Based on target-of-opportunity observations collected in service mode under programme ID 078.D-0416, P.I. Vreeswijk, with the FOcal Reducer/low dispersion Spectrograph 2 (FORS2; Appenzeller et al. 1998) installed at the Cassegrain focus of the Very Large Telescope (VLT), Unit 1, Antu, operated by the European Southern Observatory (ESO) on Cerro Paranal in Chile.
GRB 070125 and the environments of spectral-line poor afterglow absorbers★
Article first published online: 31 AUG 2011
© 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS
Monthly Notices of the Royal Astronomical Society
Volume 418, Issue 1, pages 129–144, November 2011
How to Cite
De Cia, A., Starling, R. L. C., Wiersema, K., van der Horst, A. J., Vreeswijk, P. M., Björnsson, G., de Ugarte Postigo, A., Jakobsson, P., Levan, A. J., Rol, E., Schulze, S. and Tanvir, N. R. (2011), GRB 070125 and the environments of spectral-line poor afterglow absorbers. Monthly Notices of the Royal Astronomical Society, 418: 129–144. doi: 10.1111/j.1365-2966.2011.19471.x
- Issue published online: 10 NOV 2011
- Article first published online: 31 AUG 2011
- Accepted 2011 July 18. Received 2011 June 28; in original form 2011 May 12
- gamma-ray burst: individual: GRB 070125
GRB 070125 is among the most energetic bursts detected and the most extensively observed so far. Nevertheless, unresolved issues are still open in the literature on the physics of the afterglow and on the gamma-ray burst (GRB) environment. In particular, GRB 070125 was claimed to have exploded in a galactic halo environment, based on the uniqueness of the optical spectrum and the non-detection of an underlying host galaxy. In this work we collect all publicly available data and address these issues by modelling the near-infrared to X-ray spectral energy distribution (SED) and studying the high signal-to-noise ratio Very Large Telescope/FOcal Reducer/low dispersion Spectrograph afterglow spectrum in comparison with a larger sample of GRB absorbers. The SED reveals a synchrotron cooling break in the ultraviolet, low equivalent hydrogen column density and little reddening caused by a Large Magellanic Cloud type or Small Magellanic Cloud type extinction curve. From the weak Mg ii absorption at z= 1.5477 in the spectrum, we derived log N(Mg ii) = 12.96+0.13− 0.18 and upper limits on the ionic column density of several metals. These suggest that the GRB absorber is most likely a Lyman limit system with a 0.03 < Z < 1.3 Z⊙ metallicity. The comparison with other GRB absorbers places GRB 070125 at the low end of the absorption-line equivalent width distribution, confirming that weak spectral features and spectral-line poor absorbers are not so uncommon in afterglow spectra. Moreover, we show that the effect of photoionization on the gas surrounding the GRB, combined with a low N(H i) along a short segment of the line of sight within the host galaxy, can explain the lack of spectral features in GRB 070125. Finally, the non-detection of an underlying galaxy is consistent with a faint GRB host galaxy, well within the GRB host brightness distribution. Thus, the possibility that GRB 070125 is simply located in the outskirts of a gas-rich, massive star-forming region inside its small and faint host galaxy seems more likely than a gas-poor, halo environment origin.