Gamma-ray Burst 031220

(All information courtesy of the instrument teams.)

Previous IAU Circulars

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Results of Observations

GCN/BACODINE POSITION NOTICE

 
TITLE:          GCN/HETE BURST POSITION NOTICE
NOTICE_DATE:    Sat 20 Dec 03 03:30:02 UT
NOTICE_TYPE:    HETE S/C_Alert 
TRIGGER_NUM:    2976,   Seq_Num: 1
GRB_DATE:       12993 TJD;   354 DOY;   03/12/20
GRB_TIME:       12596.74 SOD {03:29:56.74} UT
TRIGGER_SOURCE: Trigger on the 6-120 keV band.
GAMMA_RATE:     466  [cnts/s]  on a 1.300 [sec] timescale 
SC_LONG:        320 [deg East] 
SUN_POSTN:      267.62d {+17h 50m 30s}  -23.42d {-23d 25' 13"}
MOON_POSTN:     220.09d {+14h 40m 22s}  -15.04d {-15d 02' 39"}  
MOON_ILLUM:     15 [%]
COMMENTS:       No s/c ACS pointing info available yet.
COMMENTS:       Probable GRB.

GCN/BACODINE POSITION NOTICE

TITLE:          GCN/HETE BURST POSITION NOTICE
NOTICE_DATE:    Sat 20 Dec 03 03:30:33 UT
NOTICE_TYPE:    HETE S/C_Update
TRIGGER_NUM:    2976,   Seq_Num: 2
GRB_DATE:       12993 TJD;   354 DOY;   03/12/20
GRB_TIME:       12596.74 SOD {03:29:56.74} UT
TRIGGER_SOURCE: Trigger on the 6-120 keV band.
GAMMA_RATE:     466  [cnts/s]  on a 1.300 [sec] timescale
SC_-Z_RA:        88 [deg]
SC_-Z_DEC:       22 [deg]
SC_LONG:        320 [deg East]
WXM_CNTR_RA:     72.760d {+04h 51m 02s} (J2000),
                 72.821d {+04h 51m 17s} (current),
                 71.982d {+04h 47m 56s} (1950)
WXM_CNTR_DEC:   +27.340d {+27d 20' 24"} (J2000),
                +27.346d {+27d 20' 47"} (current),
                +27.256d {+27d 15' 20"} (1950)
WXM_MAX_SIZE:   60.00 [arcmin] diameter
WXM_LOC_SN:     3 sig/noise (pt src in image)
WXM_IMAGE_SN:   X= 3.7 Y= 1.2 [sig/noise]
WXM_LC_SN:      X= 7.5 Y= 4.7 [sig/noise]
SUN_POSTN:      267.62d {+17h 50m 30s}  -23.42d {-23d 25' 13"}
SUN_DIST:       166.07 [deg]
MOON_POSTN:     220.09d {+14h 40m 22s}  -15.04d {-15d 02' 39"}
MOON_DIST:      147.23 [deg]
MOON_ILLUM:     15 [%]
GAL_COORDS:     174.41,-10.78 [deg] galactic lon,lat of the burst
ECL_COORDS:     74.68,4.81 [deg] ecliptic lon,lat of the burst
COMMENTS:       Probable GRB.
COMMENTS:       WXM error box is circular; not rectangular.

GCN/BACODINE POSITION NOTICE

  
TITLE:          GCN/HETE BURST POSITION NOTICE
NOTICE_DATE:    Sat 20 Dec 03 03:32:35 UT
NOTICE_TYPE:    HETE S/C_Update
TRIGGER_NUM:    2976,   Seq_Num: 3
GRB_DATE:       12993 TJD;   354 DOY;   03/12/20
GRB_TIME:       12596.74 SOD {03:29:56.74} UT
TRIGGER_SOURCE: Trigger on the 6-120 keV band.
GAMMA_RATE:     466  [cnts/s]  on a 1.300 [sec] timescale 
SC_-Z_RA:        88 [deg]
SC_-Z_DEC:       22 [deg]      
SC_LONG:        320 [deg East] 
WXM_CNTR_RA:     72.760d {+04h 51m 02s} (J2000),
                 72.821d {+04h 51m 17s} (current),
                 71.982d {+04h 47m 56s} (1950)
WXM_CNTR_DEC:   +27.340d {+27d 20' 24"} (J2000),
                +27.346d {+27d 20' 47"} (current),
                +27.256d {+27d 15' 20"} (1950)
WXM_MAX_SIZE:   60.00 [arcmin] diameter
WXM_LOC_SN:     3 sig/noise (pt src in image)
WXM_IMAGE_SN:   X= 3.7 Y= 1.2 [sig/noise]
WXM_LC_SN:      X= 7.5 Y= 4.7 [sig/noise]     
SUN_POSTN:      267.62d {+17h 50m 30s}  -23.42d {-23d 25' 13"}
SUN_DIST:       166.07 [deg]   
MOON_POSTN:     220.09d {+14h 40m 22s}  -15.04d {-15d 02' 39"}
MOON_DIST:      147.23 [deg] 
MOON_ILLUM:     15 [%]
GAL_COORDS:     174.41,-10.78 [deg] galactic lon,lat of the burst
ECL_COORDS:     74.68,4.81 [deg] ecliptic lon,lat of the burst
COMMENTS:       Probable GRB.
COMMENTS:       WXM error box is circular; not rectangular.

GCN/BACODINE POSITION NOTICE

  
TITLE:          GCN/HETE BURST POSITION NOTICE
NOTICE_DATE:    Sat 20 Dec 03 03:33:22 UT
NOTICE_TYPE:    HETE S/C_Update
TRIGGER_NUM:    2976,   Seq_Num: 4
GRB_DATE:       12993 TJD;   354 DOY;   03/12/20
GRB_TIME:       12596.74 SOD {03:29:56.74} UT
TRIGGER_SOURCE: Trigger on the 6-120 keV band.
GAMMA_RATE:     466  [cnts/s]  on a 1.300 [sec] timescale     
SC_-Z_RA:        88 [deg]
SC_-Z_DEC:       22 [deg]      
SC_LONG:        320 [deg East] 
WXM_CNTR_RA:     72.760d {+04h 51m 02s} (J2000),
                 72.821d {+04h 51m 17s} (current),
                 71.982d {+04h 47m 56s} (1950)
WXM_CNTR_DEC:   +27.340d {+27d 20' 24"} (J2000),
                +27.346d {+27d 20' 47"} (current),
                +27.256d {+27d 15' 20"} (1950)
WXM_MAX_SIZE:   60.00 [arcmin] diameter
WXM_LOC_SN:     3 sig/noise (pt src in image)
WXM_IMAGE_SN:   X= 3.7 Y= 1.2 [sig/noise]
WXM_LC_SN:      X= 7.5 Y= 4.7 [sig/noise]     
SUN_POSTN:      267.62d {+17h 50m 30s}  -23.42d {-23d 25' 13"}
SUN_DIST:       166.07 [deg]   
MOON_POSTN:     220.09d {+14h 40m 22s}  -15.04d {-15d 02' 39"}
MOON_DIST:      147.23 [deg] 
MOON_ILLUM:     15 [%]
GAL_COORDS:     174.41,-10.78 [deg] galactic lon,lat of the burst
ECL_COORDS:     74.68,4.81 [deg] ecliptic lon,lat of the burst
COMMENTS:       Probable GRB.
COMMENTS:       WXM error box is circular; not rectangular.

GCN/BACODINE POSITION NOTICE

  
TITLE:          GCN/HETE BURST POSITION NOTICE
NOTICE_DATE:    Sat 20 Dec 03 03:33:25 UT
NOTICE_TYPE:    HETE S/C_Last  
TRIGGER_NUM:    2976,   Seq_Num: 5
GRB_DATE:       12993 TJD;   354 DOY;   03/12/20
GRB_TIME:       12596.74 SOD {03:29:56.74} UT
TRIGGER_SOURCE: Trigger on the 6-120 keV band.
GAMMA_RATE:     466  [cnts/s]  on a 1.300 [sec] timescale     
SC_-Z_RA:        88 [deg]
SC_-Z_DEC:       22 [deg]      
SC_LONG:        320 [deg East] 
WXM_CNTR_RA:     72.760d {+04h 51m 02s} (J2000),
                 72.821d {+04h 51m 17s} (current),
                 71.982d {+04h 47m 56s} (1950)
WXM_CNTR_DEC:   +27.340d {+27d 20' 24"} (J2000),
                +27.346d {+27d 20' 47"} (current),
                +27.256d {+27d 15' 20"} (1950)
WXM_MAX_SIZE:   60.00 [arcmin] diameter
WXM_LOC_SN:     3 sig/noise (pt src in image)
WXM_IMAGE_SN:   X= 3.7 Y= 1.2 [sig/noise]
WXM_LC_SN:      X= 7.5 Y= 4.7 [sig/noise]     
SUN_POSTN:      267.62d {+17h 50m 30s}  -23.42d {-23d 25' 13"}
SUN_DIST:       166.07 [deg]   
MOON_POSTN:     220.09d {+14h 40m 22s}  -15.04d {-15d 02' 39"}
MOON_DIST:      147.23 [deg] 
MOON_ILLUM:     15 [%]
GAL_COORDS:     174.41,-10.78 [deg] galactic lon,lat of the burst
ECL_COORDS:     74.68,4.81 [deg] ecliptic lon,lat of the burst
COMMENTS:       Probable GRB.
COMMENTS:       WXM error box is circular; not rectangular.

GCN/BACODINE POSITION NOTICE

  
TITLE:          GCN/HETE BURST POSITION NOTICE
NOTICE_DATE:    Sat 20 Dec 03 05:16:08 UT
NOTICE_TYPE:    HETE Ground Analysis
TRIGGER_NUM:    2976,   Seq_Num: 6
GRB_DATE:       12993 TJD;   354 DOY;   03/12/20
GRB_TIME:       12596.74 SOD {03:29:56.74} UT
TRIGGER_SOURCE: Trigger on the 6-120 keV band.
GAMMA_RATE:     466  [cnts/s]  on a 1.300 [sec] timescale     
SC_-Z_RA:        88 [deg]
SC_-Z_DEC:       22 [deg]      
SC_LONG:        320 [deg East] 
WXM_CNTR_RA:     69.971d {+04h 39m 53s} (J2000),
                 70.024d {+04h 40m 06s} (current),
                 69.294d {+04h 37m 11s} (1950)
WXM_CNTR_DEC:    +7.949d {+07d 56' 55"} (J2000),
                 +7.956d {+07d 57' 23"} (current),
                 +7.852d {+07d 51' 07"} (1950)
WXM_CORNER1:     70.0630   7.3510 [deg]
WXM_CORNER2:     69.7290   7.4240 [deg]
WXM_CORNER3:     69.8770   8.5460 [deg]
WXM_CORNER4:     70.2140   8.4740 [deg] 
WXM_MAX_SIZE:   72.53 [arcmin] diameter  
WXM_LOC_SN:     13 sig/noise (pt src in image)
WXM_IMAGE_SN:   X= 4.4 Y= 2.2 [sig/noise]
WXM_LC_SN:      X= 12.5 Y= 5.6 [sig/noise]
SUN_POSTN:      267.62d {+17h 50m 30s}  -23.42d {-23d 25' 13"}
SUN_DIST:       157.11 [deg]
MOON_POSTN:     220.09d {+14h 40m 22s}  -15.04d {-15d 02' 39"}
MOON_DIST:      149.86 [deg]
MOON_ILLUM:     15 [%]         
GAL_COORDS:     189.05,-24.56 [deg] galactic lon,lat of the burst
ECL_COORDS:     69.53,-14.08 [deg] ecliptic lon,lat of the burst
COMMENTS:       Definite GRB.
COMMENTS:       Burst_Validity flag is true.  
COMMENTS:       WXM data refined since S/C_Last Notice.

GCN/BACODINE POSITION NOTICE

  
TITLE:          GCN/HETE BURST POSITION NOTICE
NOTICE_DATE:    Sat 20 Dec 03 08:17:43 UT
NOTICE_TYPE:    HETE Ground Analysis
TRIGGER_NUM:    2976,   Seq_Num: 7
GRB_DATE:       12993 TJD;   354 DOY;   03/12/20
GRB_TIME:       12596.74 SOD {03:29:56.74} UT
TRIGGER_SOURCE: Trigger on the 6-120 keV band.
GAMMA_RATE:     466  [cnts/s]  on a 1.300 [sec] timescale     
SC_-Z_RA:        88 [deg]
SC_-Z_DEC:       22 [deg]      
SC_LONG:        320 [deg East] 
SXC_CNTR_RA:     69.893d {+04h 39m 34s} (J2000),
                 69.947d {+04h 39m 47s} (current),
                 69.219d {+04h 36m 53s} (1950)
SXC_CNTR_DEC:    +7.373d {+07d 22' 25"} (J2000),
                 +7.381d {+07d 22' 52"} (current),
                 +7.276d {+07d 16' 35"} (1950)
SXC_CORNER1:     70.0570   7.3140 [deg]
SXC_CORNER2:     70.0630   7.3600 [deg]
SXC_CORNER3:     69.7290   7.4330 [deg]
SXC_CORNER4:     69.7230   7.3870 [deg] 
SXC_MAX_SIZE:   20.77 [arcmin] diameter  
SXC_LOC_SN:     3 sig/noise (pt src in image)
SUN_POSTN:      267.62d {+17h 50m 30s}  -23.42d {-23d 25' 13"}
SUN_DIST:       156.65 [deg] 
MOON_POSTN:     220.09d {+14h 40m 22s}  -15.04d {-15d 02' 39"}
MOON_DIST:      149.76 [deg]
MOON_ILLUM:     15 [%]
GAL_COORDS:     189.52,-24.95 [deg] galactic lon,lat of the burst
ECL_COORDS:     69.37,-14.64 [deg] ecliptic lon,lat of the burst
COMMENTS:       Definite GRB.  
COMMENTS:       Burst_Validity flag is true.    
COMMENTS:       WXM data refined since S/C_Last Notice.
COMMENTS:       SXC data refined since S/C_Last Notice.

GCN notice #2494
M.K.Yang (NCU), C.P.Chin (NCU)
K.Y. Huang, Y. Urata, W.H. Ip, T. Tamagawa
on behalf of Lulin GRB collaboration report:
" We have observed the entire SXC error box of GRB031220(=H2976) in R-band at Lulin observatory(National Central University, Taiwan) beginning at 11:49 UT on Dec. 20. There are no candidate in these images comparison with DSS-II images down to 20.9 mag.
Further analysis and observation are in progress."
This message may be cited.
GCN notice #2495
E. S. Rykoff, R. Quimby, C. W. Akerlof and B. E. Schaefer report on behalf of the ROTSE collaboration:
The ROTSE-IIIb telescope at McDonald Observatory, Texas, responded automatically to HETE Trigger 2976 (GRB031220). Our first image of the final SXC localization was taken 6 seconds after the receipt of HETE Trigger 2976-6 on December 20, 05:16:14 UT, 1.77 hours after the burst. The system took 10x5 sec, 10x20 sec and 160x60 sec exposures. After co-adding sets of ten images, comparison to the USNO-B catalog reveals no new sources in the SXC error box. Furthermore, visual comparison to the DSS-2 Red and IR Plates reveal no new sources to an unfiltered limiting magnitude of 19.2 at 1.9 hours after the burst.
GCN notice #2496
R. Sato and N. Kawai (Tokyo Tech) report:
"We have observed the field covering the entire SXC error box of GRB 031220 (HETE trigger 2976; trigger time 03:29:56.7 UT) with the unfiltered CCD camera on the 30-cm telescope at Tokyo Tech, beginning at 09:23:19 UT on Dec. 20. After co-adding a set of 10 images of 10 sec exposures, comparison with the USNO A2.0 catalog revealed no new sources in the SXC error box. Furthermore, visual inspection of the images revealed no obvious new source compared with the DSS-2 Red to an approximate limiting magnitude of 16.5."
GCN notice #2497
G. Kosugi, Y. Ohyama, T. Yamada, K. Sekiguchi, T. Sasaki, M. Akiyama, and N. Kawai report on behalf of the Subaru GRB team:
"We have observed the field covering most of the SXC error box of GRB 031220 (=H2976) in I band with FOCAS on the Subaru 8.2m telescope on Mauna Kea beginning at 11:20 UT on Dec 20, approximately 8 hours after the burst. The field was covered with 5 pointings of 10 min exposure, with average seeing of 0.9 arcseconds.
Visual comparison with DSS-2 images revealed no obvious counterpart candidates."
GCN notice #2498
M. Uemura, R. Ishioka, T. Kato (Kyoto U.), C. Nakayama and H. Yamaoka (Kyushu U.) report on behalf of VSNET-GRB collaboration:
"We observed the position of GRB 031220 (=H2976) starting at Dec. 20.39 UT (6.0 hours after burst) with the 0.25-m telescope (+unfiltered CCD) at Kyoto University. The stacked image (32 x 30s, during 09:43 - 10:07 UT) covers the central part (about 45%) of the entire SXC error box. Visual comparison of the stacked image to the DSS 2 red image shows no new object down to a limiting magnitude of about 17.8 (estimated with USNO-A2.0 red magnitudes of many field stars).
This message can be cited."
GCN notice #2499
D.B. Fox, A.M. Soderberg and E.Berger (Caltech) report on behalf of a larger collaboration:
"We have observed the HETE-2/SXC localization region for GRB 031220 through intermittent clouds with the Palomar 200-inch (5-m) Hale Telescope and Large Format Camera (LFC) from 08:30 to 10:00 UT on 20 Dec 2003. With a subset of these data we have created two coadded r' images with exposures of approximately 24-min over the full SXC region and with mean epochs of 08:50 UT (5:20 after the GRB) and 09:47 UT (6:17 after the GRB), respectively. The limiting magnitudes of these images, as referenced against the R-band (F-emulsion) photometry of the GSC-2, are R~23 and R~22.5, respectively, with the later observations having been more strongly extinguished by cloud cover.
Comparison of these images to the Digitized Sky Survey reveals no new objects brighter than the DSS limit. Moreover, PSF-matched image subtraction of the two images reveals no strongly variable sources significantly brighter than our limits. Since a t^(-1) decay would have produced a 0.2 mag decline between the two epochs of exposure, we estimate that any afterglow of GRB 031220 had R>21 at our epoch of first observation.
Our coadded r' images, and their subtraction, may be found at: http://www.astro.caltech.edu/~derekfox/grb031220/
GCN notice #2502
GRB031220(=H2976): Observation with the Chandra X-ray Observatory
M. De Pasquale, B. Gendre (also CESR, Toulouse), L. Piro, E. Costa, M. Feroci (IASF-INAF/Rome), M. Garcia (Harvard, CFA), L. A. Antonelli (OaR/INAF-Rome), G. Garmire (PSU), G. Ricker (MIT) on behalf of a lager collaboration report:
On 25 Dec. 18:18 UT, the Chandra Observatory targeted the field of GRB031220, which was localized by the HETE satellite (HETE trigger 2976). The 40 ksec observation spanned the interval 5.6 - 6.1 days after the burst. More than 99% of the HETE/SXC error region was contained within the field-of-view of the Chandra ACIS-I chip.
From a preliminary analysis of the data we detected seven sources in this region in the 0.2-10 keV band:
```
#      Chandra Name              RA           DEC     dR(s) dD(") Cts
6   CXOU J043946.5+072220   4 39 46.47   7 22 20.19  0.005  0.13  89
27  CXOU J044003.8+072055   4 40  3.76   7 20 55.26  0.016  0.26  79
2   CXOU J043939.8+072319   4 39 39.77   7 23 18.64  0.005  0.07  45
37  CXOU J043857.1+072449   4 38 57.14   7 24 48.84  0.06   0.7   33
17  CXOU J043954.8+072149   4 39 54.83   7 21 49.33  0.018  0.27  25
7   CXOU J043946.1+072256   4 39 46.14   7 22 56.46  0.011  0.16  25
1   CXOU J043944.3+072037   4 39 44.35   7 20 36.74  0.009  0.13  21
```
Source #37 is near the border of the chip.
None of these source is present in the SIMBAD and 2-MASS catalogues.
At this stage of the analysis, the astrometry is dominated by the systematic error in the attitude reconstruction. The nominal attitude reconstruction has been checked by using 3 stars with detectable X-ray emission, giving an average systematic error of about 1", that has to be added in quadrature to the statistical error reported above.
A second observation with Chandra should be carried out around mid January.
We thank the personnel at the Chandra Science Center for the assistance in the acquisition and preliminary processing of these data.
[GCN OPS: This Circular was delayed in distribution 7.5 hrs due to an address problem.]
GCN notice #2503
L.A. Antonelli, V. Testa (INAF-OAR, Rome), D. Fugazza (INAF-OABr, Merate), M. Pedani (INAF-TNG, La Palma), L. Piro (INAF-IASF, Rome), S. Covino (INAF-OABr, Merate), N. Masetti, (INAF-IASF, Bologna), D. Malesani (Sissa, Trieste) on behalf of the larger Italian GRB Collaboration report:
"On 28 Dec. we imaged the field of the X-ray rich GRB031220 (HETE trigger 2976) with the Telescopio Nazionale Galileo and DOLORES instrument operated in imaging mode. The observation started at 00:54 UT and the field was imaged for a total exposure time of 1 hour in the R-band. Seeing varied during the observation from 1.1'' to 1.4''.
At the position consistent with four of the X-ray sources observed by Chandra and reported by De Pasquale et al. (GCN 2502) we found the following optical counterparts:
```
Chandra 
  ID#     RA          DEC          Rmag   Err(mag)  

  6       4 39 46.49  7 22 19.30   20.61  0.05    
 27       4 40 03.71  7 20 55.98   19.42  0.05
 17       4 39 54.82  7 21 49.05   23.10  0.07
  7       4 39 46.13  7 22 56.26   23.48  0.11
```
(for errors on positions we assume a conservative value of 0.6")
For the other two sources detected by Chandra (#2 and #1 see GCN 2502) we did not find evidence of optical counterparts. We derived a 3 sigma limiting magnitude of R>24 for both sources.
Source #37 (CXOU J043857.1+072449) is out of the field of view.
A further NIR observation of this field is planned at TNG.
We thank the personnel of TNG for their enthusiastic effort to the GRB program.
This message may be cited"
GCN notice #2510
C.Fukami, S.maeno, K.Sakamoto, E.Sonoda, M.Yamauchi (Univercity of Miyazaki)
"We have observed the field covering the entire SXC error box of GRB 031220 (HETE trigger 2976;triger time 03:29:56.7 UT) with the unfiltered CCD camera on the 30-cm telescope at Univercity of Miyazaki, beginning at 10:05:59 UT on Dec.20. After co-adding a set of 2 images of 30 sec exposure and CCD's field of view is 43arcmin. We compared with the USNO-A2.0 catalog. There is no new source brighter than 17.51 mag in the SXC error box.
[GCN OPS NOTE (07jan04): This Circular was delayed 10.5 hours until a new account could be set up.]
GCN notice #2513
J. Gorosabel (STScI/IAA-CSIC), J. Rhoads (STScI), A. Fruchter (STScI), D. Bersier (STScI), J.M. Castro Ceron (STScI), A. Levan (U. of Leicester), C. Smith (CTIO), report:
We have acquired 5x480s R-band images of GRB 031220 (HETE trigger #2976) field with the CTIO 4-m Telescope (+Mosaic2) on 2003 Dec. 21.1787--21.2113 UT. The large field of view of Mosaic2 allowed us to simultaneously image the positions of the 7 X-ray sources detected by Chandra (GCN #2502).
Fixing our zero point to the R-band magnitude of source #27 (based on the R-band images taken with the TNG, GCN #2503) we obtain the following magnitudes:
```
    Chandra          R +/- ER                   R +/- ER
    ID #    (CTIO, Dec. 21 ~04:30 UT)   (TNG, Dec. 28 ~01:24 UT)
    -------------------------------------------------------------
     1           24.40 +/- 0.18                    -
     2                  %                          -
     6           20.61 +/- 0.01             20.61 +/- 0.05
     7           23.03 +/- 0.04             23.48 +/- 0.11
     17          23.06 +/- 0.04             23.10 +/- 0.07
     27          19.42 +/- 0.01             19.42 +/- 0.05
     37          25.15 +/- 0.24                    *
    -------------------------------------------------------------
     - No counterpart brighter than R = 24 (3 sigma).
     % No counterpart brighter than R = 25.3 (3 sigma).
     * Out of the FOV.
    -------------------------------------------------------------
```
Sources #6, #17 and #27, remain constant within photometric uncertainties. No comparison was possible for objects #1, #2 and #37, which are not present in the TNG images. Object #7 shows an R-band decay at a 3.8 sigma level, which makes it a suggestive afterglow candidate.
However, the potential fading associated with object #7 would yield a power-law decay index Alpha = 0.21 +/- 0.05, quite unusual (but not unprecedented) for GRB afterglows.
We encourage further deep R-band imaging to clarify the potential fading of source #7
GCN notice #2523
GRB031220: 2nd Chandra observation and X-ray afterglow candidate
B. Gendre, M. De Pasquale, L. Piro, E. Costa, M. Feroci (IASF-INAF/Rome), M. Garcia (Harvard, CFA), L. A. Antonelli (OaR/INAF-Rome), G. Garmire (PSU), G.Ricker (MIT), G. Tagliaferri (Oss.Astr.Merate/INAF-Mi), J. in 't Zand (SRON-Utrecht) report:
On 17 Jan. 14:47 UT, the Chandra Observatory targeted the field of GRB031220 for a second time for a total exposure time of 20 ksec. We have performed a detailed analysis of both observations (preliminary results of the 1st observation were reported in GCN2502).
Of the seven sources reported in GCN2502 (i.e. the brightest contained in the HETE2 error box) we find that only 2 of them show a decrease by more than a factor of 2 (at 2 sigma level). In particular source #1 is not detected (with a 2 sigma upper limit of about 6 counts) and source #7 is marginally detected. In the following table we report for sources #1 and #7 the observed counts (corrected for the point spread function) in the two observations. Note that the first observation was twice as long.
```
 #     RA           DEC          Cts (obs1)  Cts (obs2)  variation factor
 1     4:39:44.35   7:20:36.99  31.4+/- 6.5   ND            >2.6
 7     4:39:46.19   7:22:56.17  31.9+/- 6.5   7+/- 4         2.3
```
Source #7 showed an optical variation (GCN 2503, GCN 2513) with a decay index (0.21 +/- 0.05, GCN 2513) consistent, within the errors, with the observed X-ray slope. This slope is much flatter than usually observed in afterglows (unless the optical flux is being dominated by a constant contribution by e.g. the host galaxy). Assuming a (rather flat) decay slope of -1, the expected variation between the two observations is a factor of 4.5. The most likely afterglow candidate on the basis of X-ray data is then source #1.
We also note that the suspect source #37 reported in the previous GCN near the border of the CCD chip is excluded by this refined analysis.
In addition to the bright sources found in the error box reported in the previous GCN, we list the fainter sources detected with this refined analysis in the error box during the 1st observation and not detected in the second one. Note, however, that we cannot derive any significant conclusion on the variability of these sources.
```
 #	RA		DEC	Cts(Obs1)
 82   4:38:55.23   7:24:57.79  10.3+/- 3.7
 66   4:39:27.23   7:23:51.04  13.3+/- 4.1
 55   4:39:29.20   7:23:14.88  18.6+/- 5.2
 78   4:39:34.49   7:21:24.16  13.4+/- 4.4
 58   4:40:12.58   7:19:45.20  22.6+/- 6.2
```
Finally, we list all the sources outside the error box that underwent a decrease by more than a factor of 2. We add in table their distance from the nearest side of the error box
```
 #	RA	    Dec		Cts(Obs1)  Cts(Obs2)  variation   D
 E28   4:39:27.62  7:24:49.04  37.4+/-7.3   9.2+/- 3.2   2.0       22"
 E10   4:39:07.31  7:21:23.07  76.2+/-9.8   10.7+/-4.8   3.5       1'
 E21   4:39:12.65  7:26:14.00  43.2+/-7.7   10.6+/-3.5   2.0       1'
 E5    4:39:57.00  7:18:20.92  110.9+/-11.7 23.4+/-5.7   2.4        1'10"
 E22   4:39:31.37  7:26: 1.26  33.7+/- 6.6  6.0+/-2.6    2.8        1'40"
 E4    4:39:39.38  7:16:55.21  122.3+/-12.0 25.5+/-6.5   2.4        3'30"
 E2    4:39: 6.88  7:17: 1.27  244.6+/-17.9  54.2+/-9.2  2.2         5'5"
 E13   4:39:20.23  7:16: 3.25  61.6+/- 9.2   9.1+/-4     3.4        5'15"
```
This message may be cited.
astro-ph/0510604 from 20 Oct 2005
Melandri: Multi-wavelength analysis of the field of the dark burst GRB 031220
We have collected and analyzed data taken in different spectral bands (from X-ray to optical and infrared) of the field of GRB031220 and we present results of such multiband observations. Comparison between images taken at different epochs in the same filters did not reveal any strong variable source in the field of this burst. X-ray analysis shows that only two of the seven Chandra sources have a significant flux decrease and seem to be the most likely afterglow candidates. Both sources do not show the typical values of the R-K colour but they appear to be redder. However, only one source has an X-ray decay index (1.3 +/- 0.1) that is typical for observed afterglows. We assume that this source is the best afterglow candidate and we estimate a redshift of 1.90 +/- 0.30. Photometric analysis and redshift estimation for this object suggest that this GRB can be classified as a Dark Burst and that the obscuration is the result of dust extinction in the circum burst medium or inside the host galaxy.

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Jochen Greiner, last update: 30-Oct-2005 [Disclaimer]