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Astron. Astrophys. 327, 562-568 (1997)

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6. X-ray properties

NGC 1427A is clearly detected with ROSAT PSPC (observation RP600043) as an X-ray source with 445 [FORMULA] net photons in the full spectral range of the ROSAT PSPC (0.1 to 2.4 keV). The spectrum appears to be relatively soft with most of the photons at energies lower than 1 keV. Neither a fit with a Raymond-Smith thin plasma, nor a power law gave an acceptably good fit to the spectrum. This may imply that the spectrum is a composite of emission from stars, supernovae, and true diffuse hot gas, typical for an irregular galaxy (see e.g. Bomans et al. 1997). We can estimate a total luminosity in the 0.5-2.0 keV band of the ROSAT PSPC to be 5 [FORMULA] ergs s-1, adopting our distance modulus and a foreground absorption of 6 [FORMULA] cm-2. This appears somewhat higher than the only published X-ray luminosity of the LMC (Wang et al. 1991), but both the Einstein IPC data of the LMC and the unsatisfactory fit to our ROSAT PSPC spectrum of NGC 1427A leave a significant margin of uncertainty. We conclude, that both the X-ray luminosity and hardness of NGC 1427A are not atypical for a large irregular galaxy (Bomans et al. 1997) and similar to the values of the LMC. Much deeper and higher spatial and spectral resolution data (with the upcoming X-ray satellites AXAF and XMM) are necessary to investigate the origin of the X-ray emission of NGC 1427A further. Also an integrated spectrum and a total X-ray luminosity of the LMC derived from the ROSAT All Sky Survey would be of great value for future studies of the X-ray properties of more distant star forming galaxies.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998