GRB 030725 at the Bronberg Observatory / CBA Pretoria

By LAG (Berto) Monard

The observing instrumentation at the Bronberg Observatory / CBA Pretoria is always running during clear nights, which is nearly every night during the highveld winters.

Timeseries photometry on cataclysmic variables (CVs) takes the largest time slot. In the 'spare' times, snapshots are taken of faint CVs, programme galaxies in search of supernovae or in response to requests for follow up on novae or rarely outbursting objects. But alerted transients, including gamma ray bursts, get right of way.

Most often a GRB counterpart has already been located by professional observatories and it is of interest that we follow up on these afterglows. In some cases no candidate for the counterpart is known at the time of the observation. I have gone after several of those GRB error fields, usually in vain with afterglows fainter than 19....20 magnitude. In most cases the GRB had taken place more than 12 hours earlier, sometimes more than 24 hours. But nearby ones like GRB 030329 were still reachable after 4 nights.

Most X ray transients in our MW are heavily obscured by dust and nebulae, and are often hidden in regions with strong IR sources. Some of those IR sources show up very strongly on unfiltered CCD images but are very faint or even not present on reference images like those of the DSS including those of generation 2/red. GRBs on the other hand are (mostly) extragalactic and occur about equally in each direction, and often outside Milky Way patches. This makes it much easier to detect any new sources.

The error box of GRB 030725 was in such a direction and the search would be short and clean.

Some 7 hours after the burst trigger, around 20:45 LT, I started to take unfiltered CCD images of the field which had risen more than 30 degrees above the reasonably dark South Eastern horizon. With 45 sec exposures and 15 sec downloads one image was acquired every minute. Tracking errors on production telescopes will make some of the images unusable. But with this GRB at declination -50, those errors would not be such a big problem. I usually aim for about 15 'good' images, which under the local sky conditions gives depth of fields comparable to that of DSS-2/red images, about magnitude 20.5R. More than 20 images were taken.

Dark subtraction and flat fielding takes less than two minutes with the software. Three minutes after that last image, the telescope was already tracking an equatorial target, an exciting 'new' CV from the recent SDSS report, which was meant to be the main target for the night.

Timeseries imaging using DOS software allows dataprocessing to take place simultaneously. The calibrated images from the GRB field were stacked and inspected while the photometry of the CV was going on.

The inspection of the resulting image didn't show any new objects in this 'easy' field, but there was a smudge towards the North of the image, which looked like it might be an artefact or possibly an IR source. To investigate the latter I had to leave the observatory and to download a DSS-2/IR image from the internet with the home pc. It turned out there was no IR source at that position and the only way to find out if it was no image defect, more imaging had to be done. The measured brightness of this 'object' was 18.8CR. This is normally too faint a mark to be seen on the individual images, but inspection showed that it was present on a few of probably the sharper ones. Maybe there was something there. Follow up was certainly needed.

I wrote my first observation report to the AAVSO-GRB report list, mentioning no counterpart visible but that there was a need for further investigation on something and more observations.

The suspect object was actually a bit outside the quoted error box of 90% confidence, but now it was placed near the centre of the new series of images that were done around 22:00 LT, interrupting the ongoing timeseries of the CV.

This time the stacked image showed the object clear and starlike. A new report was sent, referring to the still presence of that new object, that it had slightly faded and with the announcement of further follow-up.

I have found several SNe over the past couple of years and acquired a strong sense of knowing whenever this happened. This feeling was present with this GRB afterglow detection, right there with the confirmation image. The object magnitude was now 19.0CR, measured against the same UCAC1 comparison stars.

The next imaging occurred around 00:30 LT. The GRB field was close to culmination, the weather was still fine and the Sagittarius cloud, the centre of the MW had already moved West of zenith. The new images had to bring final prove.

The object was still there and had faded down to 19.6CR. The final report for the night was sent off with the promise to send the images next morning.

It was now time to sleep, others would probably confirm and follow up. No-one actually did, but the AAVSO GRB office, by deed of A Henden, did send out the GCN notice despite no images had reached them, for which I am thankfull.

The next follow up on GRB 030725 was done on 28/7/03 .... at the Bronberg Observatory. The transient had disappeared from the sky down to a magnitude of 21CR.

Professional confirmation of this fading transient was obtained with the Danish 1.5m telescope on July 29.38 and reported in GCN 2335 by astronomers from Spain and Denmark.

Berto Monard / MLF
Bronberg Observatory / CBA Pretoria

Thanking Prof J Patterson of the Astronomy Dept of the Columbia University, New York, for helping arrange the observing instrumentation. I also appreciate the ad hoc interest, insight and support demonstrated by AAVSO-GRB consultant, A Henden.

• More information about this GRB is available here