GRB 991106

GRB991106: BeppoSAX/BATSE results

G. Gandolfi and P. Soffitta (IAS/CNR,Roma), J. Heise and J. in 't Zand
(SRON, Utrecht), L.Amati (ITESRE/CNR, Bologna) on behalf of BeppoSAX GRB
Team and R. M. Kippen and P. M. Woods (University of Alabama in
Huntsville) and C. A. Meegan (NASA/MSFC) on behalf of the BATSE GRB team
report:

The BeppoSAX 4-sigma detection ((9.5 +/- 2.4)E-8 erg/cm2/s in the
40-700 keV range) of a short gamma peak of about 1 s of duration in
correspondence with the X-ray transient reported in GCN 443
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  (GRB991106)
was not confirmed by BATSE observations. The WFC source position was
observable to BATSE (i.e., not occulted by the earth). The on-board
trigger was disabled, but the continuous data with 1.024 and 4.096 second
resolution shows no evidence for significant excess flux above background.
Upper limits (2-sigma), based on these data are:

   F < 6.6e-9 erg/cm^2/s  (25- 50 keV)
   F < 1.7e-8 erg/cm^2/s  (50-300 keV)
   F < 2.7e-8 erg/cm^2/s  (40-700 keV; the BeppoSAX GRBM energy range)

We therefore conclude that the BeppoSAX-GRBM detection was apparently
either a statistical fluctuation or a spurious event of a different origin
(e.g. Cosmic Rays).
On the other side, the faint WFC X-ray transient, with a (2.0 +/- 0.4)E-8
erg/cm2/s peak flux in the 2-26 keV range, shows marginal evidence
of a high hardness ratio.
These BeppoSAX/BATSE combined results suggest a peculiarly short and
hard X-ray burst or an anomalous GRB. The lower limit to the ratio of
X-ray to gamma peak flux is about 0.75, a factor 5 higher
than average values found for BeppoSAX prompt GRB counterparts,
indicating in the latter case a X-ray rich event, i.e. a candidate member
of the X-ray-GRB class.

Javier Gorosabel, Univ of Amsterdam and LAEFF-INTA (Madrid)
 Evert Rol and Paul Vreeswijk, Univ of Amsterdam
 Alberto Castro-Tirado, LAEFF-INTA (Madrid) and IAA-CSIC (Granada)
 Antonio Aparicio, David Martinez Delgado, Sebastian L. Hidalgo, 
 Otilia de La Rosa, Instituto de Astrofisica de Canarias (IAC), 
 Marco Azzaro, Isaac Newton Group (ING),
 Katherine F. Gunn, University of Southampton, 
 Chryssa Kouveliotou (USRA/MSFC),
 Marco Feroci on behalf of the BeppoSAX team, IAS (Frascati)
 
 report:
 
 "We report on I band observations of the BeppoSAX WFC position of GRB
 991106 (Gandolfi et al., GCN 435
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 ), carried out on Nov 6.856-6.868 UT
 (roughly 9.6 hours after the burst) and on Nov 11.821-11.833 UT with
 the 2.5-m INT telescope at the Observatorio del Roque de los Muchachos
 at La Palma. On each of the two nights we used the Sloan I band filter
 and exposed for a total of 900s. The limiting magnitudes are I~23.5
 and I~23.7, respectively, for the first and second epoch.

 At R.A.= 22:24:27.85, decl.=+54:21:54.4 (J2000), we find a 5-sigma
 detection in our first epoch image (using a circular aperture with a
 radius equal to the FWHM). This position is ~2.2" away from the radio
 source reported by Frail et al (GCN 444
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 ). The detection is not present
 on the second epoch image.  We estimate the error in our astrometry to
 be about 1.5".  However, all the flux of the object is concentrated in
 2 or 3 pixels, while the FWHM is 4.3 pixels, so it does not appear to
 be an actual source.

 We checked for variability of all the objects we detected inside the
 WFC and NFI error circles (GCN 435
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  and 445
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 , respectively). None of the
 sources brighter than I=22 vary by more than about 0.4 mag. For
 sources with I > 22 the large photometric errors do not allow us to
 discern any variability. In conclusion, no reliable candidates
 have been found. 

 We find that the two red objects reported by Stecklum et al. (GCN 446
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 )
 are constant in brightness between our two epochs within the
 measurement errors. We measure the preliminary magnitudes I=19.8 +/-
 0.2 and I=20.1 +/- 0.2 for the objects located at R.A.= 22:24:32.4,
 decl.= +54:23:51 (J2000) and R.A.= 22:24:39.7, decl.= +54:21:46
 (J2000), respectively."

 This message can be cited.

B. Stecklum, S. Klose (Thueringer Landessternwarte Tautenburg), 
O. Fischer (Universitaets-Sternwarte Jena),
R. Lenzen (Max-Planck-Institut fuer Astronomie, Heidelberg),
F. J. Vrba, A. A. Henden, C. B. Luginbuhl,
   B. Canzian, S. E. Levine, H. H. Guetter, J. A. Munn
   (U.S. Naval Observatory, Flagstaff),
A. Castro-Tirado (LAEFF-INTA, Madrid, and IAA-CSIC, Granada), 
J. Greiner (AIP Potsdam), 
J. Gorosabel (LAEFF-INTA, Madrid, and University of Amsterdam), and
A. Riffeser (Universitaets-Sternwarte Muenchen) 

report:


The error box of GRB 991106 was imaged with the Calar Alto 3.5-m
telescope on November 7.799 - 7.841 UT using the near-infrared camera 
Omega Cass (see http://www.mpia-hd.mpg.de/). The frames were
taken in the course of a project whose goal is to measure the degree of
linear polarization of GRB afterglows (Klose et al., proceedings 5th
Huntsville symposium, to be submitted). The limiting magnitude of the
K'-band image is about K'=19 after adding all images taken at
different position angles of the wire-grid polarizer. Since the
error box is at low Galactic latitude, we report here on the results 
of a search for very red objects. Polarimetric data will be published 
at later times.

A comparison of the combined K'-band image with I-band images obtained
on November 7.9 UT at the  Calar Alto 1.23-m telescope and on November
8.2 at the USNO 1.0-m telescope shows an object at coordinates RA
(J2000) = 22:24:32.4, DEC = 54:23:51 (+/- 1 arcsec) which has a large
I-K' color. The object is inside the original 3.2 arcmin BeppoSAX
error circle (Piro et al., GCN #435
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 ), but about 10 arcsec outside the
BeppoSAX NFI 1.5 arcmin error circle (Antonelli et al., GCN
#445
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 ). There is one object inside the NFI error circle which is seen
both in K' and I, and which is very red. This object is at RA (J2000)
= 22:24:39.7, DEC = 54:21:46 (+/- 1 arcsec). Both objects have very
faint counterparts on the DSS2 Digitized Sky Survey. They seem to be 
constant between the two I-band epochs. This cannot be stated with 
certainty, however.

There is no object visible in K' at the position of the radio source
reported by Frail et al. (GCN #444
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 ). Any such source must be fainter than
about K'=19.

The preliminary K'-band image is posted on the Tautenburg Web page at
http://www.tls-tautenburg.de/research/grb991106.html.

This message is citeable.

L.A. Antonelli, Osservatorio Astronomico di Roma;
G. Gandolfi, M. Feroci, Istituto di Astrofisica Spaziale, CNR, Rome,
D. Ricci, C. De Libero, M. Capalbi, BeppoSAX Science Operations Center, 
Telespazio, Rome; F. Frontera, Istituto Tecnologie e Studio Radiazioni 
Extraterrestri, CNR, Bologna, J. in 't Zand and J. Heise, Space Research 
Organization Netherlands, Utrecht, on behalf of the BeppoSAX Team report:

"The region of GRB 991106 (GCN #434
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 ) was observed with the Narrow 
Field Instruments (NFI) on board BeppoSAX from Nov. 6.781 to 7.565 U.T. 
(starting about 8 hrs after the burst trigger time) for a total net 
exposure time of 32 ks. In the 2-10 keV image of all data from both 
Medium-Energy Concentrator Spectrometer units 2 and 3, a faint point 
source (1 SAX J222441+5422.7) is detected, with a chance probability 
for the source being a background fluctuation of 5E-4, within the WFC 
error box (Gandolfi et al, GCN #435
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 ). The average source flux in 
2-10 keV band was (1.7+-0.4)x10E-3 counts/s corresponding to 
1.25x10E-13 erg cmE-2 sE-1, assuming a crab-like spectrum. 
The source position is R.A. = 22h24m40.8s, Dec.= +54d22'45" (Eq. 2000.0) 
and the error radius is 1'.5. 
The radio source reported by Frail (GCN #444
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 ) is 2'.1 from this X-ray 
source position and it lies outside the NFI error circle. The data do 
not allow to draw a useful constraint about the variability of this source. 
The probability of detecting a foreground source at this flux level or 
brighter in the WFC error circle is expected to be ~10% as suggested by the 
BeppoSAX LogN-LogS (Fiore et al., 1999).

This message is citable."

D. A. Frail (NRAO) reports on behalf of a larger collaboration:

"Beginning 1999 November 7.34 UT we imaged the field of GRB 991106,
centered on the refined BeppoSAX position (GCN #435
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 ) with the Very
Large Array (VLA). A frequency of 8.46 GHz was used, which provides a
field-of-view of 5.3 arcmin to half power (FWHM). A quick-look
analysis of the data shows a single unresolved source near the western
edge of the WFC error circle at R.A.= 22 24 27.65, dec.=+54 21 53.0
(J2000), with conservative errors of 1 arcsec in each dimension. The
probability of detecting an unrelated background source in the WFC
error circle is not insignificant (~10%). Further radio observations
are underway to determine whether the source is variable or not."

This message is citeable.

GRB991106: the first BeppoSAX short GRB detection?

G.Gandolfi, E.Costa, Istituto di Astrofisica Spaziale (IAS)/CNR, Roma, L.
Di Ciolo, G.Tarei, BeppoSAX Scientific Operation Centre, Telespazio, Roma,
M.J.S. Smith, J. In't Zand, Space Research Organization of the Netherlands
(SRON), Utrecht, report:
"The BeppoSAX Gamma Ray Burst Monitor (GRBM) and Wide
Field Camera (WFC unit 1) observed an untriggered, faint and possibly
short gamma-ray burst on November 6.4545 UT. A preliminary analysis
reveals a duration of about 1 s in the GRBM, with a peak photon countrate
of 120 cts/s in the 40-700 keV energy band. In the WFC, the duration is
about 5 s and the peak flux is 0.3 Crab (2-26 keV). The position of the
X-ray counterpart is R.A. = 22h24m49s Decl. =  +54d21'.9 (equinox 2000.0)
with an 99% error radius of  3.2'. An analysis of the 70 ksec NFI
follow-up, performed about 8 hours after, is being carried on."

#----------------------------------#------------------------------------------#
 Giangiacomo Gandolfi              | E-mail:
 Istituto Astrofisica Spaziale     | gandolfi@hal9000.ias.rm.cnr.it
 C.N.R.                            | 
 Via Fosso del Cavaliere           | phone: -39-06-4993-4005
 I-00133 Roma                      | 
 Italy                             | fax(CNR)  : -39-06-20660188
#__________________________________#__________________________________________#

B. L. Jensen, H. Pedersen, J. Hjorth, S. Larsen (University of
Copenhagen), and E. Costa (CNR, Rome; on behalf of the BeppoSAX
consortium) report:

Using the ALFOSC instrument at the 2.5-m Nordic Optical Telescope, La
Palma, we have obtained five 2x2 mosaics, covering the error circle
(Piro, GCN #435
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 ). All exposures were taken in the R band and lasted 300
s. The first data (three mosaics) were obtained from 6.872 to 6.921 Nov
1999 UT, i.e. starting ~10 hours after the event, and the last (two
mosaics) were obtained from 7.845 to 7.884 Nov 1999 UT. On the first
night the seeing was ~1.4", on the second ~1.4-1.8".

We have compared the exposures both internally and to the DSS-2 (POSS-II
R), finding no variable objects. The limiting magnitude of the first
night's data is approximately R ~ 23.5. The corresponding figure for the
second night is R ~ 22.0 (due to variable cloud cover). Comparison
between the data of the first and second night sets a limit of R ~ 22
(Nov 7.8 1999 UT) on possible optical transients varying by more than
0.5 magnitude (relative to Nov 6.9 Nov 1999 UT).

A .jpg-image of the field can be found at:
http://www.astro.ku.dk/~brian_j/grb/grb991106

S. Mitronova, A. Kopylov, S. Zharikov, T. Fatkhullin, I. Karachentsev, V.
Sokolov, S. Kaisin (SAO-RAS), N. Masetti (ITeSRE/CNR, Bologna), on behalf of
the BeppoSAX GRB collaboration, communicate:


"We acquired on Nov 6.776 UT one R-band (300s exposure) image of
the central part of the GRB991106 error box (GCN #434
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 , #435
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 ) using the CCD
camera of the 6m SAO-RAS telescope. The 3-sigma limiting magnitude
of the image, estimated using USNO-A1.0 field stars, is R~23.5.
Comparison of our image with the DSS did not reveal any new object
brighter than the DSS limit (R~21) 7.7 hours after the GRB trigger. 
The FITS file of the R frame as well as a .gif image can be found at the
URL http://www.sao.ru/~zhar/home/GRB/991106.html .".


This message can be cited.

G. G. Williams (Clemson Univ.), H. S. Park, R. Porrata (LLNL)
report on behalf of the LOTIS collaboration:

During a routine sky patrol the LOTIS telescope observed the field of GRB
991106 in two 10 s images at 2.65 minutes and 3.02 minutes after the burst
(10:57:06 UT and 10:57:28 UT). There was no real-time BATSE trigger for
this event. LOTIS also obtained 12 sky patrol images of the position prior
to the burst during the interval 4:23:00 UT to 9:51:16 UT.  Comparison of
the images obtained after the burst with the previous sky patrol images,
the Digital Sky Survey, and the Guide Star Catalog reveal no OT within the
BeppoSAX WFC error box (Gandolfi et al. GCN 435
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 ) brighter than m = 13.4
(10-sigma). No astronomical filter was used during these observations.  
Further analysis is in progress.

This message may be cited.

GRB 991106, optical observations
--------------------------------------

Alberto Castro-Tirado, LAEFF-INTA (Madrid) and IAA-CSIC (Granada)
Javier Gorosabel, LAEFF-INTA (Madrid) and University of Amsterdam (UoA)
Eloy Rodriguez and Justo Sanchez del Rio, IAA-CSIC
Christian Vanderiest and Dominic Proust (Observatorio de Meudon, Paris)
Marco Feroci, IAS (Frascati), on behalf of the BSAX team

report:

"We have obtained sixteen exposures of the BeppoSAX position of
GRB 991106 (Gandolfi et al. GCN 435
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 ) with the 1.5-m telescope at
the Observatorio de Sierra Nevada (IAA-CSIC) near Granada.  The
images were taken starting 9.1-hr after the event (Nov 6.83-6.97 UT)
through R- and I-band filters. After a visual comparison with the
Digital Sky Survey, no sources varying by more than 0.3 mag were
seen to the DSS limit within the BSAX WFC error box."

This report can be cited in other publications.

BeppoSAX refined position for GRB991106 

 GB991106 refined coordinates from BeppoSAX WFC are:
  R.A.(2000)= 336.202
  DEC(2000)= 54.365

  with an error radius of about 3.2'

Giangiacomo Gandolfi on behalf of
BeppoSAX Mission Scientist
                                                                               



#----------------------------------#------------------------------------------#
 Luigi Piro                        | E-mail:
 Istituto Astrofisica Spaziale     |              piro@ias.rm.cnr.it
 C.N.R.                            | SAX program: saxsci@ias.rm.cnr.it 
 Via Fosso del Cavaliere           | phone: -39-06-4993-4007 
 I-00133 Roma                      | phone BeppoSAX center: -39-06-4079-6393
 Italy                             | fax(CNR)  : -39-06-20660188
#__________________________________#___________________________________________#

BeppoSAX GRB Alert: GRB991106

On Nov.6, 10:54:27 UT a GRB (GRB991106) was detected
by BeppoSAX WFC/GRBM.

  Preliminary coordinates from WFC are:
  R.A.(2000)= 336.211
  DEC (2000)= 54.365

  with an error radius of about 4'

 Giangiacomo Gandolfi on behalf of
 BeppoSAX Mission Scientist