UT Oct 11, 2022: Photometry of afterglow of GRB221009A

Michael Richmond
Oct 12, 2022

On the night of Oct 10/11, 2022, I observed the optical emission from an unusual gamma-ray burst known as GRB 221009A. I was alerted to the event by fellow members of the Kilonova Catcher collaboration. Although this was a tough test, due in part to the nearly full moon, I managed to detect the target and make at least a rough measurement.

GRB 221009A

Many gamma-ray bursts are detected each year, but this one was particularly bright, and its photons particularly energetic. You can find a large collection of reports from observatories around the world at

• https://gcn.gsfc.nasa.gov/other/221009A.gcn3

The burst was first detected by the Fermi satellite at JD 2459862.05346. I heard about it the next day, so my measurements took place after the optical peak brightness, while the source was fading. The reported position was

  RA  =  19:13:03.48  = 288.26452
  Dec = ++19:46:24.6  =  19.77350

This was near the meridian after sunset, so had several hours to observe it at a reasonable airmass. I decided to use our ASI 6200MM camera, which is somewhat more sensitive that our ATIK 11000, attached to the 12-inch telescope. I didn't put any filter in the optical path, in order to detect faint sources.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ASI 6200MM CMOS camera
• no filter, exptime 60 seconds
• images binned 4x4
• guiding, exptime = 5 seconds, X aggr 0.2, Y aggr 0.0
• focus values:
  • clear: 1.620 at T = 51/10
  • clear: 1.648 at T = 45/7 (made things worse - whoops)
  • clear: 1.607 at T = 43/6
  • clear: 1.597 at T = 41/5
• camera ran at T = -20 C

Notes from the night:

• sky was clear
• nearly full Moon made the sky very bright, unfortunately
• I had to use the silver knob to make large focus adjustment at the start of the night, since the ASI and ATIK cameras have different focal positions. To go from ATIK to ASI, turn knob CCW.
• no creatures
• many of the images showed a small amount of East-West elongation, due to imperfect tracking. It would probably be better to use 30-second images in order to achieve a tighter PSF in the future.

The object is located at

  RA = 19:13:03.48  Dec = +19:46:24.6    (J2000)

A closeup of the DSS red image around the position of the GRB is shown below. The size of the chart is about 5 x 5 arcminutes. The field is pretty rich, and the position of the GRB marked by a red arrow.

I collected 60-second images for about 3.45 hours, from 7:58 PM to 11:44 PM. Many of them were somewhat trailed. After discarding the bad ones, I combined the remaining 116 images using a median algorithm.

Here's a closeup around the position of the GRB in my median frame. This is a bit smaller in size than the chart above, being only about 3 x 3 arcminutes.

I've marked the position of the target, and a set of comparison stars, in the graphic below.

The GRB is clearly detected, but it's very close to a brighter star (marked "G" in the graphic). I used aperture photometry with a radius of 3 pixels = 3.12 arcsec to measure the brightness of the target and all the labelled reference stars. In order to estimate a magnitude, I tried two methods.

• the bright reference stars (A, B, C, D) all appear in the APASS 9 catalog. Using the V-band values for these stars, and no color corrections, I derive a magnitude V = 19.15 +/- 0.35 for the target.
• the faint reference stars (F, G, H, K) appear in the USNO B1.0 catalog. I averaged their R1 and R2 entries to compute an R-band magnitude for each one, then computed an average offset from the instrumental magnitudes. In this way, I derive R = 17.75 +/- 0.40

The Sony IMX455 sensor inside our camera has a peak sensitivity around 530 nm, so perhaps it matches V-band better ...

The images cover the span from JD 2459863.49917 to 2459863.65488. Using the mean value for all 116 images, one ends up with the following measurement from RIT:

   JD 2459863.57421      CV = 19.15 +/- 0.35
                     or
                         CR = 17.75 +/- 0.40

This is about 1.5 days after the GRB was first detected. As far as I can tell, these measurements lie close to those made by others at roughly the same time, but we'll have to wait for someone to collect all the reports and create a definitive light curve to be sure.

I reported my measurement, and submitted my median image, to the Kilonova Catcher Project. At the time of this writing, the Project had collected 85 images from collaborators around the world.