Gamma Ray Burst GRB 000926 - Optical Afterglow

Observations from Nyrölä Observatory September 28, 2000 by A. Oksanen

This Gamma Ray Burst (GRB000926) was detected September 26, 2000 at 23:49 UT by three spacecrafts Ulysses, Konus, and NEAR. The approximate coordinates were trianguled and posted to the GCN Observation Report Circulars (GCN#801) about 20 hours later. The first optical detections were reported just three hours later by Calar Alto Observatory (GCN#803) and Nordic Optical Telescope (GCN#803) giving very accurate coordinates. This night was cloudy in Finland.

Opportunity rises

The next evening was clear and decided to try to image the afterglow as it was well positioned high on the western sky. I arrived to Nyrölä Observatory as sun was setting. After setting up the 16-inch Meade LX200 telescope and SBIG ST7E CCD-imager I slewed to the reported GRB position. After checking the field from POSS2 image I searched a suitable guide star for the active optics and found one enabling 2 Hz correction rate. I started series of 240 second unfiltered exposures at 18:18 UT. There was a few technical clitches and some clouds on the sky, but I managed to get 20 frames before sky was completly cloudy at 19:40 UT.

I was ready go home after I took darks and flats and wrote all data to a CD-Rom disk. At home I quickly reduced all images and combined all 17 good frames. I compared the summed image to POSS image, but could not see any new objects. I mailed the image to Dr. Taichi Kato of university of Kioto, Japan and to Guy Hurst, editor of The Astronomer magazine in England for further analysis before going to bed.

The combined image.
Unfiltered 17 x 240s exposure,
CCD: SBIG ST7E 2x2 binned + AO7 guiding @ 2 Hz
Telescope: 0.40 m Meade LX200 at f6.3
Note: North is down and east is left because AO7 produces mirror image.

Click to see a larger version.


Next morning I went to work and soon received an email from Japan. Hitoshi Yamaoka, Kyushu University had examined the image and had found a dim object that was missing from POSS. I examined the image again and agreed with him that there was indeed a new object. I requested all POSS plates from US Naval Observatory in Flagstaff to be sure this was the GRB afterglow.

One of the POSS images images showing the position of the Optical transient. Click to see the whole image.

I cropped the interesting part of my image and there was very little doubt that the object was the afterglow of GRB 000926. The transient was easily seen. But still I had not seen any other images to be absolutelly sure about it.

Enlarged part of the combined image showing the transient just a bit left from the center of the frame.


Report to GCN

Hitoshi Yamaoka had measured the object position and the result agreed very well with the other reports. Magnitude was determined to 20.0 magnitudes. The results were then reported to GCN network by dr. Kato. (GCN#813 & GCN#817)

The precise astrometry using 43 USNO_A2.0 stars yields:
R.A. = 17h04m09s.71 +/- 0s.05, Decl. = +51o47'10".0 +/- 0".5
(2000.0), which shows quite good agreement with the reported
positions.  Oksanen reports that every POSS scan examined at USNO
server does not show this object.  Thus it is highly probable that
this object is the same object as a candidate of the afterglow of GRB 000926.

The magnitude was about 20.0CR at Sept. 28.8 UT
(comparison star is USNO_A2.0 star at 17h04m01s.21, +51o46'19".9
(2000.0), rmag = 17.1, same as Halpern et al. (GCN 806)).
Note that it is somewhat brighter than Hjorth et al. (GCN 809) report.

Sincerely Yours,
Hitoshi Yamaoka, Kyushu Univ., Japan

Final confirmation from NOT

The Nordic Optical Telescope image was then published to web page and that at last dissolved all doubts. I had successfully imaged a GRB afterglow after two years of trying!

The NOT image of GRB 000926 Optical transient
Note the image in not mirrored.
Click the image to see it in full resolution.

The first ever Gamma Ray Burst Optical Afterglow imaged by an European amateur astronomer.
Imaged at Nyrölä Observatory, Finland on September 28, 2000 at 19:0 UT by Arto Oksanen.

GRB observers of Sirius

Sirius GRB team has been established 1999 and has a group of members who are responding to alerts. We have established our own alert network via GSM mobile telehone network. We are receiving pager alerts from GCN Network and redistribute them by a email distribution list to our observers GSM telephones via SMS messages. Our response time is less than 10 minutes from the alert if we are observing with CCD at the observatory. We are also part of AAVSO GRB network and subscribe VSNET GRB mailing list.
Soo far this has been our only successfull GRB-observation.