Gamma-ray Burst 981220

A hard X-ray flare was detected with the RXTE/ASM on 1998 December 20, 21:52:21 UT and confirmed to be a Gamma-Ray Burst by a simultaneous BeppoSAX GRB monitor detection. The triangulation ring with Ulysses crosses the ASM error box nearly perpendicular, thus reducing the error box to a few arcmin on each side.

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN notice #159
  D. A. Smith (MIT) reports on behalf of the RXTE/ASM Team at MIT and NASA/GSFC:
  "The RXTE All-Sky Monitor has detected a hard X-ray flare, beginning within a few seconds of 1998 December 20, 21:52:21 (UTC) and lasting for about 20 s. The flare consisted of a single symmetric peak that reached a maximum flux (1.5-12 keV; 1-s bins) of 1.2+-0.1x10^-7 erg cm^-2 s^-1. The event was detected in a single camera and was localized to an error box 5.0' x 60.0' (full width; 90% confidence including a rough estimate of systematic error due to calibration drift), centered on the position: R.A. = 03h43m01s, Decl. = +17o09'.0 (J2000.0) at a position angle of 75.54 degrees west of north. This position is inconsistent with any known X-ray sources. See http://xte.mit.edu/grb/grb981220/ for a sky map and other relevent plots."
  M. Feroci and B. Preger (IAS/CNR, Rome), L. Amati, F. Frontera and M. Orlandini (ITESRE/CNR, Bologna) report on behalf of the BeppoSAX/GRBM Team:
  "The BeppoSAX Gamma Ray Burst Monitor was triggered on 1998 December 20, 21:52:25.89365 UTC on a gamma-ray burst of about 15 s duration. The preliminary estimated fluence in the 40-700 keV energy band is 1.0+-0.2 erg cmE-2 sE-1, while its peak flux is about 10+-2 photons cmE-2 sE-1. We therefore confirm that the event reported by the RXTE/ASM Team was indeed a real GRB."
• GCN notice #160
  K. Hurley (UC Berkeley), on behalf of the Ulysses/GRB team, T. Cline (NASA-GSFC), E. Mazets, and S. Golenetskii (Ioffe Physico-Technical Institute), on behalf of the KONUS-Wind team, and D. A. Smith (MIT) report:
  "Ulysses and KONUS detected GRB981220. A preliminary triangulation annulus for this event is centered at RA(2000)=347.373 degrees, Decl(2000)=+7.336 degrees, with a radius 67.134 +/- 0.020 degrees (3 sigma). This annulus intersects the RXTE/ASM error box at

  RA(2000)        Decl(2000)
  
  3h 42m 28.5s    17o 06.3'
  3h 42m 29.0s    17o 11.1'
  3h 42m 39.0s    17o 11.6'
  3h 42m 38.5s    17o 06.8'
  

  to form a ~2.4' x 4.5' error box with approximately 11 sq. arcmin. area. The IPN annulus width can be reduced by a factor of 2 to 4. An image may be found at http://ssl.berkeley.edu/ipn3/newdata.html, under 981220 (and is reproduced below).
  
• GCN notice #162
  R. Mark Wagner and Sumner Starrfield report:
  We visually examined the region bounded by the ASM error box and IPN annulus for GRB 981220 on 1998 December 23.12 with the intensified acquisition camera (FOV: 2 arcmin) on the Boller and Chivens CCD spectrograph at the Steward Observatory 2.3-m telescope. Comparison with the Digitized Sky Survey does not reveal any new object brighter than about VR = 19.5 in the intersection of the error regions.
• GCN notice #163
  J. Halpern & E. Gotthelf (Columbia U.), C. Martin & B. Kern (Caltech), report:
  We obtained a 600 s R-band image of the joint RXTE/IPN error region of GRB 981220 (GCN 159,160) on the Palomar 5m telescope on Dec 23.15. The limiting magnitude was about 22.7. Preliminary analysis shows no new object in comparison with the digitized POSS II plate to a limit of R=20.5. Our image is posted at http://www.astro.columbia/~evg/grb981220.html.
• GCN notice #164
  The U.S. Naval Observatory GRB team (C. B. Luginbuhl, F. J. Vrba, A. A. Henden, S. E. Levine, J. Munn, B. Canzian, H. H. Guetter), D. H. Hartmann (Clemson Univ.), and M. C. Jennings (IGPP, UCR visitor) report I-band observations of the GRB 981220 localization formed by the intersection of the RXTE (GCN 159) and IPN (GCN 160) error boxes. Observations were obtained with the USNO 1.55-m telescope on UT 1998 December 23 consisting of 6 dithered 10-min exposures taken between UT 06:05:50 and UT 07:21:06 with a Tek 2K CCD having a field of view of approximately 6x6 arcmin. The combined, median filtered, and high-pixel rejected image can be viewed at the USNO public website: http://www.nofs.navy.mil/news/grb/grb981220.html. Although the seeing was rather poor (1.5-1.8 arcsec, FWHM), each of the 10-min exposures is deeper than the Digital Sky Survey POSS-I image and reveals many more objects than on the DSS. No obvious counterpart is revealed in comparing to the DSS image. Additionally, the six frames were blinked to search for variable objects, but none were obvious.
• GCN notice #165
  Bradley E. Schaefer (Yale) reports:
  "I have obtained 19 images in the V and R bands of the joint RXTE/IPN region for GRB981220. The total integration time is 2.5 hours in the V-band and 2.25 hours in the R-band, with the filters alternating in time, for a total time span of just over 5 hours. These images were obtained from 23 December 1998 00:52 to 05:55 UT with the Yale 1m telescope on Cerro Tololo. No variable source was identified in or near the GRB981220 region. Comparisons were made with the Digital Sky Survey, for which no 'new' stars appear. Also, comparisons were made between the first and last images, separated by roughly 5 hours of time. For the latter comparison, the limiting magnitude for detecting change is close to V=22.0 mag. As always, further analysis can push somewhat deeper, yet it is clear that any associated optical transient must be close to this quoted limit if not fainter."
• GCN notice #166
  The U. S. Naval Observatory GRB team (C. B. Luginbuhl, F. J. Vrba, A. A. Henden, S. E. Levine, J. Munn, B. Canzian, H. H. Guetter), D. H. Hartmann (Clemson Univ.), and M. C. Jennings (IGPP, UCR visitor) report follwup I-band observations of the GRB 981220 localization reported in GCN 164. A second set of 6 dithered 10-min I-band exposures was obtained with the USNO 1.55-m telescope and Tek 2K CCD on UT 1998 December 24 between UT 05:45:47 and UT 07:00:12. The combined, median filtered, and high-pixel rejected image can be viewed at the USNO public website: http://www.nofs.navy.mil/news/grb/grb981220.html beginning sometime 24 December. The seeing was again rather poor (1.7-1.9 arcsec, FWHM). No obvious fading or brightening sources are detected to a preliminary estimated limiting magnitude of I=22. The source marked "1" on the ARC 3.5m image taken by Dierks and Deutsch ( http://www.astro.washington.edu/deutsch/grb/grb981220/) seems approximately constant in this preliminary analysis. Complete photometric reduction of the two nights' USNO observations to determine the variance vs. magnitude relation and look for outliers is underway, and will be reported in a followup GCN later today.
• GCN notice #167
  F. Frontera and L. Amati (ITESRE/CNR, Bologna), M. Feroci and E. Costa (IAS/CNR, Rome) report on behalf of the BeppoSAX/GRBM Team:
  "From the high time resolution data the GRBM lightcurve shows a global three peak structure, with minor (~20-30 ms) structures within each peak. The time-resolved spectral analysis shows a weak spectral evolution. The time-averaged spectral power law photon index in the 40-700 keV energy range is (2.1+-0.3). The peak flux and fluence in the same band are (2.4 +/- 0.4)e-6 ergs/cm2/s and (1.0+/-0.2)e-5 ergs/cm2, respectively. Please note that the latter value corrects a typo occurred in Feroci et al. (GCN 159)."
• GCN notice #168
  T.J. Galama, P. Vreeswijk (U. of Amsterdam), J. van Paradijs (U. of Amsterdam and U. of Alabama in Huntsville), C. Kouveliotou (USRA/MSFC), R. Strom (NFRA and U. of Amsterdam), G. de Bruyn (NFRA and U. of Groningen) report:
  "We have observed the error box of GRB 981220 (Smith et al. 1998, GCN Circ 159; Hurley et al. 1998, GCN Circ 160) with the Westerbork Radio Synthesis Telescope (WSRT). We observed for 7 hours switching between 4.88 and 1.38 GHz on Dec 22.98 UT, 1998, and for 12 hours at 4.88 GHz on Dec 23.88 UT, 1998. We find no sources in the errorbox at 1.38 GHz down to 200 microJy (4 sigma). At RA = 3h42m28.98s +- 0.07s, Dec = +17d09'14.7'' +- 1.6'', we detect a source at 4.88 GHz at 260 +- 90 microJy (Dec 22.98), which is not detected at 1.38 GHz. On Dec 23.88 we measure 200 +- 50 microJy (4.88 GHz). The location of the source is at the edge of the error box of GRB 981220. Whether or not this source is related to GRB 981220 is unclear. We will continue monitoring the error box at 4.88 GHz."
• GCN notice #170
  D. A. Frail (NRAO), and S. R. Kulkarni (Caltech) report on behalf of a larger NRAO/Caltech collaboration:
  "We have observed the error box of GRB 981220 (GCN #159, GCN #160) with the Very Large Array (VLA) for 12 minutes each at 4.86 GHz and 8.46 GHz on Dec 23.14 UT, 1998. On this day we detected a source at RA = 3h42m28.94s +- 0.01s, Dec = +17d09'14.6'' +- 0.2'' with a 4.86 GHz flux density of 200 +- 45 microJy and a 8.46 GHz flux density of 370 +- 43 microJy. This is the same radio source reported by Galama et al. (GCN #168). Noting its unusual rising spectrum we carried out another set of observations on Dec 27.21. We observed the field for 18 minutes at each frequency and measured a 4.86 GHz flux density of 540 +- 40 microJy and a 8.46 GHz flux density of 460 +- 36 microJy. These wide swings of flux and spectral index resemble the behavior of radio afterglows at early times. Other sources in the field have remained constant during this time. We suggest that J034228.94+170914.6 is the radio afterglow of GRB 981220. Our position is accurate to 0.2 arcsec (1-sigma) and should enable deep optical studies. Further radio observations are in progress."
• GCN notice #171
  The U.S. Naval Observatory GRB team (F. J. Vrba, C. B. Luginbuhl, A. A. Henden, S. E. Levine, J. Munn, B. Canzian, H. H. Guetter), D. H. Hartmann (Clemson Univ.), and M. C. Jennings (IGPP, UCR visitor) report the results of photometric reductions of the I-band observations of of GRB 981220 taken at the U. S. Naval Observatory 1.55-m telescope. (See preliminary reports and details of observations in GCN 164 and 166.) One hour integrations on two consecutive nights were centered in time at UT 1998 December 23.280 and 24.266 and reached limiting magnitudes on each night of I = 24.5 for photometric errors of < 0.20 mag. Standardization was accomplished by observation of two nearby Landolt standards and assuming a mean extinction coefficient for the site, adding about 0.008 mag error to the total error budget (details can be obtained by contacting the authors at fjv@nofs.navy.mil). Photometry was obtained for all objects in an area centered on the intersection of the 5.0-arcmin wide (90% confidence) RXTE localization (GCN 159) and the 2.4-arcmin wide (99% confidence) IPN annulus (GCN 160), but with dimensions 3.4-arcmin by 10.5-arcmin in order to be confident of covering the actual GRB location. Photometry for a total of 267 objects brighter than I = 24.5 was obtained, of which approximately 100 objects lie within the formal RXTE/IPN localization intersection. No object was found to vary by more than 2-sigma of its standard error of unit weight (approximately 0.01 mag for I < 21.0; 0.05 mag for I = 23.0, 0.15 mag for I = 24.0). Specifically, the object suggested as a possible counterpart and marked "1" on the ARC 3.5-m image taken by Dierks and Deutsch (http://www.astro.washington.edu/deutsch/grb/grb981220/) had I-band magnitudes of 21.674+/-0.020 and 21.667+/-0.020 on UT Dec. 23 and 24, respectively.
  We find no counterpart to the detection limit of our frames estimated to be I = 25 for the radio source reported in GCN 168 and suggested to be the radio afterglow in GCN 170.
• GCN notice #175
  C. Bartolini, S. Bernabei, I. Bruni, A. Guarnieri, A. Piccioni, University of Bologna, and N. Masetti and G. Pizzichini, ITEsre-CNR, Bologna, communicate: "Four 40 min. R-band and one 40 min. I-band CCD frames of the error box of GRB 981220 (Hurley et al., GCN #160) were obtained with the 1.5-m telescope of the University of Bologna from 1998 December 22.86 UT to 23.02 UT. A preliminary analysis of the R band frames shows no variable object within 0.3 mag. In the I band frame no new evident objects with respect of the Palomar Digitized Sky Survey were observed inside the mentioned error box. The seeing was 3.8"; the measurable limiting magnitude was R ~ 22 (for the sum of the four CCD frames) and I ~ 19.7.
• GCN notice #176
  A. C. Eichelberger, J. S. Bloom, S. R. Kulkarni, R. R. Gal (Caltech), E. G. Martin (UCB), and D. A. Frail (NRAO) report:
  On December 23, 1998 (UT) we obtained four 300-s R-band imaging of the field of GRB 981220 with LRIS on the Keck II 10-m telescope. Based on astrometry from the USNO-A2 catalogue, at the position of the transient radio source (RT; GCN #170) we find a faint source in the stacked image. Our astrometry is good to 0.3 arcsec (rms) in each coordinate. Lacking observations of standard stars, we have used the standard R-band zeropoint of LRIS. Using a 1.2 arcsec aperture we find the object at the position of the RT to have R= 26.7 +/- 0.7 mag on 23.4 Dec 1998 UT. For reference, this zeropoint implies that the USNO star at ra: 03:42:28.32, dec: +17:09:42.60 (J2000) has a magnitude of R=18.35. Although the object appears somewhat extended in our 0.8 arcsec (FWHM) seeing, deeper imaging is required to firmly establish that this object is indeed the host galaxy of the GRB.
• GCN notice #179
  N. Masetti, E. Palazzi, E. Pian, F. Frontera (ITESRE-CNR, Bologna), T. Galama, P. Vreeswijk, J. van Paradijs (Univ. of Amsterdam), O. Hainaut (ESO), M. Fridlund (ESA/ESTEC), on behalf of a large collaboration, report:
  We have imaged the error box of GRB981220 (GCN N. 159 and 160) with the ESO New Technology Telescope equipped with EMMI plus B and I filters on 1998 Dec 23.04-23.07 UT, and with EMMI plus R filter on 1998 Dec 28.07-28.08 UT, in seeing conditions of ~1.6 arcsec (FWHM). A preliminary analysis of the 900-seconds B-band exposure shows a marginal (3-sigma) detection of a slightly extended source (~4 arcsec across) at a position (J2000) RA = 3h 42m 28.8s, Dec = +17o 09' 13.3" (3-sigma error of 1 arcsec), consistent with the position of the radio source detected by Galama et al. (GCN 168) and Frail and Kulkarni (GCN 170), identified with the radio afterglow of the GRB, and with the position of the faint source detected in the R-band by Eichelberger et al. (GCN 176). The detection of extended emission is consistent with the suggestion that at least part of the observed optical signal originates from a host galaxy. The source has a total magnitude of B ~ 24. Our 1200-seconds R-band exposure and 600-seconds I-band exposure yield no source detection at the same position, down to 3-sigma limiting magnitudes of R = 23.5 and I = 23, respectively. Our B measurement and the R magnitude reported in GCN 176 imply that the detected emission has a very blue spectrum (alpha ~ 5, with f_nu = k*nu^{alpha}), consistent with no detection in the I-band down to I = 25 (GCN 171). If this emission is dominated by the contribution of an underlying galaxy, this result can be compared with the finding of Fruchter et al. (1998, ApJ, in press, astro-ph/9807295) of remarkably blue colors for the host galaxy of GRB970228. The B-band image is posted in the site http://tonno.tesre.bo.cnr.it/~masetti/grb981220.html (and is reproduced below).
  
• GCN notice #186
  Sylvio Klose, Thueringer Landessternwarte Tautenburg, Germany, reports:
  "The error box of GRB 981220 was observed with the Tautenburg Schmidt telescope in the night from Dec. 23 to 24. For any OT at the position of the radio counterpart of GRB 981220 (Galama et al., GCN #168; Frail et al., GCN #170) the following preliminary data point can be provided: Dec. 23.717 UT, Gunn z >~ 17 +/- 1 mag (observations from 16:48 to 17:36 UT). A more accurate limit will be published, when photometric standards are available. Observations have also been performed in the I-band, but do not reach the limiting magnitude reported by Vrba et al. (GCN #171). Images are posted on the WWW at http://www.tls-tautenburg.de/research/grb.html.
  The error box of GRB 981226 was observed with the Tautenburg Schmidt telescope only 6.5 hours after the burst, but under very critical observing conditions (zenith distance ~75 deg). For the proposed NIR counterpart of GRB 981226 (Castro-Tirado et al., GCN #173), the object noted by Galama et al. (GCN #172), as well as for the variable object reported by Wozniak et al. (GCN #177) and Bloom et al. (GCN #182), the following data point can be provided: Dec. 26.682 UT, I >~ 16 (observations from 16:07 to 16:40 UT). This magnitude limit is based on the Hipparcos star 115941 which has I=7.9 (see http://astro.estec.esa.nl/hipparcos_scripts/ HIPcatalogueSearch.pl?hipId=115941) and which is very close to the GRB error circle."
• GCN notice #187
  I. A. Smith (Rice University), R. P. J. Tilanus and F. Baas (Joint Astronomy Centre) report on behalf of the James Clerk Maxwell Telescope (JCMT) GRB collaboration:
  We used the SCUBA sub-millimeter continuum bolometer array on the JCMT to observe the variable radio source J034228.94+170914.6 in the error box of GRB 981220 (Galama et al. GCN #168, Frail et al. GCN #170). The observation, performed in mediocre weather, started UT 1998 Dec 30.24 and lasted 3.8 hours. No source was detected at this location: the 850 micron flux density was 0.1 +/- 1.6 mJy.
• GCN notice #191
  M. R. Metzger, C. Martin, and B. Kern (Caltech) report:
  Images of the GRB 981220 joint localization errorbox (GCN 159,160) were obtained on UT Dec 23.15 (VRI, GCN 163) , UT Dec 24.2 (BVRI), and UT Dec 27.2 (VRI) at the Palomar 5m telescope using COSMIC. The seeing varied between 1.4 and 2.0 arcsec (FWHM). Photometric calibration was perfomed on UT Dec 24 using Landolt standards, and astrometry was computed using a solution of 70 nearby stars from USNO A2.0. For reference, on this system we find the following magnitudes for calibrating objects near the Galama et al (GCN 168) radio source:

                                     mag(+-) 
        Pos (J2000)           B         V         R         Ic 
  03 42 28.69 +17 09 26.1  23.81(14) 22.98(13) 22.34(09) 22.00(15) 
  03 42 28.90 +17 09 33.0  21.61(03) 20.75(04) 20.12(03) 19.56(04) 
  03 42 30.51 +17 09 20.9  22.12(06) 20.58(06) 19.65(04) 18.61(05) 
  03 42 28.33 +17 09 42.8  19.46(02) 18.47(04) 17.97(02) 17.36(02) 
  

  At the radio position of Frail and Kulkarni (GCN 170), no source was detected on Dec 23.15 to the limits (2-sigma) I > 23.0, R > 23.8, V > 24.2. The object marked "1" on the APO image referred to by Vrba et al on GCN 171 has I = 19.6 +- 0.1 on Dec 23.15 and Dec 24.2 at rough position 03h42m31.7 +17:10:04; it is fairly red, with (V-I) ~= 2. This disagrees with the photometry reported on GCN 171, and could be due to a difference in photometric zero point or different object.
  We can also place B-band upper limits at the radio position. On Dec 24.2, there is no significant detection (though possibly a very faint peak), which puts a limit of B >= 25.4 (1-sigma). Any host is less blue and fainter than implied on GCN 179 given the Echelberger et al. (GCN 176) magnitude. The detection of flux by Masetti et al. (GCN 179) with B ~= 24 on Dec 23 would imply variability, and thus may be the optical afterglow of GRB 981220. The V limit on Dec 23 is consistent with a blue spectrum, and the power law time decay in B would be steeper than alpha = -1.7 (though uncertainties are large). No significant variability of objects (other than asteroids) is detected over the Dec 23 to Dec 27 interval in V, R, or I, consistent with other reports. A B-band image from Dec 24 may be found at http://astro.caltech.edu/~mrm/grb981220.html (and reproduced below).
  
• GCN notice #192
  H. Pedersen, B. Lindgren, J. Hjorth (University of Copenhagen), M. I. Andersen (Nordic Optical Telescope), A. O. Jaunsen (University of Oslo), J. Sollerman (Stockholm Observatory), and J. Smoker, C. Mooney (Queens University, Belfast) report:
  Images of GRB 981220 (GCN 159, 160), including the position of the proposed radio counterpart (Galama et al., GCN 168; Frail and Kulkarni, GCN 170) were obtained with the 2.5-m Nordic Optical Telescope, La Palma, on 1998 Dec 22.927 UT (900 sec, R, FWHM = 1.4"), and with the 1.5-m Danish telescope, La Silla, on 1998 December 23.110 UT (300 sec, R, FWHM = 1.3").
  No optical source was detected at the position of the radio source. The upper limits for 3-sigma detections are m(R) > 23.8 on December 22.927, and m(R) > 23.0 on December 23.110.
• GCN notice #193
  N. Masetti, E. Palazzi, E. Pian (ITESRE-CNR, Bologna), by apologizing with the GCN readers, would like to notify them about a change in GCN #179 (optical ESO-NTT observations of GRB981220): the observation time of the B and I observations is incorrect, and should be modified from 'Dec 23.04-23.07 UT' to 'Dec 24.04-24.07 UT'.
  Therefore, the upper limit of B > 25.4 given in GCN #191 implies a variation of more than 1 magnitude in ~3.5 hours, namely a much steeper lower limit for the power-law decay index than that suggested in GCN #191.
• GCN notice #194
  F. J. Vrba reports, on behalf of the U.S. Naval Observatory GRB team, that an error was made in the calibration of GRB 981220 photometry reported in GCN 171. An incorrect exposure time was used with the result that all photometry reported in GCN 171 should be brightened by 1.945 magnitudes. For convenience, we provide a corrected version below and apologize for any confusion that may have resulted.
  We report the results of photometric reductions of the I-band observations of of GRB 981220 taken at the U. S. Naval Observatory 1.55-m telescope. (See preliminary reports and details of observations in GCN 164 and 166.) One hour integrations on two consecutive nights were centered in time at UT 1998 December 23.280 and 24.266 and reached limiting magnitudes on each night of I = 22.6 for photometric errors of < 0.20 mag. Standardization was accomplished by observation of two nearby Landolt fields comprising nine standard stars and assuming a mean extinction coefficient for the site, adding about 0.008 mag error to the total error budget (details can be obtained by contacting the authors at fjv@nofs.navy.mil). Photometry was obtained for all objects in an area centered on the intersection of the 5.0-arcmin wide (90% confidence) RXTE localization (GCN 159) and the 2.4-arcmin wide (99% confidence) IPN annulus (GCN 160), but with dimensions 3.4-arcmin by 10.5-arcmin in order to be confident of covering the actual GRB location. Photometry for a total of 267 objects brighter than I = 22.6 was obtained, of which approximately 100 objects lie within the formal RXTE/IPN localization intersection. No object was found to vary by more than 2-sigma of its standard error of unit weight (approximately 0.01 mag for I < 19.1; 0.05 mag for I = 21.1, 0.15 mag for I = 22.1). Specifically, the object suggested as a possible counterpart and marked "1" on the ARC 3.5-m image taken by Dierks and Deutsch (http://www.astro.washington.edu/deutsch/grb/grb981220/) had I-band magnitudes of 19.729 +/- 0.020 and 19.722 +/- 0.020 on UT December 23 and 24, respectively.
  We find no counterpart to the detection limit of our frames estimated to be I = 23.1 for the radio source reported in GCN 168 and suggested to be the radio afterglow in GCN 170.
• GCN notice #196
  J. S. Bloom, S. G. Djorgovski, S. R. Kulkarni (CIT), J. Brauher (IPAC), D. A. Frail (NRAO), R. Goodrich, F. Chaffee (CARA) report on behalf of the Caltech-NRAO-CARA GRB Collaboration:
  On 14 January 1999 UT we imaged the region of the radio transient (GCN #168; GCN #170) of GRB 981220 (GCN #159; GCN #160) with the Keck II 10-m telescope. Total integration time in R-band was 1300-s in ~0.6 arcsec seeing (FWHM). An astrometric plate solution was obtained on the stacked image using the USNO-A2.0 catalogue. The r.m.s. errors of the 26 tie stars surrounding the radio position was 0.24 arcsec (in ra) and 0.18 arcsec (in dec). A Landolt standard field (PG0231+051) was also observed for photometric calibration.
  Our Keck images can be found at the web location, http://astro.caltech.edu/~jsb/GRB/grb981220.html. There are two sources in the vicinity of the radio transient, 'J' and 'K'. Source 'J' coincides with the radio position as reported earlier (GCN #176). On Jan 14.30, the magnitudes of source 'J' and 'K' are respectively 26.4 (+/- 0.3 statistical; +/- 0.4 zero-point error) and R = 25.6 (+/- 0.2 statistical; +/- 0.4 zero-point error); the zero-point error includes uncertainties in the local sky level determination, aperture correction, and the photometric zero-point of the standard stars.
  We note that on Dec 23.4 1998 (GCN #176) both 'J' and 'K' have comparable brightness whereas 'J' is clearly fainter than 'K' in the Jan 14.30 image. We conclude that the optical source coincident with the radio transient position has faded by ~0.8 mag between the two epochs. Since the inferred decay is shallower than that found in other GRB afterglow, we suggest that the first epoch flux measure of the optical transient (GCN #176) was partly contaminated by the host galaxy.
  Masetti et al. (GCN #179) had claimed that the source at the radio position had a 4-arcsec extension. As can be seen from our images, the extension is likely an artifact resulting from poorer seeing in the B-band image of Masetti et al.
  We conclude that source 'J' is the host galaxy of GRB 981220 and it is not unusually large but has typical angular size of other host galaxies.
• GCN notice #269
  D. A. Frail (NRAO), S. R. Kulkarni (Caltech) and G. B. Taylor (NRAO) report:
  We have been monitoring the radio source claimed to be a possible afterglow candidate for GRB 981220 based on early observations at WSRT (GCN #168) and the VLA (GCN #170). Observations made over a period of 75 days shows that the source has increased in flux density at 4.9 GHz by nearly a factor of four. However, at 15 GHz the source has remained constant, while at 8.5 GHz there are only small (<20%) variations. The behavior of the radio light curves for this source does not resemble that of previous radio afterglows. Further observations are planned with the Very Long Baseline Array to ascertain the nature of this unusual radio variable.
• GCN notice #270
  K. Hurley (UC Berkeley Space Sciences Laboratory) on behalf of the Ulysses GRB team, and M. Feroci (IAS - Rome) on behalf of the BeppoSAX GRB Team, report:
  A refined IPN annulus for GRB981220 has been obtained by triangulation between Ulysses and BeppoSAX. It is centered at RA(2000)=347.398 degrees, Delta(2000)=+07.340 degrees, with radius 67.118 degrees, and 3 sigma half-width 0.015 degrees. The radio source reported by Frail et al. (GCN 170) and by Galama et al. (GCN 168) lies 0.027 degrees from the center line of the annulus, and is therefore outside it. Further refinement in the annulus half-width is possible, but we can state with some certainty that the position of the radio source is incompatible with that of the gamma-ray burst.
• GCN notice #287
  G. B. Taylor (NRAO), D. A. Frail (NRAO), and S. R. Kulkarni (Caltech) report:
  On 1999 March 10.01 we carried out VLBA observations of the radio source, J0342+1709, claimed to be a possible afterglow candidate for GRB 981220 based on early observations at WSRT (GCN #168) and the VLA (GCN #170). Within the 0.9" x 0.9" error box set by VLA observations at 8.4 GHz we detect an extended source with a peak flux density at 5 GHz of 404 +/- 60 microJy. The position of the radio source is ra=03h42m28.9611s dec=17d09'14.669" (equinox J2000) with an uncertainty of 0.05 arcsec in each coordinate. The integrated flux density of this source is 528 +/- 90 microJy. The flux density at 5 GHz measured by the VLA on March 8.96 was 510 +/- 26 microJy. The VLBA image can be viewed at http://www.nrao.edu/~gtaylor/G981220.html.
  The standard models and redshift distribution of GRB afterglows predict that the radio counterpart to G981220 should be unresolved by our VLBA observations. The "core-jet" morphology of the VLBA image therefore makes it unlikely that J0342+1709 is associated with G981220. A more plausible explanation is that it is a highly variable background intraday-variable (IDV) source. Such sources are known to vary on timescales of days to weeks, and consist of a strong core and one-sided jet. This explanation is supported by the location of J0342+1709 outside of the refined IPN localization (GCN #270) for G981220.