GRB 000131

K. Hurley, on behalf of the Ulysses GRB team, and T. Cline and
E. Mazets, on behalf of the KONUS-WIND and NEAR GRB teams, report:

Ulysses, Konus, and NEAR observed a gamma-ray burst on January 31,
2000 at 14:59:13 UT.  Its T90 duration was ~50 s and its 25-100
keV fluence was ~10^-5 erg/cm^2.  At present, it can only be localized
to two alternate ~55 sq. arcmin. error boxes given by the following 
preliminary centers and corners (~3 sigma confidence):

 ERROR BOX CENTER:   6 h 13 m 32.72 s    -51 o 55 '  36.77 " 
 ERROR BOX CORNER 1:  6 h 13 m  8.83 s    -52 o  2 '  20.83 " 
 ERROR BOX CORNER 2:  6 h 13 m 25.43 s    -51 o 50 '  11.43 " 
 ERROR BOX CORNER 3:  6 h 13 m 40.09 s    -52 o  1 '   1.94 " 
 ERROR BOX CORNER 4:  6 h 13 m 56.59 s    -51 o 48 '  52.07 "
 
     OR

 ERROR BOX CENTER:  10 h 48 m 50.99 s      4 o 34 '   1.70 " 
 ERROR BOX CORNER 1: 10 h 49 m 21.03 s      4 o 35 '  42.55 " 
 ERROR BOX CORNER 2: 10 h 48 m 31.31 s      4 o 36 '  35.98 " 
 ERROR BOX CORNER 3: 10 h 49 m 10.65 s      4 o 31 '  26.95 " 
 ERROR BOX CORNER 4: 10 h 48 m 20.91 s      4 o 32 '  19.87 " 

There is a fair chance that further processing will allow one
of these positions to be ruled out.

R. M. Kippen (UAH/MSFC) reports on behalf of the BATSE GRB Team:

GRB000131 was detected by BATSE as trigger #7975 at 53877.497 s UT on
2000 January 31.  BACODINE/GCN locations were not issued because the
trigger began in a real-time telemetry gap.  The burst lasts more than
100 s and is therefore undoubtedly the same event detected by KONUS,
Ulysses and NEAR (GCN 529
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 ) some 76 s after the BATSE trigger time
(corresponding to the brightest pulse of the burst).  The final BATSE
location is (R.A., Dec. J2000) = (94.0, -52.0 deg), with a total
(statistical + systematic) error radius of 2.3 deg (68% Conf.).  This
location rules out one of the two possible IPN error boxes (GCN 529
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 ),
which means that the most probable combined BATSE+IPN error-box is:

                          RA(2000)            DEC(2000)
 ERROR BOX CENTER:    6 h 13 m 32.72 s    -51 o 55 '  36.77 " 
 ERROR BOX CORNER 1:  6 h 13 m  8.83 s    -52 o  2 '  20.83 " 
 ERROR BOX CORNER 2:  6 h 13 m 25.43 s    -51 o 50 '  11.43 " 
 ERROR BOX CORNER 3:  6 h 13 m 40.09 s    -52 o  1 '   1.94 " 
 ERROR BOX CORNER 4:  6 h 13 m 56.59 s    -51 o 48 '  52.07 "

The center of this box is within about 0.4 deg of the independent
BATSE location centroid.

-eof-

VLT Detection of Fading Optical Source in the GRB 000131 Error Box


H. Pedersen, B. L. Jensen, J. Hjorth, L. Fogh Olsen (U. of Copenhagen),
M. I. Andersen (U. of Oulo), J. Gorosabel (DSRI, Copenhagen),
M. Vestergaard (Ohio State U.), L. Kaper (U. of Amsterdam), and
A. O. Jaunsen (U. of Oslo), on behalf of a European GRB consortium, report:

"Using the 8.2-m Antu telescope, Paranal, and the 1.54-m Danish telescope, 
La Silla, we have performed optical observations, covering the entire error 
box of GRB 000131 (GCN #529
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 , #530
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 ), as follows:

Antu:

2000 February 4.04 UT; B, V, R ; 240 s each filter ; FWHM = 0.9" 
2000 February 4.22 UT; B, V, R ; 240 s each filter ; FWHM = 0.9" 
2000 February 6.10 UT; B, V, R ; 240 s each filter ; FWHM = 1.2" 
2000 February 6.22 UT; B, V, R ; 240 s each filter ; FWHM = 1.2" 

1.54-m Danish:

2000 February 4.19 UT; I ; 3600 s ; FWHM = 1.3"

For Antu, two separate pointings were obtained using FORS1; the exposure
times apply to each of these. For the 1.54-m, the error box was covered by 
a single pointing with DFOSC.

Intercomparison of the Antu images reveals a single, unresolved object, which 
has faded significantly between the two observing nights. The variation is 
noticed on the V and R exposures, while the B exposures do not show the object 
at any epoch. The object is detected in the combined 1.54-m Danish I-band 
image.

Its position is RA(J2000) = 6:13:31.0, Dec(J2000) = -51:56:40, with an
uncertainty of about 2".

The object has R ~ 24 in the Feb 4 images and has faded 0.6+-0.1 mag
on Feb 6, corresponding to a power law decay index of -1.2+-0.2.

Sections of the images can be seen at

http://www.astro.ku.dk/~brian_j/grb/grb000131/

jointly with a DSS finding chart.

We appreciate the kind assistance given by the VLT Service Mode team."

S.G.Bhargavi, A.K. Pati, G. SelvaKumar and R.Cowsik 
(Indian Institute of Astrophysics,Bangalore, India 560034)
 
 We observed the error box of GRB000131 (GCN #529
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 , #530
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 ) on Feb 4th and
 5th between 15-18UT using the 2.34m as well as 1.02m telescopes of
 Vainu Bappu Observatory, Kavalur, India. The sky conditions were
 non-photometric with poor transparency and seeing (fwhm >2".5).
 
 The fading optical counterpart reported by Peredson et al ( GCN #534
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 ) is
 not detected in a single 30min V image (limiting magnitude = 21.0) taken 
 on Feb 4.68 UT with 2.34m telescape.

GRB 000131: HST Detection of the Host Galaxy

J. S. Bloom, D. Fox, S. R. Kulkarni, T. J. Galama, on behalf the
larger Caltech-NRAO-CARA GRB Collaboration:

"We observed the field of GRB 000131 (Hurley et al., GCN #529
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 ; Andersen et
al., 2000, A&A, 364, L54) on 2001 August 17-19 UT using the Hubble Space
Telescope.  With a total integration time of 24200 s in the WFPC/F814W
filter, we detect a faint galaxy system coincident with the position of
the optical transient using differential astrometry with the VLT/FORS1
images (GCN #534
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 )."

An image of the field and the putative host system is linked from

   http://www.astro.caltech.edu/~jsb/GRB/grb000131

The 3-sigma uncertainty contour of the OT position is shown in this
image.

This message may be cited.