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Description
A very sensitive X-ray investigation of the giant H II region N11 in the Large Magellanic Cloud (LMC) was performed using the Chandra X-ray Observatory in which 165 point sources were detected. The 300-ks observation reveals X-ray sources with luminosities (if at the 50 kpc distance of the LMC) down to 10<sup>32</sup> erg s<sup>-1</sup>, increasing the number of known point sources in the field by more than a factor of five. Among these detections are 13 massive stars (3 compact groups of massive stars, 9 O stars, and one early B star) with log(L<sub>X</sub>/L<sub>BOL</sub>) ~ -6.5 to -7, which may suggest that they are highly magnetic or colliding-wind systems. On the other hand, the stacked signal for regions corresponding to undetected O stars yields log(L<sub>X</sub>/L<sub>BOL</sub>) ~ -7.3, i.e., an emission level comparable to similar Galactic stars despite the lower metallicity. Other point sources coincide with 11 foreground stars, 6 late-B/A stars in N11, and many background objects. This observation also uncovers the extent and detailed spatial properties of the soft, diffuse emission regions, but the presence of some hotter plasma in their spectra suggests contamination by the unresolved stellar population. The Chandra ACIS-I observations of N11 were made in six separate segments within two months in 2007. As summarized in Table 1, the exposure time of each segment was 42-49 ks and the roll angle ranged from 130 to 188 degrees Cleaning of significant background flares, together with a correction for the dead time of the six observations, resulted in a total of 280 ks useful exposure for the subsequent analysis. A combination of source detection algorithms (wavelet, sliding-box, and maximum likelihood centroid fitting) were applied to unsmoothed data in three bands: soft (S): 0.5-2.0 keV, hard (H): 2-8 keV, and total (T): 0.5-8 keV. The final source list contains 165 sources with local false detection probability P < 10<sup>-6</sup> in at least one band (Poisson statistics were used in calculating the significance of a source detection above the local count background). The source detection, though optimized for point-like sources, includes a few strong peaks of diffuse X-ray emission, chiefly associated with the SNR N11L, which lies about ~7' west of the field center. The authors calculated the net (background-subtracted) count rates in four sub-bands (S1 = 0.5-1.0 keV, S2 = 1-2 keV, H1 = 2-4 keV and H2 = 4-8 keV, which were later added to form the count rates in the broader bands (S, H, and T). Source counts for each sub-band were then extracted within the 70% energy-encircled radius (EER) of the PSF, whose size depends on the off-axis angle of the source in the exposure and of the energy band under consideration. A background correction was also applied. Finally, count rates were derived by dividing source net counts by their effective exposure times (values at the source positions in the exposure map of the energy band under consideration), leading to equivalent on-axis values. It should be noted that the presented count rates have thus been corrected for the full PSF and for the effective exposure, which accounts not only for the telescope vignetting, but also for the degradation of the detector sensitivity over time. Therefore, the actual number of counts in a detection aperture is not simply a count rate multiplied by an exposure of 280 ks. The difference could be up to a factor of ~2, depending on a source's spectral shape. The authors searched for counterparts to their X-ray sources in several catalogs: the USNO-B1.0 Catalog (Monet et al. 2003), the Guide Star Catalog V2.3.2 (GSC, Lasker et al. 2008), the 2MASS All-Sky Catalog of Point Sources (Cutri et al. 2003), the Magellanic Clouds Photometric Survey (MCPS; Zaritsky et al. 2004, AJ, 128, 1606), the IRSF Magellanic Clouds Point Source Catalog (Kato et al. 2007, PASJ, 59, 615)), the DENIS Catalogue toward Magellanic Clouds (DCMC; Cioni et al. 2000, A&AS, 144, 235), and JHK<sub>s</sub> photometry of N11 young stellar objects ([HKN2006]; Hatano et al. 2006, AJ, 132, 2653). A best correlation radius of 1" was found to be optimal and was thus used to derive the final list of optical and infrared counterparts to the Chandra X-ray sources: 71 of the 165 sources have at least one counterpart within 1". The HEASARC has modified the counterpart names given in this table compared to those given in the reference paper so that they comply with the forms recommended by the CDS Dictionary of Nomenclature of Celestial Objects. This table was created by the HEASARC in August 2014 based primarily on the contents of Tables 2, 3 and 4 from the reference paper, machine-readable versions of which were obtained from the ApJS web site. Some information from Table 8 of the reference paper, viz., a number of the spectral types quoted for individual stars, was also used in populating the HEASARC-created class parameter. This is a service provided by NASA HEASARC .
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