New optical spectra and general discussion on the nature of ULX's

Abstract

We present spectroscopic observations of three Ultra Luminous X-ray sources (ULX's). Two of them are very close to the active galaxy NGC 720 and the other is near NGC 1073. The two around NGC 720 turn out to be quasars at z=0.964 and z= 2.216, the one near NGC 1073 seems to be associated to an HII region at the redshift of NGC 1073. We concentrate our analysis on the two quasars and analyze them in conjunction with a set of 20 additional X-ray sources close to nearby galaxies which also fit the criteria of ULX's and which also have been identified as quasars of medium to high redshift. This sample shows an unusually large fraction of rare BL Lac type objects. The high redshifts of these ULX's and their close proximity to their low redshift, supposedly parent galaxies is a surprising result in the light of standard models. We describe the main properties of each of these objects and their parent galaxy, and briefly discuss possible interpretations.

submitted to A&A, 8 pages, 5 ps/eps figures

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• The Discovery of a High Redshift X-ray Emitting QSO Very Close to the Nucleus of NGC 7319
• Optical Sources Near the Bright X-Ray Source in NGC 1073
• Toward a Clean Sample of Ultra-Luminous X-ray Sources
• Multiwavelength study of the bright X-ray source population in the interacting galaxies NGC5774/NGC5775
• Suzaku Observation of the Metallicity in the Interstellar Medium of NGC 4258
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• Optical Counterparts of Two Candidate Ultraluminous X-ray Sources in NGC 4536
• Quasars, Gamma Ray Bursters and BL Lacertids