SIMBAD references

We present an analysis of a 35.6 ks observation made with the Chandra X-Ray Observatory of the nearby spiral Scd galaxy NGC 2403. Forty-one discrete sources are detected at a significance of 3 σ or greater within the portion of the galaxy sampled by the Chandra detectors. The luminosities of the discrete sources range from ∼10³⁶ through ∼10³⁹ ergs.s^–1 over the energy range of 0.5-10 keV. We have also analyzed Einstein, ROSAT, and ASCA observations of this galaxy to search for time variability from the detected sources. One luminous X-ray source seen in an Einstein observation of this galaxy is not detected by Chandra or in a separate observation made with ASCA, while a luminous source observed by Chandra was not detected by Einstein or ASCA. For the four most luminous X-ray sources detected by the Chandra observation–including one previously identified as an ultraluminous X-ray source–spectral fits to these sources provide the best broadband CCD spectra to date. We have searched for counterparts at optical and radio wavelengths. Only one of the 35 optically identified supernova remnants (SNRs) in this galaxy, MFBL 31, is clearly associated with an X-ray source in our sample. Three other optically identified SNRs–MFBL 15, MFBL 17, and MFBL 29–may also have X-ray counterparts, but the associations are far less robust. A similar search for X-ray counterparts to known discrete radio sources seen toward NGC 2403 yields a single coincidence between an X-ray source and a candidate radio SNR, TH-2. None of the detected X-ray sources are coincident with either candidate globular clusters or massive star clusters associated with NGC 2403. The only historical supernova in NGC 2403, SN 1954J, is not detected; neither is an X-ray counterpart to the galaxy's optical nucleus detected. Finally, we confirm the diffuse emission results of Fraternali et al.