GRB fades so fast that if you don't image it within a few hours it's too faint to show up. This GRB, on the other hand, is so close that even after fading at least 4 magnitudes during the first 24 hours it was still at a respectable Mr = 16.5. This initial 4 magnitude fading is probably the light curve of a polar jet that happened to be pointed at the Earth. The polar jet produced gamma rays as well as optical radiation. Then, after 24 hours, the uderlying supernova rose in brightness to "take over" the light curve, reaching a peak at 1.6 days and Mr = 16.4. A typical SN will reach maximum brightness within about week of burst, then slowly fade about 1/2 magnitude per week. This GRB SN faded at a much faster rate after the small Day 1.6 peak, showing a loss of ~1.0 magnitude per day for several days. Then, after about 9 days, when the light curve in Fig. 3 ends, the rate of fading decreased to something more appropriate for a typical SN. That's why I was able to image it after 23 days.
For a couple years I've been trying to get an image of a GRB optical afterglow, and I finally did it!
This site opened: April 2, 2003. Last Update: April 29, 2003