letters to nature 17. Yamazaki, R., Yonetoku, D. & Nakamura, T. An off-axis jet model for GRB 980425 and low-energy viewed off-axis. Like GRB 980425, GRB 031203 appears to be an gamma-ray bursts. Astrophys. J. 594, L79–L82 (2003). g 18. Soderberg, A. M., Frail, D. A. & Wieringa, M. H. Constraints on off-axis gamma-ray burst jets in type intrinsically sub-energetic -ray burst. Such sub-energetic events Ibc supernovae from late-time radio observations. Astrophys. J. 607, L13–L16 (2004). have faint afterglows. We expect intensive follow-up of faint 19. Paciesas, W. S. et al. The fourth BATSE gamma-ray burst catalog (revised). Astrophys. J. Suppl. Ser. 122, bursts with smooth g-ray light curves8,9 (common to both GRB 465–495 (1999). 031203 and 980425) to reveal a large population of such events. 20. Mazets, E. P. & Golenetskii, S. V. Recent results from the gamma-ray burst studies in the KONUS experiment. Astrophys. Space Sci. 75, 47–81 (1981). 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A hard X-ray survey of the Galactic-Center region with the IBIS telescope of the discovered a radio source at right ascension a(J2000) ¼ 08 h INTEGRAL observatory: a catalog of sources. Astron. Lett. 30, 430–435 (2004). 0 00 ^ 25. Mazets, E. P. et al. Cosmic gamma-ray burst spectroscopy. Astrophys. Space Sci. 82, 261–282 (1982). 02 min 30.18 s and declination d(J2000) ¼ 2398 51 03.51 ( 0.1 26. Band, D. et al. BATSE observations of gamma-ray burst spectra. I - Spectral diversity. Astrophys. J. 413, arcsec in each axis), well within the 6-arcsec radius error circle of the 281–292 (1993). XMM source. A subsequent XMM observation12 confirmed the 27. Schaefer, B. E. Gamma-ray burst Hubble diagram to z ¼ 4.5. Astrophys. J. 583, L67–L70 (2003). gradual decay of the X-ray source. From our analysis of the XMM 20.4 Acknowledgements This work is based on a Core Programme pointed observation (PI: S.Yu.S.) data, we find the flux / t between the two epochs and the b with INTEGRAL, an ESA project with instruments and science data centre funded by ESA spectral flux density, F n,X / n , is fitted by b ¼ 20.81 ^ 0.05 with member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), 21 22 an absorbing column density, N H ¼ 6.2 £ 10 cm .Taken Czech Republic and Poland, and with the participation of Russia and the USA. We thank M. Revnivtsev and E. Churazov for help in the data analysis, and S. Kulkarni for suggestions. Competing interests statement The authors declare that they have no competing financial interests. Correspondence and requests for materials should be addressed to S.Yu.S. ([email protected]). .............................................................. The sub-energetic g-ray burst GRB 031203 as a cosmic analogue to the nearby GRB 980425 A. M. Soderberg1, S. R. Kulkarni1, E. Berger1, D. W. Fox1, M. Sako2, D. A. Frail3, A. Gal-Yam1, D. S. Moon4, S. B. Cenko4, S. A. Yost4, M. M. Phillips5, S. E. Persson5, W. L. Freedman5, P. Wyatt5, R. Jayawardhana6 & D. Paulson6 1Caltech Optical Observatories 105-24, California Institute of Technology, Pasadena, California 91125, USA Figure 1 Radio light curves of the afterglow of GRB 031203. All measurements (circles) 2Stanford Linear Accelerator Center, 2575 Sand Hill Road M/S 29, Menlo Park, are summarized in Table 1 and include 1j error bars. Triangles represent 2j upper limits. California 94025, USA The solid lines are models of synchrotron (afterglow) emission from spherical ejecta 3National Radio Astronomy Observatory, PO Box 0, Socorro, New Mexico 87801, expanding into a uniform circumburst medium19. The models include a contribution from USA the host galaxy, which is well fitted by F < 0.4(n/1.4 GHz)20.6 mJy (dashed lines) and 4 host Space Radiation Laboratory 220-47, California Institute of Technology, is consistent with the star-formation rate inferred6 from optical spectroscopy of the host. In Pasadena, California 91125, USA applying the models, the X-ray observations are considered upper limits because they are 5Carnegie Observatories, 813 Santa Barbara Street, Pasadena, California 91101, probably dominated by (non-synchrotron) emission arising from the associated supernova USA 6Department of Astronomy, University of Michigan, 830 Dennison Bldg, SN 2003lw, as evidenced by the unusually slow flux decay at early time and the flat 20.4 20.8 21 21.3 Ann Arbor, Michigan 48109, USA spectral index (F n,X / t n as opposed to / t n for GRBs). This was also 1 20.2 21 ............................................................................................................................................................................. the case for the X-ray emission of GRB 980425/SN 1998bw (F n,X / t n ). For our 1 2 Over the six years since the discovery of the g-ray burst GRB best-fit model, we find xr ¼ 8:9 (38 degrees of freedom), dominated by interstellar 2 980425, which was associated with the nearby (distance scintillation. The blastwave transitions to the non-relativistic regime at t NR < 23 d. From 8 ,40 Mpc) supernova 1998bw, astronomers have debated fiercely the derived synchrotron parameters (at t ¼ 1 d): n a < 3.2 £ 10 Hz, 12 the nature of this event. Relative to bursts located at cosmological n m < 3.6 £ 10 Hz and F na < 0.04 mJy we find an isotropic afterglow energy, 49 1=4 3=4 23 distance (redshift z < 1), GRB 980425 was under-luminous in E AG;iso < 1:7 £ 10 nc;15:5 erg, a circumburst density n < 0:6 nc;15:5 cm and the g-rays by three orders of magnitude. Radio calorimetry3,4 showed fractions of energy in the relativistic electrons (energy distribution N(g) / g 2p with 1=4 25=4 that the explosion was sub-energetic by a factor of 10. Here we p < 2.6) and magnetic field of ee < 0:4 nc;15:5 and eB < 0:2 nc;15:5; respectively. 15 report observations of the radio and X-ray afterglow of the recent Here, n c ¼ 3 £ 10 n c,15.5 is the synchrotron cooling frequency, which is roughly GRB 031203 (refs 5–7), which has a redshift of z 5 0.105. We constrained by the (non-synchrotron) SN 2003lw X-ray emission. Extrapolation of the demonstrate that it too is sub-energetic which, when taken synchrotron model beyond n c underestimates the observed X-ray flux by a factor of &10, together with the low g-ray luminosity7,suggeststhatGRB which is comparable to the discrepancy for SN 1998bw (found by extrapolating the radio 4 23 031203 is the first cosmic analogue to GRB 980425. We find model by Li and Chevalier (p ¼ 2.5, e B ¼ 10 ) beyond n c and comparing with the no evidence that this event was a highly collimated explosion X-ray data1 at t , 12 d). 648 © 2004 Nature Publishing Group NATURE | VOL 430 | 5 AUGUST 2004 | www.nature.com/nature letters to nature Table 1 Radio observations made with the Very Large Array Epoch (UT) Dt (d) F 1.43 (mJy) F 4.86 (mJy) F 8.46 (mJy) F 22.5 (mJy) ................................................................................................................................................................................................................................................................................................................................................................... 2003 Dec. 5.52 1.60 – – 0.540 ^ 0.062 – 2003 Dec. 7.52 3.60 – – 0.249 ^ 0.043 – 2003 Dec. 8.35 4.43 – 0.393 ^ 0.060 0.053 ^ 0.052 – 2003 Dec. 12.38 8.46 – – 0.280 ^ 0.049 – 2003 Dec. 15.37 11.45 – – 0.304 ^ 0.042 – 2003 Dec. 17.38 13.46 – 0.520 ^ 0.050 0.448 ^ 0.039 0.483 ^ 0.083 2003 Dec. 21.35 17.43 – – 0.457 ^ 0.041 – 2003 Dec. 23.37 19.45 – – 0.811 ^ 0.040 – 2003 Dec. 26.40 22.48 – 0.583 ^ 0.054 0.467 ^ 0.046 – 2003 Dec. 31.33 27.41 – – 0.675 ^ 0.045 – 2004 Jan. 4.33 31.41 – 0.728 ^ 0.055 0.459 ^ 0.047 – 2004 Jan. 8.26 35.34 – 0.624 ^ 0.050 0.308 ^ 0.043 – 2004 Jan. 12.29 39.37 1.011 ^ 0.113 0.598 ^ 0.063 0.647 ^ 0.045 – 2004 Jan. 15.35 42.43 0.689 ^ 0.136 0.749 ^ 0.063 0.664 ^ 0.061 – 2004 Jan. 25.24 52.32 0.710 ^ 0.082 – 0.450 ^ 0.044 – 2004 Jan. 26.34 53.42 – 0.556 ^ 0.058 – – 2004 Feb. 7.24 65.32 0.937 ^ 0.112 0.751 ^ 0.045 0.533 ^ 0.028 0.273 ^ 0.066 2004 Feb. 15.22 73.30 0.756 ^ 0.147 0.576 ^ 0.050 0.517 ^ 0.042 – 2004 Feb. 28.13 86.21 – – 0.517 ^ 0.047 0 ^ 0.114 2004 Mar.