Gamma-ray Burst 991216

On December 16, 1999, 16:07:01 UT a very bright GRB was detected by BATSE (trigger 7906), and subsequently scanned by RXTE/PCA which detected a previously unknown X-ray source at 5 mCrab - interpreted to be the X-ray afterglow.

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

BACODINE alert:

TITLE:         BACODINE FINAL BURST POSITION NOTICE
NOTICE_DATE:   Thu 16 Dec 99 16:08:06 UT
NOTICE_TYPE:   Final
TRIGGER_NUM:   7906
GRB_RA:         76.32d {+05h 05m 17s} (J2000),
                76.32d {+05h 05m 17s} (current),
                75.63d {+05h 02m 32s} (1950)
GRB_DEC:        +9.80d {+09d 48' 00"} (J2000),
                +9.80d {+09d 48' 00"} (current),
                +9.73d {+09d 43' 57"} (1950)
GRB_ERROR:     0.1 [deg radius, statistical only]
GRB_FLUENCE:   668562 [cnts]    Peak=119404 [cnts/sec]  Del_t=21.8 [sec]
GRB_TIME:      58021.36 SOD {16:07:01.36} UT
GRB_DATE:      11528 TJD;   350 DOY;   99/12/16
GRB_SC_AZ:     165.46 [deg]
GRB_SC_EL:     -65.90 [deg]  {Zenith_angle=155.90}  {Scan=-156.59}
SC_X_RA:       197.71 [deg] (J2000)
SC_X_DEC:       24.68 [deg]
SC_Z_RA:       274.10 [deg]
SC_Z_DEC:      -27.10 [deg]
SUN_POSTN:     263.74d {+17h 34m 58s}  -23.31d {-23d 18' 50"}
SUN_DIST:      164.74 [deg]
MOON_POSTN:      3.66d {+00h 14m 39s}   -3.28d {-03d 16' 53"}
MOON_DIST:      73.53 [deg]
PROG_VERSION:  5.33
PROG_LEVEL:    1
COMMENTS: Definite GRB.  
COMMENTS: This notice is based on 18 1.024-sec samples of the light curve.

TITLE:         BACODINE/HUNTSVILLE/LOCBURST BURST POSITION NOTICE
NOTICE_DATE:   Thu 16 Dec 99 17:02:43 UT
NOTICE_TYPE:   Hunts_Locburst
TRIGGER_NUM:   7906
GRB_RA:         79.81d {+05h 19m 15s} (J2000),
                79.81d {+05h 19m 14s} (current),
                79.12d {+05h 16m 28s} (1950)
GRB_DEC:       +10.79d {+10d 47' 24"} (J2000),
               +10.79d {+10d 47' 24"} (current),
               +10.74d {+10d 44' 21"} (1950)
GRB_ERROR:     0.5 [deg radius, statistical only]
GRB_INTEN:     100000 [cnts]    Peak=100000 [cnts/sec]
GRB_TIME:      58021.37 SOD {16:07:01.37} UT
GRB_DATE:      11528 TJD;   350 DOY;   99/12/16
GRB_SC_AZ:     160.24 [deg]                        {XScan=7.01}
GRB_SC_EL:     -68.84 [deg]  {Zenith_angle=158.84}  {Scan=-159.98}
SC_X_RA:       197.71 [deg] (J2000)
SC_X_DEC:       24.69 [deg]
SC_Z_RA:       274.10 [deg]
SC_Z_DEC:      -27.10 [deg]
SUN_POSTN:     263.74d {+17h 34m 58s}  -23.31d {-23d 18' 50"}
SUN_DIST:      166.93 [deg]
MOON_POSTN:      3.66d {+00h 14m 39s}   -3.28d {-03d 16' 53"}
MOON_DIST:      77.05 [deg]
PROG_VERSION:  1.42
PROG_LEVEL:    3
COMMENTS:  Locburst Coordinates.  
COMMENTS:  Definite GRB.  
COMMENTS:  Meets the preliminary criteria for RXTE-PCA follow-up
COMMENTS:  observation.
COMMENTS:  Possible RXTE-PCA ToO observation to follow.  
COMMENTS:  Intensity is really 101648 counts/sec.

TITLE:           GCN/RXTE_PCA BURST POSITION NOTICE
NOTICE_DATE:     Thu 16 Dec 99 22:49:22 UT
NOTICE_TYPE:     RXTE-PCA
TRIGGER_NUM:     7906
GRB_DATE:        11528 TJD;   350 DOY;   99/12/16
GRB_TIME:        58021.37 SOD {16:07:01.37} UT
GRB_RXTE_RA:      77.38d {+05h 09m 31s} (J2000),
                  77.38d {+05h 09m 31s} (current),
                  76.69d {+05h 06m 45s} (1950)
GRB_RXTE_DEC:    +11.19d {+11d 11' 24"} (J2000),
                 +11.19d {+11d 11' 24"} (current),
                 +11.13d {+11d 07' 39"} (1950)
GRB_RXTE_ERROR:  0.040 [deg radius, statistical only]
GRB_RXTE_INTEN:  5.00 [mCrab]
SUN_POSTN:       263.74d {+17h 34m 58s}  -23.31d {-23d 18' 50"}
SUN_DIST:        166.45 [deg]
MOON_POSTN:        3.66d {+00h 14m 39s}   -3.28d {-03d 16' 53"}
MOON_DIST:        74.72 [deg]
PROG_LEVEL:      3
COMMENTS:   RXTE-PCA GRB Coordinates.  
COMMENTS:   The RXTE-PCA observation started at 99/12/16 19:55:27.00 UT.  
COMMENTS:   A previously unknown xray source of   
COMMENTS:   about 5 mCrab level was detected   
COMMENTS:   during the two scans with RXTE/PCA    
COMMENTS:   around the BATSE/LOCBURST position.   
COMMENTS:   Two scans are parallel or near parallel   
COMMENTS:   to equator, so the determination of R.A    
COMMENTS:   is much better than Decl. The Decl. is   
COMMENTS:   determined assuming the source was   
COMMENTS:   constant during the two scan peaks   
COMMENTS:   separated by about 240-sec.  
COMMENTS:   Short-term variability of the source   
COMMENTS:   could lead to an error of a few tenths  
COMMENTS:   of a degree in Decl.  
COMMENTS:   T.Takeshima, C.B.Markwardt, & F.E.Marshall (GSFC)  
COMMENTS:   and T.Giblin & R.M. Kippen (UAH/MSFC)

GCN notice #463
R. M. Kippen, R. D. Preece and T. Giblin (University of Alabama in Huntsville) report on behalf of the BATSE GRB team:
An extremely bright gamma-ray burst was detected by BATSE on 1999 December 16.671544 UT (trigger number 7906). The event began with a weak precursor pulse lasting about 3 seconds, followed about 15 seconds later by an intense multi-peaked complex. The main emission lasted about 20 seconds, but a long decay tail persisted for another 30 seconds. Using preliminary rapid burst response data (not corrected for deadtime), we estimate that the peak flux was >60 photons cmE-2 sE-1 (50-300 keV; integrated over 1.024 s), and the total fluence was >2 x 10E-4 erg cmE-2 (>20 keV). This burst is thus one of the brightest (if not the brightest) ever detected by BATSE. The spectral properties appear to be typical of other GRBs.
The BATSE rapid burst response location of this event is (Ra, Dec J2000) = (79.81, 10.79) degrees, with a total uncertainty radius of 2.4 deg (68% conf.). Given that more precise localizations from RXTE-PCA and IPN and probable, we strongly encourage observations by wide-field telescopes. A location sky-map and lightcurve for this event are available at http://www.batse.msfc.nasa.gov/~kippen/batserbr/
GCN notice #465
J. Zhu, on behalf of the Beijing Astronomical Observatory GRB team, reports:
A combined field of about 1.8 degrees in radius centered at the BACODINE_FINAL_POSITION of GRB991216 (also given in GCN #463) was observed with the BAO 60/90 cm Schmidt telescope starting from about 1 hour after the GRB event. For the observed 12 fields of each in f.o.v. of 58' X 58', 3 150-s exposures were taken in different epoch. The limiting magnitude for combined images is expected to be fainter than R=20, and will be checked when I arrive the observation site this evening.
However, even for the quite large coverage, it's a pity that the later RXTE_PCA_POSITION of the GRB was out of the observed area.
We are planning to observe the RXTE_PCA_POSITION tonight with one 58' X 58' fields in much longer exposure time from about Dec. 17.5 UT. Any comments and suggestions are welcomed to zj@bac.pku.edu.cn (with a copy to zhu_jin@263.net for urgent message).
GCN notice #466
J. Mattox reports:
I began observing GRB 991216 at 02:46:09 on 12/17 UT with the 1.8m Perkins Telescope at Lowell Observatory. Will initially cover the region indicated by the RXTE/PCA scan - dRA=0.04 deg, dDEC=0.2 deg (Takeshima, Markwardt, Marshall, Giblin, & Kippen, GCN Notice, 12/16/99) with a series of 5'x5' frames covering this DEC range with 200s exposures and an Rc filter. I am prepared to do UBVRI photometry until 11:00 UT if the optical transient can be localized.
GCN notice #467
Bradley E. Schaefer (Yale) and Patrick Seitzer (Michigan) report:
We have obtained R-band and V-band images of GRB991216 and find no transient source in a comparison with the Digital Sky Survey. The first images were taken at 23:45 UT as 60 second V-band and R-band images with the Schmidt telescope on CTIO. The next image was a 10 minute R-band image with the Yale 1m telescope on CTIO starting at 23:52 UT (over a 10.2'X10.2' FOV). These images start 7.6 hours after the burst. The comparison with the Digital Sky Survey, has the usual difficulties that (a) the DSS does not go as deep as our images, (b) color effects near the DSS threshold make for a fuzzy limit.
We have already taken BVRI images with the CTIO Schmidt and UBVRIK images with the Yale 1m. We are now repeating these sequences to allow for the detection of variable sources.
With GRB991216 being one of the all-time brightest BATSE bursts, the ratio of the gamma-ray luminosity to the optical afterglow luminosity is at least roughly one order of magnitude larger than for any other known burst.
It is often convenient to have a 'proper name' for sources frequently discussed, so whimsically, we propose that GRB991216 be informally called the 'Beethoven Burst' in honor of the composer's birthday.
GCN notice #469
P. Garnavich, K. Stanek, M. Garcia, S. Jha and A. Szentgyorgyi (CfA) report:
We imaged a 15'x15' field centered at the RXTE localization of the GRB991216 (T.Takeshima et al.) with the FLWO 1.2m telescope and IR Camera on Dec 17.23. We find one object in our images that is not on the digitized sky survey. The object is stellar with a J magnitude of approximately 17. The position based on the Guide Star Catalog is 05:09:24.29 11:10:39 (2000). From the difference in color between the DSS and the J-band, this may be an extremely red star and we are monitoring it to look for variability. Shorter wavelength observations are suggested. A finder chart can be found at http://cfa-www.harvard.edu/~peterg/grb991216.gif which is reproduced below:

GCN/RXTE_PCA_POSITION

TITLE:           GCN/RXTE_PCA BURST POSITION NOTICE
NOTICE_DATE:     Fri 17 Dec 99 07:25:18 UT
NOTICE_TYPE:     RXTE-PCA
TRIGGER_NUM:     7906
GRB_DATE:        11528 TJD;   350 DOY;   99/12/16
GRB_TIME:        58021.37 SOD {16:07:01.37} UT
GRB_RXTE_RA:      77.38d {+05h 09m 31s} (J2000),
                  77.38d {+05h 09m 31s} (current),
                  76.69d {+05h 06m 44s} (1950)
GRB_RXTE_DEC:    +11.31d {+11d 18' 36"} (J2000),
                 +11.31d {+11d 18' 36"} (current),
                 +11.25d {+11d 14' 51"} (1950)
GRB_RXTE_ERROR:  0.080 [deg radius, statistical only]
GRB_RXTE_INTEN:  1.10 [mCrab]
SUN_POSTN:       263.74d {+17h 34m 58s}  -23.31d {-23d 18' 50"}
SUN_DIST:        166.55 [deg]
MOON_POSTN:        3.66d {+00h 14m 39s}   -3.28d {-03d 16' 53"}
MOON_DIST:        74.73 [deg]
PROG_LEVEL:      3
COMMENTS:  RXTE-PCA GRB Coordinates.  
COMMENTS:  The RXTE-PCA observation started at 99/12/17 02:15:37.00 UT.  
COMMENTS:  Previously reported RXTE/PCA X-ray  
COMMENTS:  source was detected again during  
COMMENTS:  followup scans six hours later, a  
COMMENTS:  factor of ~5 fainter (2-10 keV).  In  
COMMENTS:  this observation scans were primarily  
COMMENTS:  along Decl., and thus the R.A. was fixed  
COMMENTS:  at its previously reported value. Position  
COMMENTS:  uncertainties are estimated to be  
COMMENTS:  0.04 deg in R.A. and 0.08 deg in Decl.  
COMMENTS:  C.B.Markwardt, T.Takeshima & F.E.Marshall (GSFC)  
COMMENTS:  and T.Giblin & R.M. Kippen (UAH/MSFC)

GCN notice #470
A. Diercks, S. G. Djorgovski, J. S. Bloom, A. Mahabal, and R. R. Gal (Caltech) report on behalf of the Caltech-NRAO-CARA GRB collaboration:
The source by Garnavich et al. (GCN #469) is well detected in the N-band DPOSS survey plate (IVN + RG9) and thus unlikely to be related to GRB 991216. Unfortunately, the photometric calibration of this plate is not available at present. However, based on an average zero-point, we estimate the brightness of this source to be approximately I ~ 18.5 +/- 0.5. The source is not detected in either the F (red) or J (blue) plates.
GCN notice #471
Bradley E. Schaefer (Yale) reports:
I have obtained new B-, R- and K-band images which show the revised RXTE position (J2000 05:09:31 +11:18:36), with no optical transient apparent in a comparison with the Digital Sky Survey. The start time of the R-band image is 17 December 07:47 UT. The R-band image shows stars which are much fainter than the limit of the DSS. As always, I can identify stars in the error box for which a [weak] case could be made that the source should be visible on the DSS whereas it is not - but no such cases suggests any confidence.
The RXTE position is now too low in the sky to take any further images from CTIO.
The source identified by Garnavich et al. (GCN 469) is seen in our earlier R and I images as a red star missing from our U, B, and V images.
In an earlier notice (GCN 468), the new RXTE position was just outside the old field-of-view. Also, the start times of the exposures were stated in local time not UT, so the actual start time was 4:45 UT on 17 December (12.7 hours after the burst).
GCN notice #472
Robert Uglesich, Nestor Mirabal, J. Halpern (Columbia U.), Susan Kassin, & Sebastino Novati (Ohio State U.) report on behalf of the MDM Observatory GRB follow-up team:
We find a probable afterglow of the extremely bright BATSE burst GRB 991216 (Kippen et al. GCN #463) in a series of R-band observations taken by S. Kassin and S. Novati on the MDM 1.3m telescope. An object at (J2000) 05:09:31.2, +11:17:07.2 faded from approximately R = 18.8 on Dec. 17.142 to R = 19.4 on Dec 17.372. The corresponding power-law decay index is -1.4. This object is not visible on the digitized POSS II plate. Its position is consistent with the latest RXTE scan (Markwardt et al. GCN/RXTE_PCA notice).
This preliminary photometry is based on several USNO stars. We used a nonstandard broad R filter which transforms very well with Landolt R.
A finding chart is posted at http://www.astro.columbia.edu/~rru/GRB991216.html
GCN notice #473
A. Henden (USRA/USNO), C. Luginbuhl, F. Vrba, H. Harris, B. Canzian, J. Munn, S. Levine, H. Guetter (USNO), D. H. Hartmann (Clemson Univ.), M.C. Jennings (IGPP, UCR visitor) report:
We confirm the fading optical source identified by Halpern, et. al (GCN 472). Accurate position from a large set of USNO-A2.0 reference stars is
```
                    05:09:31.29  +11:17:07.4 (J2000)
```
(internal error +/- 0.1arcsec)
R-band observations, referenced to the star at 05:09:39.30 +11:16:59.5 (J2000)
```
  UT     r    telescope
  0409  3.54   1.0-m
  0511  3.81   1.3-m
  0702  3.87   1.3-m
```
with photometric errors around 0.05mag. It has been non-photometric here for the past few weeks, but as soon as the weather improves, we will calibrate this field in UBVRI.
We have additional R-band and V-band data, and will be observing this field again this evening. Our infrared camera went on the 1.55-m this morning, so we may attempt H-band observations as well.

GCN/IPN Notice for GRB991216

TITLE:          GCN/IPN/HUNTSVILLE LOCALIZATION
NOTICE_TYPE:    Updated (based on the BATSE-LOCBURST location)
NOTICE_DATE:    Fri Dec 17 18:14:57 GMT 1999
TRIGGER_NUM:    7906
GRB_DATE:       99/12/16
GRB_TIME:       16:07:01.36 UT
BACO_RA:        82.33d {+05h 29m 19s}   (J2000)
BACO_DEC:       +18.83d {+18d 49' 48"}
HUNT_RA:        79.79d {+05h 19m 10s}   (J2000)
HUNT_DEC:       10.78d {+10d 46' 46"}
HUNT_ERR:       2.06 [degees radius, statistical plus systematic]
B_H_DELTA:      8.41 [deg]
HUNT_SC_AZ:     160.24 [deg]
HUNT_SC_EL:     -68.84 [deg]  {Zen_angle=158.84}
SUN_POSTN:      263.74d {+17h 34m 58s}   -23.31d {-23d 18' 50"} (Current)
SUN_B_DIST:     175.33 [deg]
MOON_POSTN:     3.66d {+00h 14m 39s}   -3.28d {-03d 16' 53"}   (Current)
MOON_B_DIST:    80.37 [deg]
ANNULUS_RA:     346.5053d {+23h 06m 01s}   (J2000)
ANNULUS_DEC:    31.2297d {+31d 13' 47"}
ANNULUS_RADIUS: 84.916 [deg]
ANNULUS_WIDTH:  0.072 [deg]  (Total width)
COMMENTS:       The event began with a weak precursor pulse lasting
COMMENTS:       about 3 seconds, followed about 15 seconds later
COMMENTS:       by an intense multi-peaked complex.  The main emission
COMMENTS:       lasted about 20 seconds, but a long decay tail
COMMENTS:       persisted for another 30 seconds.
COMMENTS:       This IPN annulus segment is available at T+26.5 hrs.

This IPN localization is preliminary.  The "total width" of the annulus
was selected to include all known uncertainties and systematics.  The final
analysis will reduce this width.  All follow-up queries should be addressed to
Kevin Hurley (UC Berkeley, khurley@sunspot.ssl.berkeley.edu).

The center of the IPN annulus is tabulated below along the segment
that intersects the Huntsville error circle.  The Distance column is
the sky-angle distance between the Huntville location and the sampled position
on the arc segment.  The position of maximum probability plus
the 1-, 2-, & 3-sigma containment probabilities (statistical+systematic)
are marked.

A detailed description of the format, content, and meaning of this
document is given in the URL: 
http://gcn.gsfc.nasa.gov/gcn/ipn.html

     (J2000)
   RA       DEC       Distance

  74.817    7.048     6.17   <=== +3-sigma probability
  74.903    7.190     6.02
  74.988    7.332     5.86
  75.074    7.474     5.71
  75.160    7.616     5.56
  75.246    7.758     5.41
  75.332    7.900     5.26
  75.418    8.043     5.11
  75.504    8.186     4.96
  75.590    8.328     4.81
  75.677    8.471     4.67
  75.763    8.615     4.52
  75.850    8.758     4.38
  75.936    8.901     4.24
  76.023    9.045     4.10   <=== +2-sigma probability
  76.110    9.189     3.96
  76.197    9.332     3.82
  76.284    9.476     3.69
  76.372    9.621     3.56
  76.459    9.765     3.43
  76.547    9.909     3.31
  76.635   10.054     3.19
  76.722   10.199     3.07
  76.810   10.344     2.96
  76.899   10.489     2.86
  76.987   10.634     2.76
  77.075   10.780     2.67
  77.164   10.925     2.58
  77.253   11.071     2.51
  77.341   11.217     2.44
  77.430   11.363     2.39
  77.520   11.509     2.34
  77.609   11.655     2.31
  77.698   11.802     2.29
  77.788   11.949     2.28
  77.788   11.949     2.28   <=== Max probability
  77.788   11.949     2.28
  77.878   12.096     2.29
  77.968   12.243     2.31
  78.058   12.390     2.34
  78.148   12.537     2.38
  78.239   12.685     2.44
  78.330   12.833     2.50
  78.421   12.981     2.58
  78.512   13.129     2.66
  78.603   13.277     2.75
  78.694   13.426     2.85
  78.786   13.574     2.96
  78.878   13.723     3.08
  78.970   13.872     3.19
  79.062   14.021     3.32
  79.154   14.171     3.45
  79.247   14.320     3.58
  79.340   14.470     3.72
  79.433   14.620     3.86
  79.526   14.770     4.00
  79.619   14.921     4.14   <=== -2-sigma probability
  79.713   15.071     4.29
  79.807   15.222     4.44
  79.901   15.373     4.59
  79.995   15.524     4.75
  80.090   15.675     4.90
  80.184   15.827     5.06
  80.279   15.979     5.22
  80.375   16.131     5.38
  80.470   16.283     5.54
  80.566   16.435     5.71
  80.662   16.588     5.87
  80.758   16.741     6.03
  80.855   16.894     6.20   <=== -3-sigma probability

GCN notice #476
S. Jha, R. Kirshner, K. Stanek, P. Garnavich, M. Garcia, A. Szentgyorgyi (CfA), and J. Tonry (U. Hawaii) report:
We imaged the optical transient reported by Uglesich et al. (GCN 472) with the U. Hawaii 2.2-m telescope on Dec 17.61 UT. The source is clearly detected in each of two 300s R-band exposures. Though conditions were photometric, the observations were made at high airmass (~3), making precise absolute photometry difficult.
Differential photometry with respect to 30 USNO-A2.0 stars in the images yields a magnitude estimate for the transient of R = 19.4 +/- 0.1 (internal error). This is the same magnitude derived by Uglesich et al. for their Dec. 17.312 UT observation, but it is possible that our two zeropoints are not identical.
With respect to the reference star used by Henden et al. (GCN 473), we find an R magnitude difference of 4.29 magnitudes. This together with the data of Henden et al. gives an R-band power-law decay index of 1.0 +/- 0.2.
Given the lack of a precise calibration for comparison stars in the field, we also list the R magnitude difference between the transient and two stars very near the transient from our Dec 17.61 UT images:
```
         Star                   Position (J2000)        Transient - Star 
A (USNO-A2.0 975.01388002)  05:09:29.799 +11:17:08.47       +4.12 mag
B (anonymous)               05:09:32.132 +11:17:23.75       -0.04 mag
```
A finder chart with the proposed comparison stars labelled is available at http://cfa-www.harvard.edu/cfa/oir/Research/GRB/grb991216.jpg and reproduced below:
GCN notice #477
A. Diercks (Caltech), L. Ferrarese (UCLA), J. S. Bloom (Caltech) report on behalf of the Caltech-NRAO-CARA GRB collaboration:
We imaged the field of GRB 991216 in Gunn-i on UT Dec. 17.214 and Dec. 17.462 UT using the Palomar 200-inch with the COSMIC imager under photometric conditions. At each epoch we took three exposures of 300s each centered on the RXTE-PCA error circle (Takeshima et al. 1999). The OT identified by Halpern (GCN #472) is well detected in each image and faded 0.48 +/- 0.04 mag between epochs. This corresponds to a power-law decay index of alpha = -1.2 +/- 0.1.
Based on observations of Landolt standards in SA 94 and conversion to Gunn-i magnitudes (Wade et al., PASP 91:35), we estimate the brightness at i = 19.1 +/- 0.2 on Dec. 17.214 UT and i = 19.6 +/- 0.2 on Dec. 17.463 UT. Refinement of these absolute magnitudes will be possible as soon as better absolute calibrations are available. We note that the dust maps of Schlegal, Finkbinder, and Davis (ApJ, 500, 525) give E(B-V) = 0.634 at this position, (l,b) = (190.44,-16.63), corresponding to A(Gunn-i) = 1.3 for extra-galactic sources.
GCN notice #478
T. Takeshima (USRA/GSFC), C. Markwardt (U.Md./GSFC), F. Marshall (GSFC), T.Giblin & R.M. Kippen (UAH/MSFC) report:
RXTE has discovered an X-ray afterglow from the extremely bright gamma-ray burst detected with BATSE on 1999 Dec. 16.672 UT (Kippen et al., GCN Circ. 463, trigger number 7906). Initial results from the RXTE observations have been distributed in two GCN Notices entitled "GCN/RXTE_PCA BURST POSITION NOTICE". This circular summarizes the results of the RXTE observations.
The first RXTE observation searched for an X-ray afterglow by scanning the region around the BATSE LOCBURST position with the PCA, which has 1-degree field-of-view. In two of the scans a bright X-ray source was detected, the first detection occurring at 1999 Dec. 16.840 UT or 4.02 hours after the burst trigger, and second occurring about 200 seconds later. During both detections the pointing direction of the PCA was changing most rapidly in R.A. With such a scan pattern the source Declination cannot be determined with as high a precision as the R.A. The best-fit position from this observation was distributed through GCN at 22:49 UT on Dec. 16.
RXTE scanned across the same source again at Dec. 17.126 UT or 10.90 hours after the burst. During this observation the PCA scanned primarily in the Declination direction. The best-fit source position for both observations is R.A. = 77.38 +/- 0.04 and Decl. = 11.30 +/- 0.05 (J2000). This position is consistent with the optical transient detected by Uglesich et al (GCN 472).
The best-fit source intensities for the both observations were 1.24 +/- 0.04 and 0.25 +/- 0.01, in units of 1e-10 erg/s/cm2 in the 2-10 keV band. Assuming a power law decay, the evolution of the intensity can be predicted (in the same units) by f = 0.065 x t**(-1.64) where t is the time in days after the burst trigger.
Preliminary analysis of the source spectrum indicates a power-law shape with a photon index of about 2.1.
GCN notice #479
D. Andrew Howell, Michael Ward, Lifan Wang, and J. Craig Wheeler of the University of Texas at Austin report:
The field of GRB991216 was imaged with the 0.8m telescope Prime Focus Camera at McDonald Observatory. Three 0.49 sq. deg. 600s Rc exposures were obtained centered on the RXTE position reported by Takeshima et al. The images were taken at Dec 17.153, 17.162, and 17.173 UT under conditions of intermittent clouds and high cirrus, with a very rough limiting magnitude of 19.2. The OT reported by Uglesich et al. (GCN Circular #472) is confirmed, but is so near our magnitude limit that reliable photometry cannot be done. We can say that from visual inspection the OT appears brighter than most stars listed at 19.1 magnitude in the USNO (red) catalog, but fainter than stars around 18.5 (the USNO red photgraphic magnitudes are derived from scans of POSS-I plates and highly uncertain). Therefore our data are consistent with the first epoch data reported by Uglesich et al. (R=18.8 at Dec. 17.142).
GCN notice #483
G. B. Taylor (NRAO) and E. Berger (Caltech) report on behalf of a larger collaboration:
Beginning on December 18.16 UT we observed the field containing the bright BATSE burst GRB 991216 (Kippen et al. GCN 463) using the VLA at 8.5 GHz. We detect a 938 +/- 60 microJy radio source coincident with the optical transient reported by Uglesich et al. (GCN 472). No other radio sources are detected within 2 arcminutes above a 5sigma flux density limit of 300 microJy. The radio afterglow is at ra = 05h09m31.297s dec = +11d17'07.25" (equinox J2000) with a conservative error of 0.1 arcsec in each coordinate. Additional radio observations with the VLA and VLBA are planned.
GCN notice #484
K. Hurley, on behalf of the Ulysses GRB team, and M. Feroci, on behalf of the BeppoSAX GRB team, report:
We have obtained a refined IPN annulus for GRB991216, approximately two times narrower than the one reported in the automated GCN/IPN/ Huntsville localization circulated earlier today. The annulus center is at RA(2000), Decl.(2000) = 346.505, +31.230, and the radius is 84.915 +/- 0.016 degrees (3 sigma). The annulus reduces the area of the RXTE error box reported in the GCN/RXTE_PCA burst position notice circulated earlier today, and its center line passes approximately 3 arcseconds from the optical transient position reported by Halpern et al. (GCN 472). This annulus can be refined further.
Due to hacker activity, a map cannot be posted immediately on the IPN website. Contact khurley@sunspot.ssl.berkeley.edu for one.
GCN notice #485
Bradley E. Schaefer (Yale) reports:
I have observed the afterglow of GRB991216 with the Yale 1m telescope on CTIO with B, R, and K filters last night and with B, V, R, I, and K filters tonight. The afterglow has faded by 1.15 +- 0.10 mag in the R-band between times 15.7 hours and 34.0 hours after the burst. This corresponds to a power-law decline of index -1.37 +- 0.12.
The images show the optical transient to not be a point source, but to has some excess 'fuzziness' and extent. This could well be the underlying host galaxy. If so, then this might have one of the brightest hosts known, leading to possibilities of sharply defining the local environment of the progenitor in conjunction with the accurate radio position. A bright host would also suggest a relatively small red shift, perhaps appropriate for one of BATSE's brightest bursts.
Last night, I obtained substantial UBVRIK images covering the preliminary RXTE error box. At the end of this time, the RXTE reported a revised error box that was just outside my FOV. By this time, the position was at airmass of 2.5, and I could only get one R-band, one B-band, and some K-band images. With the late arrival of the revised RXTE error box, it was up to observatories farther west to identify the afterglow by its variability. On the second night, I am in the middle of taking BVRIK images in two batches (to allow interpolation to a common time). Here is a listing of the YALO data for the first two nights which shows the afterglow source:
```
        1999 Dec 17  07:47 UT     R-band    10 minutes  
        1999 Dec 17  07:58 UT     B-band    10 minutes
        1999 Dec 17  07:47 UT     K-band    19 x 1 minute
        1999 Dec 18  01:58 UT     K-band    30 x 1 minute
        1999 Dec 18  01:58 UT     I-band    5 minutes
        1999 Dec 18  02:05 UT     R-band    5 minutes
        1999 Dec 18  02:12 UT     V-band    10 minutes
        1999 Dec 18  02:23 UT     B-band    10 minutes
        1999 Dec 18 ~06:00 UT     K-band    30 x 1 minute
        1999 Dec 18 ~06:00 UT     I-band    5 minutes
        1999 Dec 18 ~06:00 UT     R-band    5 minutes
        1999 Dec 18 ~06:00 UT     V-band    10 minutes
        1999 Dec 18 ~06:00 UT     B-band    10 minutes
```
Photometric standards were obtained also, but these data have not been reduced yet.
GCN notice #486
Chris Dolan (U. Wisconsin), Ian Dell'Antonio (KPNO/Brown U.), Buell Jannuzi (NOAO/KPNO), and James Rhoads (STScI) report on behalf of the KPNO GRB follow-up team:
We report observations of the optical transient reported by Uglesich et al. (GCN #472) probably corresponding to the gamma-ray bright burst GRB 991216 (Kippen et al., GCN #463). We note that numerous independent confirming observations have now been reported starting with those of Henden et al. (GCN #473).
Our observations consist of six 180 second R-band exposures taken in two groups of three with the KPNO 0.9m telescope and Mosaic camera on December 17, 1999 UT. Photometric calibration was performed using observations of Landolt stars in SA93, SA97, and SA98 over a range of airmasses from 1.1 to 2.0 to estimate the photometric zeropoints (referenced to airmass=0; the extinction coefficient was measured to be 0.085 mag/airmass in R for the night) of the images with an accuracy of about +- 0.03 mag (based on uncertainties in the flatfielding). This systematic uncertainty is in addition to our measurement uncertainty listed below. Our reported coordinates of the optical transient were astrometrically calibrated to the USNO-A2 catalog using approximately 6500 star positions, and should be accurate to better than +-0.15 arcsec.
R-band observations from the KPNO 0.9m+Mosaic:
```
 RA  (J2000)    DEC        UT_start on    Exp  Mag  Error(statistical)
                           Dec. 17, 1999
05:09:31.29  +11:17:07.5    03:33:13     180s 18.63   0.02
05:09:31.29  +11:17:07.5    03:38:50     180s 18.64   0.02
05:09:31.28  +11:17:07.5    03:44:38     180s 18.64   0.07*
05:09:31.27  +11:17:07.6    10:44:24     180s 19.25   0.03
05:09:31.29  +11:17:07.5    10:50:06     180s 19.23   0.08*
05:09:31.28  +11:17:07.5    10:55:49     180s 19.28   0.03

* near edge of chip boundary, measurement is more uncertain.
```
The inferred power-law decay index is -1.2, consistent with that found by Uglesich et al.
Henden et al. (GCN #463) reported photometry relative to a star at 05:09:39.30 +11:16:59.5 (J2000) In our observations this star had a R magnitude of 15.187 +- 0.004
Similarly, our measured R magnitudes for the stars measured by Jha et al. (GCN #476) are:
```
  RA (J2000)    DEC(J2000)   R mag
05:09:29.799   +11:17:08.47  15.36+-0.01
05:09:32.132   +11:17:23.75  19.45+-0.03
```
GCN notice #487
Using the KPNO GRB follow-up team's photometry (GCN #486) for Henden's star H (GCN #473), and stars A & B of Jha et al. (GCN #476), the GRB 991216 data obtained so far with the 1.8m Perkins Telescope at Lowell Observatory provides the following Rc band measurements.
```
December (UT)  Exposure  Seeing   R
17.156         200s      3.0"     18.8 +/- 0.1
18.191         600s      1.7"     21.1 +/- 0.2
```
GCN notice #488
L. Wang, D. A. Howell, M. Shetrone, J. C. Wheeler, M. Eracleous and P. Meszaros on behalf of the HET GRB follow-up team report: GRB 991216 was observed with the Low Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. Three sets of data were obtained on UT December 18.167, 18.183, and 18.356. The data cover a wavelength range of 410-1000 nm. The data show no obvious emission or absorption lines. The flux peaks at around 6000 A.
GCN notice #489
Guy Pooley, MRAO, University of Cambridge reports
that an observation of GRB991216 with the Ryle Telescope at 15 GHz from 1999 Dec 17.81 to 18.18 measured a flux density of 1.1 +- 0.25 (sigma) mJy, at a position consistent with that reported for the optical transient and radio source detected by Frail et al. (GCN 472, 483).
GCN notice #491
E. Rol, P.M. Vreeswijk (U. of Amsterdam), R. Strom (NFRA, U. of Amsterdam), C. Kouveliotou (USRA, NASA/MSFC), E. Pian (ITESRE, Bologna), A. Castro-Tirado (LAEFF-INTA, Madrid), J. Hjorth (U. of Copenhagen) and J. Greiner (AIP, Potsdam) report on behalf of a larger European collaboration:
We have performed observations with the Westerbork Synthesis Radio Telescope (WSRT) at the position of the optical counterpart of GRB991216 (Uglesich et al., GCN 472), on Dec. 17 and 18 at 4.8 GHz and 1.4 GHz, for 6 hours at each frequency. At 4.8 GHz, we detect a 6 sigma radio source, coincident with the position of the radio afterglow reported by Taylor et al. (GCN 483) and by Pooley (GCN 489). At 1.4 GHz, we do not detect anything at this position, with an rms upper limit of 0.17 mJy. This upper limit can be reduced further.
If we split the observation into four epochs, we detect significant variability of the flux intensity of the source at 4.8 GHz, as can be seen from the following table:
```
UT Dec 1999     flux (mJy)      sigma (mJy)
(mid obs.)
17.783          0.82            0.13
17.997          0.84            0.13
18.044          1.62            0.13
18.153          0.70            0.16
```
GCN notice #492
P.M. Vreeswijk, E. Rol (U. of Amsterdam), C. Kouveliotou (USRA, NASA/MSFC), E. Pian (ITESRE, Bologna), A. Castro-Tirado (LAEFF-INTA, Madrid), H. Pedersen (KUO, Copenhagen) and J. Greiner (AIP, Potsdam) report on behalf of a larger European collaboration:
Around Dec. 18.13 UT, we imaged the optical transient to GRB991216 (Uglesich et al., GCN #472) in the infrared J, H and Ks bands with the New Technology Telescope (NTT) at the European Southern Observatory (ESO, Chile). The counterpart is clearly detected in each filter. We calibrated the field by observations of the NICMOS standard 9115, and we estimate the error in this calibration to be 0.1 mag. We obtain the following magnitudes (and measurement error) for the IR counterpart:
```
         J=17.56 (0.02), H=16.74 (0.02), Ks=16.76 (0.02).
```
GCN notice #494
W. Keel (Univ. of ALabama, Tuscaloosa) observed the GRB 991216 transient using the ONIS system on the MDM 2.4m telescope. From observations at 0400-0430 UT on 18 December, the K magnitude was 16.3 from a preliminary reduction with respect to UKIRT faint standard 2, in accord with the ESO NTT observations. The K image shows no extension with respect to stellar images in 1.3" seeing. J and H data were also obtained but have not yet been fully reduced.
GCN notice #495
P. Garnavich, S. Jha, K. Stanek, M. Pahre, M. Garcia, A. Szentgyorgyi (CfA) and J. Tonry (U. Hawaii) report:
The object reported by Uglesich et al. (GCN 472) as a possible afterglow of GRB991216 is detected on 5 sets of 120 sec. exposures taken with the FLWO 1.2m+IRCam on Dec. 17 (UT) and on two sets of 540 sec. images taken Dec 18. A preliminary calibration provides the following brightness estimates:
```
  UT Date     J     err
Dec. 17.35  16.99  0.05
Dec. 18.30  18.25  0.06
```
The power-law index derived from these data is -1.33. Our Dec. 18. point is significantly fainter than the J-band observation reported by Vreeswijk et al. (GCN 492) (UT 18.13).
The afterglow was again observed with the Hawaii 2.2-m in the R band and calibrated using star "B" (Jha et al. GCN 476) with R=19.45 (Dolan et al. GCN 486). These new brightness estimates are:
```
  UT Date     R    err
Dec 18.32  20.32  0.05
Dec 18.56  20.57  0.05
```
and when combined with previous R-band observations reported by Dolan et. al (GCN 486), Henden et al. (GCN 473) and Jha et al. (GCN 476), give a power-law index of -1.23+/-0.05. The Dec. UT 18.191 R-band magnitude of 21.1 reported by Mattox (GCN 487) is significantly fainter than our measurements.
GCN notice #496
P.M. Vreeswijk, E. Rol (U. of Amsterdam), J. Hjorth (U. of Copenhagen), C. Kouveliotou (USRA, NASA/MSFC), E. Pian, E. Palazzi (ITESRE, Bologna), H. Pedersen (U. of Copenhagen), J. Gorosabel, A. Castro-Tirado (LAEFF-INTA, Madrid) and J. Greiner (AIP, Potsdam) report on behalf of a larger European collaboration:
At Dec. 18.17 UT, we took two 30-minute low resolution spectra of the optical counterpart to GRB991216 (GCN #472) with the VLT-Antu telescope at the European Southern Observatory (ESO, Chile). These spectra roughly cover the wavelength range 400-900nm.
We detect several absorption features that we can identify with the following lines: FeII(at 237.45nm, 238.28nm, 258.67nm and 260.02nm), MgII(279.64nm/280.35nm) and MgI(285.30nm), assuming there exist three absorption systems at the redshifts z=0.77, z=0.80 and z=1.02.
No clear emission features are detected.
We are planning to take more spectra around Dec. 19.2 UT, which should allow us to verify these preliminary results.
GCN notice #497
A. Diercks, J. S. Bloom, S. G. Djorgovski, S. R. Kulkarni, B. T. Soifer, D. J. Thompson, R. R. Gal, and A. Mahabal report on behalf of the Caltech-NRAO-CARA GRB group:
Spectra of the OT associated with GRB 991216 were obtained at the WMKO Keck-II 10-m telescope by B. T. Soifer and D. J. Thompson on Dec. 18.3 UT, using LRIS instrument (Oke et al. 1995). Grating with 150 l/mm was used, giving the useful coverage of ~ 5600 - 10400 Ang, with a mean dispersion of ~ 4.86 Ang/pixel.
Despite a strong continuum detection, no significant absorption or emission features (other than atmospheric) have been detected in the summed spectrum. Absorbers at z ~< 1, as reported by Vreeswijk et al. (GCN #496), would be outside the wavelength range of our data.
In addition, we measure R = 20.30 +/- 0.06 at Dec 18.40 UT from 5 150s imaging exposures with LRIS. The OT was calibrated against star "B" (Jha et al. GCN #476) with R=19.45 (Dolan et. al., GCN #486). The images were taken in 0.75" seeing and the pixel scale is 0.21 arc-seconds/pixel. At this resolution, the PSF of the OT is consistent that with neighboring stars, and we find no evidence for the excess 'fuzziness' reported by Schaefer (GCN #485).
No host galaxy is detected on the Digital POSS-II (DPOSS) scans, corresponding to approximate limits of g > 21, r > 20.5, and i > 20 mag (conservative limits) in the Gunn system. This is consistent with the absence of flattening of the light curve so far, and the lack of detectable extension or PSF residuals in our Keck LRIS images.
GCN notice #498
B. L. Jensen, H. Pedersen, J. Hjorth, J. Pritchard (U. of Copenhagen), T. Abbott (NOT), A. J. Castro-Tirado (LAEFF-INTA, Madrid), E. Pian (ITESRE, Bologna), P. Vreeswijk (U. of Amsterdam) and J. Greiner (AIP, Potsdam) report on behalf of a larger European collaboration:
We have obtained R-band exposures of the optical counterpart (GCN #472) using the Danish 1.5m at La Silla (Dec. 18) and the 2.5m Nordic Optical Telescope on La Palma (Dec. 19). When measured relative to star "A" (GCN #476, #486) we obtain the following magnitudes:
```
Date (UT)      R               Exp    FWHM
 
Dec. 18.11 :  20.12 +-0.1     3x300s  1.8"
Dec. 18.32 :  20.40 +-0.1     3x300s  1.9"
Dec. 19.10 :  20.90 +-0.1     4x300s  1.8"  
```
When analyzed jointly with observations available from GCNs (excluding the Rc~21.1 from GCN #487) we deduce a power-law decay index of -1.17+-0.10. The corresponding plot is available at: http://www.astro.ku.dk/~brian_j/grb/grb991216/ .
GCN notice #499
E.M. Leibowitz reports on behalf of the Wise Observatory team - U. Giveon, B. Bilenko, E. Ofek and Y. Lipkin:
With the 1 m telescope of the Wise Observatory we have obtained 5 CCD R frames of GRB991216, each one of 5 minutes exposure. The mean UT time of all frames is Dec 17.7333. The R magnitude of the OT, as measured on the combined frame relative to 84 USNO-A1.0 catalogue stars, is 19.95 +-0.15. When measured relative to star "A" (GCN 476, 486, 498) the resulting magnitude is 19.89+-0.13. The combined image is placed at http://wise-obs.tau.ac.il/~eran/GRB991216/
GCN notice #500
L. Piro (IAS/CNR, Rome), G. Garmire (Penn State University), M. Garcia and the CXC team (CfA, Cambridge), F. Marshall & T. Takeshima (GSFC) report on behalf of a larger collaboration:
The Chandra X-ray Observatory imaged the field of GRB991216 for 10ks starting on Dec 18.208 (UT), i.e. about 37 hrs after the GRB (Kippen et al. GCN n. 463). From a preliminary analysis of the data we found a single unknown point source, that we name CXO J050931.4+111706, in the center of the field. The position is RA = 05h09m31.35s, Dec = 11d 17' 05'.7, consistent with the position of optical counterpart (Uglesich etal, GCN 472). The difference of 1".7 between the two positions is in fact within the error present at this stage of the analysis.
The flux observed in the ACIS-S plus High Energy Transmission Grating zero-order image is equivalent to 2.5 x 10**-12 ergs/cm**2/sec over 2-10keV, assuming a power law spectrum with a slope of 2.1 and the galactic Nh of 2.0x10**21 cm**-2. Given the preliminary nature of the reduction, this is consistent (within 20%) with the extrapolation of the decay slope observed with XTE of -1.64 (Takeshima etal, GCN 478). We note that the x-ray decay slope may be steeper than the optical slope (-1.23 +/- 0.05, Garnavich etal GCN 495; -1.17+/-0.10, Jensen etal GCN 498). A more detailed analysis of the data is in progress.
GCN notice #503
As suggested by Garnavich et al. (GCN 495), the Lowell/Perkins magnitude reported by Mattox in GCN 487 was indeed to faint by 0.8 magnitudes (due to the use of a preliminary flat with a large photometry radius).
A series of 400s integrations were done in R band centered at 20.30 Dec. UT (under marginal conditions, 2.6" FWHM seeing, and with the nearly full moon 25 deg away). GRB 991216 is apparent in the sum of 14 images. A preliminary R magnitude is 22.3 +/- 0.2, which is fainter than the extrapolation of the power law of Jensen et al. (GCN 498). This discrepancy could result from an error in our preliminary reduction of these Lowell/Perkins data, or it could indicate a break in the decay slope due to evolution of a jet.
This message should not be cited in publication - it is provided here to encourage observations which could confirm a break from the power law decay - a final reduction of these data will be linked to http://gamma.bu.edu/~mattox/grb.html when available.

GCN notice #504
R. M. Kippen (University of Alabama in Huntsville) reports on behalf of the BATSE GRB team the updated standard BATSE parameters for GRB 991216 (trigger 7906):

Duration (seconds)
     T50:  6.2720 +/- 0.0905
     T90: 15.1680 +/- 0.1109

Peak Flux (photons / cm^2-s, 50-300 keV)
    64ms: 91.4654 +/- 1.053570     (3rd brightest in BATSE catalog)
   256ms: 82.0787 +/- 0.498275     (3rd brightest in BATSE catalog)
  1024ms: 67.5055 +/- 0.226543     (2nd brightest in BATSE catalog)
 
Peak Flux (ergs / cm^2-s, 50-300 keV)
    64ms: 1.99303e-05 +/- 2.39823e-07
   256ms: 1.78031e-05 +/- 1.12832e-07
  1024ms: 1.45472e-05 +/- 1.27662e-07

Fluence in 4 discriminator channels (ergs / cm^2)
    1: 1.72637e-05 +/- 5.28282e-08    ( 20.6  -  64.7 keV)
    2: 2.29693e-05 +/- 5.43826e-08    ( 64.7  - 118.1 keV)
    3: 6.50656e-05 +/- 1.36158e-07    (118.1  - 345.5 keV)
    4: 1.50204e-04 +/- 1.16799e-06    (       > 345.5 keV)
 
Total fluence (ergs / cm^2, > 20.6 keV)
       2.55503e-04 +/- 1.17834e-06    (13th largest fluence in 
                                      BATSE catalog)

High time resolution lightcurves of this burst are available at http://gammaray.msfc.nasa.gov/batse/grb/lightcurve/

GCN notice #505
K. Hurley, on behalf of the Ulysses GRB team, T. Cline, on behalf of the NEAR GRB team, and C. Kouveliotou, on behalf of the BATSE GRB team, report:
We have obtained a ~24 sq. arcmin. (3 sigma) Ulysses-BATSE-NEAR error box for GRB991216 ((BATSE 7906: GCN 463, GCN 502). The corners of the box are:
```
 RA(2000)          Decl.(2000)
 5 h 9 m 34 s      11 o 16 ' 41 "
 5 h 9 m 03 s      11 o 03 ' 54 "
 5 h 9 m 52 s      11 o 27 ' 06 "
 5 h 9 m 21 s      11 o 14 ' 40 "
```
This box includes the optical transient (GCN 472), and further refinements to it are possible. Maps of this box and the earlier SAX-Ulysses annulus (GCN 472) may be found at ssl.berkeley.edu/ipn3/991216.
GCN notice #506
Robin Corbet (GSFC/USRA) and Don Smith (MIT) report for the RXTE All-Sky Monitor team at GSFC and MIT:
Using the RXTE ASM we have extracted an X-ray light curve for the position of the optical afterglow from GRB991216 (Uglesich et al., GCN 472) and we find evidence suggesting a detection of the X-ray afterglow (2-12 keV) at only one hour after the burst.
Observations of this location were obtained as part of the normal ASM sky-monitoring program. Standard ASM observations ("dwells") are 90 seconds long, and the ASM rotates between dwells such that a large fraction (~80%) of the sky is observed over 90 minutes (Levine et al. 1996, ApJ, 469, L33).
The first ASM dwell that covered the location of GRB991216 was obtained 0.99 hours after the peak of the burst (Kippen et al. GCN 463) and a total of 7 dwells were obtained during the next 11 minutes. From these 7 dwells we derive a mean flux of 32 +/- 8 mCrab (1 sigma error). A second sequence of 10 dwells covering this location was obtained starting 2.57 hours after the burst peak in an interval of about 15 minutes. This second sequence yields a mean flux of 12 +/- 4 mCrab. For comparison, we note that two other later clusters of dwells, centered on times of ~5.8 hours and ~7.5 hours after the burst peak, yield mean fluxes of 4 +/- 4 and 0.3 +/- 4 mCrab respectively.
Takeshima et al. (GCN 478) report a power-law fit to the two X-ray afterglow measurements made with the RXTE PCA at 4 and 11 hours after the burst. An extrapolation of this fit to earlier times predicts fluxes at 1.1 and 2.7 hours after the burst peak of 42 and 10 mCrab respectively. These are completely consistent with the mean ASM fluxes given above. The average spectrum of the afterglow during the first ASM sequence seems to be slightly steeper than that of the Crab nebula, with a spectral index of 1.8 +/- 0.3. This measurement is consistent with the spectral index of 2.1 determined from the PCA (ibid.).
While GRB afterglows are generally faint and thus difficult for the RXTE ASM to study, due to its modest collecting area and short observation times, we believe that this unusually bright afterglow has indeed been detected. These observations will require more detailed analysis, but the indications are that the RXTE ASM is providing a measurement of the X-ray afterglow light curve at times which have previously not been studied.
GCN notice #510
S. G. Djorgovski (CIT), R. Goodrich (CARA), S. R. Kulkarni, J. S. Bloom, A. Dierks, F. Harrison (CIT), and D. A. Frail (NRAO) report on behalf of the Caltech-CARA-NRAO GRB collaboration:
We obtained R-band images of the field of GRB 991216 using the ESI instrument on the WMKO Keck-II telescope, on UT 1999 Dec 29.41. The OT is well detected with an estimated magnitude R = 23.6, marginally fainter than the extrapolation of the early power-law light curve, possibly indicative of a change in the decline rate. We detect a galaxy extending out to ~ 1 arcsec to the W of the OT, with an estimated magnitude R ~ 24.5, not including any portion of the galaxy covered by the OT image. We propose that this is the host galaxy of the GRB, which may be also responsible for the z = 1.02 absorption system reported by Vreeswijk et al. (GCN #496). Its low-resolution spectra show no obvious strong line emission in a very rough, preliminary reduction. Another galaxy of a comparable magnitude (R ~ 24.8 mag) is detected 2.4 arcsec to the SE of the OT; it may be responsible for another absorption system reported in the spectrum of the OT. These magnitudes have the zero-point uncertainty of at least 0.3 mag, due mainly to the aperture corrections. Images of the field will be posted at http://astro.caltech.edu/~george/grb/grb991216.html and reproduced below:

CCD image in the R band obtained at the Keck-II telescope on UT 1999 Dec 29.41. The field shown is approximately 19.6 arcsec square, and the image is tilted w.r.t. the standard orientation by about 7.2 degrees. Galaxy A (R ~ 24.5 mag) is the probable host galaxy of the burst.
GCN notice #511
Dick Joyce (KPNO), James Rhoads (STScI), Babar Ali (IPAC), Ian Dell'Antonio (KPNO/Brown), and Buell Jannuzi (NOAO) report:
Joyce and Ali obtained J band spectrophotometry of the afterglow of GRB 991216 at 1999 Dec 18.372 UT, using the Kitt Peak National Observatory 2.1m telescope + Cryogenic Spectrometer (CRSP) with a 3.9 arcsec slit. The spectral resolution obtained was about 40 Angstroms. We detect a weak continuum (s/n approx. 4 per resolution element). No clearly significant emission or absorption features are detected. The continuum flux level rises from ~150 uJy at 1.12 um to ~220 uJy at 1.23 um, and remains approximately constant at ~220 uJy out to 1.33 um. It is thus not particularly well described by a single power law, suggesting either that not all of the near-IR flux seen is due to synchrotron emission, or that a break in the synchrotron spectrum was near 1.25 microns at the time of observation. The most obvious alternative flux source is a host galaxy, although this host would have to be luminous and red for this explanation to work, given the redshift constraint (z>1, Vreeswijk et al, GCN #496) and faint optical flux of the candidate host galaxy (Djorgovski et al, GCN #510). Wavelength and flux calibration were based on observations of a J=5.5 A0 star. The overall flux calibration is good to about 30%.
GCN notice #514
G. B. Taylor and D. A. Frail (NRAO) report on behalf of a larger collaboration:
Following the detection of the radio afterglow on Dec 18.16 (Taylor & Berger, GCN 483) from the bright BATSE burst GRB 991216 (Kippen et al., GCN 463) we observed with the Very Long Baseline Array (VLBA) for 2 hours beginning on December 18.32 UT. The flux density measured at 8.4 GHz was 705 +/- 85 microJy. The source is unresolved by these observations with a size less than 1 mas (size < 6.6 pc given the probable redshift of 1.02 reported by Vreeswijk et al. in GCN 496). The VLBA position is at ra = 05h09m31.2983s dec = +11d17'07.262" (equinox J2000) with a conservative error of 0.001 arcsec in each coordinate. This position is within 0.02 arcsec of the radio position derived by Taylor & Berger (GCN 483) who claim an uncertainty of 0.1 arcsec, but is 0.321 arcsec from the optical afterglow position reported by Dolan et al. (GCN 486) who claim an uncertainty of 0.15 arcsec. Given that the optical and radio afterglow should be coincident, we suggest that the optical astrometric position given by Dolan et al. suffers from a systematic error.
GCN notice #517
Bradley E. Schaefer (Yale) reports:
I have obtained deep R-band images with the 3.5m WIYN telescope on Kitt Peak starting on January 6, 2000 at 04:35 UT. The images had 0.6-0.8" FWHM seeing and used the Harris-R filter on the Mini-Mosaic camera. After standard processing, IRAF APPHOT photometry (with 0.6" radius aperture) was used. The only calibrated star which was unsaturated in my images is Star B, with R=19.45+-0.03 (Dolan et al. GCN 486). The optical transient is still visible with a SNR~7 within 0.3" of the radio position. My two measures of the optical transient magnitude are R=24.20+-0.15 and R=24.24+-0.20.
These observations were taken around the time when a possible underlying supernova would be at peak. However, if the red shift is 1.02 or greater (Vreeswijk et al. GCN 496) and the supernova is like SN1998bw, then the supernova light should be fainter than R~25.0 (Bloom et al. 1999, Nature, 401, 453).
An extrapolation of the afterglow light curve from the first few days of the burst gives either 23.45+-0.14 (Garnavich et al. GCN 495; index=-1.23+-0.05) or 23.66+-0.25 (Jensen et al. GCN 498; index=-1.17+-0.10). Thus, my combined magnitude (R=24.21+-0.12) is two-sigma fainter than the faintest of these extrapolations. (A similar result was found by Djorgovski et al. [GCN 510] for an observation on December 29, 1999.) An index of -1.40+- 0.06 since several days after the burst is needed to satisfy my observed magnitudes. It is possible that this is a break in the afterglow light curve (like for GRB990510) due perhaps to the evolution of a jet.
GCN notice #518
A. Henden (USRA/USNO), H. Guetter and F. Vrba (USNO) report on behalf of the USNO GRB team:
We have acquired UBVRcIc all-sky photometry for the field of GRB991216 with the USNOFS 1.0-m telescope on four photometric nights, but with some nights having poor seeing. This field is reddened, with a mean color (B-V) ~ 1.2. Because of this, many of the fainter sources do not have U or B magnitudes. We have therefore created two files on our anonymous ftp site: ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb991216.dat ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb991216f.dat where the first file contains multicolor observations, and the second file contains a smaller set of fainter stars surrounding the OT position, but with only R magnitudes listed. In particular, the R magnitudes we obtain for stars A and B of Jha (GCN 476) and the comparison star of Henden (GCN 463) agree with those obtained by Dolan, et. al. (GCN 486) within the quoted errors. The astrometry in these files is based on linear plate solutions with respect to USNO-A2.0. The internal errors are less than 100mas.
We also observed this field with the USNO IRCAM on the 1.55-m telescope on a single photometric night to obtain JHK photometry for 6 stars near the OT position. Observations were obtained of 11 standard stars to put the measures on the CIT photometric system. The errors reflect uncertainties in the in the extinction and color transformations. The data were reduced independently in K and H magnitudes and J-H and H-K colors:
```
RA (J2000)     DEC           K          H          J-H       H-K
77.375279   11.288480  11.77+/-.03 11.92+/-.03 0.69+/-.03 0.19+/-.03  
77.374150   11.285729  12.48   .03 12.62   .03 0.70   .04 0.19   .03
77.392377   11.290707  14.00   .04 14.19   .03 0.82   .04 0.27   .03 
77.399284   11.290875  14.68   .08 14.85   .10 0.50   .10 0.25   .10
77.377129   11.297201  14.61   .07 14.65   .05 0.61   .04 0.11   .05
```
from Halpern etal (astro-ph/000626)

Optical light curve

X-ray light curve
GCN notice #751
P.M. Vreeswijk, A. Fruchter, H. Ferguson and C. Kouveliotou report for a larger HST GRB Collaboration:
The afterglow of GRB 991216 (c.f. Kippen et al. 1999 ; Uglesich et al. 1999) was observed using HST/STIS at approximately UT 2000 April 17.6 through the clear (50CCD) and long pass (LP) filters, each for a total of 4790s. The pipeline reduced images were drizzled onto output images with pixels one-half native scale, or approximately 0."025 on a side.
We have projected the OT position from an early VLT image taken 1.5 days after the burst, to the frame of the HST drizzled images. Four bright nearby reference stars were used, and the estimated error in the resulting position is 0."1, corresponding to 4 pixels.
The position coincides with the visible extent of a faint galaxy, presumably the host of GRB 991216. The galaxy appears irregular, with a diameter of about 0."3. Another, probably separate, faint galaxy is located 0."4 to the southwest of the afterglow position. These two objects may explain the presence of two MgI absorption line systems in the VLT spectrum of 991216 (Vreeswijk et al. 1999), while one of the other galaxies visible at a separation of about 2" could potentially be responsible for the third absorption line system.
Using an aperture of diameter 0."4, we measure R=26.9 +/- 0.2 for the probable host of GRB 991216. The galaxy to the southwest has R = 26.1 +/- 0.2 inside an aperture of diameter 0."6. The large errors reflect the difficulty of matching the colors of these objects -- all objects in the field are quite red, perhaps indicating that the foreground extinction is even higher than the A(R)=1.6 mags predicted by the Schlegel et al. (1998) model. Additionally, it is probable that the small apertures used underestimate the total magnitudes of these galaxies by at least a couple of tenths.
The transient afterglow may still be present in these observations, but the low signal to noise does not allow unambiguous identification of the bright patch at the edge of the galaxy as a point source. We estimate that any remaining OT is no brighter than R=27.6. Assuming the single power law decay index, alpha = -1.36, of Garnavich et al. (2000), the afterglows expected magnitude at the time of our observations is R ~ 27 (not corrected for Galactic extinction). Our observations therefore suggest a break in the light curve, as already inferred by Halpern et al. (2000). A supernova of type SN1998bw at a redshift of z=1.02 would have R>30 at the epoch of our observations.
Images of the host and surrounding region can be found at: http://www.stsci.edu/~fruchter/GRB/991216
```
References:

Garnavich et al. 2000, ApJ, in press, preprint astro-ph/0003429
Halpern et al. 2000, ApJ, in press, preprint astro-ph/0006206
Kippen et al. 1999, GCN Circ. No. 463
Schlegel et al. 1998, ApJ, 500, 525
Uglesich et al. 1999, GCN Circ. No. 472
Vreeswijk et al. 1999, GCN Circ. No. 496
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