New optical spectra and general discussion on the nature of ULXs

¹ Max-Planck-Institut für Astrophysik, Karl Schwarzschild-Str.1, Postfach 1317, 85741 Garching, Germany
² Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain e-mail: cgc@ll.iac.es
³ Astronomisches Institut der Universität Basel, Venusstrass 7, 4102 Binningen, Switzerland e-mail: martinlc@astro.unibas.ch

Corresponding author: H. Arp, arp@mpa-garching.mpg.de

Received: 16 September 2003
Accepted: 29 January 2004

Abstract

We present optical spectroscopic observations of three Ultra Luminous X-ray sources (ULXs). 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  and , 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 22 additional X-ray sources close to nearby galaxies which also fit the criteria of ULXs 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 ULXs 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.