Gamma-ray Burst 970228: Score !

The BeppoSAX Gamma Ray Burst Monitor (GRBM) was triggered on Feb. 28.12362 UT. The moderate intense Gamma Ray Burst lasted about 80 sec. While BATSE was behind the Earth, Ulysses and TGRS have also detected this GRB. Quick follow-up pointings with the NFI on BeppoSAX (8 hours and 3 days after the GRB) find for the first time a fading X-ray source within the GRB error box! In addition, also a variable optical counterpart has been discovered! Here you find some information on the follow-up observations of the BeppoSAX GRB 970228.

Previous IAU Circulars

• IAUC 6572 Discovery with WFC onboard BeppoSAX
  
• IAUC 6574 Westerbork observations: 0.7 mJy upper limit at 5 GHz; V and I images show no variable optical object brighter than V=19
• IAUC 6576 bright X-ray source SAX J0501.7+1146 near border of GRB error box which declined by a factor of 20 within 2.5 days; radio source of 0.8 mJy at 20 cm at X-ray position
• IAUC 6577 TGRS results; 1.2 mJy upper limit at 86.5 GHz; R=21 mag object at radio position
• IAUC 6578 OSSE results; Ulysses/BeppoSAX triangulation ring
• IAUC 6580 ROSAT all-sky survey upper limit; R=21 mag object is galaxy
• IAUC 6581 te-1 decay follows prediction of saturated Comptonization model; R=21 mag galaxy confirmed; spectrum implies z=0.5
• IAUC 6582 no optical variability by >0.2 mag of objects brighter than B=19 and R=20; 30 optical objects down to B = 24.5 or R = 24, several are galaxies; z=0.498 for galaxy at radio position
• IAUC 6584 optical star-like object at R.A. = 5h01m46s.70, Decl. = +11o46'53".0 was fading from V=21.3 and I=20.6 on Mar. 1.0 to V>23.6 and I>22.2 on Mar. 8.9.
• IAUC 6588 quiescent object at optical transient position has R=24 and is extended (galaxy)
• IAUC 6593 improved Ulysses/Wind IPN triangulation ring; ASCA detection on Mar. 7
• IAUC 6594 refined Ulysses/SAX triangulation
• IAUC 6598 difference in optical/X-ray luminosity to GRB 970111
• IAUC 6606 HST F606W and F814W detection of the optical counterpart at 25.7+/-0.3 and 24.2+/-0.3 within an embedded extended source
• IAUC 6611 J band detection (J=21.0 mag) and H and K band upper limits
• IAUC 6618 optical detection on Mar 3.1, 1997: no fast fading
• IAUC 6619 J and K band detections (J=23.5+/-0.2, K=22.0+/-0.2) on Mar 30/31; HST F606W and F814W detection on Apr. 7, 1997: point source has 26.0+/-0.3 and 24.6+/-0.3 in these bands
• IAUC 6620 optical spectra of transient on Mar. 31 - Apr. 2, 1997: no 'obvious' emission lines
• IAUC 6629 optical transient shows proper motion of 540 mas/yr corresponding to a transverse velocity of 260 km/s at 100 pc
• IAUC 6631 detection of point source on Apr. 5/6 at R=24.9+-0.3 (faded by 1 mag), and fading of extended object below detection limit (not specified)
• IAUC 6635 broadband imaging of Feb. 28.8 and Mar. 4.8 imply fading by >2.7 mag between these 4 days
• IAUC 6637 ROSAT HRI detection of fading X-ray source 11 days after GRB yields 10" accurate position; optical transient is within this new error circle
• IAUC 6643 Analysis of HST images reveal no fading of extended emission
• IAUC 6732 no emission lines in Keck II LRIS spectrum of object at optical transient position; nebulosity seen at R~25.5
• IAUC 6747 V = 28.0 +/- 0.25 for OT and V = 25.7 +/- 0.15 for the nebula from HST STIS observation on Sep. 4, 1997.

• The BeppoSAX pointing on Feb. 28 reveals a new, bright X-ray source within the 3 arcmin GRB error box at position
  1SAX J0501.7+1146
  The source flux is (2.8 +/- 0.4) x 10E-12 erg cmE-2 sE-1 in the MECS (2-10 keV) and (4.0 +/- 0.6) x 10E-12 erg cmE-2 sE-1 in the LECS (0.5-10 keV).

  

• Optical transient fades within a week after the GRB as images with the WHT and INT show (Groot et al., IAUC 6584).
• HST press release

  Original HST data of from 26 Mar 1997 and 7 Apr 1997.

  Two Hubble Public-Domain Images of the GRB Transient Region

  The Wide-field Planetary Camera Image of the Region in the Visual Band. The Optical Transient is at the center of the photograph.

  A Close-Up of the Optical Transient shows both a point-like source (the bright emission) plus the extended emission (below and to the right) from what may be the distant host-galaxy

• J band image taken on 1997, March 17.8 UT, Calar Alto, DSAZ, Spain (3.5-m telescope, MAGIC camera (J. Klose, Tautenburg Observatory, IAUC 6611). The marked area approximately corresponds to the public HST image.

  
  

• Spectrum of the optical transient obtained with the LRIS spectrograph on the Keck II 10-m telescope (bottom figure) on March 31 through April 2 (IAUC 6620) and the position of the slit overlayed over the HST F606W image of March 26 (top figure, image rotated). The transient source is weakly detected in the 5500 sec long summed spectrum and shows no obvious emission features.

  
  
  

• Decay lightcurve in the optical (astro-ph/9807295):
  
• GCN notice #289
  S. G. Djorgovski, S. R. Kulkarni, J. S. Bloom (Caltech), and D. A. Frail (NRAO) report on behalf of the Caltech-CARA-NRAO GRB Collaboration:
  Spectra of the host galaxy of GRB 970228 were obtained in the course of several runs at the Keck-II 10-m telescope. A prominent emission line at 6319 A was observed in at least 3 independent data sets, along with a very blue continuum, showing no apparent breaks between about 4000 and 8900 A. We interpret this emission line as [O II] 3727 at z = 0.695 +- 0.002. Moreover, possible emission lines are detected corresponding to [Ne III] 3869 and [O III] 5007 at the same redshift. We thus consider this redshift to be practically secure.
  Assuming a cosmology with H_0 = 65 km/s/Mpc, Omega_0 = 0.2, and Lambda_0 = 0, we derive the luminosity distance of 1.28 e28 cm. The measured BATSE fluence of 4.3 e-6 erg/cm2 then implies E(gamma,isotropic) = 5.2 e51 erg.
  We detect continuum flux in the V band of about 0.12 uJy, uncertain by at least 20%. This is in an excellent agreement with the published ground-based and HST photometry, corresponding to V ~ 26 mag.
  The [O II] line has the equivalent width 75 +- 15 A (observed), or 44 +- 9 A (restframe), typical for star-forming galaxies at comparable redshifts. The very blue continuum suggests little restframe extinction in this galaxy.
  In the following estimates, we assume a Galactic extinction correction A(V) = 0.75 mag. From the [O II] line flux, we estimate SFR = 0.47 Msun/yr, and from the continuum power at 2800 A (rest), SFR = 0.35 Msun/yr. Both of these are uncertain by at least 30%. The absolute restframe B band luminosity of the galaxy corresponds to about 0.05 to 0.1 L_* today. Thus, neither the SFR nor the SFR per unit mass are especially high.
• 0705.2415from 16 May 2007
  Bernardini: GRB970228 as a prototype for short GRBs with afterglow
  Abstract: GRB970228 is analyzed as a prototype to understand the relative role of short GRBs and their associated afterglows, recently observed by Swift and HETE-II. Detailed theoretical computation of the GRB970228 light curves in selected energy bands are presented and compared with observational BeppoSAX data.
• 0712.0550 from 4 Dec 2007
  Bernardini: GRB970228 and the class of GRBs with an initial spikelike emission: do they follow the Amati relation?
  Abstract: On the basis of the recent understanding of GRB050315 and GRB060218, we return to GRB970228, the first Gamma-Ray Burst (GRB) with detected afterglow. We proposed it as the prototype for a new class of GRBs with "an occasional softer extended emission lasting tenths of seconds after an initial spikelike emission". Detailed theoretical computation of the GRB970228 light curves in selected energy bands for the prompt emission are presented and compared with observational BeppoSAX data. From our analysis we conclude that GRB970228 and likely the ones of the above mentioned new class of GRBs are "canonical GRBs" have only one peculiarity: they exploded in a galactic environment, possibly the halo, with a very low value of CBM density. Here we investigate how GRB970228 unveils another peculiarity of this class of GRBs: they do not fulfill the "Amati relation". We provide a theoretical explanation within the fireshell model for the apparent absence of such correlation for the GRBs belonging to this new class.