Molecular Gas Near Gamma-Ray Bursts

If a GRB occurs in or near a molecular cloud, the ultraviolet radiation from the GRB will dissociate and ionize the initially molecular gas. In the process, large column densities of vibrationally-excited H2 can be created (Draine 2000).

We have recently carried out detailed calculations of the propagation of the ionization/dissociation front through the molecular gas (Draine and Hao 2002), and have calculated transmission spectra showing the rich spectrum of absorption lines between 1750 A and 1110 A arising from vibrationally-excited levels of H2. These absorption lines, normally too weak to be detected in the interstellar medium, could be prominent in spectra of GRB afterglows if there is H2 on the sightline which is not too far from the GRB.

I provide here absorption spectra for the ionization/dissociation/dust destruction fronts calculated by Draine and Hao (2002). Transmission spectra are calculated for various different instrumental resolutions R = lambda/FWHM. The transmission factor I/I_0 includes the effects of continuous absorption by H2+, and line absorption by H and H2. 28765 different allowed (Lyman and Werner band) ultraviolet transitions of H2 are included in these calculations. If you would like to compare observations carried out using a different instrumental resolution R, contact <draine@astro.princeton.edu>; I may be able recalculate the theoretical spectrum for the appropriate value of R.

References:

• Draine, B.T. 2000, "Gamma Ray Bursts in Molecular Clouds: H_2 Absorption and Fluorescence", ApJ, 532, 273-280 (pdf)
• Draine, B.T., & Hao, L. 2002, "Gamma Ray Burst in a Molecular Cloud: Destruction of Dust and H_2, and Emergent Spectrum," ApJ, 569, 780-791 (pdf)

Model results:

• GRB in cloud with n_H = 10^3 cm^{-3}, GRB spectrum L_nu propto nu^{-0.5}:
  • h2abs_list_n3_b5_1000.gz (gzipped file with list of individual H2 lines with equivalent widths)
  • h2abs_spec_n3_b5_260_1000.gz (gzipped file with absorption spectrum calculated for resolution R=260)
  • h2abs_spec_n3_b5_350_1000.gz (gzipped file with absorption spectrum calculated for resolution R=350)
  • h2abs_spec_n3_b5_1e3_1000.gz (gzipped file with absorption spectrum calculated for resolution R=1000)
  • h2abs_spec_n3_b5_3e3_1000.gz (gzipped file with absorption spectrum calculated for resolution R=3000)
  • h2abs_spec_n3_b5_1e4_1000.gz (gzipped file with absorption spectrum calculated for resolution R=10000)
  • h2abs_spec_n3_b5_5e4_1000.gz (gzipped file with absorption spectrum calculated for resolution R=50000)
• GRB in cloud with n_H = 10^4 cm^{-3}, GRB spectrum L_nu propto nu^{-0.5}

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