Multiwavelength study of Chandra X-ray sources in the Antennae

Abstract

We use Wide-field InfraRed Camera (WIRC) infrared (IR) images of the Antennae (NGC 4038/ 4039) together with the extensive catalogue of 120 X-ray point sources to search for counterpart candidates. Using our proven frame-tie technique, we find 38 X-ray sources with IR counterparts, almost doubling the number of IR counterparts to X-ray sources that we first identified. In our photometric analysis, we consider the 35 IR counterparts that are confirmed star clusters. We show that the clusters with X-ray sources tend to be brighter, K s ≈ 16 mag, with (J− K s ) = 1.1 mag.We then use archival Hubble Space Telescope (HST) images of the Antennae to search for optical counterparts to the X-ray point sources. We employ our previous IR-to-X-ray frame-tie as an intermediary to establish a precise optical-to-X-ray frame-tie with <0.6 arcsec rms positional uncertainty. Due to the high optical source density near the X-ray sources, we determine that we cannot reliably identify counterparts. Comparing the HST positions to the 35 identified IR star cluster counterparts, we find optical matches for 27 of these sources. Using Bruzual-Charlot spectral evolutionary models, we find that most clusters associated with an X-ray source are massive, and young, ∼10 6 yr.

X-Ray Binaries and Star Clusters in the Antennae: Optical Cluster Counterparts

“…Sources X45 and X49 are in our list but not in Clark et al (2011). They are coincident with clusters that have ages of ≈50 Myr and are somewhat fainter than most of the very young clusters in our sample, and hence may not have been detected in the WIRC observations used by Clark et al (2011).…”

“…In the remaining cases, no obvious optical counterpart to the candidate XRBs is observed. Clark et al (2011) found IR counterparts (within 2 ) to 32 of the X-ray sources listed in the Zezas et al (2006) catalog that are within the main body of the Antennae (see their Figure 1). Excluding sources that are beyond the field of view of the ACS images used here, close to the northern nucleus, and those that may be detections of diffuse plasma, we have 17 sources in common with Clark et al (2011).…”

“…This is similar to, although somewhat smaller than, the ≈0. 6 positional uncertainty between the X-ray positions and the WIRC IR images used in the Clark et al (2011) study.…”

“…1 pixel −1 ). Clark et al (2011) found 32 likely IR counterparts to the X-ray sources. They estimated ages for a subset of 10 sources by comparing integrated UBVIJK photometry with predictions from the STARBURST99 stellar population models , and derived ages for all 10 clusters of log(τ/yr) ≈ 6.9-7.5.…”

“…The last two scenarios, which lead to a situation where XRBs are close to but not coincident with star clusters, will be studied in a second paper (Paper II). Clark et al (2011) recently compared X-ray sources in the Antennae compiled by Zezas et al (2006) with near-infrared images from the WIRC camera on the Palomar 5 m telescope, and optical images from the WFPC2 camera on the Hubble Space Telescope (HST). The FWHM of the IR images is approximately 1 , and that of the optical images is ≈0.…”