Compact Optical Counterparts of Ultraluminous X-ray Sources

📝 Abstract

Using archival Hubble Space Telescope (HST) imaging data, we report the multiband photometric properties of 13 ultraluminous X-ray sources (ULXs) that have a unique compact optical counterpart. Both magnitude and color variation are detected at time scales of days to years. The optical color, variability, and X-ray to optical flux ratio indicate that the optical emission of most ULXs is dominated by X-ray reprocessing on the disk, similar to that of low mass X-ray binaries. For most sources, the optical spectrum is a power-law, $F_{\nu} \propto \nu^{\alpha}$ with $\alpha$ in the range 1.0 to 2.0 and the optically emitting region has a size on the order of 1e12 cm. Exceptions are NGC 2403 X-1 and M83 IXO 82, which show optical spectra consistent with direct emission from a standard thin disk, M101 ULX-1 and M81 ULS1, which have X-ray to optical flux ratios more similar to high-mass X-ray binaries, and IC 342 X-1, in which the optical light may be dominated by the companion star. Inconsistent extinction between the optical counterpart of NGC 5204 X-1 and the nearby optical nebulae suggests that they may be unrelated.

💡 Analysis

Using archival Hubble Space Telescope (HST) imaging data, we report the multiband photometric properties of 13 ultraluminous X-ray sources (ULXs) that have a unique compact optical counterpart. Both magnitude and color variation are detected at time scales of days to years. The optical color, variability, and X-ray to optical flux ratio indicate that the optical emission of most ULXs is dominated by X-ray reprocessing on the disk, similar to that of low mass X-ray binaries. For most sources, the optical spectrum is a power-law, $F_{\nu} \propto \nu^{\alpha}$ with $\alpha$ in the range 1.0 to 2.0 and the optically emitting region has a size on the order of 1e12 cm. Exceptions are NGC 2403 X-1 and M83 IXO 82, which show optical spectra consistent with direct emission from a standard thin disk, M101 ULX-1 and M81 ULS1, which have X-ray to optical flux ratios more similar to high-mass X-ray binaries, and IC 342 X-1, in which the optical light may be dominated by the companion star. Inconsistent extinction between the optical counterpart of NGC 5204 X-1 and the nearby optical nebulae suggests that they may be unrelated.

📄 Content

arXiv:1106.0315v1 [astro-ph.HE] 1 Jun 2011 Draft version July 25, 2018 Preprint typeset using LATEX style emulateapj v. 11/10/09 COMPACT OPTICAL COUNTERPARTS OF ULTRALUMINOUS X-RAY SOURCES Lian Tao1,2, Hua Feng1, Fabien Gris´e2, and Philip Kaaret2 Draft version July 25, 2018 ABSTRACT Using archival Hubble Space Telescope (HST) imaging data, we report the multiband photometric properties of 13 ultraluminous X-ray sources (ULXs) that have a unique compact optical counterpart. Both magnitude and color variation are detected at time scales of days to years. The optical color, variability, and X-ray to optical flux ratio indicate that the optical emission of most ULXs is dominated by X-ray reprocessing on the disk, similar to that of low mass X-ray binaries. For most sources, the optical spectrum is a power-law, Fν ∝να with α in the range 1.0 to 2.0 and the optically emitting region has a size on the order of 1012 cm. Exceptions are NGC 2403 X-1 and M83 IXO 82, which show optical spectra consistent with direct emission from a standard thin disk, M101 ULX-1 and M81 ULS1, which have X-ray to optical flux ratios more similar to high-mass X-ray binaries, and IC 342 X-1, in which the optical light may be dominated by the companion star. Inconsistent extinction between the optical counterpart of NGC 5204 X-1 and the nearby optical nebulae suggests that they may be unrelated. Subject headings: black hole physics — accretion, accretion disks — galaxies: stellar content

1. INTRODUCTION Ultraluminous X-ray sources (ULXs) are nonnuclear X-ray sources with luminosity above the Eddington limit of stellar mass black holes (LX > 3 × 1039 erg s−1) as- suming isotropic emission. ULXs with fast variability or long-term chaotic flux variation may contain accret- ing black holes in an attached binary system. They could be powered by accretion onto either intermedi- ate mass black holes of 102 −104 M⊙at sub-Eddington rate (Colbert & Mushotzky 1999; Makishima et al. 2000; Kaaret et al. 2001) or normal or slightly massive stellar black holes of ≲100 M⊙at super or near Eddington rate (Watarai et al. 2001; Begelman 2002). In black hole binaries, the X-ray emission probes the deep potential at regions close to the event horizon while the optical emission is important to constrain their masses, interaction with environment, and evolutionary history of the binary. Optical observations of ULXs have played a key role in unveiling the nature of these sources. Many bright ULXs are found to be spatially associated with an optical nebula (Pakull et al. 2003). These nebu- lae are large with an extent of tens to hundreds pc, and expanding at a velocity in the order of 102 km s−1. Most of them are powered by strong shocks created by out- flows from the binary hitting the surrounding medium, and in a few cases, extraordinary photoionization is re- quired (Kaaret et al. 2004; Kaaret & Corbel 2009). The photoionized nebulae can be used as bolometers to ex- treme ultraviolet and soft X-ray photons, and suggest that the ULX luminosities are indeed high. Point-like op- tical objects as compact counterparts of ULXs have been identified for 10 sources in the literature. The companion star, intrinsic emission from the disk at large radii, and disk irradiation may all contribute to the optical emission and even dominate at different wavelengths. Binary pop- 1 Department of Engineering Physics and Center for Astro- physics, Tsinghua University, Beijing 100084, China 2 Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA ulation synthesis indicates that colors and magnitudes of the optical counterpart of ULXs can help distinguish be- tween stellar mass and intermediate mass black holes, and a sample of 6 sources favors the latter interpretation (Madhusudhan et al. 2008). The continuum component in the optical spectrum of NGC 5408 X-1 is consistent with that from a standard accretion disk with irradiation (Kaaret & Corbel 2009). However, this result is obtained from a ground-based telescope so the nebular contribu- tion cannot be directly resolved and has to be subtracted based on model prediction. Therefore, in most cases, HST observations are required for the study of the com- pact optical counterpart of ULXs, in particular for the continuum emission. There are 10 ULXs with a unique optical counterpart identified in the literature. We make 3 new identifica- tions in this paper. Here, we analyze all available HST imaging data for these 13 ULXs, see Table 2, to investi- gate their multiband spectra and variation at long time scales. The observations and data analysis are described in § 2, and the results are discussed in § 3 and conclusions summarized in § 4.
2. OBSERVATIONS AND DATA ANALYSIS 2.1. Sample Up to beginning of analysis work for this paper, there have been 10 ULXs with a unique optical counter- part identified: Holmberg II X-1 (Kaaret et al. 2004), Holmberg IX X-1 (Gris´e et al. 2011), IC 342 X-1 (Feng & Kaaret

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