Submitted to ApJ Supplement Ultra-luminous X-ray Sources in nearby galaxies from ROSAT HRI observations I. data analysis Ji-Feng Liu and Joel N. Bregman Astronomy Department, University of Michigan, MI 48109 ABSTRACT X-ray observations have revealed in other galaxies a class of extra-nuclear X-ray point sources with X-ray luminosities of 1039–1041 erg/sec, exceeding the Eddington luminosity for stellar mass X-ray binaries. These ultra-luminous X- ray sources (ULXs) may be powered by intermediate mass black hole of a few thousand M⊙ or stellar mass black holes with special radiation processes. In this ′ paper, we present a survey of ULXs in 313 nearby galaxies with D25>1 within 40 Mpc with 467 ROSAT HRI archival observations. The HRI observations are reduced with uniform procedures, refined by simulations that help define the point source detection algorithm employed in this survey. A sample of 562 38 43 extragalactic X-ray point sources with LX = 10 –10 erg/sec is extracted from 173 survey galaxies, including 106 ULX candidates within the D25 isophotes of 63 galaxies and 110 ULX candidates between 1–2 D25 of 64 galaxies, from × which a clean sample of 109 ULXs is constructed to minimize the contamination from foreground or background objects. The strong connection between ULXs and star formation is confirmed based on the striking preference of ULXs to arXiv:astro-ph/0501309v1 14 Jan 2005 occur in late-type galaxies, especially in star forming regions such as spiral arms. ULXs are variable on time scales over days to years, and exhibit a variety of long term variability patterns. The identifications of ULXs in the clean sample show some ULXs identified as supernovae (remnants), HII regions/nebulae, or young massive stars in star forming regions, and a few other ULXs identified as old globular clusters. In a subsequent paper, the statistic properties of the survey will be studied to calculate the occurrence frequencies and luminosity functions for ULXs in different types of galaxies to shed light on the nature of these enigmatic sources. Subject headings: catalogs – galaxies: general – X-rays: binaries – X-rays: galax- ies –2– 1. INTRODUCTION Ultra-luminous X-ray sources (ULXs) are extra-nuclear sources with luminosities in the range of 1039 1041 erg/sec in other galaxies, and have been observed by ROSAT, ASCA, − recently by XMM-Newton and Chandra Observatory in large numbers. As compared to the cases of the X-ray binaries in our Galaxy, which are powered by accretion onto neutron stars or stellar mass black holes and have luminosities of 1033 1038 erg/sec, the luminosities of 3 − 4 −2 ULXs require accreting compact objects of masses 10 – 10 M⊙ if they emit at 10 of the Eddington luminosity, typical of Galactic X-ray binaries. While the required masses could be much larger if they emit at much less than 10−2 of the Eddington luminosity, as in the cases of some low luminosity active galactic nuclei (AGN), the masses cannot be much greater than 5 10 M⊙ for these extra-nuclear sources to survive the dynamic friction over a few Gigayears (Colbert 1999). Such intermediate mass compact objects can only be black holes, and if they exist, are the missing links between stellar mass black holes and super-massive black holes in the nuclei of galaxies. While the explanation with intermediate mass black holes is simple, intriguing and astrophysically interesting, such black holes are not predicted by ordinary stellar evolution theories. It is suggested that black holes of a few hundred M⊙ can form from the death of Pop III stars, and more massive ones might form from multiple stellar interactions in some dense clusters, hencing manifest as ultra-luminous X-ray sources (Portegies Zwart et al. 2002). Alternatively, these X-ray sources could be stellar mass black holes or neutron stars whose apparent super-Eddington luminosities are due to some special processes. One suggestion is that radiation pressure-dominated accretion disks with photon-bubble instabilities are able to emit truly super-Eddington luminosities (Begelman 2002). Another suggestion is that beaming effects can produce the observed luminosities of ULXs (King et al. 2001). The leading goals in ULX studies are to determine the masses of the primary, to under- stand how and where they form, and to find out how they emit at such high luminosities. In the last few years many efforts have been made to address these questions, and important clues have been revealed. However, these studies mainly focus on some well-known objects and galaxies (e.g., M81 X-9, NGC5204 X-1, Antenna galaxy, Cartwheel galaxy) except for a few works (e.g., Colbert & Ptak, 2002, hereafter CP2002; Swartz et al. 2004; Ptak & Colbert 2004), and now it is time to define a complete sample of ULXs to study the properties of the phenomenon and test critical ideas. One resource to rely on for this purpose is the data archive of the ROSAT High Resolution Imager (HRI), which includes 5403 observations in the ten years from June 1990 to February 1999. The HRI is a multichannel plate detector of 38′ 38′ square field of view, large enough to contain all individual nearby galaxies other × than LMC, SMC, M31, and M33 in our Local Group. Its spatial resolution is suitable for –3– extra-galactic point source studies, with on-axis FWHM < 5′′ and a pixel scale of 0.′′5/pixel, adequate to resolve point sources in most cases. Also, the archival observations have rea- sonable sky coverage for survey purposes. For example, the 5393 pointed HRI observations used in the First ROSAT HRI Source Catalog (1RXH; ROSAT scientific team, 2000) covers about 2% of the sky. The large database of ROSAT HRI observations has not been thoroughly exploited for complete samples of ULXs in nearby galaxies. Roberts & Warwick (2000; hereafter RW2000) have used the HRI archive to study the X-ray properties of nearby galaxies, and detected in 83 galaxies 187 discrete X-ray sources of all luminosity ranges, among which 27 non- 39 nuclear sources have LX >10 erg/sec and can be taken as ULXs. They used the nearby galaxy sample by Ho, Filippenko & Sargent (1995), which was constructed to search for dwarf Seyfert nuclei in nearby galaxies and contains 486 bright northern galaxies. Many nearby galaxies with HRI observations are not included in this sample, and the HRI archive is far from being fully utilized for surveying ULX. Recently Colbert & Ptak (2002) made an effort to analyze the HRI observations to search for ULXs in a sample of 9999 galaxies in the Third Reference Catalog of galaxies (RC3; de Vaucouleurs et al. 1991) with cz<5000 km/sec. They found 87 ULXs in 54 galaxies, with 37 in early-type galaxies. However, many ULXs in the CP2002 catalog are projected far from the host galaxies, and may be false ULXs from foreground stars or background AGN/QSOs. For example, Irwin et al. (2004) pointed out that the radial distribution of ULXs in early-type galaxies in the CP2002 catalog is consistent with a random distribution, thus these ULXs are probably not physically associated with these early-type galaxies. Here we present our study of ULXs in nearby galaxies with the wealth of HRI archive. To fully utilize the HRI archive, we choose all observations associated with any RC3 galaxies within 40 Mpc with isophotal diameters >1 arcminute. The RC3 galaxy sample, the selected HRI observations and the survey galaxies are described in section 2. In our analysis a wavelet algorithm is used for point source detection, and in section 3 we discuss its performance on HRI images through simulations. In section 4, we describe the analysis procedures applied on the data, including the point source detection, the variability tests, astrometric corrections, associations with galaxies and computation of luminosities. These efforts lead to a catalog of 562 extragalactic X-ray sources above 3σ detection within 2 D25 of 173 out of the 313 × survey galaxies, including 106 ULX candidates within the D25 isophotes of 63 galaxies and 110 ULX candidates between 1–2 D25 of 64 galaxies. The identifications of these sources × are given in section 5. As a final product, we construct a clean sample of ULXs to include ULXs within, and a few outside the D25 isophotes, with identified foreground stars and background QSOs excluded. The comments and finding charts are given for these ULXs in section 6. In section 7 we summarize the procedures and preliminary results of the survey. –4– In a subsequent paper, we will study the occurrence rates and luminosity functions for ULXs in different types of galaxies. 2. ROSAT HRI survey of nearby galaxies To search for the ultra-luminous X-ray sources in the nearby galaxies from the ROSAT HRI archival observations, we cross correlated a nearby galaxy sample with the list of HRI archival observations to select observations with nearby galaxies within their field of views. Here we describe the nearby galaxy sample, the selected HRI observations, and the survey galaxy sample. The nearby galaxy sample is extracted from RC3, one of the most complete catalog for nearby galaxies having apparent diameters larger than 1 arcminute at the D25 isophotal level, total B-band magnitudes BT brighter than 15.5, and redshifts not in excess of 15,000 km/sec. Distances to galaxies are collected from the literature, including the HST Key Project (designated as KP; Freedman et al. 2002), the surface brightness fluctuation method (SBF; Tonry et al. 2001), the nearby galaxy flow model by Tully (1992; T92), and the Nearby Galaxy Catalog (T88; Tully 1988).