2012A&A...546A..67L

A significantly higher incidence of strong (rest equivalent width W_r>1Å) intervening MgII absorption is observed along gamma-ray burst (GRB) sight-lines relative to those of quasar sight-lines. A geometrical explanation for this discrepancy has been suggested: the ratio of the beam size of the source to the characteristic size of an MgII absorption system can influence the observed MgII equivalent width, if these two sizes are comparable. We investigate whether the differing beam sizes of the continuum source and broad-line region of Sloan Digital Sky Survey (SDSS) quasars produce a discrepancy between the incidence of strong MgII absorbers illuminated by the quasar continuum region and those of absorbers illuminated by both continuum and broad-line region light. We performed a semi-automated search for strong MgII absorbers in the SDSS Data Release 7 quasar sample. The resulting strong MgII absorber catalog is available at the CDS. We measured the sight-line number density of strong MgII absorbers superimposed on and off the quasar CIVλ 1550 and CIII]λ 1909 emission lines. We see no difference in the sight-line number density of strong MgII absorbers superimposed on quasar broad emission lines compared to those superimposed on continuum-dominated spectral regions. This suggests that the MgII absorbing clouds typically observed as intervening absorbers in quasar spectra are larger than the beam sizes of both the continuum-emitting regions and broad line-emitting regions in the centers of quasars, corresponding to a lower limit of the order of 10¹⁷cm for the characteristic size of an MgII absorbing cloud.