Multiwavelength observations of the extreme X-ray-selected BL Lacertae object PG 1553+11 (1ES 1553+113)

PG 1553+11 was the target of a coordinated 3 week multiwavelength campaign during 2003 April and May. A significant X-ray flare was observed during the second half of this campaign. Although no optical flare was recorded during the X-ray campaign, optical observations obtained immediately prior to the campaign displayed a flux higher than that recorded during the campaign. An optical flare was observed a few days after the end of the X-ray campaign and may be related to the X-ray flare. Radio observations were made at three frequencies, with no significant changes in flux detected near the times of the optical and X-ray flares. The spectral energy distributions and flux ratios in different wave bands observed for this object are compared to other X-ray-selected blazars to demonstrate that PG 1553+11 is an extreme member of this group.