Gamma-ray Burst 970828

The RXTE All Sky Monitor has detected the gamma ray burst reported as BATSE trigger #6350 (burst at 1997 August 28, 17:44:36.8 UT). The burst was detected by ASM camera #1 with a mean flux of 756 mCrab (2-12 Kev) averaged over the 40 s in which the burst was seen before the camera was rotated. Ninety seconds later, ASM camera #2 scanned over the source and determined a crossing line of position and an average flux of 238 mCrab, declining in an exponential-like fashion with a 30 s time constant. The X-ray emission began roughly simultaneously with the gamma ray trigger, but was detectable with the ASM over at least 160 s. The combined camera detections yield an X-ray position (J2000) of 18h 08m 29s +59 18' with uncertainty (1 sigma) of +/- 2.5' in RA. and +- 1' in Dec. (courtesy R. Remillard, A. Wood, D. Smith, and A. Levine (MIT)).

Previous IAU Circulars

• IAUC 6726 Location with ASM/RXTE
  
• IAUC 6727 Detection and flux from PCA/RXTE scanning
  
• IAUC 6728 Refinded ASM error box and Ulysses/BATSE triangulation
  
• IAUC 6729 no optical variable down to POSS level; ASCA source
  
• IAUC 6730 no optical variations of >0.3 mag brighter than 22.5; radio source with possible variability; optical object at radio position
• IAUC 6731 no optical variations of >0.5 mag brighter than POSSII on Kiso Schmidt plates during Aug. 29.46-29.52 and 30.45-30.63 UT
• IAUC 6732 revised ASCA position and flux; X-ray source fades within observation at t^-1.4
• IAUC 6733 no optical variations of >0.2 mag brighter than R=21 on Aug. 29.86, 30.84, and 31.83 UT
• IAUC 6735 no optical variations of >0.2 mag/day at R<24.5 on Aug. 30, 31, and Sept. 1.
• IAUC 6756 survey down to K' = 20 mag; one object with large R-K' colour
• IAUC 6757 ROSAT location of X-ray afterglow; no optical variable within 10" error box down to 25th mag

• The RXTE/ASM scanning observations have reduced the GRB error box to about 5'*2' arcmin: The error box is an ellipse within a rhombus with the following corners: R.A. = 271.92, Decl. = +59.30; 272.10, +59.321; 272.31, +59.297; 272.14, +59.279 (equinox 2000.0).
• Scetch of the refined ASM error box with the IPN arc overlayed (courtesy Don Smith)
  
• A radio source (IAUC 6730) is detected at R.A. = 18h08m25s.51 +/- 0s.05, Decl. = +59o18'08".5" +/- 0".4 (equinox 2000.0) at a mean flux of 0.36 and 0.27 mJy on Aug. 31.19 and Sept. 1.27 UT (upper limit of 0.18 mJy on Aug. 28.88).
• An object with R = 24.5 +/- 0.5 (IAUC 6730) is detected at the position of the radio source.
• A short summary report on optical follow-up observations led by H. Pedersen
  updated on Oct. 22, 1997; V and R images available
• I and K band images (from Klose, Eislöffel, and Stecklum; IAUC 6756):
  Left: I-band image taken on 1997, Aug. 31.9 UT using the prime focus CCD camera on the 1.34-m Schmidt telescope of the Thüringer Landessternwarte Tautenburg (limiting magnitude about I = 20). Note that the I-band image is slightly affected by a bad sky flat. Right: K'-band image taken on Sept. 9.8 UT, Calar Alto Observatory, Spain, using the 3.5-m telescope equipped with the MAGIC near-infrared camera (wide-field mode, 0.81" pixels). The circle approximately represents the ASCA error circle (radius 30 arcsec) reported by Murakami et al. (IAUC 6732). The object mentioned in IAUC 6756 is indicated. The K'-band image was reduced using a special sampling technique, resulting in a pixel scale of 0.4" per pixel.
• HRI image of the ROSAT observation:
  
  The ellipse represents the RXTE error box, the blue straight lines the IPN triangulation arc and the green circle the ASCA position. ROSAT sources are encircled (red) and numbered.
  
  In the following two images the ROSAT position is plotted over: Left: R band image taken at the 4.2m WHT about 3.5 hrs after the GRB. The limiting magnitude is about 23.5 mag. The large circle is the ASCA position (30" radius) and the small circle is the 10" ROSAT position. Right: The K band image from Klose et al. (IAUC 6756)
  
  And finally a close-up view of the optical content in stacked WHT images (from Aug. 28, 29 and Sep. 3, 4, 5). Note that East is to the right!
• X-ray images from ASCA . The whole time image is reproduced below:
• Note that there are two different redshift estimates on the 'market':
  (1) z=0.33 from the X-ray spectrum (ASCA paper)
  (2) z=0.958 from host galaxy; see http://online.itp.ucsb.edu/online/gamma_c99/djorgovski/oh/01.html
• A reanalysis of the ASCA spectrum leads to a new interpretation of the X-ray spectral feature as being radiative recombination edge + continuum (Yoshida et al 2001, ApJ Lett.; astro-ph/0107331 + astro-ph/0107330 ). Thus, the earlier redshift estimate of z=0.33 has been withdrawn.
• from Djorgovski et al. (astro-ph/0107539)
  Image of the field of GRB 970828 from the R band data taken at the Palomar 200-inch telescope on 1997 August 30 UT. The field size shown is 72.5 arcsec square, with North up and East to the left. The ROSAT error circle of the X-ray afterglow, with a 10 arcsec radius is shown. The position of the radio afterglow is indicated by the cross. Proposed host galaxy components (A, B, C) are indicated. The offsets from star 1 to the brighter component of the host galaxy component A are: 10.1 arcsec E, and 14.9 arcsec N.
  Close-up region of GRB 970828 in the R band (left) and K_s band (right). The images are 33 x 33 arcsec^2 with North up and East to the left. Galaxies A, B, and C are labeled as well as the offset star 1. The small ellipse at the center of the R band image is the 1-sigma error contour of the position of the radio transient. The transient appears nearly coincident with galaxy B but may also have arisen in the region between galaxies A and B, potentially a dust lane intersecting a single, larger galaxy or a merging system of with components A, B, and possibly C. A comparison of the R and K images demonstrates the red colors of galaxies A and B; in contrast, galaxy C appears to be very blue.
  A weighted average spectrum of the component A of the host galaxy of GRB 970828 obtained at the Keck. Prominent emission lines of [O II] 3727 and [Ne III] 3869 are indicated; the remaining apparent features are artifacts of night sky subtraction, with the sky spectrum shown in the bottom panel for comparison. In addition to the emission lines, the spectrum also shows the Balmer break at the same redshift, z = 0.9578.
• GCN notice #1134
  GRB 970828: HST Observations of the Host Galaxy
  J. S. Bloom, S. R. Kulkarni, T. J. Galama, D. A. Frail and S. G. Djorgovski report on behalf the larger Caltech-NRAO-CARA GRB Collaboration:
  "We observed the field of GRB 970828 (Remillard et al., IAUC 6726; Murakami et al., IAUC 6732; Groot et al. 1998, ApJ, 493, 27) on 2001 August 16-18 UT using the WFPC F450W, F606W, F814W filters on the Hubble Space Telescope. The total integration time in each filter was 8800 s. The 3-component host system seen in deep Keck imaging (Djorgovski et al. 2001, astro-ph/0107539, accepted) appears to be spatially resolved, with no detected emission connecting the components. However, given the signal-to-noise, we would be unable to detect low-surface brightness features and cannot therefore make a strong claim about the geometry of dust and stars in the system.
  Astrometrically comparing the F606W images with the Keck localization, we identify component "B" (from figure 3 of Djorgovski et al.) as the host galaxy. The offset location and uncertainty (which is large due to the poor astrometric tie between radio and optical images) are consistent with those reported in Bloom, Kulkarni & Djorgovski (2001; astro-ph/0010176).
  An image of the host may be found at:
  http://www.astro.caltech.edu/~jsb/GRB/grb970828-host-reg.ps
  The 3-sigma uncertainty contour of the OT position is shown in this image as well as the galaxy component labels from Djorgovski et al."
  This message may be cited.
• 1205.6651 from 31 May 12
  L. Izzo et al.: On the thermal and double episode emissions in GRB 970828
  Following the recent theoretical interpretation of GRB 090618 and GRB 101023, we here interpret GRB 970828 in terms of a double episode emission: the first episode, observed in the first 40 s of the emission, is interpreted as the proto-black-hole emission; the second episode, observed after t$_0$+50 s, as a canonical gamma ray burst. The transition between the two episodes marks the black hole formation. The characteristics of the real GRB, in the second episode, are an energy of $E_{tot}^{e^+e^-} = 1.60 \times 10^{53}$ erg, a baryon load of $B = 7 \times 10^{-3}$ and a bulk Lorentz factor at transparency of $\Gamma = 142.5$. The clear analogy with GRB 090618 would require also in GRB 970828 the presence of a possible supernova. We also infer that the GRB exploded in an environment with a large average particle density $ \, \approx 10^3$ part/cm$^3$ and dense clouds characterized by typical dimensions of $(4 - 8) \times 10^{14}$ cm and $\delta n/n \propto 10$. Such an environment is in line with the observed large column density absorption, which might have darkened both the supernova emission and the GRB optical afterglow.
• 1311.7432 from 2 Dec 13
  R. Ruffini et al.: Induced Gravitational Collapse in the BATSE era: the case of GRB 970828
  Following the recently established "Induced Gravitational Collapse" (IGC) paradigm, we here interpret GRB 970828 in terms of the four episodes typical of such a paradigm. The "Episode 1", up to 40 s after the trigger time t_0, with a time varying thermal emission and a total energy of E_{iso,1st} = 2.60x10^{53} erg, is interpreted as due to the onset of a supernova Ib/c in a tight binary system with a companion neutron star. The "Episode 2", observed up t_0+90 s, is interpreted as a canonical gamma ray burst, with an energy of E^{e^+e^-}_{tot} = 1.60x10^{53} erg, a baryon load of B = 7x10^{-3} and a bulk Lorentz factor at transparency of \Gamma = 142.5. From this Episode 2, we infer that the GRB exploded in an environment with a large average particle density ~ 10^3 particles/cm^3 and dense clouds characterized by typical dimensions of (4 - 8)x10^{14} cm and \delta n / n ~ 10. The "Episode 3" is identified from t_0+90 s all the way up to 10^{5-6} s: despite the paucity of the early X-ray data, typical in the BATSE, pre-Swift era, we find extremely significant data points in the late X-ray afterglow emission of GRB 970828, which corresponds to the ones observed in all IGC GRBs-SNe sources. The "Episode 4", related to the Supernova emission, does not appear to be observable in this source, due to the presence of darkening from the large density of the GRB environment, also inferred from the analysis of the Episode 2.