{
  "bibcode": "2003GCN..2469....1R",
  "body": "Reichart, D. (U. North Carolina) comments:\n\nAssuming that the new, bright source that Zerbi et al. (GCN 2466) have\nidentified in H and Ks in the very small (<6\") S1 XMM-Newton error circle\n(Santos-Lleo & Calderon, GCN 2464) is indeed the afterglow of GRB 031203\n(Gotz et al., GCN 2459), the deep limits that Bailyn et al. (GCN 2468)\nmeasure at the location of this source in I and J do indeed imply a sharp\nspectral break between the H and J bands:  J - H > 5 mag and H - Ks ~ 1 mag\nwhen corrected for Galactic extinction (Bailyn et al., GCN 2468).\n\nAlthough one explanation for this dropout signature might be that GRB\n031203 is at redshift z ~ 10, this is difficult to reconcile with the\nbrightness of the afterglow (Price, private communication), which is\nprobably in the top 10% of all afterglows.\n\nNor can so sharp a spectral break be explained by extinction by regular\ndust, whether Galactic or in the source frame.\n\nHowever, the dust along the line of sight, at least within hundreds of\nparsecs of the GRB, should *not* be regular.  Although dust within parsecs\nto tens of parsecs of the GRB will be returned to the gas phase by\nsublimation, dust at greater distances -- out to hundreds of parsecs --\nwill be repeatedly fragmented by grain charging and Coulomb explosions\n(Waxman & Draine 2000, Fruchter, Krolik & Rhoads 2001, Reichart 2001).\n\nIf most of the dust fragments down to PAH sizes, its visual absorption edge\n-- which can be quite sharp -- will shift to the source-frame R or I band\n(the exact wavelength depends on the size of the grains; Li & Draine 2001).\nLonger-wavelength light will pass through this dust as if were not even\nthere.\n\nFor a GRB at a typical redshift of z ~ 1, this visual absorption edge will\nbe redshifted to between the J and H bands.  Such a redshift would also be\neasier to reconcile with the brightness of the GRB 031203 afterglow in the\nH and Ks bands.\n\nSo actually, every highly extinguished, low- to moderate-redshift GRB\nafterglow might look like this in the NIR.\n\nFor GRB 031203, this hypothesis can be tested by (1) pursuing NIR\nspectroscopy tonight while the afterglow is still sufficiently bright, but\nbe warned that if at z ~ 1 H-alpha might be blueward of the spectral break,\nand/or (2) pursuing deep imaging blueward of the spectral break in hopes of\nidentifying a typical-redshift host galaxy, but again be warned:  since the\nGalactic A_V ~ 3 mag and possibly more along this line of sight, J, z, and\nI bands are preferred.\n\nThe Follow-Up Network for Gamma-Ray Bursts (a.k.a. the FUN GRB\nCollaboration), the resources of which are currently concentrated more in\nthe northern hemisphere, will not be able to pursue either of these\nobserving strategies tonight.",
  "circularId": 2469,
  "createdOn": 1070604377000,
  "email": "reichart@physics.unc.edu",
  "subject": "GRB 031203: z ~ 10 or Fragmented Dust in the Circumburst Environment?",
  "submitter": "Daniel E. Reichart at U.North Carolina  <reichart@physics.unc.edu>",
  "eventId": "GRB 031203"
}