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Description
Archival Chandra observations are used to study the X-ray emission associated with star formation in the central region of the nearby (D = 3.2 Mpc, 1 arcminute = 1 kpc) SAB(s)cd galaxy NGC 2403. The distribution of X-ray emission is compared to the morphology visible at other wavelengths using complementary Spitzer, Galaxy Evolution Explorer, and ground-based H-alpha imagery. In general, the brightest X-ray emission is associated with H II regions and to other star-forming structures, but is more pervasive, existing also in regions devoid of strong H-alpha and UV emission. NGC 2403 was observed in full-frame mode with the Chandra ACIS-S on four occasions for a total of ~ 180 ks, on 2001 Apr 17, 2004 Aug 13, 2004 Oct 03 and 2004 Dec 22. The source-finding tool described by Tennant (2006, AJ, 132, 1372) was applied to all 4 individual data sets and to the merged data set in order to search for discrete X-ray sources. The search was limited to the cnetral 6' x 6' (6 kpc x 6 kpc) region and to events within the full Chandra energy range 0.3-8.0 keV. Fifty eight point sources were detected in the merged data set with a signal-to-noise ratio (S/N) above 2.8 and with a minimum of 5 sigma above background uncertainty (corresponding to a detection limit of 8-10 counts for a typical on-axis source). These sources were listed in Table 2 of the reference paper and and are contained in the present HEASARC table. They can be selected by specifying source_type = 'Point Source'. The X-ray data were also examined to see if there was emission from known SNRs and H II regions after masking out the afore-mentioned X-ray point sources (see Section 2.1 of the reference paper for full details). Events falling within the areas defined by 24 optically identified SNRs that were imaged on the S3 chip in the first three observations were used to construct a composite spectrum. This stacked spectrum was fit by an absorbed 1-T APEC model with the hydrogen column density as a free parameter in XSPEC which was then used to translate the observed net count rates into X-ray luminosities. Only 4 or 5 of these SNRs are likely to be 'truely' detected X-ray sources. The SNRs can be selected in the present HEASARC table by specifying source_type ='SNR'. A similar procedure was used to search the X-ray data for the presence of X-ray emission at the locations of 47 H II regions in NGC 2403. Events falling within the areas defined by 47 H II regiuons that were imaged on the S3 chip in the first three observations were used to construct a composite spectrum. This stacked spectrum was fit by an absorbed 2-T APEC model with the hydrogen column density as a free parameter in XSPEC which was then used to translate the observed net count rates into X-ray luminosities. Only the most X-ray-luminous H II regions are likely to be 'truely' detected X-ray sources. The H II regions can be selected in the present HEASARC table by specifying source_type ='HII Region'. This table was created by the HEASARC in June 2012 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/AJ/139/1066">CDS Catalog J/AJ/139/1066</a> files table2.dat, table5.dat and table7.dat. This is a service provided by NASA HEASARC .
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