Gamma Ray Burst 971214 observations at NOAO

James Rhoads observed the optical transient associated with GRB 971214 on the night of 971217 UT (971216 - 971217 local time), using the 0.9 meter telescope. This source was first reported by Halpern, Thorstensen, Helfand, Costa, and the BeppoSAX team in IAU Circular 6788. It was confirmed by contemporaneous observations at Apache Point Observatory (Diercks et al) and by observations with the Keck Telescopes on Mauna Kea (Kulkarni et al).

We observed in I band, obtaining a total of forty-four 300 second exposures before morning twilight became noticeable. The combined final image shows the counterpart, though it is at the limit of what the data can do and our present estimate is a 30-40% flux error. (There are another 30 minutes or so of miscellaneous data- shorter exposures and exposures in morning twilight- but they are unlikely to significantly improve our results.)

Our final images can be obtained in FITS format by anonymous FTP from ftp://gemini.kpno.noao.edu/pub/grb/971214. If anyone has a good reason to want the individual frames, we'd be happy to provide them as well, but the transient is much too faint to be seen in any small subset of our data.

If you use this data in a publication, please at least cite our communication to the GCN (Gamma Ray Burst Coordinate Network) on Dec 17 1997 (Rhoads et al), or our report in the IAU Circulars (IAUC 6788). A standard issue Kitt Peak acknowledgement may also be in order in some cases, e.g. if you wish to reproduce the image. That acknowledgement is something like: This work uses photometry obtained by James Rhoads for the Kitt Peak National Observatory GRB followup team using the KPNO 0.9 meter telescope. Kitt Peak National Observatory, National Optical Astronomy Observatories, is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.

KPNO image of GRB 971214 on 971217 UT

Summary of Results

The most recent and succinct summary of our results is the following excerpt from IAU Circular 6793:

GRB 971214
J. Rhoads, Kitt Peak National Observatory (KPNO), for the KPNO gamma-ray-burst follow-up team, reports: "We imaged the field surrounding GRB 971214 with the KPNO 0.9-m telescope in the I band from Dec. 17.37 to 17.55 UT. The proposed optical counterpart (Halpern et al., IAUC 6788) was detected in a combined image with 220 min of total integration time. The transient was at mag I = 22.9 +/- 0.4, based on stellar magnitudes and positions provided by J. Halpern (priv. comm.) for stars of I = 15.93 and 18.46 at R.A. = 11h56m29s.86, Decl. = +65o12'15".9 (equinox 2000.0), and 11h56m25s.80, +65o11'35".9, respectively. Images and further details are available at http://www.noao.edu/noao/grb/971214.html."

More detail (but no absolute photometry) is available in our message submitted to the GRB Coordinate Network on 17 December 1997:
On 17 December UT, we observed the candidate optical counterpart to GRB 971214 that was reported by Halpern, Thorstensen, Helfand, Costa, et al (IAU Circular 6788). We used the Kitt Peak National Observatory 0.9 meter telescope with a Harris I band filter. The transient source is seen in the clipped average of forty-four 300 second exposures, though it is quite near the detection limit. We also detect the two nearby faint sources that are reported by Kulkarni et al and apparent in the R band images by Diercks et al. The brighter of the two is clearly nonstellar in our data. The observations spanned the period UT 08:55 to 13:18 on 971217. The point spread function of the final image has approximately a 2 pixel (approx 1.6 arcsec) full width at half maximum.

Photometry with a 2 pixel (approx 1.6 arcsec) radius aperture yields an I magnitude difference of +4.4 +- 0.4 magnitudes between the optical transient and to the bright star ~ 26 arcseconds SSW of the transient.

Other magnitudes relative to this same reference star are
-2.52 +- 0.015 Bright star ~ 26 arcsec NE of transient;
+0.33 +- 0.02 Bright star ~ 55 arcsec NW of transient;
+4.35 +- 0.35 Fainter nearby fuzzball, ~ 5 arcsec N of transient;
+3.48 +- 0.15 Brighter nearby galaxy, ~ 5.5 arcsec SW of transient;
+3.15 +- 0.15 same object, but comparing 3 pixel radius apertures.
We do not have absolute photometric calibration for our data. A crude estimate suggests that the 3 sigma limiting magnitude of the data set ought to be in the range 23 < I < 24.

The combined image will be linked to the KPNO GRB followup web page at http://www.noao.edu/noao/grb/971214.html .

This message is quotable in publications.

-James Rhoads, on behalf of the Kitt Peak National Observatory GRB followup team.

Comments on the images

procmos1.fits is the composite image formed by averaging forty-four 300 second dithered 0.9 meter telescope I band exposures.
procmos1m19.fits is a spatially filtered version of procmos1.fits that is less affected by scattered light from the nearby bright star SAO 15663. I first obtained a smoothed local sky + scattered light estimate by running a 19x19 pixel median filter over procmos1.fits, and then subtracted this sky + scattered light image to produce procmos1m19.fits.

General comments: The pixel scale is very approximately 0.8 arcsec per pixel. The instrument used was the T5HA 512x512 pixel CCD camera. The optical transient is the source located at pixel coordinates (333.92,465.26). North is to the left, and East is at the top.

The effective exposure time varies greatly across the final mosaic, since we were offsetting the telescope quite a lot to minimize the effects of scattered light. If you need to reconstruct the exposure time for a particular pixel, you can do so from the FITS header, which contains keywords XORIG_N and YORIG_N to indicate where pixel (1,1) of each input image ended up in the final image. This is probably not important information for the optical transient, which by construction was included in every frame [as were the three bright stars at pixels (313.98, 492.93), (366.04, 460.70), (291.55, 414.18)].

Other GRB 971214 information on the Web

As of 971217, there are some good links right at the top of the GCN homepage. This includes a record of what measurements have been reported on the mailing lists, etc.

The first optical ID was by a team at the MDM observatory. They observed in I band, so their data is most directly comparable to ours.

The Apache Point Observatory 3.5 meter telescope has reported an R band detection of the optical transient.

Caltech people are going after it with Keck, but I don't know if they have anything on the web yet.

IAU Circular links here are only accessible from the NOAO network; sorry.
IAU Circulars numbers 6794, 6793, 6792, 6791, 6789, 6788, and 6787.


[GRB logo] GRBs [NOAO logo] NOAO KPNO logo KPNO WIYN logo WIYN

National Optical Astronomy Observatories, 950 North Cherry Avenue, P.O. Box 26732, Tucson, Arizona 85726, Phone: (520) 318-8000, Fax: (520) 318-8360

jrhoads@noao.edu - Last updated: 21 December 1997