Gamma-ray Burst 980425

The Gamma-Ray Burst was detected by BeppoSAX GRBM on 1998 April 25, 21:49:11 UT. With the BeppoSAX WFC the location could be deduced with an 8 arcmin error circle centered at R.A. = 19h34m54s, Decl. = -52o49'.9 degrees (2000.0).
A follow-up observation with the BeppoSAX NFI instruments was initiated within 12 hrs.
A ROSAT TOO observation is not possible due to Sun angle constraints.
(All information courtesy of the instrument teams.)

Previous IAU Circulars

• IAUC 6884 BeppoSAX WFC detection and localisation
• IAUC 6895 variable object in spiral galaxy ESO 184-G82
• IAUC 6896 variable object has z=0.0085; strong and variable radio source
• IAUC 6898 correction of optical position
• IAUC 6899 transient still brightened: May 8.3 UT, V = 13.87; R = 13.84; I = 13.98
• IAUC 6901 transient is SN-Ib before maximum
• IAUC 6903 SN interpretation implies relativistic expansion (see GCN#70)

• BeppoSAX GRB MAIL N. 98/11
  The BeppoSAX Wide Field Camera error box of GRB 980425 (IAUC 6884) was observed with the BeppoSAX Narrow Field Instruments (NFI) starting about 10.0 hrs after the burst. Preliminary analysis of the first 27 hr shows a previously unknown x-ray source in the MECS and LECS at:

   R.A. =  19h 35m 04s
  Decl. = -52d 48' 34'' 
  

  (equinox 2000.0) with an error radius of 1'. This position is 2'.0 from the centroid of the WFC position. The source did not significantly fade during the observation. The average flux (2-10 keV)was a few x 10E-13 c.g.s. Further BeppoSAX observations are already planned to monitor the behavior of the source and to establish its association with GRB980425.
• BeppoSAX GRB MAIL N. 98/12
  The first TOO pointing of GB980425 with the NFI was completed with a total duration of about 2 days. The unknown x-ray source quoted in the previous mail (98/11), reporting preliminary results of the first 27 hr, remains at about the same flux as before. A more detailed analysis shows another unknown X-ray source at

   R.A.  =  19h 35m 21s
   Decl. = -52d 52' 19"
  

  (equinox 2000.0) with an error radius of 1'. This source is visible only during the first day of observation at a flux of about 10E-13 c.g.s. (2-10 keV). The upper limit in the second part of the observation implies a decay by about a factor of two or more.
• GCN notice #60
  T.J. Galama, P.M. Vreeswijk, P.J. Groot, B. Stappers (University of Amsterdam); E. Pian, F. Frontera, E. Palazzi, N. Masetti (CNR, Bologna); L. Nicastro (IFACI-CNR, Palermo); M. Feroci (CNR, Roma); R.G. Strom (NFRA and U of Amsterdam); C. Kouveliotou (USRA/MSFC); J. van Paradijs (U of Amsterdam and U of Alabama in Huntsville) report: "Comparison of Digitized Sky Survey (DSS) and ESO NTT images obtained on April 28.37 UT (1998) shows a point source in the BeppoSAX WFC error box of GRB 980425 (IAUC 6884) which is not visible in the DSS. The object is also detected in red and blue band images taken at the 50in telecope at the Australian National University's Mt. Stromlo Observatory on April 26.63 UT (1800s), April 26.81 UT (1800s) and Apr 28.68 UT (1800s). It is located at RA 19:35:03.17, DEC -52:50:46.1 (J2000), offset from the nucleus of the barred spiral galaxy ESO 184-82 (in the DN 1931-529 group of galaxies; Duus & Newall, ApJS 35, 209, 1977) and coincident with the spiral arms. The object varies by less than 0.2 magnitudes between April 26.63 and 28.83 and has a blue magnitude of 16.2 +/- 0.5. It is therefore not clear whether the source is related to GRB 980425, or whether it is, e.g., a supernova. Images of the object and the SB galaxy can be found at http://www.astro.uva.nl/titus (and reproduced below). Comparison of the Mt. Stromlo April 26.63 UT and Apr 28.68 UT red band images at the location of the X-ray source (BeppoSAX GRB MAIL N. 98/11) shows no variation > 0.2 mag down to about 21 mag."

  
  

• GCN notice #61
  E. Pian, Istituto TESRE, CNR, Bologna, Italy, L. A. Antonelli, M. R. Daniele, S. Rebecchi, SAX-SDC, Rome, Italy, V. Torroni, SAX-SOC, Rome, Italy, G. Gennaro, SAX-OCC, Rome, Italy, M. Feroci, L. Piro, IAS, CNR, Rome, Italy, report: "The BeppoSAX Wide Field Camera error box of GRB980425 (IAUC 6884) was observed with the BeppoSAX Narrow Field Instruments (NFI) starting about 10.0 hr after the burst in the period April 26.31-28.16 UT. Preliminary analysis of the data shows two previously unknown X-ray sources in the LECS and MECS: 1SAXJ1935.0-5248 and 1SAXJ1935.3-5252, located at RA = 19h35m04s, Dec = -52o48'33'' and at RA = 19h35m21s, Dec = -52o52'19'' (equinox 2000.0), respectively, with an error radius of 1' each. These positions are 2' and 4'.7 away from the centroid of the WFC error box, respectively. During the first 27.7 hours of observation, the average count rates in the two MECS units (1.6-10 keV) were (4.5 +- 0.7)x10E-3 cts/s, corresponding to (3.0 +- 0.4)x10E-13 erg sE-1 cmE-2 in the 2-10 keV range, for 1SAXJ1935.0-5248 and (2.4 +- 0.5)x10E-3 cts/s for 1SAXJ1935.3-5252, equivalent to (1.6 +- 0.3)x10E-13 erg sE-1 cmE-2 (2-10 keV). The measurements during the subsequent 16.6 hours showed that 1SAXJ1935.0-5248 did not vary significantly from the first epoch. 1SAXJ1935.3-5252 was not detected at the second epoch, with a 3 sigma upper limit of 1.8x10E-3 cts/s. This indicates a decay by a factor of 1.3 or more in 22 hours. A further BeppoSAX observation is already planned to monitor the behavior of both sources."
• GCN notice #62
  T.J. Galama, P.M. Vreeswijk, P.J. Groot (University of Amsterdam); E. Pian, F. Frontera, E. Palazzi, N. Masetti (CNR, Bologna); L. Nicastro (IFACI-CNR, Palermo); M. Feroci (CNR, Roma); R.G. Strom (NFRA and U of Amsterdam); C. Kouveliotou (USRA/MSFC); J. van Paradijs (U of Amsterdam and U of Alabama in Huntsville) report: "Comparison of red band images of the error box of GRB 980425 (Soffitta et al. 1998; IAUC 6884) taken at the 50in telescope at the Australian National University's Mt. Stromlo Observatory on April 26.63 UT (1800s) and April 28.68 UT (1800s) shows no variation > 0.2 mag down to about 21 mag at the location of the transient BeppoSAX NFI X-ray source 1SAXJ1935.3-5252 (Pian et al. 1998; GCN #61)."
• GCN notice #63
  M. Wieringa (ATNF), D.A. Frail (NRAO), S.R. Kulkarni (Caltech), J.L. Higdon (ATNF), R. Wark (ATNF), and the BeppoSAX GRB Team report: Radio observations were made with the Australian Telescope Compact Array beginning on April 28.73 and April 28.98 UT at 6cm and 3cm, centered on the position of 1SAXJ1935.0-5248, an X-ray source detected by the NFI on BeppoSAX (Pian et al. 1998; GCN #61). In addition, radio observations of the transient BeppoSAX NFI source 1SAXJ1935.3-5252 were made (also at 6cm and 3cm) on April 29.79 and April 30.00 UT No radio sources were detected in the 1' error radius of the two NFI sources. Typical three-sigma limits were 0.26 mJy and 0.3 mJy at 3cm and 6cm, respectively. Three radio sources are detected in the 8' error radius of the BeppoSAX WFC. There is one object just outside the error circle of 1SAXJ1935.0-5248 at RA 19:34:56.49, Dec -52:49:04.9 (J2000) with a flux density of 2.4 mJy at 6cm and 1.1 mJy at 3cm. The brightest radio source in the field is at RA 19:35:03.31, Dec -52:50:44.7 (posn. error +/-0.1"). It coincides within the likely optical astrometric errors to the candidate supernova proposed by Galama et al. (1998, GCN# 60) in the barred spiral Galaxy ESO 184-82. Approximate flux densities (corrected for primary beam attenuation) on April 28 were 9.3 mJy and 17 mJy at 6cm and 3cm, respectively.
• GCN notice #64
  J.S. Bloom, S.R. Kulkarni, S.G. Djorgovski (Caltech), P. McCarthy (OCIW) and D.Frail (NRAO) report on behalf of the Caltech-OCIW GRB effort: "Kaspar von Braun (U. Mich) imaged the field of the X-ray source reported in IAUC 6884 (Soffitta et al.) on the 40-inch at Las Campanas Observatory in Chile. Two sets of images were obtained in the R band at April 27.4 UT and April 28.4 UT. We scrutinized the images in the vicinity of the two BeppoSAX NFI sources reported by L. Piro and Pian et al. (GCN #61). In the 1-arcmin error circle of the fading X-ray source SAXJ1935.3-5252 we found no source with variability greater than 0.2 mag for R < 21 mag. We thank Kaspar von Braun and Mario Mateo for their help on these observations. "
• GCN notice #65
  P.M. Vreeswijk, T.J. Galama, P.J. Groot (University of Amsterdam); F. Frontera, E. Palazzi, N. Masetti (CNR, Bologna); L. Nicastro (IFACI-CNR, Palermo); E. Costa, L. Piro (CNR, Roma); R.G. Strom (NFRA and U of Amsterdam); C. Kouveliotou (USRA/MSFC); J. van Paradijs (U of Amsterdam and U of Alabama in Huntsville); J.F. Gonzalez, V. Doublier (ESO, Chile) report: "Comparison of R band images of the error box of GRB 980425 (Soffitta et al. 1998; IAUC 6884) taken at the ESO NTT telescope on April 28.37 UT (900s) and May 1.33 UT (900s) shows no variation > 0.3 mag down to 22.8 mag at the location of the transient BeppoSAX NFI X-ray source 1SAXJ1935.3-5252 (Pian et al. 1998; GCN #61)."
• The optical spectrum (see below) of the variable object in a spiral arm of the small galaxy ESO 184-G82 shows some broad absorption lines, which indicates that this is an explosive event. The very intense radio emission (IAUC 6896) can be interpreted as evidence for interaction with with dense circumstellar matter.
  
• GCN notice #67
  R. M. Kippen (University of Alabama in Huntsville) reports on behalf of the BATSE GRB team: GRB 980425 (IAUC 6884) was detected by BATSE on Apr. 25.90913 UT as trigger number 6707. The event consisted of a single pulse lasting about 40 s with little resolvable structure. Its peak flux (50-300 keV; integrated over 1 s) and fluence (> 20 keV) are 0.96 (-/+ 0.05) photons cmE-2 sE-1 and 4.0 (-/+ 0.6) x 10E-6 erg cmE-2, respectively, ranking it near the middle of the BATSE burst flux/fluence distribution. The BATSE location is consistent with that of the BeppoSAX-WFC and the reported SAX-NFI x-ray counterpart (GCN 61). Please note that this report has been delayed due to an interruption of BATSE data delivery from GSFC. Bursts are normally fully processed within a few days. The BATSE lightcurve and location map can be found at http://www.batse.msfc.nasa.gov/~kippen/batserbr/
• optical lightcurve (from http://astro.caltech.edu/~jsb/ltcurve_980425.ps; IAUC 6899)
  
• GCN notice #69
  E. Pian, F. Frontera, Istituto TESRE, CNR, Bologna, Italy, L.A. Antonelli, SAX-SDC, Rome, Italy, L. Piro, IAS, CNR, Rome, Italy, also on behalf of the BeppoSAX team report: "A second Target-of-Opportunity observation of the BeppoSAX Wide Field Camera (WFC) error box of GRB980425 (IAUC 6884) was done with the BeppoSAX Narrow Field Instruments (NFI) in May 2.604-3.646 UT. Preliminary analysis of the MECS data shows that the source 1SAXJ1935.0-5248 reported in GCN N. 61 has a count rate of (3.0 +- 0.5)x10E-3 cts/s in the 1.6-10 keV range (1.9x10E-13 erg sE-1 cmE-2 in 2-10 keV), and therefore has decreased only with marginal significance with respect to the TOO observation performed in April 26.31-28.16 UT. A 3-sigma upper limit of 1.5x10E-3 cts/s in the 1.6-10 keV range is found for 1SAXJ1935.3-5252. If one assumes for the X-ray emission of this source a power-law temporal decay f ~ (t - t0)^{-alpha}, the measured upper limit is consistent with the alpha >0.4 fading observed between the first and second part of the first TOO NFI pointing (GCN N. 61), as well as with the alpha ~1.4 decrease between the 2-10 keV flux preliminarily estimated from the WFC detection (~2.6x10E-08 erg sE-1 cmE-2, Pian et al. 1998, in preparation) and that observed in the first 27.7 hours of the first NFI pointing (GCN N. 61). This variability behavior is typical of X-ray afterglows of GRBs so far detected.
• GCN notice #70
  S. R. Kulkarni, J. S. Bloom, California Institute of Technology, D. A. Frail, National Radio Astronomy Observatory, R. Ekers, M. Wieringa, R. Wark, J. L. Higdon, Australian Telescope National Facility report: Within the localization of GRB 980425 (IAUC 6884) Galama et al. (IAUC 6895) reported a possible supernova candidate for which Wieringa et al. (IAUC 6896) saw a brightening radio source. The object appears to be an unusual supernova based on its spectrum (IAUC 6895). The continued brightening in the optical (IAUC 6899) suggests that the supernova is young and is compatible with an explosion on or around April 24, 1998, the epoch of GRB 980425. For an assumed expansion speed of 20,000 km/s and a distance of 44 Mpc to the host galaxy of the supernova (from the redshift given in IAUC 6896) we derive a brightness temperature of 3x10^14 K from the observed 39 mJy at 6 cm on May 5 (IAUC 6896). This is in excess of the usual Compton limit of 10^12 K. Despite this, no X-ray emission is seen (GCN #69). Thus we are forced to invoke relativistic expansion speed which results in a larger source size and correspondingly smaller brightness temperature. We suggest that the radio emission arises in a relativistic shock and the optical emission in a standard low velocity shock. The model predicts that the radio source should not exhibit diffractive scintillation. We urge observers to carry out higher frequency radio observations and IR observations as these directly measure the particle spectrum that gives rise to the radio emission. The urgency is that the radio emission may cease once the relativistic shock runs into denser ambient gas. Parenthetically, we note that it is possible that such a fast moving shock could generate an initial burst of gamma-rays."
• GCN notice #71
  M. H. Wieringa, R. M. Wark, J. L. Higdon, Australia Telescope National Facility, D. A. Frail, National Radio Astronomy Observatory, S. R. Kulkarni, J. S. Bloom, California Institute of Technology report: Monitoring of the time-variable radio source (GCN 63, IAUC 6896, GCN 70) coincident with a supernova candidate proposed by Galama et al. (GCN 60, IAUC 6895) has continued with the Australia Telescope Compact Array at 20, 13, 6 and 3 cm. The radio source may have reached a peak on May 7 1998 at 6 and 3 cm of 45 and 49 mJy, respectively. A radio light curve, spectra, and individual maps are available at http://www.narrabri.atnf.csiro.au/public/grb/grb980425.html (lightcurve and spectrum are reproduced below).

  
  

  
  

• GCN notice #155
  L. Piro, IAS/CNR, Rome, Italy, R.C. Butler, ASI, Rome, Italy, F. Fiore, A. Antonelli, BeppoSAX SDC, Roma, E. Pian, ITesre, Bologna, Italy, report:
  The majority of BeppoSAX observations were performed with the star tracker Z (i.e., the one co-aligned with the NFI) in the attitude control loop. However, a few observations could not be performed in this configuration, but had to be carried out with the X and Y star trackers only. We have found out that, in these very infrequent cases, the attitude reconstruction could have an error of up to 3'. This problem does not affect observations accomplished after May 5, 1998. Prior to this date, the only GRB which was observed by the NFI in this configuration is GB980425. The revised coordinates (equinox 2000.0) of the two sources found in the WFC error box (Pian et al., GCN 61) are: 1SAXJ1935.0-5248, RA = 19h35m05.9s, Dec =-52o50'03"; 1SAXJ1935.3-5252, RA = 19h35m22.9s, Dec =-52o53'49", with a conservative error radius of 1.5'. Both updated positions are still within the WFC error box (Soffitta et al., IAU Circ. No. 6884). However, 1SAXJ1935.0-5248 is at 50" from SN1998bw (Galama et al. 1998, Nature, in press, astro-ph/9806175; Kulkarni et al., Nature, in press, astro-ph/9807001) and therefore consistent with it, while the variable source 1SAXJ1935.3-5252 is 3' away, i.e., inconsistent.
• GCN notice #156
  J. P. Halpern, Columbia University, reports:
  I obtained optical spectra of objects in the error circle of the variable X-ray source 1SAXJ1935.3-5252 (Pian et al. GCN 61,69) using the CTIO 1.5m telescope on Oct. 17-21. A total of 24 objects in the original error circle (Pian et al. GCN 61) and the revised error circle (Piro et al. GCN 155) were observed, completing this spectroscopic survey to a limiting magnitude of 18. No compelling candidate for identification was found at this level. Thus, 1SAXJ1935.3-5252 remains a plausible candidate for the afterglow of GRB980425. The brightest galaxy in this field, of approximate magnitude 17.5, has narrow emission lines of H-alpha and [N II] 658 nm at z = 0.142, but no evidence of nuclear activity. Its position is RA = 19h35m27.6s, Dec = -52o53'00" (J2000), which is within both the old and the new error circles. Deeper spectroscopy in this field using larger telescopes would be worthwhile, since the optical counterpart of 1SAXJ1935.3-5252 could easily be fainter than the limit surveyed here. If a definitive identification cannot be obtained, more X-ray observations of 1SAXJ1935.3-5252 are needed to determine if this X-ray source recurs.
• Decay lightcurves from Iwamoto etal (astro-ph/9810400):
  
• GCN notice #158
  E. Pian, ITESRE, CNR, Bologna, Italy, L. A. Antonelli, Osservatorio di Monteporzio, Rome, Italy, L. Piro, and M. Feroci, IAS, CNR, Rome, Italy report:
  The field of GRB980425 (IAUC 6884) was re-observed with the BeppoSAX Narrow Field Instruments on 1998 November 10.754-12.004 UT. Preliminary analysis of the MECS data shows that the source 1SAXJ1935.0-5248 reported in the GCN N. 61 and 69, the position of which (see updated coordinates in GCN N. 155) is consistent with that of the supernova SN1998bw (Galama et al. 1998, Nature 395, 670), has a count rate of (1.8 +- 0.4)x10E-03 cts/s in the 1.6-10 keV range, and therefore has decreased by approximately a factor of two with respect to the average level observed in April-May (see GCN N. 61 and 69). This suggests the presence of variable X-ray emission from the supernova. This preliminary analysis shows also that the source 1SAXJ1935.0-5248 is slightly extended.
  The source 1SAXJ1935.3-5252 (see GCN N. 61, 69, 155) is not detected down to a 3-sigma limit of 1.4E-03 cts/s.
• GCN notice #286
  Bradley E. Schaefer and Eric H. McKenzie (Yale University) report:
  We have obtained 139 photometric observations in B, V, and I of the late time light curve for SN1998bw associated with GRB980425. These demonstrate a strikingly linear decline in magnitude versus time.
  Our data was taken with the Yale 1.0-m telescope on Cerro Tololo between 27 June and 28 October 1998. We used standard IRAF reduction and the comparison stars of Galama et al. (http://www.astro.uva.nl/~titus/grb980425/grb980425chart.html). During the time interval of our observations, the light from the underlying galaxy (ESO 184-G82) was insignificant in our photometry.
  Our first and last data nights had measured photometry as follows:

    JD2450992.9   B=16.68+-0.03, V=15.79+-0.02, I=15.16+-0.03
    JD2451115.6   B=18.35+-0.06, V=18.09+-0.05, I=17.33+-0.06
  

  Between these two nights, all our photometry is perfectly consistent with an exactly linear decline in magnitudes (hence an exponential decline in luminosity). Our measured uniform decline rates are as follows:

    B   0.0141+-0.0002 mag/day for equivalent half-life of 53.4+-0.8 days
    V   0.0184+-0.0003 mag/day for equivalent half-life of 40.9+-0.7 days
    I   0.0181+-0.0003 mag/day for equivalent half-life of 41.6+-0.7 days
  

  The observed exponential decline is in contradiction to the theoretical predictions of Iwamoto et al. (1998, Nature, 395, 672) and Iwamoto (1999, ApJ, astro-ph/9810400), which claims that the decline will be as a power law. The observed exponential decline has a rate similar to that expected from the decay of radioactive cobalt as modified by the effects due to the expansion of the shell. So it is reasonable to conclude that the late-time light curve of SN1998bw is being powered by radioactive cobalt. This then implies that the underlying explosion mechanism must create large masses of radioactive cobalt.
  The light curve of SN1998bw is significantly different from all previously known supernovae. For a comparison with Type Ia events, the decline rate of SN1998bw is the same in B but not V and I as for Type Ia events, while SN1998bw does not show the bump in the I band light curve from 20-50 days after peak. A comparison with Type Ic events is difficult since their lights curves are not well defined, yet the late time decline rate of Type Ic events is substantially smaller than for SN1998bw.
  Our observations of SN1998bw are continuing with the Yale 1.0-m telescope. However, since the source has come out from behind the Sun, the light from the galaxy provides an increasingly significant obstacle to accurate photometry. Perhaps future accurate photometry must await the complete fading of SN1998bw to allow for subtraction of the galaxy light.
• GCN notice #704
  Stephen Holland, Johan Fynbo, Bjarne Thomsen (University of Aarhus), Michael Andersen (University of Oulu), Gunnlaugur Bjornsson (University of Iceland), Jens Hjorth (University of Copenhagen), Andreas Jaunsen (University of Oslo), Priya Natarajan (University of Cambridge, & Yale), and Nial Tanvir (University of Hertfordshire)
  We have obtained 1240 seconds of STIS images with the 50CCD (clear) aperture and 1185 seconds with the F28X50LP (long pass) aperture of the host galaxy of GRB 980425. This data was taken as part of the Survey of the Host Galaxies of Gamma-Ray Bursts (Holland et al. GCN 698) approximately 778 days after the burst. Combined images are now available at "http://www.ifa.au.dk/~hst/grb_hosts/data/index.html" and a GIF image of the host galaxy is available at "http://www.ifa.au.dk/~hst/grb_hosts/data/grb980425_colour.gif".
  An astrometric solution from VLT V-, R-, and I-band images (P.I., B. Libundgut) suggests that the supernova is located at X = 987 +/- 2, Y = 1064 +/- 2 on our drizzled long pass image. This is in an extended object with AB magnitudes of 26.2 +/- 0.1 in the STIS clear filter and 26.3 +/- 0.1 in the STIS long pass filter. An extrapolation of the V-band late-time light curve of SN 1998bw (McKenzie & Schaefer 1999, PASP, 111, 964) suggests that the SN remnant will have a magnitude of ~28.4 in the clear filter, which is near the detection limit of our data. The extended object has a full-width at half-maximum, in the long pass image, of 0.13 arcseconds. When the resolution limit (0.088 arcsec) is taken into account the object has a diameter of, at most, ~0.09 arcsec. For z = 0.0085, and a cosmology with H0 = 65, Omega_matter = 0.2 and Omega_lambda = 0, this corresponds to a diameter of less than ~17 pc. The colour and size of the object is consistent with it being a young star cluster. The extended object is embedded in a large extended feature (possibly an HI region) with a diameter of ~75 pc and an estimated colour of V-R ~= 0.7 +/- 0.3. A GIF image of the region containing the supernova is available at "http://www.ifa.au.dk/~hst/grb_hosts/data/grb980425cd_sfr.gif". A detailed analysis of these images is in progress (Holland et al. 2000, in preparation).
• ESA press release
  
• from Fynbo et al.: ApJ 542, L89:
  
• astro-ph/0605431 from 17 May 2006
  Ghisellini: Are GRB 980425 and GRB 031203 real outliers or twins of GRB 060218?
  GRB 980425 and GRB 031203 are apparently two outliers with respect to the correlation between the isotropic equivalent energy E_iso emitted in the prompt radiation phase and the peak frequency E_peak of the spectrum in a vF(v) representation (the so-called Amati relation). We discuss if these two bursts are really different from the others or if their location in the E_iso-E_peak plane is the result of other effects, such as viewing them off-axis, or through a scattering screen, or a misinterpretation of their spectral properties. The latter case seems particularly interesting after GRB 060218, that, unlike GRB 031203 and GRB 980425, had a prompt emission detected both in hard and soft X-rays which lasted ~2800 seconds. This allowed to determine its E_peak and total emitted energy. Although it shares with GRB 031203 the total energetics, it is not an outlier with respect to the Amati correlation. We then investigate if a hard-to-soft spectral evolution in GRB 031203 and GRB 980425, consistent with all the observed properties, can give rise to a time integrated spectrum with an E_peak consistent with the Amati relation.
• 0707.0931 from 6 Jul 2007
  Daigne: The low-luminosity tail of the GRB distribution: the case of GRB 980425
  Abstract: (abridged) The association of GRB 980425 with SN 1998bw at z=0.0085 implies the existence of a population of GRBs with an isotropic-equivalent luminosity which is about 10^4 times smaller than in the standard cosmological case. We investigate two scenarios to explain a weak GRB : a normal (intrinsically bright) GRB seen off-axis or an intrinsically weak GRB seen on-axis. For each of these two scenarios, we first derive the conditions to produce a GRB 980425-like event and we then discuss the consequences for the event rate. If we exclude the possibility that GRB 980425 is an occurence of an extremely rare event observed by chance during the first eight years of the 'afterglow era', the first scenario implies that (i) the local rate of standard bright GRBs is much higher than what is usually expected; (ii) the typical opening angle is much narrower than what is derived from observations of a break in the afterglow lightcurve. In addition, we show that the afterglow of GRB 980425 in this scenario should have been very bright and easily detected. For these reasons the second scenario appears more realistic. We show that the parameter space of the internal shock model indeed allows GRB 980425-like events, in cases where the outflow is only mildly-relativistic and mildly-energetic. The rate of such weak events in the Universe has to be much higher than the rate of standard bright GRBs to allow the discovery of GRB 980425 during a short period of a few years. However it is still compatible with the observations as the intrinsic weakness of these GRB 980425-like bursts does not allow detection at cosmological redshift. We finally briefly discuss the consequences of such a high local rate of GRB 980425-like events for the future prospects of detecting non-electromagnetic radiation, especially gravitational waves.
• 0807.3554from 22 Jul 2008
  Christensen: IFU observations of the GRB 980425/SN 1998bw host galaxy: emission line ratios in GRB regions
  Abstract: The collapsar model predicts that the progenitors of Gamma-ray Bursts (GRBs) are metal poor in Fe group elements. The existence of low metallicity stellar populations could manifest itself through special characteristics of the immediate environment of the GRB site in the host galaxy. We analyse the strong emission lines from the sub-luminous host galaxy of GRB 980425, which showed the first connection with a supernova explosion (SN 1998bw). The host has a sufficient size to allow detailed resolved spectroscopy of individual HII regions and to look for regions with special properties close to the the GRB site. Using integral field spectroscopy with VIMOS we cover most of the high surface brightness part of the host including the HII region where the supernova and GRB occurred. The star formation rate, reddening, equivalent width and stellar mass in the GRB region is similar to other HII regions in the host. Extreme values arise in the only region that shows emission lines from Wolf-Rayet stars; a region that lies 800 pc in projection from the GRB site. Strong emission line diagnostics of all HII regions imply oxygen abundances between 0.3 and 0.8 solar with the lowest values arising in the WR and GRB regions. Comparing the measured emission line ratios for a sample of low redshift GRB hosts to theoretical models and to observations of field galaxies, we find that GRBs arise in a range of metallicity environments while the regions are consistently very young. The similar line ratios of GRB hosts compared to the WR region can arise in spatially unresolved galaxies with bright HII regions near the GRB location. (Abridged)
• 0809.0508from 2 Sep 2008
  Michalowski: The properties of the host galaxy and the immediate environment of GRB 980425 / SN 1998bw from the multi-wavelength spectral energy distribution
  Abstract: We present an analysis of the spectral energy distribution (SED) of the galaxy ESO 184-G82, the host of the closest known long gamma-ray burst (GRB) 980425 and its associated supernova SN 1998bw. We use our observations obtained at the Australia Telescope Compact Array (the third >3 sigma radio detection of a GRB host) as well as archival infrared (IR) and ultraviolet (UV) observations to estimate its star formation state. We find that ESO 184-G82 has a UV star formation rate (SFR) and stellar mass consistent with the population of cosmological GRB hosts and of local dwarf galaxies. It has however a higher specific SFR (per unit stellar mass) and lower molecular gas-to-dust ratio than luminous spiral galaxies. The mass of ESO 184-G82 is dominated by an older stellar population in contrast to the majority of GRB hosts. The Wolf-Rayet region ~800 pc from the supernova site experienced a starburst episode during which the majority of its stellar population was built up. Unlike that of the entire galaxy, its SED is similar to those of cosmological submillimeter/radio-bright GRB hosts with hot dust content. These findings add to the picture that in general, the environments of GRBs on 1-3 kpc scales are associated with high specific SFR and hot dust.
• 1101.4208 from 24 Jan 11
  A. Corsi et al.: PTF 10bzf (SN 2010ah): a broad-line Ic supernova discovered by the Palomar Transient Factory
  Kasliwal, Y. Green, D. Murray, D. Xu, S. Ben-ami, J. S. Bloom, B. Cenko, N. M. Law, P. Nugent, R. M. Quimby, V. Pal'shin, J. Cummings, V. Connaughton, K. Yamaoka, A. Rau, W. Boynton, I. Mitrofanov, J. Goldsten We present the discovery and follow-up observations of a broad-line type-Ic supernova (SN), PTF 10bzf (SN 2010ah), detected by the Palomar Transient Factory (PTF) on 2010 February 23. The SN distance is \cong 218 Mpc, greater than GRB 980425 / SN 1998bw and GRB 060218 / SN 2006aj, but smaller than the other SNe firmly associated with gamma-ray bursts (GRBs). We conducted a multi-wavelength follow-up campaign with Palomar-48 inch, Gemini-N, Keck, Wise, Swift, the Allen Telescope Array, CARMA, WSRT, and EVLA. Here we compare the properties of PTF 10bzf with those of SN 1998bw and other broad-line SNe. The optical luminosity and spectral properties of PTF 10bzf suggest that this SN is intermediate, in kinetic energy and amount of 56Ni, between non GRB-associated SNe like 2002ap or 1997ef, and GRB-associated SNe like 1998bw. No X-ray or radio counterpart to PTF 10bzf was detected. X-ray upper-limits allow us to exclude the presence of an underlying X-ray afterglow as luminous as that of GRB 980425. Early-time radio upper-limits do not show evidence for mildly-relativistic ejecta. Late-time radio upper-limits rule out the presence of an underlying off-axis GRB, with energy and wind density similar to the SN-associated GRB 030329 and GRB 031203. Finally, by performing a search for a GRB in the time window and at the position of PTF 10bzf, we find that no GRB in the IPN catalog could be associated with this SN.
• 1106.1695 from 10 Jun 11
  Alejandro Clocchiatti et al.: The Ultimate Light Curve of SN 1998bw/GRB 980425
  We present multicolor light curves of SN 1998bw which appeared in ESO184-G82 in close temporal and spacial association with GRB 980425. They are based on observations done at Cerro Tololo Inter-American Observatory and data from the literature. The CTIO photometry reaches ~86 days after the GRB in $U$ and ~160 days after the GRB in BV(RI)_C. The observations in U extend by about 30 days the previously known coverage, and determine the slope of the early exponential tail. We calibrate a large set of local standards in common with those of previous studies and use them to transform published observations of the SN to our realization of the standard photometric system. We show that the photometry from different sources merges smoothly and provide a unified set of 300 observations of the SN in five bands. Using the extensive set of spectra in public domain we compute extinction and K corrections, and build quasi-bolometric unreddened rest frame light curves. We provide low degree piecewise spline fits to these light curves with daily sampling. They reach ~86 rest frame days after the GRB with U band coverage, and ~498 rest frame days after the GRB without U.
• 1111.0970 from 7 Nov 11
  Davide Lazzati et al.: Unifying the Zoo of Jet-Driven Stellar Explosions
  Astrophysics (astro-ph.SR) We present a set of numerical simulations of stellar explosions induced by relativistic jets emanating from a central engine sitting at the center of compact, dying stars. We explore a wide range of durations of the central engine activity, two candidate stellar progenitors, and two possible values of the total energy release. We find that even if the jets are narrowly collimated, their interaction with the star unbinds the stellar material, producing a stellar explosion. We also find that the outcome of the explosion can be very different depending on the duration of the engine activity. Only the longest-lasting engines result in successful gamma-ray bursts. Engines that power jets only for a short time result in relativistic supernova explosions, akin to observed engine-driven SNe such as SN2009bb. Engines with intermediate durations produce weak gamma-ray bursts, with properties similar to nearby bursts such as GRB 980425. Finally, we find that the engines with the shortest durations, if they exist in nature, produce stellar explosions that lack sizable amounts of relativistic ejecta and are therefore dynamically indistinguishable from ordinary core-collapse supernovae.
• 1111.1234 from 8 Nov 11
  E.Le Floc'h et al.: The first Infrared study of the close environment of a long Gamma-Ray Burst
  We present a characterization of the close environment of GRB980425 based on 5-160mic spectro-imaging obtained with Spitzer. The Gamma-Ray Burst GRB980425 occurred in a nearby (z=0.0085) SBc-type dwarf galaxy, at a projected distance of 900pc from an HII region with strong signatures of Wolf-Rayet (WR) stars. While this "WR region" produces less than 5% of the B-band emission of the host, we find that it is responsible for 45+/-10% of the total infrared luminosity, with a maximum contribution reaching 75% at 25-30mic. This atypical property is rarely observed among morphologically-relaxed dwarves, suggesting a strong causal link with the GRB event. The luminosity of the WR region (L_8-1000mic=4.6x10^8 Lsol), the peak of its spectral energy distribution at <~100mic and the presence of highly-ionized emission lines (e.g., [NeIII]) also reveal extremely young (<5Myr) star-forming activity, with a typical time-scale of only 47Myr to double the stellar mass already built. Finally, the mid-IR over B-band luminosity ratio in this region is substantially higher than in star-forming galaxies with similar L_IR, but it is lower than in young dust-enshrouded stellar clusters. Considering the modest obscuration measured from the silicate features (tau_9.7mic ~ 0.015), this suggests that the WR region is dominated by one or several star clusters that have either partly escaped or cleared out their parent molecular cloud. Combined with the properties characterizing the whole population of GRB hosts, our results reinforce the idea that long GRBs mostly happen within or in the vicinity of relatively unobscured galactic regions harboring very recent star formation.
• 1112.3462 from 16 Dec 11
  Rodrigo Negreiros et al.: Cooling of young neutron stars in GRB associated to Supernova
  Recent observations of the late ($t=10^8$--$10^9$ s) emission of Supernovae associated to GRBs (GRB-SN) show a distinctive emission in the X-ray regime consistent with temperatures $10^7$--$10^8$ K. Similar features have been also observed in the two Type Ic SN 2002ap and SN 1994I, not associated to GRBs. We advance the possibility that the late X-ray emission observed in GRB-SN and in isolated SN is associated to a hot neutron star (NS) just formed in the SN event, here defined as a neo-NS. We discuss the thermal evolution of neo-NS in the age regime that spans from $\sim 1$ minute (just after the proto-NS phase) up to <10-100 yr. We examine the key factor governing the neo-NS cooling emphasizing on the neutrino emission. A phenomenological heating source and new boundary conditions are introduced to mimic the high temperature of the atmosphere for young NSs. We thus match the neo-NS luminosity to the late X-ray emission of the GRB-SN events: URCA-1 in GRB980425-SN1998bw, URCA-2 in GRB030329-SN2003dh, and URCA-3 in GRB031203-SN2003lw. By calibrating our additional heating source at early times to $\sim 10^{12}$--$10^{15}$ erg/g/s, we find a striking agreement of the luminosity obtained from the cooling of neo-NSs with the prolonged ($t=10^{8}$--$10^{9}$ s) X-ray emission observed in GRB-SN. It seems appropriate to revise the boundary conditions usually used in the thermal cooling theory of NSs, in order to match the proper conditions of the atmosphere at young ages. Additional heating processes not yet studied within this context, e.g. e+e- pair creation by overcritical fields and nuclear fusion and fission energy release, might also take place under such conditions and deserve further analysis. The observation of GRB-SN has evidenced the possibility of witnessing the thermal evolution of neo-NSs. A new campaign of dedicated observations of GRB-SN and isolated Type Ic SN is recommended.
• 1205.4239 from 22 May 12
  Michal J. Michal/owski et al.: The optically unbiased GRB host (TOUGH) survey. VI. Radio observations at z<1 and consistency with typical star-forming galaxies
  The objective of this paper is to determine the level of obscured star formation activity and dust attenuation in a sample of gamma-ray burst (GRB) hosts; and to test the hypothesis that GRB hosts have properties consistent with those of the general star-forming galaxy populations. We present a radio continuum survey of all z<1 GRB hosts in The Optically Unbiased GRB Host (TOUGH) sample supplemented with radio data for all (mostly pre-Swift) GRB-SN hosts discovered before October 2006. We present new radio data for 22 objects and have obtained a detection for three of them (GRB 980425, 021211, 031203; none in the TOUGH sample), increasing the number of radio-detected GRB hosts from two to five. The star formation rate (SFR) for the GRB 021211 host of ~825 Mo yr^-1, the highest ever reported for a GRB host, places it in the category of ultraluminous infrared galaxies. We found that at least 63% of GRB hosts have SFR < 100 Mo yr^-1 and at most 8% can have SFR > 500 Mo yr^-1. For the undetected hosts the mean radio flux (<35 uJy 3sigma) corresponds to an average SFR < 15 Mo yr^-1. Moreover, ~92% of the z<1 GRB hosts have ultraviolet dust attenuation A_UV < 6.7 mag (visual attenuation A_V < 3 mag). Hence we did not find evidence for large dust obscuration in a majority of GRB hosts. Finally, we found that the distributions of SFRs and A_UV of GRB hosts are consistent with those of Lyman break galaxies, Halpha emitters at similar redshifts and of galaxies from cosmological simulations. The similarity of the GRB population with other star-forming galaxies is consistent with the hypothesis that GRBs, a least at z<1, trace a large fraction of all star formation, and are therefore less biased indicators than once thought.
• 1508.03342 from 17 Aug 15
  Maryam Arabsalmani et al.: First measurement of HI 21cm emission from a GRB host galaxy indicates a post-merger system
  We report the detection and mapping of atomic hydrogen in HI 21cm emission from ESO 184-G82, the host galaxy of the gamma ray burst 980425. This is the first instance where HI in emission has been detected from a galaxy hosting a gamma ray burst. ESO 184-G82 is an isolated galaxy and contains a Wolf-Rayet region close to the location of the gamma ray burst and the associated supernova, SN 1998bw. This is one of the most luminous HII regions identified in the local Universe, with a very high inferred density of star formation. The HI 21cm observations reveal a high HI mass for the galaxy, twice as large as the stellar mass. The spatial and velocity distribution of the HI 21cm emission reveals a disturbed rotating gas disk, which suggests that the galaxy has undergone a recent minor merger that disrupted its rotation. We find that the Wolf-Rayet region and the gamma ray burst are both located in the highest HI column density region of the galaxy. We speculate that the merger event has resulted in shock compression of the gas, triggering extreme star formation activity, and resulting in the formation of both the Wolf-Rayet region and the gamma ray burst. The high HI column density environment of the GRB is consistent with the high HI column densities seen in absorption in the host galaxies of high redshift gamma ray bursts.
• 1509.07124 from 25 Sep 15
  Maryam Modjaz et al.: The Spectral SN-GRB Connection: Systematic Spectral Comparisons between Type Ic Supernovae, broad-lined Type Ic Supernovae with and without Gamma-Ray Bursts
  We present the first systematic investigation of spectral properties of 17 Type Ic Supernovae (SNe Ic), 10 broad-lined SNe Ic (SNe Ic-bl) without observed Gamma-Ray Bursts (GRBs) and 10 SNe Ic-bl with GRBs (SN-GRBs) as a function of time in order to probe their explosion conditions and progenitors. We analyze a total of 396 spectra, which were drawn from published spectra of individual SNe as well as from the densely time-sampled spectra data of Modjaz et al. (2014). In order to quantify the diversity of the SN spectra as a function of SN subtype, we construct average spectra of SNe Ic, SNe Ic-bl without GRBs and SNe Ic-bl with GRBs, along with standard deviation and maximum deviation contours. We find that SN~1994I is not a typical SN Ic, in contrast to common belief, while the spectra of SN 1998bw/GRB 980425 are representative of mean spectra of SNe Ic-bl. We measure the ejecta absorption and width velocities (as traced by FeII 5169) and find that SNe Ic-bl with GRBs, on average, have quantifiably higher absorption velocities, as well as broader line widths than SNe without observed GRBs. We interpret this to indicate that SNe Ic-bl without observed GRBs may have had lower energy, chocked jets that imparted lower velocities to the SN ejecta. Moreover, we address the He-problem in SNe Ic-bl, namely whether the puzzling lack of He lines in SN Ic-bl spectra could be due to their He lines being too broadened by the high velocities present, and thus smeared out. We show that the absence of clear He lines in optical spectra of all SNe Ic-bl, and in particular of SN-GRBs, is not due to them being too smeared out. This implies that the progenitor stars of SN-GRBs are probably He-free, in addition to being H-free, which puts strong constraints on the stellar evolutionary paths needed to produce such SN-GRB progenitor stars at the observed low metallicities. (Abridged)