Gamma-ray Burst 970508

A faint Gamma-Ray Burst was detected with the GRBM and WFC on May 8.904, 1997 lasting 15 sec (peak flux about 1 Crab). This burst was detected as trigger 6225 by BATSE.

Previous IAU Circulars

• IAUC 6649 Discovery with WFC onboard BeppoSAX
  
• IAUC 6654 revised WFC position; optical brightening during first 2 days
  
• IAUC 6655 further brightnesses for optical variable; Keck spectrum shows set of absorption lines with z = 0.835.
  
• IAUC 6656 new X-ray source 1SAX J0653.8+7916 detected with NFI 5.7 hrs after GRB
  
• IAUC 6657 further brightness measurements and optical spectra
  
• IAUC 6658 brightness measurements for 7 optical objects within the 50" error box; optical variable starts fading on May 11
  
• IAUC 6660 further fading; faint galaxy (r = 24.8) which may be responsible for the absorption lines seen in the spectrum; BATSE fluxes and spectral shape; further brightness measurements
  
• IAUC 6661 further brightness measurements
  
• IAUC 6662 flaring radio source at R.A. = 6h53m49s.45, Decl. = +79o16'19".5 (equinox 2000.0; error 0".1)
  
• IAUC 6663 no detection at 3.5 mm; further multi-colour brightnesses
  
• IAUC 6664 K and I band photometry
  
• IAUC 6666 upper limit of 3.9 and 6.8 mJy/beam at 86.8 GHz during May 10 to 15
  
• IAUC 6670 faint detection at 15 GHz; VLBA detection at 8.4 GHz
  
• IAUC 6671 further multi-colour measurement of May 22
• IAUC 6674 HST observation on June 2: R = 23.10 +/- 0.15, H = 20.3 +/- 0.3; no diffuse emission brighter than R = 24.3
• IAUC 6676 Keck photometry and spectroscopy of June 5: R = 23.4 +/- 0.2; emission line at 684.0 nm is interpreted as [O II] 372.8-nm emission at z = 0.835 +/- 0.001; consistent with constant line flux but decreasing continuum
• IAUC 6848 R = 25.20 +/- 0.25 obtained from WHT imaging on March 20/21, 1998 suggests that the optical light is dominated by the host galaxy.

• The BeppoSAX pointing on May 9.1375 reveals a new X-ray source
  1SAX J0653.8+7916
  (R.A. = 6h53m46s.7, Decl. = +79o16'02" (equinox 2000.0; error radius 50") within the revised, 3 arcmin GRB error box. The previously known source, 1RXS J065213.8+790855 (lying outside the revised WFC error box) is also detected at R.A. = 6h52m14s.8, Decl. = +79o09'16", consistent within the errors with the ROSAT measurement. The countrates of 1SAX J0653.8+7916 were 0.0070 +/- 0.0007 and 0.0043 +/- 0.0006 count/s in the MECs (2-10 keV) and LECS (0.5-5 keV), respectively, corresponding to fluxes of (6.3 +/- 0.6) and (7.0 +/- 0.8) x 10E-13 erg cmE-2 sE-1.
  
  
• VLBA observations result in coordinates with 0".001 error:
  R.A. = 6h53m49s.4513, Decl. = +79o16'19".513 (equinox 2000.0)
• Finding charts are reproduced below:
  WHT image from P. Groot:
  
  Image obtained with the 1.52-m telescope at Loiano on May 10 (Mignoli et al):
  
  Images from Palomar (G. Djorgovski):
  A comparison of the images of the gamma-ray burst field, as its brightness in the visible light was still rising. The images were obtaines at the Palomar 200-inch telescope. The image on the left was taken on May 9, and the image on the right on May 10, 1997. The optical counterpart is marked with the arrows.
  
  A section of a deep r band image (average of May 9 and 10 images), centered on the OT (left side). The field shown is 1 arcmin square, with N at the top, and E to the left. A faint, 24.5~mag galaxy (g1) is seen 4.3~arcsec E and 3.5~arcsec N of the OT. It may be associated with the host galaxy of the OT itself, or with one of the absorption line systems seen in its spectrum. The right figure shows the field of GRB 970508 (320~arcsec square) together with the GRB and X-ray source error circles, from an r band image obtained at Palomar on UT May 10 (N at the top and E to the left). The proposed optical counterpart of the burst (OT) is indicated. We also indicate two nearby offset stars, labeled as A (2.7~arcsec W and 13.0 arcsec N of the OT, $r = 19.85$~mag) and N (28.9~arcsec W and 58.3~arcsec S of the OT, r = 17.4 mag). The dotted circle centered on the "x", with a 40~arcsec radius, indicates the error circle of the X-ray transient. The dashed circle, with a radius of 3 arcmin, indicates the final error circle of the GRB
  
• Optical lightcurve (from G. Djorgovski):
  
  Optical light curve in the r band, as a function of time starting at UT May 8.904, from the Palomar data (solid squares). The data have been corrected for the Galactic extinction and converted to specific fluxes. These numbers supersede our preliminary measurements. We also include a measurement {mig97} of R_C = 19.78 +/- 0.05 mag (in the Kron-Cousins system) at the mean epoch of May 10.85, close to the peak of the light curve (open circle). The decline in brightness seen after May 10 is consistent with a power-law t^-1.
• Optical lightcurve (from H. Pedersen, astro-ph/9710322):
  R-band lightcurve of the OT of GRB 970508: Following a period of constancy, a peak of optical emission was reached about 40 hrs after the GRB. During the subsequent 35 day interval the emission decayed approx. as a power law. The possible contribution of a constant source with m_R = 25.5 is shown by the curved line.
• Radio observations of GRB 970508 (Frail etal, Nature 389 (1997)261) shows quenching of diffractive scintillation about 4 weeks after GRB. The image below is taken from Waxman etal (astro-ph/9709199) and shows the radio lightcurve together with a model curve:
• Optical lightcurve including the HST observation (Pian etal: astro-ph/9710334 = ApJ 492, 1998, L103):
• HST STIS image of GRB 970508 (Pian etal: astro-ph/9710334 = ApJ 492, 1998, L103). The size is 22.5 arcsec * 16.25 arcsec, and the limiting magnitude V = 27.4 (in a 0.5"*0.5" box):
• GCN3 notice #030
  J. S. Bloom, S. R. Kulkarni, S. G. Djorgovski, Caltech and D. A. Frail, NARO report on behalf of the Caltech GRB effort:
  "Optical follow-up of GRB 970508 on the Keck-II 10m was conducted on the nights of Nov 28-29, 1997 and Feb 22-23, 1998. In both cases, imaging observations were obtained with the Low Resolution Imaging Spectrograph (LRIS). Coincident with the position of the optical transient, a faint source is easily detected in both B- and R-bands. Differencing photometry with a number of field objects provided by V. Sokolov (see their paper at http://xxx.lanl.gov/astro-ph/9802341), suggest that the light curve has levelled off from the initial power-law decline seen from 2-100 days from the time of the burst (Sokolov et al. ibid). We find: B (UT Nov 29.6, 1997) = 26.32 +/- 0.26 mag (integ., 2400s), B (UT Feb 23.4, 1998) = 26.27 +/- 0.14 mag (integ., 2400s), R (UT Nov 28.6-29.5, 1997) = 25.09 +/- 0.14 mag (integ., 5400s), and R (UT Feb 22.4, 1998) = 25.29 +/ 0.16 mag. (integ., 3600 s). The systematic uncertainty of the Sokolov photometric zero-point (0.05 mag) is not included in the error analysis. Had the power-law decline continued the predicted brightness at the time of observations would be [B,R] 26.65,25.63 (Nov) and 27.11,26.07 (Feb). The 1-sigma errors for the power-law light curve are estimated at 0.25 and 0.2 for B and R, respectively, at each epoch. At the second epoch, then, the source is significantly brighter then predicted by ~0.8 mags in each band. Thus, although the object does not appear extended (consistent with the HST findings of Fruchter et al. IAUC 6674) beyond that which is expected from the seeing (~0.8"), we interpret the level-off as due to the faint host galaxy--itself responsible for the persistent [OII] emission. In fact, the lack of extent of the galaxy may be connected with its previously inferred underluminous nature (Fruchter et al., and Natarajan et al. 1997). Furthermore, the B-R color (+1) is consistent with an irregular galaxy at z=0.8 (Fukugita et al. 1995). We thank V. Sokolov for collegial help in providing photometry of secondary stars fainter than those reported in their Huntsville poster.
• GCN3 notice #31
  S. V. Zharikov, V. V. Sokolov, SAO RAS and Yu. V. Baryshev, St.Petersburg University report on behalf of the SAO RAS optical follow-up team:
  "Optical observations of the GRB 970508 optical remnant were continued with the 6-m telescope of SAO RAS in the standard B, V, Rc, Ic bands in October-December 1997 and in January 1998. We found:

  Rc (UT Oct.09.94, 1997) = 24.30+/-0.20  (integ., 2100sec.)
  Rc (UT Nov.10.04, 1997) = 24.70+/-0.15  (integ., 2400sec.)
  V  (UT Nov.10.04, 1997) = 25.10+/-0.17  (integ., 2700sec.)
  Rc (UT Nov.25.97, 1997) = 24.70+/-0.14  (integ., 2500sec.)
  Ic (UT Nov.25.99, 1997) = 23.90+/-0.14  (integ., 4800sec.)
  V  (UT Nov.26.06, 1997) = 25.50+/-0.35  (integ., 2100sec.)
  B  (UT Dec.01.91, 1997) = 25.75+/-0.30  (integ., 2400sec.)
  Rc (UT Jan.24.87, 1998) = 24.96+/-0.17  (integ., 2580sec.)
  V  (UT Jan.24.87, 1998) = 25.44+/-0.25  (integ., 2400sec.)
  

  B band images have been obtained by A. I. Kopylov. In determination of parameters of the faint host galaxy we used the results of our BVRcIc photometry of May-August, 1997 (see at http://xxx.lanl.gov/astro-ph/9802341), the data of recent observations with Keck-II (Bloom et al., 1998, GCN Circular #30) and WHT telescopes (Castro-Tirado et al., IAU Circular No. 6848), and also the data of Rc and B bands photometry obtained by other authors based on our secondary standards: Pedersen H. et al., 1997, astro-ph/9710322; Garsia M.R., et al., 1997, astro-ph/9710346; Galama T.J. et al., 1998, astro-ph/9802160. The level-off from the initial power-law decline seen in the first months after the burst is observed in all bands. The effect is the most strong in the Ic band: the difference at the last date of Ic observations achieves ~1.3 mags. The best X^2-fits of the data in each of 4 bands point to the presence of a faint constant source with Ic = 24.13 +/- 0.28, Rc = 25.55 +/- 0.19, V = 25.80 +/- 0.14, B = 26.68 +/- 0.14. The fitted magnitudes of the constant component and small angular size of underlying host galaxy (Pian et al. 1998, ApJ 492, L103) correspond to a dwarf elliptical galaxy with an absolute magnitude of M_B = -16.76 +/- 0.14 mags for z = 0.835 (Metzger et al. 1997, Nature 387, 879). Corresponding figures for all 4 photemetrical bands can be seen in http://www.sao.ru/~zhar/home/GRB/970508.html.
• GCN3 notice #151
  A. Fruchter (STScI), E. Pian (ITESRE) and an international collaboration report:
  The field of GRB970508 was imaged by HST with the STIS CCD in open filter mode (50CCD) on 1998 August 5.78-6.03 for a total exposure time of 11,568 seconds. An extended object, which we believe to be the host galaxy of GRB970508, was detected at the astrometric position of the optical transient of GRB970508. The galaxy has high signal-to-noise in our data and is clearly resolved, with a major axis of approximately 0."5 . Including an uncertain contribution from the optical transient, the galaxy has a magnitude of V= 25.25 +/- 0.20, where the primary source of uncertainty is due to the wide bandpass (and thus poor color resolution) of the detector. This agrees reasonably well with previous photometric estimates (Bloom et al. 1998, Zharikov et al. 1998). The image of the galaxy is well described by an exponential disk with a scale length of approximately 0."07, and an ellipticity of 0.3. However, the FWHM of the PSF is comparable in size to the observed scale length; thus an accurate measurement of the surface brightness profile of the galaxy will require modelling and/or deconvolution. We have obtained an astrometric solution with an r.m.s. uncertainty of approximatgely 0."01 which maps the previous STIS image (Fruchter et al. 1997, Pian et al 1998) onto the new data. The position of the OT in the June 1997 STIS image, and the center of the galaxy on the August 1998 image, agree to within the astrometric error.
  Sections of the images are available on the web from http://www.stsci.edu/~fruchter/GRB/970508.html (and reproduced below).
  

  Bloom, J.S., Djorgovski, S.G., Kulkarni, S.R. and Frail, D.A., 1998,
                Ap. J. (Lett.), submitted, astro-ph/9807315.
  Fruchter, A., Bergeron, E., and Pian, E., 1997, IAUC 6674.  
  Pian, E., et al., 1998, Ap. J. (Lett.), 492, L103.  
  Zharikov, S.V., Sokolov, V.V.  and Baryshev, Yu.V., 1998, GCN 31.
  

  During June 1997 (left) the host was overwhelmed by the light of the GRB, while the image from August 1998 (right) now reveals the host galaxy. The width of the images on the sky is about 12.5 arcseconds. North is up; east to the left. The major-axis of the galaxy is about 0.5 arcseconds in length.
• 1512.02865 from 10 Dec 15
  Mariusz Tarnopolski: Testing the anisotropy in the angular distribution of $Fermi$/GBM gamma-ray bursts
  Phenomenology (hep-ph); Space Physics (physics.space-ph); Applications (stat.AP) Gamma-ray bursts (GRBs) were confirmed to be of extragalactic origin due to their isotropic angular distribution, combined with the fact that they exhibited an intensity distribution that deviated strongly from the -3/2 power law. This finding was later confirmed with the first redshift, equal to at least z=0.835, measured for GRB970508. Despite this result, the data from CGRO/BATSE and Swift/BAT indicate that long GRBs are indeed distributed isotropically, but the distribution of short GRBs is anisotropic. Fermi/GBM has detected 1669 GRBs up to date, and their sky distribution is examined in this paper. A number of statistical tests is applied: nearest neighbour analysis, fractal dimension, dipole and quadrupole moments of the distribution function decomposed into spherical harmonics, binomial test, and the two point angular correlation function. Monte Carlo benchmark testing of each test is performed in order to evaluate its reliability. It is found that short GRBs are distributed anisotropically on the sky, and long ones have an isotropic distribution. The probability that these results are not a chance occurence is equal to at least 99.98\% and 30.68\% for short and long GRBs, respectively. The cosmological context of this finding and its relation to large-scale structures is briefly discussed.