Gamma-ray Burst 980326

The Gamma-Ray Burst (BATSE trigger No. 6660, 2000 cts/s peak intensity) was detected by BATSE and BeppoSAX WFC on 1998 March 26, 21:18:53 (26.888125) UT. With the BeppoSAX WFC the location could be deduced to a 8 arcmin error circle centered at R.A. = 08h36m26s, Decl. = -18o53'.0 degrees (2000.0).
No follow-up observation with the BeppoSAX NFI instruments is possible within the next days due to sun angle constraints. For the same reason, the GRB location is not reachable for ROSAT for the next 4 weeks. (All information courtesy of the instrument teams.)

Previous IAU Circulars

• IAUC 6851 BeppoSAX WFC detection and localisation
• IAUC 6852 optical transient at RA = 8h36m34s.28, Decl. -18o51'23".9 (2000) declined from R=21.7 on Mar 27.42 UT to R=23.4 mag on Mar 28.17 UT
• IAUC 6856 details of BATSE measurement

• GCN3 notice #32
  P.J. Groot, P.M. Vreeswijk, T.J. Galama (University of Amsterdam); E. Pian, F. Frontera, E. Palazzi (CNR, Bologna); M. Feroci (CNR, Roma); C. Kouveliotou and C. Robinson (USRA at NASA/MSFC) on behalf of the CTIO GRB collaboration; J. van Paradijs (U of Amsterdam and U of Alabama in Huntsville); C. Tinney, Phillips, Driver, Smith, Jones, Windridge (AAO); M. Keane, P. Hall, M. Smith (NOAO at CTIO); J.F. Gonzalez, C. Lidman (ESO) report: "Comparison of optical R-band observations of the error box of GRB980326 (IAUC 6851) taken at the AAT at Mar 27.42 UT (480s), the CTIO 4m-telescope at Mar 28.04 UT (600s) and the ESO 3.5m NTT at Mar 28.01 UT (ESO1, 600s) and Mar 28.17 UT (ESO2, 600s) shows a clearly variable object located at RA 08:36:34.28, Decl -18:51:23.9 (J2000). Differential photometry with respect to three local stars shows a decrease between the AAT and ESO1 of 1.2(1) mag, AAT and CTIO of 1.6(1) mag and AAT and ESO2 of 1.7(1) mag. The three local standards have coordinate end-figures: 1) 36:35.41, 51:41.6; 2) 36:31.00, 51:17.3; 3) 36:34.03, 50:53.2. Their R-band magnitudes are: 1) 18.4(1); 2) 21.7(1); 3) 20.3(1), as determined with respect to Landolt SA 98, no. 626, 624, 634, 590. At the time of the AAT observation (Mar. 27.42 UT) the R-band magnitude of the variable was R=21.7, equal to that of comparison star 2. A finding chart of the variable can be found at: http://www.astro.uva.nl/paulgr/grb/grb980326.html.

  
  

• GCN3 notice #33
  A. C. Eichelberger, S. R. Kulkarni, S. G. Djorgovski (California Institute of Technology) report on behalf of the Caltech-Keck GRB effort:
  On UT March 27, 1998, A. V. Filippenko, D. C. Leonard and A. G. Reiss (U. C. Berkeley), observed the WFC localization region (IAUC 6851) with the Low Resolution Imaging Spectrograph (LRIS) on the Keck II telescope. On UT March 28, 1998, H. Spinrad, A. Bunker, D. Stern (U. C. Berkeley), A. Dey (JHU) and S. A. Stanford (IGPP), observed the same field with LRIS. For both nights images were obtained in the R band. We report the following magnitudes for the variable object reported by Groot et al. (IAUC 6852; also previous GCN note #32):

        R(UT 1998 March 27.31) = 21.04 mag.
        R(UT 1998 March 28.25) = 23.54 mag.
  

  The zero point was set by assuming R=21.7 magnitude for the object designated as "comparison star 2" by Groot et al. (IAUC 6852; R=21.7). Combining the two Keck points with the R magnitude from the AAT observations (IAUC 6852) we find the variable source exhibits a power law decay with index = -2.0 This is considerably steeper than those exhibited by optical transients of previous GRBs.
• GCN3 notice #34
  Bruce Grossan, Robert Knop, Saul Perlmutter (Lawrence Berkeley National Laboratory and CfPA, U.C. Berkeley), and Isobel Hook(ESO) report that the optical transient counterpart to GRB980326 was observed with 900 seconds of exposure at Keck II with the LRIS instrument on UT 1998 March 30.2. Relative photometric measurements were made with respect to the second and third stars given in IAUC6582, and the average of these two relative measurements yielded approximately R=24.5 mag. A 1.5" aperture was used in 0.9" seeing conditions for these preliminary results.
• GCN3 notice #34 addendum
  Bruce Grossan, Robert Knop, Saul Perlmutter (Lawrence Berkeley National Laboratory and CfPA, U.C. Berkeley), and Isobel Hook(ESO) report that in the observations of UT Mar 30.2 using the LRIS instrument on Keck II (see GCN#34), OT980326 (see IAUC6582) is irregular in shape, with some elongation roughly NE to SW. We have therefore analyzed the OT photometry with a 1.5" aperture to include all the light up to the point where the curve-of-growth flattens. The OT, with 900 seconds of exposure, was delta R= 3.1 and 4.7 mag fainter than stars 2 and 3 of IAUC6582, where these reference stars were measured with a larger 3.0" aperture. This yields R = 24.8 or 25.0 mag for the OT using the absolute calibrations for stars 2 and 3. Note that the OT has a neighboring object currently 0.7 mag brighter, approximately 1.4" E, 0.7" S of the OT.
• BATSE location map at http://www.batse.msfc.nasa.gov/~kippen/batsebr
• GCN3 notice #53
  K. Hurley (UCB) on behalf of the Ulysses GRB team, M. Feroci (IAS/CNR, Rome) and F. Frontera (ITESRE/CNR, Bologna) on behalf of the BeppoSAX/GRBM Team report: We have derived a preliminary IPN annulus for this burst using Ulysses and the BeppoSAX Gamma-Ray Burst Monitor. The annulus is described by a circle of radius 40.771 degrees centered at RA(2000) = 154.714 degrees, Declination (2000) = 13.166 degrees. The half-width of the annulus is 0.042 degrees. The annulus intersects the WFC error circle (IAUC 6851) and reduces its area. The intersection points are:

                  RA(2000)                Decl (2000)
       129.194 o = 8 H 36 M 47 S     -18.981 o =-18 o 58 ' 53 "
       128.988 o = 8 H 35 M 57 S     -18.817 o =-18 o 49 ' 01 "
       129.242 o = 8 H 36 M 58 S     -18.911 o =-18 o 54 ' 38 "
       129.054 o = 8 H 36 M 13 S     -18.761 o =-18 o 45 ' 38 "
  

  This error box includes the optical transient (GCN #32, IAUC 6852). Further processing will reduce the annulus width. An image may be found at http://ssl.berkeley.edu/ipn3/980326/ (and is reproduced below).
  
• GCN notice #56
  Frank Valdes, Buell Jannuzi, and James Rhoads (on behalf of the NOAO Deep Widefield survey team and the KPNO GRB followup team) report: We observed the location of the GRB 980326 optical transient on 1998 April 7.15 UT using the KPNO 4 meter telescope and the CCD Mosaic camera. The total integration time was 3300 seconds. The final coadded image shows no source at the location of the optical transient. The flux obtained in a 5 pixel (1.30 arcsecond) radius aperture and calibrated by comparison to Groot et al's reference star 1 (IAU Circular 6852) corresponds to -0.44 +- 0.33 microJansky at R band. The corresponding 3 sigma limiting magnitude is R > 24.3 ; the 2 sigma limit is R > 25.3. For a 7 pixel aperture, the flux is -0.52 +- 0.38 microJansky. We also placed an aperture on the location of Grossan et al's nearby source (GCNC #35). The fluxes in 5 and 7 pixel apertures are 0.14 +- 0.32 microJansky and 0.51 +- 0.37 microJansky respectively. Corresponding 3 sigma limits are R > 23.6 and 23.2 ; two sigma limits are R > 24.0 and 23.4 respectively. These results are marginally consistent with Grossan et al's magnitude measurement of R=24.1 or 24.3 for the neighboring source. More detail, including a link to a 104 by 91 arcsecond section of the final image, is available at http://www.noao.edu/noao/grb/980326.html . The Mosaic camera has a 36 arcminute field of view, and so covers the entire BeppoSAX error box (16 arcminute diameter). We can make data for this full region available, with the caveat that further data processing might be required. Contact one of us if interested.
• GCN3 notice #57
  S. G. Djorgovski, S. R. Kulkarni, P. Cote, J. Blakeslee, J. S. Bloom, and S. C. Odewahn (Caltech), on behalf of the Caltech GRB collaboration, report: We obtained R band images of the field of GRB 980326, using Keck-II 10-m telescope, on the night of 17 April 1998 UT. We clearly detect a galaxy coincident to within the astrometric errors (less than 0.3 arcsec) with the previously reported optical transient (cf. Groot et al. IAUC 6852, and Kulkarni et al. GCN Circ. 33). Using the magnitudes of stars 2 and 3 from Groot et al., we derive for this galaxy R = 25.5 +- 0.5 mag (preliminary reductions). This is about 0.6 magnitudes fainter than the tentative detection by Grossan et al. (GCN Circ. 35), and is consistent with the upper limits reported by Valdes et al. (GCN Circ. 56). We interpret this object as the host galaxy of GRB 980326, probably with a negligible contamination from the OT itself. The difference in brightness from the measurements by Grossan et al. on 30 March 1998 UT suggests that some contribution from the OT was still present at that time, with the estimated OT magnitude of R ~ 25.8 at that epoch. Images of the field can be found at http://astro.caltech.edu/~george/grb/grb980326.html .
  Further observations of this field are in progress.
• GCN notice #58
  F.E. Marshall (Goddard Space Flight Center) and T. Takeshima (USRA at GSFC) report: X-ray emission from GRB980326 was not detected during two observations with the Proportional Counter Array (PCA) on the Rossi X-ray Timing Explorer (RXTE). The first observation began at 05h31m UT on March 27th (about 8.2 hours after the burst) and lasted about 1850 s. The second observation began 12 days later and lasted about 1600 s. During both observations, the PCA alternated between viewing the GRB and a nearby background region. Assuming that any X-ray afterglow was negligible during the second observation and that there were no other variable sources in the field-of-view, the observed change in counting rate from the direction of the apparent optical counterpart of the GRB (IAUC 6852) of -0.03 +/- 0.15 (90% confidence) counts/sec/detector is a measure of the intensity of the X-ray afterglow 8.5 hours after the burst. An upper limit of 0.15 counts/sec/detector corresponds to a flux of 1.6e-12 ergs/s/cm**2 in the 2-10 keV band for an assumed Crab-like spectrum.
• GCN report #161
  J. S. Bloom and S. R. Kulkarni report on behalf of the Caltech GRB effort report:
  "On the nights of 17 and 18, December 1998 we carried out imaging observations of the field of GRB 980326 with LRIS instrument on Keck II. In the R-band we obtained 8 images each with an integration time of 300 s. These images were debiassed, flat-fielded, registered and median stacked. In the resulting image we do not detect the host galaxy at the position of the optical transient of this GRB (Groot et al., ApJ 502, L123, 1998). We determined the zero-point calibration using the reference stars quoted in Groot et al. (1998). The faintest object that was readily detected and in the general vicinity of the OT was ~27.3 mag (with aperture radius of 1.5 arcsec). This object and others at similar brightness levels are clearly visible to the eye in the stacked image. We can place a firm lower limit on the magnitude of the host galaxy at this level.
  Previous magnitudes for the the magnitude of the host galaxy were derived using data when the OT was bright and could have been affected by the assumed decay model and photometric errors.
  The true host of GRB 980326 is thus fainter than R=27.3 mag or that the OT/GRB is not coincident with its host (e.g. the GRB was ejected from the host galaxy or is of intergalactic origin). If the former is true then the host galaxy of GRB 980326 is by far the faintest host and this faintness is unlikely due to dust extinction given the detection of the optical transient in the B band (Groot et al. 1998)."
• Supernova indication from light curve (Bloom etal, Nat. 401, 1999, p. 453)
  
• GCN notice #1029
  Andrew Fruchter (STScI), Paul Vreeswijk (University of Amsterdam) and Peter Nugent (LBNL) report on behalf of a larger HST GRB collaboration:
  We observed the field of GRB 980326 with HST/STIS on 31 December 2000, just over 1000 days after the burst. The total exposure time was 7080 seconds in the 50CCD aperture. The images were reduced by the standard HST pipeline, and drizzled to a combined image with a pixelscale of 0."0254.
  We projected the location of the early afterglow from a KeckII/LRIS R band image, taken by Filippenko, Leonard and Riess on March 27, 1998 (see GCN 33). We estimate the 1-sigma error radius to be less than 1.5 drizzled HST pixels, which includes both the scatter in the transformation (0.9 pixels, using 6 reference stars) and the measurement error of the transient in the Keck image (0.8 pixels).
  Within one pixel of the estimated position, there was evidence of a small source. We therefore convolved the image with a gaussian with a FWHM equal to that of the PSF (3.5 drizzled pixels). This procedure uses the PSF as a matched filter to enhance the the signal-to-noise ratio of unresolved sources. In the convolved image an object is found, again offset by one pixel from the expected position with a significance of ~4.5 sigma. We estimate a magnitude (in the broad STIS 50ccd filter) of V = 29.25 +/- 0.25.
  Bloom et al., (1999, Nature, 401, 453) have suggested that the unusual light curve of this GRB might be explained by an underlying supernova, and on the basis of this estimated a redshift of about z~1. A supernova at this redshift (which in this model peaks around a magnitude of 25) would be expected to be significantly fainter than V=29 by now (as would the rapidly decaying GRB afterglow). Therefore, we may be detecting a faint host galaxy. Deep HST images have in all other cases detected a host underlying the GRB, when good astrometry was available for the GRB. If we are observing a host galaxy, then the above magnitude could underestimate its brightness by as much as a few tenths of a magnitude, due to extended emission missed in this measurement. A galaxy at a z~1 with this observed magnitude would lie a rather remarkable 7 magnitudes below L*, the knee of the of the galaxy luminosity function at that redshift.
  Given the apparent point-source nature of the detected object, and the lack of color information, we cannot exclude the possibility (which we consider remote) that we are observing a light echo, from either the GRB or the supernova.
  Images of the field can be found at
  http://www.stsci.edu/~fruchter/GRB/980326