Gamma-ray Burst 991106

On November 6, 1999, 10:54:27 UT a GRB was detected by the BeppoSAX GRBM and WFC instruments and localized to 4 arcmin accuracy. The burst had a duration of only 1 s in the GRBM, and thus could be the first BeppoSAX short GRB detection.

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• BeppoSAX GRB alert n. 99/31 and GCN notice #434
  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'.
• BeppoSAX GRB alert n. 99/33 GCN notice #435
  Refined coordinates from WFC are:

    R.A.(2000)= 336.202
    DEC(2000)= 54.365
  

  with an error radius of about 3.2'
• GCN notice #436
  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) 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.
• GCN notice #437
  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) brighter than m = 13.4 (10-sigma). No astronomical filter was used during these observations. Further analysis is in progress.
• GCN notice #438
  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, #435) 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.
• GCN notice #440
  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). 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
• GCN notice #443
  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.
• GCN notice #444
  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) 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.
• GCN notice #445
  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) 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). 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) 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).
• GCN notice #446
  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), but about 10 arcsec outside the BeppoSAX NFI 1.5 arcmin error circle (Antonelli et al., GCN #445). 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). 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.
  
  K'-band image of the GRB 991106 error box. The two objects mentioned in the text are indicated.
• GCN notice #447
  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), 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). 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 and 445, 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) 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.
• GCN notice #448
  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 (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.